Symposium Organizers
Norbert Koch Humboldt-Universitaet zu Berlin
Egbert Zojer Technische Universitaet Graz
Saw-Wai Hla Ohio University
Xiaoyang Zhu University of Texas-Austin
B3: Molecular Scale Electronics III
Session Chairs
Avik Ghosh
Latha Venkataraman
Tuesday AM, April 14, 2009
Room 2001 (Moscone West)
9:30 AM - **B3.1
Imaging the Local Properties of Graphene: a New Platform for Molecular Electronics.
Michael Crommie 1 , Y. Zhang 1 , V. Brar 1 , C. Girit 1 , A. Zettl 1
1 Physics Dept. and Materials Science Division, LBNL, UC Berkeley, Berkeley, California, United States
Show AbstractGraphene, a single atomic layer of carbon, provides an exciting new platform for molecular electronics due to its chemical and structural flexibility as well as its novel electrical, mechanical, and magnetic properties. Scanning tunneling microscopy (STM) is an ideal tool to study the properties of graphene at lengthscales necessary to evaluate its utility for molecule-scale device applications. We have used STM to explore backgated graphene flakes, and we observe surprisingly strong electron-phonon coupling in electronic tunneling spectra. We also find that the graphene charge neutral point (the Dirac point) manifests itself as a clear feature in tunnel spectra, and it can be shifted by applied gate voltage. By spatially mapping the Dirac point we are able to map out electron density inhomogeneities in graphene with a spatial resolution at the nm scale. Using this new technique we have observed molecule-induced charge inhomogeneities that coexist with energy-dependent electronic interference patterns in graphene, giving us new insight into the microscopic mechanisms that determine graphene electron mobility.
10:00 AM - B3.2
Metal/Insulator/Metal Thin Film Electrodes for Molecular Conduction.
Bing Hu 1 , Pawan Tyagi 1 , Bruce Hinds 1
1 Chemical and Materials Engineering, Univ. of Kentucky, Lexington, Kentucky, United States
Show AbstractProducing reliable electrical contacts with gaps having the dimensions of molecular lengths is a difficult challenge for molecular electronics. As a promising alternative to break-junctions, we use conventional film deposition and photolithography to form an exposed edge of a thin film multilayer structure (metal/insulator/metal). Molecules can self-assemble on the exposed edge offering an alternative conduction path through the molecules with angstrom-scale dimensional control. Critical to this approach is to have minimal background tunnel current through the insulator layer sandwiched between metal layers. Robust electrodes with aluminum oxide insulator layer are found on alloys of Al/Au and Ta/Au. The readily oxidized Al/Ta reduce surface energy for a mechanically stable interface with oxide but allow molecular contact with metallic gold at the pattern edge. Electrodes were successfully fabricated with this strategy with current measured through a metal coordination compound cluster composed of a cube with cyano linked Ni or Fe at the corners. Thiolacetate ligand tethers come off of the cluster core and bind the complex to the metal leads, allowing the molecule to span the insulator gap on the surface of the etched pattern. Molecules that do not bridge the gap are not electrically active. Along the 10um pattern edge approximately 6000 molecules are involved in conduction. 10nA per molecule is seen at 10mV bias. Tunnel current through the molecules is analyzed with Simmons model and barrier height is found to be 1.1 eV and tunnel length of 1.2nm.
10:15 AM - B3.3
Dipole Effects on Electron Transport through Helical Peptides Immobilized on Gold.
Shunsaku Kimura 1 , Tomoyuki Morita 1
1 Dept. of Material Chemistry, Kyoto University, Kyoto Japan
Show AbstractElectron injection or extraction from helical peptides to gold was studied to evaluate the dipole effects of Au-S linkages and helical peptides. The helical lengths of the helical peptides exceeds over 3 nm, where the electron hopping mechanism is prevailing. The molecular terminal has a ferrocene group as a redox species, and the electron transfer from the ferrocene unit to gold was evaluated by electrochemical methods. The electron transfer process comprises three regions; the hopping through the helical peptide under the peptide dipole, through the linker, and through the covalent connection between Au and the peptide under the Au-S dipole. Not only the dipoles but also the physical properties of the helical peptide SAMs were found to influence the electron transfer processes at the interface. Eventually, we try to extract the dipole effects on the electron transport through the helical peptide SAMs.
10:30 AM - B3.4
Creating and Characterizing Robust, Large Area Molecular Electronic Junctions.
Michael Preiner 1 , Nicholas Melosh 2
1 Applied Physics, Stanford University, Stanford, California, United States, 2 Materials Science and Engineering, Stanford University, Stanford, California, United States
Show AbstractWhile recent years have seen numerous advances in creating single-molecule electronic junctions, practical molecular electronic devices will almost certainly require junctions with large numbers of molecules. We demonstrate a technique for creating large area, electrically stable molecular junctions. We use atomic layer deposition to create nanometer thick passivating layers of aluminum oxide on top of self-assembled organic monolayers with hydrophilic terminal groups. This layer acts as a protective barrier and allows simple vapor deposition of the top electrode without short circuits or molecular damage. This method allows nonshorting molecular junctions of up to 9 mm2 to be easily and reliably fabricated. The effect of passivation on molecular monolayers is studied with Auger and x-ray spectroscopy, while electronic transport measurements confirm molecular tunneling as the transport mechanism for these devices. Using a similar processing method, we also demonstrate the ability to rapidly characterize (and passivate) single defects on hydrophobic SAMs.
10:45 AM - B3.5
Atomic Level Analysis of Polythiophene by the Scanning Atom Probe.
Osamu Nishikawa 1 , Masahiro Taniguchi 2 , Hitoshi Kato 3 , Satoru Tanemura 4
1 Chemistry & Biology, Kanazawa Institute of Technology, Nonoichi Japan, 2 Chemistry & Biology, Kanazawa Institute of Technology, Nonoichi Japan, 3 Physics of Materials, Kanto Gakuin University, Yokohama Japan, 4 Physics of Materials, Kanto Gakuin University, Yokohama Japan
Show AbstractThe atom probe (AP) is known to be an ultimate micro mass analyzer that allows atom-by-atom mass analysis. However, the analyzable area of the conventional AP is limited to a minute hemispherical area at the apex of an extremely sharp and long tip because a high field required to field evaporate surface atoms as positive ions can be generated by a relatively low voltage. The fabrication of such a filamentary long tip is extraordinary difficult for many materials. Accordingly, the preparation of the specimens, such as polymers, is another barrier for wide range applications of the atom probe. In order to overcome this difficulty, a funnel-shaped micro-extraction electrode is introduced in the conventional AP. Since this electrode scans over a planar specimen surface, this atom probe is named as a scanning atom probe (SAP). The extraction electrode confines the high field required for field evaporation of surface atoms into a small space between an apex of a minute cusp on the planar surface and an open end of the electrode. Thus, the SAP can analyze not only a sharp slender tip but also an apex area of the cusp. Thin films of conductive polythiophene are fabricated by electrochemically polymerizing thiophene monomers on an ITO substrate. Film thickness is about 10μm-30μm and the dopant is BF4-. A small piece of the polythiophene films is peeled off from the substrate and inserted into a narrow gap between two small Nichrome nails. Then the film is clipped by the nails. The Nichrome holder is introduced in the SAP and placed in front of the electrode. DC voltages and pulsed YAG laser beams are applied to the specimen. The pulse width of the pulsed laser is 5 ns and its wavelength is 532 nm. The field evaporated ions pass through the open hole of the electrode and enter the reflectron type mass analyzer. Mass resolution of the analyzer m/dm is better than 1000. Mass spectra of the analyzed polythiophene exhibit a large mass peak of SC4Hn2+, the radical of the polythiophene, and various singly charged fragment ions such as C2H+ and C3Hn+. The doubly charged radical ions indicate that the radicals are strongly bound. All sulphur atoms are detected as S-C clusters such as SC+, SC2+ and SC3Hn+. Although no fluorine ions are detected, most boron atoms are detected as the cluster ions with S and C such as SCB2+ and C4HB+. Presently the mass analysis of the polythiophene films containing C60 molecules are under progress. The distribution of C60 and dopants in the polythiophene films will be discussed.
11:30 AM - B3.6
Monitoring Dynamic Molecular Electronic Processes at Buried Interfaces with Ångstrom Resolution using X-Ray Reflectivity.
Jason Fabbri 1 , Michael Toney 1 , Nazanin Davani 1 , Ken Shimizu 1 , Nicholas Melosh 1
1 , Stanford University, Stanford, California, United States
Show AbstractMolecular electronics has been proposed as a successor to conventional CMOS technology as device dimensions continue to shrink. Interesting electrical behavior has been observed in molecular junctions and structural and spectroscopic probes are needed to understand and improve upon it. We demonstrate the utility of X-ray reflectivity as an in-situ probe of molecular electronic junctions under electrical bias. In the present study, we have examined chlorophyll monolayers deposited on silicon bottom electrodes using the Langmuir-Blodgett technique. The spot size of the X-ray beam necessitates large area junctions (~ cm^2) posing a formidable fabrication challenge. Using a soft top contact deposition technique we have developed we obtain functioning molecular junctions. From the X-ray reflectivity curves, we obtain precise structural characterization of the junction. In addition, by applying a pulsed voltage sequence we detect changes in reflected intensity as a function of applied electric field. Control experiments and modeling show that these changes can be attributed to polarization of the molecular layer. Further modeling allows us to extract the voltage drop across the monolayer, an important parameter for molecular electronic devices. Finally, we will discuss the general utility of this technique as a highly spatially sensitive probe of electronic changes in thin film devices.
11:45 AM - B3.7
Single Nanometric Memory Unit Based On a Protein-Nanoparticle Hybrid.
Izhar Medalsy 1 , Arnon Heyman 2 , Or Dgany 2 , Oded Shoseyov 2 , Danny Porath 1
1 Physical Chemistry , The Hebrew University, Jerusalem Israel, 2 Agriculture, The Hebrew University, Jerusalem Israel
Show AbstractProtein as a versatile isolating template on one hand and a nanoparticle (NP) as an electric storage component on the other hand have long been investigated as independent entities. The ability to combine thsee two species to form a single addressable unit cell isolated from the conductive surface and from adjacent NPs gives rise to a wide range of nanoelectronic device applications. Here we demonstrate the means to achieve an ultra dense memory unit using individual protein-NP hybrids by Conductive Atomic Force Microscopy (C-AFM).SP1 (Stable Protein 1) is a boiling-stable (melting temperature, Tm~109 oC) ring-shaped protein complex, 11 nm in diameter. Mutants of SP1 were synthesized by means of genetic engineering, allowing its selective attachment to gold or silica surfaces (SiO2). The SP1-gold affinity is controlled by the cys-group at different positions on the protein structure. Furthermore a switchable silica binding SP1 mutant was engineered. Thru solvent condition changes, silica-binding peptides (serving as anchors) are gradually and controllably exposed, thus creating a tunable silica-binding scaffold while significantly reducing nonspecific surface binding. The mutants and their corresponding attachment to the surfaces were characterized by gel electrophoresis and AFM.In order to serve as an addressable memory device, 2D arrays of the SP1 protein were formed using different methods such as phospholipids trough and Langmuir Blodgett and characterized using TEM and AFM.In addition to the capability of selectively attaching to different surfaces and forming ordered 2D arrays, SP1 was connected to gold and Si-SiO2 NP. This setup of an isolating unit connected to a chargeable NP over a conductive surface enables selective charging of the NP. Each NP holds three charging states: natural, positive and negative. The charging of the conjugated Si-SiO2 NP was induced and tested using C-AFM, revealing life times of the charged states of 10 min in ambient and hours in vacuum. Using this setup, and the relative long charging time, we were able to apply a read and write operations on individual 5nm Si-SiO2 NP embedded in a stable protein.Having a stable and well ordered array of SP1-NP hybrids capable of charge storage at a three state setup for long times will enable to implement ultra high density memory array.
12:00 PM - **B3.8
Transition from Tunneling to Hopping Transport in Long, Conjugated Molecular Wires
C. Frisbie 1
1 Chemical Eng & Materials Science, University of Minnesota, Minneapolis, Minnesota, United States
Show AbstractIn this talk I will describe a series of comprehensive electrical transport measurements on conjugated molecular wires up to 10 nm in length, grown off of gold electrodes. The wires are grown using stepwise imine-forming reactions; using this chemistry the length of the wires can be precisely controlled from 1-10 nm. Electrical conduction of ~100 parallel wires is measured using metal coated AFM tips to make the second contact. Both the temperature and length dependence of the wire resistance indicates a transition from tunneling to hopping transport at approximately 4 nm in wire length. In addition, the current-voltage characteristics reveal that field emission can occur at higher bias voltages. An important benefit of the length dependent transport measurements is that the role of contact resistance can be assessed directly and separated from the wire resistance. Overall, these measurements combined with the synthesis chemistry open unprecedented opportunities to probe the connection between tailored molecular structure and charge conduction in small bundles of conjugated molecular wires.
12:30 PM - B3.9
Formation of Conjugated Monolayers on Metallic Nanoparticles for Single-Molecule Transport Studies.
Alexander Neuhausen 1 , David Goldhaber-Gordon 2 , Chris Chidsey 3 , Zhenan Bao 4
1 Electrical Engineering, Stanford University, Palo Alto, California, United States, 2 Physics, Stanford University, Palo Alto, California, United States, 3 Chemistry, Stanford University, Palo Alto, California, United States, 4 Chemical Engineering, Stanford University, Palo Alto, California, United States
Show AbstractWe present studies of the formation of monolayers of conjugated organic molecules on metallic nanoparticles. After monolayer formation, we have used click chemistry to link azide-terminated monolayers with dialkyne bridge molecules. With appropriate tuning of azide density and the concentration of linking molecules, we make the case for single-molecule linking of metallic nanoparticles to form a dimer or "dumbbell" structure, and propose using nanoscale lithography to fabricate leads to the nanoparticles, allowing for single-molecule transport and surface-enhanced Raman spectroscopy measurements.
12:45 PM - B3.10
Sub-10 nm Nanoimprint Lithography for the Application to Molecular and Organic Electronic Devices.
Andrew Bonifas 1 2 , Richard McCreery 2 3
1 Materials Science and Engineering, The Ohio State University, Columbus, Ohio, United States, 2 , National Institute for Nanotechnology, Edmonton, Alberta, Canada, 3 Chemistry, University of Alberta, Edmonton, Alberta, Canada
Show AbstractAs the fields of molecular and organic electronics mature, progression from fundamental experiments to the incorporation of molecular/organic components into “real world” devices has become of primary interest. Numerous methods have been proposed to facilitate this progression which include, but are not limited to, top contact fabrication on a preassembled molecular layer through direct or indirect metal evaporation, conducting polymer intermediate, and soft contact methods. Although these methods incorporate standard semiconductor processes, the degree of structural damage to the molecular layer is highly debatable. To circumnavigate damage to the molecular layer, methods where both electronic contacts are fabricated prior to the molecular layer formation can be employed. The limitation with this approach is the difficulty to fabricate on the length scale of several aromatic molecules for molecular devices or the polaron delocalization length in conducting polymer devices. This presentation will outline a novel technique to fabricate sub-10 nm gaps with nanoimprint lithography (NIL) and its application to molecular/organic electronic devices. The inherent 1:1 pattern transfer from the NIL mold to the substrate limits NIL to the resolution of the mold fabrication process. A straight forward NIL mold fabrication technique incorporates electron beam lithography (EBL) followed by reactive ion etching (RIE) of the NIL mold. The resolution of this technique is limited by the EBL process which is on the order of 15-20 nm. Our approach is an extension of the standard EBL/RIE process where the RIE of a Si mold is followed by a controlled Si oxidation process. The formation of SiO2 narrows the gap between the NIL mold’s features and provides a surface to allow the attachment of a silane based anti-adhesion layer. Since the mold’s features are fabricated from a Si single crystal, the oxidation process is highly uniform. This technique is intended to make sub-10 nm nanoimprint lithography accessible to a large range of researchers. To illustrate applications of this technique to molecular/organic electronic devices, experimentally measured electronic properties of conduction polymers are presented as the gap of the prefabricated contacts approaches the polaron delocalization length. Electronic differences between spin-coated and direct-polymerized polymer layers are discussed as a function of the contact gap dimension, fabrication method, and applied gate bias.
B5: Poster Session I
Session Chairs
Saw-Wai Hla
Norbert Koch
Xiaoyang Zhu
Egbert Zojer
Tuesday PM, April 14, 2009
Exhibition Hall (Moscone West)
6:00 PM - B5.1
Charge Transfer Excitons on Organic Semiconductor Surfaces.
Qingxin Yang 1 , Matthias Muntwiler 1 , Xiaoyang Zhu 1
1 Department of Chemistry, University of Minnesota, Minneapolis, Minnesota, United States
Show AbstractCharge transfer (CT) excitons across donor/acceptor interfaces are believed to be barriers to charge separation in organic solar cells, but little is known about their physical characteristics. Here we probe CT excitons on crystalline pentacene and tetracene surfaces using time-resolved two-photon photoemission spectroscopy. We observe a series of atomic-hydrogen like CT exciton states, with the 1s CT exciton state at a binding energy of ~0.5 eV. The large binding energy of the 1s CT exciton excludes its participation in photovoltaic. Efficient charge separation in organic hetero-junction solar cells must involve a series of hot CT excitons.
6:00 PM - B5.10
Stabilizing Single Atom Contacts by Molecular Bridge Formation.
Everardus Huisman 1 , Marius Trouwborst 1 , Frank Bakker 1 , Bert de Boer 1 , Bart van Wees 1 , Sense Jan van der Molen 2
1 Zernike Institute for Advanced Materials, University of Groningen, Groningen Netherlands, 2 Kamerlingh Onnes Laboratory, Leiden University, Leiden Netherlands
Show AbstractA single molecule forms a potential electronic component, offering the perspective of true bottom up engineering of nanodevices. However, the field of molecular electronics as been troubled by difficulties in making reliable and well-defined contacts to single molecules. Fortunately, recent times have seen a significant growth of independent techniques to contact single molecules or small ensembles of molecules [1,2].A popular way to form a single metal-molecule-metal bridge is to carefully break a gold nanowire in a solution containing dithiolated molecules [3]. Surprisingly, there is little understanding on the mechanical details of the bridge formation process and specifically on the role that the dithiol molecules play themselves. We demonstrate that alkanedithiol molecules have already formed bridges between the gold electrodes before the atomic gold-gold junction is broken [4]. This leads to stabilization of the single atomic gold junction, as observed experimentally. Our data can be understood within a simple spring model.[1] Chen, F.; Hihath, J.; Huang, Z.; Li, X.; Tao, N. J. Annu. Rev. Phys.Chem. 2007, 58, 535–564.[2] Akkerman, H. B.; de Boer, B. J. Phys.: Condens. Matter 2008, 20, 013001.[3] Xu, B.; Tao, N. Science 2003, 301, 1221–1223.[4] Huisman, E.H.; Trouwborst, M.L.; Bakker, F.L.; de Boer, B.; van Wees, B. J.; van der Molen, S.J. Nano Lett. 2008, 8, 3381-3385.
6:00 PM - B5.100
Light Absorption and Charge Generation in Polymer-Fullerene Organic Photovoltaic Devices.
Paul Dastoor 1
1 Centre for Organic Electronics, University of Newcastle, Callaghan, New South Wales, Australia
Show AbstractSolar cells based on semiconducting polymer blends offer great potential for the development of low-cost printable photovoltaic arrays. Organic photovoltaic (OPV) devices fabricated from polymer-fullerene blends are of particular interest since they provide the greatest power conversion efficiency. The conventional view of device operation is that the light photons are absorbed primarily by the polymer component resulting in the generation of excitons that in turn are separated at the many polymer-fullerene interfaces that are formed during the spin-coating of these blended materials. The main role of the fullerene component is to provide the percolation network for charge conduction [1-3]. In this paper we show that for some polymer-fullerene blends this traditional paradigm of device operation needs to be reconsidered. We demonstrate that ternary blends incorporating porphyrins can be fabricated with less than 10% polymer composition and yet still exhibit full device functionality. By analysing the proportion of light absorbed by each individual component it can be shown that the majority of the light is absorbed by the fullerene component with over 50% of the photocurrent produced under AM 1.5 conditions occurring subsequent to C60-fullerene absorption [4]. New multi-wavelength near-field scanning photocurrent microscopy studies show distinct differences in the distribution of light absorbed by these devices at different wavelengths. These results provide for a consistent understanding of the origin of primary charge separation in general polymer/C60-fullerene blends.
6:00 PM - B5.101
Nano-structured ITO Electrode Applied in Polymer Solar Cell
Ming-Shin Su 1 , Kung-Hwa Wei 1 , Chia-Hua Chang 2 , Prichen Yu 2
1 Department of Material Science and Engineering, National Chiao Tung University, Hsinchu Taiwan, 2 Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu Taiwan
Show AbstractOrganic photovoltaic has become an emerging research subject recently because of their easy and low-cost fabrication process as compared to their counter parts such as silicon or compound semiconductor solar cells. In particular, bulk hetero-junction (BHJ) polymer solar cells that typically consist of an active layer of about 100nm-thick film blends constituted by P-type conjugated polymers and N-type fullerene derivatives sandwiched between two electrodes appear to attract large attentions since they can provide large surface area with limited amount of effort. Since the power conversion efficiency (PCE) of BHJ polymer solar cells still remains an area where improvements are greatly needed. Many researchers have tried to optimize the interface structures of these polymer solar cells by modifying the work function of the electrodes or adopting other transparent metal oxides as electrode, for enhancing the PCE. Nanotechnology has provided many researchers a new way to achieving this goal by applying nano-structure to enhance the performance of these cells. In the present study, we would like to use a nano-structured ITO electrode to enhance internal reflection of the incident light as well as to facilitate the charge transport in the devicesIn this paper, we report the performance of polymer solar cell devices that used the ITO electrodes with nanorods structures produced by an oblique-evaporation deposition method. These ITO nanorods have a uniform shape with diameter ranging from 120nm to 160nm and the inter-rod spacing between them is about 150nm to 300nm. The structures of the polymer solar cell device are composed of ITO glass substrate with or without ITO nanorods, PEDOT:PSS, P3HT/PC61BM, and top Al electrode. The PCE of the devices with ITO nanorods on the ITO electrode is about 3.3% under AM1.5G (100mW/cm2) standard testing condition, as compared to 3% for the devices without ITO nanorods. Additionally, we measured these two types of devices under 5 suns (500mW/cm2) incident light intensity condition. The PCE of the devices with ITO nanorods reached about 4% as compared to 3.3% for the ones without ITO nanorods under 5 suns condition. Then, we make lifetime measurement under 5 suns condition and all the tested devices are maintained at the short circuit state under illumination. It takes almost 110 minutes to decay to 80% of their original PCE value for the devices with ITO nanorods, whereas the ones without ITO nanorods takes 55 minutes to decay to the same extent.The ITO glass substrate with ITO nanorod is a proper choice to enhance polymer solar cell PCE. Polymer solar cell containing substrate with ITO nanorod has better lifetime and the electrode with nano structure can sustain such high photo current generation or high incident light intensity with lower degradation rate. The two benefits imply that the polymer solar cell with ITO nanorod electrode has the potential to be developed into a concentrator type for polymer solar cells.
6:00 PM - B5.102
Electrochemical Characterization of Tyrosinase Nanoparticles Protected by Organic-inorganic Network in Nano-biosensor.
Woo Jin Lee 1 , Dong-Hwa Yun 1 , Jun-Hyoung Chang 1 , Keum-Ju Lee 1 , Suk-In Hong 1
1 Biochemical engineering, Korea University, Seoul Korea (the Republic of)
Show AbstractSemiconductor techologies have become essential in various field of biosensors and bio-microelectromechaniical systems. They provide miniaturization and mass production of silicon-based sensors. However, the smaller the area of sensing electrode, the more and more problems, i.g., depreciation of the whole performances including low sensitivity and stability, embarrassment of enzyme or any kinds of bio-molecular immobilization occur.Tyrosinase also known as polyphenol oxidase(PPO), is a copper containing enzyme. Nowdays mushroom tyrosinase has become important because it is readily available and useful in a number of applications which environment, food industry, and medical science. Therefore, many research groups have developed tyrosinase enzzyme. Tyrosinase has a pair of copper ions with six conserved histidine residues in the active site. But Tyrosinase stability has a connected with of oxidation of cooper ions in the air. To overcome this problem, we approach to improve the enzyme stability in various nanostructures such as nanoparticles, mesoporous materials and single enzyme nanoparticles. In this works, tyrosinase nanoparticles surrounded by a polymeric organic-inorganic network resulting in stabilization of enzyme activity. They were observed by transmission electron microscope (TEM) and analyzed electrochemical properties using cycle voltametry(CV) by VMP40 Multi-potentiostat.
6:00 PM - B5.103
The Newly Designed p-type Polymer Containing alkyl-substituted Thiophene for Organic Thin Film Transistors (OTFTs)
Pengtao Kang 1 , JongWon Park 1 , SungJin Park 1 , JinUk Ju 1 , Yunhi Kim 2 , DaeSung Jung 3 , ChanEon Park 3 , SoonKi Kwon 1
1 School of Nano & Advanced Materials Science and Engineering and ERI, Gyeongsang National University, Jinju Korea (the Republic of), 2 Department of Chmistry, Gyeongsang National University, Jinju Korea (the Republic of), 3 Department of Chmical Engineering, Pohang University of Science and Technology, Pohang Korea (the Republic of)
Show AbstractA new p-type polymer was synthesized by the oxidation coupling reaction. The monomers were prepared with highly overall yields and the obtained polymer is confirmed by elemental analysis, 1H NMR, 13C NMR, FT-IR, UV absorption, photoluminescence (PL), and cyclic voltammetry (CV). The polymer is having alkyl-substituted thiophene and we are expected to have a high solubility by introducing the long alkyl-chain. Furthermore, the introduction of the alkyl chain can increase the degree of orderness by self-assembly. The thermal property of polymer was confirmed by using the thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The polymer was showed 5% weight losing temperature at 438 oC and the curve of alkyl chain was showed around 80 oC. The number-, weight-average molecular weight of the polymers were determined to be 10,820 and 15,663 gel permeation chromatography (GPC) using polystyrene standards for calibration in the eluent THF. The solubility of the polymer was showed a poor solubility in common solvents, such as chloroform, tetrahydrofuran (THF), toluene. On the other hand, the polymer was showed a good solubility in chlorobenzene at room temperature with 0.7 wt%.
6:00 PM - B5.104
Formation of Nanopatterned Polymer Blends in Photovoltaic Devices.
Ximin He 1 2 , David Hasko 3 , Ullrich Steiner 3 , Richard Friend 3 , Sven Hunter 3 , Wilhelm Huck 1 2
1 Chemistry, University of Cambridge, Cambridge, Canbridgeshire, United Kingdom, 2 The Nanoscience Centre, University of Cambridge, Cambridge United Kingdom, 3 Physics, University of Cambridge, Cambridge United Kingdom
Show AbstractThe ideal structure for polymer photovoltaic (PV) devices has been believed so far as an interpenetrating nanoscale columnar architecture that maximizes the interface between electron and hole transporting polymers, and thereby greatly improves exciton dissociation. Despite attempts at achieving such structures via block copolymers, inorganic templates, columnar liquid crystalline phases, no such structures have been achieved in efficient devices at the level of practical applications. Instead, the best polymer photovoltaic devices, based on P3HT:PCBM, rely on a complicated procedure of spincoating from the most suitable solvents and thermal or solvent annealing steps to improve the nanoscale morphology and thereby the blend performance. Here, we have developed a double nanoimprinting (NIL) process that allows the formation of nanostructured polymer blends of any composition and morphology and demonstrated how it approached the ideal polymer film architecture for PV devices. It has been conceived that we could use a pre-patterned polymer film to imprint another polymer film through NIL, with feature size down to 20 nm. The new and simple approach, which only employ solution spin coating of polymer film and NIL, was basically established by successfully obtaining very uniform and precise patterns, in quite complete large area, inside of a variety of polymer blends, ranging from conventional and semiconductor polymers to organic-polymer blend, as a photoactive layer in PV cell, including PS/PEO, PMMA/PEO, PVP/PS, F8BT/PMMA and P3HT/PCBM. Evidenced by morphology studies through AFM and SEM, this approach is able to create integrated polymer layer with controllably varied and precisely defined interdigitated interface nano-geometric structures inside, which is the ideal structure of high-efficiency PV cells, so called well-ordered bulk heterojunctions, benefiting high intersurface area and straight carrier transport pathway. Conjugated polymer-based PV cells with both 1D and 2D nanostructured donor-acceptor interface of well-ordered densely packed 20~200-nm-wide lines and 25~200-nm-wide dots were fabricated by employing this double-NIL approach. Cell devices performance enhancement was found in the nanostructured one over the conventional flat-interface double-layer one, which was believed to result from well-ordered vertically oriented heterojunction with precisely defined interdigitated nanostructured interface of electron donor and acceptor components, which facilitated charge transport as well as charge separation, ordering of conjugated polymer. Systematic study showed PV performance improved as domain size decreased and interface area increased, especially pronouncedly with domain sizes closer to carrier diffusion length, convincing the interdigitated structure as ideal polymer PV configuration. This technique of producing nanostructured polymer or organic blends are expected to be applied into more other device fabrications in the future.
6:00 PM - B5.105
Numerical Modeling for Optical Property and Charge Transport in Bulk Heterojunction Organic Solar Cells.
Young Min Nam 1 , June Huh 2 , Won Ho Jo 1
1 Materials Science and Engineering, Seoul National University, Seoul Korea (the Republic of), 2 Active Polymer Center for Pattern Integration, Yonsei University, Seoul Korea (the Republic of)
Show AbstractOrganic photovoltaic cells based on bulk heterojunction concept has been a promising alternative for conventional silicon-based solar cells, mainly due to their potential for low cost, ease of fabrication, and mechanical flexibility. However, the power conversion efficiencies of such devices remain too low for commercialization, and thus a number of researchers have devoted to improve the efficiency. Despite growing research efforts, the fundamental processes governing the performance of organic photovoltaic cells are still poorly understood and therefore a theoretical model, which deals with a whole process of optical-to-electric energy conversion, is highly demanded. Such a model should take into account not only dynamic characteristics of exciton and electron/hole transfer in the active layer but also optical interference between layers of organic solar cell so as to cover the overall conversion process in the multilayered structure of solar cell device. In this study, a device model accounting for the effect of morphology of active layer as well as the effect of layer configuration of multilayer-structured device is established to predict the current-voltage characteristics of polymer/fullerene bulk heterojunction solar cells. The model morphologies of active layers were generated by using Cahn-Hilliard-Cook diffusion dynamics which models the dynamics of phase separation between polymers and fullerenes, varying the thickness of active layer, characteristic dimension of phase-separated morphology, and long range order of phase-separated domains. The steady-state behavior of electron, hole and exciton concentrations in phase-separated morphology is captured by the drift-diffusion model for the generated morphologies of active layer, while the optical transfer matrix theory is used for optical carrier generation to consider the effects of active layer thickness and layer configuration on the light absorption efficiency in the active layer which ultimately affects the power conversion efficiency of solar cells. The model was validated by comparing the simulation results with the experimental data, such as J-V characteristics of solar cell and the thickness of each layer.Simulation results show that the power conversion efficiencies of solar cells with ideal active layer morphology are much higher than those of solar cells with randomly oriented active layer morphology. Furthermore, the performance of solar cells with randomly oriented active layer morphology changes dramatically with variation of the characteristic dimension of phase-separated morphology, while the performance of solar cells with oriented active layer morphology is less dependent to the characteristic dimension of phase-separated morphology. It is also found that the optimum thickness of active layer with oriented morphology is much larger than the optimum thickness of active layer with randomly oriented morphology.
6:00 PM - B5.107
Effect of the Surface Characteristics of TCO Thin Films on the Performance of OLED Devices.
Yu Lim Lee 1 , Kyu-Mann Lee 1
1 Materials Engineering, Korea University of Technology and Education, Cheonan-city, Chungnam Korea (the Republic of)
Show AbstractOLED device is one of the most attractive and alternative display components, which stems primarily from the self-emission, large intrinsic viewing angle, and fast switching speed. However, because of its relatively short history of development, much remains to be studied in terms of its basic device physics, manufacturing processes, and reliability etc. Especially among several issues, it should be noted that the device characteristics are very sensitive to the surface properties of transparent conducting oxide (TCO) electrode materials. Sn-doped In2O3 (ITO) thin films have been extensively studied for OLED devices because of transparency, high electric conductivity, and large work function. However, indium has some problems such as rare raw material, high cost, low stability in plasma, and toxicity. On the other hand, Al-doped ZnO (AZO) thin films have a lot of advantages, such as low cost, good stability in plasma, non-toxicity, optical transparency, and good electrical conductivity. As a result, AZO thin films are actively studied as an alternative candidate for ITO substrates.In this study, we have investigated the performance of OLED devices as a function of sheet resistance and surface roughness of TCO thin films. For this purpose, ITO and AZO thin films were deposited by r. f. magnetron sputtering under various ambient gases (Ar, Ar+O2 and Ar+H2, respectively). The micro-structural observation and crystal orientation of the TCO thin films were evaluated using X-ray diffraction and FESEM (field emission scanning electron microscope), respectively. The optical transmittance and the film thickness were measured using ultraviolet spectrophotometer (Varian, cary-500) and surface profile measurement system, respectively. The electrical properties such as sheet resistance, charge carrier concentration, and mobility of TCO films were measured using four point probe system and hall effect measurement (HMS-3000), respectively. In order to control the surface roughness of TCO films, we have proceeded with photo-lithography and reactive ion etching processes. The micro-size patterned mask was used and the etching depth was regulated by changing etching time. The surface morphology of the TCO films was observed by FESEM and atomic force microscopy (AFM). And then, organic materials and cathode electrode were sequentially deposited on the TCO thin films. Device structure was TCO/α-NPD/DPVB/Alq3/LiF/Al. The DPVB was used as a blue emitting material. The electrical characteristics such as current density vs. voltage and luminescence vs. voltage of OLED devices were evaluated by using spectrometer (minolta CS-1000A).
6:00 PM - B5.108
Substrate Effect on Energy Level Alignment at the CuPc-C60 Interface in Organic Solar Cells.
Zengtao Liu 1
1 , City University of Hong Kong, Hong Kong China
Show AbstractThe interface energy level alignment between copper phthalocyanine (CuPC) and fullerene (C60) on indium-tin oxide (ITO) and Mg substrate was investigated. The CuPC/C60 interface deposited on ITO shows a nearly common vacuum level, but a dipole and band-bending exist, resulting in a 0.8 eV band offset at the same interface on Mg. Significantly, this observation indicates that the energy difference between the highest occupied molecular orbital of CuPC and the lowest unoccupied molecular orbital of C60, which dictates the open circuit voltage of the CuPC/C60 OPV, can be tuned by the work function of the substrate. Furthermore, the substrate effect on the energy alignment at the donor/acceptor interface can satisfactorily explain that a device with an anode of a smaller work function would provide a higher open circuit voltage.
6:00 PM - B5.109
New Multi-branched π-conjugated Molecules bearing Benzothiadiazole-based Peripheral Moieties and Their Electrical Properties
Ki Won Lee 1 , Dae Cheol Kim 1 , Tae Wan Lee 1 , Kyung Hwan Kim 1 , Min Ju Cho 1 , Dong Hoon Choi 1 , Jae-Woong Yu 2
1 chemistry, Korea University, Seoul Korea (the Republic of), 2 , Korea Institute of Science and Technology, Seoul Korea (the Republic of)
Show AbstractPhotovoltaic cells based on polymer semiconductors are of great interest as a low cost approach to solar energy conversion. Although numerous polymers have to date been explored as the donor in polymers / fullerene bulk heterojunction solar cells, not many new functional materials were demonstrated for the efficient photovoltaic devices. For the development of ideal donor material, π-conjugated small molecules, dendrimers, oligomers, and polymers were employed because of their strong potential applications to electronics and optoelectronics.We recently reported the facile synthesis of new p-type multi-branched semiconducting molecules and their organic photovoltaic cells applications including the device performance of organic thin film transistors. For this presentation, new conjugated crystalline multi-branched molecules bearing benzothiadiazole-based peripheral moieties have been synthesized through various cross-coupling reactions. A good correlation between theoretical calculations performed on model compounds and the experimental HOMO, LUMO, and band gap energies of the serial materials has been obtained. These materials was characterized by UV-visible spectroscopy, photoluminescence spectroscopy, cyclic voltammetry, thermogravimetric analysis and differential scanning calorimetry. They also display a p-type semiconducting behavior and their electrical properties are investigated in detail. We investigated the performance of bulk heterojunction type photovoltaic device and capability of molecular heterojunction or supramolecule through self-association in solution or in the bulk.
6:00 PM - B5.11
Carbon Nanotube Enabled Vertical Organic Field Effect and Light Emitting Transistors.
Mitchell McCarthy 1 2 , Bo Liu 2 , Youngki Yoon 3 , Do Young Kim 1 , Zhuangchun Wu 2 , Franky So 1 , Paul Holloway 1 , John Reynolds 4 , Jing Guo 3 , Andrew Rinzler 2
1 Materials Science and Engineering, University of Florida, Gainesville, Florida, United States, 2 Physics, University of Florida, Gainesville, Florida, United States, 3 Electrical and Computer Engineering, University of Florida, Gainesville, Florida, United States, 4 Chemistry, University of Florida, Gainesville, Florida, United States
Show AbstractActive matrix organic light emitting diode (AMOLED) displays are expected to compete with liquid crystal displays in the coming years due to the promise of increased power efficiency, larger contrast ratio, wider viewing angle and thinner design. Currently there are technological limitations to the widespread use of AMOLED displays such as limited lifetime of the OLED as well material limitations preventing further technological advances, such as making AMOLED displays flexible. All-organic AMOLED displays are required for use on flexible substrates; however, the low mobility of organic materials, while enabling flexibility, hinders the overall device performance by the requirement of larger driving voltages and smaller aperture ratios leading to reduced power efficiency and lifetime. In this study, nanoscale materials are shown to provide a promising path forward in the realization of industrially feasible all-organic flexible AMOLED displays. In this collaborative study between the Departments of Computer Science and Engineering, Materials Science and Engineering, Chemistry and Physics at the University of Florida, carbon nanotube enabled vertical organic light emitting transistors (VOLETs) are fabricated and characterized. Single walled carbon nanotube (CNT) networks possess unique properties which make the networks ideally suited for use in the VOLET architecture. The low density of states of the CNTs allows significant modulation of their Fermi level, resulting in transconductance due to modulation of the hole injection barrier between the CNTs and the active layer. It is shown by experiment and simulation that the unique properties afforded by the nanoscopic nature of carbon nanotubes are responsible for the transconductance observed. Preliminary devices using CNT networks show superior performance when compared to VOLETs fabricated from other materials.
6:00 PM - B5.110
Numerical Study for the Morphological Effect on the Performance of Organic Solar Cells.
Sungjun Kong 1 , Dongchoul Kim 1
1 Mechanical Engineering, Sogang University, Seoul Korea (the Republic of)
Show AbstractThe conversion of solar energy into electricity indicates the route to reduce the concern over global warming and satisfy our global energy needs. Polymer solar cells have potential to be alternative to silicon-based solar cells which are impractically expensive. To improve the performance of polymer solar cells, significant efforts have been made to decrease energy loss from the internal morphology in a bilayer system. Here, we present a three-dimensional model to study the effect of device morphologies on photovoltaic performance. The diffuse interface model is employed and incorporates photovoltaic mechanism. The semi-implicit Fourier spectral method and preconditioned biconjugate-gradient method are applied for high efficiency and numerical stability. The simulations demonstrate the improved performance of the organic solar cells with designed morphologies.
6:00 PM - B5.111
Conjugated Polymers Based on Pyridazine and Benzothiadiazole for Photovoltaic Applications.
Sangwon Ko 1 , Rajib Mondal 2 , Sanghyun Hong 2 , Hector Becerril 2 , Zhenan Bao 2
1 Chemistry, Stanford University, Stanford, California, United States, 2 Chemical Engineering, Stanford University, Stanford, California, United States
Show AbstractA new class of conjugated copolymers based on pyridazine and benzothiadiazole was designed and synthesized for application in polymer solar cells (PSCs). The absorption spectra, electrochemical, field effect carrier mobility, and photovoltaic properties of these polymers were investigated. The broad absorptions (300-600 nm), and low band gaps (~ 2.0 eV), and high hole mobility of the polymers make them promising for solar cell application. The changing copolymerization units leaded further red-shifted λ(max) broadening of the wavelengths at which high photoconversion efficiencies can be achieved. The results indicate that the polymers consisting of further improved the absorption properties are promising polymer photovoltaic materials.
6:00 PM - B5.113
Predicting the Efficiency of Organic Photovoltaics Based on Optical Properties.
Katherine Hurst 1 , Nathan Tomlin 1 , John Lehman 1
1 , National Institute of Standards and Technology, Boulder, Colorado, United States
Show AbstractImproving the quantum efficiency of organic photovoltaics is a requirement for large-scale commercial use. It has been established that doping of conjugated-polymer films with single walled carbon nanotubes (SWCNTs) facilitates exciton dissociation and electron transport. Accurate measurement of important intrinsic device properties such as absorbance with respect to film thickness and SWCNT concentration is necessary for informing our expectations for extrinsic photovoltaic efficiency. This is also the basis for modeling the role of the photoactive layer and maximizing the efficiency of the device. In this work, we explicitly measured the absolute absorbance of photoactive films by depositing them on a pyroelectric detector and modeled the optical function of the doped films based on Kramers-Kronig analysis. Raman spectroscopy, UV-VIS absorption and four-point probe measurements provide further characterization of nanotube concentration and homogeneity of our films. Our study provides direct quantitative measurements traceable to NIST standards, which demonstrates variations in the intrinsic efficiency of the photoactive layer directly correlate to the extrinsic efficiency of the SWCNT-polymer photovoltaics.
6:00 PM - B5.115
N-type Semiconducting Organic Materials for High-Performance and Air-Stable Organic Field-Effect Transistors and Circuits.
Yoonyoung Chung 1 , Joon Hak Oh 2 , Boris Murmann 1 , Zhenan Bao 2
1 Electrical Engineering, Stanford University, Stanford, California, United States, 2 Chemical Engineering, Stanford University, Stanford, California, United States
Show AbstractUnlike inorganic electronic devices which have higher electron mobility than hole mobility, organic electronic devices show superior performance in p-type where holes dominate the current. Although p-type organic field-effect transistors (OFET) have been widely investigated, air-stable and high-performance n-type OFETs have been less developed. A major drawback of n-type materials is the existence of oxygen and water molecule which have high electron affinity. Considering many advantages of complementary circuit design, there is no doubt about the necessity of fast and reliable n-type OFETs. We report the fabrication of simple electronic circuits, such as inverters and ring oscillators, using high performance air-stable n-type OFETs, based on Perylene Diimides (PTCDI) derivatives and Naphthalene Diimides (NTCDI) derivatives, together with p-type pentacene OFETs. Their electrical characteristics such as bias-stress effect and air-stability have also been investigated. The field-effect mobility of the n-type OFETs was close to 1 cm2/Vs in air.
6:00 PM - B5.116
A Crystalline Dielectric Surface-modification Layer for High Performance Organic Thin-film Transistors.
Ajay Virkar 1 , Stefan Mannsfeld 1 , Yutaka Ito 1 , Michael Toney 2 , Zhenan Bao 1
1 , Stanford University, Stanford, California, United States, 2 , Stanford Synchrotron Radiation Lab , Menlo Park, California, United States
Show AbstractIn organic thin film transistors (OTFTs),the vast majority of the current flows within the first few monolayers of the semiconductor at the dielectric interface. To improve performance it is very common to modify the SiO2 dielectric with an octadecylsilane (OTS) monolayer. We found that the phase (order) of the underlying OTS is a critical device parameter which must be controlled in order to optimize OTFTs. We found a crystalline, dense octadecylsilane (OTS) surface modification layer promotes two-dimensional growth in a variety of organic semiconductors. Higher mobility is achieved for OTFTs with a crystalline OTS layer compared to a disordered one, with mobilities as high as 5.3 cm2/Vs and 2.2 cm2/Vs for C60 and pentacene, respectively. Moreover, we developed a simple, scalable spin-coating method to produce crystalline OTS. This work demonstrates a significant step towards a general approach for morphological control of organic semiconductors which is directly linked to their performance in OTFTs. Finally, nucleation and thin film growth of semiconductors on the different OTS surfaces will be discussed with particular attention to the relevant energetics which must be considered to design ideal dielectric surfaces.
6:00 PM - B5.117
Energetics and Stability of Pentacene Thin Films on Methyl Terminated Surfaces
Ajay Virkar 1 , Stefan Manssfeld 1 , Zhenan Bao 1
1 , Stanford University, Stanford, California, United States
Show AbstractIncreasingly there has been interest in analyzing the nucleation, growth, and morphology of pentacene thin films on methyl terminated surfaces. We recently discovered that pentacene has a high nucleation density and a 2D growth mode on dense methyl terminated surfaces compared to loosely packed methyl surfaces. The desired 2D growth gave rise to much better performance pentacene transistors. In this work we discuss the relevant energetics of pentacene nucleation and thin film growth on methyl surfaces. We also demonstrate how to design the dielectric surface so that pentacene thin films are stable. These findings are important for monolayer transistors and ultrathin organic transistor based sensors.
6:00 PM - B5.118
Three-dimensional Anisotropic Electronic Properties of Solution Grown Organic Single Crystals Measured by Space-Charge Limited Current (SCLC).
Beatrice Fraboni 1 , Fabio Brigidi 1 , Anna Cavallini 1 , Alessandro Fraleoni-Morgera 2
1 Physics, University of Bologna, Bologna Italy, 2 , Sincrotrone ScpA, Trieste Italy
Show AbstractCharge transport processes in organic materials are highly sensitive to the material deposition and device fabrication conditions that, in turn, affect to a major extent the molecular packing and interface defective states. In order to correlate the electrical properties of organic semiconductors to their molecular functionality, it is necessary to control the effects of impurity and disorder induced defects. It is has been shown that organic single crystals offer the possibility of studying the intrinsic properties of organic molecules thanks to their high purity and molecular order, and single crystal field effect transistors (SCFET) are a powerful tool in this respect. The observation of anisotropic transport in single crystals, due to the anisotropic packing of the organic molecules, provides an assessment of the occurrence of band-like transport processes.We studied the three-dimensional anisotropic charge transport properties of solution-grown organic single crystals based on a dipolar molecule 4HCB (4-hydroxy-cyanobenzene) by Space Charge Limited Current (SCLC), by spectral photocurrent (PC) and by X-ray diffraction analyses. Most reports on organic single crystals deal with crystals grown by vacuum deposition methods, while only a few are available on high quality single crystals obtained by growth from solution [1,2].We have measured the charge carrier mobility along the three crystallographic axes of single crystals by SCLC analyses obtaining highly reproducible and markedly anisotropic values (i.e. 5x10-2 cm2/Vs for the main axis a, 3x10-3 for the axis b and 3x10-6 cm2/Vs for the axis c, along the crystal thickness). These results fully confirm values we have previously obtained by measuring the carrier mobility using FET devices [2]. In addition, a modelization of the obtained data allowed to evaluate the concentration and activation energy of the dominant deep traps along each crystallographic direction.We observed how the exposure to visible light induced a different effect on the transport properties along the two directions, that we have attributed to the presence and alignment of the electron-attractor cyano group. We suggest that the presence of an intrinsic molecular dipole differently affects the flow of charge carriers along the two main planar crystal axes, thus altering the charge transport anisotropy induced by the molecular π-orbitals stacking. A band of electrically active deep traps was identified by PC analyses and we suggest that this lattice structure may be ascribed to the intrinsic 4HCB molecular electric dipole that may originate trapping centers that behave like deep donors and that are more efficient along one of the two main planar axes. [1] S. C. B. Mannsfeld, J. Locklin, C. Reese, M. E. Roberts, A. J. Lovinger, Z. Bao, Adv. Funct. Mater., 17, 1617 (2007)[2] B. Fraboni, R. DiPietro, A. Castaldini, A. Cavallini, A. Fraleoni-Morgera, L. Setti, I. Mencarelli, C. Femoni, Organic Electronics, 9, 974 (2008)
6:00 PM - B5.119
Photocurrent Studies of Sexythiophene–based OFETs.
Beatrice Fraboni 1 , Riccardo DiPietro 1 , Anna Cavallini 1 , Piero Cosseddu 2 , Annalisa Bonfiglio 2 , Jorg Vogel 3 , Jurgen Rabe 3
1 Physics, University of Bologna, Bologna Italy, 2 Electrcal Engineering, University of Cagliari, Cagliari Italy, 3 Physics, Humboldt University , Berlin Germany
Show AbstractPhotocurrent (PC) spectroscopy is proposed as a reliable tool in the investigation of the transport properties of organic thin film transistors (OFETs). We have applied PC analyses to the study of the electronic density of states distribution (DOS) of the OFETs with crystalline mixed films of two derivatives of a conjugated molecule [α-sexithiophene (6T), and its alkylated analogue α,ω-dihexylsexithiophene (DH6T)] as the active semiconductor. The charge carrier mobility in organic semiconducting materials depends on the effective charge carrier density, which can be modulated, e.g., by gate voltage induced charge in a transistor geometry. We investigated the modifications in the DOS distribution associated to variations in the carrier density in the OFET channel and we detected the formation of deep electrically active states in the below-band-gap region, associated to polaron states induced by a prolonged exposure of the device to atmosphere. A clear correlation between the PC results and the electrical characteristics of the corresponding FET devices has been observed
6:00 PM - B5.12
Atomic-Scale Scanning Tunneling Microscopy and Spectroscopy Studies of Nanometer-Sized Graphene Flakes on Semiconducting Surfaces.
Justin Koepke 1 2 , Kevin He 1 2 , Joseph Lyding 1 2
1 Electrical and Computer Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois, United States, 2 , Beckman Institute for Advanced Science and Technology, Urbana, Illinois, United States
Show AbstractWe have used ultrahigh vacuum scanning tunneling microscopy to perform atomic level studies of graphene on semiconducting surfaces. We used a dry contact transfer technique (DCT) developed by Albrecht and Lyding [1] to deposit mechanically exfoliated graphene in-situ onto atomically clean semiconducting surfaces [2]. The DCT technique deposits predominantly single and double layers of atomically clean graphene with lateral dimensions ranging from 2 - 60 nm. We observe varying degrees of transparency of the graphene monolayers depending on the substrate. This is most pronounced for graphene on the cleaved InAs(110), GaAs(110), and Si(111)-7x7 surfaces, where the substrate atomic structure is clearly seen through the graphene. On the Si(111)-7x7 surface, the substrate atomic structure can also be seen through graphene bilayers. We believe that electronic structure of a graphene monolayer on the InAs(110) and GaAs(110) surfaces leads to the transparency of monolayers and the opacity of bilayers similar to the findings of Rutter, et al [3]. We suspect that the transparency of graphene bilayers on the Si(111)-7x7 surface is due to a similar effect, while the transparency of graphene monolayers on the same surface also has a topographic component. Room-temperature scanning-tunneling spectroscopy (STS) measurements of the graphene monolayers and bilayers on the Si(111)-7x7 surface show predominantly metallic behavior. STS measurements of graphene features on the InAs(110) and GaAs(110) surfaces also show predominantly metallic behavior, but Semiconducting behavior is observed for the smaller features.[1] P.M. Albrecht and J.W. Lyding, Appl. Phys. Lett. 83, 5029 (2003).[2] K.A. Ritter and J.W. Lyding, Nanotechnology 19, 015704 (2008).[3] G.M. Rutter, et al, Phys. Rev. B 76, 235416 (2007).
6:00 PM - B5.121
Solution Processable Green Polymers for Use in Solar Cells
Subbiah Jegadesan 1 , Pierre Beaujuge 2 , Kaushik Roy Choudhury 1 , John Reynolds 2 , Franky So 1
1 Dept of Materials Science and Engineering, University of Florida, Gainesville, Florida, United States, 2 Department of Chemistry, University of Florida, Gainesville, Florida, United States
Show AbstractPolymer solar cells have attracted much attention in the last few years due to their promising applications in clean and renewable energy sources. Over the years, the power conversion efficiency of the polymer solar cells has steadily been improved with the use of new materials and device architectures. Here, we report on bulk-heterojunction solar cells using a solution processable neutral green conjugated polymer as the donor and PCBM as the acceptor. We have found that the short-circuit current is very sensitive to the composition of the donor-acceptor blend. The device with a donor-acceptor ratio of 1:8 gives the best performance with a power efficiency of 1.9 %, a short-circuit current of 5.56 mA/cm2 and an open-circuit voltage of 0.77 V. The incident photon-to-current efficiency (IPCE) of the green solar cells shows two bands, one with a maximum of 57% in the UV region corresponding to the absorption of PCBM and a second one with a maximum of 42% in the visible-near IR region corresponding to the absorption of the green polymer.
6:00 PM - B5.13
Gold Work Function Reduction by 2.2 eV with a Molecular Donor Layer.
Benjamin Broeker 1 , Ralf-Peter Blum 1 , Johannes Frisch 1 , Antje Vollmer 2 , Oliver Hofmann 3 , Ralph Rieger 4 , Klaus Muellen 4 , Juergen Rabe 1 , Egbert Zojer 3 , Norbert Koch 1
1 Institut für Physik, Humboldt-Universität zu Berlin, Berlin Germany, 2 , Berliner Elektronenspeicherring-Gesellschaft für Synchrotronstrahlung mbH, Berlin Germany, 3 Institut of Solid State Physics, Graz University of Technology, Graz Austria, 4 , Max Planck Institut für Polymerforschung, Mainz Germany
Show AbstractUltraviolet photoelectron spectroscopy was used to investigate neutral methyl viologen (1,1'-dimethyl-1H,1'H-[4,4']bipyridinylidene, MV0) deposited on Au(111) surfaces. As a result of molecule-to-metal electron transfer, the work function of Au(111) was decreased from 5.5 eV to 3.3 eV. The energy levels of electron transport layers deposited on top of modified Au surfaces were shifted to higher binding energy compared to layers on pristine Au, and the electron injection barrier was reduced by 0.8 eV for tris(8-hydroxyquinoline)aluminum (Alq3) and by 0.7 eV for C60. The air-stable donor MV0 can thus be used to facilitate electron injection into organic semiconductors even from high work function metals.This work is financially supported by European Community project "IControl" (EC-STREP-033197).
6:00 PM - B5.14
Electronic Properties at Gold/Conjugated Polyelectrolyte Interfaces.
Jung Hwa Seo 1 , Renqiang Yang 1 , Jasek Z. Brzezinski 1 , Bright Walker 1 , Thuc-Quyen Nguyen 1 , Guillermo C. Bazan 1
1 , UC Santa Barbara, Santa Barbara, California, United States
Show AbstractThe electronic properties of conjugated polyelectrolytes (CPEs) with poly(fluorene-co-phenylene) backbones and different counterions and charges have been investigated using absorption, x-ray photoemission (XPS) and ultraviolet photoelectron spectroscopy (UPS). The optical energy band gap of CPEs depends mainly on their conjugated backbone and nearly insensitive to the charges or counterions. XPS and UPS measurements reveal that electron injection from Au to polymers with cationic groups is more efficient than for the neutral and anionic counterparts. The vacuum levels of CPEs were also shifted toward higher or lower binding energy, relative to that of Au depending on the charge and counterion presence, and provide insight into the general alignment of dipoles at the metal/organic interface. This finding shows that counterions and backbone charges enable control of the electronic and chemical nature of critical device interfaces.
6:00 PM - B5.15
Study of Solid/liquid Interfaces in Organic Field-effect Transistors with Ionic Liquids.
Shimpei Ono 1 , Kazumoto Miwa 1 , Shiro Seki 1 , Jun Takeya 2
1 Materials Science Research Laboratory, Central Research Institute of Electric Power Industry, Tokyo Japan, 2 Graduate School of Science, Osaka University, Osaka Japan
Show AbstractThere has been significant interest to develop new kinds of organic transistors such as those using electric double layers (EDLs) of electrolytes. It is reported that the EDL gating can be a promising technology not only to realize high performance organic field-effect transistors (OFETs) [1-4], but also to drive phase transitions in strongly correlated electron systems to a metal [5] or a superconductor [6]. However, there is no detailed study of microscopic mechanisms of the charge transport in the vicinity of the solid/liquid interfaces. In this work, we report the two-dimensional charge transport at the interface with the use of various ionic liquids as the electrolyte layer in OFETs. Since the use of ionic liquid electrolytes in OFETs enables high-density carrier doping with minimum gate voltages without scarifying the carrier mobility [7], higher performances are likely to emerge by elaborate search for compounds incorporated in OFETs. We have formed a well structure of polydimethylsiloxan elastomer on which rubrene single crystal is electrostatically attached and each ionic liquid is poured underneath a rubrene single crystal by the capillary force, so the EDL in the ionic liquid can induce high-density carriers at the surface of the crystal. The achieved reproducibility permits one to observe that the mobility of the charge carriers systematically increases with decreasing the dielectric constant of the ionic liquids and becomes as high as 9.5 cm2/Vs in 1-ethyl-3methylimidazolium bis(fluorosulfonyl)imide, which is only half of the value of the air-gap transistors. These results suggest that the mobility of carriers in OFETs is an intrinsic property of the solid/liquid interface between organic semiconductors and the ionic liquids.[1] M. Panzer, C. D. Frisbie et al., Appl. Phys. Lett. 88 203504 (2006).[2] J. Takeya et al., Appl. Phys. Lett. 88 112102 (2006).[3] H. Shimotani et al., Appl. Phys. Lett. 89 203501 (2006).[4] J. Lee, C. D. Frisbie et al., J. Am. Chem. Soc. 129 4532 (2007).[5] H. Shimotani, et al., Appl. Phys. Lett. 91 082106 (2007).[6] K. Ueno et al., Nature Materials (2008) in press.[7] S. Ono et al., Appl. Phys. Lett. 92 103313 (2008).
6:00 PM - B5.16
Liquid-crystalline Semiconducting Copolymers with Intramolecular Donor-acceptor Building Blocks for High-stability Polymer Transistors.
