Symposium Organizers
David L. Andrews University of East Anglia
Kenneth P. Ghiggino University of Melbourne
Theodore Goodson University of Michigan
Arthur J. Nozik University of Colorado-Boulder
M1: Dye Sensitization
Session Chairs
Monday PM, December 01, 2008
Republic A (Sheraton)
11:15 AM - **M1.1
Energy Transfer in Single Conjugated Polymer Chains.
John Lupton 1
1 Department of Physics, University of Utah, Salt Lake City, Utah, United States
Show AbstractSingle molecule spectroscopy enables the study of intrinsic electronic properties of large macromolecules by screening out ensemble disorder effects. This approach has become particularly interesting in the study of materials for plastic electronics, such as conjugated polymers, which are characterized by poorly defined physical shapes and sizes. Using a variety of time and frequency domain techniques, interchromophoric coupling phenomena can be identified. Both the fundamental spectroscopy of individual chromophores on a polymer exhibits a stunning diversity, as does the physical coupling mechanism between chromophores. Developing a microscopic understanding of intramolecular energy transfer as required by theory necessitates both low temperature techniques to study intrinsic chromophore linewidths as well as model donor-acceptor polymer systems. Dye endcapped polymers are particularly versatile models, revealing how excitation of one part of the molecule impacts emission of another. By scanning the excitation wavelength we can perform excitation spectroscopy and identify ultrafast intramolecular energy transfer giving rise to extremely broad transitions.
11:45 AM - M1.2
Charge Transfer Mechanisms in Excitonic Solar Cells at the Single Molecule Level One Molecule at a Time.
Oliver Monti 1 , Laura Schirra 1 , Michael Blumenfeld 1 , Brandon Tackett 1
1 Chemistry, The University of Arizona, Tucson, Arizona, United States
Show AbstractWe present a novel approach to studying interfacial charge transfer dynamics in organic and hybrid organic/inorganic photovoltaic cells. In order to reduce the complexities of heterogeneity at the bulk heterojunction of thin film organic solar cells and dye-sensitized solar cells, charge transfer is investigated from single fluorescent molecules to a highly defined single-crystalline wide-bandgap semiconductor using confocal fluorescence microscopy under ultrahigh vacuum conditions. First results permit the observation of the distribution of charge transfer rates as a function of the density and type of interband states, molecular orientation and distance between donor and acceptor.
12:00 PM - M1.3
Electronic Properties of Dye-Sensitized Solar Cells Using Long-range Corrected Density Functional Theory.
Bryan Wong 1
1 , Sandia National Laboratories, Livermore, California, United States
Show AbstractRecent advances in solar energy technology using dye-sensitized solar cells (DSCs) have attracted attention as low-cost alternatives to conventional photovoltaic devices. In particular, a rapidly-growing area in DSC technology is the use of organic dyes to function as charge-transfer groups for achieving efficient solar-to-electric power conversion. Despite the numerous experimental studies on DSCs, the progress of theoretical calculations has been slower in quantitatively describing their unique electronic structure. To address this problem, we have used a new density functional which remedies the deficiencies of other functionals by incorporating a long-range correction for describing charge-transfer behavior. Based on extensive tests consisting of several DSC molecular sensitizers, we demonstrate that this long-range corrected functional provides an accurate description of both light absorption and emission. The results of these calculations enable a guided approach to maximizing the light-harvesting properties and potential capabilities of future dye sensitizers.
12:30 PM - M1.5
Post-treatment of Titanium Dioxide Nanoparticles on TCO Glass for High Efficiency Dye-sensitized Solar Cells.
Mukul Dubey 1 , Hongshan He 1 , Pavel Dutta 1 , David Galipeau 1 , Venkat Bommisetty 1
1 Department of Electrical Engineering, South Dakota State University, Brookings, South Dakota, United States
Show AbstractThe nano-sized titanium dioxide semiconductor material on transparent conductive oxide (TCO) glass is critical for adsorption of light-harvesting agents (also called dyes), electron injection from dyes and electron migration for efficient energy conversion in dye-sensitized solar cells (DSSCs). Research shows that post-treatment of semiconductors is important for achieving high efficiency of energy conversion. In this presentation, we will report the new method for treatemnt of titanium dioxide with transparent titanium tetrachloride solution and nano-sized titanium dioxide in sol-gel from microwave-assistant reactions. The microscopic (SEM and AFM) studies of titanium dioxide films and detailed study of photovoltaic performance of these films in DSSCs will be presented.
12:45 PM - M1.6
Non Coherent Ultra-low Power Up-conversion in Multi-component Organic Systems.
Angelo Monguzzi 1 2 , Franco Meinardi 1 2 , Marcello Campione 1 , Riccardo Tubino 1 2
1 Scienza dei Materiali, Univ. Milano Bicocca, Milano, Milano, Italy, 2 , INFM-CNR, Milano Italy
Show AbstractThe generation of high-energy emission starting from photons with lower energy is a well known phenomenon. This up-conversion processes such as second-harmonic generation or two-photon absorption, usually require very high excitation power densities of the order of MW/cm2 or GW/cm2, being therefore effective only by using laser sources. Recently it has been proposed a completely new approach based on the exciton triplet-triplet annihilation (TTA) in organic molecules, giving rise to an up-conversion induced delayed fluorescence, indirectly excited via energy transfer from a second moiety acting as a light harvesting. In such a way due to the high efficiency of the energy transfer between the two species, the requested power density is reduced to the ultra-low level of the solar emission power (as low as ~ 0.1 W/cm2) for which an efficient up-conversion can be obtained. A fundamental advantage of the TTA supported bimolecular up-conversion process is its inherent independence on the coherence of the excitation light. The optical excitation of the active system occurs by resonant single photon absorption: consequently, the efficiency of the up-conversion process depends only on the materials properties but does not in any way depend upon the coherence of the photons used for excitation. This makes these processes suitable for the huge market of the photovoltaic, which calls for methods for the conversion of the low energy tail of the solar spectrum into a spectral range efficiently exploited by the solar cells, and of that of the organic LEDs, which requires efficient and low cost strategies to tune the device emission. The up-conversion induced fluorescence is the result many intermediate photophysical processes and the overall yield of the mechanism outlined above is clearly influenced by many factors. By the general approach proposed to describe the kinetics of these system and by experimental results, we were able to define the threshold excitation power density necessary to have the maximum yield in up converted light generation, as a function of intrinsic characteristics of the molecular species involved. Moreover the up-conversion process was not only investigated in liquid solutions but also in solid amorphous films and crystals for practical application in devices.
M2: OLED Materials
Session Chairs
Monday PM, December 01, 2008
Republic A (Sheraton)
2:30 PM - **M2.1
Luminescence Quenching Processes in pi-Conjugated Materials and Organic Light-Emitting Diodes (OLEDs).
Joseph Shinar 1 2 , Ying Chen 1 2 , Min Cai 1 2
1 Physics and Astronomy, Iowa State University, Ames, Iowa, United States, 2 Ames Laboratory, USDOE, Iowa State University, Ames, Iowa, United States
Show AbstractNumerous studies have provided strong evidence for nonradiative singlet exciton (SE) quenching by metal electrodes, polarons, other SEs, and triplet excitons (TEs) in pi-conjugated materials and OLEDs. This talk will review the extensive optically detected magnetic resonance (ODMR) studies that have provided evidence for quenching of SEs by polarons, TEs, and trions (i.e., bipolarons stabilized by a counterion), and of TEs by polarons. The latter in particular is believed to be of paramount importance, since the polaron and TE populations are typically much higher than that of SEs. Indeed, it is now widely accepted that these quenching processes are the main cause of OLED efficiency roll-off at the high current. Quenching by trions may also be very significant, since recent studies demonstrate that interfaces, which typically induce trapped charges, and consequently trions as well, are detrimental to device stability. *Ames Laboratory is operated by Iowa State University for the US Department of Energy under Contract DE-AC 02-07CH11358.
3:00 PM - M2.2
Exploiting Dual Fluorescence in Fluorene Co-polymers for OLED Applications Including White-Light Emission.
Martin Bryce 1 , Igor Perepichka 1 3 , Kiran Kamtekar 1 , Fernando Dias 2 , Simon King 2 , Andrew Monkman 2 , Irene Perepichka 1 , Mustafa Tavasli 1
1 Chemistry, Durham University, Durham United Kingdom, 3 Centre for Materials Science, University of Central Lancashire, Preston United Kingdom, 2 Physics, Durham University, Durham United Kingdom
Show AbstractWe will discuss 9,9-dialkylfluorene(F)-based co-oligomers and co-polymers which incorporate dibenzothiophene-S,S-dioxide (S) as an electron-deficient unit in the backbone. S has a highly emissive singlet excited state and has a LUMO energy level 1.04 eV lower than that of F. F-S co-oligomers are better electron acceptors than the corresponding oligofluorenes, they can be reversibly p- and n-doped, and they possess high photoluminescence in both solution and solid state.[1] Detailed photophysical studies reveal that F-S co-oligomers give highly efficient dual emission from molecular excited states which have both local excitonic (LE) and charge-transfer (CT) character.[2,3] This strategy has the benefit of removing unwanted Forster energy transfer between multiple chromophores. New results establish that combined LE/CT emission from F-S copolymers can be exploited to give efficient PLEDs with scope for color tuning including white-light emission and solid-state lighting applications, utilizing simple solution processable fabrication routes. Optimization of molecular design, photophysical properties and PLED architectures will be presented. [1] Perepichka, I.I.; Perepichka, I.F.; Bryce, M.R.; Palsson, L.-O. Chem. Commun. 2005, 3397.[2] Dias, F.B.; Pollock, S.; Hedley, G.; Palsson, L.-O.; Monkman, A.; Perepichka, I.I.; Perepichka, I.F.; Tavasli, M.; Bryce, M.R. J. Phys. Chem. B, 2006, 110, 19329.[3] Dias, F.B.; King, S.; Monkman, A.P.; Perepichka, I.I.; Kryuchkov, M.A.; Perepichka, I.F.; Bryce, M.R. J. Phys. Chem. B, 2008, 112, 6557.