Do Hwan Kim 1 , Bang-Lin Lee 1 , Hyunsik Moon 1 , Eun-Jeong Jeong 1 , Jeong-Il Park 1 , Kuk-Min Han 1 , Byung Wook Yoo 1 , Bon Won Koo 1 , Joo Young Kim 1 , Wi Hyung Lee 2 , Kilwon Cho 2 , Hector Becerril 3 , Zhenan Bao 3 , Sangyoon Lee 1
1 Display Laboratory, Samsung Advanced Institute of Technology, Samsung Electronics Co., LTD. , Yongin Korea (the Republic of), 2 Department of Chemical Engineering, Pohang University of Science and Technology, Pohang Korea (the Republic of), 3 Department of Chemical Engineering, Stanford University, Stanford, California, United States
Show AbstractOrganic field-effect transistors (OFETs) based on π-conjugated polymers have recently attracted significant attention because of their potential use as soft channel materials in organic/printed optoelectronic devices such as active-matrix flat panel displays (AMFPDs), electronic paper, RFID tags, and chemical/bio-sensors. The solution-processability of π-conjugated polymers has also stimulated interest in their utilization in active electronic elements for low-cost, large-area, and flexible active matrix display backplanes, with performances that are comparable to those of hydrogenated amorphous silicon (a-Si:H)-based thin-film transistors. However, when OFETs based on π-conjugated polymers are fabricated and tested under ambient conditions, their electrical performance decreases remarkably, which is primarily due to the sensitivity of the polymer chains to atmospheric O2/H2O and structural defects. In particular, OFETs composed of solution-processed π-conjugated polymers commonly exhibit some electrical instability under external bias stress due to the less-ordered molecular structure of their semiconductor films; charge trapping instability under bias-stress and environmental stability are problems for these materials. In order to be comparable to a-Si-based TFTs, OFETs should exhibit similar performance with respect to electrical bias stress. Although there have been a few studies aimed at enhancing the environmental/electrical stability of π-conjugated polymers under external bias stress, an adequate understanding of the relationship between crystalline nanostructure and bias stress driven electrical instability on the microscopic scale still eludes us. Here we report a novel charge-transfer type liquid-crystalline semiconducting copolymer, poly(didodecylquaterthiophene-alt-didodecylbithiazole), which contains both electron-donating quaterthiophene and electron-accepting 5,5’-bithiazole units, and exhibits unprecedented electrical characteristics such as field-effect mobilities as high as 0.33 cm2/Vs and good bias-stress driven electrical stability that is comparable to that of amorphous silicon (a-Si). Liquid-crystalline thin films with structural anisotropy form spontaneously through the self-organization of the individual polymer chains as a result of intermolecular interactions in the liquid-crystalline mesophase, and adopt preferential well-ordered inter-molecular π-π stacking parallel to the molecular surface. This bottom-up assembly of the liquid-crystalline semiconducting copolymer enables the facile fabrication of highly-ordered soft channel layers with a minimal concentration of charge traps, as well as unidirectional and delocalized transport with hitherto unreported electrical stability.
6:00 PM - B5.17
Reversible Electro-optical Switch of Self Assembled Monolayers of azobenzene-derivatized Oligothiophenes Grafted on Gold.
Dominique Vuillaume 1 , Kacem Smaali 1 , Stephane Lenfant 1 , Dominique Deresmes 1 , Sandrine Karpe 2 , Maitena Ocafrain 2 , Philippe Blanchard 2 , Jean Roncali 2
1 , IEMN-CNRS, Villeneuve d'Ascq France, 2 , CIMA-CNRS, Angers France
Show AbstractThe grafting of azobenzene-based molecules on surfaces is an attractive approach for optical switches [1;2], molecular machines [3] and biosensors [4].Here, we report the preparation of a self-assembled monolayer (SAM) on gold surface obtained by grafting molecules based on an azobenzene moiety associated to a bithiophene unit (see inset below). The structure of the SAM was characterized using various techniques such as ellipsometry, X-ray Photoelectron-spectroscopy, cyclic voltammetry and contact angle measurement. This azobenzene moiety can switch between two isomeric configurations (trans and cis) when irradiated by visible (480 nm) and UV (360 nm) light respectively. The electrical properties of the SAM in each configuration were studied by conducting-AFM and by eutectic drop contact (GaIn). We clearly observed the effect of this photo-isomerization on the current, at the nano-scale by conducting-AFM and at macroscopic scale using GaIn. The current (ION) measured by C-AFM at 1.5V for the cis isomer was higher by a factor ~2000 compared to the current (IOFF) for the trans isomer. By measuring with GaIn contact, the value of this ION/ IOFF ratio was 223 at 0.5V (see graph).These results represent significant improvement compared to those reported by Mayor and coworkers namely ~ 30 at 0.3V by mercury drop contact [2] and ~ 20 at 0.5V by C-AFM [3]. Various hypotheses to explain this conductivity variation will be discussed, in particular an I-V analysis (Fowler-Nordheim plot) that suggests an injection barrier reduction (lowering LUMO).[1] Kumar A.S. et al., Nanoletters 8(6), 1644-1648 (2008)[2] Mativetsky et al., JACS 130(29), 9192-9193 (2008)[3] Ferri V. et al., Angew. Chem. Int. Ed. 47, 3407-3409 (2008)[4] Dietrich P. et al., Appl. Phys. A 93, 285-292 (2008)
6:00 PM - B5.19
Extended Lifetime of Organic Field-Effect Transistors Encapsulated with Titanium Sub-Oxide as an `Active’ Passivation/Barrier Layer.
Shinuk Cho 1 , Kwanghee Lee 2 , Alan Heeger 1
1 Center for Polymers and Organic Solids, University of California at Santa Barbara, Santa Barbara, California, United States, 2 Department of Materials Science and Engineering, Gwangju Institute of Science and Technology, Gwangju Korea (the Republic of)
Show AbstractDespite significant improvements in the performance of organic electronic devices, the operating lifetimes are limited by the intrusion of oxygen (O2) and water vapor (H2O). High performance encapsulation with low-cost is therefore required to achieve lifetimes sufficiently long to enable commercialization of plastic electronics technology. We report that a thin capping layer of titanium sub-oxide (TiOx), prepared by sol-gel synthesis from titanium alkoxides, extends the lifetime of organic FETs. The TiOx layer functions as an ‘active’ passivation/barrier layer that actually removes oxygen and water vapor from the organic semiconductor. The results demonstrate a significant improvement in the lifetime of organic field effect transistors when exposed to air.
6:00 PM - B5.2
Optical and Structural Properties of rrP3HT on Highly Dense Vertically Aligned CNTs.
Wei-Chao Chen 1 2 , Chien-Hung Lin 3 , Hsiang-Ting Lien 4 , Kuei-Hsien Chen 1 , Li-Chyong Chen 2
1 Institute of Atomic and Molecular Science, Academia Sinica, Taipei, Taipei, Taiwan, 2 Center for Condensed Matter Science, National Taiwan University, Taipei, Taipei, Taiwan, 3 Graduate Institute of Photonics and Optoelectronics and Department of Electrical Engineering, National Taiwan University, Taipei, Taipei, Taiwan, 4 Institute of Organic and Polymeric Materials, National Taipei University of Technology, Taipei, Taipei, Taiwan
Show AbstractInfluence of interchain-interactions in rrP3HT/VACNTs (vertically aligned carbon nanotubes) composites have been investigated via optical and structural analyses. The composite was synthesised through dip coating in different concentration or deposition time. The interchain coupling can strongly influence the electronic properties of polymer chains. On the surface of highly dense VACNTs, it was observed the pristine rrP3HT [regioregular poly(3-hexylthiophene)] naturally induces the self-assembling organization with high-symmetry cofacial configurations, particularly stronger edge-on orientation. From Raman spectrum, the outmost surfaces of CNTs possessing electron-rich property strongly induce the non-specific covalent interactions (π-π or CH-π) with rrP3HT. Via strong π-π interactions between polymer-backbone and CNTs-surface, or the multiple CH-π interactions between the alkyl side chains of conjugated polymer and outmost graphene of CNTs, the high-symmetry cofacial orientation can be induced. The organized structure, as observed by XRD, exhibits a highly lamellae orientation of P3HT in parallel with VACNTs template on a-face [100] Photoluminescence excitation (PLE) mapping of this solid structure of rrP3HT/VACNTs also reveals a blue shift of radiative emission from 0-1 to 0-0 transition, (or this 2-dimentional conjugated orientation of rrP3HT leads to the blue shift of radiative emission at 0-1 and 0-0 transitions. These results suggest decreasing in the polaron-binding energy of rrP3HT/VACNTs. The self-assembling 2-dimentional structure of rrP3HT is naturally achieved on VACNTs, via highly cofacial orientation, without any requirement of surface-treatment.
6:00 PM - B5.20
peri-Xanthenoxanthene Thin-Film Transistors.
Norihito Kobayashi 1 , Mari Sasaki 1 , Noriyuki Kawashima 1 , Kazumasa Nomoto 1
1 Advanced Materials Laboratories, Sony Corporation, Atsugi Japan
Show AbstractWe have synthesized and characterized stable organic semiconductors (OSCs), 3,9-diphenyl-peri-xanthenoxanthene (Ph-PXX) and its soluble derivatives of 3,9-bis(p-alkylphenyl)-peri-xanthenoxanthene (CnPh-PXX) for organic thin-film transistors (OTFTs). A π-system is stabilized against oxidation by introduction of hetero-atoms and phenyl groups into the reactive sites in the π-system. This strategy for stabilization does not suffer from the conventional trade-off between environmental stability and efficient carrier injection, which appears when OSCs with deeper highest occupied molecular orbitals (HOMOs) are applied. UV-Vis spectra of an air-saturated solution of Ph-PXX were unchanged over 120 hours, indicating that the molecule has great environmental stability. A HOMO level of Ph-PXX molecule was estimated to be only 5.1 eV below vacuum level, achieving efficient carrier injection from Au electrodes. In fact, OTFTs with Ph-PXX showed high apparent mobility over 0.4 cm2/Vs without demonstrating nonlinear behavior of source-drain ohmic contacts, and have been stable over five months under ambient conditions. In addition, the OTFTs showed great thermal stability at temperatures up to 150°C in air. These characteristics have been also achieved with a solution-processed OTFT with a soluble CnPh-PXX. In this presentation, we will talk about the molecular design with the passivation of the reactive cites leading to stable molecules and the characteristics of its OTFTs with efficient carrier injection between metal electrodes and OSCs.
6:00 PM - B5.21
Self Assembly and Activation of Phosphonic Acid Monolayers on GaN and AlGaN for Biosensing Applications.
Soonwook Hong 2 , B. Simpkins 1 , R. Stine 1 , M. Mastro 1 , C. Eddy Jr. 1 , P. Pehrsson 1
2 , Thomas Jefferson High School, Alexandria, Virginia, United States, 1 , Naval Research Lab, Washington, District of Columbia, United States
Show AbstractSelf assembled monolayers of 6-phosphonohexanoic acid and n-octadecylphosphonic acid (ODPA) were formed on silicon dioxide, gallium nitride (GaN) and aluminum gallium nitride (AlGaN) surfaces. Deposition and long-term stability of these phosphonic acids was verified through water contact angle and X-ray photoelectron spectroscopy (XPS) measurements. The ODPA monolayers reduced the conductivity of AlGaN/GaN HFET structures by ~30%. This effect was evaluated in the context of the dipole contribution due to the PO3 end group found on ODPA. We also report a novel method to activate ODPA monolayers for biological applications by forming active carbonyl groups through exposure to microwave oxygen plasma. This activation enables immobilization of 2-mercaptoethylamine (MEA) on functionalized silicon oxide substrates. This research is of direct relevance to the operation of field effect transistor-based biochemical sensors, and provides a fundamental basis for further applications of Si, GaN and AlGaN in biosensing and microelectronics technologies.
6:00 PM - B5.22
Anomalous Tunneling in Carbon/Alkane/TiO2/Au Molecular Electronic Junctions: Energy Level Alignment at the Metal/Semiconductor Interface.
Haijun Yan 1 2 , Richard McCreery 2 3
1 Department of Chemistry, The Ohio State University, Columbus, Ohio, United States, 2 , National Institute for Nanotechnology, Edmonton, Alberta, Canada, 3 Department of Chemistry, University of Alberta, Edmonton, Alberta, Canada
Show AbstractCarbon/TiO2/Au electronic junctions show slightly asymmetric electronic behavior, with higher current observed in current density (J)-voltage (V) curves when carbon is biased negative with respect to the Au top contact. When a ~1 nm thick alkane film is deposited between the carbon and TiO2, resulting in a carbon/alkane/TiO2/Au junction, the current increases significantly for negative bias and decreases for positive bias, thus creating a much less symmetric J-V response. Similar results were obtained when SiO2 was substituted for the alkane layer, but Al2O3 did not produce the effect. The observation that by adding an insulating material between carbon and TiO2, the junction becomes more conductive is unexpected and counterintuitive. Kelvin probe measurements revealed that while the apparent work function of the PPF electrode is modulated by surface dipoles of different surface-bound molecular layers, the anomalous effect is independent of the direction of the surface dipole. We propose that by using a nanometer thick film with a low dielectric constant as an insertion layer, most of the applied potential is dropped across this thin film, thus permitting alignment between the carbon Fermi level and the TiO2 conduction band. Provided the alkane layer is sufficiently thin, electrons can directly tunnel from carbon to the TiO2 conduction band. Therefore, the electron injection barrier at the carbon/TiO2 interface is effectively reduced by this energy level alignment, resulting in an increased current when carbon is biased negative. The modulation of injection barriers by a low-κ molecular layer should be generally applicable to a variety of materials used in micro- and nano-electronic fabrication.
6:00 PM - B5.23
Single and Mixed Self-Assembled Monolayers of Phenyl Species on Silicon with Various Ring to Ring Interactions
Virginie Gadenne 1 , Simon Desbief 1 , Lionel Patrone 1
1 Institut Supérieur de l’Electronique et du Numérique, IM2NP, CNRS, IM2NP (UMR 6242), Maison des Technologies, Place Georges Pompidou, F-83000 Toulon France
Show AbstractPreparation of self-assembled monolayers (SAM) [1] of aromatic conjugated molecules is a key point in molecular electronics [2]. Moreover, regarding potential applications, it is important to be able to prepare nano-islands of such active molecules on silicon. Nevertheless few works addressed this subject [3]. In a first part of our work, in order to control the formation of conjugated molecular nano-domains on native oxide covered silicon, we studied how various tri-functionalized silane molecules bearing a phenyl cycle, modified or not, interact during their self-assembly [4]. For phenyl rings without alkyl chain, SAM growth is shown to occur in a single step: chemisorption on the surface. This step is thermally activated and does not depend on ring to ring interactions. We show that adding a short alkyl chain (3-4 carbon atoms) to the phenyl ring gives the molecules enough flexibility to generate an additional second growth step. The latter is independent from the deposition temperature and corresponds to the arrangement between molecules. We found that this packing step is accelerated by replacing phenyl by pentafluoro-phenyl rings, possibly due to quadrupolar interactions between fluorinated cycles. Furthermore we demonstrate that mixing phenyl and pentafluoro-phenyl molecules leads to an even faster packing step which is accounted for by hydrogen bonding CH...FC in a face to face phenyl/pentafluoro-phenyl arrangement [5]. We believe these results allow improving charge delocalization over conjugated molecular domains. In a second part, we studied the phase separation between phenyl-alkylsilane and octadecyltrichlorosilane (OTS) molecules. Improving the phase separation was studied using two parameters: ring to ring interactions afore-analyzed and reactive heads with different grafting kinetics. Using the same trichlorosilane grafting moiety for phenyl molecules as for OTS, we show that phase separation is improved and OTS islands are smaller with phenyl species that involve stronger ring to ring interactions. The best case is obtained with mixing phenyl and pentafluoro-phenyl rings using hydrogen bonds for packing together the aromatic species of the SAM. Small phenyl species islands (40-100 nm in diameter) could be obtained inside the OTS SAM using a less reactive grafting head for the aromatic molecules. These two cases demonstrate an improved control of SAM composition and morphology essential to further use the obtained islands for building molecular devices. [1] F. Schreiber, Progress in Surf. Sci. 2000, 65, 151. [2] H.B. Akkerman et al., Nature 2006, 441, 69. [3] F. Fan et al., Langmuir 2003, 19, 3254. [4] J. Moineau et al., Langmuir 2004, 20, 3202. [5] V.R. Thalladi et al., J.Am.Chem.Soc. 1998, 120, 8702 ; J.D. Dunitz, ChemBioChem 2004, 5, 614; S. Zhu et al., Tetrahedr. Lett. 2005, 46, 2713.
6:00 PM - B5.24
The Effects of Octanedithiol Additive on the Three-Dimensional Nanoscale Organization of Highly Efficient Conjugated Polymer Bulk Heterojunction Solar Cells.
Mark Dante 1 , Andres Garcia 1 , Thuc-Quyen Nguyen 1
1 , University of California, Santa Barbara, Santa Barbara, California, United States
Show AbstractIt was recently shown that the addition of a small percentage of 1,8-octanedithiol to the solution from which polymer:fullerene bulk heterojunction films are spin-coated leads to solar cell power conversion efficiencies of greater than 5%. This “additive” approach circumvents the need of post-deposition processing that improve efficiencies, such as thermal or solvent annealing, and thus has the potential to greatly simplify the fabrication methods; an important consideration when comparing polymer solar cell devices versus their inorganic counterparts. This effect was first observed in poly(3-hexylthiophene) and [6,6]-phenyl C61-butyric acid methyl ester (P3HT:PCBM) bulk heterojunction solar cells. The increase in efficiency of these devices has been attributed to an increase in the crystallinity of the polymer phase, resulting in a higher charge carrier mobility. Although similar increases in efficiency have been observed when incorporating additives into bulk heterojunction blends containing the amorphous conjugated polymer poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b’]-dithiopene)-alt-4,7-(2,1,3-bezothiadiazole)], and [6,6]-phenyl C71-butyric acid methyl ester (PCPDTBT:C71-PCBM), the lack of crystallinity of the polymer phase with or without additive treatment and no increase in the charge mobility indicate a different mechanism is responsible for the performance improvement. Scanning probe examination of cross sections of PCPDTBT:C71-PCBM bulk heterojunctions reveal a structural change in the internal morphology of the hole and electron transporting networks when the film is cast from a solution containing 2% by volume 1,8-octanedithiol. Phase separation of the nanoscalar domains becomes more defined and the average sizes of hole and electron transporting networks double upon addition of the additive. The increase in the size of the domains likely gives rise to less charge recombination.
6:00 PM - B5.25
Maskless Patterning of Active Layer for Organic Thin-Film Transistor by Transfer-Printing Based Lift-Off Technique.
Wonsuk Choi 1 , Min-Hoi Kim 1 , Won-Ho Kim 1 , Kyungmo Koo 1 , Sin-Doo Lee 1
1 School of Electrical Engineering, Seoul National University, Seoul Korea (the Republic of)
Show AbstractOrganic thin film transistors (OTFTs) have great potential to replace conventional silicon-based inorganic semiconductor devices as a mainstream of ubiquitous flexible electronic applications. Although there has been a tremendous progress in the device performance of OTFTs during the past several years, there still remains a critical issue on the methods of precisely patterning organic active materials. For instance, accurate patterning of active semiconducting material in the OTFTs is inevitably required for the enhanced device performances.Here, we present a high-resolution patterning method of organic materials using transfer-printing based lift-off (TPLO) technique. The patterning processes using the TPLO are described as: (i) a fluoro-polymer layer is coated on a patterned polydimethylsiloxane (PDMS) stamp, (ii) the fluoro-polymer patterns are transfer-printed onto a substrate as a sacrificial lift-off layer, (iii) an organic material is deposited on the entire substrate having the lift-off layer, and (iv) the pattern of organic material is formed by lifting-off the organic material remaining on the fluoro-polymer using a fluoro-solvent. The TPLO technique has two distinctive advantages over existing approaches. First, the TPLO is a maskless process which is inherently suitable for large-area and high-resolution patterning so that a feature resolution down to 3 um (the resolution of conventional shadow mask technique is about 30 um) was achieved for various organic materials. Second, the TPLO has the capability of patterning organic materials onto various types of substrates including plastic substrates due to the chemical compatibility of the fluoro-solvent. As a demonstration, we fabricated pentacene patterns on the polymeric gate insulator layer using the TPLO technique to fabricate OTFT device arrays. The surface morphology and the electrical characteristics of our OTFTs are found to be comparable to those of the OTFT fabricated by a conventional shadow mask process.In summary, we developed a transfer-printing based lift-off technique (TPLO) as a general and versatile platform for precise patterning of organic materials in high resolution of a few micrometers. Our pattering process is expected to be widely used for fabricating various plastic electronic devices including high-resolution organic light emitting diode (OLED) displays and top-contact OTFTs with a very short channel length.
6:00 PM - B5.26
Electrical and Friction Properties of Stilbene Based Self Assembled Monolayers on Au (111): The Role of Molecular Ordering.
Yabing Qi 1 2 , Bas Hendriksen 2 , Xiaosong Liu 3 , Violeta Navarro 2 , Jeong Park 2 , Imma Ratera 2 , John Klopp 4 , Carine Edder 4 , Franz Himpsel 3 , Jean Frechet 4 5 , Miquel Salmeron 2 4 6
1 Applied Science and Technology Graduate Group, University of California Berkeley, Berkeley, California, United States, 2 Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California, United States, 3 Department of Physics, University of Wisconsin, Madison, Madison, Wisconsin, United States, 4 The Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, California, United States, 5 Department of Chemistry, University of California Berkeley, Berkeley, California, United States, 6 Department of Materials Science and Engineering, University of California Berkeley, Berkeley, California, United States
Show AbstractWe intend to understand charge transport mechanisms, responsible for organic/molecular electronics, by studying the relation between the structure of self-assembled monolayers of molecules that contain conjugated groups and their electrical conductivity. As an example, we investigated the electrical and friction properties of ω-(trans-4-stilbene)alkylthiol self-assembled monolayers (SAMs) on Au(111) using Atomic Force Microscopy (AFM). The sample surface prior to heating was uniformly covered with a molecular film that comprises of very small grains. Well-packed and flat islands were formed after the sample was heated in nitrogen at 120 °C for 1 h. While the lattice resolved AFM images revealed an enhanced ordering in islands, the substrate area between islands was covered with disordered molecules. The islands exhibit substantial reduction (50%) in friction supporting the existence of good ordering. The islands were ∼ 8Å higher than the surrounding disordered phase in the AFM topographic image, indicating that the molecules of islands stand more upright on the substrate. Near edge X-ray absorption fine structure spectroscopy (NEXAFS) measurements revealed an almost upright molecular orientation for samples both before and after heating, with substantial increase of ordering after heating. Conductance-AFM measurements revealed a more than 2 orders of magnitude higher conductivity on the large islands than that of the disordered phase on the heated samples. The current level on the ordered islands is also much larger than that of the non-annealed SAM. We propose that the conductance enhancement is a result of a better π-π stacking between trans-stilbene units of neighboring molecules as a result of improved ordering in islands.
6:00 PM - B5.27
High Mobility n-Channel Organic Field-Effect Transistors by Solution Process.
Peng Wei 1 , Joon Hak Oh 1 , Zhenan Bao 1
1 Chemical Engineering, Stanford University, Stanford, California, United States
Show Abstract The development of n-channel organic field-effect transistors (OFETs) by solution-process is crucial for low-cost and flexible electronics. For n-channel OFETs, the moisture and hydroxyl groups at the semiconductor-dielectric interface act as the trap sites during operation. In this work, we used double-layer dielectric with hydroxyl-free divinyltetramethyldisiloxane-bis(benzocyclobutene) (BCB) to eliminate the electron-trap. High mobility (0.1 cm2/Vs) n-channel OFET was obtained based on [6,6]-phenyl-C61-butyric acid ester (PCBM) by solution process, which is a commercial available n-channel semiconductor. In comparison with SiO2 dielectric, both the mobility and on/off ratio significantly increase with this double-layer dielectric.
6:00 PM - B5.28
Measurement and Modeling of Current in Hole-only Tandem Structures of N,N'-diphenyl-N,N'-bis(1-naphthyl)-1,1'-biphenyl-4,4'-diamine (NPB) and 4,4′,4″-tris (3-methylphenylphenylamino)-triphenylamine (m-MTDATA)
Christoph Zimmermann 1 , Manuel Bosing 1 , Florian Lindla 1 , Frank Jessen 1 , Hans-Peter Loebl 2 , Dietrich Bertram 3 , Michael Heuken 1 4 , Holger Kalisch 1 , Rolf Jansen 1
1 Chair of Electromagnetic Theory, RWTH Aachen University, Aachen Germany, 2 , Philips Technologie GmbH Forschungslaboratorien - Philips Research Laboratories, Aachen Germany, 3 , Philips Technologie GmbH, Aachen Germany, 4 , AIXTRON AG, Aachen Germany
Show AbstractOrganic light emitting diodes (OLED) have the potential to replace present light sources. The most advanced types of OLED are complex stacks of doped transport and emission layers consisting of small molecules. One advantage of multilayer OLED is the possibility to confine the recombination zone in an emission layer by proper choice of energy levels. Little effort has been spent so far to examine the charge carrier transport over energy barriers between layers of small organic molecules.We have examined the hole transport through the interface between layers of 4,4′,4″-tris(3-methylphenylphenylamino)-triphenylamine (m-MTDATA) and N,N'-diphenyl-N,N'-bis(1-naphthyl)-1,1'-biphenyl-4,4'-diamine (NPB), both common hole transport materials used in OLED.Samples with different combinations of layer thicknesses were produced by vacuum thermal evaporation. They were designed as hole-only diodes with a well injecting anode of indium tin oxide and a non-injecting cathode of aluminium on top. Temperature-dependent I-V curves were measured for all samples. Additionally, dark injection measurements were performed on the diodes to analyze their transient behaviour.Comparison of the I-V curves of the different samples showed clear evidence that the hole current is limited by the energy barrier between the two layers. The propagation time of carriers from the anode to the interface and their accumulation at the barrier could be recognized in the dark injection signals.The transient and stationary results were modeled with a numerical algorithm solving the time-dependent semiconductor device equations with arbitrary mobility models and boundary conditions at internal interfaces. The empirical findings with respect to voltage and temperature dependence of the current were compared to established theoretical assumptions about hopping transport in amorphous organic materials. Qualitative agreements concerning the shape of the transient currents and the temperature dependence of the I-V curves were found, quantitative deviations will be discussed.
6:00 PM - B5.3
Semiconductor Organic-Inorganic Nanocomposites at the Air/Water Interface and Their Performance in Thin Film Photovoltaic Devices.
Zhiqun Lin 1 , Matthew Goodman 1 , Jun Xu 1 , Jun Wang 1
1 , Iowa State University, Ames, Iowa, United States
Show AbstractOrganic-inorganic nanocomposites consisting of electroactive conjugated polymer, poly(3-hexylthiophene) (P3HT) intimately tethered on the surface of semiconductor CdSe quantum dot (i.e., P3HT-CdSe nanocomposites) at the air/water interface formed via Langmuir isotherms were explored for the first time. The P3HT-CdSe nanocomposites displayed a high pressure plateau in the Langmuir isotherm, illustrating their complex packing at the air/water interface. Furthermore, photovoltaic devices fabricated from the LB depositions of the P3HT-CdSe nanocomposites exhibited a relatively high short circuit current, ISC, while maintaining a thin film profile. These studies provide insights into the fundamental behaviors of semiconductor organic-inorganic nanocomposites confined at the air/water interface as well as in the active layer of an organic-based photovoltaic device.
6:00 PM - B5.30
Torsion Mode Conductive Atomic Force Microscopy Study on Polypyrrole Based Polyelectrolyte.
Ling Sun 1 , Jianjun Wang 1 , Elmar Bonaccurso 1 , Andreas Muehlebach 2 , Hans-Juergen Butt 1 , Gerhard Wegner 1
1 Physics of polymers, Max-planck institute for polymer research, Mainz Germany, 2 Group Research, Ciba Inc., Basel Switzerland
Show AbstractIt has been commonly understood that the properties of organic functional materials and the devices using these materials are determined by their local structure/morphological features . Conductive atomic force microscopy (c-AFM) is usually used to study the morphological and electronic properties of organic functional materials, especially conductive polymers. It measures topography of the sample surface and current flow through the bulk material with a nanometer resolution. However, conventional c-AFM has to be operated with a contact mode, which is usually destructive. This is not favored for soft materials, i.e. polymers. As an alternative, we use a torsion mode c-AFM to image the topography and current. In torsion mode c-AFM, the cantilever is oscillated at the first torsional resonance frequency by two piezos. The lateral forces that act on the tip cause a change in the torsional resonance frequency, amplitude and/or phase of the cantilever. The oscillation of the cantilever is maintained in a near-field region where electronic interaction still exists. Therefore torsion mode c-AFM could non-destructively measure the topography and current flow through the sample.Here we present the electronic property study of a newly synthesized Polypyrrole (PPy) based conductive polymer, by torsion c-AFM and Kelvin probe force microscopy (KPFM). PPy has a high conductivity and of neutral pH when doped with polystyrene sulfonate (PSS) . Thin films cast from PPy water suspension are conductive and transparent as proved in our previous study . However, the microscopic structure of the conductive species, i.e. the conductive grains, is elusive. We use torsion C-AFM to visualize the conductive grain structure. The difference in conductivity implies a phase separation which is also proved by the difference in Kelvin potential. The results suggest us the mesoscopic structure of PPy/PSS and help to correlate the material structure and its property.
6:00 PM - B5.31
Fabrication of Light-Emitting Transistor Combined with ZnO thin-film Transistors.
Hiroshi Yamauchi 1 , Yasuyuki Watanabe 2 , Masaaki Iizuka 3 , Masakazu Nakamura 4 , Kazuhiro Kudo 4
1 Faculty of Engineering, Chiba University, Chiba Japan, 2 Center of Frontier Science, Chiba University, Chiba Japan, 3 Faculty of Education, Chiba University, Chiba Japan, 4 Graduate school of Engineering, Chiba University, Chiba Japan
Show AbstractOrganic light emitting diodes (OLEDs) have much attention for flexible, low cost, and ease of processing. In this work, as one of the method for expanding effective light-emitting area for active matrix displays, a new type active light-emitting device combined with ZnO transistor is proposed. An OLED is fabricated on the transparent FET. A transparent ZnO FET has an indium-tin-oxide gate and a silicon nitride gate insulator fabricated by plasma-enhanced chemical vapor deposition. These transparent materials are expected to be promising components of high-efficiency light-emitting devices. This light-emitting device has an advantage to fabricate without the damage to organic layer by the electrode sputtering and oxidation of emitting layer with oxygen in the atmosphere. The light-emitting devices proposed here are suitable for the display element because these ZnO layers between light-emitting layer work high-transparent electron injection, electrode, and active layer of FET, therefore the aperture ratio increases and light-emitting occurs efficiently. We describe the basic characteristics of transparent FET using thin-film ZnO. The ZnO works as the active channel, and Al-doped ZnO (AZO) is used as the source/drain electrode. These films were deposited by radio frequency (rf) magnetron sputtering. In an attempt to reduce the plasma damage, we studied a method for fabricating the AZO source/drain electrode. AZO layers are fabricated by the stepped rf power deposition method. There is a correlation between rf power and resistivity and the AZO film deposited at low rf power is low conductivity We compare the static characteristics of ZnO FETs with AZO electrode deposited by uniform power deposition and those with AZO electrode deposited by stepped power deposition. The current at Vds = 60 V saturated below 3.5μA. In this experiment, Vg was varied from -10 V to +100 V with 10 V steps. Id of uniform power deposition FET shows a current saturation at low Vds and the current remains at low value of around 20 nA. On the other hand, Id of stepped power deposition increases two orders magnitude of around 3μA. These improvements are derived from low-damage and high-conductivity AZO films obtained by the new deposition method. We fabricated the active light-emitting device with ZnO transparent FET and reported on the electrical properties of ZnO FET. The low-temperature and low-damage sputtering method improves the on current in two orders magnitude. Luminance of the active light-emitting device driven by ZnO transparent FET was obtained. These experimental results indicate that the active light-emitting device using transparent ZnO thin films on a plastic substrate should be realized by optimizing the device design and the fabrication process.
6:00 PM - B5.33
Vertical-channel Organic Transistors using Step-edge Structure and its Application to Pixel Circuits.
Makoto Shirakawa 1 , Gaku Harada 1 , Toru Ishikawa 1 , Miwa Ogawa 1 , Takeshi Sano 1 , Kenichi Shibata 1
1 , Sanyo Electric Co., Ltd., Hirakata, Osaka, Japan
Show AbstractOne of the possible applications of organic thin-film transistors (OTFTs) is presumed to be a backplane for an active matrix organic light emitting diode display (AM- OLED), however, high driving voltage and low output current of OTFTs are the problems. In this work, we have introduced a step-edge structure to form a vertical-channel OTFT in order to improve driving voltage and output current for the use of driving transistor in a pixel circuit. By optimizing the structure and the fabrication process of our vertical-channel OTFT, we have obtained sufficient current for OLED which is around Id = 500 μA (@ W = 3 mm, Vd, Vg = -10 V) and high on/off ratio of 105 at a test piece using pentacene for the organic semiconductor layer. Our vertical-channel OTFT using a step-edge structure also has an advantage of a high-resolution patterning due to its self-aligned process, which is different from other types of vertical-channel transistors reported before. We successfully fabricated OTFT arrays which consist of a driving transistor (W / L = 240μm / 2μm) and a switching transistor (W / L = 120μm / 2μm) in the pixels (450μm x 450μm). A part of this work was performed under management of the OITDA supported by NEDO.
6:00 PM - B5.34
Logic Devices with p-channel Organic and n-channel Inorganic Transistors.
Yasuyuki Watanabe 1 , Hiroyuki Iechi 2 , Hiroshi Yamauchi 3 , Kazuhiro Kudo 4
1 Center for Frontier Science, Chiba University, Chiba Japan, 2 Tohoku R&D Center, Research and Development Group, RICOH Co., Ltd, Miyagi Japan, 3 Faculty of Engineering, Chiba University, Chiba Japan, 4 Graduate school of Engineering, Chiba University, Chiba Japan
Show AbstractFlexible electronic devices have been strongly attracted much attention to the potential application for the flexible displays, radio-frequency identification cards (RFIDs), etc. Especially, it is required that the high-performance organic field-effect transistors (OFETs) which are indispensable for the more advancing flexible displays because it realized the flat-panel television using organic light-emitting diodes (OLEDs) on last year. Therefore, it is important to study the characteristics of logic circuits based on p-type and n-type semiconductors. We have already reported on the characteristics of the logic devices using pentacene and ZnO FETs [1] and pentacene organic static induction transistors (OSITs) [1, 2]. In general, organic semiconductors such as pentacene show p-type properties with low-mobility and high-resistivity compared with inorganic semiconductors. In particular, it is difficult to control the electrical properties such as the mobility of organic semiconductors because the doping technology has not been established in organic semiconductors. On the other hand, ZnO materials show the wide-range properties from insulator to low conductive n-type semiconductor. In this study, we focus on the organic semiconductors with p-type properties and the inorganic semiconductors with n-type properties for the fabrication of flexible logic devices. First, the charge transport characteristics both ZnO and pentacene thin films are investigated. Next, the performances of the CMOS inverter with pentacene and ZnO FETs are controlled by matching the characteristics of ZnO to those of pentacene FETs. Finally, to improve the characteristics of ZnO/pentacene logic devices, we propose new device structures of step-edge-vertical-channel OFETs (SEVC-OFETs). The obtained results demonstrate that the hybrid complementary inverters described here have potential for use as advancing flexible displays and RFIDs.References[1] H. Iechi, Y. Watanabe and K. Kudo, Jpn. J. Appl. Phys. 46,4B, 2645 (2007).[2] Y. Watanabe, H. Iechi and K. Kudo, Thin Solid Films, 516, 2729 (2008).
6:00 PM - B5.35
Study on the Spinodal Decomposition of P3HT:PCBM Layer with Different P3HT:PCBM Ratio for High Performance Organic Solar Cells.
Woon-Hyuk Baek 1 , Tae-Sik Yoon 1 , Hyun Ho Lee 2 , Yong-Sang Kim 1 3
1 Nano Science & Engineering, Myongji University, Yongin, Gyeonggi, Korea (the Republic of), 2 Chemical Engineering, Myongji University, Yongin, Gyeonggi, Korea (the Republic of), 3 Electrical Engineering, Myongji University, Yongin, Gyeonggi, Korea (the Republic of)
Show AbstractIn organic solar cells, composites of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) are most widely used as an active layer and recorded to have the highest power conversion efficiency. Since typical range of exciton diffusion length is 4~20 nm in polymer, nanoscale interpenetrating network morphology of donor (P3HT) and acceptor (PCBM) is indispensable for efficient charge separation with extended interface and highly efficient polymer solar cell. The relation between optical absorption and chemical properties of P3HT is already well established. When P3HT has more ordered structure with longer chain length and higher crystallinity, its UV/visible absorption is shown to be red-shifted, higher, broader and more vibronic. And electro-optical and morphological characteristics of photovoltaic cells with different P3HT:PCBM ratio were also reported. In this study, it is also observed that the absorption of PCBM increases while that of P3HT decreases with increasing concentration of PCBM in active layer. Moreover, the crystallinity of P3HT characterized by peak intensity of XRD diffraction increases with increasing P3HT concentration. However, further increasing P3HT concentration results in the reduced peak intensity. This anomalous observation of crystallinity of P3HT layer depending on the ratio of P3HT and PCBM is discussed with the behavior of spinodal decomposition of P3HT and PCBM. The P3HT:PCBM layers with varied ratio are prepared on polyethylenedioxythiophene:polystyrenesulphonate (PEDOT:PSS) coated indium tin oxide (ITO) substrate by spin-coating and subsequent annealing. The behavior of spinodal decomposition is investigated by using atomic force microscopy (AFM) and transmission electron microscopy (TEM).
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Transport Properties of Graphene Field-effect Transistors with Different Metal Electrodes.
Tatsuya Saito 1 , Ryo Nouchi 2 , Hiroki Watanabe 1 , Katsumi Tanigaki 1 2
1 Department of Physics, Tohoku university, Sendai Japan, 2 WPI Advanced Institute for Materials Research, Tohoku university, Sendai Japan
Show AbstractA single-layer carbon sheet, graphene, has many interesting physical properties resulting from a zero-gap band structure with the linear energy dispersion around the Fermi energy. In addition, the highest mobility at room temperature among all materials makes graphene a major candidate for future high-speed electronic devices. On the other hand, graphene is easily affected by environment; the influences of metal contact on the electronic transport have been pointed out to be large [1]. In fact, with Co contacts, the conducting property of graphene shows an anomalous behavior [2]. In this study, we investigate the contact effect by employing field-effect transistors (FETs) with different metal electrode, where the material for the source electrode is different from that for the drain electrode (Au/Ca etc). We acquired graphene by the mechanical exfoliation method [3], and patterned electrodes onto it by using electron beam lithography and lift-off techniques. The fabricated FETs were measured in an inert atmosphere to avoid oxidation of low work function metals. In this presentation, we report on the contact doping to the graphene FETs from the view point of work function. Especially, p-n junctions formed by the contact doping are discussed.[1] G. Giovannetti et al., Phys. Rev. Lett. 101, 026803 (2008). [2] R. Nouchi et al., Appl Phys Lett. 93, 152104 (2008). [3] K. S. Novoselov et al., Proc. Natl. Acad. Sci. U.S.A. 102,10453 (2008).
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Formation of Composite Organic Thin Film Transistors with One-Dimensional Nanomaterials
Gen-Wen Hsieh 1 , Flora Li 2 , Sharvari Dalal 1 , Husnu Emrah Unalan 1 , Massimo Spina 1 , Pritesh Hiralal 1 , Arokia Nathan 3 , Paul Beecher 4 , James Stott 3 , Andrew Flewitt 1 , Gehan Amaratunga 1 , William Milne 1
1 Engineering Department, University of Cambridge, Cambridge United Kingdom, 2 Electrical and Computer Eng., University of Waterloo, Waterloo, Ontario, Canada, 3 London Centre for Nanotechnology, University College London, London United Kingdom, 4 , Nokia Research Centre, Cambridge United Kingdom
Show AbstractLarge area and flexible electronics is a rapidly expanding research area, which opens our eyes into a new version of future electronics and is expected to revolutionise the electronics industry. In this area, organic semiconductors and organic thin film transistors (OTFTs) have attracted much interest by virtue of their solution processibility, low temperature processing, and potentially low cost fabrication, with the advantage of lightweight, bendable features. Despite ongoing advancements in material functionality and processing technologies, OTFTs are somewhat limited in terms of device lifetime and mobility, which hinder their adoption on a wider scale. Here, the purpose of this study is to develop augmentative material and fabrication systems in the quest for higher performance transistors for large area and flexible applications.One-dimensional (1-D) nanomaterials, such as nanotubes and nanowires, are of interest in view of their unique optoelectronics properties, high aspect-ratio structures, high mobility and conductivity. These nanostructures that present feasible alternatives can be used as the sole material in a device structure, or can be implemented as a complement to organic materials. To combine the advantages of 1-D nanostructures with the ease of processibility offered by organic semiconducting materials, we aim to investigate means of incorporating 1-D nanomaterials into organic devices by different techniques, such as ink-jet printing, contact printing, and spin coating. A variety of carbon nanotubes (CNTs), and metallic and semiconducting nanowires (NWs) are utilised as a prospective booster to p-type or n-type organic semiconductors. The first is the fabrication of p-type OTFTs comprised of CNTs and semiconducting polymer by means of ink-jet printing. Results show that low density ink-jet printed CNT networks can enhance the performance of polythiophene-based OTFTs. For instance, the field effect mobility exhibited by composite CNT-OTFT devices is seven times higher than that of pristine organic devices. Secondly, a facile technique for large-scale parallel assembly of 1-D nanostructures by a dry shear-sliding concept is demonstrated. Parallel arrays of silicon (Si) nanowires are employed in the fabrication of bilayer composite Si-OTFTs. The electrical measurements of these p-type OTFTs show that the relative direction of aligned NWs corresponding to the current flow direction influences the field effect mobility and ON/OFF current ratio. The third strand of this work features the use of n-type semiconducting zinc oxide (ZnO) nanowires as the mobility enhancer in n-type OTFTs which represents a remarkable increase in field effect mobility by as much as a factor of 40 over OTFTs comprising pristine organics.
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Self Organization of Regioregular Poly(3-Hexyl Thiophene)-Poly(ε-Caprolactone) Block Copolymers Obtained by a Novel Controlled Synthesis Method.
Mathieu Surin 1 , Khoa Tran 2 , Olivier Coulembier 2 , Philippe Dubois 2 , Roberto Lazzaroni 1 , Philippe Leclere 1
1 Chemistry for Novel Materials, University of Mons-Hainaut, Mons Belgium, 2 Laboratory of Polymeric and Composite Materials , University of Mons-Hainaut, Mons Belgium
Show AbstractSince the initial discovery of conductive organic polymers, tremendous efforts have been devoted to the synthesis of processable compounds exhibiting high electrical conductivity or charge mobility. Among these, polythiophene-based materials have received particular attention, with applications ranging from field-effect transistors (FETs) to optical and electronic sensors, photovoltaic diodes and non-linear optical materials. Substitution at the 3-and/or 4-position of the thiophene ring not only improves the solubility and processability of poly(thiophene)s but also strongly influences the macromolecular ordering by interdigitation of the alkyl side chains between the lamellar structures of π-stacked conjugated backbones. Regioregular poly(3-alkylthiophene)s (rr-P3ATs) have been initially obtained by Mc Cullough and Rieke, who discovered that transition metal-catalyzed polycondensation lead to head-to-tail P3ATs. While side-chain functionalization has been demonstrated to be an effective way to modulate the optoelectronic properties of rr-P3ATs, the synthesis of block copolymers containing a π-conjugated polymer segment can generate unique self-assembled electronic materials with enhanced mechanical properties displaying phase separation and yielding discrete microstructures. Here we propose an original, well-controlled synthesis method, based on the association of the GRIM approach with the ring-opening polymerization (ROP) of lactones, for the design of poly(3-hexyl thiophene) (P3HT) block copolymers with poly(ε-caprolactone) (PCL) segments. This would lead to a new type of material, since PCL is: (i) crystallizable (in contrast to other segments copolymerized with P3HT, such as poly(methacrylate)s).; (ii) removable by hydrolysis, and ; (iii) biocompatible. The present communication reports on the stepwise synthesis of new P3HT-PCL di- and triblock copolymers by controlled ROP of CL from hydroxyl-terminated P3HT. Thin deposits of P3HT homopolymer compounds onto various surfaces show a fibrillar morphology, as observed with Atomic Force Microscopy (AFM). Since these deposits are formed from a good solvent for both blocks (i.e., tetrahydrofuran), the observed morphology arises from the self-assembly of the P3HT segments into π-stacked assemblies, as observed for other poly(thiophene)s and conjugated (co)polymers systems. For the P3HT-PCL block copolymers, they also self-assemble into fibrillar structures, with semiconducting nanoribbons separated by removable, insulating domains of PCL. We investigate the electrical properties of the P3HT-PCL fibrillar assemblies at the local scale via conducting probe AFM techniques. Finally, since P3HT can be chemically doped after self-assembly process and the lateral dimensions can be tuned by controlled synthesis of the polymer segments, this could lead to an elegant way for fabricating ‘nanowires’ for organic electronics.
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Temperature and Coverage Dependent Dewetting of Ultra-thin Films of Discoid Organic Molecules on Au(111) and Ag(111).
Paul Frank 1 , Norbert Koch 2 , Ralph Rieger 3 , Klaus Muellen 3 , Adolf Winkler 1
1 Institute of Solid State Physics, Graz University of Technology, Graz Austria, 2 Institut für Physik, Humboldt-Universität zu Berlin, Berlin Germany, 3 , Max Planck Institute of Polymer Research, Mainz Germany
Show AbstractUltra-thin films of hexaazatriphenylene-hexacarbonitrile (HAT-CN) were prepared on a gold(111) single crystal and on a silver(111) single crystal utilizing organic molecular beam deposition (OMBD) under well defined ultra high vacuum (UHV) conditions. The thin films were investigated by thermal desorption spectroscopy (TDS), X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). XPS in combination with TDS was applied to reveal the kinetics of the layer growth. Additionally, the HAT-CN growth characteristics were studied as a function of substrate temperature. From TDS, the pre-exponential factors and the heats of evaporation for the individual layers could be determined experimentally. Ex-situ AFM was used to determine the film morphology. A Kelvin probe (KP) was used to follow the change in the work function with increasing film thickness.The HAT-CN molecules exhibit a quite unusual layer growth behaviour and thermal stability. On the gold single crystal, the first two layers of flat lying molecules wet the surface and they are stable up to 400 K and 500 K, respectively. With increasing coverage islands form on the wetting layer, which induce a destabilisation of these layers. As a function of coverage and temperature first the 2nd and then the 1st monolayer are incorporated into the islands, as determined by thermal desorption spectroscopy. On the silver single crystal, HAT-CN shows a similar layer growth and desorption behaviour, except that the molecules in the 1st monolayer are bound more strongly and dissociate upon heating.
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Co-deposited Films of Rod-like Conjugated Molecules: Mixing versus Phase Separation.
Jörn-Oliver Vogel 1 , Ingo Salzmann 1 , Steffen Duhm 1 , Bert Nickel 2 , Juergen Rabe 1 , Norbert Koch 1
1 Institut für Physik, Humboldt-Universität zu Berlin, Berlin, Berlin, Germany, 2 Department für Physik, Ludwig-Maximilians-Universität, Munich , Bavaria, Germany
Show AbstractThe co-deposition of two conjugated molecular species can lead to novel materials with new properties. The formation of well ordered mixed layers of two different molecules can be used to continuously tune the material properties, e.g., similar to the doping of inorganic semiconductors [1] On the other hand, phase separation of donor and acceptor material is desired for organic hetero-junction solar cells. To assess the extent to which the molecular length determines either phase separation or mixing in co-deposited thin films, we investigated pairs of five different rod-like conjugated molecules: α-quaterthiophene (4T), α-sexithiophene (6T), α,ω-dihexylsexithiophene (DH6T), p-sexiphenyl (6P), and pentacene (PEN). With these molecules we realized pairs differing in length either via the size of the conjugated core (CC) alone, or additionally through the end-termination by alkyl chains. Such films were characterized with specular and in-plane X-Ray diffraction (XRD), infrared absorption spectroscopy (IR) and scanning force microscopy (AFM).The co-deposition of molecules with similarly sized CC, i.e., {6T/6P} led to well ordered mixed structures. The co-deposition of molecules with different molecular length but similarly sized CC, i.e., {6T/DH6T and 6P/DH6T} led also to ordered mixed layers. A particularly appealing feature of these films is that the interlayer distance can be controlled via the mixing ratio. The co-deposition of molecules with differently sized CC, e.g., {4T/6T} or {6P/PEN} led to pronounced phase separation of the two species in thin films.The results of this work will be useful to tailor the morphology and structure of organic co-deposited films, in particular for solar cell applications.[1] P. Cosseddu, J.-O. Vogel, B. Fraboni, J. P. Rabe, N. Koch, A. Bonfiglio, Adv. Mater., in press.
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Surface Plasmon and Semiconducting Polymer Luminescent Solar Concentrators.
Michael Griffo 1 , Sue Carter 1
1 Physics, University of California, Santa Cruz, California, United States
Show AbstractLuminescent solar concentrators use fluorescing materials with large Stokes shifts to absorb incident sunlight and waveguide the emitted light to photovoltaic cells. Although the idea behind luminescent solar concentrators is not new, they have recently been of great interest. Advances in polymer and other fluorescent organic materials have enabled some increases in efficiency; however, further improvement in the outcoupling and PL efficiency, especially of near-IR emitting materials, is needed. We use tuned surface plasmons on silver nanoparticles to enhance the fluorescence of semiconducting polymers and to increase coupling between the incident light and glass to waveguide light for luminescent solar concentrator applications. We have demonstrated a 5.6 fold increase in photoluminescence efficiency with this device structure and enhancement in collection efficiency. We analyze the collected emission spectra as a function of distance from the point the incident light is absorbed to the point the light exits the concentrator. Comparing this with a control, the incident spectra, and the absorption spectra we gain insight into the factors affecting the optical efficiencies of the structure.
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The Use of Tailored Host and Hole Blocking Materials for Improved Power Efficiencies of Blue OLEDs.
Asanga Padmaperuma 1 , Philip Koech 1 , Eugene Polikarpov 1 , Amber Von Ruden 1 , James Swensen 1 , Lelia Cosimbescu 1 , Daniel Gaspar 1
1 , Pacific Northwest National Laboratory, Richland, Washington, United States
Show AbstractThe development of stable and efficient blue OLEDs is essential in the development of organic solid state white lighting with 50% power conversion efficiency. Nationwide adoption of high-efficiency OLED lighting instead of incandescent lighting would result in significant energy savings. The simplest and lowest cost method of generating white light is to convert part of the emission from a blue light source to white light using a system of phosphors external to the light generating region. State of the art OLEDs are based on organometallic phosphors doped into wide bandgap host materials. However, poor hole injection in these materials has limited measured efficiencies, and in most cases the emitter is implicated in charge trapping. A direct consequence of emitter charge trapping is a decrease in the operating voltage with increased dopant concentration, but also reduced device efficiency at higher current densities. We have demonstrated high efficiencies at lower operational voltages by using lower dopant concentrations (~ 2% wt) in an ambipolar host material. Here we discuss the use of ambipolar charge-transporting PO host materials and hole blocking materials for blue OLEDs. Ambiploar host materials were developed by combining hole-transporting moieties with PO-based electron-transporting moieties (ETm). The use of the same ETm as in the host in designing the hole blocking materials gave rise to hosts and blocking materials with improved energy matching. Design principles, physical properties, and electronic properties of these materials will be discussed. Furthermore, we show the correlation of OLED current density and emission efficiency results to the structure of the materials.
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Solution Processable n-Channel Semiconductors for Thin-Film Transistors Based on the Pyromellitic Diimide Core.
Qingdong Zheng 1 , Jia Huang 1 , Amy Sarjeant 2 , Howard Katz 1
1 Department of Materials Science & Engineering, The Johns Hopkins University, Baltimore, Maryland, United States, 2 Department of Chemistry, The Johns Hopkins University, Baltimore, Maryland, United States
Show AbstractIn the past decade, there has been extensive effort toward developing new organic semiconductors for thin film transistors due to their applications in complementary circuits, flat-panel displays and sensors. Pyromellitic dimides (benzenetetracarboxylic diimides) are best known as segments of highly insulating polyimide dielectrics. However, the authors report here that this simple structure can be a sufficiently conjugated core for the construction of n-channel organic semiconductors. Several pyromellitic diimides with fluorinated side chains were synthesized by a one-step reaction. The field effect devices can be easily fabricated from the synthesized pyromellitic diimides through vacuum sublimation or solution deposition. For the vacuum sublimed materials, the electron mobility is found to be as high as 0.136 cm2/Vs and the on/off ratios can reach a high value of 1000000. The electron mobility is found to be ~0.01 cm2/Vs for the solution deposited materials. We also report on the effects of deposition temperature, surface treatment and chemical structure on the electron mobility of the fabricated devices. An attempt to synthesize soluble polymers based on some functionalized pyromellitic dimides is made, and the electron carrying properties of the resulting polymers are investigated and discussed. Pyromellitic dimides are more readily available than the higher rylenes (such as naphthalene or perylene tetracarboxylic diimides) already investigated, are highly transparent and chemically stable, and lead to processable and electronically tunable derivatives. These derivatives offer great opportunity for both scientific exploration and utilization in organic and polymer-based devices.
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N-type and P-type Controlled Molecular Doping of MEH-PPV with all Solution-processed Method
Yuan Zhang 1 , Mingtao Lu 1 , Bert de Boer 1 , Paul Blom 1
1 Zernike Institute for Advanced Materials, University of Groningen, Groningen Netherlands
Show AbstractIn organic light-emitting diodes (OLEDs) based on evaporated small molecules it has been demonstrated that doping of the hole- and electron transport layers strongly reduces their operation voltage.1 Furthermore, the use of doped layers makes the OLEDs less sensitive to the workfunctions of the anode and cathode. For solution-processed LEDs based on conjugated polymers, however, doped charge transport layers are hardly applied. Here we present controlled p-type and n-type doping of poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) deposited from solution. Tetrafluoro-tetracyanoquinodimethane (F4-TCNQ) and bis(pentamethylcyclopentadienyl)cobalt(II) (DMCO) are used for the p- and n-type dopants respectively. By choosing the appropriate dopant solvents and adjusting the polarity of the solution aggregation can be prevented and doped films can be deposited with a controlled carrier density. Upon the p-type doping the low voltage part of the J-V characteristics of MEH-PPV based hole-only devices is increased by several orders of magnitude and a clear Ohmic behavior appears. We find that a doping concentration of 1.0wt.% leads to a free carrier density of 2×1022 m-3. For the n-type doping it is observed that the electron transport greatly improves. The electrons from the DMCO donor fill the trap states lying below the LUMO of the MEH-PPV. For sufficiently high n-type doping trap-free electron transport is observed in PPV-based diodes. We for the first time demonstrate that in MEH-PPV the free electron mobility is equal to the hole mobility. Finally, we demonstrate the first working example of light-emitting polymer-based p-n junctions from solution processing.References:1 M. Pfeiffer et al., Organic Electronics, 4, 89-103 (2003)
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Exciton Diffusion in Organic Semiconductors.
Paul Shaw 1 , Arvydas Ruseckas 1 , Ifor Samuel 1
1 School of Physics and Astronomy, University of St Andrews, St Andrews United Kingdom
Show AbstractThe transport of charge has been extensively and elegantly studied in organic semiconductors. However, the transport of excitons has received much less attention even though it is important in a wide range of organic optoelectronic devices. In OLEDs it determines whether confinement layers are needed and the roll-off at high brightness. In lasers exciton-exciton annihilation can be a major loss mechanism. In polymer solar cells, the need for bulk heterojunctions is due in large part to the limited exciton diffusion length. Furthermore exciton diffusion determines the distance over which interfaces will strongly affect materials photophysics and device operation. Estimates of exciton diffusion lengths in organic semiconductors range from nanometres to microns, making it urgent to develop improved measurements that can guide the development of improved materials.We show that time-resolved photoluminescence (PL) measurements provide a powerful way of studying exciton diffusion. We have explored three approaches. In the first, organic semiconductor layers of a range of thicknesses are deposited on an inorganic quencher such as TiO2. Faster decay of the PL is seen for thinner samples, and the results are fitted to a one-dimensional diffusion equation. For the widely studied solar cell material, poly(3-hexylthiophene) we find that PL decays for film thicknesses ranging from 6 to 32 nm can be fitted by a single fitting parameter – the diffusion constant and we obtain a value (1.8±0.3)x10-3 cm2/s which corresponds to a diffusion length of 8.5±0.7 nm. The second method for investigating exciton diffusivity involves studying exciton-exciton annihilation. At high exciton densities the PL decays faster and this change in behaviour can be modelled as a bimolecular process, linked to the exciton diffusivity. For P3HT the results can be combined with the first method to give an estimate for the distance apart at which two excitons annihilate of 1.8±0.4 nm. In the third method, the decay of fluorescence anisotropy is used to study exciton diffusion: as excitons diffuse they move to conjugated segments of different orientation and polarisation memory is lost. These powerful techniques enable us to study how structure and processing influence exciton diffusion in organic semiconductors. For example, we have demonstrated a large reduction in exciton diffusivity in dilute copolymers. We also find that exciton diffusivity is higher in MEH-PPV than P3HT, underlining the differences between charge transport and exciton transport.
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Reduced Graphene Oxide Electrodes For Organic Thin-film Transistors.
Hector Becerril-Garcia 1 , Randall Stoltenberg 2 , Ming Lee Tang 2 , Mark Roberts 1 , Zunfeng Liu 3 , Yosheng Chen 3 , Zhenan Bao 1
1 Chemical Engineering, Stanford University, Stanford, California, United States, 2 Chemistry, Stanford University, Stanford, California, United States, 3 Key Laboratory for Functional Polymer Materials and Center for Nanoscale Science and Technology, Nankai University, Nankai, Nankai, China
Show AbstractGraphene oxide (GO) is a water-soluble nanomaterial that can be prepared in large quantities from graphite and solution-processed into conformal thin-films on diverse substrates. GO thin-films are electrically insulating, but become conductive upon removal of oxygenated functionalities from the material. We previously reported the dependence of the conductivity of reduced GO (RGO) on the reduction conditions, and the successful application of RGO films as transparent electrodes for organic solar cells. We now present our evaluation of the use of micropatterned RGO films as electrodes for organic thin-film transistors made with well characterized p- and n-channel materials. Patterning techniques will be discussed. We show that RGO films can inject electrons or holes into organic semiconductor layers. Furthermore, organic semiconductors deposited on RGO show different morphology from that observed on metal electrodes, leading to improved transistor performance. These findings illustrate the potential benefits of using carbon electrodes for carbon-based electronics.
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Pendant Polymers in Light Emitting and Organic Photovoltaic Applications.