3:15 PM - M2.3
Solution-processed Hybrid Infrared Light Emitting Devices.
Kaushik Roy Choudhury 1 , Dong Woo Song 1 , Franky So 1
1 Dept of Materials Science and Engineering, University of Florida, Gainesville, Florida, United States
Show AbstractColloidal semiconductor nanocrystals, by virtue of their quantum size-tunable bandgap, high luminance efficiency, narrow spectral emission with high photostability, and compatibility with facile solution-processing, are an attractive choice as lumophores in organic light emitting diodes. Though several hybrid device embodiments employing visible-range quantum dots with organic semiconductors have been demonstrated, very little work has been done to explore such light emitting devices with emission in the infrared. Only a few infrared electroluminescent hybrid composites have been reported, typically with low efficiency. Herein, we demonstrate a hybrid light emitting device with efficient tunable narrowband emission from PbSe nanocrystals spanning over a wide range of infrared wavelengths (1100 nm to 1900 nm). We fabricated hybrid organic/inorganic infrared light emitting diodes by incorporating colloidal PbSe nanocrystals into the matrix of the semiconducting polymer MEH-PPV in different proportions. The solution-cast devices also employed a PEDOT-PSS hole-injection layer and a bathocuproine (BCP) layer for hole blocking and exciton confinement. Devices with optimized composition and architecture exhibited broadly tunable infrared emission with peak quantum efficiency > 0.3 %, achieved with nanocrystal loading less than 10 wt%. Reduced turn-on and operating voltages indicate better power efficiency in these devices. The performance of the diodes was studied and analyzed as a function of nanocrystal loading concentration and device thickness. Current work is underway to modify surface passivation of the nanocrystal emitters with short-chain organic ligands and thin high-bandgap inorganic shells to improve carrier injection and exciton confinement and thereby enhance internal radiative efficiency.
3:30 PM - **M2.4
Site-Isolation, Intramolecular Energy Transfer, and Crosslinking in Synthetic Dendritic Quinacridones.
Dominic McGrath 1 , Gemma D'Ambruoso 1
1 Department of Chemistry, University of Arizona, Tucson, Arizona, United States
Show AbstractDendrimers incorporating a green emitter, quinacridone, for organic light emitting diodes (OLEDs) were synthesized and their photophysical and electrochemical properties were explored. Quinacridone dendrimers were synthesized for site isolation, intramolecular energy transfer, and photocrosslinking.Site-isolation of quinacridone at the core of a dendrimer was achieved by attaching bulky poly(aryl ether) dendrons to the quinacridone at the amino functional groups. Both benzyl- and t-butyl-terminated dendrimers were synthesized up to the third generation. These dendrimers showed enhanced solubility in organic solvents due to reduced aggregation and hydrogen bonding. Increased photoluminescence intensity was observed for the denderimers in the solid state indicating reduced self-quenching due to enhanced site-isolation. Preliminary incorporation of these dendrimers as dopants into OLEDs showed increased emission from the dendrimers as the doping percentage increases.When high-energy host absorbing groups, such as oligo(p-phenylene vinylene)s (oPPVs) were placed at the periphery of poly (aryl ether) dendrimers with quinacridone guest cores, intramolecular energy transfer occurs when the host periphery groups were excited. These dendrimers showed high efficiency energy transfer yields in both solution and the solid state, as well as an antennae effect which resulted in increased emission when the oPPVs were excited versus direct excitation of the quinacridone. For comparison, poly(methyl methacrylate) polymers with pendant oPPV groups were synthesized and combined both in solution and in thin films with the site-isolated 25 dendrimers to investigate the architectural requirements for energy transfer. These mixtures showed no energy transfer in solution from the polymer to the dendrimers. However, in the solid state, energy transfer increaseed with decreasing generation due to the host/guest chromophores decreased separation. Finally, poly (aryl ether) dendrimers containing photocrosslinkable cinnamate groups at the periphery and quinacridone cores were synthesized. Thin films of the higher generation dendrimers were photopolymerized via ultraviolet irradiation. The films were resistant to solvent after the polymerization step indicating a stable crosslinked network. Standard photolithography was performed on the higher generation dendrimers to achieve feature sizes as small as 5 microns as observed by fluorescence and atomic force microscopy.
4:00 PM - M2: OLEDs
BREAK
M3: Solar Cell Polymers
Session Chairs
David Andrews
Theodore Goodson
Monday PM, December 01, 2008
Republic A (Sheraton)
4:30 PM - M3.1
Synthesis of Well-Defined Electroactive Building Blocks for Organic Photovoltaic Applications.
Matthias Haeussler 1 , Ming Chen 1 , Ezio Rizzardo 1 , Scott Watkins 1 , Gerard Wilson 1
1 Molecular and Health Technologies, CSIRO Australia, Clayton South, Victoria, Australia
Show AbstractWorld demand for energy is projected to more than double by 2050 and to more than triple by the end of the century. Over the long term, the reserves of fossil fuels, including materials for nuclear power, will fall short of this demand and their continued use will produce harmful side effects such as pollution and global warming or radioactive waste. Plastic solar cells produced from organic semiconductors offer the potential to deliver efficient solar energy conversion with low-cost fabrication. The challenge to overcome is to develop materials for efficient charge separation and charge transport. The synthesis of well-defined organic conjugated chromophores is of interest as the facile tuning of their energy levels is possible through a structural engineering approach. Matching energy levels by combining two or more electroactive building blocks in a controlled fashion gives access to materials with efficient light-harvesting and energy transfer properties. In this contribution, we will present synthetic strategies for new building blocks and their transformation into well-defined block co-polymers prepared by Reversible Addition Fragmentation chain Transfer (RAFT) polymerisation techniques. The benefits of using RAFT to synthesise polymer structures where energy transfer is optimised and their application in organic solar cells will be highlighted.
4:45 PM - M3.2
Controlling Morphology in Organic Bulk Heterojunctions by the use of Block Copolymers.
David Jones 1 , Andrew Holmes 1 , Wallace Wong 1 , Qinghui Mao 1 , Weihua Tang 1 , Saif Haque 2 , Simon King 2
1 School of Chemistry, Bio21 Institute, Melbourne, Victoria, Australia, 2 Department of Chemistry, Imperial College London, London United Kingdom
Show AbstractThe active layer in 4th generation organic solar cells is formed from blends of n- and p-type organic semiconducting materials. It is becoming increasingly apparent that the generation of interpenetrating, bicontinuous phases on the scale of exciton diffusion in organic bulk heterojunction thin films is essential for the generation of highly efficient organic solar cells. Phase separation in blends is normally controlled by choice of solvent, deposition method or post film formation annealing. Supramolecular self-assembly of chain end functionalised active materials has been attempted but the resulting materials have not lead to active devices.For non-conjugated polymers exquisite control of thin film morphology has been obtained using block copolymers [Tschierke, 2007; Sergeyev et al, 2007]. The morphologies are stable and tend to the system thermodynamic minimum. To date however, attempts to control the characteristics of thin films formed from conjugated materials has not lead to devices with significant efficiency improvements and normally the device efficiencies are lower than the devices formed from blends of the homo-polymers [Scherf et al, 2008]. We were intrigued by the possibility of controlling thin film morphology by the use of block copolymers with the inherent property that device efficiency would improve as the thin film tended to a thermodynamic minimum.To this end we have been examining the use of conjugated block copolymers as the active layer in organic solar cells. A 15-20nm domain size in thin films formed from block copolymers is a direct result of the designed block length. Morphology is determined by the relative volume fraction of the blocks with the expected lamellar type structure formed when a 1:1 volume ratio of n- and p-type blocks were used. We will discuss the synthesis, thin film morphology characterisation and improved device efficiencies when conjugated block copolymers are used as the active material in organic solar cells.Scherf, U.; Gutacker, A.; Koenen, N.Acc. Chem. Res.; 2008; ASAP Article; DOI: 10.1021/ar7002539.Tschierke, C. Chem. Soc. Rev.; 2007; 36; 1930.Sergeyev, S.; Pisula, W.; gerts, Y.H. Chem. Soc. Rev.; 2007; 36; 1902.
5:00 PM - M3.3
Polymer Solar Cells: Non-Conjugated Polymers and PESA Studies of Semiconductor Energy Levels.
Scott Watkins 1 , Mark Bown 1 , Ming Chen 1 , Richard Evans 1 , Akhil Gupta 1 , Matthias Haeussler 1 , Hegedus Katalin 1 , Y. Lok 1 , Graeme Moad 1 , Ezio Rizzardo 1 , Gerard Wilson 1 , Kevin Winzenberg 1
1 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. Well-defined block-copolymers consisting of organic conjugated chromophores are advantageous as their energy levels can be tuned relatively easily through a structural engineering approach. In this contribution, we will discuss device and characterisation results for new polymer building blocks. We will also describe energy transfer in well-defined block co-polymers prepared by Reversible Addition Fragmentation chain Transfer (RAFT) polymerisation techniques.[1] Finally, we will present work on determining the energy levels of known and novel materials by Photo Electron Spectroscopy in Air (PESA) and compare these results with Ultraviolet Photoelectron Spectroscopy (UPS) and electrochemistry studies.[1] M. Chen, et al., Chem. Commun., 2008, 1112.
5:15 PM - **M3.4
Carbon Nanostructures – Integrative Components in Multifunctional Molecular Materials.
Dirk Guldi 1
1 Department of Chemistry and Pharmacy, Friedrich-Alexander-Universitat Erlangen-Nurnberg, Erlangen Germany
Show Abstract5:45 PM - M3.5
Alternative Mechanisms in Polymer Light Harvesting.
David Andrews 1 , D. Bradshaw 1 , J. Rodriguez 1
1 Nanostructures and Photmolecular Systems, University of East Anglia, Norwich United Kingdom
Show AbstractM4: Poster Session
Session Chairs
Tuesday AM, December 02, 2008
Exhibition Hall D (Hynes)
9:00 PM - M4.1
Using Nanoscale Architectures for Shaping the Emission Spectra of Semiconductor Nanocrystals.
Sergiy Mayilo 1 , Jan Hilhorst 1 , Andrei Susha 1 , Fernando Stefani 1 , Andrey Lutich 1 , Sameer Sapra 3 , Thomas Klar 1 2 , Andrey Rogach 1 , Jochen Feldmann 1
1 Department of Physics and CeNS, Photonics and Optoelectronics Group, Ludwig-Maximilians-Universität München, Munich Germany, 3 Department of Chemistry, Indian Institute of Technology Bombay, Mumbai India, 2 Institute of Physics, Department of Experimental Physics II, Technical University of Ilmenau, Ilmenau Germany
Show AbstractWe present two possibilities to shape the emission spectra of semiconductor nanocrystals: by Ca(II)-induced clustering of CdTe nanocrystals and by creating onion-like CdSe/ZnS/CdSe/ZnS nanocrystals.In the first approach, stable complexes from differently-sized water-soluble CdTe nanocrystals capped by mercaptoacid stabilizers are formed through electrostatic interactions of negatively charged carboxylic groups of capping ligands with positively charged Ca(II) ions [1]. These nanocrystal clusters composed of particles of two and three different sizes can be considered as artificial light harvesting complexes transporting the excitation energy to the nanocrystals with the largest band gap within the cluster.In the second approach, we synthesized multishell CdSe/ZnS/CdSe/ZnS onion-like nanocrystals [2] to produce two emission lines upon excitation: a red one from the CdSe core and a green from the CdSe shell. The inner ZnS layer is a spacer between the two light emitting CdSe regions and the outer ZnS layer passivates the surface. We have studied the coupling of the core and shell excitations by single nanocrystal spectroscopy to elucidate the mechanism of dual emission. References:[1] S. Mayilo, J. Hilhorst, A.S. Susha, C. Höhl, T. Franzl, T.A. Klar, A.L. Rogach, J. Feldmann, J. Phys. Chem. C, accepted (2008).[2] S. Sapra, S. Mayilo, T. A. Klar, A. L. Rogach, J. Feldmann, Adv. Mater., 19, 569-572 (2007).
9:00 PM - M4.2
Photo Electrochemical Performance of Plasma Treated Titanium-di-oxide Nanostructures.
Rajesh Sharma 1 , P. Das 2 , M. Misra 2 , V. Mahajan 2 , A. Biris 1 , S. Trigwell 3 , M. Mazumder 1
1 Applied Science, University of Arkansas at Little Rock, Little Rock, Arkansas, United States, 2 Dept of Chemical and Metallurgical Engineering, University of Nevada, Reno, Nevada, United States, 3 Electrostatics and Surface Physics Laboratory, NASA Kennedy Space Center, Cape Canaveral, Florida, United States
Show AbstractPlasma treatment has been used as a tool for modifying surface properties of materials in a variety of applications. In this study, plasma treatment was used for enhancing photoelectrochemical performance of semiconductor titania anodes. Titania (TiO2) nanotubular arrays were synthesized by electrochemical anodization of Ti thin foils. Nitrogen plasma was used to modify the surface properties of TiO2 nanostructures. The photocurrent from plasma treated samples was significantly higher than untreated samples. The open circuit potential (OCP) of the plasma treated samples turned out to be more negative implying a favorable energetics was water splitting. XPS analysis showed an increase in surface concentration of nitrogen. This increase in photoactivity could be ascribed to (1) increased absorption of visible light due to bandgap reduction, (2) efficient charge separation, (3) optimal oxygen vacancies, and (4) increased surface area and hence enhanced electrode-electrolyte area to provide maximum optical adsorption and efficient charge transfer.
9:00 PM - M4.3
Efficiency Enhancement and Optical Characteristics of Microcavity Blue Phosphorescnet 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 AbstractPreviously enhancement in external quantum efficiency (EQE) in green OLEDs has been demonstrated using microcavity structure. Here, we demonstrate by incorporating microcavity structure in blue phosphorescent OLEDs (PHOLEDs), we were able to achieve 56% enhancement in external quantum efficiency and 88% enhancement in luminous power efficiency. Microcavity blue PHOLEDs were fabricated on the two different dielectric mirrors: two-layer quarter wave stacks with reflectivity of 0.39, and 4-layer quarter wave stacks with reflectivity of 0.7 at 475nm. The thickness of each dielectric layer is designed to maximize the micro-cavity effect at 475nm by optical OLED simulation. The devices have the following structure: glass substrate (1mm)/SiO2 (79nm) /TiO2 (48nm) or SiO2 (79nm)/TiO2 (48nm)/SiO2 (79nm)/TiO2 (48nm)/ITO (50nm)/ poly(3,4-ethylenedioxythiophene)–polystyrenesulfonic acid (PEDOT:PSS) (25nm)/ 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC) (17nm)/ Iridium(III)bis [(4,6-di-fluorophenyl)-pyridinato-N,C2'] picolinate (FIrpic): 3,5'-N,N'-dicarbazole-benzene (mCP) (20nm)/2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) (40nm)/LiF (1nm)/Aluminum (100nm). FIrpic 3% is doped in emitting layer of mCP host. Noncavity devices have the same structure without the SiO2/TiO2 dielectric stacks.With the above device structures, we demonstrated that blue PHOLED shows about 50% enhancement in EQE (from 10.1% to 15.7%), 80% enhancement in luminous efficiency (10.8 lm/W to 20.4 lm/W), and more saturation in emitted blue color (CIE coordinate, from x=0.15, y=0.32 to x=0.14, y=0.22). Measured angular dependence of the spectrum intensity indicates that microcavity device has stronger directional emission pattern in the normal direction and higher out-coupling efficiency.
9:00 PM - M4.4
Mems Piezoelectric Vibration Energy Harvesters.
Woo Sik Kim 1 , Miso Kim 1 , Mathias Hoegen 1 , Seung-Hyun Kim 2 1 , Brian Wardle 1
1 Aeronautics and Astronautics, MIT, Cambridge, Massachusetts, United States, 2 , Inostek, Ansan, Gyeonggi, Korea (the Republic of)
Show AbstractA microfabricated mechanical vibration energy harvester with piezoelectric micro-cantilever is attractive for the power source of wireless sensor nodes that have application in structural health monitoring (as in the aircraft sensor targeted in this design), homeland security, and infrastructure monitoring, among others. One of Key results from prior modeling and verification efforts is the requirement of a proofmass on the beam structure to manage the beam’s resonant frequency towards the location of maximum input power. In this work, the proofmass has been treated rigorously in the electromechanical modeling, and the predicted optimum design is fabricated with MEMS technology. Progress on fabrication and testing of a MEMS harvester are discussed, particularly management of residual stresses and the comparison between the design predictions and power generation results.
9:00 PM - M4.5
Optical Excitations in Synthetic Light Harvesting Thiophene Macrocycles.
Oleg Varnavski 1 , Robert Cemborski 2 , Theodore Goodson 3 , Peter Bauerle 4 , Masahiko Iyoda 5
1 Chemistry, University of Michigan, Ann Arbor, Michigan, United States, 2 Chemistry, University of Michigan , Ann Arbor, Michigan, United States, 3 Chemistry and Macromolecular Science and Engineering, University of Michigan , Ann Arbor, Michigan, United States, 4 Organic Chemistry II, Ulm University , Ulm Germany, 5 Chemistry , Tokyo Metropolitan University, Tokyo Japan
Show Abstract9:00 PM - M4.6
Synthesis of 2,1,3-Benzothiadiazole Building Blocks for Organic Electronic Applications.
Phei Lok 1 , Kevin Winzenberg 1 , Scott Watkins 1 , Matthias Haeussler 1 , Ming Chen 1 , Katalin Hegedus 1 , Lynn Rozanski 2 , Gerard Wilson 1
1 Molecular and Health Technologies, CSIRO, Clayton , Victoria, Australia, 2 Energy Technology, CSIRO, Newcastle, New South Wales, Australia
Show Abstract2,1,3-Benzothiadiazole is a building block with useful electronic properties and is widely featured in OLED and OPV work. We will present synthetic strategies towards non-symmetrically functionalized building blocks based on this heterocycle, and the properties of the materials generated.