Akhil Gupta 1 , Scott Watkins 2 , Katalin Hegedus 3 , Melissa Skidmore 4 , Lynn Rozanski 5 , Gerard J Wilson 6 , Richard A Evans 7
1 Molecular and Health Technologies, CSIRO, Melbourne, Victoria, Australia, 2 Molecular and Health Technologies, CSIRO, Melbourne, Victoria, Australia, 3 Molecular and Health Technologies, CSIRO, Melbourne, Victoria, Australia, 4 Molecular and Health Technologies, CSIRO, Melbourne, Victoria, Australia, 5 Energy Technology, CSIRO, NewCastle, New South Wales, Australia, 6 Molecular and Health Technologies, CSIRO, Melbourne, Victoria, Australia, 7 Molecular and Health Technologies, CSIRO, Melbourne, Victoria, Australia
Show AbstractPlastic solar cells produced from organic semiconductors offer the potential to deliver efficient solar energy conversion with low-cost fabrication. The challenge is to develop materials for efficient charge separation and charge transport. We are examining the potential of using free radically synthesized polymers to replace conventional conjugated polymers such as poly(alkylthiophenes) or poly(dialkylfluorenes) in polymeric light emitting diode (PLED) and organic photovoltaic (OPV) applications. The use of free radically synthesized polymers has the potential of cheap synthesis together with the opportunity of controlling polymer structure living radical polymerization techniques such as RAFT and ATRP in order to probe structure property relationships.Our starting point was to examine conventional cyano-PPV type polymers and by using retro-synthetic analysis to make free radically polymerizable monomers with Cyano-PPV functionality as pendant substituents. A variety of analogues were prepared by the condensation of the appropriate aldehyde with p-cyanomethylstyrene. They were polymerized (uncontrolled) and their HOMO and LUMO energy levels were determined by cyclic voltametry and UV-vis spectroscopy. It was found that the incorporation of a triaryl amine group in the molecule (AG-1-78 monomer: AG1-80 polymer) allowed hole transport in PLED and OPV applications. The new material AG1-80 was evaluated in a hetero-junction organic photovoltaic device where AG1-80 replaced poly(3-hexylthiophene) (P3HT). AG1-80 is colored due to the acceptor donor nature of the molecule which induces an internal dipole. Thus AG1-80 acts as absorbing chromophore and hole transport material. An optimized device of AG1-80 and PCBM provided efficiencies of 0.2% across the entire solar spectrum. This modest efficiency would be largely due to the limited absorption of the visible spectrum of AG1-80 (λmax 406 nm). However efficiency at its absorption maxima was 10.5%. This suggests that the pendant nature of the AG1-80 was not a significant problem for actual charge transportation. Thus, we have shown free radical polymers with pendant electro-active moieties that combine charge transport and chromophore functionalities may compete with conjugated polymers although an increased absorption of the solar spectrum is essential for photovoltaic devices.This presentation will provide an overview of this research from the design, synthesis, characterization and device evaluation of these cyano-PPV derived materials within the Electro-active Nanomaterials Theme of CSIRO’s Niche Manufacturing Flagship.Acknowledgements: Australian Department of Innovation, Industry, Science and Research. International Science Linkage, CG100059 Victorian Department of Primary Industries, Sustainable Energy Research and Development Grant and Merck Speciality Chemicals for the generous donation of poly(3-hexylthiophene
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FET Characteristic of Chemically-Modified CNT
Ryotaro Kumashiro 1 , Yan Wang 1 , Naoya Komatsu 1 , Katsumi Tanigaki 2 1
1 Department of Physics, Graduate School of Science, Tohoku University, Sendai Japan, 2 World Premier International Research Center, Tohoku University, Sendai Japan
Show AbstractSemiconducting carbon nanotubes (CNT) have shown promising applications as electronic materials for nano-scale devices in the future. It is well known that the field effect transistors (FET) fabricated by CNT show high performance. Semiconducting CNT have hole- and electron-carrier type, therefore, CNT-FET usually exhibit ambipolar charge transport. In recent years, the FET structure has attracted intense research interest as a light emitting device, and the research related to organic light-emitting FET are of fundamental and practical significance. In this study, we will report the light emission properties of CNT-FET with organic light emitter. 1,3,6,8-tetraphenylpyrene (TPPy) was used as a light-emitting organic-material. TPPy/CNT-FET devices were fabricated by drop-cast method on SiO2/Si substrate. From the experimental results, it was shown that the light emission in visible-light region can be observed in FET operation and the state of light emission is changed by VG. The mechanism of light emission in TPPy/CNT-FET will also be discussed.
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Dependence of Organic Thin-film Transistors with Polymer-blend Small Molecule Semiconductors on Molecular Weights and Types of Polymers.
Takahiro Ohe 1 , Miki Kuribayashi 1 , Ryoichi Yasuda 1 , Ami Tsuboi 1 , Kotaro Satori 1 , Masao Itabashi 1 , Kazumasa Nomoto 1
1 Advanced Materials Laboratories, sony, Atsugi, Kanagawa, Japan
Show AbstractWe will discuss how performance of solution-processed organic thin-film transistors (OTFTs) with a polymer-blend 6,13-bis(triisopropylsilylethynyl)-pentacene (TIPS-pentacene) channel are affected by molecular weight (MW) and type of the polymers.It has been reported that spin-coating of a solution of TIPS-pentacene and poly(alpha-methylstyrene) (PaMS) induces phase separation, and results in a trilayer film: a TIPS-pentacene layer, a mixed layer of TIPS-pentacene/PaMS, and a TIPS-pentacene layer[1,2]. The experiments with two different MW of PaMSs[2] and with various types of polymer dielectrics[3] have been reported. It, however, has not been clear the specific dependence of phase separation phenomena, and performance of OTFTs on MW and types of polymers. In order to understand the relations among them, we have performed systematic study.First, we investigated the dependence of the filed-effect mobility on MW of polymer dielectric. The results showed that when MW of PaMS is around 20 k, 60 k, 100 k, and 800 k, the OTFTs exhibits higher mobility around 0.1 cm2/Vs. The OTFTs with PaMS with lower MW ~2 k showed lower mobility around 1×10-4 cm2/Vs. TOF-SIMS analysis with sputtering the film revealed that when MW is ~100 k (~2 k), the active layer is the separated trilayer film (a homogeneously mixed film). This structural difference can be attributed to the difference in the performance of the OTFTs.Various types of polymers have been also examined. When we use the solubility parameter of the polymer closer to that of TIPS-pentacene, for example, poly(isobutyl methacrylate) is employed as a polymer, the OTFTs showed only lower mobility and the active layer was a homogeneously distributed structure. In order to explain our results, we applied the Flory-Huggins theory to the polymer-blend organic semiconductor system. In this theory, the Gibbs free energy change ΔGm for mixing a polymer with a small molecule is given byΔGm = kNTT[(χ12φ1φ2) + (φ1lnφ1 + (φ2/x)lnφ2)] ,which depends on MW and type of polymer. The results have shown that experimentally obtained conditions for the phase separation are reasonably explained in term of the Gibbs energy calculated with this equation. This consideration will be useful to develop higher-performance OTFTs.[1] T. Ohe et al., Appl. Phys. Lett. 93, 053303 (2008).[2] J. Kang et al., J. Am. Chem. Soc., 130, 12273 (2008).[3] M.-B. Mardec et al., J. Mater. Chem., 18, 3230 (2008).
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Controlling the Temperature Dependent Electronic Properties of Organic Thin Film Transistors by Self Assembled Monolayers.
Marco Marchl 1 , Peter Pacher 1 , Andrej Golubkov 1 , Anja Haase 3 , Barbara Stadlober 3 , Stefan Possaner 2 , Ferdinand Schuerrer 2 , Karin Zojer 2 , Lucas Hauser 4 , Christian Slugovc 4 , Gregor Trimmel 4 , Egbert Zojer 1
1 Institute of Solid State Physics, TU Graz, Graz Austria, 3 Institut für Nanostrukturierte Materialien und Photonik, Joanneum Research, Weiz Austria, 2 Institute of Theoretical Physics/Computational Physics, TU Graz, Graz Austria, 4 Institute for Chemistry and Technology of Materials, Tu Graz, Graz Austria
Show AbstractManipulating the properties of the interface between the gate dielectric and the active layer in Organic Thin Film Transistors (OTFTs) is of particular interest as this allows controlling the charge carrier mobility and the threshold voltage. One possibility to tune the properties of this interface is the insertion of covalently linked functional layers.We demonstrated recently that [2-[4-(chlorosulfonyl)phenyl]ethyl]trichlorosilane and its sulfonic acid analogue (T-SC/SA) in combination with exposure to ammonia can be used to shift the threshold voltage in poly(thiophene) (rr-P3HT) based devices over more than 60 V exploiting acid/base chemistry. In this work, we compare the results for rr-P3HT based transistors with data on transistors with vacuum deposited pentacene as the active layer. To gain deeper insight into the mechanisms of carrier transport and the origin of the threshold voltage shift, the devices were studied at temperatures down to 77 Kelvin. The temperature dependent mobility could be described on the basis of the multiple trapping and release model (MTR). Furthermore a drift-diffusion based modelling has been performed to understand temperature dependent threshold voltage shifts.
6:00 PM - B5.51
Regio-regular Pentacene Containing Polymers for Organic Photovoltaic Cells.
Sanghyun Hong 1 , Joon Hak Oh 1 , Ying Jiang 1 , Rajib Mondal 1 , Hector Becerril 1 , Toshihiro Okamoto 1 , Nobuyuki Miyaki 1 , Zhenan Bao 1
1 Chemical Engineering, Stanford University, Stanford, California, United States
Show AbstractThe development of solar energy sources is necessary to decrease our dependency on fossil fuels which are contributing to climate change and the polluting our environments. To make solar energy sources inexpensive, many research groups have studied polymer photovoltaic cells. Pentacene can be a good moiety for polymer photovoltaic cells due to its good hole mobility and low bandgap property. However, pentacene containing polymers have not been widely investigated for photovoltaic cells. Recently, our group developed the synthetic methodology to make regio-regular pentacene moiety which can be easily polymerized with other co-monomers. Pentacene-dithiophene conjugated polymers and pentacene- cyclopenta[2,1-b:3,4-b']dithiophene conjugated polymers were prepared by this method. Regio-regular pentacene containing polymers showed the good hole mobility up to 10-3 cm2/Vs and below 1.7 eV bandgap properties on solid state. Their power conversion efficiency will also be discussed in this presentation.
6:00 PM - B5.52
Ion-induced Charge Screening and Significant Conductivity Enhancement of Conducting Polymers.
Yijie Xia 1 , Jianyong Ouyang 1
1 Materials Science and Engineering, National University of Singapore, Singapore Singapore
Show AbstractA novel approach will be reported to significantly enhance the conductivity of conducting polymers. The approach is based on a new concept by considering conducting polymers as complexes of polyions. Our novel approach is to reduce the Coulombic interaction between the positive charge on the polymer chain and the counter anion by ion-induced charge screening. The conducting polymer films by this approach can be used as the electrode of optoelectronic devices.
6:00 PM - B5.53
Determining the Optimum Pentacene Channel Thickness on Hydrophobic and Hydrophilic Dielectric Surface.
Sung-jin Mun 1 , Seongil Im 1 , Jeong-M. Choi 1 , Kwang H. Lee 1 , Kimoon Lee 1
1 Physics, Yonsei University, Seoul Korea (the Republic of)
Show AbstractPentacene thin-film transistors (TFTs) are extensively studied for more than a decade due to its potential advantages to replace amorphous Si TFTs: high mobility exceeding that of amorphous Si TFTs, low channel deposition temperature, and accessibility to flexible plastic substrate. Characterization of pentacene crystalline growth on various dielectric surfaces was one of the important studies, as a fundamental intrinsic study to achieve high field effect mobility from pentacene channel. Inorganic oxide dielectrics, self-assembled-monolayer (SAM)-coated oxide dielectrics, and organic polymer dielectrics were studied for finding an enhanced mobility pentacene TFT. As an extrinsic geometrical factor, optimum channel thickness in pentacene-based TFTs was also discussed but only in adopting the case of inorganic dielectrics which have generally hydrophilic surface. Now we report that the optimum pentacene channel thickness is dependent on the surface energy state of its dielectric substrate. Pentacene TFT with hydrophobic substrate displays a peak mobility at an optimum channel thickness of 50 nm, below or above which the linear mobility decreases. In contrast, the linear mobility of the TFT with hydrophilic substrate monotonically increases until the channel thickness decreases to 15 nm. According to atomic force microscopy of 15 nm-thin pentacene grown on the SiO2 and PVP dielectrics, the pentacene islands on poly-4-vinyphenol (PVP) are not perfectly interconnected unlike the case on SiO2.We briefly conclude that the pentacene TFT with hydrophobic PVP dielectric clearly has an optimum channel thickness of 50 nm for a peak mobility while the TFT with hydrophilic SiO2 would show such peak mobility at even less than 15 nm of pentacene thickness.More quantitative details on the channel thickness will be discussed in the conference.
6:00 PM - B5.54
Epitaxially Grown Pentacene on h-BN Nanomesh.
May Ling Ng 1 2 , Alexei Preobrajenski 2 , Alexei Zakharov 2 , Alexander Vinogradov 3 , Sergey Krasnikov 4 , John Beggan 4 , Anthony Cafolla 4 , Nils Martensson 1 2
1 Department of Physics, Uppsala University, Uppsala Sweden, 2 MAX-lab, Lund University, Lund Sweden, 3 Institute of Physics, St.-Petersburg State University, St. Petersburg Russian Federation, 4 School of Physical Sciences, Dublin City University, Dublin Ireland
Show AbstractThe advancement of lithography and imprinting technology is gradually approaching saturation while the needs for faster and smaller electronics continue to grow exponentially. Therefore, it is interesting, if not necessary, to explore alternative opportunities in organic and hybrid organic-inorganic electronics. Pentacene is an interesting organic molecule because it crystallizes in a well-ordered structure, resulting in high charge carrier mobility (comparable to amorphous silicon). However, the very first challenge in realizing the concept of pentacene organic thin film transistors or photovoltaic cells is to find an insulating substrate for the long range epitaxial growth of structurally perfect pentacene. Hexagonal boron nitride (h-BN) can be a good choice for such substrate due to its 2D nature, i.e. high surface mobility, insulating property and ease of preparation by CVD.In the present study, we have investigated the structure of pentacene grown by thermal evaporation on h-BN/Rh(111), alternatively known as h-BN nanomesh, at RT. Each deposition step (from 0.06 ML to 4 ML) is characterized by recording the C 1s spectra at grazing (20°) and normal incidence (90°), and C 1s PE spectra at normal emission. In addition, we have used LEEM and micro-LEED to reveal the influence of the h-BN monolayer on the pentacene growth mode. We have also checked the structure of the multilayer pentacene/nanomesh with STM.For a small amount of pentacene (up to 0.3 ML), the pentacene molecules first fill the pores of the nanomesh because the pores act as physical traps for the molecules that have only weak bonding to h-BN (PES C 1s profile resembles the one of free pentacene molecule). These molecules are lying flat due to the lack of pentacene neighbours to form crystal structure. On the contrary, for 0.8 ML and above, pentacene molecules start to re-arrange among the neighbouring pentacene molecules with hydrogen bonding and become more vertically aligned, i.e. imitating its natural herringbone crystalline structure.With LEEM we observe pentacene grows in large 2D islands on h-BN while on clean rhodium it grows randomly with high nucleation due to the strong bonding between pentacene and Rh. The lateral profile of the 2D pentacene crystals on h-BN/Rh is modulated by the nanomesh buckling, as reflected in the micro-LEED patterns.
6:00 PM - B5.55
Tuning the Ionization Energy of Organic-semiconductor Films: The Role of Intramolecular Polar Bonds.
Ingo Salzmann 1 , Steffen Duhm 1 , Georg Heimel 1 , Martin Oehzelt 2 , Rolf Kniprath 1 , Robert Johnson 3 , Juergen Rabe 1 , Norbert Koch 1
1 Institut für Physik, Humboldt-Universität zu Berlin, Berlin Germany, 2 Institut für Experimentalphysik, Johannes Kepler Universität, Linz Austria, 3 Institut für Experimentalphysik, Universität Hamburg, Hamburg Germany
Show AbstractFor the prototypical conjugated organic molecules pentacene and perfluoropentacene, we demonstrate that the surface termination of ordered organic thin films with intramolecular polar bonds (e.g., -H versus -F) can be used to tune the ionization energy. The collective electrostatics of these oriented bonds also explains the pronounced orientation dependence of the ionization energy. Furthermore, mixing of differently terminated molecules on a molecular length scale allows continuously tuning the ionization energy of thin organic films between the limiting values of the two pure materials. Our study shows that surface engineering of organic semiconductors via adjusting the polarity of intramolecular bonds represents a generally viable alternative to the surface modification of substrates to control the energetics at organic/(in)organic interfaces.
6:00 PM - B5.56
Metal-contact Induced Anomaly in Transfer Characteristics of Graphene Field-effect Transistors.
Ryo Nouchi 1 , Tatsuya Saito 2 , Hiroki Watanabe 2 , Katsumi Tanigaki 1 2
1 WPI Advanced Institute for Materials Research, Tohoku University, Sendai Japan, 2 Department of Physics, Tohoku University, Sendai Japan
Show AbstractGraphene is a name given to a one-atomic carbon sheet which forms a honeycomb structure. This material shows a charge carrier mobility of higher than 200,000 cm2 V-1 s-1 at room temperature and is regarded as a major candidate for the future high-speed electronics. In addition to the strikingly high mobility, a weak spin-orbit interaction in graphene makes it a pivotal material in the field of molecular spin electronics. In order to construct such electronic devices, metallic materials should make a contact with graphene. When a molecule comes into contact with a metal, the electronic structure of the molecule should be affected by a chemical interaction with the metal. Recently, we observed anomalous distorted transfer characteristics in Co-contacted graphene devices [1]. The present study aims at clarifying the microscopic mechanism of the anomaly in the carrier transport properties. Graphene layers were formed onto a highly doped Si substrate with a 300 nm thick thermal oxide layer using conventional mechanical exfoliation [2]. These layers were confirmed to be single layer graphene from the optical contrast. Metal electrodes were fabricated onto the graphene layers by electron beam lithography and lift-off techniques. While graphene devices with inert metals (e.g. Au) displayed conventional transfer characteristics, those with more reactive metals (e.g. Ni) showed distorted characteristics. The distortion appeared only in short channel devices, suggesting that it is a consequence of the metal contact. Carrier conduction through short graphene channels can be ballistic [3]. Possible mechanisms of the anomaly, including one based on ballistic conduction, will be presented. [1] R. Nouchi et al.: Appl. Phys. Lett. 93 (2008) 152104. [2] K. S. Novoselov et al.: Proc. Natl. Acad. Sci. U.S.A. 102 (2005) 10451. [3] F. Miao et al.: Science 317 (2007) 1530.
6:00 PM - B5.59
Ultrasensitive Solution Processed Polymer Photodetectors
Xiong Gong 1 , Alan Heeger 1 , Minghong Tong 1 , Yong Cao 1
1 UCSB and CBrite, UCSB and CBrite, Santa Barbara, California, United States
Show Abstract Semiconducting polymeric optoelectronic and electric devices have evolved as a promising cost-effective alternative to silicon-based devices. Organic photodetectors have been the subject due to several inherent advantages. Some of the important advantages of these so-called “plastic” electronics include large-area detection, low cost of fabrication, ease of processing, mechanical flexibility and versatility of chemical structure due to the advancements in organic chemistry. Full-color, fast-response and positive sensitivity organic photodetectors have been reported. However, there are few reports on organic photodetectors whose performances are comparable with inorganic countparts. will report ultrasensitive solution processed photodetectors fabricated by different semiconducting polymers as the electron donors and various fullerences derivatives and/or inorganic quantum dots as the electron acceptors. Polymer photodetectors with different photo-response and detectivity were demonstrated. One example is that polymer photodetectors have photo-response from 300nm to 1450nm, the detectivity larger than 1012 cm Hz1/2/W, and linear dynamic range larger than 120 dB. All these values are comparable to or even better than their inorganic counterparts.
6:00 PM - B5.6
``Column-like" Structure of the Cross-Sectional Morphology of Bulk Heterojunction Materials.
Ji Sun Moon 1 , Jae Kwan Lee 1 , Alan Heeger 1
1 Center for Polymers and Organic Solids, University of California, Santa Barbara, Santa Barbara, California, United States
Show AbstractTransmission electron microscopy (TEM) through thin sections cut from films of the bulk heterojunction (BHJ) material comprising rr-poly(3-hexylthiophene), rrP3HT, and [6,6]-Phenyl-C61 butyric acid methyl ester (PCBM) provides information on the cross-sectional morphology. Previous studies of the BHJ morphology mostly have been carried out on or through the top surface of the film (with Atomic Force Microscopy or with TEM, respectively). However, subsequent to photo-induced charge separation, the photogenerated carriers must move toward the electrodes by traveling across the film thickness rather than parallel to the film surface. Thus, there is limited information on direct correlation between the cross-sectional morphology of the BHJ material and the solar cell device performance. We report here the observation of “column-like” structures in the defocused cross-sectional TEM images. These “column-like” structures provide the required pathways for charge transport across the film thickness. We calculate the power spectral density and the autocorrelation function of the vertical pathways and thereby obtain information on the length scale of the nanometer scale phase separation.
6:00 PM - B5.60
Photo-crosslinkable Polythiophenes for Efficient Thermally Stable Organic Photovoltaics.
Yoshikazu Miyamoto 1 2 , Bumjoon Kim 3 , Biwu Ma 2 , Jean Frechet 1 2
1 Department of Chemistry and Chemical Engineering, University of California, Berkeley, California, United States, 2 The Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, California, United States, 3 Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology, Daejeon Korea (the Republic of)
Show AbstractPolymer based organic photovoltaics have attracted a great deal of attention due to their potential as cost-effective, light-weight and flexible solar cells. Here we report a series of photo-crosslinkable poly(3-hexylthiophene-co-3-(6-bromohexyl)thiophene) (P3HT-Br) for use in solution processed organic photovoltaics. P3HT-Br copolymers were synthesized from two different monomers, 2-bromo-3-hexylthiophene and 2-bromo-3-(6-bromohexyl)thiophene, where the ratio of the two monomers was carefully controlled to achieve a UV photo-crosslinkable layer while leaving the π-π stacking feature of conjugated polymers unchanged. We demonstrate efficient thermally stable organic photovoltaics by using P3HT-Br copolymers as electron donors in both bulk heterojunction (BHJ) and bilayer type devices. Our photo-crosslinking approach stabilizes the nanophase separated morphology in the crosslinked P3HT-Br:PCBM BHJ with the minimum disturbance in π-π stacking, which leads to photovoltaics with comparable power conversion efficiency as compared to those of P3HT:PCBM and remarkably enhanced thermal stability (over 2 days at 150°C). We have further applied these photo-crosslinkable P3HT-Br copolymers to making efficient solution processed bilayer devices. Benefited from the little disturbance in π-π stacking by crosslinkable units, P3HT-Br/PCBM bilayer devices show high power conversion efficiency of over 2.2% and excellent thermal stability (over 3 days at 150°C), which represents one of the highest performing bilayer devices fabricated by solution processing.
6:00 PM - B5.61
Solution Processing of Small Molecules for Efficient Organic Photovoltaics
Biwu Ma 1 , Claire Woo 2 , Yoshikazu Miyamoto 2 1 , Jean Frechet 2 1
1 The Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, California, United States, 2 Department of Chemistry and Chemical Engineering, University of California, Berkeley, Berkeley, California, United States
Show AbstractThin film organic photovoltaics (OPVs) based on polymeric materials or small molecules have attracted tremendous research attention due to their potential application as low cost solar energy conversion devices. Efficient OPVs have been fabricated by either solution processing of polymeric materials or vapor deposition of thermally stable small molecules. To date, there has not been much success in solution processing of small molecules for OPVs due to several factors, such as limited solubility and poor film formation.Here I present our research efforts on solution processing of small molecules for efficient OPVs. The materials of interest are soluble conjugated organic subphthalocyanines. Due to their unique structural and photophysical properties, including high solubility, low tendency of aggregation and strong light absorption, we have been able to prepare amorphous thin films with high charge transporting and light harvesting properties via simply solution casting. By using these materials as donor and fullerenes as acceptor, we have demonstrated simple planar heterojunction OVPs with power conversion efficiencies over 1.5 %, which represent the highest performing OPVs based on solution processable small molecules to date. Our work clearly shows that solution processing of light harvesting small molecules has great potential in low cost thin film organic photovoltaics.
6:00 PM - B5.63
Energy Level Alignment of Model Molecular Electronic Systems on Silicon 111 7x7 Surfaces
Conan Weiland 1 , Liu Yang 2 , Dimitri Skliar 3 , Brian Willis 3 , Doug Doren 2 , Robert Opila 1
1 Department of Materials Science and Engineering, University of Delaware, Newark, DE, Delaware, United States, 2 Department of Chemistry and Biochemistry, University of Delaware, Newark, Delaware, United States, 3 Department of Chemical Engineering, University of Delaware, Newark, Delaware, United States
Show AbstractMolecular electronics promises great advances in device technology. With specialized chemical functionalizations, molecular devices offer the prospect of novel device properties, while promising reduced power consumption, inexpensive manufacturing and integration into everyday products. Electron transport through single molecules has been well studied; however, the role of the interface in charge transport in molecular electronic/substrate systems is not yet well understood. This talk will focus on understanding the role of energy level alignment and hence charge transfer at the molecule/silicon (111) 7x7 interface. Phenylacetylene and styrene were used in this study as model molecular electronic systems as both molecules are conjugated over the full molecule and are similar to the oligo-phenylene-vinylene moieties used in many molecular electronics studies. Bonding between the molecule and surface was verified using X-ray and UV photoelectron spectroscopy (XPS, UPS) as well as scanning tunneling microscopy (STM). These measurements were also compared with density functional theory calculations. Both molecules are seen to bind in a similar configuration - between the 2 carbon atoms of the substituent and a silicon surface adatom-rest atom pair. Energy level alignment between molecule and substrate was measured with UPS and bias dependent STM. The highest occupied molecular orbital of both phenylacetylene and styrene were found to lie 0.7 eV below the silicon Fermi level. The lowest unoccupied molecular orbital (LUMO) of phenylacetylene was found to be about 0.7 eV above the silicon Fermi level, while the styrene LUMO was greater than 2 eV above. This result shows that the specific chemical bond between molecule and substrate plays a large role in the energy level alignment and hence charge transfer between molecule and substrate.
6:00 PM - B5.64
Enhanced Performance of Inverted Bulk-heterojunction Photovoltaic Cells by Interface Modification with Self Assembled Monolayers
Steven Hau 1 , Hin-Lap Yip 1 3 , Kevin O'Malley 2 , Kung-shih Chen 1 , Jingyu Zou 1 , Hong Ma 1 3 , Alex K.-Y. Jen 1 2 3
1 Materials Science and Engineering, University of Washington, Seattle, Washington, United States, 3 Institute of Advanced Materials and Technology, University of Washington, Seattle, Washington, United States, 2 Department of Chemistry, University of Washington, Seattle, Washington, United States
Show AbstractCompared to the conventional photovoltaic architecture which uses low work function metals, inverted bulk-heterojunction photovoltaic cells have the advantage of using a more air stable high work function metal (Au, Ag) as the hole collecting back electrode and a thin n-type metal oxide (ZnO, TiO2) layer at the ITO interface as the electron selective contact. However, inverted structures have had lower photocurrent density and fill factor compared to the normal device structures due to the poor electrical coherence between the inorganic metal oxide and organic active layers. Self-assembled monolayers (SAMs) have been shown to significantly change the interfacial properties of various oxide and metallic surfaces. The adhesion, compatibility, morphology and electrical coherence at the inorganic/organic interface can be tuned by assembling different functional SAMs. Modifying the interface of the electron selective ZnO nanoparticles layer with a C60 functionalized SAM in inverted bulk-heterojunction photovoltaic cells lead to an improvement in photocurrent density and fill factor. This monolayer can serve multiple functions including the passivation of inorganic surface traps, enhancing interfacial exciton dissociation efficiency, optimizing the upper organic layer morphology and stabilizing the interface against dewetting.1 In addition, these unencapsulated inverted devices have excellent ambient stability retaining over 80% of its original conversion efficiency after 40 days of exposure.2
1. Hau, S., Yip, H.-L., Acton, O., Baek, N. S., Ma, H., Jen, A.K-Y., J. of Mater. Chem. 18, 5113 (2008).
2. Hau, S., Yip, H.-L., Baek, N. S., Jen, A.K-Y., Appl. Phys. Lett. 92, 253301 (2008).
6:00 PM - B5.65
Investigation on Optical and Optoelectronic properties of Blue Emission Conjugated Polymer Optical Gain Media
Cora E. C. Cheung 1 , Boon Kar Yap 1 , Ruidong Xia 1 , Xuhua Wang 1 , Colin R. Belton 1 , Paul N. Stavrinou 1 , Donal D. C. Bradley 1
1 Department of Physics, Imperial College London, London United Kingdom
Show AbstractConjugated polymer semiconductors combine the processing and mechanical characteristics of plastics with the desirable optical and electronic properties of semiconductors. A broad range of devices including LEDs, field effect transistors, photodiodes and optical amplifiers and lasers are under development with application potential in displays, lighting, RFID, display driver circuits, imaging, solar energy conversion and data communications.Our focus here is on gain media, where conjugated polymers are of interest for laser and optical amplifier development, with visible spectrum emission, wavelength tunability and polymer optical fibre compatibility. Extensive research has been undertaken on laser design and performance and it has become evident that not all of the conjugated polymers optimised for LED operation are good gain media. This presentation focuses on an investigation of the optical and optoelectronic properties of a series of blue emission conjugated polymers, motivated by a desire to establish an effective means to optimise polymer chemical structure for optical gain.Nine different polyfluorenes have been selected for comprehensive characterisation of their optical properties, including amplified spontaneous emission (ASE) measurements on asymmetric slab waveguides. The chosen materials are from two groups of polyfluorenes coded as the SCB group (SCB3, SCB9, SCB11 and SCB18) and the SC group (SC005, SC006, SC007, SC008 and SC010), both from Sumitomo Chemical. The S1 to S0 0-1 vibronic peaks (where gain is typically maximised) in the polymer photoluminescence spectra range from 450nm (SCB11) to 463nm (SC006) and their refractive indices range from 1.75 to 1.96. Three of the nine polymers (SCB3, SCB11 and SC006) were unable to generate ASE at low pump energy (<30µJ per pulse, 10ns, 10Hz). All three of these non-ASE polymers have long singlet excited state lifetimes (0.32-1.7ns) whilst the polyfluorenes that do give ASE have significantly shorter lifetimes (134-250ps). In addition, the highest photoluminescence quantum efficiency (PLQE) polymer is SC006 (96%) and lowest SCB18 (22% with an ASE threshold of 1.7µJ/pulse). It is therefore evident that high steady state PLQE and long excited state lifetime are insufficient for good optical gain properties. SC006 is, moreover, the best LED material (highest efficiency) in the SC family whilst SC007 (which does give ASE) is the worst. Similar trends apply to the SCB group, evincing an anti-correlation between optimised LED and optical gain characteristics. The optimised LED materials have low charge carrier mobility (10-8cm2/Vs for holes in SC006 c.f. 10-2cm2/Vs for SC007) consistent with charge trapping due to the incorporation of a fraction of relatively low ionisation potential arylamine moieties within the polymer chain that also bestow an excited state charge transfer character. The nature of the differences between LED and optical gain optimised polymers will be discussed in detail.
6:00 PM - B5.66
Ultrathin Self-Assembled Organophosphonic Acid Monolayers/Hafnium Oxide Hybrid Dielectrics for Low-Voltage Organic Thin Film Transistors
Orb Acton 1 , Hong Ma 1 2 , Guy Ting 3 , Hin-Lap Yip 1 2 , Balaji Purushothaman 4 , Itaru Osaka 5 , Tomek Kowalewski 5 , Richard McCullough 5 , John Anthony 4 , Alex K.-Y. Jen 1 2 3
1 Materials Science & Engineering, University of Washington, Seattle, Washington, United States, 2 Institute of Advanced Materials and Technology, University of Washington, Seattle, Washington, United States, 3 Department of Chemistry, University of Washington, Seattle, Washington, United States, 4 Department of Chemistry, University of Kentucky, Lexington, Kentucky, United States, 5 Department of Chemistry, Carnegie Mellon University, Pittsburgh, Pennsylvania, United States
Show AbstractOrganic thin film transistors (OTFTs) based on pi-conjugated materials are envisioned for use in ubiquitous low-cost flexible electronic devices, such as displays, sensors and electronic barcodes. A prerequisite for realizing practical applications lies on the development of stable gate dielectrics with low leakage current, low interface trap density, and high capacitance that afford low-voltage OTFT operation with high performance. However, standard OTFTs still require rather high operating voltages, often exceeding 20 V.By using organophosphonic acid self-assembled monolayers (SAMs) on low-temperature solution processed hafnium oxide (HfOx) as ultrathin hybrid gate dielectrics, we have realized low-voltage high performance OTFTs with low leakage currents, and high charge carrier mobilities. In the demonstrated OTFTs, the following device characteristics have been achieved: 1) low leakage current density - down to 1 nA/cm2; 2) large capacitance density - up to 630 nF/cm2; 3) low operating voltage (<2 V); 4) high charge carrier mobility - up to 2.5 cm2/(Vs); 5) small subthreshold slope - down to 100 mV/decade; 6) low-hysteresis device operation. This is achieved by using well-packed and dense organophosphonic acid SAMs (<4 nm) on HfOx (<5 nm) as ultrathin hybrid dielectrics and through interfacial engineering using an appropriate SAM to control the chemical, electrical, and morphological structure at the semiconductor/dielectric interface.Furthermore, this hybrid dielectric system is shown to be generally applicable for solution processed organic semiconductors by achieving stable device operation for polymer and soluble acene semiconductors with mobilities >0.1 cm2/(Vs). This work represents a major advancement towards developing large-area low-cost solution-processed/printed, flexible organic electronic devices.
6:00 PM - B5.68
Structured Interfaces in the Assembly and Performance of Organic Field-Effect Transistors
Gregory Whiting 1 , Rene Kist 1 , Tse Nga Ng 1 , Sanjiv Sambandan 1 , Beverly Russo 1 , Brent Krusor 1 , Ana Arias 1
1 Electronic Materials and Devices Laboratory, Palo Alto Research Center (PARC), Palo Alto, California, United States
Show AbstractInterfaces play an important role in the operation of organic thin-film field-effect transistors (FETs). For example, many reports have shown that the composition and quality of the dielectric/semiconductor or electrode/semiconductor interface has a significant effect on the overall device performance. Control over the chemistry of surfaces can also greatly aide the fabrication of solution-processed devices and the patterning of larger arrays of these devices to form electronic components. This control is particularly useful when deposition methods like ink-jet printing are used, as parameters such as the placement and drying of a drop can be influenced. This report will focus on both n- and p-type solution processed FETs. Ink-jet printing is used to pattern small molecule organic semiconductors, as well as silver source, drain and gate contacts. By careful modification of surface energy, particularly through the use of self-assembled monolayers, well-defined metal contacts can be produced, and semiconductor morphology can be controlled. Device parameters can also be strongly affected by these interfaces. For example, electrode work functions can be tailored to achieve good contact between the printed silver and organic semiconductor. Both top- and bottom-gate architectures have been used for this study, which present different benefits and challenges for additive solution processing. In addition, ink-jet printed complementary circuits using these materials and methods will be described.
6:00 PM - B5.69
Impact of Interfacial Modification on Polymer Morphology, Photoexcitation Dynamics, and Device Performance in P3HT/ZnO Solar Cells.
Matthew Lloyd 1 , Rohit Prasankumar 2 , Michael Sinclair 3 , Alex Mayer 4 , Julia Hsu 1
1 Surface and Interface Sciences, Sandia National Laboratories, Albuquerque, New Mexico, United States, 2 Center for Integrated Nanotechnologies, Los Alamos National Laboratory, Los Alamos, New Mexico, United States, 3 , Sandia National Laboratories, Albuquerque, New Mexico, United States, 4 Department of Materials Science and Engineering, Stanford Univeristy, Stanford, California, United States
Show AbstractShort-circuit current, which is determined by interfacial charge separation and recombination, in ZnO/poly 3-hexylthiophene (P3HT) solar cells is found to improve by intentional modification of the heterojunction interface. To understand the origin of this enhancement, we investigate the morphology of the interfacial polymer layer and decay dynamics of photoexcited species in P3HT deposited on glass, bare ZnO, and ZnO modified with an alkanethiol monolayer. These results are correlated with the characteristics of P3HT/ZnO and P3HT/alkanethiol-modified ZnO bilayer photovoltaic devices. Synchrotron x-ray diffraction spectra of pristine P3HT and P3HT on an alkanethiol-modified ZnO surface point to a more crystalline P3HT interfacial layer, while an amorphous interfacial layer of P3HT is found on unmodified ZnO. To investigate the decay dynamics of initial photoexcited states in the ZnO/P3HT system, the composite samples are interrogated by pump-probe spectroscopy with sub-picosecond resolution. Transient photoinduced absorption spectra are collected for photoexcited species in the range of 1090 nm to 3034 nm after excitation with a 550 nm pump. Compared to P3HT/ZnO composite films, the decay behavior for both polarons and excitons over a 500 ps time interval becomes significantly slower with alkanethiol modification, indicating a reduction in of early-stage charge recombination. Accompanying the decrease in recombination, we find an increase in the short-circuit current in the alkanethiol-modified ZnO devices in spite of the electron tunneling barrier presented by the alkanethiol monolayer. External quantum efficiency measurements in alkanethiol-modified devices also exhibit a clear signature of crystalline P3HT within an exciton diffusion length from the heterojunction interface. As these experiments demonstrate, charge injection efficiency and device performance can be improved by control of the polymer morphology at the heterojunction interface. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company for the United States Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.
6:00 PM - B5.7
Layer Cross-Fading at Organic/Organic Interfaces in OVPD-Processed Red Phosphorescent Organic Light Emitting Diodes as a New Concept to Increase Current and Luminous Efficacy.
Florian Lindla 1 , Manuel Bosing 1 , Christoph Zimmermann 1 , Frank Jessen 1 , Philipp van Gemmern 2 , Dietrich Bertram 2 , Dietmar Keiper 3 , Nico Meyer 3 , Michael Heuken 1 3 , Holger Kalisch 1 , Rolf H. Jansen 1
1 , Chair of Electromagnetic Theory, RWTH Aachen University, Aachen Germany, 2 , Philips Technologie GmbH, Aachen Germany, 3 , AIXTRON AG, Aachen Germany
Show AbstractOrganic light emitting diodes (OLED) have the potential to play a dominant role in solid state lighting. Most small molecule based OLEDs are at present processed either by vacuum thermal evaporation or organic vapor phase deposition (OVPD), with the latter offering unique features. Most important, several growth parameters (substrate temperature, deposition chamber pressure, carrier gas flows) can be controlled individually and growth rates are stable over a long period of time.These features make it possible to control the ratios in mixtures of organic materials during the deposition process precisely by adjusting the carrier gas flows through different organic sources. This is used to investigate the impact of an introduced layer cross-fading at organic/organic interfaces on current and luminous efficacy of phosphorescent red OLEDs. Layer cross-fading describes linearly decreasing the fraction in growth rate of an organic layer during deposition over a certain period of time while increasing the fraction in growth rate of the following layer. The result is a cross-fading zone of controlled thickness.The base structure of the investigated OLED consists of an ITO anode with a 20 nm hole injection layer (HIL), a 20 nm hole transport layer (HTL), a 40 nm host/guest system as red emission layer (EL) and a 30 nm electron transport layer (ETL) followed by a LiF/Al cathode. Devices with cross-faded interfaces are compared with the basic layer-by-layer processed structure with sharp interfaces and OLEDs in which the cross-fading zone is replaced by a mixed interlayer with constant ratios of organic materials.The layer-by-layer processed OLED shows a current and luminous efficacy of 18.8 cd/A and 14.1 lm/W (at 1000 cd/m2). A cross-fading zone of 10 nm thickness from either the HIL to HTL or the EL to ETL has no impact on the efficacies. Whereas with a rising cross-fading zone thickness of 40 nm at the HTL to EL interface, 29.3 cd/A (+56%) and 25.9 lm/W (+84%) can be measured, compared to a 40 nm mixed interlayer which shows 24.8 cd/A (+32%) and 20.4 lm/W (+45%). Different cross-fading zone and mixed interlayer thicknesses are compared. The efficacy of OLEDs with a layer cross-fading always exceeds the one with mixed interlayers.As result, layer cross-fading significantly increases both efficacy figures. The observation of a lower driving voltage will be discussed in terms of an interpenetrating network created in the cross-fading zone, which might improve charge injection and transport. A better mixing of charge carriers would broaden the recombination zone and increase the current efficacy. With an OVPD-based process, it is possible to realize nearly any kind of cross-fading profile. Different profiles will be studied in future. Furthermore, the lifetime of OLEDs with cross-faded interfaces will be investigated. An improved charge injection and transport should have a positive impact here as well.
6:00 PM - B5.70
Shelf Lifetime Study of Unencapsulated Organic and Hybrid Photovoltaic Devices.
Matthew Lloyd 1 , Dana Olson 2 , Matthew Reese 2 , Erica Fang 1 , Diana Moore 3 , James Voigt 3 , Julia Hsu 1
1 Surface and Interface Sciences, Sandia National Laboratories, Albuquerque, New Mexico, United States, 2 , National Renewable Energy Laboratory, Golden, Colorado, United States, 3 , Sandia National Laboratories, Albuquerque, New Mexico, United States
Show AbstractAs organic photovoltaics (OPVs) approach commercially viable power conversion efficiencies, in depth investigation of degradation mechanisms and methods for their mitigation becomes increasingly important. To address this issue, we monitored the difference in degradation rates for polymer:fullerene bulk heterojunctions (BHJ) and polymer/ZnO hybrid devices. The device performance was measured over the course of 6 weeks, storing the devices in the dark in ambient atmosphere between measurements. We found that the performance of BHJ devices employing silver electron-extracting contacts degraded rapidly (with a half-life of three days). BHJ devices with calcium/aluminum electron-extracting electrodes also degrade, but at a slower rate. These results are in contrast with the behavior of “inverted” BHJ and hybrid polymer/ZnO devices, where an ITO/ZnO electrode serves as the electron-extracting contact and silver serves to extract holes. Initially, the efficiency of these inverted devices increased; notably, the inverted BHJ device improved by more than 50% at day nine and maintained an efficiency greater than the initial value for the duration of the degradation study. These results indicate that the cause of OPV shelf life degradation is not, as commonly believed, the instability of the active organic materials. We find that the changes in the contact are a more important cause. Specifically, as silver oxidizes in air, its work function increases and loses selectivity as an electron-accepting electrode. A larger work function is found to advantageously increase open-circuit voltage and short-circuit current in both inverted BHJ and hybrid devices where Ag functions as the hole-collecting electrode. In addition, we have measured the change of many polymer/ZnO bilayer and nanorod devices over a longer period of time and found that, over 450 days, the devices still perform at 50% of their initial efficiency. In summary, we found that changes in the metal contact have a larger effect than changes in the active material on the degradation of organic and hybrid solar cells.Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company for the United States Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.
6:00 PM - B5.72
Nanotube Based High Current Transistor with Good on/off Ratios.
Islamshah Amlani 1 , Rudy Emrick 1
1 Applied Research and Technology Center, Motorola, Tempe, Arizona, United States
Show AbstractAligned and randomly networked ensembles of single-walled carbon nanotubes (SWNT) represent a potentially powerful platform for developing thin film semiconductor technologies such as flexible electronics, optoelectronics, RF electronics and sensing. Electrical characteristics of SWNT based devices that contain ensemble of nanotubes in the channel exhibit a cumulative effect of the characteristics due to varying chiralities of the constituent nanotubes. For instance, in an aligned SNWT device the lower limit of the off state leakage current characteristics will depend on the fraction of metallic SWNTs in the channel. One way to improve the on/off ratio post-fabrication is by selective joule heating of metallic and high-leakage ambipolar semiconducting nanotubes. A typical approach is to bias the semiconducting tubes in the off-state by applying an appropriate gate bias and apply a sufficiently high drain current to electrically stress and break the conducting nanotubes. If not properly configured however, on-current can also diminish substantially in this process due to undesirable burning of semiconducting nanotubes. Here, we present an automated electrical burning process that preferentially breaks down both metallic and high-leakage semiconducting SWNTs in a nanotube ensemble to obtain on/off ratios higher than three orders of magnitude. Typical on-state current of our back gated devices with interdigitated source drain geometry is in the milliampere range due to a large number of nanotubes in the channel and the reduction in on-state current at the end of the electrical burning process is minimal (~ 30%) with on/off ratios of 1000 or greater. These results are among the best that have been reported. However, we were not able to obtain electrical burning results on top gated geometry with similar efficiency.We perform two different experiments to elucidate that nanotubes break at the center when electrically stressed, the main reason for only marginal success with top gated devices. In the first experiment, the channel of the back gated devices are completely passivated by PMMA to purposely block off oxygen at the nanotube interface. Electrical burning result in this case is rather poor as lack of oxygen availability at the nanotube interface leads to non-preferential breakdown of both metallic and semiconducting nanotubes. In the second experiment we expose the center portion consisting of ~30-50% of the channel by performing electron beam lithography and developing the PMMA resist. Electrical burning results on these devices yield similar to those without any passivation. This indirectly suggests that breakdown indeed takes place in the center of the nanotubes and good on/off ratios can be obtained using this approach as long as the middle of the channel is exposed to air. This has important consequences for top-gated devices where the center portion of the channel is passivated by gate dielectric and metal.
6:00 PM - B5.73
Control of Pentacene Growth and Its Effects on Organic TFT Characteristics
Jong Sun Choi 1 , Jaehoon Park 1 , Jong Won Lee 1 , Dong Wook Kim 1 , Hyung Tak Kim 1 , Dong Myung Shin 2
1 Dept. of Electrical, Information and Control Engineering, Hongik University, Seoul Korea (the Republic of), 2 Dept. of Chemical Engineering, Hongik University, Seoul Korea (the Republic of)
Show AbstractRecently, organic semiconductor devices have been extensively investigated and steady progresses in device performances are continuously being obtained with ever increasing range of applications. One of the most promising organic semiconductors is pentacene, mainly due to the high hole mobility in pentacene-based TFTs. And many researchers continue to develop high-performance pentacene thin-film transistors (TFTs), focusing on improving the electrical conduction in the pentacene active layer.In this study, we have fabricated blends of two different polyimides with varying the composition ratio and investigated the growth of pentacene grains on each blend film. It is observed that the pentacene grain size pronouncedly reduced with increasing the content of hydrophobic polyimide in the blend film. This result can be explained by the interaction between the adsorbed pentacene molecules and the protrusion of hydrophobic component in the blend film. Indeed, atomic force microscopy images show that the surface diffusion of pentacene molecules on the blend film was limited by the hydrophobic protrusions. Accordingly, it is confirmed that the fabricated blend films can be applied to control the pentacene grain growth. We extended these results to investigate the effects of pentacene grain size on the TFT characteristics. Organic TFTs with larger pentacene grains exhibited improved device performances, which might be attributed to less grain boundaries. And experimental results show that grain boundaries act as trap sites during device operation and the trapped charges at grain boundaries can be activated by increasing the gate-source field. However, the threshold voltage shift upon a gate-voltage sweep direction was more pronounced for the device with larger pentacene grains, even strongly depending on the delay time of gate-voltage step. These results are considered to be attributed to a strong interaction between the pentacene layer and the concomitant blend film owing to its polar feature. Of note, grain boundaries can be identified as a limiting factor to the charge transport in organic TFTs and the interface between the organic semiconductor and insulator layer plays a significant role in the operational stability. Further investigations are focused on evaluating the activation energies of trapped charges at grain boundaries as well as the interface. These results will be presented.
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Effects of Alignment Layers on Pentacene Molecular Orientation and Thin-Film Transistor Characteristics
Jae-Hoon Kim 1 , Jongseung Kim 1 , Hyunsuck Kim 1 , Jaehoon Park 2 , Jong Sun Choi 2 , Dong Myoung Shin 3
1 Department of Electronics and Computer Engineering, Hanyang University, Seoul Korea (the Republic of), 2 Dept. of Electrical, Information and Control Engineering, Hongik University, Seoul Korea (the Republic of), 3 Dept. of Chemical Engieering, Hongik University, Seoul Korea (the Republic of)
Show AbstractPentacene, a fused-ring polycyclic aromatic hydrocarbon, is one of the most intensively investigated systems among various organic semiconductors due to great mobility and good semiconducting behavior. The electrical conductivity in this material strongly depends on the direction of applied electrical field to its long molecular axis. And also, it is known that the vertical alignment of pentacene molecules to the gate insulator surface provides a strong π-π* overlap and increases the electrical conductivity in the direction of perpendicular to the long-axis. These have motivated several studies of the effects of pentacene molecular orientations on the performance of organic thin-film transistors (OTFTs).In the present work, we use different alignment layers to investigate their effects on pentacene molecular orientation and the concomitant performance of organic TFTs. For the fabrication of morphological alignment layer, polyimide films were formed by spin-coating and then rubbed in the parallel and vertical directions to conducting channel in OTFTs. And also, liquid crystal (LC) material was used for the fabrication of molecular alignment layer. LC molecules were aligned in the parallel and vertical directions to channel direction. Experimental results show that the transistor characteristics were dependent on the directions of alignment layers. The OTFTs with the morphological alignment layer exhibited an increase in the drain current compared to the device without rubbing treatment, independent on the rubbing directions. Dichroic ratio of the drain current was about 1.2, which is defined as the ratio of current for the device with the parallel alignment layer to that with the vertical alignment layer. On the other hand, the OTFTs with the molecular alignment layer showed a significant dependence of drain current on the direction of alignment layer: the drain current in the parallel direction increased compared with that for the device with unaligned LC layer, but the drain current in the vertical direction even deteriorated. In this case, dichroic ratio was about 2.1. These results indicate that the morphological effect on the pentacene molecular orientation is intrinsically different from that of a prior molecular orientation. We will report the detailed growth mechanism of pentacene molecule on these alignment layers, combining with the electrical characteristics of OTFTs.
6:00 PM - B5.75
Nanostructure-Assisted Hole Injection in Schottky Diodes and Application in Organic TFTs
Hyunsuck Kim 1 , Jongseung Kim 1 , Jae-Hoon Kim 1 , Jaehoon Park 2 , Jong Sun Choi 2
1 Department of Electronics and Computer Engineering, Hanyang University, Seoul Korea (the Republic of), 2 Dept. of Electrical, Information and Control Enigeering, Hongik University, Seoul Korea (the Republic of)
Show AbstractOrganic thin-film transistors (OTFTs) have shown great promise for a variety of electronic applications, including flexible displays, chemical sensors, and low-cost microelectronics. The characteristics of OTFTs have been remarkably advanced and even surpass those of TFTs with amorphous Si. Most of efforts for OTFTs have been mainly focused on the improvements of electrical performance. But the basic study in the interfacial characteristics related with device performance is not so much progressed, which is also necessary for understanding device physics and further improvements in device characteristics.The interface between an organic material and a metal is one of critical factors for the device performance. Recently, some groups have investigated the importance of the interfacial characteristics and attempted to enhance the interfacial characteristics. Therefore, the interfacial properties should be investigated and must be improved in order to make OTFTs competitive with more conventional amorphous Si and poly-Si TFTs.In this work, we have fabricated the conic-nanostructures on the Al-bottom electrode and investigated the growth of pentacene molecules on the rough conic-nanostructures, combing with the barrier heights for hole injection in Schottky diodes. For the fabrication of conic-nanostructures, the H1 solution, in which polyurethane was dissolved into acetone solvent, was spin-coated onto the Al-bottom electrode. X-ray diffraction results show that the conic-nanostructures can contribute to the ordered growth pentacene molecules. And the barrier height for hole injection from the top-Au electrode was calculated by Fowler-Nordheim theory and found to be lowered for the Schottky diode with conic-nanostructures. This injection barrier lowering can be explained by the ordered growth of pentacene molecules under the influence of conic-nanostructures. We also introduced these structures into the interface bewteen the pentacene layer and gate insulator of OTFTs. It is observed that the electrical characteristics of the OTFT with conic-nanostructures were higher for the device without nanostructures. In particular, the field-effect mobility was significantly improved by using conic-nanostructures, calculated to be about 2.94 cm2/Vs. Consequently, we conclude that the nanostructure-assisted hole injection facilitated the barrier lowering, thereby contributing to achieving high performances in OTFTs. These results will be discussed.
6:00 PM - B5.76
Structural Phase Transition and Molecular Electron Tunneling in Self-Assembled Monolayers
Kyoungja Seo 1 , Hyoyoung Lee 1
1 , ETRI, Daejeon Korea (the Republic of)
Show AbstractThe electrical and chemical nature of organic molecules in self-assembled monolayers (SAMs) on metal or semiconductors have been studied for applications of molecular electronics[1]. As a model system, alkanethiols have been studied extensively by current-voltage (I-V) characteristics in molecular junctions (metal-molecule-metal). Electron transport across the alkanethiol monolayers is influenced by a tunneling pathway through the σ-bonded alkyl chain (through-bond tunneling) and through the intermolecular charge transport (through-space tunneling)[2]. Strong electronic coupling of a sulfur atom to a gold atom by chemisorption enhances the electron transport via through-bond tunneling across the interfaces of alkanethiol and gold. A molecular tilt in alkanethiol SAMs enhances contribution of through-space tunneling by the intermolecular coupling, and molecular conductance across the SAM relatively decreases[3].Molecular orientation on alkanethiol SAMs is changed with structural phase transition by thermal annealing in the SAMs. Molecular electron transport characteristics will be varied by different intermolecular coupling induced in each structural phase. However, tunneling characteristics such as an electron tunneling barrier of molecular junctions has not been reported with different structural phases, yet. In addition, it was reported that the thermal and electrostatic effects influence conductance across molecular junctions, dependent of the local molecular environment induced by neighboring molecules. Thus, we expect that the structural phase transition-induced a difference in molecular electron tunneling characteristics will depend on different intermolecular coupling effects created in a large molecular junction and an individual molecular junction. In this study, we demonstrate structural phase dependency of conductance across the thiolate (e.g., alkanethiol and alkanedithiol) SAMs, dependent of a junction area in size. The molecular electron tunneling characteristics for each structural phase are compared with I-V curves obtained in a scanning tunneling microscope (STM)-based individual molecular junction and a micropore-based large molecular junction. A tunneling barrier is proved as a measure of the intermolecular coupling on different structural phases of the thiolate SAMs. It is the first examination of the molecular electron tunneling in different structural phases of the thiolate SAMs and dependence of the molecular electron tunneling on a junction area in size. [1]A. Salomon, D. Cahen, S. Lindsay, J. Tomfohr, V. B. Engelkes, C. D. Frisbie, Adv. Mater. 2003, 15, 1881-1890.[2]X. D. Cui, X. Zarate, J. Tomfohr, O. F. Sankey, A. Primak, A. L. Moore, T. A. Moore, D. Gust, G. Harris, S. M. Lindsay, Nanotechnology 2002, 13, 5.[3]H. Song, H. Lee, T. Lee, J. Am. Chem. Soc. 2007, 129, 3806-3807.
6:00 PM - B5.77
Semi-transparent Flexible Photo-detector using Tetracene/ZnO hybrid p-n Junction.
Aaron Park 1 , Seongil Im 1 , Kimoon Lee 1 , Kwang H. Lee 1
1 physics, Yonsei university, Seoul Korea (the Republic of)
Show AbstractOver the last few decades, organic thin-film devices, such as organic light-emitting diodes (OLED), organic thin-film transistors (OTFT), solar cells, and photodetectors have made steady progresses in their performances. Among organic materials polyacenes like tetracene, which is one of the important organic molecules composed of four benzene rings, have been studied for applying to optoelectronic devices due to their good optical properties. We report on the fabrication of p-type organic/n-type ZnO hetero-junction diode and its applications. To construct the organic/inorganic hetero-junction, 50 nm-thick ZnO was deposited on ITO-coated flexible PET substrate at 100 °C and then 100 nm-thick tetracene as a p-type organic semiconductor was evaporated on the ZnO layer. Not only to make an ohmic contact with p-type organic layer but also to investigate the photo-response properties of the p-n junction, we adopt the semi-transparent NiOx electrode with large work-function value of ~5.1 eV and transmittance of ~30 %.Organic/inorganic hetero-junction of p-type tetracene/n-type ZnO showed rectifying behavior with current-voltage (I-V) characteristic and exhibited quite a high current density under forward bias. Also the tetracene/ZnO p-n diode has excellent photo-response properties under green, blue, and weak UV illuminations. We actually fabricated pentacene/ZnO p-n diode using the similar method, to compare with the tetracene/ZnO diode. Although the pentacene/ZnO diode displayed a little higher forward current than that of the tetracene/ZnO, we now regard that our p-tetracene/n-ZnO diode has clear advantages over pentacene/ZnO as an optoelectronic device due to its excellent photodetecting potentials. More details on the comparison will be discussed in the conference meeting.
6:00 PM - B5.78
Characteristics of Organic Thin-Film Transistors with Anodized Aluminum Oxide as a Gate Insulator.
Jong Won Lee 1 , Dong Wook Kim 1 , Jaehoon Park 1 , Hyoung Tak Kim 1 , Dong Myoung Shin 2 , Jong Sun Choi 1
1 Dept. of Electrical, Information and Control Engineering, Hongik University, Seoul Korea (the Republic of), 2 Dept. of Chemical Engineering, Hongik University, Seoul Korea (the Republic of)
Show AbstractOrganic thin-film transistors (OTFTs) have been expected by switching devices for flexible display. Aluminum gate electrode is usually adopted to reduce the delay time because of the property of low resistivity. And anodized aluminum oxide film is considered as a good insulator because it is resistant to various chemical solvent and has a high dielectric constant (high-k) contributing to reduction of the threshold voltage. The reduction of threshold voltage can lead to lowering supply voltage and thus resulting in lowering power dissipation. In general, anodized oxide films form two types of morphology, i.e. barrier-type and porous-type films, by altering the condition of electrolytes. Barrie-type films in the pH range of 5 to 7 are utilized as electrolytic capacitors due to thin, compact, and non-porous structure, while porous-type films are applied to nano-templates due to its thick and porous structure. In this work, we fabricated two types of anodizing aluminum oxide films with varying the pH values of electrolyte. Atomic force microscopy images of anodized aluminum oxide films show that the surface of film forming the barrier-type structure grown at pH 6.3 is smoother than that for the porous-like structure grown at pH 4.1. The difference in the height of these two films was about 10-30 nm. And the OTFT with the barrier-type aluminum oxide insulator exhibited the mobility of 0.09 cm2/Vs, the subthreshold slope of 1.1 V/decade, and the on/off ratio of 10_4, which are superior to those for the device with the porous-type insulator. It is thought that the smooth surface of the barrier-type film might contribute to a long range hopping of charge carriers in the conducting channel. Indeed, we found that the activation energy for device with the porous-like anodized insulator was much larger than that for the barrier-type structure, which demonstrates that the characteristic improvement in the device with the barrier-type structure was attributed to low activation energy for carrier transport in the conducting channel. These results will be discussed.
6:00 PM - B5.79
Scanning Probe Analysis of Poly(3-Hexylthiophene) Thin Films.
Rajiv Giridharagopal 1 , Kevin Kelly 1
1 Electrical and Computer Engineering, Rice University, Houston, Texas, United States
Show AbstractThe electronic behavior of conducting polymers at the polymer-electrode interface is of great interest, both technologically and in terms of basic materials science. We have used scanning tunneling microscopy (STM), including spectroscopic extensions such as work function imaging and alternating current STM (ACSTM), to probe conducting polymer thin films and monolayers with high spatial resolution in both ambient and ultrahigh vacuum environments. In our studies we focus on poly(3-hexylthiophene) (P3HT) films, each approximately one to three monolayers in thickness, deposited on highly-ordered pyrolytic graphite (HOPG) and molybdenum disulfide (MoS2) substrates. P3HT is one of a number of conducting polymers, and STM reveals that P3HT deposited on HOPG or MoS2 self-assembles to form a highly-ordered structure with symmetry commensurate with that of the underlying lattice. Additionally, we have used ACSTM to analyze spatial variations in the charge carrier density in such layers, thus shedding light on substrate-dependent charge transfer in P3HT films on different materials. Understanding such effects is of great importance for improving P3HT devices and conducting polymer-metal interfaces in general.