Symposium Organizers
David L. Andrews University of East Anglia
Kenneth P. Ghiggino University of Melbourne
Theodore Goodson University of Michigan
Arthur J. Nozik University of Colorado-Boulder
M5: Energy Transfer Processes
Session Chairs
Tuesday AM, December 02, 2008
Republic A (Sheraton)
9:00 AM - **M5.1
Effective Light-energy Delivery to Molecules using Superfocusing of Surface Plasmon Polaritons.
Akira Otomo 1 , Kazuhiro Yamamoto 1 , Kazuyoshi Kurihara 1 , Ryo Naraoka 1 , Yukito Naitoh 1 , Toshiya Kamikado 1
1 Kobe Advanced ICT Research Center, National Institute of Information and Communications Technology, Kobe, Hyogo, Japan
Show AbstractLow energy consuming devices are the keys for sustainable development of information and communication systems. Nano-scale devices operating with single quantum, such as an electron, a photon, a flux quantum and etc., the switching energy can be as small as single quantum. The nano-scale photonic devices have been demonstrated using energy transfer in a molecular system and in a quantum-dot system. The molecular system has advantages over the others on design flexibility, fabrication feasibility and integration capability. However, optical excitation efficiency for a molecular device is limited by the diffraction limit of focusing light due to large discrepancy in scale between molecules and the diffraction limit of the light focus. Superfocusing of surface plasmon polaritons (SPPs) that propagate towards tapered metal tips recently attracts attention due to the effective delivery of electromagnetic energy to nanoscale. The extraordinary enhancement of electromagnetic field of SPPs has known as surface or tip enhanced Raman scattering. Superfocusing mode of SPPs propagating on metal tip surface is considered as a possible mechanism of such enhancement. We investigate effective delivery of light energy to molecules using the electric field enhancement of the SPP’s superfocusing modes. We have developed analytical method to describe the superfocusing mode using so called quasi-separation of variables approach. Optimum structures are also examined to achieve largest enhancement of the field using FDTD simulations. In addition to that, effective layout between a metallic structure and a molecule will be discussed in order to prevent a reverse energy-flow from molecules to metal tips.
9:30 AM - M5.2
Ultrafast Dynamics of Excited Molecular Semiconductors Coupled to Surface Plasmon Active Materials.
Justin Johnson 1 , Thomas Reilly 1 , Jao van de Lagemaat 1
1 , National Renewable Energy Laboratory, Golden, Colorado, United States
Show AbstractMolecular chromophores placed in close proximity to surface plasmon active materials can lead to unique photophysical effects such as enhanced absorption/emission and altered charge carrier relaxation dynamics. If the nature of these effects can be understood such that they can be utilized to produce a particular result (such as the efficient transfer of energy from one photoexcited state into another preferred state), the implications for light harvesting could be profound. Our approach to study such effects involves achieving reasonably strong resonant coupling between a thin crystalline polyacene film and a metallized nanohole array. The coupling strength between polyacene/plasmon was varied both by altering the distance between the metal and polyacene layers and by tuning the peak surface plasmon energy by controlling the size of the nanoholes in the array. Steady state optical characterization suggests that enhanced molecular absorption is achieved in the spectral region overlapping the surface plasmon. Ultrafast transient absorption measurements reveal both new photoexcited species possibly involving molecule/plasmon hybrid states as well as altered yields of native molecular excitations, especially the ratio of singlets to triplets. Additional characterization and kinetic modeling further support the notion that improved control over such hybrid systems could lead to enhanced photocurrent in next generation photovoltaic devices.
9:45 AM - M5.3
Novel Hybrid Organic/Inorganic Structures Utilizing Non-Radiative Förster Energy Transfer.
Grigorios Itskos 1 , Colin Belton 2 , George Heliotis 2 , Pavlos Lagoudakis 3 , Ian Watson 4 , Martin Dawson 4 , Ray Murray 2 , Donal Bradley 2
1 Physics, University of Cyprus, Nicosia Cyprus, 2 Physics, Imperial College London, London United Kingdom, 3 Physics, University of Southampton, Southampton United Kingdom, 4 Institute of Photonics, University of Strathclyde, Glasgow United Kingdom
Show AbstractFörster resonant energy transfer (FRET) from a semiconductor quantum well (QW) to an organic material has been predicted to become an efficient process when the two materials are placed in close proximity and there is a large spectral overlap of the QW emission and the organic absorption. We have fabricated hybrid layered structures based on combinations of inorganic InGaN/GaN QWs and semiconductor polyfluorene films. Two types of structures were developed. Type I structures consist of thin layers of the polyfluorene material F8DP spin-coated onto MOVPE-grown InGaN/GaN QWs that differ only in the thickness of the GaN cap above the QW. Type II structures used the same QW structures on top of which polymer blends were deposited. The blends consisted of the blue emitting F8DP material doped with green emitting F8BT and red emitting Red F to achieve white light emission. Type I structures were studied using a combination of spectrally- and temporally-resolved optical measurements, supported by structural characterization [1, 2]. Photoluminescence measurements (PL) in the 8–300 K range, exhibited a reduction in the QW intensity and a simultaneous increase in the polymer emission with decreasing QW cap thickness. Time-resolved measurements revealed a clear cap-dependent reduction of the well recombination lifetime in the presence of the polymer layers which confirms the presence of an efficient non-radiative transfer of energy from the QW to the polymer layer. These experiments also found that the energy transfer rate increases with a power law dependence on the QW-polymer spacing which indicates a plane-to-plane Förster resonant coupling (FRET) between the excitations in the two layers. Type II structures were fabricated to achieve white light generation. Light emission in these structures is a result of a complicated process that includes FRET from the QW to the blend and energy transfer processes within the blend that control the relative proportions of blue, green and red light emission. In common with the type I hybrids, these structures reveal a dependence of the emission intensity on the average separation of the well and blend excitations which is indicative of the presence of FRET from the QW to the blend. The emission characteristics of the white light including its dependence on the excitation density are controlled by a complex interaction between the various energy transfer processes from the QW to the blend and within the blend itself [3]. This work shows that it is possible to engineer novel hybrid light emitting devices which make use of efficient Förster resonant coupling between excitations in their inorganic and organic components.[1] G. Heliotis et al., Advanced Materials 18, 334, (2006) [2] G. Itskos et al., Physical Review B 76, 035344, (2007) [3] C. R. Belton et al., Journal of Physics D: Applied Physics, 41, 094006, (2008)
10:00 AM - M5.4
Measurement of the Exciton Diffusion Length of Organic Semiconductors by Spectrally Resolved Photoluminescence Quenching.
Richard Lunt 1 2 , Jay Benziger 1 , Stephen Forrest 2
1 Chemical Engineering, Princeton University, Princeton, New Jersey, United States, 2 EECS and Physics, University of Michigan, Ann Arbor, Michigan, United States
Show AbstractExciton lifetime and diffusion length are critically important to the understanding and control of energy transport in thin film electronic devices, particularly solar cells. A number of methods have been used to extract the diffusion length in molecular systems, most notably, thickness dependant photoluminescence quenching and spectrally resolved photocurrent response of Schottky diodes. Unfortunately, the former method can be dominated by incomplete coverage of very thin films required, and optical interference effects, while the latter can introduce significant errors due to energy transfer at the metal contacts and other effects. Here we show that spectrally resolved photoluminescence quenching can overcome many of the drawbacks of previously reported techniques, providing quick and accurate measurements of the exciton diffusion length. Using this method we measure the singlet diffusion length of amorphous N,N’-diphenyl-N,N’-bis(1-naphthyl)-1,1’biphenyl-4,4’’diamine (α-NPD), and boron subphthalyocyanine (SubPc), as well as polycrystalline films of 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) and diindenoperylene (DIP). We find that, in general, the diffusion length of singlets is constant over a large spectral range in most materials, contrary to other reports where the separate effects of singlet and triplet transport could not be separately determined[1]. We show the dependence of crystalline order on exciton diffusion length in PTCDA, where the diffusion length increases from 82Å (±5Å) for 300Å diameter crystallites to 106Å(±5Å) for 550Å crystallites. Moreover, we show that in polycrystalline vapor-phase grown films of DIP, there is significant anisotropy in the exciton diffusion length for various stacking directions. For example, the DIP exciton diffusion length was found to be 165Å(±4Å) for the molecular plane oriented normal to the surface, while it is 218Å(±6Å) in the flat-lying direction despite a smaller crystallite size. [1]A. K. Ghosh, T. Feng, J. Appl. Phys. 1978, 49, 5982.
10:15 AM - **M5.5
Probing Quasiparticle Correlations and Coherent Exciton Dynamics in Photosynthetic Complexes by Multidimensional Optical Spectroscopy.
Darius Abramavicius 1 , Dmitri Voronine 1 , Shaul Mukamel 1
1 Chemistry, UCI, Irvine, California, United States
Show AbstractM6: Supramolecular Systems
Session Chairs
Tuesday PM, December 02, 2008
Republic A (Sheraton)
11:15 AM - **M6.1
Intrachain Energy Transfer in a Conjugated Polymer: Coherence and the Intermediate Coupling Limit.
Gregory Scholes 1 , Elisabetta Collini 1
1 Department of Chemistry, University of Toronto, Toronto, Ontario, Canada
Show AbstractImportant questions still to be addressed in understanding electronic energy transfer (EET) concern the so-called intermediate coupling case. The intermediate coupling case lies somewhere in between the well known limiting examples of strong (excitons) and weak (Förster energy transfer) coupling, such that the electronic coupling is of the same order as the dephasing rate. It has perplexed researchers for some time because it is not obvious how to identify this regime using experimental probes, nor is it easily described precisely. We define intermediate coupling as an situation wherein there is competition between the formation of eigenstates caused by electronic coupling and their destruction, caused by coupling to bath fluctuations. This regime is of particular interest because the excitation moves in space—which is a deterministic, classical, attribute—yet a preferred path can be chosen because the wavefunction is delocalized—a quantum characteristic that requires some degree of phase preservation. Together these attributes may in principle conspire to confer a special quality onto excitonic systems. That is that phase information can be transferred through space. It is hence that EET may be ‘steered’ by the electronic Hamiltonian of the entire system. By learning how to observe the intermediate coupling case and henceforth understanding its properties, we will be able to learn how to control excitation waves.Conjugated polymers, highly conjugated linear macromolecules that have recently attracted an increasing interest owing to their unique electronic and optical properties. While these systems are fully conjugated, the presence of twists, bends or chemical defects breaks the conjugation of the backbone into individual segments, thus producing a chain of linked chromophores (or conformational subunits). EET within and among conjugated polymers has been of considerable interest, both from a practical point of view because EET is a significant source of electroluminescence quenching, and a fundamental point of view as researchers aim to elucidate how excitation evolves and migrates as a function of chain conformation and packing. We report a new kind of ultrafast nonlinear optical experiment that reveals incisively coherent EET dynamics in intrachain EET in a prototypical conjugated polymer at room temperature. The interpretation and analysis of this observation will be described.
11:45 AM - M6.2
Design and Characterization of Light Harvesting Macromolecules.
Ken Ghiggino 1 , Ming Chen 2 , Gerard Wilson 2
1 School of Chemistry, University of Melbourne, Parkville, Victoria, Australia, 2 , CSIRO Molecular and Health Technologies, Clayton South, Victoria, Australia
Show AbstractRecent advances in synthesis have enabled the design of polymers with well controlled architectures suitable for light harvesting and energy conversion applications. In particular the ability to place chromophores at well defined locations within a polymer chain can provide control over the processes of excitation energy migration and transfer. We describe the application of controlled living free radical polymerization using reversible addition-fragmentation chain transfer (RAFT) to produce linear and star-shaped light harvesting homopolymers and block copolymers of defined structure and low polydispersity. With appropriate synthetic strategies, polymers incorporating an energy gradient of chromophores can facilitate directed energy transfer and trapping while addition of hydrophilic monomers can confer water solubility. The polymers incorporate light absorbing energy donor chromophores (acenaphthalene, styryl coumarin) and luminescent energy traps (diphenylanthracene, ruthenium trisbipyridyl). Examples will be provided of the role of location in the chain (i.e. centre, end-group) of the energy accepting chromophore on transfer efficiencies and the effect of polymer chain length on energy migration dynamics and excimer formation. The results can be utilized to propose optimum macromolecule structures for efficient light harvesting and energy conversion.M. Chen, K.P. Ghiggino, E. Rizzardo, S.H. Thang, G.J. Wilson, Chem. Commun., 2008, 1112-1114.
12:00 PM - **M6.3
Photo- and Electro-active Concave-convex Supramolecular Nanostructures.
Nazario Martin 1
1 Organic Chemistry, Universidad Complutense de Madrid, Madrid Spain
Show AbstractThe combination of supramolecular and electronic reciprocity between exTTF as strong electron donor systems and Fullerenes as well-known electron acceptors suggest that this novel host-guest system is a good candidate to be utilized in the self-organization of electroactive materials. Considering these principles, we have prepared a series of novel electroactive TTF-type derivatives as supramolecular receptors for fullerenes and CNTs. Several bowl and belt-shaped fullerene receptors based on this concave-convex complementarity principle have already been reported.. Herein, we report novel electroactive TTF-type receptors as supramolecular partners for fullerenes. Firstly, we describe the first exTTF-based receptor for fullerene, based on a synthetically straightforward tweezer-like design. Despite its structural simplicity, our receptor associates fullerene efficiently in both chlorobenzene and CHCl3/CS2 mixtures (Kassoc > 103 M-1). Moreover, it shows a unique solvent-controlled positive homotropic cooperative binding behavior. Other related supramolecular assemblies endowed with exTTF concave geometry and covex fullerene surfaces will be also presented. In addition, we report the synthesis, X-ray, spectroscopic, electrochemical and theoretical characterization of a new family of π-extended TTF derivatives as bowl-shaped electroactive monotopic receptors for fullerenes. Single wall carbon nanotubes (SWCNs) have also been interacted with electron donor exTTFs through covalent and supramolecular linkage and photoinduced electron transfer processes were observed. [1]E. M. Pérez, L. Sánchez, G. Fernández, N. Martín, J. Am. Chem. Soc., 2006, 128, 7172.[2]E. M. Pérez, M. Sierra, L. Sánchez, M. R. Torres, R. Viruela, P. M. Viruela, E. Ortí, N. Martín, Angew. Chem. Int. Ed., 2007, 46, 1847.[3]M. A. Herranz, Ch. Ehli, S. Campidelli, M. Gutierrez, G. L. Hug,, K. Ohkubo, S. Fukuzumi, M. Prato, N. Martín, Dirk M. Guldi,, J. Am. Chem. Soc., 2008, 130, 66.[4] E. M. Pérez, M. Sierra, L. Sánchez, M. R. Torres, N. Martín, Angew. Chem. Int. Ed., 2008, 47, 1094.
12:30 PM - **M6.4
The Efficiency of Photosynthetic Light-Harvesting and Its Regulation.
Rienk van Grondelle 1
1 Physics and Astronomy, VU University, Amsterdam Netherlands
Show AbstractM7: Charge Transfer Processes
Session Chairs
Tuesday PM, December 02, 2008
Republic A (Sheraton)
2:30 PM - **M7.1
Electron and Energy Transfer Reactions in Designed Donor-bridge-acceptor systems.
Bo Albinsson 1
1 , Chalmers University of Technology, Göteborg Sweden
Show AbstractUnderstanding how the electronic coupling is promoted through molecules is important for modeling the electron and energy transfer processes in biological and biomimetic systems and also for the future construction of molecular scale electrical and logical networks. We have designed different sets of tri-chromophoric donor-bridge-acceptor (DBA) molecules in which the rate of singlet excitation energy transfer,[1] triplet excitation energy transfer,[2] and electron transfer[3] are studied as a function of different factors such as: donor-acceptor distance, bridge conformation, height of the tunneling barrier, and the electronic structure of the bridging chromophore. This presentation will give an overview of our recent activities in this area of research.References:[1] Pettersson, K.; Kyrychenko, A.; Rönnow, E.; Ljungdahl, T.; Mårtensson, J.; Albinsson, B. J. Phys. Chem. A 2006, 110, 310-318. [2] Eng, M.; Albinsson, B. Angew. Chem. Int. Ed. 2006, 45, 5626-5629. Eng, M. P.; Mårtensson, J.; Albinsson, B. Chem. Eur. J. 2008, Accepted. [3] Kilså, K.; Kajanus, J.; Macpherson, A. N.; Mårtensson, J.; Albinsson, B. J. Am. Chem. Soc. 2001, 123, 3069-3080. Pettersson, K.; Kilså, K.; Mårtensson, J.; Albinsson, B. J. Am. Chem. Soc. 2004, 126, 6710-6719. Winters, M.; Pettersson, K.; Mårtensson, J.; Albinsson, B. Chem. Eur. J. 2005, 11, 562-573. Wiberg, J.; Guo, L. J.; Pettersson, K.; Nilsson, D.; Ljungdahl, T.; Mårtensson, J.; Albinsson, B.. J. Am. Chem. Soc. 2007,129, 155-163. Winters, M. U.; Dahlstedt, E.; Blades, H. E.; Wilson, C. J.; Frampton, M. J.; Anderson, H. L.; Albinsson, B. J. Am. Chem. Soc. 2007, 129, 4291-4297.
3:00 PM - M7.2
Electrochemical Gate Tuned Charge Transport in Single Molecule.