6:00 PM - B5.8
Solution Processed Low-Voltage Organic Field-Effect Transistors
Paul Woebkenberg 1 , James Ball 1 , Florian Colleaux 1 , Donal Bradley 1 , Thomas Anthopoulos 1
1 Blackett Laboratory, Imperial College London, London United Kingdom
Show AbstractThe field of organic microelectronics has evolved rapidly during the past twenty years and is now producing the first commercial applications. While recent progress in the area has been astonishing, some major technology bottlenecks still remain and hinder practical implementation of organic microelectronics in large-volume, low-end applications. One such technology bottleneck is the large operating voltages of state-of-the-art organic field-effect transistors (OFETs) and the resulting high power consumption. For example, the majority of OFETs reported in literature operate at voltages in excess of 20 V. This makes organic transistor technology unsuitable for use in a wide range of future applications including portable, battery-powered devices where organic microelectronics could potentially play a dominant role. A very promising approach towards low-voltage operation is the use of self-assembled monolayer (SAM) nanodielectrics. Unfortunately, the majority of SAM based organic transistors have so far been restricted to device architectures incorporating evaporated organic semiconductors. Demonstration of similar low-voltage devices fabricated via solution processing has also been attempted with limited success. It is the aim of this work to address both low-voltage operation and solution processing of OFETs that are suitable for use in integrated circuits. We present a simple method for fabricating <5 nm thin gate dielectrics from solution at room temperature utilising suitably designed phosphonic-acid SAMs. We previously demonstrated that solution processing of organic semiconductors on methyl terminated octadecylphosphonic acid is incompatible with most organic semiconductors due to the SAM's low surface energy characteristics. By modifying the terminal group (i.e. end-group) of the alkyl chain in these molecules we are able to tailor the SAM’s surface properties, consequently enabling solution processing of a much wider range of organic semiconductors. Based on this approach we have successfully demonstrated solution-processed hole (p-channel) and electron transporting (n-channel) OFETs operating at voltages below |1.5| V. To demonstrate the potential of the technology for use in practical applications, the transistors are integrated to form low-voltage, low-power logic circuits such as unipolar and complementary voltage inverters. Based on the same approach we also realise ambipolar SAM transistors and complementary-like inverters employing a single semiconductor material. The potential application of these low-voltage ambipolar organic transistors in light-sensing applications is also discussed. This work is a crucial step towards the production of solution processed low-voltage, low-power organic circuits and sensor arrays utilising low manufacturing cost methodologies.
6:00 PM - B5.80
Hybrid Nanostructures «Semiconductor/Organic Dye J-Aggregate» In Reverse Micelles.
Vladimir Razumov 1 , Lubov Nikolenko 1 , Sergey Brichkin 1
1 , Institute of Problems of Chemical Physics RAS, Chernogolovka Russian Federation
Show AbstractHybrid nanostructures such as «molecular aggregate/semiconductor nanoparticle» or core/shell structures in which core is the semiconductor nanocrystal and shell is the organic dye are very important for the charge separation at light absorption. These structures are useful for development of light-emitting devices, thin-film transistors, and optical memory or solar cells. Organic/inorganic semiconductor interfaces play an important role for effective charge separation. There are different methods for design of hybrid nanostructures. One of these methods is a self-assembly using of reverse micelles. The main procedure includes three stages. The first is synthesis of semiconductor nanoparticles by chemical reaction controlled by intermicellar exchange in «water in oil» microemulsions. The second stage is molecular dye aggregation and the third stage is selfassembly of organic/inorganic nanostructures.Self-assembly of hybrid “nanocrystal/J-aggregate” nanostructures after mixing two reverse micelle AOT/water/hexane solutions: one containing J-aggregates of pyridinium salt of betaine 3,3'-di(γ-sulfopropyl)-4,5,4',5'-dibenzo-9-ethylthiacarbocyanine and the other - AgI nanocrystals was shown. During the hybrid nanostructure formation new dye absorption bands with λmax≈673 and 695 nm appeared, these belong to J-aggregates of different structures adsorbed on nanocrystals. It was found that the excess of iodide ions during AgI nanocrystal synthesis is the main factor influencing the hybrid nanostructures self-assembly, so AgI crystal lattice is of great importance in this process. Taking into account that AgI nanocrystals synthesized in iodide excess have hexagonal crystal lattice it can be concluded that J-aggregates effective absorb on β-AgI and do not absorb on γ-AgI.There occurs a “symbiosis” of the hybrid structure components: nanocrystals raise the photostability of J-aggregates, and adsorbed J-aggregates efficiently stabilize the nanocrystal size. It was shown that the stable hybrid nanostructures may be extracted from micellar solution without further aggregation. Spectral and structure investigations of these hybrid systems were carried out depending on average size of reverse micelles using of light absorption and TEM techniques.
6:00 PM - B5.81
Morphological Stabilization of Polymer Photovoltaic Cells by Using Cross-linkable Polythiophene
Shoji Miyanishi 1 , Keisuke Tajima 1 , Kazuhito Hashimoto 1 2
1 Enginnering, Graduate School of The Univerity of Tokyo, Tokyo Japan, 2 , JST-ERATO, Tokyo Japan
Show AbstractPolymer photovoltaic cells draw considerable attention these days for their potential of low cost fabrication of large area devices by simple means of painting or printing from the polymer solutions. The most commonly used structure for the polymer photovoltaic cells now is a bulk heterojunction, which is a physical mixture of donor and acceptor materials. Recent research has suggested that control of the mixing morphology of the donor and the acceptor in films is of high importance to achieve highly efficient charge separation and transport. Especially in the case of the combination of poly(3-hexylthiophene) (P3HT) and (6,6)-phenyl C61 butyric acid methyl ester (PCBM), thermal annealing of the films has been used to control this phase separation. During the thermal annealing, both the polymer and PCBM crystallize to form an interpenetrating network in nanoscale, resulting in drastic enhancement of the device performance. However, even if such phase-separated nanostructure is constructed, this morphology is not thermally stable and may gradually undergo changes during the device operation since the polymer and the PCBM thermodynamically prefer to segregate from each other. In fact, a prolonged thermal treatment of the device induces the formation of large aggregations of PCBM in the films that lowered the device performance significantly. Therefore, morphology control that only depends on thermal annealing might be insufficient for achieving a stable and reliable mixing morphology of the donor and the acceptor.In this work a new cross-linkable regioregular poly(3-(5-hexenyl)thiophene) (P3HNT) was synthesized for the purpose of stabilizing the film morphology in polymer photovoltaic cells. The vinyl group at the side chains of P3HNT is expected to conduct cross-linking reaction by thermal treatment. After the process of cross-linking is complete, the diffusion of PCBM into the film can be lowered to suppress the formation of large aggregations. As a result, the thermal stability of the cells is expected to improved in comparison to non-cross-linkable P3HT:PCBM bulk heterojunction.P3HNT was characterized by NMR, GPC, UV-vis absorption spectra, XRD spectra and DSC. XRD and UV-vis spectra showed that P3HNT has similar crystallinity to P3HT. As a result, the performance of the P3HNT:PCBM bulk heterojunction devices showed comparably high efficiency over 3% similar to P3HT:PCBM cells. Furthermore, P3HNT was certainly cross-linked during thermal treatment, confirmed by the insolubility of the films in organic solvents. This cross-linkability was also confirmed even in the mixture films with PCBM. As the result, the formation of large aggregations of PCBM was prevented in P3HNT:PCBM films even after prolonged thermal annealing. In addition, the deterioration of the photoconversion performance at a high temperature was suppressed in the polymer solar cells compared to the control cells with P3HT:PCBM.
6:00 PM - B5.82
Low-Voltage Self-Supported Ion Conductive Membrane Based Transistors
Nikolai Kaihovirta 1 3 , Carl-Johan Wikman 2 , Tapio Makela 1 , Carl-Eric Wilen 2 , Ronald Osterbacka 1
1 Center for Functional Materials and Department of Physics, Åbo Akademi University, Turku Finland, 3 Graduate School of Materials Research, Turku Universities, Turku Finland, 2 Center for Functional Materials and Laboratory of Polymer Technology, Åbo Akademi University, Turku Finland
Show AbstractIon enhanced organic transistors are promising candidates for large-scale fabrication of low-voltage applications [1-3]. In the ion enhanced organic transistor, an ionic insulator replaces the traditional dielectric insulator. Two types of ion enhanced polymer transistors have been presented in the literature: Electrochemical transistors [1, 4] and electric double layer (EDL) gated transistors [2, 3, 5, 6].We present a novel EDL-gated transistor using a thick (> 50 μm), ion conducting membrane (MemFET) [7]. The membrane acts both as gate insulator and as mechanical support. The fabrication of the membrane starts with a PVDF-film as base material. The PVDF-film is functionalized by the roll-to-roll suitable electron beam irradiation induced grafting technique [8]. For comparison, we have also used the commercially available, proton-conducting Nafion®-membrane, as received. The MemFETs are fabricated by standard laboratory fabrication techniques using soluble polymers. As semiconductor we apply the regioregular P3HT. The highly conducting polymers PANI or PEDOT:PSS are used for the gate electrode, while evaporated gold is chosen, for convenience, for the source and drain electrodes. The MemFETs operate at low voltages (1 V) with a high current throughput. By using the electron beam irradiation induced grafting technique we can fabricate membranes to conduct different ions and/or simply tailor-make membranes to be locally ion conducting in whatever pattern needed. MemFETs fabricated on different membranes will be presented. Furthermore, the ion conducting membrane allows for fabrication of two (or more) devices on the same membrane in a simple and cost-effective matter. This will be shown by driving an electrochromic display pixel with a MemFET, both fabricated on the same membrane.[1] R. Mannerbro, M. Ranlöf, N. Robinson, R. Forchheimer, Synth. Met. 158 (2008), 556-560.[2] D. Tobjörk, N. J. Kaihovirta, T. Mäkelä, F. S. Pettersson, R. Österbacka, Org. Electron. 9 (2008), 931-935.[3] J. H. Cho, J. Lee, Y. Xia, B. Kim, Y. He, M. J. Renn, T. P. Lodge, C. D. Frisbie, Nat. Mater. 7 (2008), 900-906.[4] D. Nilsson, M. Chen, T. Kugler, T. Remonen, M. Armgarth, M. Berggren, Adv. Mater. 14 (2002), 51-54.[5] H. G. O. Sandberg, T. G. Bäcklund, R. Österbacka, H. Stubb, Adv. Mater. 19 (2004), 1112-1115.[6] M. J. Panzer, C. D. Frisbie, Adv. Funct. Mater. 16 (2006), 1051-1056.[7] N. J. Kaihovirta, C, -J. Wikman, T. Mäkelä, C. -E. Wilén, R. Österbacka, Adv. Mater. (2008) in press.[8] T. Lehtinen, G. Sundholm, S. Holmberg, F. Sundholm, P. Björnbom, M. Bursell, Electrochim. Acta 43 (1998), 1881-1890.
6:00 PM - B5.83
Understanding Aminated Silane Monolayer Formation Kinetics for Use in Organic Electronics
Justin Opatkiewicz 1 , Melbs LeMieux 1 , Zhenan Bao 1
1 Chemical Engineering, Stanford University, Stanford, California, United States
Show AbstractAminated silane films are being used in a wide variety of fields: from biological sciences to semiconductor research. Amines with varying degrees of substitution are common in nature and hence can be used to modify surfaces to interface with biological systems. 3-aminopropyltriethoxysilane (APTES) is a very common silane and has been characterized in many studies. It is generally accepted that the amine catalyzes monolayer formation by electrostatic attraction of the amine to a hydrophilic surface. Silanes with very similar structures, however, have not been analyzed in as much depth. Similar molecules varying simply by methyl-substitutions, such as N-methylaminopropyltrimethoxysilane (MAPS) and (N,N-dimethylaminopropyl)trimethoxysilane (DMAPS), can potentially be used alongside APTES in a variety of applications such as biological linkages and carbon nanotube separation. Here, we compare the kinetics of MAPS and DMAPS to APTES surface reactions and determine the influence of the methyl substitution on monolayer formation. After the kinetics of the three silanes are understood, their abilitly to separate carbon nanotubes (CNTs) are analyzed.
6:00 PM - B5.84
Enhanced Performance of Organic Light Emitting Diodes Using LiF Buffer Layer.
Omkar Vyavahare 1 , Richard Hailstone 2
1 Materials Science and Engineering, Rochester Institute of Technology, Rochester, New York, United States, 2 Imaging Science, Rochester Institute of Technology, Rochester, New York, United States
Show AbstractSince the invention of organic electroluminescent devices a great deal of effort has been made to improve their performance. Reducing the barrier and optimizing charge injection is crucial for efficient and bright Organic Light Emitting Diodes (OLEDs). We report improvement in the performance of OLEDs with ITO/TPD/Alq3/Al structure by inserting LiF both at electrode-organic interface and organic-organic interface. In this paper, we elucidate the mechanism of LiF buffer layer inserted at different interfaces. These devices show improved luminescence and steeper IV characteristics.
6:00 PM - B5.85
Transparent Photo-stable Complementary Inverter with Organic-inorganic Nano-hybrid Dielectrics.
Min Suk Oh 1 , Yong Hoon Kim 1 , Sung Kyu Park 1 , Jeong In Han 1 , Byoung H. Lee 2 , Myung M. Sung 2 , Kimoon Lee 3 , Kwang Lee 3 , Sung Hoon Cha 3 , Seongil Im 3
1 Flexible Display Research Center, Korea Electronics Technology Institute, Seongnam Korea (the Republic of), 2 Department of Chemistry, Hanyang University, Seoul Korea (the Republic of), 3 Institute of Physics and Applied Physics, Yonsei University, Seoul Korea (the Republic of)
Show Abstract“Transparent” electronics has been one of the key terminologies forecasting the ubiquitous technology era. Several researchers have thus extensively developed transparent oxide-based thin-film transistors (TFTs) on glass and plastic substrates although in general high voltage operating devices have been mainly studied considering transparent display drivers. However, low voltage operating oxide TFTs with transparent electrodes are very necessary if we are aiming at logic circuit applications, for which transparent complementary or one-type channel inverters are required. The most effective and low power consuming inverter should be a form of complementary p-channel and n-channel transistors but real application of those complementary TFT inverters also requires electrical- and even photo-stabilities. Since p-type oxide TFTs have not been developed yet, we previously adopted organic pentacene TFTs for the p-channel while ZnO TFTs were chosen for n-channel on sputter-deposited AlOx film. As a result, decent inverting behavior was achieved but some electrical gate instability was unavoidable at the ZnO/AlOx channel interface. Here, considering such gate instability issues we have designed a unique transparent complementary TFT (CTFTs) inverter structure with top n-ZnO channel and bottom p-pentacene channel based on 12 nm-thin nano-oxide/self assembled monolayer laminated dielectric, which has a large dielectric strength comparable to that of thin film amorphous Al2O3. Our transparent CTFT inverter well operate under 3 V, demonstrating a maximum voltage gain of ~20, good electrical and even photoelectric stabilities. The device transmittance was over 60 % and this type of transparent inverter has never been reported, to the best of our limited knowledge.
6:00 PM - B5.86
Pentacene- and Anthradithiophene-Containing Conjugated Polymers for Organic Photovoltaics.
Ying Jiang 1 , Toshihiro Okamoto 2 1 , Hector Becerril 1 , Sanghyun Hong 1 , Zhenan Bao 1
1 Chemical Engineering, Stanford University, Stanford, California, United States, 2 Functional Soft Matter Eng. Laboratory, The Institute of Physical and Chemical Research (RIKEN), Saitama Japan
Show AbstractMaterial innovation is a key component of the research towards enhancing organic solar cells performance. We have taken the new approach of incorporating acenes such as pentacene and anthradithiophene into conjugated copolymers, based on their superior thin-film transistor performance. In this work, we extend our research to report two classes of novel acene-containing polymers. The first class consists of the regioregular 2,9- and 2,10-pentacene-diethynylbenzene copolymers. The two polymers are observed. to differ in absorption properties owing to foreseeably different conformations. The second class of materials consisting anthradithiophene-cyclopentadithiophene copolymers shows excellent film absorption properties as well as solubility in common organic solvents. High field effect mobilities of up to 10-3 cm2/Vs are measured for anthradithiophene-4,4-bis(2-ethylhexyl)cyclopentadithiophene deposited in top-contact geometry on a silane treated silicon oxide surface. The material also achieves a power conversion efficiency of 0.59% in preliminary solar cell devices.
6:00 PM - B5.87
Synthesis and Optical properties of Perylene Bisimide Incorporated Low Bandgap Polymers for Photovoltaics.
Sivamurugan Vajiravelu 1 , Valiyaveettil Suresh 1
1 Department of Chemistry, National University of Singapore, Singapore, Singapore, Singapore
Show AbstractSynthesis and development of a broad range light absorbing molecules with high extinction coefficient as suitable metarials to improve the solar cell efficiency is exciting due to the great demand in energy for our day to day life. Perylene diimide (PDI) derivatives are most attractive molecule owing to high charge mobility, greater electron affinity and act as good n-type materials. In this investigation, we focused on the synthesis of alternative donor-acceptor conjugated systems prepared using Suzuki polymerization of N,N’-didodecyl 1,7-dibromoperylene diimide (PDI) with diboronic acids of flurene and dithiophene respectively. The polymers were designed to achieve molecular heterojunction through charge transfer from donor to acceptor. The polymers were characterised using GPC, 1H, 13C NMR and elemental analysis. TGA and DSC techniques were used to identify thermal stability and phase changes of the polymers. The absorption spectra of polymers covered whole range of visible absorption region from 300 to 800 nm in solution and the absorption maximum shifts to higher wavelength in solid state.
6:00 PM - B5.88
Multifunctional Organic Field-effect Transistor Based on Polydiacetylene.
Jaehui Ahn 1 , Doo Ho Yang 1 , Chunzhi Cui 1 , Jihyun Kim 1 , Dong June Ahn 1
1 Chemical & Biological Eng., Korea Univ., Seoul Korea (the Republic of)
Show AbstractRecently, the interest of low cost and reliable organic materials for multifunctional device is increased. Organic field effect transistors (OFETs) are suitable for multifunctional devices because of their potential applications. In our research, PCDA based FET was used. Among a lot of conjugated polymer, the hole mobility of single crystal polydiacetylene is founded to be 1-10 cm2/V*s by time of flight method[1], it is good property as candidate of OFETs material. Another unique property of PDA is electrical property to be varied as change of PDA’s phase (so called blue and red phase), which can be controllable by UV and external stimuli. We demonstrated that the single device performed as both UV and gas sensor which shows that PCDA based FET has potential application in multifunctional device.In this research, we investigated electrical property of both red phase and blue phase PDA. First, we made thin film of 10,12-pentacosadiynoic acid (PCDA) by the Langmuir-Blodgett (LB) deposition on SiO2/Si substrate that has predefined source and drain metal. And then, we performed polymerization to obtain PDA thin film, followed by thermal treatment to obtain red phase PCDA. Then, we measured specific contact resistance by the transmission line modeling (TLM) method and the characterization of Field Effect Transistor. We found that the red phase of PCDA can be obtained by specific species by the surface modification to increase the selectivity. In our experiment, the surface of PCDA was modified to have selectivity for NH3 over N2. The details about the fabrication process and sensing mechanism will be presented.
6:00 PM - B5.9
Properties of Fluorenyl Silanes in Organic Light Emitting Diodes.
Wei Wei 1 , Peter Djurovich 1 , Mark Thompson 1
1 Department of Chemistry, University of Southern California, Los Angeles, California, United States
Show AbstractFluorene derivatives have been developed as potential charge transporters for OLEDs because of their high charge transport mobilities and thermal stabilities. Here, we have designed and synthesized four different fluorene silicon derivatives (PhnSi(DMFL)4-n, DMFL= 9,9-dimethylflouren-2-yl, n= 0, 1, 2 and 3) with increasing number of fluorene units, in order to provide a systematic study for investigating the changes of the molecular as well as the device properties when the fluorene ratio increases in the molecule. All of these compounds possess high triplet energies, large HOMO-LUMO gaps and high glass transition temperatures. Both glass transition and sublimation temperatures increase linearly as the fluorene ratio increases. In contrast, there is no apparent change in their electrochemical or photophysical properties, which indicates that fluorene moieties have been conjugatively isolated by the central silicon. These molecules exhibit ambipolar transport characteristics in undoped OLED devices (ITO/NPD/mCP/PhnSi(DMFL)4-n/Alq3/LiF/Al) and the conductivity of the device is enhanced by the molecules with higher fluorene ratios. Hence, the Si(DMFL)4 was used as the host of Ir(ppy)3 and PQIr phosphorescence devices, respectively, and high device external efficiencies were achieved.
6:00 PM - B5.90
Doping of Perylene Diimide Derivatives n-type Semiconductor Layer and Interface Modification for Organic Thin Film Transistor.
Heng-Wen Ting 1 , Tri-Rung Yew 1
1 Department of Materials Science and Engineering, Nationl Tsing Hua University, Hsinchu Taiwan
Show AbstractDoping of air-stable solution-processed tetrachloroperylene tetracarboxyldiimide based n-type semiconductor (TC-PDI-F) layer by dipolar molecules and ionic compounds is demonstrated to enhance the electrical properties of organic thin film transistors (OTFT). Besides, the interface modifications between semiconductor and insulator layers by dipolar molecules with phenyl, alcohol and fluoro-functional groups were applied to improve the molecules packing order and the electrical performance of doped TC-PDI-F OTFTs. The electronic structures were characterized by cyclic voltammetry (CV), UV-visible optical absorption spectroscopy (UV-Vis) and photoluminescence spectroscopy (PL). The surface properties were also inspected by atomic force microscope (AFM), contact angle system and X-ray diffraction spectroscopy (XRD). The electrical properties of OTFTs were also measured. All processes were fabricated and measured in air.
6:00 PM - B5.91
Self-Assembled Hydrophobin Protein Membranes on Silicon Platforms
Jouni Ahopelto 1 , Markku Kainlauri 1 , Jani Kivioja 1 , Paivi Laaksonen 2 , Arja Paananen 2 , Markus Linder 2
1 Micro and Nanoelectronics, VTT, Espoo Finland, 2 Nanobiomaterials, VTT, Espoo Finland
Show AbstractWe report on formation and characterisation of ordered single layer protein crystals formed by directed self-assembly on hydrophobic substrates, such as graphite and silicon platforms. The hydrophobin proteins form an ordered two-dimensional crystal at air-water interface with the protein molecules all having a well defined orientation and position [1]. From the air-water interface the crystal membranes are transferred onto surface of highly oriented pyrolytic graphite or on patterned silicon substrates. The thickness of the membrane is about 3 nm and it has a hexagonal-like lattice with lattice constant of about 6 nm. On silicon substrates selectivity can be obtained between hydrophilic oxide and hydrophobic silicon areas, providing means to integrate protein membranes with silicon microelectronics [2]. By decorating the proteins with Au nanoparticles, nanoelectrodes for electrical measurements and, on the other hand, plasmonic devices can be envisaged. The effect of nanoelectrodes, as measured by conducting AFM, can be seen as largely enhanced conductivity through the protein membrane. Also, on glass slides made hydrophobic by silanization, clear plasmon band absorption arising from the Au nanoparticles can be seen. [1] G. Szilvay, A. Paananen, K. Laurikainen, E. Vuorimaa, H. Lemmetyinen, J. Peltonen, M. Linder, Self-assembled hydrophobin protein films at the air-water interface: structural analysis and molecular engineering, Biochemistry . Vol. 46 (2007) 2345 – 2354.[2] P. Laaksonen, J. Kivioja, A. Paananen, M. Kainlauri, K. Kontturi, J. Ahopelto, M. B. Linder, Selective nanopatterning using citrate stabilized Au nanoparticles and NCysHFBI fusion protein, submitted 2008.
6:00 PM - B5.92
A Self-patterned Polymer Dielectric for Low Voltage Pentacene Thin Film Transistor.
Hui-Chen Huang 1 , Ting-Hsiang Huang 1 , Zingway Pei 1
1 Graduate Institute of Optoelectronic Engineering, Department of Electrical Engineering, National Chung Hsing University, Taichung Taiwan
Show AbstractOrganic thin-film transistors (OTFTs) based on pentacene has attract much attention due to low temperature and low cost fabrication process. Although mobility of pentacene is comparable to the a-Si:H having potential to replace a-Si:H TFT in display applications, the high driving voltage is still a problem. Self-assembled monolayer (SAM) or ultra thin film are general methods used to reduce the operation voltage to about 2~ 5 volts. However, SAM methods require unique coating technique and the thickness of the ultra thin film is hard to control during the coating process limit theses method only demonstrated by some very specific group. In this work, a random copolymer, PS-r-PMMA, is used as dielectric material for a pentacene OTFT. The PS-r-PMMA dielectric with 10 nm thick can be uniform coated on the surface with hydroxyl groups in a very simple spin coating process. The PS-r-PMMA is initially coated in roughly 50 nm thick. After thermal annealing, a layer of PS-r-PMMA will connect to the OH- group forming a 10nm thick layer no matter how thick of the initial layer. Besides the thickness control, the PS-r-PMMA only coated on the surface having OH- groups, the self-patterning is possible to prevent the chemical etching process during the organic circuit fabrication. The designed OTFT is bottom gate and top contact structure. The substrate is glass and gate material is aluminum. After the deposition of the Al, the UV/ozone process is performed to convert the surface of Al into Al2O3 having OH- groups. After this, PS-r-PMMA is spin coated and heated sequentially. Finally, pentacene and gold contact electrodes are deposited sequentially by thermal evaporation to finish the OTFT. By using this self-patterning thin dielectric, the pentacene OTFT exhibits operation voltage as low as 5V and an on/off ratio large than 105.
6:00 PM - B5.94
Controlling the Electrical Properties of Molecule-Terminated, Non-Oxidized Silicone by the C-C Bond Nearby the Surface
Sreenivasa Puniredd 1 , Ilia Platzman 1 , Hossam Haick 1 2
1 Chemical Engineering, Technion- Israel Institute of Technology, Haifa Israel, 2 Russell Berrie Nanotechnology Institute, Technion- Israel Institute of Technology, Haifa Israel
Show AbstractThe ability to exert systematic control over the electronic properties of Si is an important factor for the realization of nanoelectronic devices. Mostly, such controllability can be achieved by placing molecules, whose dipole can be changed systematically, at the device surface and/or interface. In this study, we present an approach for controlling the electrical properties of Si by inducing deliberate interaction between the energy levels of organic molecules and Si, without the need for a dipole modification. We illustrate this approach by functionalizing 50±2% of Si atop sites of n- and p-type Si (111) oxide-free surfaces with various organic molecules having similar (3C) backbone but different in their C-C bond close to the Si surface (i.e., C-C vs. C=C vs. C≡C bonds). These molecules have nearly similar dipole moment (0.9-1.6 Debye). Electrical characterizations of molecule/Si surfaces and Hg/molecule/Si junctions showed a systematic control over the work function and Schottky barrier of the Si, respectively, in the range from 0.2 to 0.8 eV, as compared to H-terminated Si. Our results show that the control over the electrical properties of n- and p-type Si is dominated by systematic electron transfer from the Si surface to the organic monolayers. The results indicate that the extent of electron transfer depends on the difference between the energy levels of the organic molecules and the Si surface. This finding, for which we will present a detailed explanation, has very significant implications as it suggests the ability to control the electrical properties of semiconductors with minimal depolarization effects.
6:00 PM - B5.95
Ultrathin and Printable Conjugated Films for Organic Electronics.
Li Tan 1
1 Engineering Mechanics and Nebraska Center for Materials and Nanoscience, University of Nebraska-Lincoln, Lincoln, Nebraska, United States
Show AbstractUltrathin organic films or self-assembled monolayers (SAMs) can provide molecular or organic electronics unmatched processing convenience and some knowledge in device operation. Conventional concepts often come from a head-tail molecular design, where the head carries the function of charge transport and the tail is capable of self-assembly. Amorphous nature of the monolayers and rather significant defects along grain boundaries, however, all inhibit their device performances at high current density and high temperature conditions.Toward this end, we are investigating a new family of ultrathin organic thin films, i.e., self-organized nanolayers. This type of molecules enjoys a polymeric backbone, branched with an extended pi-pi system. Rather rigid branches, plus the chain-chain cooperation allow the polymer form long-range ordered lamellae in solution. Printed films have a super dense, multistack morphology over tens of microns. Unprecedented thermal stability is also realized due to an enhanced molecular interaction between the pi-pi units, plus a strong interlayer binding between the layers. Research efforts with scientific and engineering importance are worth of reporting, including 1) strategy of the molecular design and synthesis; 2) film casting and characterizations; and 3) molecular modeling to interpret the structuring mechanism of the nanolayers. Merits of well-ordered packing, plus easiness to chemically functionalize these nanolayers, all promise great applications in molecular or organic electronics.
6:00 PM - B5.96
Probing Nanoscale Electric Field Fluctuations: Towards a Local Measurement of Carrier Mobility in Organic Semiconductors.
Showkat Yazdanian 1 , Seppe Kuehn 2 , Nikolas Hoepker 1 , Roger Loring 1 , John Marohn 1
1 , Cornell University, Ithaca, New York, United States, 2 , Rockefeller University, New York, New York, United States
Show AbstractA rich array of dynamic physical processes can be modelled as fluctuating electric fields. These processes include electron transfer, enzyme kinetics, ionic motion in solution and the glass transition. We have shown that cantilever friction can be used to study dielectric fluctuations in the vicinity of the cantilever resonance frequency. Here we show that cantilever frequency fluctuations (“jitter”) can be used to probe low frequency dielectric fluctuations over a much broader frequency window. We present measurements of dielectric fluctuations in polymers in tandem with a theory predicting the shape and magnitude of the effect. We also present our progress towards employing this technique to measure the local carrier mobility in organic semiconductor systems.
6:00 PM - B5.97
Porphyrins and N-confused Porphyrins as Spectral Dopants in Organic Solar Cells.
Warwick Belcher 1 , Nathan Cooling 1 , Paul Dastoor 1
1 Physics, University of Newcastle, Callaghan, New South Wales, Australia
Show AbstractOrganic photovoltaic cells show immense promise as a new alternative for renewable energy. However, one major problem with these devices is that the polymers typically employed in their design absorb light in only a limited part of the solar spectrum (usually <600nm). One way to further extend the spectral response of these devices is via the addition of complimentary chromophores. It has been shown that the absorption of light by the Q-bands of porphyrins incorporated into MEH-PPV/PCBM blends contributes up to 20% of the total photocurrent generated by the device by utilising light that would not normally be absorbed [1]. Furthermore, N-confused porphyrins could be used to extend the absorption spectra of these devices even further, since they have Q-band absorptions extending well beyond the range of a standard porphyrin. Despite this promise porphyrin aggregation within these ternary devices has been observed to lead to disruption of the crucial morphology of the active layer and lowered device efficiency.Porphyrin aggregation can be controlled by controlling the steric bulk of peripheral substituents on the porphyrin. A series of substituted tetraphenylporphyrins and N-confused tertraphenylporphyrins have been prepared in which the steric bulk of the peripheral substituents, and thus the degree to which aggregation occurs, was varied. These materials have been used to prepare a series of MEH-PPV/Porphyrin/PCBM and PPV/N-confused Porphyrin/PCBM ternary organic photovoltaic devices. Furthermore, we have observed that the porphyrin molecules act as “hole traps” within the devices due to the basicity of the pyrollic nitrogens, lowering device efficiency. Reduction of this basicity has been achieved by metallation and alkylation of these sites. The effect that these structural changes have on device performance will be presented.References[1]P.C. Dastoor, C. R. McNeill. H. Frohne, C, J, Foster, B. Dean, C.J. Fell, W. J. Belcher, W. M. Campbell, D.L. Officer, I. M. Blake, P. Thordarson, M.J. Crossley, N.S. Hush, R. Jeffrey, J. Phys. Chem. 111(42) (2007), 15415-15426.
6:00 PM - B5.98
Near Infrared Fluorescent and Phosphorescent Organic Light-Emitting Devices.
Yixing Yang 1 , Richard Farley 2 , Timothy Steckler 2 , Jonathan Sommer 2 , Sang-Hyun Eom 1 , John Reynolds 2 , Kirk Schanze 2 , Jiangeng Xue 1
1 Department of Materials Science and Engineering, University of Florida, Gainesville, Florida, United States, 2 Department of Chemistry, Center for Macromolecular Science and Engineering, University of Florida, Gainesville, Florida, United States
Show AbstractThere has been a growing interest in the development of near-infrared (NIR) organic light-emitting devices (OLEDs) due to their potential applications in defense, biomedical sectors, and telecommunications. For example, NIR OLEDs can be used as illumination and signaling sources for night vision and friend-foe identification, and have advantages in light weight, low thermal signature, low power consumption, and compatibility with large area and flexible substrates. Existing NIR OLEDs have been mostly based on lanthanide-based organometallic complexes, which generally have low external quantum efficiencies (EQE) (<0.1%). Only recently were high efficiency NIR OLEDs reported, in which peak emission at λ≈770 nm and a maximum EQE up to 8.5% were achieved using a phosphorescent Pt-porphyrin complex.1 Nonetheless, alternative materials and devices that exhibit longer emission wavelengths (λ>800 nm) with high efficiencies are still needed.Here we report fluorescent and phosphorescent NIR OLEDs with peak emission wavelengths up to 892 nm and EQE up to 3.8%. First, OLEDs based on two NIR-emitting fluorescent donor-acceptor-donor (DAD) oligomers, BEDOT-TQMe2 and BEDOT-BBT, are demonstrated. In these molecules, the energies of the highest occupied and lowest unoccupied molecular orbitals are controlled by the donor and the acceptor portion, respectively. A maximum EQE of ηEQE = 1.6% and a maximum power efficiency of ηP = 7.0 mW/W are achieved in devices based on BEDOT-TQMe2, with the electroluminescence (EL) peaked at 692 nm but extending to well above 800 nm. BEDOT-BBT based OLEDs show red-shifted emission peaking at 815 nm (and extending to as far as 950 nm), although the maximum efficiencies were reduced to ηEQE = 0.51% and ηP = 2.1 mW/W due to the significantly lower fluorescent quantum yield of the NIR emitter.The efficiencies of these fluorescent OLEDs were further increased by two to three times by incorporating a phosphorescent sensitizer in the emissive layer to funnel the triplet excitons formed on the host molecules to the fluorescent emitters, which are not utilized in fluorescent devices. Using this sensitized fluorescence structure, we achieved maximum efficiencies of ηEQE = 3.1% and ηP = 12 mW/W for BEDOT-TQMe2 based devices, and ηEQE = 1.5% and ηP = 4.0 mW/W for BEDOT-BBT based devices.Finally, a phosphorescent NIR emitter, platinum tetraphenyltetranaphtho[2,3]porphyrin (Pt-TPTNP), was synthesized. The more extended conjugation on this molecule compared to the Pt-porphyrin complex used in Ref. 1 leads to a red shift in the emissive wavelength by more than 100 nm. In the Pt-TPTNP-based OLEDs, we obtain peak emission at 892 nm, and maximum efficiencies of ηEQE = 3.8% and ηP = 19 mW/W, much higher than those of the sensitized fluorescence OLEDs based on DAD oligomers.1 Y. Sun et al., Appl. Phys. Lett. 90, 213503 (2007)
6:00 PM - B5.99
Non-Volatile Molecular Monolayer Memory.
Sangkwan Kim 1 2 , Hojong Chang 1 2 , Junghyun Lee 1 , Gyeong Sook Bang 1 , Hyoyoung Lee 1
1 Center for Smart Molecular Memory, Electronics and Telecommunications Research Institute(ETRI), Deajeon Korea (the Republic of), 2 Next Generation Device Engineering, University of Science and Technology, Deajeon Korea (the Republic of)
Show AbstractMolecular monolayer memory is considered to be one of the best solutions to the scaling limit problem in the silicon industry. Organic self-assembled monolayers (SAMs) as active memory elements in electronic devices are capable of highly integrated density in metal-molecule-metal (MMM) types of devices. Newly designed dialkylthiolate-tethered metal complex SAMs (RuII(tpy(CH2)nS)2) are introduced. These SAMs using a conducting polymer, PEDOT:PSS, as a soft part of top electrode provide a stable and reproducible molecular monolayer memory device that shows hysteretic I-V characteristics, write-multiple read-erase-multiple read pulse cycles and good retention time. As alkyl chain lengths of metal complexes increase from 7 to 13, molecular monolayer device yield improves up to 81% at a micro-scale well and simultaneously its retention time increases from 200 to 390 seconds. The non-volatile molecular monolayer memory is non-destructive for more than 300 write-multiple read-erase-multiple read cycles. A conduction mechanism is direct tunneling with little temperature dependence
Symposium Organizers
Norbert Koch Humboldt-Universitaet zu Berlin
Egbert Zojer Technische Universitaet Graz
Saw-Wai Hla Ohio University
Xiaoyang Zhu University of Texas-Austin
B8: Charges & Transport II
Session Chairs
Thursday AM, April 16, 2009
Room 2001 (Moscone West)
9:30 AM - **B8.1
The first principles measurement of charge mobility of organic semiconductors with UPS.
Satoshi Kera 1 , Hiroyuki Yamane 2 , Shunsuke Hosoumi 1 , Shin-ichi Nagamatsu 1 , Nobuo Ueno 1
1 Graduate School of Advanced Integration Science, Chiba University, Chiba Japan, 2 , Institute for Molecular Science, Okazaki Japan
Show Abstract Most of the over 86 million registered materials are organic materials, and many of them form solid by weak intermolecular interaction. Organic semiconductor is the representative of electronic function brought by the weak intermolecular interaction coupled with individual molecular characteristics, and has been increasingly studied for device application. As organic semiconductor films show various ‘faces’ depending on molecular packing structure as well as on individual molecular structure, many mysteries exist to be elusive. A key issue of organic FET devises is how one can improve the carrier mobility (μ) of organic thin films. As the electrical conductivity (σ) is given by σ=nqμ, where n is the carrier concentration and q is the charge of the carrier concerned, one must know principal mechanism which dominates μ, namely coherent band conduction or incoherent hopping conduction. In relation to organic transistors, many electrical measurements have been performed to investigate the charge mobility. Unfortunately, however, most of the works have directed to obtain phenomenological μ from I-V measurements. The coherent conduction is dominated by the band dispersion with a mean free path of the carrier much longer than intermolecular distance, and the hopping conduction is specified by two physical parameters, the transfer integral (t) and the charge reorganization energy (λ). t is the measure of the intermolecular interaction, and λ is related to charge-vibration coupling. To go into the mobility, one needs to measure t and λ experimentally. t is given by observing the energy band dispersion or the energy level splitting in finite molecular stacks, and λ is obtained by measuring hole- or electron-vibration coupling in organic systems at low temperature (namely not in gas phase but in a film). UPS can in principle measure these two targets. At this conference we will present our recent challenges on UPS of the thin films that give (1) the energy level splitting and the band dispersion, both of which offer the value of t, and (2) λ obtained directly from the measurement of the HOMO hole-vibration coupling in various organic semiconductor films. From these we can obtain the ultimate hole mobility for the coherent band conduction and the hopping conduction.
10:00 AM - B8.2
Charge Transport and Microstructure Correlations using Anisotropic Polythiophene Thin Films Fabricated via Directional Crystallization.
Leslie Jimison 1 , Alberto Salleo 1
1 Materials Science and Engineering, Stanford University, Stanford, California, United States
Show AbstractIn recent years much research has been devoted to the improved fabrication and understanding macroelectronics: a category of devices that include thin film transistors, photovoltaics and light emitting diodes. Polymeric semiconductors have great potential as the active layer for such large scale systems, with a key attribute being their ability to be dissolved in solvents to make printable “semiconducting inks.” While polymer semiconductors are beginning to reach performance levels that enable them to be competitive in low-cost electronics, what is lacking is an understanding of the fundamental charge transport processes in relation to microstructure. We have used a means of controlling the orientation and size of crystallites in the plane of the substrate to produce films with well-known grain-boundary types. As a result we are able to explore the relationship between trap density within grain boundaries and charge transport. Regioregular poly-3-hexylthiophene (P3HT) is the material under investigation. We have fabricated anisotropic films on glass and silicon substrates via directional solidification, using 1,3,5-trichlorobenzene first as a solvent and then as a substrate for epitaxy. The technique enables us to create films consisting of large (mm2) domains of uniform extinction under crossed polarizers, suggesting long range orientation of the polymer chain axis. Using a film lift-off technique, atomic force microscopy reveals the lamellar microstructure at both the polymer/air interface and polymer/substrate interface. Film microstructure was further characterized at the Stanford Synchrotron Radiation Laboratory (SSRL). We have collected 2D diffraction patterns, specular diffraction patterns, and grazing incidence diffraction patterns, confirming the unique in-plane and out-of-plane texture of the polymer films. Charge transport in the directionally crystallized films was probed by measuring temperature-dependent mobilities using thin film transistors with the oriented film as the active layer. Devices were made with different relative orientations between the channel and the polymer film. Transport measurements as a function of charge density and temperature for different orientations of our film confirm mobility anisotropy but no activation energy anisotropy. Temperature dependent transport measurements also reveal increased effect of bias stress between 220-280°C. Furthermore, there is marked anisotropy of the bias stress effect between the different device orientations. Because our films are anisotropic in the type of grain boundary present, this strongly suggests that bias stress in semicrystalline organic thin films is dependent on the microstructure at the grain boundaries.
10:15 AM - B8.3
Scanning Kelvin Probe Measurements on Pentacene based Field-Effect Transistors with UV-modified Gate Dielectric
Christopher Siol 1 , Niels Benson 1 , Christian Melzer 1 , Heinz von Seggern 1
1 Institute of Material Science, Technische Universitaet Darmstadt, Darmstadt Germany
Show AbstractThe use of organic field-effect transistors (OFETs) in organic electronics is often hampered by the fact that solely unipolar logic is implemented. In view of a performance improvement, it is aspired to use complementary metal oxide semiconductor (CMOS) -like techniques to benefit from the efficiency of complementary logic circuits. In recent publications we have demonstrated a CMOS-like inverter based on pentacene n- and p-type OFETs that comprise identical device layouts thus facilitating the production of CMOS-like elements. Solely the treatment of the used PMMA gate dielectric with UV light prior to the pentacene deposition allowed for an inversion of the operation mode from the usual n-type to a unipolar p-type behaviour. Even though the stable device performance of the thus produced inverter demonstrates the potential of the proposed UV treatment the exact mechanism allowing for the polarity change is not known in detail.In this contribution, we try to reveal this mechanism by discussing the results of Scanning Kelvin Probe Measurements (SKPM) performed in the channel of UV-treated OFETs in combination with the analysis of the OFET performance. From the current-voltage characteristics it is supposed that the UV treatment itself results in the formation of electronic trap states, in particular in electron traps. Since the establishment of the p-type behaviour requires, besides the UV treatment, a precedent operation of the transistor in electron accumulation, it is speculated that the electron traps have to be negatively charged. Indeed, SKPM disclose that the accumulation of negative charge carriers at the PMMA / pentacene interface results in a distinct and stable trapping of electrons, which proves the existence of efficient electron traps in UV-modified PMMA. In what way this charging helps to convert the n-type transistor to a p-type transistor, in particular to inject holes from the employed low workfunction metal Ca, will be discussed in detail. The stably trapped negative areal charge density leads to a gate-field enhancement at the contacts. It is supposed that this field enhancement is sufficient to inject holes from Ca into the channel. Under current flow, the channel in the vicinity of the source will again partly deplete from the compensating holes allowing anew for a field-enhanced hole injection from Ca.
10:30 AM - B8.4
Trap-Dominated Charge Transport in Organic Transistors as Investigated by Field-Induced ESR Spectroscopy.
Hiroyuki Matsui 1 2 , Tatsuo Hasegawa 1
1 PRI, AIST, Tsukuba Japan, 2 Department of Advanced Materials Science, University of Tokyo, Tokyo Japan
Show Abstract Recently we reported that motional narrowing effect can be observed in field-induced electron spin resonance (ESR) spectra of high-mobility pentacene thin-film transistors (TFTs). We found that the methods are quite useful in elucidating the carrier dynamics in organic TFTs since the narrowed linewidth allows us to estimate the average residence time of carriers at respective sites. In particular the analyses afford a clear evidence for the considerably long trap residence time or the trap-dominated conduction in pentacene TFTs. Meanwhile, motional narrowing effect is directly evidenced by temperature-dependent and gate-field-dependent single-Lorentzian ESR spectra. However, the temperature-dependent feature deviates from the simple motional narrowing regime in high (T > 200 K) and low (T < 50 K) temperature ranges. Here we discuss the whole picture of field-induced ESR spectra in the temperature range of 300 - 20 K on the basis of continuous wave saturation experiments. The method enables us to check the homogeneity of ESR spectra as well as to estimate spin-lattice relaxation time T1. It is found from the temperature dependence of ESR linewidth that the linewidth increases as temperature decreases with activation energy of about 15 meV at 50 K < T < 200 K, while it deviates from the feature at higher and lower temperature ranges. First we examined the saturation behavior of ESR signals, which allows us to separate the contribution of spin-lattice relaxation from motionally-narrowed inhomogeneous linewidth. The motionally-narrowed and the spin-lattice relaxation components compete with each other at around 200 K. From this we conclude that the small increase of linewidth at higher than 200 K with increasing tempeature should be attributed to the spin-lattice relaxation. At the temperature lower than 50 K, on the other hand, the linewidth tends to converge at about 0.18 mT. The ESR spectra do not show broadening under saturation at high microwave power, demonstrating the inhomogeneity of the ESR absorption. It means that motional narrowing is no longer effective in the low temperature range because of long residence time at trap states. Transport and localization of field-induced carriers will be discussed on the basis of these experimental results.
10:45 AM - B8.5
Electrochemical Transistors: New Platforms to Study Interfaces in Liquids.
Fabio Cicoira 1 2 , Sang Yang Yoon 1 , DeFranco John 1 , George Malliaras 1
1 MSE, Cornell UNiversity, Ithaca, New York, United States, 2 Institute of Photonics and Nanotechnology, CNR, Trento Italy
Show AbstractThe considerable research efforts in organic electronics have led to the development of a number of devices like organic light emitting diodes, solar cells and organic thin film transistors that are nowadays in production or prototype stage. Along with these well-established fields, exciting emerging applications are taking advantage of the mixed ionic/electronic transport in organic electronics devices [1]. Along this line, the application of organic semiconductor devices to chemical and biological sensors seems to be a great fit. A promising approach towards organic-based sensors involves the use of organic electrochemical transistors (OECTs). OECTs consist of source and drain electrodes, and a channel containing the organic active material in ionic contact with a gate electrode via an electrolyte solution. These devices can be operated in aqueous environment as efficient ion-to-electron converters, thus providing an interface between the worlds of biology and electronics and also a unique platform for the study of organic/organic and organic/metal interfaces in liquids.Although electrochemical transistors have been known since long time [3], they received little attention in the scientific community until the recent resurgence due to their application in biosensors. Therefore a great deal of work is needed to understand the fundamental processes that take place in these devices, essential for their use in sensing applications. In this presentation we intend to address this important issue. Using photolithography, surface engineering and micro fluidics we have developed several technique to fabricate OECTs having different geometries. This allows us to study the basic electronic properties and the sensing response of devices in order to understand their mechanism of operation [4] [5]. We studied how the dimensions of the transistors (in particular on the gate/channel area ratio) and the gate electrode material (metal or polymer) can be used to tune the device response. The effect of the electrolyte on device response was evaluated studying transistors in aqueous electrolytes and ionic liquids. The detection limit of OECTs based sensors having different geometry, was analyzed for glucose and hydrogen peroxide (a species involved in glucose sensing). [1]J. M. Leger, Adv. Mater. 2008, 20, 837. [2]M. J. Panzer, C. D. Frisbie, Adv. Mater. 2008, 20, 3177.[3]D. Vanmaekelberg, A. J. Houtepen, J. J. Kelly, Electrochem. Acta 2007, 53, 1140.[4]D. A. Bernards, G. G. Malliaras, Adv. Funct. Mater. 2007, 17, 3538.[5]D.A. Bernards, G. G., Malliaras, D. J. Macaya, M. Nikolu, J. A. DeFranco, S. Takamatsu, G. G. Malliaras, J. Mater. Chem. 2008, 18, 116.
11:30 AM - **B8.6
Electronic Structure of Organic Heterointerfaces.
Henning Sirringhaus 1
1 , University of Cambridge, Cambridge United Kingdom
Show AbstractCharge injection at metal-semiconductor interfaces as well as transport in the accumulation layer of organic FETs is critically determined by the electronic structure at these interfaces. In this presentation we will review our current understanding of the factors that determine the molecular structure, energetic disorder and polaronic relaxation processes at interfaces and discuss recent experiments that have yielded information about such processes with high interfacial sensitivity. A thorough understanding of interfacial electronic structure is needed in order to identify the factors that limit performance of state-of-the-art organic FETs.
12:00 PM - B8.7
Morphology and Energy Levels in Conjugated Polymers: A Theoretical View on P3HT.
Georg Heimel 1 , Juergen Rabe 1
1 Insitut für Physik, Humboldt-Universität zu Berlin, Berlin Germany
Show AbstractIn the field of organic electronics, π-conjugated polymers bear great promise for solution-processible, flexible applications such as organic light-emitting devices (OLEDs), organic field-effect transistors (OFETs), or organic photovoltaics. In addition to the bulk material properties of the active organic part, the interfaces between organic and inorganic components are well acknowledged to be of paramount importance for device performance and functionality. Most importantly, the energetic position of the conducting states in the organic material crucially impacts the energy barriers for charge injection into the device. These barriers not only limit the overall injection rates but also give rise to often undesirably high onset voltages below which the device remains inactive.Preparation conditions are know to crucially impact the thin-film morphology of the prototypical π-conjugated polymer region-regular poly(3-hexylthiophene) and, consequently, also the device characteristics of, e.g., OFETs based on this material (rr-P3HT). In such devices, it has been shown that an edge-on configuration of highly ordered polymer chains favorably impacts transistor performance while a face-on morphology proved detrimental; in the former, the preferential direction for charge transport in these systems, the π-stacking direction in co-facial polymer chains, is aligned with the direction of current flow in an OFET device geometry.In our contribution, we present density-functional theory (DFT) based band-structure calculations on highly ordered rr-P3HT monolayers of different morphology, i.e., edge-on, face-on, and intermediate regimes. Supported by electrostatic modeling, we find that the backbone orientation importantly influences the energy-level positions in these thin films due to intra-molecular surface dipoles. As a consequence, also the hole-injection barriers (HIB) can be expected to depend critically on thin-film morphology. Our calculations suggest that the HIB into rr-P3Ht films with edge-on morphology can be up to 0.5 eV lower than into films of face-on orientation.In addition to underlining the importance of morphology control in polymer-based organic electronic devices, understanding the impact of intra-molecular surface dipoles also paves the way towards novel strategies for material design. To that end, we extend our investigations towards rr-P3HT with end-fluorinated alkyl side-chains. Our calculations reveal that, due to the strongly negative surface termination in such films, the energy levels in the polymer could be lowered by as much as 1.5 eV in the edge-on configuration compared to face-on. This scenario would lead to significantly reduced electron injection-barriers and, thus, favor device operation involving negative charge-carriers. Consequently, our results imply that such materials, together with improved control over morphology and electron traps, could potentially serve as active organic components in n-type polymer OFETs.
12:15 PM - B8.8
Influence of Contact Effects on the Switching Behavior of Organic Transistors
Arne Hoppe 1 , Dietmar Knipp 2 , Benedikt Gburek 1 , Marko Marinkovic 2 , Veit Wagner 1
1 Molecular and Nanoelectronics Laboratory, Jacobs University Bremen, Bremen Germany, 2 Electronic Devices and Nanophotonics Laboratory, Jacobs University Bremen, Bremen Germany
Show AbstractThe influence of contact effects on the switching behavior of organic transistors was studied. High switching frequencies can be achieved by using short channel transistors with high charge carrier mobilities. However, most of the short channel transistors exhibit a distinct drop of the device charge carrier mobility. The reduced device mobility is caused by the influence of contact effects. In this study the influence of the drain/source contacts and the device geometry on the switching frequency of high mobility dihexyl-7-thiophene (DH7T) oligothiophene thin film transistors was investigated. The transistors were realized with channel lengths ranging from 50 nm to 50 μm. The transistors exhibit high charge charier mobilities of 0.1 cm2/Vs and high switching frequencies between 200 kHz and 2 MHz at low operating voltages of 5 V. The maximum of the switching frequency is limited by the specific contact resistance and the overlap capacitance between drain/source and gate electrodes. The normalized contact resistance of the oligothiophene transistor was determined to be 3.5 kΩcm. An upper limit of the switching frequency for organic thin film transistors was derived, which is determined by the specific contact resistance between the drain and source electrodes and the organic channel material. The charge carrier mobility does not affect the upper limit of the switching frequency. Different strategies will be discussed to maximize the cut-off frequency.
12:30 PM - B8.9
Polymer-Small Molecule Semiconductor Blends for Integrated Circuits with a 712 ns Single Stage Delay.
Jeremy Smith 1 , Richard Hamilton 2 , Donal Bradley 1 , Iain McCulloch 2 , Martin Heeney 3 , Dago de Leeuw 4 , John Anthony 5 , Thomas Anthopoulos 1
1 Physics, Imperial College London, London United Kingdom, 2 Chemistry, Imperial College London, London United Kingdom, 3 Materials, Queen Mary University of London, London United Kingdom, 4 , Philips High-Tech Campus, Eindhoven Netherlands, 5 Chemistry, University of Kentucky, Lexington, Kentucky, United States
Show AbstractSolution processed semiconducting blends of acene based small molecules with amorphous polymers provide a means of combining the high mobility of the former with the ease of processing and thin-film uniformity of the latter. We have demonstrated organic field-effect transistor (OFET) mobilities of up to 2.4 cm2/Vs with low device-to-device variation[1]. This level of performance qualifies the system for use in integrated circuits with the possibility of future applications such as organic radio frequency identification tags and electronic paper. High mobility and optimal device architecture is required to attain a high operation frequency. In this work logic inverters and ring oscillators have been used to evaluate the dynamic performance of difluorinated triethylsilylethynyl anthradithiophene blended with a poly(triarylamine) and to demonstrate that the technology can be transferred from single transistors to multi-transistor circuits such as ring oscillators. The latter are often used as prototype circuits for the evaluation of new semiconductors and deposition processes. They allow the determination of an inverter stage delay, t (i.e. the switching time between a high and a low state) using the equation t=1/(2nf), where n is the number of inverting stages and f the oscillation frequency. This delay is proportional to the time taken for charging or discharging at the inverter input/output node and is in turn determined by the charge carrier mobility, the supply voltage used and the transistor channel length. Stage delay thus provides a useful indication of the speed of integrated circuit operation.Ring oscillators were measured with a variety of design rules, specifically channel lengths, and over a range of supply voltages resulting in the expected trends for oscillation frequency. To obtain high performance circuits, an understanding of these effects and the ability to control film morphology within the transistor channels are very important. The highest mobility transistors employ a top-gate design due to vertical phase separation of the acene molecule to the semiconductor-dielectric interface[1]. However, a bottom-gate, bottom-contact circuit architecture is much easier to construct[2]. Hence surface energies and thin film formation become critical to maintaining a high enough mobility for low t devices. By using short channel transistors employing gold source-drain contacts functionalised with a self-assembled monolayer (SAM) as hole injecting electrodes, we have fabricated 7-stage ring oscillators with a single stage delay of less than 800 ns which, to our knowledge, is one of the fastest organic integrated circuits to date[3]. This level of performance makes the polymer-small molecule blend an excellent candidate for use in a wide range of low-end organic electronic applications.[1] Hamilton, R. et al., Adv. Mater., (2008) in press.[2] Gelinck, G. H. et al., Nature Mater. 3 (2004) 106.[3] Smith, J. et al., (2008) submitted.
12:45 PM - B8.10
Charge Transport in Organic Transistors Based on Interconnected Polythiophene Nanofibrillar Network Embedded in Insulating Polymer
Kilwon Cho 1 2 , Longzhen Qiu 1 , Jung Ah Lim 1 , Juhyun Kim 2 , Wi Hyoung Lee 1
1 Chemical Engineering, Pohang University of Science and Technology, Pohang Korea (the Republic of), 2 School of Environmental Science and Engineering, Pohang University of Science and Technology, Pohang, Kyungbuk, Korea (the Republic of)
Show AbstractSemiconducting and insulating polymer blends has brought a new way to tune the electronic properties of devices and combine the electronic properties of semiconducting polymers with the low-cost and excellent mechanical characteristics of insulating polymers. However, the presence of the insulating component tends to degrade the device performance by diluting the current density of the film. In this study, we demonstrated that organic field-effect transistors (OFETs) with excellent electronic performance can be achieved in blends of poly(3-hexylthiophene) (P3HT) and amorphous polystyrene (a-PS) at very low P3HT content by controlling the solubility of solvent using a marginal solvent or solvent mixture. The controlled solubility allows the P3HT chain in blends to form highly crystalline, interconnected nanowire networks embedded in PS matrix. In particular, the precise control of solvent composition of P3HT/PS blend by using solvent mixture causes the inkjet-printed blend deposits to form a uniform film-thickness and highly ordered P3HT nanowire structure in PS matrix. The transistors based on inkjet-printed P3HT/PS blends exhibits the improved electrical performance even though a single droplet deposit with super-low content of P3HT (tens of pictogram) is used as an active material. This finding may offer a excellent route to the direct-write fabrication of OFETs with low semiconductor cost, high environmental stability, and good mechanical properties.Acknowledgement. This work was supported by a grant (F0004021-2008-31) from the Information Display R&D Center under the 21st Century Frontier R&D Program, and Creative Research Initiative(CRI)-Acceleration Research (R17-2008-029-01000-0).
B9: Photovoltaics
Session Chairs
Emil List
Henning Sirringhaus
Thursday PM, April 16, 2009
Room 2001 (Moscone West)
2:30 PM - B9.1
Charge Extraction in Planar Heterojunction Organic Photovoltaics.
Cody Schlenker 1 , Mark Thompson 1
1 Chemistry, University of Southern California, Los Angeles, California, United States
Show AbstractThe buffer layer between the acceptor and cathode in multi-heterojunction organic photovoltaic (PV) devices has the potential to act as powerful light management tool. However, conventional buffer materials rely on metal mediated charge transport that is generally limited to ~10 nm, precluding their use as versatile optical spacers. The complex tris(β-diketonato)ruthenium(III) has shown promising thickness tolerance resulting from reciprocal carrier transport in the archetypical device anode/CuPC/C60/buffer/cathode. We examined the energetic constraints on reciprocal carrier transport in the following series of 6 ruthenium diketonate complexes: 1,3-CH3 (acac); 1,3-C6H5; 1-CF2H,3-C6H5; 1-CF3,3-C4H3S; 1-CF3,3-C10H7; 1-CF3,3-C6H5. These compounds were selected to adjust the carrier injection barriers as their oxidation and reduction potentials vary by 680 mV and 830 mV respectively. The current density-voltage (J-V) characteristics of PV devices were recorded in the dark, and under both white and monochromatic illumination. The fill factor and maximum power output correlate with the oxidation potential of the Ru complex. Due to concavity changes in the J-V trace, we attribute this to an interfacial contact resistance arising from an increased hole injection barrier at the buffer/cathode interface. Charge carrier injection, energy level alignment, and electrical contacts to organic semiconductors will be discussed, as well as prospects for further study.
2:45 PM - B9.2
Energy Level and Morphology Optimization of Small Band Gap Polymers for Photovoltaics.
Arjan Zoombelt 1 2 , Jan Gilot 1 , Rene Janssen 1 2
1 , Eindhoven University of Technology, Eindhoven Netherlands, 2 , Dutch Polymer Institute, Eindhoven Netherlands
Show AbstractMany small band gap polymers have been synthesized over the past few years to be utilized in bulk heterojunction solar cells. The small band gap, in the range of 1.8-1.3 eV, would lead to an improved overlap of the polymer absorption with the solar emission spectrum, which peaks around 700 nm (1.77 eV), leading to an increase of absorbed photons and therefore would enhance the efficiency of solar cells.A successful and flexible strategy to achieve small band gap conjugated polymers involves the alternation of electron-rich (Donor) and electron-deficient (Acceptor) units in the polymer chain. The chemical nature of the building blocks and side chains determines energy level positions. These need to be judiciously positioned with respect to those of phenyl-C61 butyric acid methyl ester (PCBM), the most commonly used acceptor, to enable efficient charge separation and maintain the highest possible open-circuit voltage (Voc). Furthermore, the polymer’s solubility, molecular weight, crystallization behavior, and miscibility with the acceptor are key factors in determining photovoltaic performance.A series of new conjugated polymers will be presented with band gaps ranging from 1.8 to 1.3 eV and energy levels that are well matched for optimal performance. The focus will be on the effect of side chain position, head-to-head and head-to-tail coupling, on Voc and short circuit currents (Jsc) and the influence of solubility and aggregation behavior upon the processing conditions of the active layer. We will show that a more planar backbone results in a lower Voc, but increases Jsc due to a slightly improved absorption and hole mobility. In addition, we will present the optimization of morphology of the active layer for a variety of polymers and show how different polymers having a different solubility require distinct processing conditions.
3:00 PM - B9.3
A Systematic Approach to the Design and Synthesis of New Acceptors for Organic Photovoltaics.
John Anthony 1 , Ying Shu 1 , Sean Parkin 1 , Yee-Fun Lim 2 , George Malliaras 2
1 Chemistry, University of Kentucky, Lexington, Kentucky, United States, 2 Materials Science and Engineering, Cornell University, Ithaca, New York, United States
Show AbstractHigh-performance organic photovoltaic reports are dominated by bulk-heterojunction structures where semiconducting polymers are blended with the fullerene derivative PCBM (or higher-order fullerene derivatives of this molecule). We wondered whether the fullerene-based acceptor could be modified to yield improved open-circuit voltage and short-circuit current. Using an approach similar to that used to engineer the solid-state order of pentacene, we have found a simple route to a series of crystalline fullerene derivatives that can be prepared quickly and in high yield. Our initial studies have shown that changes in the aromatic portion of the substituent on the fullerene can yield significant changes in open-circuit voltage, arising from changes to the fullerene LUMO arising from close contact with these pendant aromatic groups. Alternatively, changes to the hydrocarbon portion of the substituent on the fullerene lead to alteration of the crystal packing of the fullerene, which profoundly influences the short-circuit current of these devices. The ability to independently tune both voltage and current is allowing us to explore wide substitution space in preparing design rules for this class of fullerene-based acceptors.The high energy requirements for fullerene synthesis may make these materials less desirable as components of “green” energy sources. We are concurrently exploring alternative acceptors based on nitrile-functionalized pentacenes. Here again, functionalization can be used to tune both photovoltage and photocurrent, and our current best-performing material yields power conversion efficiency > 1% in photovoltaic cells using poly(3-hexylthiophene) as donor. Our progress in this material class will also be discussed.
3:15 PM - B9.4
Morphological Model of Polymer:Fullerene Solar Cells.
Klara Maturova 1 , Martijn Kemerink 1 , Svetlana van Bavel 2 , Rene A.J. Janssen 1
1 Dept. of Applied Physics, Eindhoven University of Technology, Eindhvoen Netherlands, 2 Laboratory of Polymer Technology, Eindhoven University of Technology, Eindhoven Netherlands
Show AbstractResearch on organic bulk heterojunction solar cells, which are promising candidates for low cost energy harvesting, leads to yearly improvements in efficiency. Further improvement requires better understanding of limiting factors of current devices. It is widely accepted that for polymer:polymer or polymer:fullerene solar cells the morphology and phase separation are of high importance. Most of the current device models treat the organic solar cells as homogeneous devices. Moreover, it is commonly accepted that field dependent exciton dissociation is responsible for features in high field regime of I-V curves. Here we present results from 1D as well as 2D modeling of organic solar cells which are based on solving the coupled drift-diffusion, continuity and Poisson equations. We are able to predict the I-V curves in the operational and the high field regime. We have chosen three systems: MDMO-PPV:PCBM, P3HT:PCBM and PF10TBT:PCBM, which represent the improvement in the field of organic solar cells in going from an efficiency of 1% to 4%. Depending on spincoating conditions, solvent and composition, and thus on morphology, two distinct situations can be distinguished: one that can be described by a 1D model, i.e. one where phase separation is irrelevant and one that requires a 2D model, i.e. inclusion of phase separation. A typical representative of a 2D structure is MDMO-PPV:PCBM spincoated from toluene on glass/ITO/PEDOT:PSS substrates It has a coarse phase separation consisting of 500 nm large PCBM clusters surrounded by a blend of MDMO-PPV and PCBM. We have found that in this type of systems the short circuit current Jsc and the characteristic linear part at intermediate bias in a double-log I-V curve are determined by the length scale of the phase separation. The latter feature was previously assigned to field-dependent exciton dissociation. The coarse phase separation and high electron mobility in the PCBM cause that lateral charge transport at low fields remarkably contributes to device current. Also the unannealed 1:1 (mass ratio) P3HT:PCBM and 1:4 PF10TBT:PCBM cells fall in this 2D category.On the other hand, the annealed 1:1 P3HT:PCBM and the PF10TBT:PCBM in weight ratio 4:1 (both spincoated from chlorobenzene) have very fine phase separation and are found to behave mostly 1D-like. P3HT:PCBM after annealing at 130°C has 20 nm large nanocrystals of P3HT surrounded by the blend of P3HT and PCBM, and can be modeled by a 1D model. However to be able to determine the Jsc 2D model has to applied.
3:30 PM - B9.5
Polymer Solar Cells using Self-Assembled Monolayers Modified Metal oxide/Metal as Electron Collecting Electrode
Hin-Lap Yip 1 , Steve K. Hau 1 , Hong Ma 1 , Alex K.-Y. Jen 1
1 Materials Science and Engineering, University of Washington, Seattle, Washington, United States
Show AbstractFor efficient electron collection in polymer solar cells, thin films of low work-function metals such as Ca/Al and LiF/Al are commonly used as the cathode. However, the performance of those devices is limited by their poor stability due to the vulnerable contact between the polymeric layer and the reactive metals. Here a simple method was developed to tune the interface of the cathode in polymer solar cells. This was achieved by inserting a layer of metal oxide/self-assembled monolayer (SAM) between the active poly(3-hexylthiophene) (P3HT) : [6,6]-phenyl-C61 butyric acid methyl ester (PCBM) bulk-heterojunction film and the top metal cathode. We found that the device performance could be significantly altered depending on the magnitude and direction of dipole, and chemical bonding between the SAM and metals. With appropriate choice of SAMs, devices showed dramatically improved efficiencies and even high work-function metals such as Ag and Au could be used as electron collecting electrodes. This finding provides an efficient method for interface engineering in organic-based optoelectronic devices.
3:45 PM - B9.6
Charge-transfer Excitons in Strongly Coupled Supramolecular Semiconductors.
Carlos Silva 1
1 Department of Physics, Université de Montréal, Montreal, Quebec, Canada
Show AbstractTime-resolved and temperature-dependent photoluminescence measurements on one-dimensional sexithiophene lattices reveal intrinsic branching of photoexcitations to two distinct species: self-trapped excitons and dark charge-transfer excitons (CTX; 5% yield), with radii spanning 2–3 sites. The significant CTX yield results from the strong charge-transfer character of the Frenkel exciton band due to the large free exciton bandwidth (∼ 400 meV) in these supramolecular nanostructures.
4:30 PM - B9.7
Dye Sensitized Solar Cells Fabricated Using Transparent Vertically Aligned Titania Nanotube Arrays up to 18 µm in Length Grown on FTO Coated Glass.
Maggie Paulose 1 , Oomman Varghese 1 , Craig Grimes 2 1
1 Materials Research Institute, The Pennsylvania State University, University Park, Pennsylvania, United States, 2 Department of Electrical Engineering, The Pennsylvania State University, University Park, Pennsylvania, United States
Show AbstractFilms comprised of a random interpenetrating network of titania nanoparticles, approximately 10-15 µm thick, have been the foundation of dye sensitized solar cells since the potential of such films were revealed by Graetzel and co-workers. Our research efforts to obtain ordered nano-architectures of titania with superior light transmission and charge transport characteristics yielded highly ordered nanotube arrays on titanium foils up to several hundred microns in length. However the opaque Ti substrate necessitates a reversal in the solar cell illumination geometry, which limits the solar cell efficiency due to loss of light at the counter electrode and electrolyte. Fabrication of transparent nanotube array films, several microns thick, on FTO-coated glass has been difficult to achieve. Requirements include the necessity of a thick and well-adhered starting Ti film on the FTO glass substrate, obtaining suitable transparency in the anodized film, and achieving a film robust enough to withstand various thermal, physical and chemical processes during the solar cell fabrication. We have successfully fabricated robust, highly ordered vertically aligned transparent nanotube array films on FTO glass by anodizing titanium films of up to ≈ 20 µm thickness sputter deposited onto FTO glass. The transparent films consist of nanotube arrays up to about 18 µm in length. We will discuss the effects of anodization chemistry as well as physical and chemical surface treatments on the performance of dye sensitized solar cells fabricated using these transparent nanotube array films.
4:45 PM - B9.8
Carbon Nanotube Based Near-Infrared Photodetectors with >1% External Quantum Efficiency.
Jeramy Zimmerman 1 , Michael Arnold 2 , Xin Xu 3 , Christopher Renshaw 4 , Christine Austin 1 , Richard Lunt 5 , Stephen Forrest 1 2 6
1 Physics, University of Michigan, Ann Arbor, Michigan, United States, 2 Electrical Engineering and Computer Science, University of Michigan, Ann Arbor, Michigan, United States, 3 Electrical Engineering, Princeton, Princeton, New Jersey, United States, 4 Applied Physics, University of Michigan, Ann Arbor, Michigan, United States, 5 Chemical Engineering, Princeton, Princeton, New Jersey, United States, 6 Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan, United States
Show AbstractOrganic photodetectors have seen limited success in the near infrared (NIR) region where detection has had limited success beyond 1000 nm.i Carbon nanotubes (CNT) hold great promise for extending the NIR sensitivity because they can be readily produced with diameters that absorb between 1000 and 2000 nm and have carrier mobilities exceeding 105 cm2-V-1s-1.ii Due to the inherent presence of metallic tubes and difficulties in processing, successful demonstrations of photovoltaic action have been limited to single CNT devices.iiiThe CNTs are wrapped in the polymer MDMO-PPV. Films are then doctor bladed onto indium tin oxide coated glass, and followed by deposition of a compound acceptor/cathode structure by vacuum thermal evaporation. These devices show CNT based photovoltaic response up to wavelengths of 1450 nm with external quantum efficiencies of over 1.5% at 1155 nm and 1300nm. By adjusting the processing conditions, dark current rectification ratios of over four orders of magnitude are achieved. A spectrally resolved specific detectivity of >1010 cm-Hz1/2W-1 from 400 nm to 1400 nm is realized. These devices significantly extend the detection capabilities of existing organics into the NIR and indicate a promising new direction for CNT based optoelectronics.i R. Kroon et al., Polym. Rev. 48, 531 (2008)ii T. Durkop et al., J. Phys.-Condes. Matter 16, R553 (2004)iii P. Avouris et al., Nat. Photonics 2, 341 (2008)
5:00 PM - B9.9
Template-Assisted Fabrication of Free-Standing Nanorod Arrays of Semi-Conducting Polymers for the Preparation of Organic Solar Cells.
Niko Haberkorn 1 , Patrick Theato 1
1 Johannes Gutenberg University, Institute of Organic Chemistry, Mainz Germany
Show AbstractArrays of free-standing nanorods are of great interest for the fabrication of high efficient bulk heterojunction organic solar cells. The ideal structure that has been proposed for organic solar cells is a bicontinuous and interpenetrating network of donor and acceptor phase, with the interfacial distance being smaller than the exciton diffusion length in the polymer (~ 10-20 nm). Such a nanostructured interdigitated network should result in an efficient exciton separation at the interface between the donor and acceptor phase and the perculated pathways would ensure a high mobility charge carrier transport to the electrodes. [1][2]In this study, we present a template-assisted approach to pattern semi-conducting polymers to build up a photoactive layer with a well-defined morphology that would fulfill the above mentioned requirements for photovoltaic devices. Anodized aluminum oxide (AAO) membranes with a highly ordered nanoporous structure were fabricated by a controlled anodization process and used as templates.[3] [4] These membranes with pore diameters down to 20 nm and pore lengths of several hundred nanometers were filled with cross-linkable triarylamines by a solution wetting process. Thermal curing and selective etching of the AAO template resulted in free-standing nanorod arrays of the cross-linked triarylamines attached to a conductive substrate. To overcome aggregation and collapse of the nanorods after removal of the template, their aspect ratio was optimized and a freeze-drying technique was applied after the etching step.Afterwards, an electron-conducting material, e.g. perylene bisimide derivative, was used to fill the void between the hole-conducting nanorods, which resulted in a bicontinuous photoactive layer with a large donor-acceptor interface. The polymeric interpenetrating network was analyzed by cross-sectional transmission electron microscopy (TEM). Further measurements concerning the performance of organic photovoltaic devices that have been prepared by this template assisted approach will be discussed.References[1] X. Yang, and J. Loos, Macromolecules, 2007, 40, 1354.[2] S. Günes, H. Neugebauer and N. S. Sariciftci, Chem. Rev., 2007, 10, 1324.[3] M. Masuda, and K. Fukuda, Science, 1995, 268, 1466. [4] M. Steinhart, J. H. Wendorff, A. Greiner, R. B. Wehrspohn, K. Nielsch, J. Schilling, J. Choi and U. Gosele, Science, 2002, 296, 1997.
5:15 PM - B9.10
Vertical Phase-Separation Due to Differences in Surface Energies in Bulk Heterojunction Polymer Solar Cells.
Sarah Mednick 2 1 , Anshuman Roy 2 , Ji Sun Moon 2 1 , Sung Heum Park 2 , Alan Heeger 2
2 Center for Polymers and Organic Solids, University of California, Santa Barbara, Santa Barbara, California, United States, 1 Materials Science & Engineering, University of California, Santa Barbara, Santa Barbara, California, United States
Show AbstractThe synthesis and testing of new photoactive polymers is steadily improving the light conversion efficiencies of organic bulk heterojunction (BHJ) solar cells. Understanding the physical interactions between the polymer donor material and the electron acceptor is critical in controlling and optimizing the morphology of the blend. While interactions between the donor and acceptor in the blend determine the scale and stability of lateral phase separation, interactions between the constituents of the blend and the neighboring device layers are equally important. In this work, we demonstrate that bulk heterojunction constituents in a polymer solar cell, i.e. the electron donating material and the electron accepting material, tend to vertically phase-separate due to differences in surface energies. Using a combination of cross-sectional transmission electron microscopy (TEM), variable angle spectroscopic ellipsometry (VASE), and a contact angle study, we probe the vertical phase separation in poly(3-hexylthiophene) : [6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) and poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] : [6,6]-phenyl-C71-butyric acid methyl ester (PCDTBT:PC70BM). Finally, we demonstrate the relevance of vertical phase separation in understanding the device level physics of bulk heterojunction polymer solar cells.
5:30 PM - B9.11
Hybrid Organic/Quantum Dot Photodetector.
Tim Osedach 1 3 , Scott Geyer 2 , John Ho 1 , Alexi Arango 1 , Moungi Bawendi 2 , Vladimir Bulovic 1
1 Department of Electrical and Computer Engineering, MIT, Cambridge, Massachusetts, United States, 3 School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts, United States, 2 Department of Chemistry, MIT, Cambridge, Massachusetts, United States
Show AbstractWe describe a heterojunction photodetector of lateral geometry that consists of molecular organic and colloidal quantum dot (QD) films. The interface between the organic and QD films creates a type-II heterojunction that dissociates photo-generated excitons. Carriers are collected at lateral in-plane electrodes in the presence of an applied electric field on the order of 10^5 V/cm. In contrast to organic photodetectors (PDs) and photovoltaics (PVs) of the more familiar sandwich geometry, this unique device structure allows for independent optimization of optical absorption and charge transport through the selection of the heterojunction materials. Optical sensitivity can be controlled via the selection and sizing of the colloidal QDs and electronic transport is dominated by the choice of organic film. We present numerical modeling results to elucidate device operation as well as experimental data characterizing performance for devices consisting of CdSe QDs and a variety of organic hole-transporting materials including Spiro-TPD, NPD and MEO-TPD. Current is found to follow Child's Law for space-charge-limited conduction and high photon-to-electron conversion efficiencies are measured under illumination.
5:45 PM - B9.12
Molecular Semiconductor Blends: Microstructure, Charge Carrier Transport and Application in Photovoltaic Cells.
Andreas Opitz 1 , Julia Wagner 1 , Bernhard Ecker 1 , Marcel Goetzenbrugger 1 , Ulrich Hoermann 1 , Markus Bronner 1 , Michael Kraus 1 , Wolfgang Bruetting 1 , Alexander Hinderhofer 2 , Frank Schreiber 2
1 Institute of Physics, University of Augsburg, Augsburg Germany, 2 Institute of Applied Physics, University of Tübingen, Tübingen Germany
Show AbstractBlends of organic electron and hole conductive materials are widely used for ambipolar charge carrier transport and photovoltaic cells. Many investigations have reported an increase of the solar cell efficiency by optimizing the balance between charge carrier transport in phase-separated structures and exciton dissociation at the interface between these phases. Here we show the implications of blending molecular materials for structural, optical and electrical properties in two model systems. These are (i) fullerene C60 combined with copper-phthalocyanine (CuPc) and (ii) CuPc in combination with fluorinated CuPc. The analysis of X-ray diffraction measurements shows the formation of phase-separated nanocrystals for blends from C60/CuPc and indicates the formation of mixed crystals for the CuPc/F16CuPc blends. The absorption spectra for the nano-phase separated blends scale with the concentration of the individual components whereas the absorption spectra for the mixed crystals show a decreasing interaction between the F16CuPc molecules. The formation of mixed crystals is a new feature for organic blends, which has not yet been explored in organic solar cells.Additionally the charge carrier transport and the electronic structure were analyzed for the C60/CuPc blends. The exponential decrease of the mobility by dilution of the respective transport material indicates that percolation is a crucial feature in mixtures. Photoelectron spectroscopy measurements show that mixing of the organic materials reduces the intermolecular gap between the highest occupied molecular orbital of the donor and the lowest unoccupied molecular orbital of the acceptor.The C60/CuPc system was analyzed also in solar cells. The comparison between bilayered and blended cells shows a higher open circuit voltage of bilayered cells, which is related to the higher intermolecular gap in this system. Nevertheless, the blended solar cells reach higher short circuit currents based on the larger donor/acceptor interface even though the mobility in the mixed system is much lower. This indicates that other solar cell geometries might be required to combine a high open circuit voltage and a high short circuit current.
B10: Poster Session II
Session Chairs
Saw-Wai Hla
Norbert Koch
Xiaoyang Zhu
Egbert Zojer
Friday AM, April 17, 2009
Salon Level (Marriott)
9:00 PM - B10.1
Micro-cavity Effect on Light Extraction Efficiency of Blue Phosphorescent Organic Light Emitting Devices.
Jaewon Lee 1 , Neetu Chopra 1 , Franky So 1
1 Dept of Materials Science and Engineering, University of Florida, Gainesville, Florida, United States
Show AbstractRecently we reported high efficiency blue phosphorescent organic light emitting devices (PHOLEDs) (49 cd/A, 23% EQE) by controlling the device charge balance. In this paper we demonstrated a Iridium(III)bis [(4,6-di-fluorophenyl)-pyridinato-N,C2] picolinate (FIrpic) based blue emitting micro-cavity PHOLEDs . Our results show that in addition to a slightly blue shift with spectral narrowing resulting in a more saturated blue color, there is also an enhancement in current efficiency. Micro-cavity blue PHOLEDs were fabricated on two different dielectric mirror substrates: two-layer quarter wave stacks (2QWS) with reflectivity of 0.39, and 4-layer quarter wave stacks (4QWS) with reflectivity of 0.7 at 475nm. The design of the dielectric stacks is to maximize the micro-cavity effect at 475nm. The devices have the following structure: glass substrate (1mm)/SiO2 (79nm)/TiO2 (48nm) or SiO2 (79nm)/TiO2 (48nm)/SiO2 (79nm)/TiO2 (48nm)/ITO (50nm)/ 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC) (45nm)/mCP (10nm)/ [1,4-bis(triphenylsilyl)benzene] (UGH-2) (20nm) doped with 20 wt % FIrpic/ tris[3-(3-pyridyl)-mesityl]borane (3TPYMB) (40nm)/LiF (1nm)/Aluminum (100nm). Non-cavity devices were also fabricated on ITO coated substrates without the SiO2/TiO2 dielectric stacks. We found that the micro-cavity devices show enhancement in current efficiency (61 cd/A compared with 49 cd/A in non-cavity device) in addition to a more saturated blue color in micro-cavity devices (CIE coordinates of x=0.12 and y=0.26 for the cavity device compared with x=0.15 and y=0.32 for non-cavity device). Furthermore, we also measured the amount of substrate guided modes and found that the 4QWS cavity device has the strongest substrate modes compared with the 2QWS cavity device and the non-cavity device. Simulation results also confirmed the experimental data and the substrate modes are coming from the secondary cavity mode at higher angles.
9:00 PM - B10.10
Spin Transport and Magnetoresistance in Organic Semiconductors.
Mohammad Yunus 1 , Paul Ruden 1 , Darryl Smith 2
1 Electrical and Compueter Engineering, University of Minnesota, Minneapolis, Minnesota, United States, 2 , Los Alamos National Laboratory, Los Alamos, New Mexico, United States
Show AbstractSmall molecules and polymers of π-conjugated organic semiconductors are of interest for the fabrication of spintronic devices, primarily because the charge carrier spin relaxation times are expected to be long and integration with extremely spin polarized (half-metallic) materials is feasible. Half-metallic contacts (or ferromagnetic contacts with spin selective tunnel barriers) can inject spin polarized charge carriers into organic semiconductors. Detection of the resulting spin polarization of the current is, in principle, possible through the measurement of the magneto-resistance of a spin valve device. Indeed, evidence for large MR effects in organic spin valves has been reported in the literature.[1,2] However, the absence of MR, spin injection, and transport in organic semiconductors has also been reported.[3] In this work, we model spin injection, transport, and magneto-resistance (MR) in structures consisting of an organic semiconductor layer sandwiched between two ferromagnetic contacts. We explore the different regimes of diffusion dominated and drift dominated current, the effects of contact polarization ranging from that of conventional ferromagnetic metals to half-metals, and the effectiveness of spin polarized tunnel injection and extraction for the observation of MR. Carrier transport in organic semiconductors is modeled with spin dependent device transport equations in drift-diffusion approximation. We find that half-metallic contacts can inject strongly polarized charge carriers, but that the resulting spin current does not necessarily manifest itself in a measureable MR. Small MR effects may be observable at low bias. Furthermore, even in the case of conventional ferromagnetic metal contacts, spin injection can be greatly enhanced if (spin dependent) tunneling is the limiting process for carrier injection. However, the spin current by itself does not give rise to measurable MR. In order to obtain a strong MR effect, we find that a second tunnel barrier at the extracting contact is necessary.* This work was supported in part by NSF-ECCS. [1] Z. H. Xiong, Di Wu, Z. Valy Vardeny, and J. Shi, Nature 427, 821 (2004). [2] S. Majumdar, H. Huhtinen, H. S. Majumdar, R. Laiho, and R. Österbacka, J. Appl. Phys. 104, 033910 (2008). [3] J. S. Jiang, J. E. Pearson, and S. D. Bader, Phys. Rev. B 77, 035303 (2008).
9:00 PM - B10.100
Self-Assembled Monolayers to Support the Growth of Inorganic Films
Diana Mars 1 , Shirin Usmani 1 , Andrew Ichimura 1
1 Chemistry & Biochemistry, San Francisco State University, San Francisco , California, United States
Show AbstractSelf-assembled monolayers (SAMs) provide a direct route to modify the interfacial chemistry between a metal surface such as gold to organic or inorganic films. SAM chemistry and formation on gold substrates is well-known and organo-sulfur compounds such as hexadecanethiol self-assemble to form densely packed quasicrystalline 2D arrays. Depending on the terminal functional groups, SAMs can be used to passivate a surface, control macroscopic surface properties such as wetting and friction, and block or permit charge transport across the film. Despite advances in SAM chemistry in recent years, considerable opportunities to develop new monolayers remain. For example, increasing the thermal stability of SAMs and tailoring the terminal group to specific add-layers would extend the range of applications. In this work, we have prepared SAMs through the reaction of SiCl4, TiCl4, P(O)Cl3, and P(S)Cl3 with densely packed -OH terminated monolayers. The result is a trithiolate or tripod SAM that has three thiolate bonds to the gold surface. The tripod SAM proves to have a higher thermal stability than single thiolate-Au bonds as measured by temperature programmed desorption (TPD) even for very short alkyl chains. By terminating the tripod SAM with a SiOH, TiOH, P=O, or P=S functional group, these films can be used as supports for the growth of inorganic films such as silica zeolites, TiO2, or other chalcogonide based films. The structures and properties of the monolayers were elucidated by fourier transform infrared (FTIR) spectroscopy, single wavelength ellipsometry (SWE), TPD, and density functional theory (DFT) and will be reported in this paper.
9:00 PM - B10.101
Solvent Induced Structural Transition in Continuous and Nanopatterned Pentacene Thin Films.
Aram Amassian 1 , Vladimir Pozdin 1 , Alexander Zakhidov 1 , Detlef Smilgies 2 , George Malliaras 1
1 Materials Science and Engineering, Cornell University, Ithaca, New York, United States, 2 Cornell High Energy Synchrotron Source, Cornell University, Ithaca, New York, United States
Show AbstractSolvent vapor annealing is increasingly used to induce structural and morphological changes in polymer systems with the goal of increasing intermediate and long range order. In this paper, we demonstrate that solvent vapor annealing can be used in small-molecule polycrystalline thin films of pentacene to control its structure and polymorphism. Using acetone solvent vapor we have transformed pentacene films which form in the “thin film” polymorph into the bulk phase. In situ time-resolved grazing incidence X-ray diffraction measurements suggest that the phase transformation occurs throughout the depth of the film. Closer inspection of diffraction patterns reveals smearing and “dissolution” of the lattice followed by formation of the bulk lattice, indicating a first-order phase transition. This enables fabrication of OTFTs with the bulk phase films of pentacene and makes it possible to compare charge transport measurement from the thin film and bulk polymorphs. This was previously impossible due to the fact that as-deposited pentacene films are dominated by the “thin film” phase. We have gone a step further and performed solvent-vapor annealing on nanopatterned pentacene films to investigate size effects and to produce arrays of well-ordered crystalline microdomains for transistor fabrication.
9:00 PM - B10.102
Novel Morphological Relationship between Field-Effect Mobility and Molecular Layer Population at the Semiconductor-Dielectric Interface.
Aram Amassian 1 , Tushar Desai 2 , Vladimir Pozdin 1 , Stefan Kowarik 4 , Sukwon Hong 2 , Arthur Woll 3 , Detlef Smilgies 3 , Frank Schreiber 4 , George Malliaras 1 , James Engstrom 2
1 Materials Science and Engineering, Cornell University, Ithaca, New York, United States, 2 School of Chemical and Biomolecular Engineering, Cornell University, Ithaca, New York, United States, 4 Institute für Angewandte Physik, Universität Tübingen, Tübingen Germany, 3 Cornell High Energy Synchrotron Source, Cornell University, Ithaca, New York, United States
Show AbstractWe report a novel relationship between the morphology of the semiconductor-dielectric interface and the field-effect mobility of top-contact organic thin film transistors (OTFTs). We have achieved a variation of the field effect mobility by 4 orders of magnitude (from 10-5 to 10-1 cm2-V-1-s-1) in small-molecule thin films of diindenoperylene (DIP) by controlling the morphology and molecular layer population of the DIP films near the semiconductor-dielectric interface. The morphological differences appear to be related to the degree to which molecular layers near the semiconductor-dielectric interface are populated during deposition of the molecular semiconductor. Control of layer population at the semiconductor-dielectric interface was achieved by depositing DIP films in a variety of processing conditions, including by vacuum sublimation, by supersonic molecular beam deposition, on bare SiO2 and on SiO2 treated with hexamethyldisilazane (HMDS). To measure variations in layer population, we have used a combination of in situ time-resolved X-ray reflectivity and ex situ non-contact atomic force microscopy. The histogram of surface height obtained independently from these two methods hold crucial clues about the morphology of the interfacial region and can be directly linked to the performance of top-contact OTFTs. In particular, the asymmetry or skewness of these distributions can be related directly to the transport properties of OTFTs. When the histograms are strongly skewed toward the interface (layer population at the interface is deficient), mobility is substantially lower than when the histogram is symmetric or skewed toward the surface (layer population at the interface is adequate). Remarkably, the lateral size of crystalline and morphological features does not appear to have a strong bearing on the performance of devices. Our study suggests that layer population near the semiconductor-dielectric interface may be influencing the degree of interconnectivity between crystallites, i.e., at grain boundaries.
9:00 PM - B10.103
Highly Soluble Polymer Containing Thiophene and Alkyl-substituted Fluorene for Organic Thin Film Transistors (OTFTs)
Jae Wan Jang 1 , Jin Uk Ju 1 , Peng Tao Kang 1 , Qinghua Zhao 1 , Soon-Ki Kwon 1
1 Polymer Engineering, Gyeongsang National University , JinJu Korea (the Republic of)
Show AbstractHighly soluble conjugated a new p-type copolymers (PBTADF and PD5TADF) were synthesized by the Suzuki coupling reaction. The newly designed copolymers are expected to have high mobility because long-linked thiophene and long alkyl chains can give a high charge density, π-stacking, and orderness. The monomers were prepared with highly overall yields. Chemical structures and optoelectronic properties of the copolymers were characterized by elemental analysis, 1H NMR, FT-IR, UV absorption, cyclic voltammetry (CV), and photoluminescence (PL). In the case of the PBTADF, the absorption maximum was observed at 455 nm for solution and 520 nm for film, respectively. The PL maximum was observed at 561 nm for solution and 610 nm for film, respectively. The thermal properties of copolymers were characterized by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The copolymers were showed 5% weight losing temperature at 407 oC for PBTADF and 419 oC for PD5TADF, respectively. The weight-average molecular weight of the copolymers were determined to be 41,202 for PBTADF and 29,322 for PD5TADF by gel permeation chromatography (GPC) using polystyrene standards for calibration in the eluent THF with polydispersity indexes of 1.66 for PBTADF and 1.29 for PD5TADF, respectively. The solubility of the copolymers were important property for conjugated polymers used in OTFTs. Especially, the copolymers were easy to be soluble in common solvents, such as chloroform, tetrahydrofuran (THF), toluene, chlorobenzene, and dichlorobenzene., this is due to the introduction of long alky chains.
9:00 PM - B10.104
Synthesis and Characterization of High-Performance Semiconductors based on Alkoxylnaphthyl for Organic Thin Film Transistors
Dong Hee Lee 1 , Qinghua Zhao 1 , Jong-Won Park 1 , Sung Ouk Jung 1 , Yun-Hi Kim 2 , Soon-Ki Kwon 1
1 , GNU, Jinju Korea (the Republic of), 2 department of chemistry, Gyeongsang national university, Jinju Korea (the Republic of)
Show AbstractOrganic semiconductors that consist of conjugated oligomers or polymers have attracted much interest in recent years, due to their potential applications in organic thin-film transistors (OTFTs). OTFTs can be fabricated at a low cost, over large areas, using flexible substrates. Over the past decade, many researchers have made great progress in increasing the performances (charge-carrier mobility, on/off ratio) of organic semiconductors for OTFTs, by designing new fused aromatic compounds. Suzuki et al. reported oligo(2,6-anthrylene) derivatives with a mobility of 0.18 cm2V-1s-1. Frisbie et al. demonstrated high mobility thiophene/acene hybrid semiconductors, showing anthracene end-capped oligomers with stable device performance with a mobility of up to 0.12 cm2V-1s-1. Moreover, µTFT values of up to 0.40 cm2V-1s-1 were reported for naphthyl end-capped quarterthiophene, 5,5’’’- bis(naphth-2-yl)-2,2’:5, 2’’:5’’, 2’’’-quaterthiophene (NaT4), by Tian et al. Recently, we reported thienothiophene end-capped new oligo acene derivatives, 2,6-bis(5’-hexyl-thieno[3,2-b]thiophen-2’-yl)anthracene (DH-TAT), which showed a hole mobility of 0.14 cm2V-1s-1 , and an on/off current ratio of 6.3 x 106. In this paper, we synthesized a series of new organic semiconductors, one of them showed a field effect mobility value higher than 0.50 cm2V-1s-1, on/off ratios greater than 7.5 x 105 along with a low threshold voltage of 4.4 V and subthreshold slope of 0.8 V/dec. Their properties of optical, thermal stability and device performance would be discussed further.
9:00 PM - B10.105
A Nanoengineering Path Towards All-printed DH4T-based Thin Film Transistors by Soft Lithography.
Dana Serban 1 , Alexandru Vlad 1 , Constantin Dutu 1 , Sorin Melinte 1 , Pierpaolo Greco 2 , Massimiliano Cavallini 2 , Fabio Biscarini 2 , Stefano Zacchini 3 , Maria Iapalucci 3
1 Electrical Engineering, Universite catholique de Louvain, Louvain-la-Neuve Belgium, 2 , Istituto per lo Studio dei Materiali Nanostrutturati, Bologna Italy, 3 Dipartimento di Chimica Fisica e Inorganica, Universita di Bologna, Bologna Italy
Show AbstractOrganic semiconductors have received utmost attention recently due to their increasing role in emergent electronic devices and optoelectronics. As the deposition method dictates key physical characteristics of the semiconductor such as molecular packing and charge carrier mobility, various techniques have flourished in the quest of fulfilling various performance requirements. Yet, the need of innovative approaches should not be limited to the semiconductor deposition, but ought to also touch other transistor components.Here we report on a simple and versatile method of fabricating dihexylquaterthiophene (DH4T)-based coplanar bottom-gate thin film transistors by means of soft lithography. Namely, microinject molding in capillaries (MIMIC) [1] has been used by turns to define the electrodes and the active layer. In a first set of experiments, metallic stripes - 3 mm long spaced by 200 µm - acting as source and drain have been deposited from Pt salts [2] on heavily-doped Si wafers bearing 200-nm-thick silicon dioxyde dielectrics. Electrical conductivity measurements of the Pt MIMIC stripes have been consistently reproduced upon several voltage-sweep cycles. Finally, the devices were completed by drop-casting a solution of DH4T diluted at 0.2% wt. in tetrahydrofuran. On the other hand, we have employed the MIMIC approach to pattern the DH4T on top of conventional transistors [3]. These devices have electron-beam evaporated Pd source and drain electrodes, channel width of 1 mm and length of 10 µm. We formed the active layer by depositing the DH4T using a poly(dimethylsiloxane) stamp accommodating 800-nm-wide channels. Samples have been analyzed by Polarized Optical Microscopy, Tapping-Mode Atomic Force Microscopy and X-Ray Reflectometry. They uncovered the formation of multidomains with preferential orientation along the long axis of the stripes, as well as the organization in (32Å-high) monomolecular terraces [3]. Electrical characterisation of the transistors revealed the key role of the nanostructuration: higher field effect mobilities are achievable when the semiconductor is deposited by MIMIC with respect to homogeneous films. We explain the improved mobility by a preferential charge transport taking place inside the stripes, as their increased internal order provides a better pathway for the hole carriers.Our findings that nanoengineered transistors exceed in performance those fabricated by standard approaches are very encouraging in the light of fabricating fully-patterned devices from solution processable materials.[1] E. Kim, Y. Xia, G. M. Whitesides, Nature 1995, 376, 581; M. Cavallini and F. Biscarini, Nano Lett. 2003, 3, 1269.[2] P. Greco et al., J. Am. Chem. Soc. 2008, 130, 1177.[3] D. A. Serban et al., Appl. Phys. Lett. 2008, 92, 143503.
9:00 PM - B10.106
Ultrathin High Efficiency Solar Cell Based on Vapor Deposited Squaraine Dye.
Siyi Wang 1 , Elizabeth Mayo 2 , Dolores Perez 1 , Laurent Griffe 1 , Guodan Wei 2 , Brian Lassiter 2 , Peter Djurovich 1 , Stephen Forrest 2 , Mark Thompson 1
1 Chemistry, university of southern california, Los angeles, California, United States, 2 Electrical Engineering and Computer Science, Physics, Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan, United States
Show AbstractSquarains are a family of dyes with sharp and intense absorption in solution, which exhibit panchromatic absorption in the solid state. Squaraines have highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO) energy levels comparable to the common OPV donor material copper phthalocyanine (CuPc). 2,4-Bis [4-(N,N-diisobutylamino)-2,6-dihydroxyphenyl] squaraine (SQ1) is demonstrated to be a successful donor like material in organic heterojunction photovoltaics. Optimal SQ1 device is fabricated as ITO/SQ1 /C60 /BCP /Al, which generates high short circuit current Jsc of 6.54 mA/cm2, and open circuit voltage Voc of 0.80 V , resulting in power conversion efficiency η of 3.4 %, at 1 sun incident irradiation (AM1.5). Compared with the 2% power conversion efficiency of our conventional CuPc device [ITO/CuPC (400 Å)/C60 (400 Å)/BCP (100 Å)/Al (1000 Å)]. It is possible to tune the absorption spectra of squaraines to near infrared region by varying the substituent groups. The demonstration of sublimable SQ1 as a donor material opens up a new family of compounds for small molecular heterojunction photovoltaics.
9:00 PM - B10.108
Towards Greatly Improved Efficiency of Polymer LED.
Zhang-Lin Zhou 1 , Lihua Zhao 1 2 , James Brug 1 , Xia Sheng 1 , Gary Gibson 1 , Sity Lam 1 , K. Nauka 1
1 , Hewlett Packard Labs, Palo Alto, California, United States, 2 , University of California, Berkeley, California, United States
Show AbstractOrganic/polymer light-emitting diodes (OLEDs) show great promise of revolutionizing display technologies. Hence, these devices and the materials that render them functional are the focus of intense scientific and technological interest. The archetypical multilayer OLED heterostructure introduces numerous chemical and physical challenges to the develoment of efficient and robust devices. This requires placing a nanoscale feature at desired sites within the thin film stack. These layered structures are formed from solution based spin-casting or printing with subsequesnt removal of the solvent. However, solvent from the freshly deposited film can dissolve or partially dissolve the underlying layer resulting in loss of the desired structure and corresponding device functionality. Undesirable redistribution of the nanoscale features within the polymer is another detrimental effect associated with solvent removal. In this paper, we will describe a new solution to the above problem by embedding hole transporting materials (HTLs) in a cross-linked polymer matrix. We have successfully demonstrated this technique to deposit the HTL in our OLED device. In this device, the HTL is deposited together with cross-linkable monomers on the HIL that is pre-deposited over the patterned ITO anode. The next layer for the stacking process is EML (PFO) that is followed by the deposition of the ETL. Since solvents that are used for EML polymer are commonly shared by the under-layer HTL polymer, embedding the HTL molecules into the inert cross-linked polymer network is a good way to minimize the undesirable impact from solvents that are used by the EML polymer. There are many options that could be employed to form the cross-linked inert polymer network: a mixture of cross-linkable monomer, oligomers, and polymers, in addition to cross-linking agent and an initiator. The cross-linking agent could be a 2-branch, 3-branch, or 4-branch cross-linker. Cross-linking could be activated using appropriate energy sources such as UV-exposure or thermal process. Device fabrication together with the total luminance and external quantum efficiency of such devices will be presented.
9:00 PM - B10.109
Hysteresis Mechanism and Reduction in the Bis(triisopropylsilylethynyl)Pentacene Thin Film Transistors with Polymer Blend, Cross-linked Poly(vinyl phenol).
Jin Young Oh 1 , Young Bum Yoo 1 , Man Hyeop Han 1 , Byeong har Hwang 1 , Hyeon Seok Hwang 1 , Joo Hee Kim 2 , Hong Ku Baik 1
1 material science, Yonsei university, seoul Korea (the Republic of), 2 chemistry, Iwha woman university, seoul Korea (the Republic of)
Show AbstractWe have studied the electrical stability of cross-linked Poly(Vinyl Phenol) dielectric and active layer with polymer blend for the Bis(triisopropylsilylethynyl)Pentacene[tips-pentacene] thin film transistors. the hysteresis occurs mainly due to the dielectric surface -OH group and the rough surfaces which are induced by trap sites at the interface in OTFTs. In order to reduce hysteresis, we used an OH free and smooth dielectric and active layer with Polymer blend(PaMS). As a result, Increased mobility and decreased hysteresis are observed in Tips-pentacene thin film transistors with polymer blend(PaMS) and PVP dielectric.
9:00 PM - B10.11
Novel Nanocomposites of Covalently Bonded Multi-Wall Carbon Nanotubes to Conducting Polymers
Yeong Kim 1 , Changhyun Park 1 , Jae Song 2
1 Chemistry, Chungnam National University, Daejeon Korea (the Republic of), 2 Chemistry, Sunchon National University, Sunchon Korea (the Republic of)
Show AbstractWe describe here a synthesis of novel nanocomposites of covalently bonded multi-wall carbon nanotubes (MW-CNT) to conducting polymers such as polypyrrole. The surface modification of MW-CNT was done by the reaction with bis(2-picolyl)amine after oxidation. The Fe(III) ions were able to coordinate to the surface functionalized MW-CNT and were used as in-situ catalysts for the polymerization of pyrrole. According to TEM images, polymers cover the MW-CNT very uniformly and the width of resulting nanocomposite can be controlled by the reaction conditions. We will discuss the physical properties of novel nanocoposites.
9:00 PM - B10.110
π-Conjugated Molecules based on Thiophene Derivatives as Organic Semiconductors for Thin Film Transistor Applications.
Ki Hwa Jung 1 , kyung Hwan Kim 1 , Min Ju Cho 1 , Dong Hoon Choi 1
1 , Korea university, Seoul Korea (the Republic of)
Show AbstractOrganic semiconductor materials based on extended linear-conjugated systems have been very intriguing and significant development has been achieved in these materials over the one decade. In the exploration of the application as organic semiconductors in organic field effect transistor (OFET) is an important component for developing future flexible displays. Accordingly, a number of researchers have attempted to synthesize π-conjugated small molecules, dendrimers, oligomers, and polymers because of their strong potential applications to electronics and optoelectronics such as in organic light-emitting diodes (OLEDs), organic field effect transistors (OFETs), and photovoltaic cells. In organic semiconductors, the intrinsic carrier mobility depends critically on the degree of molecular orientation and on the extent of the intermolecular interaction. In this study, new conjugated crystalline cruciform molecules have been synthesized through the Sonogashira coupling reaction. Introduction of thiophene-based peripheral moiety into an aromatic core improve the solubility in organic solvent for facilitating the solution device fabrication. They display a p-type semiconducting behaviors and their electrical properties are investigated in detail. We investigated the UV-visible spectroscopy, photoluminescence spectroscopy, cyclic voltammetry, thermogravimetric analysis and differential scanning calorimetry properties of the new π-conjugated molecules. Finally, we fabricated thin film transistor to investigate the carrier mobility and its photosensitivity under light illumination.
9:00 PM - B10.112
Fused Aromatic Thienopyrazine Copolymers for Photovoltaic and Thin Film Transistor Applications.
Rajib Mondal 1 , Nobuyuki Miyaki 1 , Hector Becerril 1 , Jack Parmer 2 , Michael McGehee 2 , Zhenan Bao 1
1 Chemical Engineering, Stanford University, Stanford, California, United States, 2 Materials Science and Engineering, Stanford University, Stanford, California, United States
Show AbstractDevelopment and tuning the fundamental properties of polymeric semiconducting materials have become an active area of research in recent years due to their potential uses in light weight and flexible optoelectronic devices, such as organic photovoltaics (OPVs) and organic thin film transistors (OTFTs). In this regard, several fused aromatic thienopyrazine (TP) copolymers were synthesized and investigated. The fused aromatic TP unit provides a planar and rich π-face, which promotes the π-π stacking between the polymeric chains and enhances the device efficiencies. The electronic properties of these polymers can also be tuned significantly using different aromatic group used in TP unit. For example, band gap of TP-fluorene copolymer was reduced to ~1.4 eV from ~1.7 eV by replacing phenanthrene from acenaphthyl. Various alkyl chains were used at different positions of the polymers to achieve the morphological control. Power conversion efficiency more than 1% in OPV devices and hole carrier mobility of ~0.2 cm2/Vs in OTFT devices have already been achieved using some of these polymers. Further optimization of the newer polymers is currently underway.
9:00 PM - B10.114
Synthesis of Donor-Acceptor Diblock Copolymer Based on Regioregular Poly(3-hexylthiophene) and Fullerene and Its Use as a Compatibilizer in P3HT/PCBM Bulk Heterojunction Solar Cells.
Jea Uk Lee 1 , Jae Woong Jung 1 , Won Ho Jo 1
1 Materials Science and Engineering, Seoul National University, Seoul Korea (the Republic of)
Show AbstractAmong various organic photovoltaics, the conjugated polymer/fullerene bulk heterojunction system is one of the most promising ones for achieving high performance of polymer photovoltaics. Recently, it has been reported that thin-film bulk heterojunction solar cells fabricated by simple blending of regioregular poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) show high power conversion efficiency. Despite remarkable recent progress, the bulk heterojunction system still has some problems to be solved for commercialization. The most serious problem is that this approach is largely dependent upon the solid state phase morphology of the two components (donor/acceptor) in the photoactive layer because the exiton diffusion length is limited within 10 nm. Although nanometer scale phase separation of donor/acceptor blend can be attained through optimized device conditions in laboratory (specific blending ratio, very short thermal annealing time), exposure to sun light for long time will cause macrophase separation in the blend of conjugated polymer and fullerene derivatives. The macrophase separation may limit charge separation and thus lower the power conversion efficiency in the photovoltaic device.For the purpose to solve this problem, in this study a novel diblock copolymer composed of regioregular poly(3-hexylthiophene) and fullerene derivates (P3HT-b-C60) was synthesized and used as a compatibilizer for P3HT/PCBM bulk heterojunction system. It has been well known that a proper compatibilizer can reduce the domain size of phase separation and also retard the rate of phase separation in the blend system. With varying the fabrication conditions, the change of domain size and the power conversion efficiencies of P3HT/PCBM/P3HT-b-C60 blends were measured. First, to examine the effect of compatibilizer on the performance of heterojunction solar cells, P3HT/PCBM bulk heterojunction devices were fabricated with varying the amount of P3HT-b-C60 compatibilizer. Second, the effect of the thermal annealing time and temperature on the performance of P3HT/PCBM/P3HT-b-C60 bulk heterojunction solar cells was investigated. Third, the solvent annealing (slow drying) method was also applied to the P3HT/PCBM/P3HT-b-C60 blend in order to obtain the desirable morphology.AFM and TEM images of blend films with compatibilizer exhibited uniform morphology with finer domain size after annealing as compared with the blends without compatibilizer, when a small amount of compatibilizer was added to the blend. The power conversion efficiency of blend with compatibilizer was improved as compared with the blend without compatibilizer. Particularly, the short circuit current (Jsc) of the blend with the compatibilizer was higher than the blend without the compatibilizer due to improvement of phase morphology of P3HT/PCBM/P3HT-b-C60 blend.
9:00 PM - B10.115
High Electron Carrier Mobility Electron Transport Materials for OLED Devices
Heh-Lung Huang 1 , Teng-Chih Chao 1 , Mei-Rurng Tseng 1
1 , Industrial Technology Research Institute, Chutung, Hsinchu Taiwan
Show AbstractWe developed a series of new electron transport materials. These materials contain heterocyclic related core structure and with other electron withdrawing group. The electron mobility of thermal deposition for these materials are greater than 10-4 cm2/Vs. We are now investigating the effect of new electron transport materials for the OLED devices and lifetime. Currently, the current efficiency and brightness of green phosphorescent OLED devices with new electron transport materials are 46.0 cd/A and 82123 cd/m2 respectively. We are still investigating and optimize the OLED devices with those new electron transport materials. With the increasing mobility of the new electron transport materials, we can apply on the flexible plastic substrate to fabricate the flexible OLED device.
9:00 PM - B10.116
Multilayer Formation Using a Lamination Process Toward Efficient Polymer Solar Cells
Keisuke Tajima 1 , Motoshi Nakamura 1 , Erjun Zhou 2 , Chuhe Yang 2 , Kazuhito Hashimoto 1 2
1 Department of Applied Chemistry, The University of Tokyo, Tokyo Japan, 2 HASHIMOTO Light Energy Conversion Project, Exploratory Research for Advanced Technology (ERATO), Japan Science and Technology Agency (JST), Tokyo Japan
Show AbstractPolymer solar cells (PSCs) are drawing much attention these days because of their potentials for the production of flexible and large-area solar cells at dramatically low costs. In the fabrication method commonly used now, however, an metal electrode is deposited on the polymer layer by vacuum evaporation, which is both costly and time-consuming process. Furthermore, it is suggested that the device performance and stability is detracted by physical or chemical damage to the organic active layer form the evaporated metals.In this presentation, first we report the use of a lamination method to fabricate the PSCs. Two different polymer layers are coated on conductive substrates (ITO covered with TiO2 and Au with PEDOT:PSS) and the substrates were simply laminated by applying pressure (2-6 MPa) and heat (100-150 °C) to fabricate the devices. By this simple method, we obtained physically and electronically connected interface between the polymer layers. The photovoltaic performances of the devices with P3HT:PCBM bulk heterojunction are compared to those of the conventional devices with the electrode deposited by vacuum evaporation. As the result, the two devices showed almost identical J-V curves, and the PCE reached 3.3% in both devices under the AM1.5 irradiation. This result opens up the possibility of low-cost mass production of the PSC through a roll-to-roll lamination system. Since we can apply this method to the heterointerface of the semiconducting polymers, it is possible to enhance the light absorption of the PSCs by combining the two polymer layers with different absorption ranges. For this purpose, a novel low band gap polymer, PDTPDTBT was synthesized with an alternating structure of N-substituted dithieno[3,2-b:2’,3’-d]pyrrole (DTP) as a new donor, and 4,7-dithien-2-yl-2,1,3-benzothiadiazole as an acceptor group. The new polymer has a low optical band gap (1.46 eV) and broad absorption with peaks at 451 and 697 nm in the film, which is complementary to the absorption of P3HT. Preliminary results showed that the power conversion efficiency of bulk heterojunction type PSC with PDTPDTBT reached 2.18% in combination with PCBM. The combination of P3HT and PDTPDTBT layers using the lamination method gave the devices that respond to the broad range of the solar spectrum. Further optimization of the layer components would give us a possibility to reach higher conversion efficiency.[1] Nakamura, M.; Tajima, K.; Yang, C.-H.; Hashimoto, K. Control of the Active Layer/Metal Electrode Interface Using a Lamination Process toward Efficient Polymer Solar Cells. Submitted.[2] Zhou, E.J.; Nakamura, M.; Nishizawa, T.; Zhang, Y.; Wei, Q.S.; Tajima, K.; Yang, C.H.; Hashimoto, K. Synthesis and Photovoltaic Properties of A Novel Low Band Gap Polymer Based on N-substituted dithieno[3,2-b:2’,3’-d]pyrrole. Macromolecules in press.
9:00 PM - B10.117
Synthesis and Molecular Orientation Control of Oligo(p-phenylenevinylene) Derivatives for the Application to Organic Photovoltaic Devices.
Keisuke Tajima 1 , Takeshi Nishizawa 1 , Hady Kesuma Lim 1 , Kazuhito Hashimoto 1 2
1 Department of Applied Chemistry, The University of Tokyo, Tokyo Japan, 2 HASHIMOTO Light Energy Conversion Project, Exploratory Research for Advanced Technology (ERATO), Japan Science and Technology Agency (JST), Tokyo Japan
Show AbstractNanoscale morphology of the donor and acceptor is of high significance to achieve a high efficiency in the organic bulk heterojunction (BHJ) solar cells. Namely, large interface and interpenetrating network of the donor and acceptor realize the efficient charge separation and transport in the organic film, respectively. In the solution-processed BHJ solar cells, formation of such a nanostructure is simply achieved by spin-coating and thermal annealing and it is affected by many factors such as solvent, donor/acceptor mixing ratio, character of the materials used, etc. Important characters of the materials are crystallinity and molecular orientation that facilitate the formation of the network to achieve the efficient charge transport.In this presentation, we first report the synthesis of novel oligo(p-phenylenevinylene)s (OPV) to demonstrate the effect of the crystallinity of the donor on the morphology and solar cell efficiency in the OPV and fullerene derivative (PCBM) BHJ solar cells. The crystallinity of the OPVs is tuned by changing the alkyl side chain length. AFM images revealed that the film with highly crystalline OPV showed a significantly different morphology and prevented the large PCBM aggregation that was observed in the films with the lower crystalline OPVs. The solar cell with the highly crystalline OPV showed the improvement in the short circuit current and the fill factor. These results suggest that the highly crystalline OPV formed the donor network and prevented the PCBM aggregation, resulting in the improved efficiency in the solar cells.Similarly, the crystallinity of the donor group in the donor-acceptor dyad is also of high importance to achieve a high efficiency in the solar cells [1, 2]. We also report the synthesis of novel oligo(p-phenylenevinylene)-fullerene dyads and their solar cell efficiency. The strong pi-pi interaction in the highly crystalline donor groups enhances the intermolecular charge hopping, resulting in the improved fill factor and efficiencies over 1%.Finally, highly uniaxial alignment of the OPVs were demonstrated in the as-cast films spin-coated from the solutions on mechanically rubbed polymer alignment layers such as poly(3,4-ethylenedioxythiophene):poly(styrene sulfonic acid) (PEDOT:PSS) films. High dichroic ratio (Ap / An) of 41.0 and order parameter ([An - Ap] / [An + 2Ap]) of 0.93 were estimated from the polarized absorption spectra of the films. The molecular orientation was also investigated by in-plane X-ray diffraction measurements, revealing the highly uniaxial alignment of the molecules with its long axis parallel to the rubbing direction. This spontaneous alignment could be a new method useful to control the electronic/optoelectronic properties of the semiconducting materials in the thin films.[1] T. Nishizawa, K. Tajima, K. Hashimoto, J. Mater. Chem. 2007, 17, 2440–2445.[2] T. Nishizawa, K. Tajima, K. Hashimoto, Nanotechnology, 2008, 19, 424017.
9:00 PM - B10.118
Solution Processed High Mobility Tetrathienoacene Polymer Semiconductors with a Long Shelf Life in Ambient Environment.
H. H. Fong 1 , Vladimir Pozdin 1 , Aram Ammassian 1 , George Malliaras 1 , Mingqian He 2 , Susan Gasper 2 , Feixia Zhang 2 , Michael Sorensen 2
1 Materials Science and Engineering, Cornell University, Ithaca, New York, United States, 2 , Corning Incorporated, Corning, New York, United States
Show AbstractWe demonstrate that high performance, soluble polymeric semiconductors can be achieved by increasing the rigidity of the thiophene monomer through the use of an alkyl-substituted core that consists of four fused thiophene rings. We report on a member of the family of di-alkylated tetrathienoacene copolymers, namely poly(2,5-bis(thiophene-2-yl)-(3,7-ditridecanyltetrathienoacene) (P2TDC13FT4), that can be deposited from a 1,2-dichlorobenzene solution into highly ordered films with a field-effect hole mobility exceeding 0.3 cm2/V/s. Devices prepared on vapor phase deposition of silane based oxide gate oxide show a remarkably long life-time of one year in ambients, with no significant change on the performance.We show that di-alkylated tetrathienoacene copolymers represent a new class of organic semiconductors that can be easily processed from solution into ordered, high mobility thin films. The straightforward processing and the relatively low temperatures involved make them particularly attractive for the development of electronics on plastic substrates.
9:00 PM - B10.119
Photolithographic Micropatterning of Organic Electronic Materials.
H. H. Fong 1 , Jin-Kyun Lee 1 , Alexander Zakhidov 1 , Priscilla Taylor 1 , John DeFranco 1 , Christopher Ober 1 , George Malliaras 1
1 Materials Science and Engineering, Cornell University, Ithaca, New York, United States
Show AbstractWe developed a class of organic electronic materials that can be processed using fluorinated solvents. These materials can function as imaging materials for photolithographic applications and light-emitting applications for full color display.We demonstrate a critical technique of patterning multi-layer organic devices by different orthogonal development media including hydrofluoroethers and supercritical carbon dioxide. Small molecules and polymer materials can be patterned on top of different substrates such as metal, transparent conducting oxide, and even polymer, with a spatial resolution down to a micron. Using supercritical carbon dioxide, we fabricated a patterned polymer light-emitting diode. A molecularly-doped polymer light emitting diode is patterned on top of Poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS). This green-emitting device exhibits an efficiency of 22 cd/A and 8 lm/W at 100 cd/m2, which is compatible to the typical device performance. Using hydrofluoroethers as development media, we developed a novel photosensitive material that allows us to pattern traditional organic electronic materials without any damage. A stack structure of patterned multi-layers is demonstrated. Furthermore, P3HT and pentacene transistors with patterned Au top contacts using lift-off method are demonstrated with decent performance.We also fabricate a class of highly fluorinated red, green, and blue light-emitting materials that can be only processed using fluorinated solvents. These materials show superior stability in common organic solvents including xylene, toluene, acetone, isoproponal and chlorinated solvents. Preliminary results show a current efficiency of >5 cd/A. We can therefore pattern these solution processed light-emitting materials using traditional photolithography under aqueous based media and further aim for fabricating RGB pixels.