Fang Chen 1 , Xiulan Li 1 , Josh Hihath 1 , Nongjian Tao 1
1 Electrical Engineering, Arizona State University, Tempe, Arizona, United States
Show AbstractIn the field of molecular or bio-electronics, one of the main tasks is to study the fundamental charge transport behavior, and further control the transport by external stimulus, such as gated induced carrier injection, and photons excited current boost. In this work, we demonstrate the utilization of electrochemical gate, to control the charge transport in two conjugated molecule systems: one is single layer of graphene, consisting of one-atom-thick sp2-bonded carbon sheet; the other one is perylene tetracarboxylic diimide (PTCDI) derivatives (PTCDI) terminated with thiol groups. For single PTCDI molecules, we use STM tip to bridge them, and then we study the current flowing from the STM tip (source) to the substrate (drain) as a function of electrochemical gate voltage. We found that the source-drain current increases 2 or 3 orders of magnitude when a negative gate voltage is applied and reaches a peak near the redox potential of the molecules. Electron transport in this molecule depends on the temperature in the aqueous electrolyte but is independent of temperature in a nonpolar solvent, suggesting a strong coupling of the redox states to the polarized water molecules. We have obtained thermal activation energy of the electron transport as a function of gate voltage, from which the reorganization energy associated with the polarization of water molecules was extracted. On the other hand, we use one level of photolithography to make planar contacts on graphene. If a gate voltage is applied through the SiO2, the source-drain current shows well-known bipolar behavior and is almost linearly dependent on the gate voltage, except for the region close to the minimum conductivity where the external impurities or the defects are dominating the charge transport. However, when an electrochemical gate is applied through aqueous solution, the source-drain current behaves very different. The current-voltage curve is linear at low voltage region and becomes sublinear at higher voltage, which originates from the high gate efficiency of electrochemical gate, thus more charge transport nature or physical quantities are revealed in this process, i.e. the quantum capacitance starts to contribute to the total capacitance of device.
3:15 PM - M7.3
High-Efficiency Thermoelectric Oxide Semiconductors.
Matthew Scullin 1 5 , Jayakanth Ravichandran 2 5 , Subroto Mukerjee 3 5 , Joel Moore 3 5 , Arun Majumdar 4 5 , R. Ramesh 1 3 5
1 Materials Science and Engineering, University of California, Berkeley, Berkeley, California, United States, 5 Materials Sciences Division, Lawrence Berkeley National Laboratories, Berkeley, California, United States, 2 Applied Science and Technology, University of California, Berkeley, Berkeley, California, United States, 3 Physics, University of California, Berkeley, Berkeley, California, United States, 4 Mechanical Engineering, University of California, Berkeley, Berkeley, California, United States
Show AbstractOxides, historically studied as thermoelectrics for their high-temperature stability, have been limited in their thermoelectric efficiency zT because of high thermal conductivities. In addition, oxide semiconductors show high power factors in the limits converse to those of typical semiconductor systems, in that PF is maximized at high carrier concentrations due to low-mobility, high-effective mass carriers which yield thermopowers as large as several hundred µV/K at 1020 electrons/cm3. Presented here are the effects of double-doping in the strontium titanate system, where the addition of La in place of Sr offers electrical resistivities less than 1 mΩ-cm and the introduction of oxygen vacancies increases electron effective mass and in turn thermopower. These doped thin-films also show a four-fold reduction in thermal conductivity versus bulk strontium titanate systems. Through selecting the combination of dopant concentrations that maximizes both effective mass and carrier concentration, zT in this system has been pushed as high as 0.25 at 300K and increases monotonically with temperature, expected to increase to 0.8 at 1000K and making it a viable material for high-temperature energy conversion applications.
3:30 PM - **M7.4
The Mechanism of Enhancement in Novel Two-PhotonAbsorption Branched and other Macrocyclic Materials.
Theodore Goodson 1
1 Department of Chemistry, University of Michigan, Ann Arbor, Michigan, United States
Show AbstractM8: Piezoelectrics
Session Chairs
Tuesday PM, December 02, 2008
Republic A (Sheraton)
4:30 PM - M8.1
Modeling and Experimental Verification of Piezoelectric Mechanical Vibration Energy Harvesters for MEMS Applications.
Miso Kim 1 , Mathias Hoegen 1 , Woosik Kim 1 , Seung Hyun Kim 1 2 , John Dugundji 1 , Brian Wardle 1
1 Dept. of Aeronautics and Astronautics, MIT, Cambridge, Massachusetts, United States, 2 , INOSTEK Inc., Ansan, Gyeonggi, Korea (the Republic of)
Show AbstractEnergy harvesting is an attractive alternative to current power generation based on fossil fuels or nuclear fuels since it utilizes ubiquitous environmental energy from wind, solar power, and mechanical vibration. Our research focuses on mechanical vibration energy harvesters using a cantilevered beam configuration with piezoelectric elements. Both uni-morph and bi-morph piezoelectric energy harvesters are modeled along with {3-1} and {3-3} modes of operation. In order to adjust resonance frequency of interest, a proof mass is introduced and suitable models are developed to predict not only mechanical but also electrical responses that can be obtained from energy harvester devices with or without a proof mass. This model encompasses most of key testing and device parameters. Rigorous experimental work is performed to verify our improved model. Simulated responses from this power-optimized model are found to be in excellent agreement with experimental results. Enhanced power-optimized modal analysis, details of the experimental set-up, and experimental observations are presented. While the models are scale-independent, their use in design targets microfabricated devices where significant ambient mechanical energy exists to power MEMS sensors. Design of a MEMS harvester for an automotive application using the verified models is presented.
5:00 PM - M8.3
Energy Harvesting using Nanowires?
Marin Alexe 1 , Stephan Senz 1 , Markus Andreas Schubert 1 , Dietrich Hesse 1 , Ulrich Goesele 1
1 , Max Planck Institute of Microstructure Physics, Halle Germany
Show Abstract5:15 PM - M8.4
Flexoelectricity and Energy Harvesting from Electroactive NanoFibers and NanoArchitectures.
Cheng Huang 1 , Jim Holbery 1 , Jim West 2
1 Chemical, Materials, and Electrical Engineering, Energy & Environment Directorate, Battelle, Pacific Northwest National Laboratory, Richland, Washington, United States, 2 Electrical and Computer Engineering Department, Whiting School of Engineering, Johns Hopkins University, Baltimore, Maryland, United States
Show AbstractWe report enhanced electromechanical response from electroactive polymer nanofibers and nanoarchitectures. The high apparent piezoelectric coefficient up to 50 pC/N was measured. The phenomena can be attributed to flexoelectricity. We demonstrate the approaches to improve flexoelectricity by designing of these materials: selection of high polar molecular materials and fabrication of nanostructures. These electroactive nanofibers and nanoarchitectures by designing of piezoelectricity are typical candidates for artificial materials with mechanical energy harvesting. This work was supported by ONR and NRL, as well as a Center for Advanced Energy Materials.
Symposium Organizers
David L. Andrews University of East Anglia
Kenneth P. Ghiggino University of Melbourne
Theodore Goodson University of Michigan
Arthur J. Nozik University of Colorado-Boulder
M9: Quantum Dots
Session Chairs
Wednesday AM, December 03, 2008
Republic A (Sheraton)
9:00 AM - M9.1
Spectroscopy and Exciton Dynamics of Highly Luminescent CdTe/CdSe and ZnTe/CdSe Heteronanocrystals.
Celso de Mello Donega 1 , Veronique Gevaerts 1 , Evelien van Schrojenstein Lantman 1 , Esther Groeneveld 1
1 Debye Institute for Nanomaterials Science, Utrecht University, Utrecht Netherlands
Show AbstractSemiconductor heterostructures can show different behavior regarding charge carrier localization after photoexcitation (type-I or type-II), depending on the energy offsets between the valence and conduction band levels of the materials that are combined at the heterointerface. In the type-I case both carriers are primarily confined in the same part of the heterostructure, while in the type-II case electrons and holes are spatially separated on different sides of the heterojunction, leading to the formation of an indirect exciton. The relative energy offsets in heteronanocrystals can be tuned by controlling the composition, size and shape of each component, since the energy levels of semiconductor nanocrystals (NCs) are strongly size- and shape-dependent, and may also be affected by electronic coupling between the components. This offers the possibility of directly controlling the electron-hole wavefunction overlap, and consequently the material optoelectronic properties (e.g. emission wavelength, exciton radiative lifetimes, etc.), with important consequences for a number of potential applications. Solar cells in particular may benefit in several ways: more efficient charge carrier separation/extraction, enhanced conversion efficiency (up- and down-conversion, biexciton generation) and improved harvesting efficiency (larger absorption cross-sections in the UV-Visible range associated with reduced emission reabsorption). In this work we investigate the optical properties of CdTe/CdSe and ZnTe/CdSe colloidal heteroNCs by a number of spectroscopic techniques (viz. absorption, emission and excitation spectra, PL quantum yields, exciton lifetimes). The preparation methodology developed yields highly efficient (PL QY 40-80% at 300 K) anisotropic heteroNCs, with shapes tunable from prolate to rod or branched (bipods and multipods) heteroNCs. Depending on the growth conditions quasi-spherical core/shell QDs can also be obtained. The exciton PL gradually shifts to lower energies as the dimension of the CdSe part increases. The emission red-shift is accompanied by a large increase in the exciton radiative lifetime and a decrease in the absorption oscillator strengths at the emission energies, clearly indicating the progressive reduction of the electron-hole wavefunction overlap, ultimately leading to the formation of an indirect exciton. High PL QYs associated with long (indirect) exciton lifetimes imply low defect concentrations, therefore attesting the high-quality of the heteroNCs prepared in this work and their potential as efficient energy harvesting materials.
9:15 AM - M9.2
The Nucleation and Growth of CdSe Quantum Dots - Theory & Experiment.