9:00 PM - B10.12
Self-assembled Nanostructured Molecular Memory
Hyoyoung Lee 1
1 , ETRI, Daejeon Korea (the Republic of)
Show AbstractFor the realization of nano-structured molecular monolayer devices, it is essential to synthesize and characterize active switching and memory materials that have hysteretic I-V characteristics in the solid state, develop nano-sized conducting organic electrodes and patterning of active memory materials on the nano-sized array electrodes.In this presentation, we like to introduce thiol-terminated metal (Ru, Co, and Fe)-terpyridine, metal-to-ligand charge transfer (MLCT) complexes for molecular switches and memory devices. Bis(terpyridine)-transition metal complexes exhibit superior chemical and electronic stability toward redox reactions due to their octahedral configuration in coordination.[1] We like to also introduce how to improve molecular device yields using self-assembly technique.[2] For patterning-of organic electrodes, we like to introduce a self-assembly process for an immobilization of the conducting polymer materials.[2] Finally we like to present nano-patterning for molecular memory array devices using nanoimprint lithography.[3] References1.Seo, K., Konchenko, A. V., Lee, J., Bang, G. S., Lee, H. Molecular Conductance Switch-On of Single Ruthenium Complex Molecules. Journal of the American Chemical Society, 130, 2553–2559 (2008).2.Bang, G. S., Chang, H., Koo, J.-R., Lee T., Advincula R., Lee, H., The high-fidelity formation of molecular junction device using a thickness controlled bilayer architecture, Small, 4, 1399-1405 (2008).3.Jung, M.-H., Lee, H., Patterning of conducting polymers using charged self-assembled monolayers, Langmuir, 24, 9825-9831 (2008).
9:00 PM - B10.120
Charge Injection in Polyfluorene Films using High Conductivity Thiophene Copolymers.
H. H. Fong 1 , Arunabh Batra 1 , George Malliaras 1
1 Materials Science and Engineering, Cornell University, Ithaca, New York, United States
Show AbstractFluorene-based copolymer is considered to be one of the most promising hole transporting and blue light-emitting conjugated polymers used in polymeric light-emitting diodes (PLEDs) due to its high-lying highest occupied molecular orbital (HOMO) of arylamine moiety and high carrier mobilities. It is therefore desirable to boost up the charge injection.Our work attempts to provide a comprehensive understanding on charge transport of fluorine-arylamine copolymers and examine new approaches to improve carrier injection. Time-of-flight (TOF) technique has been employed to evaluate the charge drift mobility at the thick film regime (1-10 micron) and shows the polymer possesses superior mobility of > 0.01cm2V-1s-1. We examine the hole injection in the polyfluorenes using chemically doped thiophene copolymers. Furthermore, the effect using molecular additives on tuning the conductivity of these thiophene polymers is also investigated. Results show a significant improvement of charge injection efficiency by tuning the conductivity of the doped thiophene hole injecting layer. Moreover, the charge injection in this polyfluorene with high-lying HOMO can be further enhanced by using vapor phase polymerization of thiophene contained monomers that increases the injection efficiency to ~ 0.1 for a 100nm polyfluorene film.Results will shed light on the enhancement of device efficiency and stability in the future polymer electronic devices.
9:00 PM - B10.121
Spin-Cast High-κ Non-Hysteresis Barium Titanate Nanoparticle Thin Film as Gate Dielectric in Organic Thin Film Transistors.
Zhang Jia 1 , Limin Huang 1 , Stephen O'Brien 1 , Ioannis Kymissis 2
1 Applied Physics and Applied Mathematics, Columbia University, New York, New York, United States, 2 Electrical Engineering, Columbia University, New York, New York, United States
Show AbstractWe demonstrate the usage of spin-cast 8nm-diameter high-κ BaTiO3 nanocrystals as gate dielectric with parylene C coating as surface treatment in the fabrication of high-performance pentacene organic field effect transistors (OFETs). Thanks to the extremely small size of BaTiO3 nanocrystals, the hysteresis seen in bulk material is quenched. This structure enhances the OFETs performance while retaining all the processing advantages of organic semiconductors: low temperature processing (<60 Celsius), low cost, applicability for large area casting, spraying and printing on various substrates, etc. Pentacene mobility in the linear region was enhanced from ~0.03 cm2/(Vs) at gate bias -20V in Parylene OFETs to ~0.35 cm2/(Vs) at gate bias -20V in BaTiO3/Parylene OFETs under identical conditions, resulted from increased concentration of accumulated carriers in the latter.
9:00 PM - B10.122
Photocurrent Study of Traps States in Pentacene Thin Film Transistors.
Zhang Jia 1 , Laura Banu 2 , Ioannis Kymissis 2
1 Applied Physics and Applied Mathematics, Columbia University, New York, New York, United States, 2 Electrical Engineering, Columbia University, New York, New York, United States
Show AbstractWe demonstrate the gate-bias dependent and wavelength dependent photocurrent study of both UV-ozone treated and air-free pentacene transistors with bottom gate top contact structure. Our unique facility enables us to fabricate the pentacene transistors without the exposure to air in any phase of the fabrication process. Traps states are intentionally introduced by treating the dielectric surface with UV ozone before the pentacene deposition. The photocurrent measurement is carried out utilizing chopped excitation with wavelength between 300~800nm and a lock-in amplifier with the frequency locked to the chopping frequency. The wavelength dependent photocurrent spectrum shows extra peaks from the UV-ozone sample in the range of 350nm to 470nm, which correspond to energy transitions larger than the pentacene HOMO-LUMO gap. This indicates extra trap states are introduced by UV-ozone treatment. The gate dependent photocurrent spectrum shows the photocurrent intensity is proportional to the square root of source-drain current at various gate bias in the saturation region. It also shows that the UV-ozone treated samples have a positively shifted threshold voltage, which is confirmed by direct electrical measurements. A more detailed transport model is proposed based on the trap and release model and these photocurrent studies.
9:00 PM - B10.13
Material Design of Triarylamine-Based Amorphous Polymers for Organic Field-effect Transistors
Takeshi Yasuda 1 2 , Takao Suzuki 3 , Mitsuru Takahashi 3 , Tetsuo Tsutsui 4
1 Innovative Materials Engineering Lab, National Institute for Materials Science, Tsukuba, Ibaraki Japan, 2 , PRESTO-JST, Tokyo Japan, 3 Nanyo Research Laboratories, Tosoh Corporation, Syunan, Yamaguchi Japan, 4 Institute for Materials Chemistry and Engineering, Kyushu University, Kasuga, Fukuoka Japan
Show AbstractThere has been a recent surge of interest in organic semiconductors with high carrier mobility through ordering at the molecular level, as in single-crystalline, polycrystalline, and liquid-crystalline films. However, thin films of organic semiconductors with an amorphous state have some merits compared to those with the ordered state mentioned above. Most importantly, morphological effects, which often lead to a poor reproducibility for the performance of organic field-effect transistors (OFETs) using polycrystalline or liquid-crystalline films, are not a major consideration. In this study, we have designed and synthesized twenty eight kinds of polytriarylamine (PTAA)1)-based amorphous semiconductors for OFETs. The triarylamine-based polymers which we developed in this study have different side-alkyl chain systems and conjugated main-chain each.
The molecular weights with polydispersity index, the glass transition temperature, HOMO energy levels and the optical bandgap were estimated as the physical and optical properties of polymers. Effects of the molecular design on the performance of OFETs were also investigated. We fabricated OFETs having a top contact geometry using amorphous polymers. The polymers as organic semiconductors were spin coated from toluene solution onto a gate insulator of poly-chloro-p-xylylene (Parylene-C). A shadow mask was attached onto the film to form Au source-drain electrodes. The field-effect mobility was calculated from the saturation drain currents.
All triarylamine-based polymers in this study exhibited p-channel behaviors in OFETs and filed-effect hole mobilities were ranging from 10-6 to 10-3cm2/Vs. For example, OFETs using a polymer with anthracene unit in the conjugated main-chain showed a field-effect hole mobility of 6.1×10-6cm2/Vs. On the other hand, a compound having naphthalene unit in the conjugated main-chain exhibited 1.6×10-4cm2/Vs. This study on the field-effect hole mobilities in synthesized twenty eight kinds of triarylamine-based polymers provides a good start point for molecular design of amorphous organic semiconductors for OFETs.
1) J. Veres, S. D. Ogier, S. W. Leeming, D. C. Cupertino and S. M. Khaffaf, Adv. Funct. Mater. 13 (2003) 199
9:00 PM - B10.14
Fabrication of Pentacene Organic Thin Film Transistor with a Trapezoid-Shaped Active Layer for Large Driving Drain Current.
Min-Hoi Kim 1 , Jin-Hyuk Bae 1 , Won-Ho Kim 1 , Chang-Min Keum 1 , Sin-Doo Lee 1
1 Electrical Engineering, Seoul National University, Seoul Korea (the Republic of)
Show AbstractOver the last decade, organic thin-film transistors (OTFTs) have attracted much attention for a variety of low cost and large area electronic applications including chemical sensors, flat panel displays, and smart cards. Although a pentacene based top contact OTFT shows good carrier mobility comparable to an amorphous silicon based TFT, a drain current is still low for realizing practical applications. In order to enhance the drain current in the top contact OTFT, reduction of the channel length is one of simple ways since the drain current is inversely proportional to the channel length. In principle, for a top contact OTFT, it is known to be difficult to fabricate a short channel without deteriorating the organic active layer during patterning the channel by lithography. In this work, we fabricate a top contact OTFT with trapezoid-shaped active layer for a large drain current by using a short channel. Our approach is to use a trapezoid-shaped structure of the organic active layer tilted from the substrate. The channel length in our trapezoid-shaped OTFT is reduced as short as 1.5 um, which is 30 times smaller than the channel length (typically, about 50 um) in a shadow mask-processed top contact OTFT with the help of a ladder support. Note that the channel in our trapezoid-shaped OTFT is self-patterned by slanted deposition without a shadow mask. Due to the short channel, a drain current per channel width in our trapezoid-shaped OTFT is found to reach at about 100 uA/mm, which is at least 10 times larger than that of the conventional top contact OTFT. Furthermore, our trapezoid-shaped OTFT is operated at a drain voltage as low as -3V. In conclusion, we fabricated a short channel trapezoid-shaped OTFT for a large drain current. By slanted deposition, the short channel length of our trapezoid-shaped OTFT was naturally produced. Our concept of using a ladder support in the OTFT would be a viable platform for realizing active driving elements in a variety of organic electronic systems that require large drain currents in a low voltage regime.
9:00 PM - B10.15
In Depth Investigations of State of the Art Molecules used to Tune Surface Properties.
Gerold Rangger 1 , Oliver Hofmann 1 , Anna Track 1 , Ferdinant Rissner 1 , Lorenz Romaner 1 2 , Benjamin Broeker 3 , Ralf-Peter Blum 3 , Norbert Koch 3 , Egbert Zojer 1
1 Institute of Solid State Physics, University of Technology Graz, Graz Austria, 2 Department of Materials Physics, Montanuniversität Leoben, Leoben Austria, 3 Institut für Physik, Humboldt-Universität zu Berlin, Berlin Germany
Show AbstractThe deposition of organic (sub)monolayers on metals is of particularly interest for modifying surface properties, such as carrier injection barriers, for applications like chemical sensing, and in the field of molecular electronics. Here, we present a joint theoretical and experimental study of a number of prototypical acceptor molecules, such as, for example, F4TCNQ. In the experiments, pronounced changes in the UPS spectra and an increase of the surface work functions are observed. Density-functional theory based band-structure calculations as well as X-ray standing wave experiments reveal a heavily distorted adsorbate structure. The modeling provides also additional information on the intrinsic properties of the interfaces. For example, employing certain projection techniques reveals that the experimentally observed (partial) filling of the molecular π-LUMO is to a significant extent compensated by back-transfer from localized sigma-states. With the help of the crystal orbital overlap population (COOP) we identify especially favored substituents that act as docking groups and, thus, are of particular relevance when designing new molecules for surface treatments. In addition to varying the adsorbate molecules, the role of the substrate metal as well as the implications of different packing densities are discussed relying on the modeling and, whenever possible, also on experimental data.Support by the European Commission through the STREP project ICONTROL (EC-STREP-033197) is gratefully acknowledged.
9:00 PM - B10.16
Structure and Electronic Properties of Anthraceneselenole SAMs - a Joint Theoretical and Experimental Study.
Anna Track 1 2 , Ferdinand Rissner 1 , Daniel Kaefer 3 , Asif Bashir 3 , Georg Heimel 4 , Gerold Rangger 1 , Oliver Hofmann 1 , Tomas Bucko 5 , Lorenz Romaner 6 , Gregor Witte 7 , Egbert Zojer 1
1 Institut für Festkörperphysik, Technische Universität Graz, Graz Austria, 2 Institut für Physik, Karl-Franzes Universität Graz, Graz Austria, 3 Lehrstuhl für Physikalische Chemie I, Ruhr-Universität Bochum, Bochum Germany, 4 Insitut für Physik, Humbold-Universität zu Berlin, Berlin Germany, 5 Insitut für Physik, Universität Wien, Wien Austria, 6 Lehrstuhl für Atomistic Modelling and Design of Materials, Montanuniversität Leoben, Leoben Austria, 7 Molekulare Festkörperphysik, Philipps-Universität Marburg, Marburg Germany
Show AbstractSelf-assembled monolayers (SAMs) provide a simple, flexible, highly ordered and convenient system to tailor and functionalize surface and interface properties of metals, metal oxides and semiconductors. In particular, SAMs of π-conjugated organic molecules have attracted significant interest in the field of molecular and organic electronics because of their considerable conductivity and there ability to change the substrate work function.We performed density-functional theory (DFT) based slab-type band-structure calculations - including geometry optimization in internal coordinates - to gain deeper insight into the energetic, chemical, and physical properties of the interface between a metallic substrate and a covalently bound organic semiconductor. In particular, we studied SAMs of anthracene-2-selenol on Au(111), which have been characterized in detail with various experimental methods including Scanning Tunneling Microscopy (STM), Ultraviolet Photoemission Spectroscopy (UPS), and Low Energy Electron Diffraction (LEED) [1]. Only the combination of experiments and theoretical calculations allows an accurate understanding of the structural ordering of the SAM on the surface. After identifying the correct structure, the electronic properties (such as the work-function modification, the interfacial charge rearrangements and the energy-level alignment) can be provided theoretically. The obtained values agree very well with the experimental data which, in turn, allow a benchmarking of the employed theoretical methods.[1] A. Bashir, D. Käfer, J. Müller, C. Wöll, A. Terfort, G. Witte, Angew. Chem. Int. Ed. 2008, 47, 5250-5252
9:00 PM - B10.17
Turning Gold into Lead – Reducing the Work Function by Charge Transfer Monolayers.
Oliver Hofmann 1 , Gerold Rangger 1 , Ferdinand Rissner 1 , Egbert Zojer 1
1 Institut für Festkörperphysik, Technical University of Graz, Graz Austria
Show AbstractApplication of (sub)monolayers of organic molecules on metal electrodes in order to tune the effective work function has become a field of significant interest. Particular electron poor or rich molecules grow charge transfer layers, which form an infinite dipole layer across the electrode, thus altering the vacuum potential above the surface and hence its effective work function.In this contribution, we investigate the interaction between the electron donor viologen and an Au(111) surface. Depending on the coverage of the monolayer, the work function of the metal can be theoretically lowered to the value of pristine lead (ca. 4.0 eV) or even that of Mg (ca. 3.7 eV). Using density functional theory, an in-depth analysis of the electronic structure at the interface is presented and compared to that of the prototypical donor tetrathiafulvalene (TTF). The work function modification in both systems is found to be determined by a subtle interplay between adsorption induced geometric distortions and electron donation from the respective molecular HOMO to the metal. Unlike carbon monoxide or strong electron acceptors, such as F4TCNQ, on noble metal surfaces, no significant back donation from the metal into the molecule is observed.Furthermore, we show that work function modification can also be tuned using spacer elements, e.g. in the form of voluminous groups enforcing a larger distance between molecule and metal. With increasing distance the transferred charge decreases. Interestingly, we find that the net interface dipole increases with increasing distance as will be shown here on the example of viologen. This work has been supported by the EC through the ICONTROL project.
9:00 PM - B10.18
Controllable Shifts in Threshold Voltage of Top-Gate Polymer Field-Effect Transistors and its Application to Organic Transistor Memory
Kang-Jun Baeg 1 3 , Yong-Young Noh 3 , Henning Sirringhaus 2 , Dong-Yu Kim 1
1 Dept. of Materials Science and Engineering, Gwangju Institute of Science and Technology, Gwangju Korea (the Republic of), 3 Convergence Components and Materials Research Laboratory, Electronics and Telecommunications Research Institute, Daejeon Korea (the Republic of), 2 Department of Physics, University of Cambridge, Cambridge United Kingdom
Show AbstractOrganic materials are attractive for many components of electronic devices such as active semiconductor layers, insulator layers, and electrodes, due to a lot of unique advantages over their inorganic counterparts. Although the organic materials are not currently suitable for applications requiring high-end performances, their low-cost and low-temperature fabrication using solution-based processing make them ideal for large-area, flexible, transparent, and disposable applications. Moreover, organic non-volatile memories are another emerging class of research fields based on the advantages of organic materials. A variety of approaches have been progressed including cross-point type organic bistable devices and organic transistor-based memories. Here we report a solution-processed polymer FET memory device with top-gate and bottom-contact device configuration. With incorporation of gold nanoparticles (NPs) inside double-layered polymer gate dielectrics, the threshold voltage (VTh) of polymer FET devices could be reversibly and systemically controlled by application of external gate fields. This reversible shifts in VTh was originated from charge trapping in Au NPs, and might be used as organic transistor based non-volatile memory devices.
9:00 PM - B10.19
Ion Irradiation Effects on The Transport Properties and Degradation Mechanisms of Organic Field-Effect Transistors.
Beatrice Fraboni 1 , Anna Cavallini 1 , Piero Cosseddu 2 , Annalisa Bonfiglio 2 , Yongquiang Wang 3 , Michael Nastasi 3
1 Physics, University of Bologna, Bologna Italy, 2 Electronic Engineering, University of Cagliari, Cagliari Italy, 3 , Los Alamos National Laboratory, Los Alamos, New Mexico, United States
Show AbstractThe remarkable advances recently made in the development of organic semiconductor field effect devices (OFET) prospect challenging applications in the field of low-cost flexible, lightweight, and conformal electronics One of the interesting features of organic active layers is their capability to respond to the environment chemical composition, but this unfortunately opens up the issue of long-term stability of devices based on organic materials, as oxidation is believed to be a major reason for early device failure. The focus of our research is to investigate the potentiality of low energy ion irradiation in the reduction and control of the degradation of the organic material due to the exposure to atmosphere (i.e. oxygen and water). We have studied the effects of N irradiation on pentacene and sexithiophene based OFETs. The damage induced by the ions can induce a rearrangement of the molecular alignment by breaking covalent bonds, crosslinking the neighboring polymer chains and forming a hydrogen-depleted three dimensional carbon network The strong molecular structure modification affects the carrier mobility and the threshold voltage of the device, but since the electrical transport in OFETs occurs in a few active molecular layers at the organic/dielectric interface, we have observed that a controlled damage depth distribution preserves the functionality of the organic active layer. We have studied the variation of the transport parameters as a function of the irradiation energy and dose by characterizing the optical and electrical properties of the material by means of electrical transport analyses and photocurrent spectroscopy. We have monitored the effectiveness of the low energy irradiation process in providing an hermetic protection to the organic active layer from the ambient conditions.
9:00 PM - B10.2
High Efficiency Blue Phosphorescent OLEDs by Tuning Charge Transport in the Emitting Layer.
Neetu Chopra 1 , Jaewon Lee 1 , Franky So 1
1 Dept of Materials Science and Engineering, University of Florida, Gainesville, Florida, United States
Show AbstractOrganic light-emitting diode (OLED) is a promising candidate for solid state lighting and display applications. While high efficiency green phosphorescent OLEDs (PHOLEDs) have been demonstrated, the efficiency of blue PHOLED is still low. Hereby, we demonstrate a high efficiency iridium (III) bis[(4,6-di-fluorophenyl)-pyridinate-N,C2`]picolinate (FIrpic) PHOLED by enhancing the carrier transport properties of the emitting layer. Charge transport and balance in OLEDs are very important for achieving high device efficiency. Apart from charge transport and balance in the charge transporting layers, charge transport in the emitting layer also plays an important role determining the device performance. In this study we investigate and tune the charge transport in the emitting layer to achieve high efficiency PHOLEDs.Wide gap materials are required as the host in blue PHOLEDs. For some wide band gap materials such as p-bis (triphenylsilyly) benzene (UGH2), they are poor carrier transporters and electro-phosphorescence occurs by charge trapping on dopant molecules. Thus, the carrier transport properties of the emitting layer can be tuned by controlling the dopant concentration in the UGH2 host. In this work, we have fabricated PHOLED devices with different FIrpic concentrations and found that the carrier transport property of the emitting layer is a strong function of FIrpic concentration. As the dopant concentration is increased, the current density of the devices increases and the turn-on voltage decreases. Furthermore, we also found that the device efficiency increases with increasing FIrpic concentration. A maximum external quantum efficiency of 25.6% (corresponding to 56 cd/A current efficiency and 40 lm/W luminous efficiency) was achieved with 20% FIrpic doping concentration. This is one of the highest efficiencies reported for blue PHOLEDs. Further increase in doping concentration decreases the device efficiency due to concentration quenching. In addition to OLED devices, single carrier devices with different FIrpic concentrations were also fabricated and the device data confirmed the enhanced carrier transport due to doping.
9:00 PM - B10.20
PN-junction Diodes made of p-type Pentacene and n-type SnO2 Nanowires.
Sung Chan Park 1 , Daeil Kim 1 , Seongmin Yee 2 , Junghwan Huh 2 , Gyu Tae Kim 2 , Jeong Sook Ha 1
1 Chemical and Biological Engineering, Korea University, Seoul Korea (the Republic of), 2 School of Electrical Engineering, Korea University, Seoul Korea (the Republic of)
Show AbstractPN-junctions are of great importance in modern electronic applications for achieving the integrated circuits and understanding the device characteristics. In our presentation, we will show the rectifying current-voltage curves and the temperature dependent diode characteristics. Hybrid pn-junction devices consisted of p-type organic semiconductor (pentacene) and n-type inorganic semiconductor (SnO2 nanowires). SnO2 nanowires can be easily synthesized or grown by VLS mechanism in CVD, showing n-type semiconducting properties with a high mobility. SnO2 nanowire networks were formed by a selective growth on the Au catalyst with various growth conditions. Onto the SnO2 nanowire networks, pentacene was deposited by thermal evaporation with various thicknesses under 7×10^-7 Torr. Ti (20 nm) or Cr (20 nm)/Au (300 nm) electrode and the 300 nm thickness of Au electrode were deposited on the SnO2 channels and pentacene channels for Ohmic contacts, respectively, which was confirmed by Ohmic current-voltage characteristics. Microscopic and chemical analyses were investigated for characterizing each component of the hybrid pn-junctions. As the temperature decreased, the current levels reduced following the diode equation of I=I0(exp(ηkT)-1) with the big ideality factor reaching 420, which indicate the big surface states at the junction parts. Considering the large ideality factor, the soft-reverse characteristics were not so significant with a good on/off ratio of 10^3 at ±60V. The possible application of organic/inorganic hybrid pn-diodes will be discussed from the point of the solar-cell or photo-detectors.
9:00 PM - B10.21
Synthesis and Characterization of High Efficiency Copolymers via Effective Energy-Transfer for Polymer Light-Emitting Diodes (PLEDs)
Qinghua Zhao 1 , Shuang Zhang 1 , Jong-Won Park 1 , Sung Ouk Jung 1 , Yun-Hi Kim 2 , Soon-Ki Kwon 1
1 school of materials science and engineering, engineering research institute (ERI),, gyeongsang national university, Jin ju Korea (the Republic of), 2 department of chemistry, gyeongsang national university, jin ju Korea (the Republic of)
Show AbstractPolymer light-emitting diodes (PLEDs) have been the subject of intense research interests recently due to their applications in large-area flat panel displays.1 To achieve highly efficient PLED devices, charge (holes and electrons) injection and transport from both the anode and the cathode should be balanced at the junction of the emitting layer to yield the maximum exciton formation.2 Although some polymer LEDs have shown high-enough efficiencies and long lifetimes, they are mainly multilayer LEDs, which involve complicated and difficult device fabrication processes, or single layer LEDs based on polymer blends. So, it is important to achieve conjugated polymers which have both functions of hole/electron affinity in the single layer LEDs. The bipolar transport characteristics of polymers are important for high EL efficiency, because the polymers offer good recombination sites for hole and electron charge carriers.3 In this communication, we designed and synthesized a series of polymers with hole transporting and electron transporting ability. Their photophysical and thermal properties would be investigated and further compared. The PLED devices would be fabricated and discussed further. These polymers are expected to obtain high efficient from PLEDs due to effective energy transfer from large bandgap site to narrow bandgap site.[1] J. H. Burroughes, D. D. C. Bradley, A. R Brown, R. N. Marks, K Mackay, R. H. Friend, P. L Burn, A. B. Holmes, Nature 1990, 347, 539.[2] F. Garten, A Hilberer, F. Cacialli, E. Essenlink, Y. van Dam, B. Schlatmann, R. H. Friend, T. M. Klapwijk, G.. Hadziioannou, Adv Mater 1997, 9, 127.[3] N. Tamoto, C. Adachi, K. Nagai, Chem. Mater. 1997, 9, 1077.
9:00 PM - B10.23
Para-sexiphenyl Based OLED Devices Grown on a Pre-patterned Polymeric Substrate.
Gerardo Hernandez-Sosa 1 , Clemens Simbrunner 1 , Thomas Hoefler 2 , Armin Moser 3 , Oliver Werzer 3 , Birgit Kunert 3 , Gregor Trimmel 2 , Wolfgang Kern 4 , Roland Resel 3 , Helmut Sitter 1
1 Institute for Semiconductors and Solid State Physics, Johannes Kepler University Linz, Linz Austria, 2 Institute for Chemistry and Technology of Materials, Graz University of Technology, Graz Austria, 3 Institute of Solid State Physics, Graz University of Technology, Graz Austria, 4 Institute of Chemistry of Polymeric Materials, Montanuniversität Leoben, Leoben Austria
Show AbstractDuring the last years organic devices became of increasing interest in many fields of electronics. A bright future for organic light emitting devices (OLED) is expected as organics provide a wide spectrum of molecules emitting at various photon energies [1]. Para-sexiphenyl represents an organic molecule which has been established as active material for blue emitting OLEDs [2, 3]. As a matter of fact, a defined control of the substrate surface properties is a good way to improve the quality of organic films. Surface properties like polarity, hydrophilicity or hydrophobicity can be tuned to change the way the film is growing. These changes can lead to an improvement on the intrinsic properties of the film and consequently have an impact on the performance of the final device. Para-sexiphenyl (PSP) has been deposited by Hot Wall Epitaxy (HWE) on poly(diphenyl bicyclo(2.2.1)hept-5-ene-2,3-dicarboxylate) (PPNB), a photosensitive polymer, which has been pre-treated by UV illumination leading to a change of the surface polarity [4]. A detailed analysis of the growth morphology as a function of substrate temperature, growth time and UV illumination of the substrate has been performed by Atomic Force Microscopy. The surface morphology and growth kinetics of PSP were found to be different depending on which surface it was deposited, as prepared or UV exposed. Furthermore, the crystallographic properties has been analysed by X-ray diffraction and Grazing Incidence X-ray Diffraction (GIXD). The crystal structure and the degree of order of the deposited PSP thin films were determined by specular scans and reciprocal space maps. Again a clear change on the structural properties between the films deposited on the as prepared and pre-treated surfaces was observed. Besides the film characterization we also report on OLEDs based on Para-sexiphenyl (PSP), which has been deposited by HWE on a UV pre-treated ITO/PEDOT:PSS/PPNB substrate. A complete quenching of the PSP electroluminescence is observed on the UV pre- illuminated regions while on the as-prepared one the characteristic PSP emission is observed. Consequently, this method has proven to be a very effective pre-patterning tool which could be fully compatible with the actual OLED production technology. [1] M. Muccini, Nature materials 5 (2006), 605[2] G. Kranzelbinder et. al.,Synthetic Metals 102 (1999), 1073-1074[3] A. Niko et. al., J. Appl. Phys. 82 (1997), 4177[4] T. Höfler et al., Polymer 48 (2007), 1930-1939
9:00 PM - B10.24
Chemical and Electronic Properties of Self-assembled Organic Monolayers on SiC Surfaces.
Ian Sharp 1 , M. Hoeb 1 , S. Schoell 1 , C. Diaz Alvarez 1 , J. Howgate 1 , M. Brandt 1 , M. Stutzmann 1
1 , Walter Schottky Institut, Technische Universität München, 85748 Garching Germany
Show AbstractWide bandgap materials are useful systems for the study of electronic processes at the organic-inorganic hybrid interface due to the large energetic tunability of the semiconductor Fermi level. Here, the group IV compound semiconductor silicon carbide (SiC) is used as a model substrate for organic monolayer self-assembly by reaction with organosilanes and alkenes on both C- and Si-polar surfaces. Comparison of the properties of both types of films on surfaces of both polarities reveals the important role of interfacial chemistry and the related binding dipole on the electronic properties at the organic-inorganic interface. In the well-known case of Si, organic functionalization is typically achieved via hydrosilylation of H-terminated surfaces or silanization of an intermediate OH-terminated thin oxide. In contrast, SiC surfaces are OH-terminated following HF etching and are thus ideally suited for silanization reactions. We demonstrate that reaction of these surfaces with octadecyltrimethoxysilane (ODTMS) yields stable, high-quality organic monolayers directly bound to the semiconductor surface without an oxide interlayer. Furthermore, we show by x-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR) spectroscopy that reaction of OH-terminated SiC with 1-octadecene also yields high quality monolayers but occurs over a bridging oxygen atom that is not present in the case of hydrosilylated Si. Spectrally resolved photocurrent measurements on functionalized surfaces reveal significant conductivity enhancements relative to oxidized surfaces, indicating a reduction of interfacial defect concentrations. These enhancements are particularly pronounced on C-polar SiC and are greater on alkene-reacted surfaces than on silanized surfaces. Transport and impedance measurements with a mercury-drop-contact across monolayer-SiC structures yield thermionic emission barriers and flat band potentials that are consistent with large dipolar contributions to interfacial band alignment and that depend on the specific chemical binding of the organic monolayers. Patterned arrays of covalently grafted, fluorescence-labeled proteins onto SiC were fabricated using both functionalization routes, thus further demonstrating the potential of the stable, biocompatible SiC surface in future biosensor and protein-integrated electronics applications.
9:00 PM - B10.25
Stability Issues of Organic Thin-Film Transistors.
Andreas Klug 1 , Raphael Pfattner 2 , Matthias Baumann 2 , Gerhild Wurzinger 1 , Arno Meingast 1 , Benjamin Souharce 3 , Michael Forster 3 , Ullrich Scherf 3 , Emil List 1 2
1 , NanoTecCenter Weiz Forschungsgesellschaft mbH, Weiz Austria, 2 Christian Doppler Laboratory Advanced Functional Materials, Institute of Solid State Physics, Graz University of Technology and Institute of Nanostructured Materials and Photonics, Joanneum Research, Graz/Weiz Austria, 3 Macromolecular Chemistry, University of Wuppertal, Wuppertal Germany
Show AbstractFor more than two decades organic thin-film transistors (OTFTs) have been investigated, being key devices for large-area, low-cost electronics fabricated with cheap and easy processing techniques such as spin-coating, inkjet-printing or soft lithography. Essential properties of the applied organic semiconductors include solution-processability, high field-effect mobility, compatibility with adjacent layers and – not to forget – stability with respect to ambient conditions. Here we report on the ambient, operational and shelf-life stability of OTFTs with a polytriphenylamine-(PTPA)-based polymer active material. The results are benchmarked against the well-established transistor polymer poly(3-hexylthiophene) (P3HT), yielding comparable mobility values around 10-4 cm2/Vs. However, upon air exposure P3HT-based devices exhibit switch-on voltage shifts of more than 30 V and a distinct off-current increase due to oxygen/moisture-induced doping. Stable air operation therefore involves expensive device encapsulation or a top-gate architecture, where P3HT is shielded by an appropriate dielectric material. The corresponding device parameters of PTPA-based OTFTs, on the contrary, remain rather stable and make device encapsulation obsolete. Moreover, we show that PTPA-based devices with polyvinylalcohol as dielectric yield improved operational stability and we present flexible OTFTs based on the two active semiconductor materials.
9:00 PM - B10.26
Para-sexiphenyl-CdSe Nanocrystals Hybrid Light Emitting Diodes with Optimized Layer Thickness and Interfaces.
Clemens Simbrunner 1 , Gerardo Hernandez-Sosa 1 , Eugen Baumgartner 1 , Juergen Roither 1 , Guenter Hesser 2 , Wolfgang Heiss 1 , Helmut Sitter 1
1 Institute of Semiconductors and Solid State Physics, Johannes Kepler University Linz, Linz Austria, 2 ZONA (Zentrum f. Oberflächen- und Nanoanalytik), Johannes Kepler University Linz, Linz Austria
Show AbstractIn the variety of molecules used for Organic Light Emitting Diode (OLED) fabrication Para-sexiphenyl (PSP) represents a well known candidate for the fabrication of high photon energy emitting devices [1,2]. The high energy gap (3.1 eV) of PSP and consequently its blue electroluminescence (EL) emission recommends PSP as a component for multi-color OLED displays and white LEDs. Recent publications promote solution-based semiconducting nanocrystals or quantum-dots (QD) integrated within the OLED structure leading to a high efficient electroluminescence emission of organic-inorganic hybrid devices [3-7]. This approach requires a very good control of the layer thickness and very well defined interface properties of the multilayer to obtain a good carrier injection and an efficient electroluminescence (EL).In this contribution we report on the optimization of the interface and thickness between PSP and CdSe nanocrystals as well as on the role of PSP as electron transport layer on Hybrid-LEDs. The devices, emitting at 422nm, 549nm and 610 nm, were fabricated by spin coating the CdSe NC from solution onto (ITO/PEDOT:PSS) anode substrates and subsequently evaporating PSP under vacuum conditions. Aluminum contacts provide electron injection in the device structure.The optimal layer thickness and well defined interface was determined by cross sectional scanning electron microscopy (SEM), while the carrier injection was studied by monitoring the EL intensity as a function of emission wavelength and applied device current. The optimized values were found to be less than 25 nm for the PSP layer and 2 monolayers for the CdSe nanocrystals, resulting in low onset voltages (3-4 V). Homogeneous layers are achieved across the whole device which is verified by the resulting homogeneous light emission.The obtained spectra of the optimized devices indicate high color purity, mainly determined by well defined emission lines, their low FWHM and the absence of parasitic emissions. Consequently the excellent device properties achieved by the optimization promote the demonstrated material system for a future candidate in the fabrication of QD-PSP based LED for displays and lighting applications [8].[1] S. Tasch, C. Brandstätter, F. Meghdadi, G. Leising, G. Froyer, L. Althoel, Adv.Mat. 9 (1), 33 (1997)[2] G. Kranzelbinder, F. Meghdadi, S. Tasch, G. Leising, L. Fasoli, M. Sampietro, Syn. Metals 102, 1073 (1999)[3] S. Coe, W. Woo, M. Bawendi, V. Bulovic, Nature 420, 800 (2002)[4] J. Zhao, J. Bardecker, A. Munro, M. Liu, Y. Niu, I. Ding, J. Luo, B. Chen, A. Jen, D. Ginger, Nano Lett. 6 (3), 463 (2006)[5] Q. Sun, Y. Wang, L. Li, D. Wang, T. Zhu, J. Xu, C. Yang, Y. Li, Nature Photonics 1, 717 (2007)
9:00 PM - B10.27
Patterning of Organic Monolayers on GaN via UV-induced Charge Transfer.
S. Schoell 1 , J. Howgate 1 , Ian Sharp 1 , W. Steins 1 , M. Brandt 1 , M. Eickhoff 1 , M. Stutzmann 1
1 , Walter Schottky Institut, Technische Universität München, Garching Germany
Show AbstractUnderstanding charge transfer processes and their mechanisms at organic-inorganic interfaces is of fundamental importance in the emerging field of organic electronics. Wide bandgap semiconductors are particularly useful for studying interfacial electronic processes since the bulk Fermi level can be systematically varied over a wide energy window. Here, the affinity of GaN to form OH-terminated surfaces by wet chemical treatments is exploited to generate functional organosilane monolayers. In particular, wet-chemically processed layers of octadecyltrimethoxysilane (ODTMS) and aminopropyl-triethoxysilane (APTES) on n-type as well as on p-type Ga-face GaN were formed. For comparative purposes, identical layers were also formed on n- and p-type 6H-SiC surfaces. The structural and chemical properties of these layers were studied by static water contact angle measurements, thermal desorption spectroscopy (TDS), X-ray photoelectron spectroscopy (XPS), Fourier transform IR spectroscopy (FTIR), and X-ray reflectivity measurements. The organic layers are smooth and change their wetting properties depending on the molecules used. Thermal desorption temperatures in the range of 500°C indicate covalent bonding of the organic molecules to the GaN surfaces. UV-induced charge transfer between the semiconductor substrate and organic monolayers was studied after irradiation with a low pressure Hg lamp at a wavelength of 253.7 nm. On n-type GaN this results in a decreased water contact angle and reduced thickness of the organic layer, saturating at an illumination time of 30 min. Furthermore, analysis via capacitance-voltage and current-voltage measurements revealed that this effect is accompanied by the accumulation of trapped charge within the organic layer. XPS data prove a decreased hydrocarbon signal on UV-irradiated n-type GaN and FTIR shows a complete loss of terminating endgroups. In contrast, the wetting behavior, as well as the XPS and FTIR results, of silanized p-type GaN, as well as n- and p-type 6H-SiC, are largely unaffected by UV irradiation, suggesting a photoinduced charge transfer through the organic/semiconductor interface which leads to the observed degradation for n-type GaN. This effect was exploited to achieve laterally patterned organic monolayers on n-type GaN by selective UV illumination through a shadow mask, followed by site-sensitive covalent grafting of proteins. We thus show that GaN is a useful platform for fundamental studies of charge transfer processes at the organic-inorganic interface as well as a promising material for future biosensor applications.
9:00 PM - B10.28
Field Effect Transistor as a Platform to Study Photoinduced Processes in a Donor-acceptor System.
K. Narayan 1 , Manohar Rao 1
1 , JNCASR, Bangalore India
Show AbstractThe existence of donor-type polymer FETs exhibiting p-type characteristics and acceptor-type molecular FETs with n-type characteristics provide an interesting possibility of a combined active bilayer system, especially under photoexcitation. We present studies of single layer p-channel P3HT, single layer n-channel PCBM, and bilayer PCBM/P3HT based FETs in dark and illuminated conditions. The single layer p type mobility was estimated to be ≈ 10-2 cm2/V-s and n type mobility in PCBM was ≈ 10-5 cm2/V-s. The bilayer structure was fabricated by introducing the P3HT film on a PCBM device using different methods. The underlying n-type characteristic of the PCBM device alters significantly upon illuminating the bilayer structure. Variety of photophysical processes arising from charge generation and interfacial processes can be followed and studied by transistor characteristics. These results are compared to the switching and relaxation effects in single layer P3HT device under photoexcitation.1,2References1)Light Responsive Polymer Field Effect transistor. Applied Physics Letter, 79, 1891 (2001).2)Nonexponential relaxation of photoinduced conductance in organic filed effect transistors. Physical Review B, 68, 125208 (2003).
9:00 PM - B10.29
Effect of Donor, Acceptor, and Hole/Exciton Blocking Layer Thickness on Power Conversion Efficiency for Small-molecular Organic Solar Cells
Su-Hwan Lee 1 , Dal-Ho Kim 1 , Ji-Heon Kim 1 , Tae-Hun Shim 1 , Jea-Gun Park 1
1 Nano SOI Process Lab., Hanyang university, Seoul Korea (the Republic of)
Show AbstractWe investigated the dependency of the power conversion efficiency on the thickness of donor (copper phthalocyanine; CuPc), acceptor (fullerene; C60), and hole/exciton blocking (2,9-Dimethyl-4,7-diphenyl-1,10-phenanthroline; BCP) layers for the organic photovoltaic (OPV) devices fabricated with double small-molecular layers. The power conversion efficiency peaked at a specific layer thickness, ~12.7 nm for the donor layer, ~17.5 nm for the acceptor layer, and ~8.0 nm for the hole/exciton blocking layer. This trend is associated with the light absorption and carrier transport resistance of the small-molecular donor layer, both of which strongly depend on the layer thickness. Experimental and calculated results showed that the short-circuit current (Jsc) due to light absorption increased with the donor layer thickness, while that due to current through the donor layer decreased with 1/R3. Since the total short-circuit current is the product of the light absorption current and current through the donor layer, there is a trade-off, and the maximum power conversion efficiency occurs at a specific small-molecular donor layer thickness (e.g., ~12.7 nm in this experiment). In addition, power conversion efficiency was determined by short-circuit-current rather than open-circuit-voltage after light absorption. Furthermore, the donor layer thickness was more sensitive than the thickness of the acceptor or hole/exciton blocking layers in improving power conversion efficiency; i.e., ~130% for the donor layer, ~118% for the acceptor layer, and ~112% for the hole/exciton blocking layers.Acknowledgement* This research was supported by "The National Research Program for Terabit Nonvolatile Memory Development” sponsored by the Korean Ministry of Knowledge Economy.
9:00 PM - B10.3
Enhanced Performance in Organic Solar Cells using Thermally-evaporated Tungsten Oxide Interlayer and its Application to ITO-free Organic Solar Cells.
Seungchan Han 1 , Won Suk Shin 2 , Myungsoo Seo 1 , Dipti Gupta 1 , Seunghyup Yoo 1
1 Department of Electrical Engineering and Computer Science, KAIST, Daejeon Korea (the Republic of), 2 Energy Materials Research Center, Korea Research Institute on Chemical Technology (KRICT), Daejeon Korea (the Republic of)
Show AbstractIn line with the recent trend that various metal oxide layers are used as an interfacial buffer layer in organic solar cells to enhance their photovoltaic (PV) performance [1,2] we explore the effect of insertion of tungsten oxide (WO3) films on the performance of organic solar cells based on poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61 butyric acid methyl ester (PCBM-60). Experimental results show that the insertion of thermally evaporated WO3 layer in between ITO and photoactive layers renders fill factor (FF) and open-circuit voltage (VOC) of P3HT:PCBM cells get significantly improved from 0.60 and 0.441 V of reference cells to 0.69 and 0.600 V with the short-circuit current density (JSC) virtually unchanged, resulting in a net improvement in power conversion efficiency by +67%. X-ray diffraction results consistently indicate that P3HT films grown on WO3 layers have a higher degree of ordering than those grown on ITO or on PEDOT:PSS layers. Such difference in growth behavior and PV performance improvement are discussed in conjunction with the contact-angle measurement which revealed the relatively low surface energy of WO3 films when compared to those of ITO and PEDOT:PSS surfaces. Finally, we demonstrate the versatility of WO3 interlayers by presenting ITO-free organic solar cells that employ a multilayer electrode in which WO3 layers play a key role in terms of both optical and electrical properties.References[1] M. D. Irwin, B. Buchholz, A. W. Hains, R. P. H. Chang, and T. J. Marks, PNAS 105, 0711990105 (2008)[2] V. Shrotriya, G. Li, C.-W. Chu, and Y. Yang, Appl. Phys. Lett. 88, 073508 (2006)
9:00 PM - B10.30
Multi-Functional Optical Thin Film Elements by an Imprinting Technique
Yong-Woon Lim 1 , Wonsuk Choi 1 , Sin-Doo Lee 1
1 Electrical Engineering and Computer Science, Seoul National University, Seoul Korea (the Republic of)
Show AbstractIn recent years, organic electro-optical materials have great interest for applications in optical components and liquid crystal displays (LCDs). However, a reliable micro-patterning technology of organic materials is not well-established so far. The imprinting lithography (IL) is a very useful method of fabricating such organic-based microstructure devices for use in electronics, optics, microfluidics, biology, and related areas.In this work, we demonstrate two optical elements of a liquid crystalline polymer (LCP) and dichroic dyes-doped LCP using an IL technique combined with the exposure of ultraviolet light. The imprinted optical elements (IOEs) can be used as both a functional optical film and an alignment layer of the liquid crystal (LC) for the LC-based optical devices. The multi-functional optical films can be used as an in-cell retarder or a viewing angle enhancement film and in-cell dye-polarizer. In addition, the microgrooves produced on the LCP through an IL process are well-defined in shape and so that they provide the uniform alignment of the LC molecules onto the IOEs without extra-coated alignment layer by taking the Berreman’s effect. One IOE with function of in-cell retarder (A-plate) was fabricated using a LCP with positive dielectric anisotropy such as RMS 03-001C (E. Merck) by an IL process. The phase retardation of the A-plate measured as a function of azimuthal angle by a photo-elastic modulation technique is about 1.6. The value was about π/2 corresponds approximately to λ/4 of the wavelength, λ=632.8 nm, used. On the other hand, another IOE was fabricated by a same IL process of fabricating the above mentioned A-plate using another LCP with negative dielectric anisotropy such as RMS 03-015 (E. Merck) instead of LCP with positive dielectric anisotropy. The hydrophobic surface wettability of used mold was induced the homeotropic aligned (HA) LCP molecules and the microstructure was transferred on the LCP layer from the mold. The HA IOE shows the dark state in any direction of optic axis. And thus, the IOE achieved wider viewing characteristics preserved microstructure on the surface of the LCP film. The other IOE with function of in-cell dye-polarizer with unique axe or multi-axes was fabricated using dichroic dyes-doped LCP by an IL process. This thin film polarizer shows excellent optical performances that can be expressed by the polarization efficiency and the extinction ratio in combination with single-piece transmittance. For using above mentioned multi-functional IOEs, we proposed two types of high-efficient LC devices. Consequently, it possesses high contrast and wider viewing characteristics due to employ two imprinted multi-functional optical films.
9:00 PM - B10.31
Comparative Study of Ni or NiO Treated Hole Injection Property for Organic Light Emitting Diodes.
Sung-Ho Woo 1 , Youngkyoo Kim 2 , Gwijeong Cho 2 , Kangpil Kim 1 , Hongkeun Lyu 1 , Jaehyun Kim 1
1 Division of Nano&Bio Technology, DGIST, Daegu Korea (the Republic of), 2 Department of chemical engineering, Kyungpook National University, Daegu Korea (the Republic of)
Show AbstractIn this letter, we compared hole injection enhancing effect for Ni and NiO. Deposition of NiO layer on ITO showed good property for both hole injection and device efficiency, while Ni was deviated from expectation, though its work function is adequately high to match with that of ITO electrode, due to dark-spot formation caused by its unstableness with respect to circumstances and adjacent layer. We obtained about 50% in-crease of device efficiency as well as 3 times increase of maximum brightness by insertion of NiO. From energy level diagram, we can suggest a bifunctional property of NiO layer to facilitate hole injection and block the leakage electron to ITO, which are come from high work function and wide band gap property, respectively. This results proposed simple method for improving efficiency of OLEDs device by insertion of thermal evaporated NiO thin layer without any addition of process facility. It is also widely applicable to other organic electronic devices such as polymer light emitting diodes, and organic photovoltaics.
9:00 PM - B10.32
Effect of Small-molecular Blocking Layer Thickness on Power Conversion Efficiency in Small-molecular Blended Polymer Solar Cells
Su-Hwan Lee 1 , Dal-Ho Kim 1 , Ji-Heon Kim 1 , Tae-Hun Shim 1 , Jea-Gun Park 1
1 Nano SOI Process Lab., Hanyang university, Seoul Korea (the Republic of)
Show AbstractWe investigated the dependency of the power conversion efficiency throughout the blended donor and acceptor layer thickness variation using poly(3-hexylthiophene (P3HT) and [6,6]-phenyl-C61 butyric acid methyl ester (PCBM), and small-molecular hole/exciton blocking (2,9-Dimethyl-4,7-diphenyl-1,10-phenanthroline; BCP) layers for organic solar cells. The power conversion efficiency peaked at a specific layer thickness, determined by ~1000 rpm for the blending layer (P3HT and PCBM). This trend is associated with the light absorption and carrier transport resistance of the blending layer, both of which strongly depends on the blending layer thickness and shows a trade-off for power conversion efficiency. Also, the blocking efficiency for the hole/exciton of the BCP layer peaked at a specific thickness (~0.5 nm) of the BCP layer. This trend is associated with the blocking efficiency and carrier transport resistance, both of which strongly depends on the blocking layer thickness and shows a trade-off for power conversion efficiency. In our work, we achieved the maximum power conversion efficiency of ~6.74% for our development small-molecular blended polymer solar cell. Acknowledgement* This research was supported by "The National Research Program for Terabit Nonvolatile Memory Development” sponsored by the Korean Ministry of Knowledge Economy.
9:00 PM - B10.33
Synthesis and Application of New Semiconducting Copolymer for Organic Devices using N-9-heptadecanyl-dithienopyrrole.
Hyung-Gu Jeong 1 , Bogyu Lim 1 , Seok-In Na 1 , Kang-Jun Beag 1 , Jin-Mun Yun 1 , Juhwan Kim 1 , Dong-Yu Kim 1
1 Dept. of Materials Science and Engineering, Gwangju Institute of Science and Technology (GIST), Gwangju Korea (the Republic of)
Show AbstractThe optical and electronic properties of conjugated organic polymers have led to their use in a variety of applications including organic light-emitting diodes (OLEDs), organic field-effect transistors (OFETs) and organic photovoltaic cells (OPVs). N-functionalized dithienopyrroles (DTPs) introduce rigidity to the backbone, resulting in increased planarity and longer conjugation length. However, some DTPs have low solubilties and low molecular weight, which greatly limits their use in devices. To make soluble polymers, we incorporated a secondary alkyl side chains. So we have synthesized N-9-heptadecanyl-dithienopyrrole based copolymers via stille coupling reaction. The polymer can be easily dissolved in common organic solvents, such as chloroform, tetrahyrofuran(THF) and chlorobenzene. The good solubility can be attributed to the bulky side chain. No obvious DSC peak was detected in the trace, suggesting the amorphous nature of polymer. In this presentation, we will discuss on the performance of OTFTs, OPVs fabricated using a new semiconducting polymer.
9:00 PM - B10.34
Hybrid Organic-Inorganic Heterojunctions made of Doped Crystalline Silicon and a Hole Conducting Polymer.
Roland Dietmueller 1 , Sabrina Niesar 1 , Helmut Nesswetter 1 , Martin Stutzmann 1
1 Walter Schottky Institut, Technische Universität München, Garching Germany
Show AbstractThere is an increasing research activity in hybrid heterojunctions made of organic semiconductors and inorganic semiconductors, due to their possible applications in new types of photovoltaic and optoelectronic devices. The use of conjugated polymers for the organic side of the hybrid heterojunction is especially interesting due to the low-cost fabrication of organic layers from solution. The inorganic semiconductor, on the other hand, can be easily doped as an n-type or p-type material at different concentrations, which opens additional possibilities for the application of hybrid heterostructures. We have studied hybrid heterojunctions as a model system made of crystalline Silicon (Si) and the organic hole conductor poly(3-hexylthiophene-2,5-diyl) (P3HT). The heterojunctions were fabricated by spin coating a P3HT layer from solution on Si wafers with an ohmic metal back contact. Afterwards, a semitransparent Aluminum front contact was applied on top of the P3HT layer. These devices were characterized via current-voltage measurements and spectrally resolved photocurrent measurements. To investigate the influence of doping on the hybrid heterojunction, we performed experiments as well with n-type Si with a Phosphorus concentration of 1.5 * 1016 - 1 * 1017 cm-3 as with p-type Si with a Boron concentration of 3 * 1015 - 2 * 1016 cm-3. Depending on the use of n-type or p-type Si, we have observed strongly different current-voltage characteristics for the P3HT/Si heterojunctions. The diode behavior of the p-type Si/P3HT heterojunction can be explained by the interplay of the p-type Si/P3HT contact and a Schottky contact at the P3HT/Al interface, which dominates the electrical characteristics of the whole structure. This finding is supported by the current-voltage characteristics under illumination with white light, when the p-type Si/P3HT heterostructure works as a solar cell with similar characteristics as a P3HT/Al Schottky solar cell. However, for the n-type Si/P3HT heterojunction a more complex current-voltage characteristic is observed, where under illumination of the heterostructure the forward direction of the diode is inverted. To explain this behavior, we take into account two rectifying interfaces with different forward directions, namely the Si/P3HT heterojunction and the P3HT/Al Schottky contact, and also the strong influence of photo-generated charge carriers on the Si/P3HT heterojunction. These investigations are important for the understanding of the fundamentals of the organic-inorganic interface in semiconductors.
9:00 PM - B10.35
Novel Water-Soluble Polyfluorenes as an Interfacial Layer in Polymer Solar Cells with High Work-Function Metal Cathodes.
Seung-Hwan Oh 1 , Seok-In Na 1 , Jang Jo 1 , Yung-Eun Sung 2 , Dong-Yu Kim 1
1 Department of Materials Science and Engineering, Gwangju Institute of Science and Technology (GIST), Gwangju Korea (the Republic of), 2 School of Chemical & Biological Engineering, Seoul National University, Seoul Korea (the Republic of)
Show AbstractWater solubility of conjugated polymers may offer many applications. Potential applications of water-soluble conjugated polymers include the polymer light-emitting diode and new materials for nano and micro hollow-capsules, and bio- or chemo-sensors. We synthesized neutral polyfluorenes containing bromo-alkyl groups by the palladium catalyzed Suzuki coupling reaction. Bromo-alkyl side groups in neutral polyfluorenes were quaternized by tri-methyl amine solution. The electrochemical and optical properties of water-soluble conjugated polymers are discussed. This novel synthesized water-soluble conjugated polymers were used as a dipole layer between active layer and metal cathode in polymer solar cell for enhancement of open-circuit voltage (Voc), which is one of the most critical factors in determining device characteristics. We also investigated the device performance of polymer solar cell with different metal cathode such as Al, Ag, Au and Cu. In polymer solar cell, novel cationic water-soluble conjugated polymers were inserted between active layer and high-work function cathode (Al, Ag, Au and Cu).
9:00 PM - B10.36
Crystallization of Organic Molecules on the Monolayers of Binaphthyl Derivatives for Thin-Film Transistors
Sang-Mi Jeong 1 2 , Ji-Woong Park 1 2
1 Materials Science & Engineering, Gwangju Institute of Science and Technology, Gwangju Korea (the Republic of), 2 , Program for integrated Molecular Systems, Gwangju Korea (the Republic of)
Show AbstractMonolayers of 1,1’-bi-2-naphthol (BN) derivatives, of which the two naphthalene rings are twisted along the carbon(1)-carbon(1’) single bond, were studied for their conformational effect on the growth of the organic molecules on their monolayer surface. We demonstrate that when pentacene was thermally evaporated on the BN monolayer, the crystallization of the pentacene molecules occurred at a low nucleation density and a high growth rate because the amorphous nature of the BN monolayer hindered the intermolecular packing between neighboring absorbates in the diffusion regime of the crystallization. We also found that the BN monolayer caused the similar effect in the solution phase crystallization of anthracenes. We fabricated organic thin film transistors (OTFTs) with the BN monolayers coated between the insulator and semiconductor layers. The electrical properties of these devices were compared with those constructed on the bare silica surface. The field-effect mobility (μ) of the device with the BN monolayer appeared 50% higher than those of silica. The morphology of the semiconductor layer was investigated using atomic force microscopy (AFM) and X-ray diffraction (XRD) method. AFM image showed that the pentacene layer on the BN monolayer consisted of larger and more interconnected grains than those on the silica surface. XRD spectra of the 50 nm thick pentacene film exhibited that the pentacene crystals on the BN monolayer consisted of mostly thin film phase (d001 = ~15.5 Å) while those on the silica surface contained both the thin film phase (d001 = ~15.5 Å) and bulk phase (d001 = ~14.5 Å).
9:00 PM - B10.37
Synthesis of Polymeric Self-Assembled Monolayer Using a Surface-Reactive Rod-Coil Diblock Copolymer.
Mingu Han 1 , Ji-Woong Park 1
1 , Gwangju Institute of Science and Technology, Gwangju Korea (the Republic of)
Show AbstractRodlike polymers which are composed of a rigid backbone with alkyl side groups can be consider as one-dimensional self-assembled monolayers (SAMs). If the rods with alkyl side groups are aligned on the surface with thickness of a single chain diameter, this SAMs become two-dimensional. Attaching a short surface-reactive coil to the end of rod, the rigid rodlike polymer can be anchored onto the surface via covalent bonding. The resulting surface tethered rodlike chains form a polymeric self-assembled monolayer (PSAM). Because the rodlike blocks of the PSAM are aligned to rotate in their azimuthal and polar anchoring angle, the PSAM may be stimuli-responsive. In our previous study, we showed that an amphiphilic rod-coil block copolymer was strongly adsorbed by the hydrogen bonding of coil blocks onto mica surface and that the rodlike PHIC blocks oriented planar to the surface to form liquid crystalline SAM[1,2]. Here we synthesize, poly(n-hexylisocyanate)-b-poly[(3-trimethoxysilyl)propyl methacrylate] (PHIC-b-PTMSPMA), a new rod-coil block copolymer which can form PSAM via covalent bonding of the alkoxysilyl containing PTMSPMA coil block to the silica substrate. The PSAM was simply produced by the immersion coating method. It exhibited uniform thickness of about 1.6 nm with low RMS roughness of below 0.4 nm. The water contact angle of the PSAM was about 100°. The PSAM showed an anisotropic morphology resulting from local ordering of grafted rods on annealing with tetrahydrofuran (THF) vapor. P3HT-based Field-Effect Transistors (FETs), which were fabricated using a PSAM-coated SiO2/Si substrate, exhibited about one order of magnitude higher charge carrier mobility than those using untreated silica dielectric layer. We also demonstrate that PSAM is micro-patternable using UV-induced photochemical cleavage of the methacrylate moieties of the anchoring blocks (PTMSPMA). Acknowlegdment : This work was supported by the Korea Science and Engineering Foundation (KOSEF) grant [R01-2008-000-12246-0] funded by the Korean goverment (MEST) and the Program for Integrated Molecular System at GIST, Korea Reference : [1] J. H. Kim, M. S. Rahman, J. S. Lee, J. -W. Park, J. Am. Chem. Soc. 2007, 129, 7756-7757 [2] J. H. Kim, M. S. Rahman, J. S. Lee, J. -W. Park, Macromolecules 2008, 41, 3181-3189
9:00 PM - B10.38
Degradation of Ir(ppy)2(dtb-bpy)PF6 iTMC OLEDs
Velda Goldberg 1 , Michael Kaplan 1 2 , Leonard Soltzberg 2 , Dolly Armira 2 , Megan Bigelow 1 , Rachel Brady 2 , Shannon Browne 1 , Bianca Dichiaro 1 , Heather Foley 1 , Lauren Hutchinson 2 , Alison Inglis 2 , Nicole Kawamoto 2 , Amanda McLaughlin 2 , Caitlin Millett 2 , Hanah Nasri 1 , Sarah Newsky 2 , Tram Pham 2 , Cassandra Saikin 2 , Mary Scharpf 2 , Melissa Trieu 2 , George Malliaras 3 , Stefan Bernhard 4
1 Physics, Simmons College, Boston, Massachusetts, United States, 2 Chemistry, Simmons College, Boston, Massachusetts, United States, 3 Materials Science & Engineering, Cornell University, Ithaca, New York, United States, 4 Chemistry, Princeton University, Princeton, New Jersey, United States
Show AbstractSimplicity of construction and operation are advantages of iTMC (ionic transition metal complex) OLEDs compared with multi-layer OLED devices. Unfortunately, lifetimes do not compare favorably with the best multi-layer devices. We have previously shown for Ru(bpy)3(PF6)2 based iTMC OLEDs that electrical drive produces emission-quenching dimers of the active species. We report evidence here that a chemical process may also be implicated in degradation of devices based on Ir(ppy)2(dtb-bpy)PF6 albeit by a very different mechanism. It appears that degradation of operating devices made with this Ir-based complex is related to current-induced heating of the organic layer, resulting in loss of the dtb-bpy ligand. (The neutral dtb-bpy ligand is labile compared with the cyclometallated ppy ligands.) Morphological changes observed in electrically driven Ir(ppy)2(dtb-bpy)PF6 OLEDs provide evidence of substantial heating during device operation. Evidence from UV-vis spectra in the presence of an electric field as well as MALDI-TOF mass spectra of the OLED materials before and after electrical drive add support for this model of the degradation process.
9:00 PM - B10.39
Industrial Applicability of Molecular Acceptors in Organic Electronic Devices using Wet-chemical Deposition.
Sibe Mennema 1 , Stephan Harkema 2 , Ralph Rieger 3 , Walter Stals 1 , Jorgen Sweelssen 1 , Hans Joachim Raeder 3 , Klaus Muellen 3 , Herman Schoo 2
1 , TNO Science and Industry, Eindhoven Netherlands, 2 , Holst Centre, Eindhoven Netherlands, 3 , Max Planck Institute for Polymer Research, Mainz Germany
Show AbstractThe relative position of the energy levels of the various materials used in organic electronic devices is of critical importance for their efficient operation. Molecular acceptors, e.g. tetracyanoquinodimethane (F4TCNQ), are molecules that have the capacity to raise the work function of a metal surface upon chemisorption. The chemisorption of such acceptors is accompanied by an electron transfer from the metal to the molecule, thus introducing local dipoles with their negative ends oriented away from the surface, and increasing the work function. The area-averaged work function of a metal surface can be adjusted by controlling the area density of such dipoles. This principle has previously been demonstrated through UHV deposition and XPS / UPS analysis [1].Here the transfer of this concept to a solution-based process, which was carried out in ambient conditions, is presented. Apart from results on F4TCNQ, also results are discussed of the purposely synthesised hexacyanohexaazatriphenylene (HATCN). HATCN is also a very strong acceptor, but is expected to persist better at the interface due its planar configuration, larger size and six instead of four binding sites. Spin-coating and dip-coating were used to deposit the electron acceptors onto the metal(oxide) surface. The area density of acceptors was controlled through the concentration of molecules in solution. Following deposition, Kelvin Probe Force Microscopy (KPFM) was used to investigate the induced work function changes. An increase of up to 0.9 eV was observed for various molecular acceptors on ITO, Ag and Au surfaces, and lower concentrations lead to smaller work function changes.Furthermore, results of devices in which molecular acceptors were incorporated are presented. Using wet-chemical processing in ambient conditions, Organic Light-Emitting Diodes (OLEDs) and Organic Photo-Voltaic Cells (OPVCs) were fabricated on lab-size 3×3 cm2 wafers as well as industrial size 15×15 cm2 wafers. The concentration of molecular acceptors had a substantial effect on the electronic and optical properties of the devices, clearly indicating that the work function changes observed by KPFM can be related to device characteristics. In the case of OLEDs, the onset voltage has been varied by a factor 2, and the luminance at a fixed voltage by an even larger factor. In the case of OPVCs, the open-circuit voltage can be controlled in a similar way. Routes towards exploiting these results in practical applications will be discussed.[1] N. Koch et al., Phys. Rev. Lett. 95 (2005) 237601
9:00 PM - B10.4
A New Physical and Electrical Model of Capacitor Consisting of Organic Semiconductor and Organic Dielectric.
Seung-Hyeon Jeong 1 , Chung-Kun Song 1
1 Electronics Engineering, Dong-A University, Busan Korea (the Republic of)
Show AbstractIn this study, we proposed a new physical and electrical model of capacitor consisting of pentacene organic semiconductor and PVP dielectric. The measured capacitances in the depletion region for low and high frequency mode as well as the capacitance in accumulation region for high frequency were identical to the theoretical values. However, for low frequency the measured capacitance in accumulation region was 40 nF/cm2 much larger than 8.6 nF/cm2 of the theoretical one. For SiO2 dielectric with pentacene this was not true. The accumulation capacitance for low frequency was identical to dielectric capacitance of 95 nF/cm2, which satisfied the theory of MOS capacitor. The reasons of the large accumulation capacitance for low frequency can be estimated from the two causes. The first one is due to the residual OH- ions in PVP, and the other on is due to the surface roughness of PVP dielectric layer. Both factors contributed to the parallel circuit effect of interface capacitance Cit to dielectric capacitance CPVP instead of serial circuit in the case of pentacene-PVP capacitor. This model was confirmed by the capacitor with PVP dielectric containing more OH- ions and rougher surface, which exhibited 55 nF/cm2 in accumulation region for low frequency mode.*This research was supported by a grant(F0004020-2008-31) from Information Display R&D Center, one of 21st Century Frontier R&D Program funded by the Ministry of Knowledge Economy of Korean government.
9:00 PM - B10.40
Comparison of Molecular Monolayer Interface Treatments in Organic-inorganic Photovoltaic Devices.
Jamie Albin 1 , Darick Baker 1 , Cary Allen 1 , Tom Furtak 1 , Reuben Collins 1 , David Ginley 2
1 Physics, Colorado School of Mines, Golden, Colorado, United States, 2 , National Renewable Energy Laboratory, Golden, Colorado, United States
Show AbstractExcitonic hybrid organic-inorganic solar cells are gaining viability as alternatives to p-n junction photovoltaics. Although hybrid cells typically have lower efficiencies than their inorganic counterparts, they are more compatible with inexpensive manufacturing techniques such as spray deposition and roll-to-roll processing, which can reduce the fabrication cost per photovoltaic watt. Polymer devices with nanostructured ZnO as the electron-accepting layer have the potential to improve carrier collection and power conversion efficiency in the bulk heterojunction approach to organic solar cells. The ZnO/polymer interface, however, is not optimal and properties such as polymer ordering and wetting at the interface need improvement. Functionalization of the ZnO surface with molecular monolayers has the potential to resolve these issues. In this study, we compare the performance of inverted planar ZnO/P3HT photovoltaic cells made from sol gel-derived ZnO that was functionalized using thiol and silane based attachment chemistries. Differences in the attachment scheme were explored using molecules with the same end group. For example, octadecyltriethoxysilane (OTES) and octadecylthiol (ODT) both yield surfaces with an 18 carbon alkyl chain termination. Both showed improved polymer ordering relative to control samples. The ODT-modified devices had higher efficiencies than OTES-treated devices, however, both treatments led to decreased short circuit current compared to optimized control devices. Similarly, the effect of the end group was explored using molecules that attach with the same chemistry but leave different, exposed, terminal groups. Phenyltriethoxysilane (PTES) treated ZnO, for example, shows significantly improved polymer wetting relative to OTES treatment and untreated surfaces. We discuss these observations in terms of the nature of the terminal group, differences in the attachment scheme (silicon vs. sulfur), and differences in surface coverage of the molecular layers. This work was supported by the National Science Foundation under Grants DMR-0606054 and DMR- 0820518.
9:00 PM - B10.41
Prediction of Dynamical Properties of Organic Field-Effect Transistors from DC Transistor Parameters.
Benedikt Gburek 1 , Veit Wagner 1
1 School of Engineering and Science, Jacobs University Bremen, Bremen Germany
Show AbstractDynamical properties of Organic Field-Effect Transistors (OFETs) are of crucial importance for almost any application. However, not direct AC data but DC measurements are usually used to optimize transistor performance. Here we present a systematic study to which extent DC parameters can actually be used to predict AC performance and AC limits of transistors.The standard FET theory for long channel devices predicts a maximum device bandwidth of ωB = µ V / L2. However, this holds only for ideal situations, e.g. without parasitic capacitances, and not too high frequencies. Furthermore, as was recently shown [1], contact resistances can pose severe additional high-frequency limits to the AC performance.To check the AC limit of a given transistor directly we perform a frequency scan up to the frequency where the gate current equals the drain current, which defines the true bandwidth of the device. The frequency-scanned AC voltage is applied at the gate while the drain contact is kept at a constant voltage.The dependence of the bandwidth on the drain-source voltage as well as on the DC offset of the gate-source voltage is analyzed in detail. Among others, this analysis allows for a more accurate determination of the threshold voltage, often difficult to be determined from standard DC transistor characteristics. In addition, it offers better insight into carrier mobility dependencies and distribution of energy states.A model which defines a correction factor to the ideal ωB = µ V / L2 behavior is proposed in order to predict the measured bandwidth from DC transistor parameters correctly. The model takes into account parasitic capacitances which occur from the source and drain contacts. Furthermore, limitations of the bandwidth due to contact resistance are presented and included in the model.[1] V. Wagner, P. Wöbkenberg, A. Hoppe, J. Seekamp, Appl. Phys. Lett. 89 (2006) 243515
9:00 PM - B10.42
Ambipolarity and Light Emission from Acene Based Transistors.
Martin Schidleja 1 , Christian Melzer 1 , Heinz von Seggern 1
1 Institute of Material Science, Technische Universität Darmstadt, Darmstadt Germany
Show AbstractPolycrystalline ambipolar light-emitting organic field-effect transistors (LEOFETs) offer new possibilities for the characterization of ambipolar devices. The main obstacles for the further development of LEOFETs is the lack of efficiency, brightness and the limited choice of materials allowing for high injection efficiency, charge carrier mobility and fluorescence yield. In our contribution the realization of low injection barrier LEOFETs will be investigated using different acenes as active layer and its relevance for organic light-emitting diodes (OLEDs) will be discussed. Devices based on F8BT (poly(9,9-di-n-octylfluorene-alt-benzothiadiazole)) as semiconductor and gold as source/drain metal show rather efficient light emission. Recent results disclose a contact dominated device behavior due to high injection barriers for both charge carrier types. In order to reduce the injection barriers Ca and Au as source and drain contacts in combination with organic materials such as pentacene, tetracene and ditetracene with electron affinities (EA) and ionization potentials (IP) matching the metal workfunctions are used. It will be demonstrated that all these device structures exhibit light emission, meaning ambipolar conduction, and how that can be utilized to investigate the injection efficiency of the individual material combinations. Although light emission verifies the presence of electrons and holes in all devices, the electron conduction in the channel deteriorates for decreasing EA of the organic semiconductor. Polycrystalline pentacene allows for rather balanced electron and hole conduction whereas the electron mobility worsens for ditetracene and tetracene. In all acene based devices the highest intensity of the emitted light is observed in the hole dominated regime, next to the Ca source electrode, indicating highly efficient electron injection from Ca into the respective acene. This behavior cannot be observed in F8BT based devices due to the high injection barriers for both charge carriers from the utilized gold electrodes. In the presented contribution the relative impact of injection compared to transport will be highlighted and its applicability to OLEDs will be discussed.
9:00 PM - B10.43
Interface Engineering in Organic Thin Film Transistors.
Philipp Stadler 1 , Anna Track 3 , Mujeeb Ullah 2 , Thockchom Singh 1 , Gebhard Matt 1 2 , Helmut Sitter 2 , Michael Ramsey 3 , N. Serdar Sariciftci 1
1 Institute for Organic Solarcells, Johannes Kepler University, Linz Austria, 3 Institute for Physics, Karl Franzens University, Graz Austria, 2 Institute for Solid State Physics, Johannes Kepler University, Linz Austria
Show AbstractInterface engineering in organic thin film transistors (oTFT) has become the key issue for optimizing the device operation. Recent effort has been made by using passivation layers on top of silicon dioxide (SiOx) and alumina (Al2O3) [1]. In this work we present a oTFT with C60 as semiconductor in bottom gate and top contact structure. We compared silicon dioxide, electrochemically grown alumina and divinyltetramethyldisiloxane-bis(benzocyclobutene) (BCB) on alumina as gate insulator [2,3,4]. The nature of the interfaces between C60 and the BCB and between C60 and the oxide insulators is studied in detail by X-ray and ultraviolet photoemission spectroscopy (XPS and UPS). The interface system fullerene and oxide is displaying large positive threshold voltages and lower drain/source currents in accumulation/depletion regime as compared to oTFT's using organic gate insulators [5]. Here we experimentally show that by introducing a thin polymeric buffer layer (BCB) between the oxide and the fullerene the threshold voltage of the transistor characteristics can be shifted from positive values in the oxide case to negative values in the polymer case [6]. The transistor is operating within one volt and we observe an enhanced on/off ratio (4 decades) and an electron mobility of approx. 1 cm2 V-1 s-1. We correlate this different transport performance to the change in the work function of the semiconductor seen in the UPS specta and the shift in binding energy of the C1s peak of the C60 in the XPS spectra respectively. [1] M. Halik, H. Klauk, U. Zschieschang, G. Schmied, C. Dehm, M. Schütz; S. Maisch, F. Effenberger, M. Brunnbauer, F. Stellacci, Nature 431, 963-966 (2004)[2] T. D. Anthopoulos, T. B. Singh, N. Marjanovic, N.S. Sariciftci, A. Ramil, H. Sitter, M. Cölle, D. de Leeuw, Appl. Phys. Lett. 89 213504-1 (2006)[3] T. B. Singh, N. S. Sariciftci, H. Yang, L. Yang, B. Plochberger, H.Sitter, Appl. Phys. Lett., 90 (2007)[4] R. Schroeder, L. A. Majewski, M. Grell, Adv. Mater. 16, 633 (2004)[5] G. Horowitz, R. Hajlaoui, R. Bourguiga, M. Hajlaoui, Syn. Met. 101, 401-404 (1999)[6] X. H. Zhang and Kippelen, Appl. Phys. Lett. 93, 133305 (2008)
9:00 PM - B10.44
New Air-Stable Organic Semiconductor for p-Channel Transistors with Large Mobility and Low-Voltage Integrated Circuits on Flexible Substrates.
Ute Zschieschang 1 , Tatsuya Yamamoto 2 , Kazuo Takimiya 2 , Tsuyoshi Sekitani 3 , Takao Someya 3 , Hagen Klauk 1
1 , Max Planck Institute for Solid State Research, Stuttgart Germany, 2 , Hiroshima University, Higashi-Hiroshima Japan, 3 , University of Tokyo, Tokyo Japan
Show AbstractThe performance of organic thin-film transistors (TFTs) often degrades upon exposure to air. This is due to the generation of charge traps as a result of the oxidation of the conjugated molecules. One strategy to improve the air stability of organic TFTs is thus the synthesis of organic semiconductors with reduced susceptibility to oxidation. For p-channel TFTs this implies a large ionization potential. However, most organic TFTs using semiconductors with large ionization potential show rather small hole mobilities, usually below 0.5 cm2/Vs [1-3]. This has been attributed to poor molecular ordering and hence poor overlap of the molecular orbitals. Recently, a six-ring fused heteroarene, dinaphtho-[2,3-b:2’,3’-f]thieno[3,2-b]thiophene (DNTT), was synthesized that has a large ionization potential (5.4 eV) and forms well-ordered films, with hole mobilities as large as 2 cm2/Vs for TFTs made on doped silicon wafers (serving as a global gate electrode) with a thermally grown SiO2 gate dielectric [4]. Here we report on the performance and stability of DNTT transistors, inverters and ring oscillators on glass and flexible polyethylene naphthalate (PEN) substrates. The TFTs and circuits use an inverted staggered (bottom-gate, top-contact) device structure with patterned aluminum gates, a thin gate dielectric based on an oxygen-plasma-grown aluminum oxide layer (3.6 nm thick) in combination with a self-assembled monolayer (SAM) of an aliphatic phosphonic acid (1.7 nm thick), a thermally evaporated DNTT layer and gold source/drain contacts. Gates, semiconductor, and source/drain contacts were patterned using shadow masks. Owing to the large capacitance of the AlOx/SAM gate dielectric (700 nF/cm2), the TFTs and circuits can be operated with low voltages of about 3 V. DNTT TFTs on glass have a mobility of 1.5 cm2/Vs, an on/off ratio of 1e7 and a subthreshold swing of 80 mV/decade. On flexible PEN, the TFTs have a mobility of 0.6 cm2/Vs, an on/off ratio of 1e7 and a subthreshold swing of 110 mV/decade. These mobilities are a factor of 2 larger than those of pentacene TFTs manufactured with the same technology [5]. Because the ionization potential of DNTT (5.4 eV) is much larger than that of pentacene (5 eV), the DNTT TFTs have significantly better air stability, showing no degradation while stored in ambient air for 3 months. Five-stage ring oscillators based on unipolar inverters with saturated load show stable oscillations for supply voltages between 2.2 and 5 V, with a signal delay of 37 µsec per stage (27 kHz) at 3 V and 18 µsec per stage (55 kHz) at 5 V. This is the highest frequency reported for flexible organic circuits at low supply voltage. [1] H. Meng et al., J. Am. Chem. Soc., vol. 123, p. 9214, 2001.[2] J. A. Merlo et al., J. Am. Chem. Soc., vol. 127, p. 3997, 2005.[3] J. Locklin et al., Adv. Mater., vol. 18, p. 2989, 2006.[4] T. Yamamoto et al., J. Am. Chem. Soc., vol. 129, p. 2224, 2007.[5] H. Klauk et al., Nature, vol. 445, p. 745, 2007.
9:00 PM - B10.45
Optical Stability of Small-molecule Thin-films Determined by Photothermal Deflection Spectroscopy.
Marco Stella 2 , Monica Della Pirriera 1 , Joaquim Puigdollers 1 , Cristobal Voz 1 , Jordi Andreu 2 , Ramon Alcubilla 1 , Joan Bertomeu 2
2 Fisica Aplicada y Optica, University de Barcelona, Barcelona Spain, 1 Electronic Engineering, Universidad Politécnica de Cataluña, Barcelona Spain
Show AbstractOrganic semiconductors represent a new interesting class of materials for several electronic applications. Organic solar cells performance have improved significantly in the last few years thanks to the optimization of the solar cell structure and, specially, to the ability to process new organic semiconductors with optimised properties. Among different deposition techniques, thermal evaporation in high-vacuum is the more suitable process to obtain small-molecule organic thin-films with well organize molecular structure. In this paper the optical absorption properties of n-type (C60 and PTCDA) and p-type (CuPc) small-molecule semiconductors are investigated by optical transmission and Photothermal Deflection Spectroscopy (PDS). Results show the usual absorption bands related to HOMO-LUMO transitions in the high absorption region of transmission spectra. PDS measurements also evidences exponential absorption shoulders with different characteristic energies (47 meV for CuPc, 50 meV for PTCDA and 87 meV for C60). In addition, broad bands in the low absorption level are observed for C60 and PTCDA thin-films. These bands have been attributed to contamination due to air exposure (1).In order to get deeper understanding of the degradation mechanisms single, bilayer and co-evaporated thin-films have been characterized by PDS. Dependence of the optical coefficient on light illumination and air exposure have been studied and correlated to the structural properties of the films (as measured by X-Ray Diffraction Spectroscopy). Results show that CuPc:PTCDA and CuPc:C60 co-evaporated films are stable after light and air exposition. However, single layers of C60 shows significant increase of the low level optical absorption coefficient.
9:00 PM - B10.46
Non-Volatile Organic Memory based on Electrically Doped Organic Heterostructures.
Frank Lindner 1 , Phillip Sebastian 1 , Bjoern Lussem 1 , Karl Leo 1
1 Institut für Angewandte Photophysik, TU Dresden, Dresden Germany
Show AbstractWithin the last few years organic memory devices have attracted considerable attention. Several different approaches for organic memory devices have been reported in literature that show bistable memory behaviour with high switching speeds and high ON/OFF ratios [1]. The drawbacks of most of these devices are a rather low stability and reproducibility and a lack of knowledge about he precise switching mechanism.Here, we report on a novel approach based on charge trapping in a two-well heterostructure device. Consisting of materials which are well known from organic light emitting devices, we obtain reproducible bistable electrical switching and memory phenomena. Depending on the number of charges stored in the wells the resistance of the device changes, i.e. the measured current-voltage characteristic shows two states of different conductivity at the same applied voltage. The ratio between the resistance in the OFF state and in the ON state can be varied by the write and erase voltage. More than 2000 Write-Read-Erase cycles are obtained without degradation. The memory state is retained for several days before reading the devices. In conclusion, the memory concept we present shows higher reproducibility and stability compared to other organic memories. Device performance tests show that the heterostructure devices are a promising candidate for low-cost, electrically addressable data storage applications.[1] J.C. Scott, L.D. Bozano, “Nonvolatile Memory Elements Based on Organic Materials”, Advanced Materials 2007, 19, 1452-1463.
9:00 PM - B10.47
Synthesis of Monofunctionalized Electroactive Molecules and their Application in Electrode Modification in Organic Devices.
Gabriele Kremser 1 , Thomas Rath 1 , Thomas Griesser 2 , Gregor Trimmel 1
1 Institute for Chemistry and Technology of Materials, Graz Universitiy of Technology, Graz Austria, 2 Institute of Chemistry of Polymeric Materials, Montanuniversität Leoben, Leoben Austria
Show AbstractThe control of the interface of organic semiconducting materials and inorganic electrodes or semiconductors is one of the crucial points for high performing organic electronic devices. Therefore, different surface treatments and/or interfacial layers are often used in the device assembly. Self assembled monolayers e.g. organosilanes, thiols or phosphonates have been employed in the modification of indium tin oxide electrodes and gate oxides in organic field effect transistors. In this contribution we present the syntheses of novel functionalized electroactive organic compounds which are capable to interact with inorganic oxide materials. These molecules consists of an electroactive unit of a stilbene, a quarterthiophene or a biphenylterthiophene moiety which is asymmetrically capped with a polar group e.g. phosphonic esters, carboxylic acid and hydroxy functionality. The PPV-compounds have been synthesized by Wittig reaction or Horner-Emmons, whereas the thiophene-based molecules have been prepared by Suzuki and Stille cross coupling reactions. All materials were identified by nuclear magnetic resonance (NMR) and infrared (IR) spectroscopy. The optical properties were investigated with UV-vis and photoluminescence spectroscopy. First result on the use of these molecules for the modification of indium tin oxide electrodes in organic electronic devices will be presented.
9:00 PM - B10.48
Bipolar Transport in Organic Field-effect Transistors: Organic Semiconductor Blends versus Contact Modification.
Andreas Opitz 1 , Michael Kraus 1 , Markus Bronner 1 , Julia Wagner 1 , Wolfgang Bruetting 1
1 Institute of Physics, University of Augsburg, Augsburg Germany
Show AbstractThe achievement of bipolar transport is an important feature of organic semiconductors, both for a fundamental understanding of transport properties and for applications such as complementary electronic devices. We have investigated two routes towards organic field-effect transistors exhibiting bipolar transport characteristics. As a first step, mixtures of p-conducting copper-phthalocyanine (CuPc) and n-conducting buckminsterfullerene (C60) were used to realize ambipolar field-effect transistors. As a second step, bipolar transport in copper-phthalocyanine was achieved by a modification of the gate dielectric in combination with a controlled variation of the electrode materials used for carrier injection. Although both routes are in principle successful, there are some characteristic differences.In molecular blends, the charge carrier mobilities decrease exponentially by dilution of the respective transport material. This indicates that percolation is a crucial feature in mixtures of both materials to achieve ambipolar carrier flow. In this case, balanced mobilities can be achieved by adjusting the mixing ratio. This is a necessity to design ambipolar inverters with symmetric transfer characteristics. In neat films of one single material, suitable contact modification allows for bipolar charge-carrier transport together with the prevention of charge carrier traps at the insulator/semiconductor interface. Here the obtained electron and hole mobilities differ by less than one order of magnitude.
9:00 PM - B10.49
Hysteresis-free Electron Currents in Conjugated Polymers.
Irina Craciun 1 , Yuan Zhang 1 , Paul Blom 1
1 Molecular Electronics, University of Groningen, Groningen Netherlands
Show AbstractCharge transport is an important issue with regard to the understanding and optimization of electronic devices made from conjugated polymers. In the last two decades a large effort has been put on the characterization of the transport of holes, which is the dominant charge carrier. It has been demonstrated that the hole transport is governed by hopping between localized states, characterized by a mobility that depends on density, electric field and temperature.[1] The transport of electrons is far less well characterized, and the strongly reduced electron currents are attributed to trapping of electrons.[2] A major problem with the investigation of the electron transport is the construction of so-called electron only devices, where hole blocking electrodes are required that are usually reactive. The resulting J-V characteristics of these devices often exhibit, next to low currents, strong hysteresis effects that strongly hinder the interpretation of, for example, a temperature scan. Major candidates responsible for the strong hysteresis are electrons trapped either in the bulk of the polymer or at the hole blocking electrode/ polymer interface. The transport of electrons in electron-only devices based on poly (2-methoxy, 5- (2’ ethyl-hexyloxy)-p-phenylene vinylene) (MEH-PPV) is investigated for various hole-blocking bottom electrodes as well as purification of the polymer. Using a variety of metallic electrodes as a bottom contact no improvement in the observed hysteresis is observed. As a next step a n-type doped PPV-based polymer was used as bottom electrode for the electron-only devices. Due to the n-type doping the J-V characteristics of the doped electron-only device are hysteresis free. However, addition of an undoped MEH-PPV layer on top directly resulted in large hysteresis, showing that it originates from trapping in the bulk of the undoped polymer. As a final step we demonstrate that by proper purification of the MEH-PPV hysteresis free electron-only currents can be obtained, enabling a further quantitative characterization of the electron transport mechanisms in this class of materials. [1] C. Tanase et al., Phys. Rev. B 70, 193202 (2004)[2] M. M. Mandoc et al., Phys. Rev. B 73, 155205 (2006)
9:00 PM - B10.51
Hole and Electron Polaron Absorptions Measured using Charge Modulation Spectroscopy on Ambipolar Polyfluorene-based FETs.
Matt Bird 1 , Ni Zhao 1 , Henning Sirringhaus 1
1 Optoelectronics Group, Cambridge University, Cambridge United Kingdom
Show AbstractWhen a charge resides on a polymer it couples strongly to a local distortion in the molecular structure; the combination is known as a polaron. The polaronic state induces new optical transitions with sub band gap energies. In combination with quantum chemical modeling, the in-situ measurement of these new transitions using charge modulation spectroscopy on operating devices can offer a unique insight into the molecular packing, the degree of polaron localization and the origin of energetic disorder [1, 2] in semiconducting polymers.We present here for the first time the charge-induced absorption spectra for both the hole and electron polarons in a range of polyfluorene-based conjugated polymers. We perform these measurements on polymer transistor structures with ambipolar charge injection and transport [3] in which one can induce a hole accumulation layer for negative gate voltage and an electron accumulation layer for positive gate voltage. By comparing the induced absorption spectra for electron and hole polarons we obtain information about fundamental electron-hole symmetries in conjugated polymers [4]. In particular, over the range of polymers tested, we can correlate shifts in the spectra with the different degrees of localization of electrons and holes on particular units of the backbone and different degrees of interchain interaction.[1] J-F. Chang, M. Giles, M. Heeney, I. McCulloch and H. Sirringhaus Phy. Rev. B 76, 205204 (2007)[2] D. Beljonne, J. Cornil, H. Sirringhaus, P. J. Brown, M. Shkunov, R. H. Friend, J.-L. Brédas, Func. Mater. 11, 229-234 (2001)[3] J. Zaumseil, C. L. Donley, J.-S. Kim, R. H. Friend, H. Sirringhaus, Adv. Mater. 18, 2708–2712 (2006)[4] K. Fesser, A. R. Bishop, and D. K. Campbell Phys. Rev. B 27, 4804 - 4825 (1983)
9:00 PM - B10.52
Fully Solution-Processed Organic Solar Cells on Metal Foil Substrates.
Whitney Gaynor 1 , Jung-Yong Lee 2 , Peter Peumans 2
1 Materials Science and Engineering, Stanford University, Stanford, California, United States, 2 Electrical Engineering, Stanford University, Stanford, California, United States
Show AbstractPolymer bulk heterojunction photovoltaic cells are an area of intense research because they show promise as a lower-cost alternative to their inorganic counterparts. Much of this cost reduction relies on the ability to use high-throughput processing techniques such as roll-to-roll coating and depositing materials from solution. However, the highest performance polymer bulk heterojunction cells require a vacuum-deposited metal contact. In this talk, we report polymer bulk heterojunction cells, based on the poly-3-hexylthiophene and phenyl-C61-butyric acid methyl ester (P3HT:PCBM) system, in which each layer in the cell, including the top transparent electrode, is processed from solution onto metal foil substrates. Silver-coated metal foil is used as the cathode, followed by a solution-processed cesium carbonate interface layer to enhance electron transport. The active layer is the bulk heterojunction P3HT:PCBM, followed by a poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) layer for hole collection, and a laminated transparent solution-processed silver nanowire film as the anode. Power conversion efficiencies of 1.6% were obtained by optimization of the cathode and anode interfaces. With continued optimization, efficiencies approaching those of conventionally fabricated organic PV cells may be reached. The metal foil substrate offers advantages such as low cost and superior barrier properties. Moreover, the metal foil can be replaced by metal fibers enabling fiber-shaped solar cells. The metal foil also offers an easy and low-cost way to obtain substrate shaping to enhance light trapping in the cell, further increasing the power conversion efficiency.
9:00 PM - B10.53
Chemical Modification of Self-assembled Monolayers: Tailor-made and Reversible Surface Functionalization Schemes.
Stephanie Hoeppener 1 , Claudia Haensch 1 , Manuela Chiper 1 , Ulrich S. Schubert 1 2
1 Lab. of Macromolecular Chemistry and Nanoscience, Eindhoven University of Technology, Eindhoven Netherlands, 2 Lab. of Macromolecular and Organic Chemistry, Friedrich-Schiller-University, Jena Germany
Show AbstractChemical surface reactions performed on self-assembled monolayers represent a versatile method to tailor the properties of surfaces. As a suitable precursor molecule 11-bromoundecyltrichlorosilane can be used, which forms reasonable well ordered monolayers on glass and silicon substrates. The bromine functionalities of such monolayers can be employed in a number of derivatization reactions, leading to a variety of functional groups, i.e. thiols, azides, or amines. Besides of the direct conversion of the bromine functions, reaction schemes, i.e. click chemistry, can be employed to further increase the variety of available surface functions and/or to covalently bind functional molecules. Examples include here the covalent binding of dye molecules or the introduction of supramolecular precursors, which are able to tailor the photochemical properties of surfaces. Terpyridine motifs represent here an example that allows to reversibly modify the optical properties of surfaces, by external stimuli. In combination with electrochemical structuring techniques it is demonstrated that different functional groups can be locally introduced in a sequential derivatization approach, which leads to the fabrication of complex structures, that can be, e.g., used to fabricate nanoelectronic device components.
9:00 PM - B10.54
Temperature Modulation Spectroscopy for Probing Excitonic Properties of Organic Semiconductors.
Abhishek Yadav 1 , Kwok Chan 3 , Max Shtein 2 , Kevin Pipe 1
1 Department of Mechanical Engineering, University of Michigan, Ann Arbor, Michigan, United States, 3 Department of Mechanical engineering, Hong Kong Polytechnic University, Hong Kong China, 2 Department of Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan, United States
Show AbstractThe performance of organic optoelectronic devices is dependent on exciton generation and transport properties that govern optical absorption and emission. Excitons in organic thin films fall into two categories: 1) Frenkel excitons in which both the hole and electron are bound on the same molecule, and 2) charge transfer (CT) excitons in which the hole and electron are bound to each other but exist on neighboring molecules. Due to disorder and self-polarization, Frenkel excitons are more common, although CT can also play an important role (e.g. in mediating charge separation in photovoltaic devices). Optical spectroscopy is widely used to study exciton transitions, spectral broadening, and oscillator strength. Simple absorption spectroscopy is better suited for Frenkel excitons, because of their larger oscillator strength. Modulation spectroscopy can be used to study CT excitons; for example, electric field modulation spectroscopy utilizes the Stark effect to provide the derivative of the absorption spectrum with respect to exciton transition level, and can be used to identify weak CT transitions. Unfortunately, the application of electric fields can create parasitic effects, including strong electric fields at interfaces and destruction of crystal symmetry, reducing the method’s utility for thin films and nanostructured samples.Here, we introduce temperature modulation spectroscopy as an alternative method to probe excitonic properties of organic semiconductors. The oscillator strength of CT excitons decreases with temperature, due to a reduced overlap of wavefunctions between neighboring molecules caused by lattice thermal expansion [1]. For molecular crystals, with a small intermolecular overlap of electronic wavefunctions, the oscillator strength of CT excitons is balanced by Frenkel exciton states [2]. Hence, the oscillator strength of Frenkel excitons is expected to increase with a decrease in oscillator strength of CT excitons [1]. We use this difference in behavior to distinguish CT and Frenkel excitons in device-relevant organic thin films, including pentacene and CuPc deposited on glass. The temperature of the film is modulated in a lock-in amplification technique by passing a sinusoidal current through a thin silver film heater deposited on top of organic film, while transmittance is measured using a photomultiplier tube. A Lorentz-Lorenz dielectric model is used to fit the spectroscopic photometry data, yielding information about excitonic transitions, oscillator strength, and broadening. To obtain the temperature dependencies of these exciton properties, we fit the derivative of the Lorentz-Lorenz model to the modulated transmission data. We show that temperature modulation spectroscopy is useful for studying the excitonic properties of nanostructured thin films, without many of the parasitic effects of other modulation spectroscopies.[1] Tanaka et al, J Chem. Phys 95: 2371, 1991[2] Hernandez et al, J Chem Phys 50: 1524, 1966
9:00 PM - B10.55
3-dimensional Organic Field Effect Transistors: Charge Accumulation in the Vertical Semiconductor Channels.
M. Uno 1 2 , I. Doi 3 , K. Takimiya 3 , Jun Takeya 1
1 , Osaka University, Toyonaka Japan, 2 , TRI-Osaka, Izumi Japan, 3 , Hiroshima University, Higashi-hiroshima Japan
Show AbstractOrganic electronics have attracted much attention as “post-silicone electronics” due to the mechanical flexibility, as well as low-cost and energy-saving fabrication processes of their devices. Among them, organic field-effect transistors (OFETs) are the key devices in which further development is necessary for such applications as full-flexible displays, incorporated in their matrix-controlling elements. So far, though much effort have been devoted to material development, even the best value of the carrier mobility μ of organic semiconductors remain in the order of 1 cm2/Vs except for single-crystal devices, resulting in unsatisfactory current amplification per pixcel. Though decreasing the channel length L and increasing the width W can be another approach, this effect is limited in the condition that the channels and the electrodes are confined in the same plane as the conventional OFETs. In this presentation, we propose a three-dimensional organic field-effect transistor (3D-OFET) to accumulate charge in its vertical semiconductor channel, so that space availability for the field-induced carriers is essentially enlarged [1].A multi-columnar structure is built to multiply the channel area, partially dry-etching Si substrates and thermally oxidizing their surfaces to form gate dielectric SiO2 layer. On the sidewall of the column, we deposit a vertical semiconductor layer of dinaphtho[2,3-b:2',3'-f] thieno[3,2-b]thiophene (DNTT), which was reported to have high μ (~ 1 cm2/Vs) and excellent air stability. Since W corresponds to the total length of all the column edges and L equals to the height of the columns, one can design the devices without constraints of the lateral space, which results in much higher ratio of W/L. Indeed, pronounced field-effect amplification is realized with 500-nm thick SiO2 gate insulator with the on-off ration of 106, in which current amplification up to 100 μA in an area of 500 μm square pixel is achieved with VG of 5 V and drain voltage VD of 5 V. Carrier mobility of the DNTT film in the vertical channel is estimated to be around 0.7 cm2/Vs.On the way for the organic flexible displays, the next challenge is to build the 3D-OFETs on plastic substrates; however it is not welcomed to complicate the fabrication process with additional deposition of dielectrics. Therefore, we propose to use an electric double-layer (EDL) capacitance CEDL of ionic liquid, of which we reported outstanding gating performance recently. Similar current-amplification capability is maintained for the ionic-liquid gated 3D-OFETs as compared to that of the above SiO2–gated devices; 0.5 V is enough for both VG and VD to obtain ID of 15 μA, resulted from much larger CEDL but smaller μ. The results already demonstrate usefulness of the 3D structure in achieving sufficient current per pixel for matrix-controlling elements, which can drive industrial development of organic flexible displays.[1] M. Uno et al., Appl. Phys. Lett. 93, 173301 (2008).
9:00 PM - B10.56
Negligible Contact Resistances in Organic Single-crystal Transistors with Secondary Gates on Source and Drain Electrodes.
K. Nakayama 1 , T. Uemura 1 , M. Uno 1 2 , Jun Takeya 1
1 , Osaka University, Toyonaka Japan, 2 , TRI-Osaka, Izumi Japan
Show AbstractTo realize the maximum device performance of organic field-effect transistors (OFETs), a significant challenge is to achieve efficient carrier injection at the electrodes. Such contact performance is the most seriously concerned for short-channel devices with the channel length typically less than sub-micrometers, though they are highly attractive because of their high-frequency response and capability of high-density integration. As compared to common silicon metal-oxide-semiconductor field-effect transistors where heavily doped carrier-rich region is incorporated next to the channel, all the reported organic field-effect transistors suffer from the carrier injection from different materials such as metals. In this presentation, we report that the influence of injection barriers can be negligible with a construction of a contact between a carrier-rich region and the channel region to be gated using the same semiconductor material, even for devices with low channel resistances such as rubrene single crystal transistors with micron-scale channel lengths.Previously, we reported a device structure with “split gates” on the source and drain electrodes buried in the gate-insulating layers, so that the carrier density in the organic semiconductors in the vicinity of the source and drain electrodes can be varied independently of the primary gate electric fields applied to the central channel in the semiconductors. Thereby, the carrier reservoirs are formed in the semiconductor locally near the electrodes by the secondary gate voltage. To fabricate the bottom contact OFETs, we first deposit the split-gate electrodes of gold on 500-nm thick SiO2 / doped silicon substrates by evaporation. An amorphous fluoropolymer (CYTOP, Asahi Glass Co.) dielectric is prepared to the thickness of 1 μm for the second gate dielectric layer, and source and drain electrodes are formed to the size slightly smaller than the split-gate electrodes, so that the split gate extends to the semiconductor channel in the vicinity of the edges of the source and drain electrodes. Finally, a thin platelet of rubrene crystal, which was independently grown by physical vapor transport technique, is laminated by natural electrostatic force. Transfer characteristics of devices with five different channel lengths are measured sweeping the primary gate voltage with the application of the maximum secondary split-gate voltages to prepare the “hole-rich” region in the rubrene crystals. As the result of the gradual channel plot, the normalized contact resistance RCW was immeasurably small, where RC and W represent contact resistance estimated from the intercept of the plot and the channel width, respectively. The result means that the value is smaller than 100 Ωcm in our present setup. At least being comparable to the smallest contact resistance reported so far, it is required to be measured with improved measurement systems. [1] K. Nakayama et al., Appl. Phys. Lett. 93, 153302 (2008).
9:00 PM - B10.57
Effect of Monochromatic Wavelength and Intensity on P3HT/PCBM Device Characteristics.
Harold Evensen 1 , Garth Berriman 2 , Warwick Belcher 2 , John Holdsworth 2 , Paul Dastoor 2
1 Chemistry & Engineering Physics, University of Wisconsin-Platteville, Platteville, Wisconsin, United States, 2 Centre for Organic Electronics, University of Newcastle, Callaghan, New South Wales, Australia
Show AbstractIn order to probe photon energy-dependent mechanisms in bulk heterojunction solar cells, high intensity LEDs were used to excite poly(3-hexylthiophene) (P3HT)/ [6,6]-phenyl C61-butyric acid methyl ester (PCBM) devices. The light sources, ranging from 450 nm to 640 nm, delivered a variable, monochromatic intensity from less than 0.5 mW/cm2 to over 50 mW/cm2, or up to eight to twenty times the equivalent solar irradiance within their bandwidths. The open circuit voltage and short circuit current varied with intensity, and the open circuit voltage exhibited distinctly different behavior under red light as compared to the other wavelengths. The results are compared with the predictions of several device models.
9:00 PM - B10.58
Permanent and Pattern-resolved Adjustment of the Surface Potential of Graphene-like Carbon Through Chemical Functionalization.
Fabian Koehler 1 , Norman Luechinger 1 , Dominik Ziegler 2 , Evagelos Athanassiou 1 , Robert Grass 1 , Antonella Rossi 3 4 , Christofer Hierold 2 , Andreas Stemmer 2 , Wendelin Stark 1
1 Chemistry and Applied Biosciences, ETH Zurich, Zurich Switzerland, 2 Mechanical and Process Engineering, ETH Zurich, Zurich Switzerland, 3 Materials, ETH Zurich, Zurich Switzerland, 4 Chimica Inorganica ed Analitica, Universita degli Studi di Cagliari, Cagliari Italy
Show AbstractThe potential use of graphene in electronics requires reliable and pattern-controlled methods to inject or remove electron density from the two-dimensional carbon honeycomb lattices. Combining well established radical chemistry at ambient conditions and classical lithography, we find that the structure of graphene layers can be permanently altered through covalent chemical functionalization. We further demonstrate how the classic Hammett concept and the Linear Free Enthalpy Relationship from organic chemistry can be used to predict the surface potential shifts in graphene-like carbon surfaces.Our study (1) reveals an astonishingly simple yet accurate method to adjust the surface potential in graphene layers. Next to the direct application of such patterning in device fabrication, the here shown covalent attachment introduces a third dimension in the two-dimensional graphene base. Such functionalization can ultimately lead to covalent attachment of molecular electronics, circuitry incorporated chemical sensors or actuators and offers an alternative approach to address the contacting problem in the nm range. The possibility to print a potential pattern onto a graphene sheet was only conceptually shown at present, but this approach offers a permanent polarization of the graphene sheets as in the case of gate-induced changes in conductivity. Latter will be of crucial importance when using a larger graphene sheet as a starting material in device fabrication (3D analogy: native Si wafer), which is then doped and partially oxidized (3D analogy: insulating silicon oxide). One might envision that for graphene, doping or oxidation to an insulating state could therefore be achieved through simple chemical processing and classical or dip-pen lithography.[1]F.M. Koehler, N.A. Luechinger, D. Ziegler, E.K. Athanassiou, R.N. Grass, A. Rossi, C. Hierold, A. Stemmer, W.J. Stark, Angew. Chem. Int. Ed., 2008, in print
9:00 PM - B10.59
Low Temperature Thermal Conductivity of Rubrene Single Crystals.
Y. Okada 1 , M. Uno 1 2 , M. Yamagishi 1 , Jun Takeya 1
1 , Osaka University, Toyonaka Japan, 2 , TRI-Osaka, Izumi Japan
Show AbstractRecently, organic crystals are gaining considerable interest linked to such practical applications as electronic semiconductor devices and nonlinear optical components; rubrene single crystal field-effect transistors exhibit one-order higher performance as compared to popular polycrystalline thin-film organic transistors and such materials as DAST shows excellent nonlinearity in response to light irradiation because of vibronic coupling to molecular polarization. However, it is concerned that density of dilutely distributed defects in these crystals is generally difficult to define, though it is recognized as crucial factor for transport or optical performances of the above devices. In this work, we measured low-temperature thermal conductivity of phonons to estimate their mean-free paths and density of crystalline defects responsible for scattering the phonons. It is known that temperature dependence of thermal conductivity in high-quality single crystals shows a pronounced peak in the temperature range below typically 30 K, below which phonons are predominantly scattered by defects. At higher temperatures, phonon-phonon Umklapp process dominates their scattering events.Thermal conductivity is measured from 0.5 K to room temperature using a steady-state one heater two thermometer technique. A set of mm-size chip resistors are mechanically attached for stand-alone crystals in case they have the length of subcentimeter. In addition, we have developed a measurement device incorporating resistive thin films of ZrN with the dimension of 100 μm, which work both for the heater and the thermometers. Employing MEMS (Micro Electro-Mechanical Systems) technique, these resistive thin films are sustained only by 1-μm thick membrane, so that the heat current is essentially restricted to the crystal. Using the latter technique, we can measure crystals of submillimeter sizes.Temperature profiles of the thermal conductivity of rubrene single crystals grown by physical vapor transport showed a well-defined peak, indicating that the crystal is indeed of exceptional quality as compared to literature for typical organic charge-transfer complex. The crystals grown from solution have smaller peaks in general, which suggests more inclusion of defects in the crystals. It turned out that it grows in proportion to T2 at low temperatures, meaning that the dominant scattering source is strain dislocations distributed in the crystals. Moreover, as the result of further quantitative estimation, typical density of the crystal dislocations is estimated to be in the order of 1016 cm-3, which is consistent with the performance of double-sided rubrene single crystal transistors interacting with each other over 1-μm distance, if we assume the dislocations themselves induce deep hole-trap levels in the bulk of the rubrene crystals.
9:00 PM - B10.6
Vertically Phase-separated Conjugated Molecules / Dielectric Bilayer Structure for High Performance Organic Transistors.
Wi Hyoung Lee 1 , Jung Ah Lim 1 , Donghoon Kwak 1 , Kilwon Cho 1
1 Chemical Engineering, POSTECH, Pohang, Kyungbuk, Korea (the Republic of)
Show AbstractWe report on the structural development and phase separation behavior of spin-cast triethylsilylethynyl anthradithiophene (TES-ADT) and poly(methyl methacrylate) (PMMA) blends and their application in field-effect transistors (FETs). The difference of surface energy between TES-ADT and PMMA causes TES-ADT–the phase with lower surface energy–segregating at the air-film surface after spin-casting. Furthermore, TES-ADT molecules in blended films can move toward the air-film surface after solvent-vapor annealing by the minimization of surface energy, and large crystals of TES-ADT are formed at the surface. The use of these phase-separated blended film as semiconducting (TES-ADT) and dielectric (PMMA) layers leads to high performance FETs (i.e., field-effect mobility as high as 0.6 cm2V-1s-1 and nearly zero hysteresis) because conducting channel is formed at the molecular interface of phase-separated TES-ADT crystals and PMMA dielectric in a one step-process. In addition, all organic FETs onto flexible substrates using phase-separated films of TES-ADT and PMMA are successfully demonstrated using all solution process.Acknowledgement. This work was supported by a grant (F0004021-2008-31) from the Information Display R&D Center under the 21st Century Frontier R&D Program, Creative Research Initiative(CRI)-Acceleration Research (R17-2008-029-01000-0), and a Grant (RTI04-01-04) from the Regional Technology Innovation Program of the MOCIE.
9:00 PM - B10.60
Low and Intermediate Coverage Self-assembled Structures of Alkanethiolates and Phenylthiolate on Au(111) Incorporating Au-adatoms
Peter Maksymovych 1 2 , John Yates, Jr. 3 2
1 Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States, 2 Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania, United States, 3 Department of Chemistry, University of Virginia, Charlottesville, Virginia, United States
Show AbstractMolecular self-assembly is a quickly developing field of nanoscience that aims to tailor self-recognizing and self-organizing properties of molecules for the bottom-up construction of complex molecular systems and implementation of designer molecular functionality. Self-assembled monolayers of alkanethiols on Au(111) are intensely studied both as a model system yielding three-dimensional, crystalline monolayers and in practical applications involving surface functionalization. Despite numerous previous attempts to understand the atomistic picture of self-assembly, most of its aspects have remained controversial. Until only recently, the gold surface has been considered a passive, unreconstructed template which provides a series of high-symmetry adsorption sites for the alkylthiolate molecules. Several recent experiments have, however, challenged this conservative view [1-4]. In this talk we will present self-assembled structures of the methylthiolate (CH3S) from the lowest up to its saturation coverage of 1/3 ML, developed based on high resolution STM images and DFT modeling. All these structures are constructed using essentially the same building block of two CH3S-species joined by a Au-adatom. Contrary to a common view that methylthiolate does not form a two-dimensional stripe-phase monolayer, we show compelling STM evidence that methylthiolate-adatom complexes do arrange into a stripe-like structure, albeit with preferentially 1D ordering. We compare the “stripe-phases” of methylthiolate, propylthiolate and phenylthiolate [4], each of which is distinct despite being constructured from constitutionally identical building blocks. The case of phenylthiolate is intriguing as the steric hindrance of the phenyl groups prevents the phenylthiolate-adatom complexes from forming well-ordered stripes as in methylthiolate. However, weak C-H…S hydrogen bonding does arrange these complexes in a locally-ordered fashion. Phenylthiolate self-assembly on the Au(111) surface is therefore strikingly similar to its collective behavior on nanoparticles comprising as few as 100 atoms. Thorough understanding of the low-coverage phases will enable resolving the self-assembled structures at higher coverages, with the symmetry of (3x4) and the √3x√3R30o for methylthiolate and c(4x2) for long-chain alkanethiolates.[1] P. Maksymovych, D. C. Sorescu, and J. T. Yates, Jr., Phys. Rev. Lett. 97 (2006) 146103.[2] M. Yu, N. Bovet, C. J. Satterley, et. al., Phys. Rev. Lett. 97 (2006) 166102.[3] A. Cossaro, R. Mazaarello, R. Rousseau, et. al., Science 321 (2008) 943.[4] P. Maksymovych, J. T. Yates, Jr., J. Am. Chem. Soc. 130 (2008) 7518.P. M. and J.T.Y: Supported by the W. M. Keck Foundation and by the Army Research Office. P.M.: Research performed in part as a Eugene P. Wigner Fellow and staff member at the Oak Ridge National Laboratory, managed by UT-Battelle, LLC, for the U.S. Department of Energy under Contract DE-AC05-00OR22725.
9:00 PM - B10.61
The Self-assembly of Thiol-modified Diamondoids on Silver and Gold
Trevor Willey 1 , Jonathan R Lee 1 , Jason Fabbri 2 , Peter Schreiner 3 , Andrey Fokin 3 , Boryslav Tkachenko 3 , Natalie Fokina 3 , Jeremy E Dahl 4 , Robert M Carlson 4 , Louis Terminello 4 , Nick Melosh 2 , Tony van Buuren 1
1 , Lawrence Livermore National Laboratory, Livermore, California, United States, 2 , Stanford University, Stanford, California, United States, 3 , Justus-Liebig University, Giessen Germany, 4 MolecularDiamond Technologies, Chevron, Richmond, California, United States
Show AbstractHigher diamondoids, hydrocarbon cages with a diamond-like structure, have largely evaded laboratory synthesis but can be purified from petroleum sources. This new class of nanometer-sized rigid hydrocarbon molecules shows promise in various areas of nanotechnology. Particularly, computation indicates individual diamondoids possess negative electron affinity, and diamondoid monolayers exhibit incredibly intense, monochromatic photoemission. Such surface-attached diamondoids have technological possibilities as high-efficiency field emitters in molecular electronics, as well as other nanotechnological applications, and fundamental studies of the properties of these monolayers are a necessary precursor.Methods to site-selectively functionalize diamondoids enable formation of controllable diamondoid self-assembled monolayers on surfaces. We have investigated a number of thiol-modified diamantanes, triamantanes, tetramantanes, and pentamantanes adsorbed on silver and gold, using near-edge x-ray absorption fine structure spectroscopy (NEXAFS) and x-ray photoelectron spectroscopy (XPS). The results illustrate how thiol position, diamondoid size, and diamondoid shape affect the diamondoid self-assembled monolayer structure.
9:00 PM - B10.63
Oligothiophene Derivatives Functionalized with a Diketopyrrolopyrrolo Core for Solution-Processed Field-Effect Transistors: Effect of Alkyl Substituents and Thermal Annealing.
Mananya Tantiwiwat 1 2 3 , Arnold Tamayo 2 3 , Ngoc Luu 2 3 , Xuan-Dung Dang 2 3 , Thuc-Quyen Nguyen 2 3
1 Physics, University of California, Santa Barbara, Goleta, California, United States, 2 Chemistry and Biochemistry, University of California, Santa Barbara, Goleta, California, United States, 3 Center of Polymers and Organic Solids, University of California, Santa Barbara, Goleta, California, United States
Show AbstractTwo new oligothiophene derivatives bearing a diketopyrrolopyrrole core, 2,5-di-n-hexyl-3,6-bis-(5''-n-hexyl-[2,2';5',2'']terthiophen-5-yl)-pyrrolo[3,4-c]pyrrole-1,4-dione (DHT6DPPC6) and 2,5-di-n-dodecyl-3,6-bis-(5''-n-hexyl-[2,2';5',2'']terthiophen-5-yl)pyrrolo[3,4-c]pyrrole-1,4-dione (DHT6DPPC12), and their use in solution processed organic field effect transistors are reported. Depending on the type of alkyl substituent and film annealing temperature, the crystal grain sizes and interlayer spacing vary as observed using atomic force microscopy and X-Ray diffractometry, respectively. These changes in film morphology and interlayer spacing lead to one order of magnitude difference in the field effect mobilities. The field effect mobilities for annealed DHT6DPPC6 and DHT6DPPC12 films are 0.02 cm2/Vs and 0.01 cm2/Vs, respectively.
9:00 PM - B10.64
Potential Mapping in the Channel of Organic Field Effect Transistors by Additional Sense Contacts.
Risha Sharma 1 , Benedikt Gburek 1 , Torsten Balster 1 , Veit Wagner 1
1 , Jacobs University Bremen, Bremen Germany
Show AbstractIn the past few years organic semiconductors have attracted considerable interest in research as well as in industrial applications. In particular, they are used in thin film transistors offering the option for cheap and large area production of electronic circuits.However, organic semiconductors exhibit a wide range of transport properties with often rather complex behaviors, which are not well understood compared to their inorganic counterparts. The analysis and understanding of these properties is often difficult if only the integral IV measurements are available. A very helpful additional information in this context is the potential distribution in the channel of the organic field-effect transistor (OFET). One very valuable option for this purpose is the Scanning Kelvin Probe method, which, however, is often restricted to vacuum environment and cannot access directly buried interfaces. To overcome these restrictions in our approach we pattern sense fingers into the channel by lithographic means in addition to the source and drain electrodes of the transistor. These sense electrodes are in direct contact with the conducting channel and their potentials can be directly accessed by high impedance electrometers. This approach works in atmospheric conditions and even for buried channels, i.e. in top gate geometry.As organic semiconductor thiophenes in form of small molecules, thiophene oligomers, and as polymers, regio-regular poly-(3-hexylthiophene) (rr-P3HT), are investigated. These materials are known to show rather differing electronic transport properties. For these systems the additional sense fingers allow to analyse contact properties to the gold contacts as well as the charge carrier density dependent mobility in detail.For the consistent interpretation of the measured current and potential data a combined model of contact resistance, parallel bulk resistance, and charge carrier density dependent mobility on the basis of the Vissenberg-Matters model is required. This theoretical model was found to be suitable to describe the various semiconducting materials investigated as well as different device designs, as there are top and bottom gate structures or the use of silicon or plastic substrates.The presented method has proved to provide valuable additional information of the internal properties of the conducting channel and the contacts and is compatible to standard production techniques and operation environments of organic field-effect transistors.
9:00 PM - B10.65
Semiconducting Polythiophenes at High Carrier Densities: Nonlinear Transport in Agreement with Luttinger Liquid Theory.
Jonathan Yuen 1 , Reghu Menon 2 , Nelson Coates 1 , Ebinazar Namdas 1 , Shinuk Cho 1 , Scott Hannahs 3 , Daniel Moses 1 , Alan Heeger 1
1 Center for Polymers and Organic Solids (CPOS), University of California, Santa Barbara, Santa Barbara, California, United States, 2 Department of Physics, Indian Institute of Science, Bangalore India, 3 , National High Magnetic Field Laboratory, Tallahassee, Florida, United States
Show AbstractWe present strong evidence that the nonlinear temperature-dependent transport data in field effect transistors (FETs), with poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT) as the electronically active semiconductor, and doped PBTTT films agrees with the predictions of the Luttinger Liquid (LL) theory. Regiosymmetric PBTTT is a planar, rigid rod conjugated polymer which exhibits a liquid crystalline phase occurring below the melting point. Thus, thin films can be fabricated with large crystalline domains ranging from hundreds of nanometers up to a few microns, exhibiting a high degree of crystallinity (90% and above). Within the domains, the PBTTT chains are straight and aligned parallel to each other, and while there is possibility for finite overlap of the π-orbitals between chains due to π- π stacking, the intrachain π-electron overlap is similar to that of cis-polyacetylene, exceeding the interchain overlap. Therefore the bandwidth that determines delocalization and, thus, the transport along the chain (W⊥) is several eV, whereas the corresponding bandwidth for interchain transport (W∥) is comparable to that of molecular crystals and therefore at least an order of magnitude smaller. Thus, based upon the structure and assuming W⊥/W∥ >> 1, PBTTT would be properly classified as a quasi-one dimensional (1D) system.
9:00 PM - B10.66
Electron Transfer/transport in 2D Nanoparticle Array/polymer Composite Thin Layers.
Shisheng Xiong 1 , Yongqian Gao 2 , John Grey 2 , Jeffrey Brinker 1 3
1 Department of Chemical Engineering, University of New Mexico, Albuquerque, New Mexico, United States, 2 Department of Chemistry, University of New Mexico, Albuquerque, New Mexico, United States, 3 Advanced Material Laboratory, Sandia National Laboratories, Albuquerque, New Mexico, United States
Show AbstractThe mechanisms of charge carrier transfer across the metal/organic interfaces and the charge transport at the nanoscale are crucial for molecular electronics and are under extensive investigation for photovoltaics, light-emitting diodes and sensors. A universal, fast and facile method to prepare robust, freestanding and patternable NP/polymer composites by evaporation-induced self-assembly on a fluid interface has been developed (JACS 2008). The thiol-capped nanoparticles (about 5.5nm in diameter) embedded within the polymer matrix are 2D monolayer arrays, ordered in a hexagonal close-packed (hcp) arrangement, enabling us to study the charge transfer/transport characteristics in a 2D configuration. Using a "shadow deposition" technique, the gold nanoparticle (GNP)/conjugated polymer films are then transferred onto CINT* Electrical Transport and Optical Spectroscopy Discovery Platforms™ with prefabricated interdigitated fingers (grid spacing around 200 microns). The absorption spectra of composite films are almost identical to the pure P3HT films. AFM scanning has verified that the composite films are conformal with the electrodes. Sheet resistance of the composite films are measured at both the ground state and excited state by charge injection through the source-drain setup using a van der Pauw four probe station. Combining both the intensity and frequency resolved spatial Raman spectra, acquired by Raman scanning spectroscopy, in terms of the C=C stretching mode of P3HT with a potential bias applied across the interdigitated electrodes, charge carrier migration and photophysical processes are interrogated. Experimental results show that the Raman intensity of the composite films transferred to the Discovery Platform is greater than films transferred to bare silicon because of possible electrochemical interactions between the film and gold fingers. Increasing the potential leads to a decrease of the intensity difference due to charge injection. Both thermal annealing and potential increase result in a downshift of the Raman frequency of the C=C stretching mode, suggesting that charge transport has been facilitated.*(Center for Integrated Nanotechnology, Sandia National Laboratories and Los Alamos National Laboratories)
9:00 PM - B10.68
Investigating the Role of Interfaces in Nanotube Polymer Composite Films by Resonant Photoconductive Decay.