Paul Mulvaney 1
1 School of Chemistry & Bio21 Institute, University of Melbourne, Parkville, Victoria, Australia
Show AbstractThe preparation of monodisperse quantum dots is essential for many important energy harvesting applications including next generation solar cells, as well as fluorescent tags in biolabelling and as active media in tunable LEDs. However to date, improvements in size control and in narrowing the size distributions, which determine the emission properties, have been largely empirical. The reason for this is that classical nucleation and growth kinetics do not apply to nanocrystals. The growth of quantum dots requires very high supersaturation, rapid mixing, the presence of high concentrations of ligands and stabilizers, rapid changes in temperature and rapid consumption of monomer. These extreme conditions are not treated in conventional colloid theories.In this talk, we compare systematic experimental data for CdSe quantum dots and CdSe/CdS core-shell nanocrystal formation with predictions from numerical simulations. We employ a standard Population Balance Model and various corrector algorithms to numerically integrate the rate equations for nucleation and growth of an ensemble of nuclei. Different regimes are identified and clear strategies for size control and narrowing the size distribution are shown. We show that the classical LSW limiting size distribution does not apply to nanocrystal systems and that the initial distribution of nuclei sizes does not play a critical role. "Early Time Ostwald ripening" is a serious problem in these systems, while nucleation agents are found to be essential for stabilization of small radius populations.The results will enable systematic synthesis of magnetic, semiconductor and metal nanocrystals.References:1. Bullen, C.R.; Mulvaney, P. Nanoletters, 4, 2303-7 (2004).2. Jasieniak, J,.; et al., J.Am. Chem. Soc., 129, 2841 (2007).3. J. van Embden et al., Aust J. Chem., 60, 457-71 (2007).
9:30 AM - M9.3
Carrier Multiplication in PbSe Nanocrystals Probed by Femtosecond Photoluminescence.
John McGuire 1 , Jeff Pietryga 1 , Jin Joo 1 , Victor Klimov 1
1 Physical Chemistry and Applied Spectroscopy, Los Alamos National Laboratory, Los Alamos, New Mexico, United States
Show AbstractCarrier multiplication (CM) is a process in which absorption of a single photon produces not just one but multiple electron-hole pairs (multiexcitons). As indicated by a number of recent spectroscopic studies, this process occurs in quantum-confined nanocrystals with efficiencies that greatly exceed those in respective bulk materials. In most of the published reports, CM was probed with femtosecond transient absorption, which allowed detection of multiexcitons based on their fast Auger recombination dynamics. Here, we use femtosecond transient photoluminescence (PL) to study CM in PbSe nanocrystals. On the basis of the measured PL time transients, we derive CM yields that are comparable to those measured previously via transient absorption. We analyze the possibility that, in addition to optically active (“bright”) biexctions, CM can also produce optically passive (“dark”) biexcitons, which may lead to apparent differences in CM efficiencies measured via PL and transient absorption. Further, we show that the properties of nanocrystal interfaces greatly affect the CM process and can lead to significant sample-to-sample variations in multiexciton yields depending on nanocrystal fabrication route, identity of the solvent, and sample conditions (e.g., stirred/flowing vs. static solution). One likely effect of the NC interface species is that they provide an additional energy relaxation channel, which competes with CM together with phonon emission.
9:45 AM - M9.4
III-V Quantum Dot Sensitized Nanoporous TiO2 for Quantum Dot-Sensitized Solar Cells.
Ravishankar Narayanan 1 , Bratindranath Mukherjee 1
1 Materials Research Centre, Indian Institute of Science, Bangalore India
Show AbstractDespite the fact that III–V semiconductor quantum dots have better electronic and optoelectronic property compared to other semiconductors due to their larger exciton radii, lesser ionic character, reduced toxicity and higher chemical stability, they have not received enough attention until recently as candidates for quantum dot sensitized solar cell. This is because of synthetic difficulty of group III–V semiconductor QDs compared to II–VI semiconductors. In the present work, we have attached III–V semiconductor QDs to TiO2 by controlled heterogeneous nucleation without a linker molecule. Electron transfer process has been investigated using femtosecond time-resolved photoluminescence spectroscopy.
10:00 AM - M9.5
Light-harvesting and Charge Carrier Relaxation in Tetrapod-Shaped Nanocrystals.
Christian Mauser 1 , Julia Baldauf 1 , Enrico Da Como 1 , Andrey Rogach 1 , Jochen Feldmann 1 , Dmitri Talapin 2
1 Ludwig-Maximilians Universitaet, Photonics and Optoelectronics Group, Muenchen Germany, 2 Department of Chemistry, The University of Chicago, Chicago, Illinois, United States
Show Abstract Semiconductor nanocrystals with branched three-dimensional morphologies are ideal nanostructures for omni-directional light harvesting. Tetrapods, which belong to this class, are nanocrystals with a tetrahedral shape. This geometry is realized by taking advantage of the polytipism of II-VI semiconductors. More interesting phenomena are expected in heterostructures, where the energy levels of different materials can be designed to funnel energy along preferential directions. In particular, novel tetrapod heterostructures with a CdSe spherical core and CdS rod-shaped arms have been successfully prepared [1]. The four CdS arms, branching out from the core in a tetrahedral symmetry, provide a huge single-particle absorption cross section. We have recently demonstrated light harvesting in all directions of space at the single-particle level [2], electing this system as an ideal absorber which mimics photosynthetic light-harvesting complexes. In this communication, our work has been primarily focused on the directional energy funneling from the peripheral arms to the central core. By femtosecond transient absorption spectroscopy, we resolve in time the charge carrier relaxation and exciton formation from the absorbing arms to the emitting core. In our model system the energetically well separated CdS arms and CdSe core give the unique opportunity to disentangle the role of geometric shape on the relaxation of the charge carriers. We monitored the population of the high energy states where the electron and hole wavefunctions are delocalized in the four CdS arms and the filling of the CdSe core states. By comparing tetrapods with arms of different length we observe large differences in the carrier relaxation time to the core. For long tetrapods (arms ~60 nm) the relaxation time increases at low temperatures suggesting a trapping mechanism during the relaxation to the core. An opposite behaviour is observed for short tetrapods (~16 nm), where a faster charge relaxation takes place. The results show the impact of dimensionality on the energy funneling in a model nanostructure with a three-dimensional architecture and are of fundamental importance for the design of artificial light-harvesting systems based on II-VI semiconductor nanocrystals.[1] D. V. Talapin, J. H. Nelson, E. V. Shevchenko, et al., Nano Lett. 7, 2951 (2007).[2] C. Mauser, T. Limmer, E. Da Como, et al., Physical Review B 77, 153303 (2008).
10:15 AM - M9.6
Utilizing Quantum Dots to Enhance Solar Spectrum Conversion Efficiencies for Photovoltaics.
Richard Savage 1 , Hans Mayer 2 , Mathew Lewis 1 , Daniel Marrujo 1
1 Materials Engineering, Cal Poly State University, San Luis Obsipo, California, United States, 2 Mechanical Engineering, Cal Poly State University, San Luis Obispo, California, United States
Show AbstractSilicon-based photovoltaics typically convert less than 30% of the solar spectrum into usable electric power. This study explores the utilization of CdSe based quantum dots as spectral converters that absorb the underutilized UV portion of the solar spectrum and fluoresce at wavelengths near the band-gap of silicon based solar cells. A flexible 1 mm thick thin-film structure that contains an array of microfluidic channels is designed and fabricated in polydimethylsiloxane (PDMS), using soft-lithographic techniques. The channels are approximately 500 microns wide by 50 microns tall and are filled with a solution containing the quantum dots. The thin-film structure can easily be attached to the surface of a single-junction solar cell. As a result, solar energy striking the coated solar cell with wavelengths less than 450 nm which would normally experience low conversion efficiency are absorbed by the quantum dots which fluoresce at 620nm. The high energy photons are converted to photons near the band-gap which increase the overall conversion efficiency of the solar cell. The quantum dots employed in this study are fabricated with a CdSe core (5.2 nm) and a ZnS outer shell and they exhibit a 25 nm hydrodynamic diameter. The UV-VIS spectral transmission properties of PDMS along with its refractive index are determined in order to characterize the spectral conversion efficiency of the thin-film structure. A model is developed to predict the optimum path length and concentration of quantum dots required to improve the power output of an amorphous silicon solar cell by 10 percent.
10:30 AM - M9.7
Modified Blinking Statistics and Non-Classical Photon Emission from Oligo-Phenylene Vinylene Functionalized CdSe Quantum Dots.
Michael Odoi 1 , Kevin Early 1 , Kevin McCarthy 1 , Ravisubhash Tangirala 2 , Todd Emrick 2 , Michael Barnes 1
1 Chemistry, University of Massachusetts, Amherst, Massachusetts, United States, 2 Department of Polymer Science and Engineering, University of Massachusetts, Amherst, Massachusetts, United States
Show AbstractThe photophysical properties of CdSe quantum dots functionalized with oligo-phenylene vinylene (OPV) have been studied with time-resolved spectroscopy and scanning probe microscopy at the single molecule level. We report significant improvement in the blinking statistics of this modified quantum dots. Specifically we have been able to observe a high degree of blinking suppression, with dark state dwell time on the order of milliseconds compared to conventional ZnS – capped CdSe quantum dots. Atomic force microscopy shows that suppression of blinking is strongly linked to the number of OPV ligands covering the surface of the CdSe dot. Time-tagged time-resolved single photon counting measurements show fluorescence lifetimes an order of magnitude lower than observed in ZnS – capped CdSe dots. We suggest, based on a modified diffusive reaction coordinate model that this is as a result of fluctuations in electron energies leading to changes in radiative and non-radiative decay rates. Single CdSe-OPV also demonstrates a clear photon antibunching signature with an average of 1.2 emissive sites. Radiative rates extracted from the antibunching result show an apparent dependence on excitation wavelength.