Katherine Hurst 1 , Richard Ahrenkiel 2 , John Lehman 1
1 , National Institute of Standards and Technology, Boulder, Colorado, United States, 2 , Colorado School of Mines, Golden, Colorado, United States
Show AbstractIncorporating single-walled carbon nanotubes into conjugated polymers composite thin films facilitates the transport of electrons and increases the overall efficiency of organic photoconductive films. The role of the interface between the nanotube and polymer is fundamental to the dissociation of photoexcited excitons, and understanding this mechanism is needed to increase the overall efficiency of devices. In this work, we apply a resonant-coupled photoconductive decay (RCPCD) method to determine the recombination lifetime of nanotube polymer composite films. The carrier recombination lifetime of carbon nanotubes is typically determined by contact-based techniques or spectroscopic methods. RCPCD is a non-contact measurement that is based on a pump-probe technique in which an optical pump and a low-frequency microwave probe are employed. This method is well suited for characterization of bulk and extrinsic material properties. Our results demonstrate the role of interfaces in regards to nanotube purity, concentration and extent of dispersion on the recombination lifetime. The mechanisms describing the interaction of photoexcited carriers and the composite material will be discussed. Raman spectroscopy, UV-VIS absorption and four-point probe measurements provide further identification and characterization of composite thin films. Finally, we report the wavelength dependence of photoconductive lifetimes.
9:00 PM - B10.69
Molecular Gating of Silicon-on-Insulator Surfaces.
Girjesh Dubey 1 2 , Gregory Lopinski 1 , Federico Rosei 2
1 Steacie Institute for Molecular Sciences, National Research Council, Ottawa, Ontario, Canada, 2 INRS-EMT (Energie, Materiaux, et Telecommunications), Universite du Quebec, Varennes, Quebec, Canada
Show AbstractThe electronic properties of semiconductors respond strongly to transverse electric fields, which penetrate susbstantially over macroscopic lengths. Nanometer-scale crystalline films are therefore attractive systems for detecting charge state occupation at molecular interfaces, inducing surface band-bending. Silicon-on-insulator (SOI) platforms are especially sensitive to this, since the thickness of the top silicon layer is comparable to the Debye length of the material. Therefore conductivity is ideally suited as a dynamic probe of events such as molecular physisorption1,chemisorption2, surface dipoles, and occupation of electrically active gap states. In this work, sheet resistance (Rs) and Hall effect measurements (VH) in high vacuum environments have been used to monitor adsorption (desorption) events on hydrogen terminated silicon-on-insulator films (H-SOI). Complementary measurements of pseudo-MOS transistor characteristics have been used to monitor shifts in threshold voltage (VT). Effects from gases in the torr range are discussed for n and p doping at room temperature. “Hole-trapping” species, such as (C5H5N:) and ammonia (:NH3) are found to mimic gate action analagous to a field effect transistor, biasing p-type surfaces into inversion. Their adsorption on n-type SOI produces a high concentration electron accumulation layer. These effects are reversible by desorption. Surprisingly, minority channels have also been formed from water vapor (H2O) isotherms. High electron affinity “electron-trapping” species, such as tetracyanoethylene (C2(CN)4), have been found to severely deplete n-type SOI even at 10 ppb exposure levels. These results demonstrate the efficacy of dc transport on SOI platforms for studies of molecular adsorption and charge transfer effects at semiconductor surfaces. References:1G. Dubey, G. P. Lopinski, and F. Rosei, Applied Physics Letters 91, 232111 (2007).2 G. P. Lopinski, B. J. Eves, O. Hul'ko, et al., Physical Review B (Condensed Matter and Materials Physics) 71, 125308 (2005).
9:00 PM - B10.7
Control of Morphology and Crystalline Microstructure of Inkjet-Printed Functionalized Pentacene via Evaporation-Induced Flows in a Drying Droplet
Jung Ah Lim 1 , Wi Hyoung Lee 1 , Donghoon Kwak 1 , Kilwon Cho 1
1 Chemical Engineering, POSTECH, Pohang Korea (the Republic of)
Show AbstractThe evaporation of inkjet-printed droplets on solid surfaces has received special attention as a key technology for controlling the morphology of dried deposits. The evaporation behavior of droplets is a function of the liquid composition of the ink, the surface properties of substrate, and the environmental conditions (i.e., temperature, moisture, and ambient pressure). Since the surface properties of gate dielectrics in organic transistors significantly influence the performance of the devices, understanding how the surface wettability of the dielectric layer influences the evaporation behavior and the distribution of molecules is very important in inkjet printing of organic semiconductors for the fabrication of organic transistors. In this study, we systematically studied the self-organization phenomena of an inkjet-printed organic semiconductor on a dielectric surface with controlled surface wettability through self-assembled monolayers (SAMs). We used 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS_PEN), which is a promising organic semiconductor due to its solution processability, and has a significantly greater π-orbital overlap and a smaller interplanar spacing than un-substituted pentacene. As a result, the different types of evaporation, induced by the various surface wettabilities of the dielectric substrates, lead to significantly different morphologies and crystalline microstructures of the deposits. Interestingly, self-aligned TIPS_PEN crystals with highly ordered crystalline structure were successfully produced on the hydrophilic surface when the contact line was pinned upon drying. Our study indicates that a control of both contact line dynamics and the evaporation behavior of inkjet-printed droplets on dielectric surfaces could become a key technology for inkjet printing of organic semiconductors. Acknowledgement. This work was supported by a grant (F0004021-2008-31) from the Information Display R&D Center under the 21st Century Frontier R&D Program, Creative Research Initiative(CRI)-Acceleration Research (R17-2008-029-01000-0), and the ERC Program of MOST/KOSEF (R11-2003-006-06004-0).
9:00 PM - B10.70
Understanding the Modulation of Semiconductor Band-bending through Transport Studies of Metal-molecule-semiconductor Device Junctions.
Archana Bahuguna 1 , Fernanda Alanis-Camacho 1 , Riya Shergill 1 , Avik Ghosh 1 , Nathan Swami 1
1 Electrical Engineering, University of Virginia, Charlottesville, Virginia, United States
Show AbstractAn understanding of the effects of molecular adsorption on the modulation of semiconductor band-bending is of great significance for the characterization of electronic coupling in a variety of device paradigms such as contacts for molecular electronics, [1], probing the passivation of semiconductor surface states,[2], and probing signal transduction for sensor schemes based on the controlled introduction of surface charge using molecular dipoles on self-assembled monolayers (SAMs) on semiconductor substrates. [3]. The fitting of experimental transport data at metal-molecule-semiconductor junctions to models on electronic coupling,[4], can enable the extraction of significant device parameters that can eventually guide the development of novel chemistries and device structures that are based on modulated semiconductor band bending.Much of the prior models do not account for tunnel barrier shape and asymmetry of the Schottky device junctions. In this study, the Non-equilibrium Green’s Function (NEGF) formalism was used to examine electronic transport and band bending at metal – molecule – semiconductor device junctions, and correlate transport characteristics to experimental I-V data. COOH-terminated SAMs of varying length (SH-CnCOOH, n=7, 10, 15) were deposited on patterned areas (7 μm) on GaAs (001) substrates; and copper complexation and electroless deposition methods were used to ensure the deposition top metal contacts, without any penetration into the underlying SAM layer [5]. In this manner, the molecular device length was varied from 0.7 nm to 2 nm, and substrate doping was varied from n+ doping (1018/cm3) to p+ doping (1019/cm3). Based on this, the effects of varying SAM length, substrate doping, and top-contact work function on electronic transport mechanisms were interpreted by fitting the model to the experimental I-V characteristics to enunciate the role of the tunnel barrier shape, semiconductor band bending, depletion width and surface charge on transport.[1] Lodha S, Carpenter P, Janes DB, J. Appl. Phys. 2006, 99.[2] Y. Liu, H. Yu, J. Phys. Chem. B 2003, 107, 7803-7811.[3] Ashkenasy, G.; Cahen, D.; Cohen, R.; Shanzer, A.; Vilan, A. Acc. Chem. Res. 2002, 35, 121.[4] Nesher G., Vilan A., Hwang J., Cohen H., Cahen D., Amy. F., Chan C., Hwan H., Kahn A., J. Phys. Chem. B 2006, 110, 14363-14371.[5] Camacho-Alanis F., Wu L., Zangari G., Swami N. Journal of Mat. Chem. DOI/10.1039/b800001a. In press.
9:00 PM - B10.71
Effects of Molecular Packing and Film Thickness on the Performance of N-Channel Organic Thin Film Transistors.
Joon Hak Oh 1 , Ya-Sen Sun 1 , Ruediger Schmidt 2 , Frank Wuerthner 2 , Zhenan Bao 1
1 Department of Chemical Engineering, Stanford University, Stanford, California, United States, 2 Institut für Organische Chemie, Universität Würzburg, Würzburg Germany
Show AbstractThe availability of high performance n-channel organic semiconductors is indispensable to practical applications such as p-n junctions, bipolar transistors and complementary circuits. Two main issues in n-channel organic thin film transistors (OTFTs) are i) high performance and ii) air-stability. To illuminate the interplays between molecular structures, electronic properties, solid-state packings, and field-effect mobilities, we have prepared OTFT devices based on a series of core-halogenated PBIs with varying fluorinated imide substituents. We present a detailed evaluation on the influence of bay and imide substituents of perylene diimide (PDI) derivatives on the crystal packing and their electrical performance in n-channel OTFTs. Furthermore, the air-stability mechanisms have been investigated in depth with PDI thin films as functions of the lowest unoccupied molecular orbital (LUMO) energy levels and the thin film thickness. Our findings provide a new insight on the performance and air-stability of n-channel OTFTs.
9:00 PM - B10.73
Increase in Open-circuit Voltage and Improved Stability of Organic Solar Cells by Inserting a Molybdenum Trioxide Buffer Layer.
Hideyuki Murata 1 , Yoshiki Kinoshita 1 , Yoshihiro Kanai 1 , Toshinori Matsushima 1
1 School of Materials Science, Japan Advanced Institute of Science and Technology (JAIST), Nomi, Ishikawa, Japan
Show AbstractPower conversion efficiency (ηP) of organic solar cells has steadily improved through the use of new materials and device structures. In particular, great efforts have been made for the enhancement of short-circuit current density (Jsc). The use of bulk heterojunctions (e.g., the composite of p-type and n-type materials) as an active layer is very effective to increase in Jsc in both polymer and small molecule-based solar cells. However, in the bulk heterojunction solar cells, it is quite challenging to precisely control the formation of interpenetrating network by fabrication process such as annealing condition. Furthermore, there is no enhancement effect of open-circuit voltage (Voc) due to the formation of interpenetrating network. For the further improvement of ηP, it is essential to enhance Voc, with maintaining the corresponding Jsc.In this study, we found that a modification of ITO surface by a high work function metal oxide (molybdenum trioxide MoO3) is very effective to increase in Voc. We demonstrate the systematic control of Voc as a function of the film thickness of MoO3 buffer layer in the organic solar cells. The open-circuit voltage increased from 0.57 to 0.97 V as the thickness of MoO3 film is increased from 0 to 50 nm in the device structure of indium-tin-oxide ITO/ MoO3 (x nm) / 5,10,15,20-tetraphenylporphine (H2TPP, 10 nm) /C60 (40 nm)/bathocuproine (10 nm) /Ag (100 nm). The values between Voc and the ionization potential of MoO3 (x nm) on ITO exhibit linear relationship, where the Ip values change from 4.92 to 5.92 eV as they increase from 0 to 50 nm. Interestingly, the enhancement of Voc was achieved without affecting the Jsc and the fill factor. Consequently, the power conversion efficiency of the device increases from 1.24% to 1.88% primarily due to the increase in Voc.We also found that a MoO3 buffer layer enhances the stability of organic solar cells (OSCs) under photo-irradiation. We have investigated the OSCs, where the structure is ITO/ MoO3 (0 or 20 nm)/ p-type layer/ C60 (40 nm)/ Bathocuproine (BCP) (10 nm)/ Ag (100). As a p-type layer, we use H2TPP and N,N’-di(1-naphthyl)-N,N’-diphenylbenzidine (α-NPD).Without MoO3 layer, the devices showed a dramatic decrease in ηp as increasing light exposure time (18 % for H2TPP and 45 % for α-NPD in initial ηp after 30 min). On contrary, the both devices with MoO3 layer showed good stability maintaining 72 % for H2TPP and 95 % for α-NPD in initial ηp at the same measurement conditions. These results clearly suggest that the ITO/p-type layer interface affects the device stability. We investigated the enhancement mechanism of the stability and found that the reaction at ITO/p-type layer was prevented by inserting MoO3 buffer layer.
9:00 PM - B10.74
Structural and Electronic Properties of Thin Films of Organic Charge-transfer Compounds.
Michael Kroeger 1 , Antoine Kahn 1
1 Department of Electrical Engineering, Princeton University, Princeton, New Jersey, United States
Show AbstractSo-called organic charge transfer complexes usually consist of a mixture of one species of molecules with very strong electron-accepting properties and one species which acts as an electron donor. In single-crystal form, these materials have been extensively studied in the 1970s and 1980s. It was shown, that in CT-compounds, donor and acceptor molecules form segregated stacks and very often exhibit one- or two dimensional charge transport along donor and acceptor planes. Further, the band gap energy of organic CT-compounds and therefore their electric properties strongly depend on the degree of charge transfer between donor and acceptor molecules. Despite offering the possibility to tailor-design electric properties by substitution or chemical modification of the constituting species, organic CT-compounds have not played a significant role as materials for organic electronic devices. This is due to the technical difficulties which arise when handling organic single crystals, and which possibly exclude manufacturing of devices based on organic single crystals. In contrast, most applications for organic semiconductors, e.g. organic light emitting diodes, organic field-effect transistors and organic photovoltaic cells, rely on thin-film deposition techniques. Therefore, we studied the thin-film growth of organic CT-compounds deposited by thermal evaporation and their electronic structure. Materials which have been used for this study are combinations of the donor molecules dibenzo-tetrathiafulvalene (DB-TTF) and bis-pentamethylene-tetrathiafulvalene (BPM-TTF) and acceptor molecules tetracyanoquinodimethane (TCNQ) and tetrafluorotetracyano-quinodimethane (F4-TCNQ). The film morphology was studied with atomic force microscopy (AFM) and x-ray diffraction (XRD). For resolving the electronic structure, ultra-violet photoelectron spectroscopy (UPS) and inverse photoelectron spectroscopy (IPES) were used. Electrical characterization of two-terminal thin-film devices was carried out in-vacuo at temperatures between 60 K and 340 K. On glass substrates (quartz), DB-TTF/TCNQ exhibits a pronounced 3-D growth and continuous thin films could not be achieved. Replacing TCNQ by F4-TCNQ yields polycrystalline but continuous thin films with grain sizes in the micron-range. A very similar thin film growth is observed for BPM-TTF/TCNQ deposits on quartz. When applying ternary CT-compounds the electrical conductivity increases by nearly two orders of magnitude, for example, comparing DB-TTF/F4-TCNQ(0.9)/TCNQ(0.1) to DB-TTF/F4-TCNQ. Whether this effect is related to a change in the electronic band structure and/or to a change in morphology will be discussed.
9:00 PM - B10.75
Structure Function Relationships of Conjugated Polyelectrolyte Electron Injection/Transport Layers in Polymer Light Emitting Diodes.
Andres Garcia 1 2 , Jacek Brzezinski 1 2 , Youngeup Jin 3 , Thuc-Quyen Nguyen 1 2
1 Department of Chemistry and Biochemistry, University of California, Santa Barbara, California, United States, 2 Center for Polymers and Organic Solids, University of California, Santa Barbara, California, United States, 3 Chemistry, Pusan National University, Busan Korea (the Republic of)
Show AbstractCharge injection and transport play an important role in organic light emitting diodes (OLEDs), in which holes are injected from the anode into the highest occupied molecular orbital (HOMO) and electrons are injected from the cathode into the lowest unoccupied molecular orbital (LUMO) of the organic semiconductor. In the absence of interfacial effects, one needs to match the energies of the HOMO and the LUMO with the work function of the anode and cathode, respectively, so to minimize charge injection barriers. Stable metals with high work functions thus typically give rise to larger electron injection barriers when used as cathodes and hence low device performance. Recently, excellent device performance have been observed in OLEDs with high work function cathodes such as Al, Ag, Cu and Au by deposition of a conjugated polyelectrolyte (CPE) electron injection/transport layer (EIL/ETL). Some CPEs in OLEDs with high work functions cathodes have been shown to lead to similar or higher performances than identical OLEDs with no or low electron injection barrier low work function cathodes, allowing the fabrication of air stable devices. While others CPEs have exhibited much lower performances. How the molecular features of these materials modify the function of the multilayer OLED performances remains to be fully understood. Here we present structure function relationship studies of CPE EILs/ETLs in polymer light emitting diodes (PLEDs) in which the influence of the conjugated backbone, appended ionic functionality and counter-ion electronic properties on electron transport and PLED device performance are investigated. In CPEs with identical appended cationic units and counter-ions but different conjugated backbone a difference of ~ 10 in electron mobility and PLED luminous efficiency is observed, with the higher electron mobility CPE exhibiting the higher device efficiency. While CPEs with identical conjugated backbones but bearing anionic units rather than cationic units, exhibit higher device efficiencies. The electronic properties of halide counter-anions in cationic CPE are also found to influence the device efficiency of PLEDs, with a correlation between the oxidative properties of the counter-ion and device efficiencies. The attractive properties of CPEs with respect to device improvement together with the uncertainties in mechanistic function argue in favor of systematic studies with the long term goal of making concrete structure-function predictions.
9:00 PM - B10.76
Fabrication of Long-Chain Organic Thin films by Physical Vapor Deposition Process for Gas Sensor Applications
Nilima Hullavarad 1 , Shiva Hullavarad 1
1 Office of Electronic Miniaturization, University of Alaska Fairbanks, Fairbanks, Alaska, United States
Show AbstractThere is ever growing need for gas detectors characterized by 4 S’s – viz., Sensitivity, Selectivity, Speed and Stability. Organic based thin films are being investigated for detector fabrication due to potential of scale up processing for commercial interest . The organic based thin films have the advantages of higher selectivity because of ease of tailoring the chemical composition of organic thin films that in turn affect the electronic properties. There have been various techniques evolved in last two decades to grow organic thin films, predominantly by solution based chemical techniques allowing to form detectors from bottom-up approach. The technique described in this paper is CMOS compatible used in standard semiconductor fabrication.In this paper, we demonstrate the simple physical deposition technique to form organic thin films. This technique is shown to be useful to deposit long chain molecules with amphiphilic nature. The long chain molecules such as stearic acid and calcium stearate are deposited on Si, quartz and alumina substrates. The morphology of thin films is found to depend on the crystalline nature of the substrates. The FTIR spectroscopy and XRD characterization reveal that the bonding and the crystalline properties change with deposition temperatures within 20C (+/- 2 C). In conclusion, the feasibility of physical vapor deposition is demonstrated for fabricating long chain organic molecules for organic electronic applications.
9:00 PM - B10.77
Precise Structure of Pentacene Monolayers on Amorphous Silicon Oxide and Relation to Charge Transport.
Stefan Mannsfeld 1 , Ajay Virkar 1 , Colin Reese 1 , Michael Toney 2 , Zhenan Bao 1
1 Department of Chemical Engineering, Stanford University, Stanford, California, United States, 2 , Stanford synchrotron radiation laboratory (SSRL), Stanford, California, United States
Show AbstractIn the field of organic semiconductors, pentacene has developed into a benchmark material because it easily and robustly yields high-performance thin film transistor (TFT) devices. The charge transport in TFT devices occurs predominantly in a few monolayers above the dielectric interface.[1] Therefore, knowledge of the precise packing in the first monolayer is important to understand the charge transport properties of pentacene TFTs.Grazing incidence X-ray diffraction (GIXD) using a synchrotron light source provides structural information on ultra-thin films down to a single monolayer. Here we present the first direct determination of the molecular packing in a pentacene monolayer on silicon oxide. Using crystallographic refinement techniques, it is found that in pentacene monolayers on silicon oxide, the pentacene molecules pack in a completely tilt-free herringbone motif, unlike in the commonly cited thin-film phase or in the bulk crystal. The charge transport in the first monolayer is discussed on the basis of density functional theory calculations. The results explain the high performance of pentacene TFTs relative to that of pentacene single crystals on the silicon oxide substrates.[1] A. Dodapalapur, L. Torsi, and H. E. Katz, Organic Transistors: Two-dimensional transport and improved electrical characteristics, Science 268, 270-917 (1995).
9:00 PM - B10.78
Probing the Nanomorphology of Annealed Conjugated Polymer Blends.
Sufal Swaraj 1 , Cheng Wang 2 , Chris McNeill 3 , Benjamin Watts 4 , Harald Ade 1
1 Department of Physics, North Carolina State University, Raleigh, North Carolina, United States, 2 Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California, United States, 3 Cavendish Laboratory, Department of Physics, , University of Cambridge, J J Thomson Ave,, Cambridge, CB3 0HE United Kingdom, 4 , Paul Scherrer Institute, 5232 Villigen PSI Switzerland
Show AbstractWe investigate the influence of annealing on initially intimately mixed polymer blends of the conjugated polymers poly(9,9'-dioctylfluorene-co-bis-N,N'(4,butylphenyl)-bis-N,N'-phenyl-1,4-phenylene-diamine) (PFB) and poly(9,9'-dioctylfluorene-co benzothiadiazole) (F8BT). Scanning Transmission X-ray Microscopy (STXM) imaging with sub-100 nm resolution was used for investigating the evolution of the morphology. An increase in domain size is observed with annealing due to phase separation. Photoluminescence (PL) quantum efficiency studies of these films indicate a direct correspondence with the domain size for blends annealed at temperatures above 180 degC. The phase separation first evolves with the evolution of relatively pure phases at length scales beyond the resolution of STXM. At higher temperatures (>=180degC) the hierarchy of phase separation is lost and the length scales can be readily imaged by STXM. Resonant Soft X-ray Scattering (RSoXS) studies show an evolution towards purer domains along with increase in domains size with annealing. At annealing temperatures below 180oC, the correlation function analysis of the RSoXS data shows the evolution and distribution of subdomains that span a range of length scales (4 - 100nm).The correlation function complements the average domain size obtained from the PL data and a Monte-Carlo model that takes the excition diffusion into account.
9:00 PM - B10.79
Probing the Buried Interface in Annealed Conjugated Polymer Bilayers.
Sufal Swaraj 1 , Cheng Wang 2 , Hongping Yan 1 , Chris McNeill 3 , Harald Ade 1
1 Department of Physics, North Carolina State University, Raleight, North Carolina, United States, 2 Advanced Light Sounce, Lawrence Berkeley National Laboratory, Berkeley, California, United States, 3 Cavendish Laboratory, Department of Physics,, University of Cambridge, J J Thomson Ave,, Cambridge, CB3 0HE United Kingdom
Show AbstractThe properties of polymer/polymer interfaces in conducting polymeric devises critically influences device performance, yet relatively few studies of such interfaces have been performed. We show that Resonant Soft X-ray Reflectivity (RSoXR) as a powerful tool for the characterization of bilayers of conducting polymers, a material class that had mostly been investigated with neutron reflectivity. The rapid changes of optical properties near the Carbon absorption edge provides selectivity to specific chemical moieties and high contrast for investigated materials. We exemplify the use of this technique for conducting polymers by characterizing the buried interface in bilayers of poly(9,9'-dioctylfluorene-co-bis-N,N'(4,butylphenyl)-bis-N,N'-phenyl-1,4-phenylene-diamine) (PFB) and poly(9,9'-dioctylfluorene-co benzothiadiazole). We investigate the influence of annealing on the polymer/polymer interface and the surface and quantify the surface and interfacial widths. RSoXR results point to an interesting strategy that will allow the interdiffusion and physical roughness at a buried polymer/polymer interface to be determined separately by diffuse scattering at an angle and photon energy where the top surface exhibits little scattering, yet the polymer/polymer interfaces will exhibit total internal reflection.
9:00 PM - B10.8
Effect of the Phase States of Self-Assembled Monolayers on Pentacene Growth and Thin-Film Transistor Characteristics
Hwa Sung Lee 1 , Do Hwan Kim 1 , Jeong Ho Cho 1 , Kilwon Cho 1
1 Chemical Engineering, POSTEH, Pohang Korea (the Republic of)
Show AbstractTo investigate the effects of the phase state (ordered or disordered) of self-assembled monolayers (SAMs) on the growth mode of pentacene films and the performance of organic thin-film transistors (OTFTs), we deposited pentacene molecules on SAMs of octadecyltrichlorosilane (ODTS) with different alkyl-chain orientations at various substrate temperatures (30, 60, and 90°C). We found that the SAM phase state played an important role in both cases. Pentacene films grown on relatively highly ordered SAMs were found to have a higher crystallinity and a better interconnectivity between the pentacene domains, which directly serves to enhance the field-effect mobility, than those grown on disordered SAMs. Furthermore, the differences in crystallinity and field-effect mobility between pentacene films grown on ordered and disordered substrates increased with increasing substrate temperature. These results can be possibly explained by (1) a quasi-epitaxy growth of the pentacene film on the ordered ODTS monolayer, and (2) the temperature-dependent alkyl chain mobility of the ODTS monolayers.Acknowledgement. This work was supported by a grant (F0004021-2008-31) from the Information Display R&D Center under the 21st Century Frontier R&D Program, Creative Research Initiative(CRI)-Acceleration Research (R17-2008-029-01000-0), and a Grant (RTI04-01-04) from the Regional Technology Innovation Program of the MOCIE.
9:00 PM - B10.80
Novel Micro and Nano Patterning Techniques for Organic Electronic Systems.
Alexander Zakhidov 1 , Jin-Kyun Lee 1 , John DeFranco 1 , Hon Hang Fong 1 , Priscilla Taylor 1 , Christopher Ober 1 , George Malliaras 1
1 , Cornell University, Ithaca, New York, United States
Show AbstractOrganic electronics and optoelectronics are fast developing branches of modern science and technology that are aiming to replace conventional inorganic materials with light, inexpensive, flexible organic materials. One of the main issues to be solved on the route to real word application is patterning and processing of thin layer active organic materials.In this report we present new reliable photolithography micro and nano patterning techniques for high resolution, high throughput patterning of solution processable organic materials [1,2]. We also demonstrate that proposed approach is completely benign [3] to majority of organic electronic materials as well as environmental friendly and can be easily adopted by industry. In order to demonstrate potential application of developed patterning technique we fabricate micropatterned top-contact organic thin film transistors and prototypes of organic light emitting displays.[1] H. S. Hwang, Al. A. Zakhidov, J.-K. Lee, X. Andre, J. A. DeFranco, H. H. Fong, A. B. Holmes, G. G. Malliaras, C. K. Ober, Journal of Materials Chemistry, 2008, 18, 3087.[2] J.-K. Lee, M. Chatzichristidi, Al. A. Zakhidov, P. G. Taylor, J. A. DeFranco, H. S. Hwang, H. H. Fong, A. B. Holmes, G. G. Malliaras, C. K. Ober, J. Am. Chem. Soc., 2008, 130, 11564.[3] Al. A. Zakhidov, J.-K. Lee, H. H. Fong, J. A. DeFranco, M. Chatzichristidi, P. G. Taylor, C. K. Ober, G. G. Malliaras, Advanced Materials, 2008, 20, 3481.
9:00 PM - B10.81
Factors Determining the Efficacy of Optical Spacers in Polymer Solar Cells: The Role of Active Layer Morphology.
Anshuman Roy 1 , Sarah Mednick 1 , Sung Heum Park 1 , Ji Sun Moon 1 , Alan Heeger 1
1 , University of California, Santa Barbara, California, United States
Show AbstractPolymer photovoltaic devices stand at the cusp of rapid commercialization today, with the maximum power conversion efficiency reported to be over 6% [J. Y. Kim et al., Science, 317 (2007) 222-225]. The photo-active layer in these devices consists of conjugated polymers and modified fullerenes that form a bulk heterojunction (BHJ) composite film about 100 nm in thickness. Additionally, a layer of TiOx (Titanium Oxide) approximately 10 nm in thickness is spin cast on top of the active layer to act as an optical spacer that maximizes the incident light intensity in the photo-active layer [Kim et al., Adv. Mater., 18 (2006) 572-576]. Using a combination of numerical modeling and experiments, including ellipsometry, transmission electron microscopy and cell efficiency, we show that the efficacy of the TiOx layer as an optical spacer is dependent not only on the thickness of the BHJ layer but also on the nano-scale structure of the BHJ layer, which in turn depends on a host of processing conditions.
9:00 PM - B10.82
Transport Anisotropy in Films of Organic n-type Semiconductor with Controlled In-plane Grain Boundary Orientation.
Jonathan Rivnay 1 , Antonio Facchetti 2 3 , Alberto Salleo 1
1 Dept. of Materials Science and Engineering, Stanford University, Stanford, California, United States, 2 , Polyera Corporation, Skokie, Illinois, United States, 3 Dept. of Chemistry, Northwestern University, Evanston, Illinois, United States
Show AbstractSolution processable small molecule organic semiconductors have gained interest due to their potential low cost processing and field effect mobilities nearing that of their vapor deposited, non-soluble counterparts. Unfortunately, pristine films of soluble small molecules suffer from poor thin-film transistor device-to-device reproducibility (mobility, VT and stability) due to the existence of grain-boundaries that are not uniformly distributed throughout the film. The effect of grain boundaries in thin films of small molecule semiconductors is relatively large compared to that of semicrystalline polymer devices for two reasons: the grain size is often on the order of the channel length preventing averaging effects and boundary regions between grains are more abrupt. Indeed, in semicrystalline polymer films, a single chain can bridge two or more adjacent crystallites, facilitating transport through amorphous-like regions/grain boundaries. Research should thus focus on understanding of the relationship between microstructure and charge transport in order to design devices that do not necessarily eliminate grain boundaries, but limit their penalty on electrical performance, while also allowing for lower device-to-device variability.To this end, in this work we use anisotropic films of the n-type small molecule N,N’-bis(n-octyl)-(1,7&1,6)-dicyanoperylene-3,4:9,10-bis(dicarboximide), (PDI8-CN2) to explore the effect of grain boundaries on field effect mobility, and understand their implication in charge transport in thin films. To fabricate samples, we use an inclined drop casting method with a heating stage. In-plane orientation and morphology are characterized with x-ray diffraction, polarized light microscopy, and AFM. Thin film transistors (TFTs) were made to probe transport as a function of charge density and temperature across two types of grain-boundaries that are formed. These PDI8-CN2-based TFTs exhibit a mobility anisotropy of two orders of magnitude depending on the relative orientation of the grains with the current flow. Parallel devices, with charge transport presumably parallel to the fast growth direction of the crystallites show room temperature mobilities above ~0.01 cm2/Vs (EA=120meV), near that of isotropic solution cast films. Perpendicular devices, on the other hand, show low mobilities of ~10-4 cm2/Vs (EA=340meV). The difference between the two orientations is much larger than a 2-3 fold difference associated with crystalline anisotropy. The similar mobilities measured in parallel and isotropic films suggest defects and grain boundaries of this device are similar to those present in ‘typical’ devices. Though the morphology and exact nature of grain boundaries in the orthogonal direction are unknown we postulate that they are host to large energetic barriers, in agreement with the larger EA, and may explain device-to-device non-uniformity in isotropic films, due to the percolative nature of charge transport.
9:00 PM - B10.83
Synthesis and Characterization of Soluble Copolymers Containing Dialkyl Quarterthiophene.
Jun Chen 1 , Sung Park 1 , Jae Jang 1 , Yun Kim 2 , Soon Kwon 1
1 School of Nano and Advanced Material Science & Engineering and ERI, Gyeongsang National University , Jinju 660-701, Gyeonsangnam-do, Korea (the Republic of), 2 Department of Chemistry and RINS, Gyeongsang National University , Jinju 660-701, Gyeonsangnam-do, Korea (the Republic of)
Show AbstractSoluble conjugated 4ThFlu and 4ThNa copolymers were synthesized by the Suzuki coupling reaction. The weight-average molecular weight of the 4ThFlu and 4ThNa copolymer were determined to be 16848 and 13505, respectively. And the polydispersity indexes obtained by gel permeation chromatography (GPC) using polystyrene standards for calibration in the eluent THF is 1.55 and 1.59, respectively. The thermal, optical and electronic properties of copolymers were investigated by thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), UV-vis absorption, photoluminescence spectroscopies, cyclic voltammetry. The absorption maximum spectra of copolymers were observed 452, 453 nm in solution state and 563, 563 nm in film state. The PL maximum spectra of copolymers were observed at 467, 503 nm in solution state and 585, 624 nm in film state, respectively. The copolymers were showed highly thermal stability of decomposition temperature over 300 oC. Especially, the copolymers were easy to be soluble in common solvents (THF, CHCl3, Toluene etc) due to the introduction of long alky chains.
9:00 PM - B10.84
Blue Organic Light Emitting Diodes from Solution Processed Small Molecules.
Bright Walker 1 , Arnold Tamayo 1 , Wesley Walker 1 , Jihua Yang 1 , Fred Wudl 1 , Thuc-Quyen Nguyen 1
1 Chemistry and Biochemistry, University of California at Santa Barbara, Santa Barbara, California, United States
Show AbstractSolution-processed organic light-emitting devices provide the potential for cheap, ultrathin, light-weight, and large-area illumination sources. In comparison with red or green light-emitting devices, efficient blue emitting diodes are typically more difficult to obtain because the emitting material requires a wider band gap for radiative recombination.Organic blue light-emitting diodes were studied using the solution processed small molecules 2,7-dipyrenyl-9 ,9-dioctyl-fluorene as well as 1,3,5-tris(7,10-diphenylfluoranthene-8-yl)benzene as emissive layers (EMLs). Devices were fabricated using the conjugated polyelectrolyte poly(9,9'-bis[6"-(N,N,N-trimethylammonium)hexyl]fluorene-alt-co-phenylene) with tetrakis(imidazolyl)borate counterions (PFN-BIm4) as an electron injecting layer and poly(9-vinylcarbazole) (PVK) as an electron blocking layer. Efficient blue light emission was observed for devices with the architecture ITO/PEDOT:PSS/PVK/EML/PFNBIm4/Al in which all organic layers were deposited by solution processing. Using PVK and PFN-BIm4 layers results in a significant improvement in device performance compared to devices without the layers or with common electron injecting layers such as lithium fluoride or barium.
9:00 PM - B10.85
Meta-stable Interfaces between Soft-contact and Closed-shell Semiconductor Surfaces.
Yang Li 1 , Wei Long 1 , Raymond Tung 1
1 Physics Department, Brooklyn College, the City University of New York, Brooklyn, New York, United States
Show AbstractThe use of a self-assembled monolayer (SAM) of molecules to modify and tailor interface electronic properties is an attractive approach for sensors and semiconductor electronic applications, because of the molecules’ functional variety and flexibility. In studies involving SAM attached semiconductor surfaces, the deposition/application of metallic contacts is geared toward minimizing disturbance to the molecular layer, i.e. for a preservation of meta-stable non-interacting molecule-metal interfaces. However, interactions between the metal and the molecular layer are often found to influence and even dominate the dipolar effect of the molecular layer. Significant barrier height inhomogeneity has also been reported. In this work, we attempt to establish a baseline for studies involving meta-stable metal-molecule interfaces. A dual-station UHV chamber was constructed for this study. Silicon surfaces terminated with different types of stable closed-shell configurations (Cl-, S- and H-) are used to simulate the stable molecular termination on SAM/Si surfaces. Meta-stable metal-Si structures are fabricated using soft-landing deposition of metal contacts (Au, Ag, and Al) with indirect deposition in an inert gas ambient or direct deposition in vacuum, and at variable temperatures. In situ characterization by surface spectroscopies and Kelvin probe techniques showed the varied levels of intermixing under different deposition conditions. Electrical measurements, by variable temperature I-V and C-V method, of Au/Si and Ag/Si diodes fabricated on n- and p-type <100> and <111> substrates also showed a significant dependence on fabrication conditions of the Schottky barrier. Lower deposition temperature led to more uniform contact between metal and semiconductor, which then led to higher SBH on n-type Si, with the expected, opposite dependence observed on p-type Si. Also, the orientation of the semiconductor surface was shown to have a significant influence on the formation of the SBH. Electrical results and results obtained from surface chemical analysis and microscopic techniques are presented with special attention paid to the possible electrical inhomogeneity in the systems. These results are compared with results involving molecular layers obtained previously in our and other groups.
9:00 PM - B10.87
Picosecond Photoexcitation Dynamics in the Poly(2,7-Carbazole) Copolymer, PCDTBT,and in Bulk Heterojunction Composites with PC70BM.
Minghong Tong 1 , Nelson E. Coates 1 , Daniel Moses 1 , Alan J Heeger 1 , Serge Beaupre 2 , Mario Leclerc 2 , Russell Gaudiana 3
1 Center for Polymers and Organic Solids, University of California, Santa Barbara, Santa Barbara, California, United States, 2 Departement de Chimie, Université Laval, Quebec City, Quebec, Canada, 3 , Konarka Technologies, Inc, Lowell, Massachusetts, United States
Show AbstractWe have studied the nature of ultrafast photoexcitations and their recombination dynamics in an alternating donor-acceptor low-bandgap Poly(2,7-Carbazole) copolymer (PCDTBT; see Blouin, N.; Michaud, A.; Leclerc, M. Adv. Mater. 2007, 19, 2295 – 2300) and in composites of that polymer with the fullerene derivative [6,6]-phenyl C70-butyric acid methyl ester (PC70BM);This class of alternating donor-acceptor copolymers (with different acceptor units) holds promise for photovoltaic applications because of the ability to tune the electronic energy levels by changing acceptor units. The implied flexibility in the synthesis can lead to both a lower bandgap that better uses the solar radiation spectrum, and a lower HOMO level that increases the open circuit voltage of photovoltaic devices. We have used transient photinduced absorption spectroscopy, over a wide spectral range in order to study the nature of the photoexcitations, and in particular the carrier generation and recombination dynamics at short time scales. A very long carrier lifetime (>>1ns) is observed in the PCDTBT-Fullerene composite.
9:00 PM - B10.88
Steady-State and Transient Photoconductivity in the Poly(2,7-Carbazole) Copolymer PCDTBT, and in Bulk Heterojunction Composites with PC70BM.
Nelson Coates 1 , Minghong Tong 1 , Daniel Moses 1 , Alan Heeger 1 , Serge Beaupre 2 , Mario Leclerc 2 , Russell Gaudiana 3
1 Physics, University of California, Santa Barbara, Goleta, California, United States, 2 Chimie, Université Laval, Quebec City, Quebec, Canada, 3 , Konarka Technologies Inc., Lowell, Massachusetts, United States
Show AbstractWe have studied the nature of carrier generation using steady-state and transient photoconductivity in an alternating donor-acceptor low-bandgap Poly(2,7-Carbazole) copolymer (PCDTBT; see Blouin, N.; Michaud, A.; Leclerc, M. Adv. Mater. 2007, 19, 2295 – 2300) and in composites of that polymer with the fullerene derivative [6,6]-phenyl C70-butyric acid methyl ester (PC70BM). This class of alternating donor-acceptor copolymers (with different acceptor units, X) holds promise for photovoltaic applications because of the ability to tune the electronic energy levels by changing X. The implied flexibility in the synthesis can lead to both a lower bandgap that better uses the solar radiation spectrum, and a lower HOMO level that increases the open circuit voltage of photovoltaic devices. In PCDTBT, the absorption band extends out to ~ 700 nm, with two distinct but broad absorption bands that are centered at ~ 400 nm and ~ 600 nm. Higher solar cell power conversion efficiency is achieved in PCDTBT devices than in P3HT based devices because of the improved light harvesting and larger open circuit voltage. We have used steady-state and transient photoconductivity to investigate the carrier generation and collection efficiency of PCDTBT and its composite with the soluble fullerene, PC70BM, after photoexcitation at each of its distinct absorption bands. In pristine PCDTBT, higher carrier quantum efficiency is observed with excitation at the high energy absorption band, but in the PCDTBT-fullerene composite, this efficiency is an order of magnitude greater and relatively wavelength independent.
9:00 PM - B10.89
High-Mobility n-Channel Organic Thin-Film Transistors
Henry Yan 1 , Antonio Facchetti 1
1 , Polyera Corporation, Skokie, Illinois, United States
Show AbstractWe report here our recent progress enabling high-performance top-gate organic thin-film transistors (OTFT). Electron mobility of ~ 1.0 cm2/Vs was achieved for top-gate bottom-contact devices tested in ambient using both the semiconductor and dielectric layers fabricated by spin-coating. To the best of our knowledge, this is first report of solution-processed top-gate n-channel TFTs with mobility higher than 1 cm2/Vs. Furthermore, we will also present updated performance of bottom-gate bottom-contact transistors using Polyera materials. Electron mobility of ~ 0.1 cm2/Vs was achieved in a bottom-gate bottom-contact structure with both the semiconductor and dielectric layers fabricated by spin-coating process. In this case, the dielectric material is an UV crosslinkable dielectric polymer having good compatibility with n-type organic semiconductors.
9:00 PM - B10.9
Tuning the Threshold Voltage in Organic Field Effect Transistors by Space Charge Polarization of Gate Dielectrics
Heisuke Sakai 1 , Koudai Konno 1 , Hideyuki Murata 1
1 , Japan Advanced Institute of Science and Technology, Nomi Japan
Show AbstractStudies of gate dielectrics in organic field effect transistors (OFETs) have been attractive because the electric properties of OFETs are susceptibly affected by the choice of the gate dielectrics. Here, we demonstrate a tunable threshold voltage in an organic field effect transistor (OFET) using an ion-dispersed gate dielectrics. By applying external electric field (Vex) to the gate dielectrics, the dispersed ions in the gate dielectrics are separated by electrophoresis and form space charge polarization. The drain current of the OFET increased over 1.9 times and the threshold voltage (Vth) decreased 22 V (from -35.1 V to -13.1 V).The shift direction of Vth was easily tuned by the polarity of the external voltage. The dielectric permittivity of the gate dielectrics and mobility of the active layer were unchanged after the polarization of the gate dielectrics. The UV-VIS differential absorption spectra of the OFETs indicate that there is no chemical doping in the active layer of the OFETs. These results indicated the shifts of threshold voltages were originated from the polarization of gate dielectrics.
9:00 PM - B10.90
Effects of Metal and Organic Impurities on Pentacene Electronic Structures
Jing Xue 1 , Geunsik Lee 1 , Kyeongjae Cho 1 2
1 Physics, University of Texas at Dallas, Richardson, Texas, United States, 2 Materials Science and Engineering, University of Texas at Dallas, Richardson, Texas, United States
Show AbstractOrganic semiconductors have received much attention due to their successful application in optical and electronic areas. Pentacene is one of the most promising materials in the organic semiconductors because of its highest hole mobility. In this work, we study the effects of metal contact and organic impurities on pentacene’s electronic structure by using ab initio density functional theory (DFT) methods. Pentacene in the electronic devices directly contacts with metal electrode, and metal atoms may diffuse into pentacene and form impurities interacting with pentacene. Therefore, the interface between metal and pentacene can be modified by the metal atomic diffusion. We calculate electronic structure of pentacene adsorbed with different metallic elements (Au, Pd, Ni) and study their effects on the carrier injection barrier. We found that the calculated energy gap for isolated pentacene molecule is 1.13 eV. The main contribution of HOMO and LUMO come from pi orbital of C-atoms. For metal adsorption, the energy gap will be decreased. Compared the binding energy and the bulk cohesive energy, Ni and Pd are easier to diffuse into the pentacene than Au. Depending on the valence electron structure of the metal atom to be adsorbed, the electronic structure of pentacene near the Fermi level has changed differently. For Au and Ni, there will form new mid gap states and the new mid-gap states decrease the contact barriers to improve the charge injection from the electrode.Organic semiconductor device characteristics is also sensitive to organic impurities which may be introduced during synthesis or by air exposure. It is worth to focus that dissociation of H2O into H and OH may form some organic defect. We consider the defect in pentacene in forms of C-H2, C=O and OH. All these organic impurities will give rise to gap states. For C-H2 or C=O defect, the 6-top site is mostly favored energetically by an additional H adsorption (C22H15)or H replacement by O (C22H13O), respectively. In both systems, the pz orbital on the perturbed C atom no longer participates in the π bonding once the defect is introduced. For H replacement by an OH, it seems that, there are no much differences between C22H14O and C22H14 because all C atoms are still participates in their π bonding. However, for additional adsorption of OH, C22H15O, the electronic structure is different from that of pentacene molecule. In addition, it is shows that the electronic structure of pentacene with the same type of defect is affected by the defect position.These calculations show the role of metal or organic impurities in generating the defect states with the HOMO-LUMO gap of the pentacene molecules. These gap states may play the critical role in limiting the mobility of the organic semiconductor channel and the efficiency of the optical device applications. Our detailed electronic structure study provides a fundamental insight on the effects of impurities in the pentacene device performance.
9:00 PM - B10.91
Interface Reaction of Aluminum and 8-hydroxyquinolatolithium.
Young Mi Lee 1 , Yeonjin Yi 2 , Jeong Won Kim 2 , Yongsup Park 1
1 Dept.of Physics, Kyung Hee University, Seoul Korea (the Republic of), 2 , Korea Research Institute of Standards and Science, Daejeon Korea (the Republic of)
Show AbstractOne of organic electron injection layer materials, Liq (8-hydroxyquinolatolithium) shows a few advantages over other inorganic materials for organic light emitting device (OLEDs). As the Liq possesses the similar structure to Alq3, the most common light emission layer material, it is believed to provide a smooth interface and be more compatible to flexible displays. The energy alignment and device performance including the Liq between Al and Alq3 have recently been demonstrated. Here the interface chemical reaction at the Liq/Al interfaces was investigated by using high resolution synchrotron radiation photoelectron spectroscopy. The different deposition sequence gives different reactions. While strong reactions are observed throughout the Liq film when Al is deposited on Liq layer, an interface localized reaction occurs just at the interface upon the Liq deposition onto Al surface. Either sequence of film stacks, Liq/Al and Al/Liq produce an interface gap state respectively at 2.1 eV and 2.8 eV below the Fermi level. Both of the highest occupied molecular orbital (HOMO) and N 1s core level peaks are shifted to the high binding energy side by 0.35 eV on Al/Liq whereas it is not the case on Al/Liq. Based on these observations, the differences in electron injection barrier and interface dipole between the two opposite deposition sequences could be drawn.
9:00 PM - B10.92
Formation of Ohmic Carrier Injection at Anode/organic Interfaces and Carrier Transport Mechanisms of Organic Thin Films.
Toshinori Matsushima 1 , Guang-He Jin 1 , Yoshihiro Kanai 1 , Tomoyuki Yokota 1 , Seiki Kitada 1 , Toshiyuki Kishi 1 , Hideyuki Murata 1
1 School of Materials Science, Japan Advanced Institute of Science and Technology, Nomi , Ishikawa, Japan
Show AbstractOrganic light-emitting diodes (OLEDs) are being developed due to their high potentials for use in low-cost, mechanically flexible, and lightweight display, and lighting applications. In typical multilayer OLEDs, a hole-injection barrier is present at the interface of an indium tin oxide (ITO) anode and a hole-transport layer (HTL) and causes an increase in driving voltage of OLEDs. Various organic and inorganic hole-injection layers (HILs) have been inserted between ITO and HTL to reduce the driving voltages. However, Ohmic contacts have never been achieved at the ITO/HIL/HTL interfaces to date. If Ohmic contacts can be formed, further improvements of driving voltages, power conversion efficiencies, and stability of OLEDs are possible.Besides the improvements in OLED performance, understanding carrier transport mechanisms in organic films is very crucial to developing fundamental science. However, the carrier transport mechanisms have not yet been elucidated, and must be further clarified to bring about maximum device performance. Large charge carrier injection barriers at anode/HTL interfaces would make clarifying the carrier transport mechanisms difficult because observed currents are governed by both carrier injection and transport. Therefore, the formation of Ohmic contacts at the interfaces is indispensable to clarifying the carrier transport mechanisms.In this study, we found that insertion of an ultrathin molybdenum oxide (MoO3) HIL between ITO and HTL provides Ohmic hole injection at the interfaces. We fabricated the hole-only devices with a glass substrate/ITO anode (150 nm)/MoO3 HIL (X nm)/organic HTL (100 nm)/MoO3 electron-blocking layer (10 nm)/Al cathode (100 nm) structure. As the HTL, we used pentacene, alpha-sexithiophene (a-6T), copper phthalocyanine (CuPc), 4',4"-tris(N-3-methylphenyl-N-phenyl-amino)triphenylamine (m-MTDATA), 4,4',4"-tris(N-2-naphthyl-N-phenyl-amino)triphenylamine (2-TNATA), N,N-di(m-tolyl)-N,N-diphenylbenzidine (TPD), rubrene, or N,N-diphenyl-N,N-bis(1-naphthyl)-1,1-biphenyl-4,4-diamine (a-NPD). Although the thickness of MoO3 HILs previously used is in the range between 5 and 50 nm, the results in the present study clearly indicated that such thick MoO3 HILs do not provide Ohmic hole injection. We found that the optimized X is smaller than 1 nm. The hole-only devices with the optimized X exhibited completely transport-limited currents, indicating that an Ohmic contact is formed at the interfaces. By analyzing the current density-voltage characteristics of the devices with a space-charge-limited current theory, we clarified the carrier transport mechanisms of the above-mentioned organic HTLs.
9:00 PM - B10.93
High Efficiency Bulk Heterojunction Solar Cells with Internal Quantum Efficiency (IQE) approaching 100% fabricated with the Poly(2,7-Carbazole) Copolymer, PCDTBT
Sung Heum Park 1 2 , Anshuman Roy 1 , Shinuk Cho 1 , Ji Sun Moon 1 , Nelson Coates 1 , Daniel Moses 1 , Kwanghee Lee 1 2 , Alan Heeger 1 2 , Serge Beaupre 3 , Mario Leclerc 3 , R. Gaudiana 4
1 Center for Polymer and Organic Solids, University of California at Santa Barbara, Santa Barbara, California, United States, 2 Heeger Center for Advanced materials, Gwangju Institue of Science and Technology, Gwangju Korea (the Republic of), 3 Departement de Chimie, Université Laval, Quebec, Quebec, Canada, 4 , Konarka Technologies, Inc, Lowell, Massachusetts, United States
Show AbstractWe report here that we have successfully fabricated a single polymer solar cell with 6% power conversion efficiency using an alternating donor-acceptor low-bandgap Poly(2,7-Carbazole) copolymer (PCDTBT; see Blouin, N.; Michaud, A.; Leclerc, M. Adv. Mater. 2007, 19, 2295 – 2300) in composite with the fullerene derivative [6,6]-phenyl C70-butyric acid methyl ester (PC70BM). This class of alternating donor-acceptor copolymers (with different acceptor units, X) holds promise for photovoltaic applications because of the ability to tune the electronic energy levels by changing X. The implied flexibility in the synthesis can lead to both a lower bandgap that better uses the solar radiation spectrum, and a lower HOMO level that increases the open circuit voltage of photovoltaic devices. The PCDTBT device performance is as follows: Jsc = 10.5 mA/cm2, Voc = 0.88 V, FF = 0.66 and ηe = 6.1% under air mass 1.5 global (AM 1.5G) illumination from a calibrated solar simulator with irradiation intensity of 100 mW/cm2. The IQE approaches 100%, implying that essentially every photon absorbed leads to a charge separated pair and that every photogenerated carrier is collected at the electrodes.
9:00 PM - B10.94
Self Sorted Carbon Nanotube Transistors and Conductive Films
Soumendra Barman 1 , Melburne Lemieux 1 , Zhenan Bao 1
1 Department of Chemical Engineering, Stanford University, Stanford, California, United States
Show AbstractSingle-walled carbon nanotubes (SWNTs) possesses great potential to be the next breakthrough for sensors, flexible computing networks and next generation electronics. The manufacture of carbon nanotubes involves the decomposition of carbon into graphitic tubes of different diameters and chiralities. Depending on the chirality of the nanotube, it behaves as a semiconductor or a metal. One of the greatest obstacles for creating the next generation of devices is the separation of nanotubes by chirality. Here, we refine the surface sorting separation technique we have developed by modifying the solution processing conditions and self assembled monolayer functionalities in an effort to understand the interactions leading to chirality selectivity. Using self sorting we have fabricated high performance SWNT thin film transistors (TFTs) and transparent conductive electrodes. UV-vis-NIR spectroscopy, Raman spectroscopy, AFM and semiconductor parameter analysis were used to characterize the results. The knowledge gained from understanding the mechanism for surface self sorting is critical for the fabrication of devices and could be important for the integration of carbon nanotubes into large scale electronics.
9:00 PM - B10.95
Accurate and Simultaneous Determination of Carrier Density and Mobility in Organic Semi-conducting Materials.
Kai Shum 1 , Jim Shi 2
1 Physics, Brooklyn College - CUNY, Brooklyn, New York, United States, 2 Chemistry, College of Staten Island - CUNY, Staten Island, New York, United States
Show AbstractAccurate and simultaneous determination of carrier mobility and density in organic semiconducting materials is very important for their optoelectronic applications including light-emitting diodes (LEDs), solar cells, and thin film field-effect transistors. In this work, we report on a unique data analysis procedure for space-charge limited currents to obtain the carrier density and carrier mobility for our newly synthesized perylene tetracarboxylic diimides with finely tuned pi-stack structures without altering the electronic characteristic of individual molecules. How pi-stack structural variations affect charge transport performance will be discussed.
9:00 PM - B10.96
High Efficiency, Solution-Processed Bulk Heterojunction Solar Cells Using Diketopyrrolopyrrole-Containing Thiophene Oligomers
Arnold Tamayo 1 , Xuan-Dung Dang 1 , Bright Walker 1 , Tyler Kent 1
1 Center for Polymers and Organic Solids, University of California, Santa Barbara, California, United States
Show AbstractBulk heterojunction (BHJ) solar cells are fabricated from blends of 2,5-di-(2-ethylhexyl)-3,6 bis-(5''-n-hexyl-[2,2';5',2'']-terthiophen-5-yl)pyrrolo[3,4-c]pyrrole-1,4-dione:[6,6]-phenyl C71 butyric acid methyl ester (SMDPPEH:C71-PCBM). Absorption and photocurrent of the blend films extend to 800 nm. A power conversion efficiency of 3.0% with a 50:50 donor-acceptor ratio is obtained under simulated 100 mW/cm2 AM 1.5 illumination with a 9.2 mA/cm<2> short-circuit current density and an open-circuit voltage of 0.72 V. The hole and electron mobilities in the 50:50 blend are fairly balanced, 1.0 × 10<-4> cm<2>/V-s and 4.3 × 10<-4> cm2/V-s, respectively. This is the highest PCE reported to date for BHJ solar cells using solution processable small molecules.
9:00 PM - B10.98
Harvesting Lost Photons: Minimizing Sub-Bandgap Losses in Organic Photovoltaic Devices byUp-conversion
Sebastien Loranger 1 , David Banville 1 , Jennifer McLeod 3 , Rosei Federico 3 , Dmytro Perepichka 2 , Clara Santato 1
1 Genie Physique, Ecole Polytechnique de Montreal, Montreal, Quebec, Canada, 3 , INRS, Varennes, Quebec, Canada, 2 Chemistry, McGill University, Montreal, Quebec, Canada
Show AbstractOrganic semiconductors (OSC) for photovoltaic applications offer advantages that include significantly lowered manufacturing costs (solutionprocessing), flexibility of the device (roll-up PV panels), and the possibility to easily tailor the optoelectronic properties through chemical synthesis. Unfortunately, the maximum conversion efficiency obtained so far in Organic PV (OPV) devices is ~5 %. Among other reasons, this is due to a largemismatch between the absorption characteristics of organic semiconductors and the solar spectrum. A significant portion of the solar photons are “lost” (can not be converted). To increase the power efficiency of OPV devices the material’s properties need to be tuned to increase light absorption.Our approach is based on the use of the well known phenomenon of up-conversion of rare earth-doped nanocrystals.We investigate conventional OPV devices where rare earth-doped nanoparticles, dispersed in the organic active layer, can transfer the up-converted energy of NIR photons to the polymer semiconductor, improving the overall efficiency. The key physical process in the proposed hybrid PV system is the energy transfer from the excited up-converting material to the organic semiconductor of the PV cell.We focus on a hybrid organic/inorganic system prepared by blending organic and up-converting nanoparticles (UCNPs) components. [1] Specifically, polythiophenes are considered as the p-type materials and functionalized fullerenes as the n-type.Systematic investigations of the energy transfer between UCNPs and conjugated polymers have been performed by means of steady-state NIR spectroscopy (UCNP-induced emission of the polymers). Interestingly, the photoluminescence of UCNP decreases with the increase of the polythiophene component in the blend. This suggests the possibility of significant energy transfer between the UCNP and the organic semiconductors. On the other hand, the fullerene has no significant effect on the UCNP emission. UCNP of different qualities are being investigated, with different crystallographic structures (cubic and hexagonal) and stabilized with different capping agents (oleic acid versus oleilamine). Time resolved spectroscopy is going to be performed to measure life-times of the UCNP excited states in presence of the organic semiconductors.To assess the effect of the UCNP on the electrical properties of the active layer of the OPV device, we performed charge transport measurements in field-effect transistor configuration, irradiating the layer with a low power NIR laser (phototransistor configuration). This approach permits to identify the optimum size of the UCNP to be incorporated in the active layer, together with their optimum concentration in the layer.[1] J.-C. Boyer,F. Vetrone, L. A. Cuccia, J. A. Capobianco J. Am. Chem. Soc. 2006, 128, 7444.
9:00 PM - B10.99
Ordered Titanium Dioxide Films Grown on Self-Assembled Monolayers.
Shirin Usmani 1 , Diana Mars 1 , Andrew Ichimura 1
1 Chemistry & Biochemistry, San Francisco State University, San Francisco , California, United States
Show AbstractTitanium dioxide finds extensive applications as pigments, in medicine, wastewater remediation, oxidative photocatalysis, and in dye-sensitized solar cells. Applications such as hybrid solar cells utilize thin films of titanium dioxide as the electron transport material. Typically, the films are prepared from TiO2 nanoparticle containing sols that are spin-coated onto substrates and subsequently sintered to induce phase transformation and interparticle contact. We have pursued a strategy of thin film preparation that involves growth of crystalline TiO2 directly onto a functionalized surface from homogeneous solutions. In this approach, a densely packed self-assembled monolayer (SAM) with a terminal Ti-OH functional group is used to chemically bond the film to the underlying gold substrate. The advantage of this method is that resultant films are highly ordered polycrystalline arrays in which monolithic crystals span the film from substrate to external surface. This arrangement may facilitate charge transport across the layer and thus decrease the probability of electron-hole recombination. In a larger sense, SAM chemistry allows us to explore avenues for controlling crystal growth through a tailoring of the surface of the substrate. In this work, anatase and rutile films grown on Ti-OH terminated SAMs from homogeneous solution will be described. Characterization methods include powder X-ray diffraction, HR-SEM, IR, UV-vis-NIR spectroscopy, and 4-probe conductivity studies.