10:45 AM - M9.8
Time-Dependent Density Matrix Model for Carrier Multiplication in Semiconductor Nanocrystals.
Andrei Piryatinski 1 , Kirill Velizhanin 1 , Victor Klimov 2
1 Theoretical division, Los Alamos National Lab, Los Alamos, New Mexico, United States, 2 Chemistry Division, Los Alamos National Lab, Los Alamos, New Mexico, United States
Show AbstractThe effect of carrier multiplication (CM) in semiconductor nanocrystals is theoretically analyzed using the non-equilibrium time-dependent density matrix approach. This approach allows us to consider single-exciton and biexciton populations induced by the finite time-resolved pump pulse, and to interpolate between the impulsive and CW excitation regimes. The Coulomb interaction between the vacuum (two valence band electrons), exciton, and biexciton states giving rise to the CM is assumed to be weak and, therefore, is accounted for using the second order perturbation theory. Using this model, we analyze both analytically and numerically the photo-induced biexciton to single-exciton ratio (CM quantum yield), and demonstrate that the biexciton population can be excited through both single-exciton and biexciton resonances.We also show that high biexciton density of states not only results in the CM enhancement but also affects the single-exciton population and as a result the CM quantum yield through the higher order Coulomb coupling terms.
M10: Nanostructures
Session Chairs
Wednesday PM, December 03, 2008
Republic A (Sheraton)
11:30 AM - **M10.1
Nanostructured Molecular and Hybrid Materials for Energy Harvesting: Meeting the Energy and Healthcare Challenges of 21st Century.
Paras Prasad 1
1 Chemistry, SUNY/at Buffalo, Buffalo, New York, United States
Show Abstract12:00 PM - M10.2
Aligned Titania Nanotube Array Thin Films for Sunlight Harvesting: Electrochemical Kinetics and Photovoltaic Characteristics.
Gopal Ganesan Pethuraja 1 , Gorun Butail 1 2 , David Duquette 2 , G. Ramanath 1 2
1 Center for Future Energy Systems, Rensselaer Polytechnic Institute, Troy, New York, United States, 2 Department of Materials Science and Engineering, Rensselaer Polytechnic Institute, Troy, New York, United States
Show AbstractIncreasing demand for renewable green energy has stimulated the development of novel materials that can efficiently generate electricity from sunlight. Titania is a excellent photocatalyst by virtue of its ability to convert photons to excitons, and has been investigated for a variety of applications including solar cells, optoelectronics devices, and biofilters. There is a great deal of interest to synthesize titania nanostructures by inexpensive methods to reap the photocatalytic activity over large surface areas for low- cost power generation. Here we report a low-cost electrochemical synthesis technique to controllably obtain nanoporous titania films with vertically aligned nanotube arrays and elucidate, for the first time, the key kinetic processes and mechanisms of nanotube formation. We further measure the photovoltaic characteristics of the porous titania filled with p-type poly-3(hexylthiophene) polymer (P3HT). Our method could pave the way for a new scalable low cost solution to harvesting solar energy for power generation.Titania nanotubes were formed by anodizing titanium films in a mixture of ethylene glycol and ammonium fluoride at chosen temperatures and potentials between 20 and 80 °C and 20 and 80 V, respectively. Cross-sectional and plan-view electron microscopy and X-ray spectroscopy reveal the formation of ordered arrays of titania nanotubes. The nanotube length increases with anodization potential and processing temperature, suggesting an electrochemically driven thermally activated process. This is confirmed by our experiments showing anodization potential-dependent activation energies for nanotube growth. For instance, increasing the potential from 30 to 60 V decreases the activation energy for nanotube growth from 0.48 eV to 0.28 eV, indicating that electrochemical driving force limits the reaction rate at high voltages, while thermal activation predominates at the low voltages. This inference is consistent with activation energy values extracted from electrochemical current density measurement at various temperatures. The nanotube diameter also is thermally activated with an activation energy of 0.3 ± 0.1 eV, but is insensitive to voltage, suggesting a purely thermal process. Based upon these results, we propose a phenomenological model describing the atomistic mechanism of nanotube formation. Finally, we will present the photovoltaic response of solar-cell test structures comprised of the nanoporous titania films filled with P3HT polymers. The effect of nanotube size and alignment on device characteristics will be discussed.
12:15 PM - M10.3
Relation of the Reverse Saturation Current and Open-Circuit Voltage to Material Properties in Organic Solar Cells.
William Potscavage 1 , Seunghyup Yoo 1 , Bernard Kippelen 1
1 Electrical and Computer Engineering, Georgia Institute of Technology, Atlanta, Georgia, United States
Show AbstractOrganic photovoltaic devices have been the subject of intense research in recent years due to their potential to lead to a new generation of low-cost portable power sources with highly flexible form factors. Despite significant advances, the power conversion efficiency of organic solar cells remains rather small with maximum values in the range of 5-6%. To compete with other thin-film technologies such as CdTe and CIGS, it is critical that progress is made towards increasing the performance of organic photovoltaic cells. To this end, a better understanding of the underlying physical processes that govern the operation of organic solar cells is required. Specifically, understanding of the processes that determine the open-circuit voltage in organic solar cells remains limited and is the subject of active research. Recent studies in small molecule multilayer heterojunction devices and in polymeric bulk heterojunctions have confirmed that the magnitude of the open-circuit voltage correlates with the energy difference between the ionization potential of the donor molecular film and the ionization potential of the acceptor molecular film that form the heterojunction. However, no analytic expression for the open-circuit voltage in organic solar cells has been provided that would yield quantitative predictions.
In this study, we present an analytical expression for the open-circuit voltage based on the equivalent circuit model where reverse saturation current is determined by the gap between the highest occupied molecular orbital of the donor and lowest unoccupied molecular orbital of the acceptor. First, we report on temperature dependent studies of the reverse saturation current in multilayer organic solar cells based on the well-known pentacene/C60 heterojunction. We find that its magnitude is thermally activated and modeled by a Schottky injection type expression with an energy barrier height equal to the energy difference between the ionization potential of the donor and the electron affinity of the acceptor. By combining this result with the expression of the open-circuit voltage derived from the Shockley equivalent circuit of solar cells, an analytical expression of the open-circuit voltage is proposed. To test the proposed model, values of the open-circuit voltages are calculated for different multilayer heterojunction devices in which the energy barrier height is varied by using donor molecules with different ionization potentials (pentacene, copper phthalocyanine, titanyl phthalocyanine) combined with the acceptor molecule C60. Good agreement is found between the measured and calculated values of the open-circuit voltage and reverse saturation current densities.
12:30 PM - M10.4
Theoretical Comparison of the Electronic Properties of Hybrids Fullerene-4-oligo-phenylenevinylene (C60-4PV) and CNT-4-oligo-phenylenevinylene (CNT-4PV) Employing the Density Functional Theory (DFT).
Alfredo Marquez 1 , Erika Lopez 1 , Luz-Maria Rodriguez 2 1 , Norma Flores 1 , Daniel Glossman 1
1 Chemistry, CIMAV, Chihuahua, Chihuahua, Mexico, 2 Facultad de Ciencias Químicas, UACH, Chihuahua, Chihuahua, Mexico
Show Abstract12:45 PM - M10.5
Controlling Fullerene/polymer Blend Morphology using Host-Guest Chemistry.
Wallace Wong 1 , Andrew Holmes 1 2 , David Jones 1 , Qinghui Mao 1 , Scott Watkins 2 , Chao Yan 1
1 , University of Melbourne, University of Melbourne, Victoria, Australia, 2 Ian Wark Laboratory, CSIRO Molecular and Health Technologies, Melbourne, Victoria, Australia
Show AbstractThe physical interaction between donor and acceptor materials in bulk heterojunction (BHJ) solar cells has a very large effect on the energy conversion efficiency of the devices. In the ideal case, the donor and acceptor materials are arranged in a bicontinuous network with phase domain size suitable for exciton diffusion, a maximum surface area for exciton dissociation and appropriate crystallinity for high charge mobility. The arrangement of components should be achieved by self-assembly and have high stability over time.Morphology of fullerene/polymer blends depend on intrinsic properties of the two components such as crystallinity and their miscibility as well as extrinsic factors like solvent choice, deposition technique, solvent evaporation rate, concentration of components and annealing conditions.1 Most research has focused on the extrinsic factors with few studies investigating non-covalent interactions between components. Since the discovery of fullerenes, their host-guest chemistry has been a topic of interest. Fullerenes interact with many electron rich molecules such as porphyrins2 and calixarenes.3 The strongest host-fullerene complex to date involves calix[5]arene derivatives.4 These calixarenes bind to fullerenes in a ball and socket manner and have been used to isolate C60 and C70 from soot.3 In this study, the influence of calix[5]arene on the fullerene/poly(3-hexylthiophene) blend morphology and BHJ solar cell performance is examined.1. B. C. Thompson, J. M. J. Fréchet, Angew. Chem. Int. Ed. 2007, 46, 2-22.2. D. M. Guldi, Chem. Soc. Rev. 2002, 2, 1425-1433.3. J. L. Atwood, G. A. Koutsantonis, C. L. Raston, Nature 1994, 368, 229-231.4. T. Haino, M. Yanase, Y. Fukazawa, Angew. Chem. Int. Ed. 1997, 36, 259-260.