Ana Claudia Arias Palo Alto Research Center
J. Devin MacKenzie Add-Vision, Inc.
Alberto Salleo Stanford University
Nir Tessler Technion-Israel Institute of Technology
O1: Synthesis of Materials I
Tuesday PM, April 10, 2007
Room 2002 (Moscone West)
9:00 AM - O1.1
Small-Molecule Design for Organic Electronics.
John Anthony 1 Show Abstract
1 Chemistry, University of Kentucky, Lexington, Kentucky, United States
The ease with which small-molecule organic semiconductors can be functionalized allows a single chromophore framework to be tuned for use in a myriad of electronic applications. Further, substitution can dramatically increase the solubility of aromatic compounds, allowing device fabrication by solution processing. Such functionalization must take careful account of how substitution of the chromophore will change intermolecular interaction in thin films and crystals, as well as the desirability of such interactions in specific applications. Using 5-ringed acenes and heteroacenes as the chromophore, this report will describe the use of a straightforward functionalization approach to create organic materials tuned for use in organic transistors, organic solar cells and organic light emitting diodes. For transistors, extensive two-dimensional π-overlap was critical to obtaining high thin film mobility, yielding µ = 1.5 cm2 / Vs for solution-cast films of a simple substituted pentacene compound. Further tuning of the aromatic chromophore, coupled with the use of arenethiol treatments on gold electrodes to direct the crystallization of the organic semiconductor, have yielded solution-cast transistors with mobility greater than 4.0 cm2 / Vs, demonstrating the unique opportunities available to enhance the properties of organic semiconductor films grown from solution. In the case of bulk heterojunction organic solar cells, donors with strong π-stacking interactions crystallized too readily, causing complete segregation of donor and acceptor leading to poor photovoltaic performance. The addition of substituents that slightly disrupted π-stacking interactions yielded solar cells with power conversion efficiencies greater than 1%. Further disruption of π-stacking in these systems again led to poor photovoltaic performance. For light-emitting diodes, any close contacts between chromophore atoms in the solid state can lead to broadening of the emission spectrum and decreased device performance. In this case, functionalization that leads to complete isolation of the chromophore yielded the best results, with red-emission efficiency greater than 3%.
9:15 AM - O1.2
Solvent Vapor Annealing Improves Device Characteristics of Transistors with Solution-Processable Triethylsilylethynyl Antradithiophene.
Kimberly Dickey 1 , John Anthony 2 , Lynn Loo 1 Show Abstract
1 Chemical Engineering, University of Texas at Austin, Austin, Texas, United States, 2 Chemistry, University of Kentucky, Lexington, Kentucky, United States
The development of organic semiconductors is driven by the promise of low-cost device applications. To fully realize cost-effective organic electronics, solution-processable materials need to be developed. While several solution-processable materials have been demonstrated, these materials often suffer from significantly reduced carrier mobilities due to defects and grain boundaries introduced during the deposition process. We have been studying triethylsilylethynyl anthradithiophene (TES ADT), a solution-processable, p-type organic semiconductor. Transistors fabricated with spun-cast TES ADT exhibit low carrier mobilities (0.002 cm2/V-s), low on-off current ratios and significant current-voltage hysterisis. Subjecting the fabricated transistors to dichloroethane solvent vapor annealing, however, yields average carrier mobilities of 0.2 cm2/V-s, high on-off current ratios (104-5), and significantly reduces the current-voltage hysterisis. This dramatic improvement in transistor performance is solvent choice dependent, and can be directly correlated with morphological transformations in the thin films. Specifically, the solvent vapor is able to partition into the organic semiconductor thin film during the annealing process to induce structural rearrangment. TES ADT crystallizes as a consequence. The improvement in device characiteristics appear to be directly correlated with the grain size within the thin films. The polarity of the solvent, on the other hand, has a dramatic impact on the threshold voltage. In general, polar solvents can induce the presence of a dipole barrier at the organic semiconductor-dielectric interface, thereby increasing the threshold voltage. Annealing with non-polar solvents, like hexanes, results in a threshold voltage that is close to zero.
9:30 AM - O1.3
Theoretical studies of P3HT, PQT, and PBTTT
John Northrup 1 Show Abstract
1 , Palo Alto Research Center, Palo Alto, California, United States
There is considerable interest in soluble polymer semiconductors such as P3HT , PQT  and PBTTT . These materials have hole mobilities exceeding 0.1 cm2/Vs. As a consequence there is an impetus to determine the atomic and electronic structure of the crystalline lamellae exhibited by these materials. It remains a very challenging problem to determine the structure from experiment alone. Energy minimization calculations were therefore performed using the density functional theory to determine the atomic structure of these materials. A tilting of the plane containing the polymer backbone is found to be energetically favorable for all three materials. It is argued that this tilting is a general feature of this class of materials, and that this tilting has a significant affect on the electronic and optical properties. The impact of the deviation from ideal cofacial pi-stacking on the hole mobility will be discussed within a simple model of the scattering. Z. Bao, A. Dodabalopur, and A. J. Lovinger APL 69, 4108 (1996).B. S. Ong, Y. Wu, P. Liu, and S. Gardner, J. Am. Chem. Soc. 126, 3378 (2004).I. McCulloch et al. Nature Materials 5, 328 (2006).R. A. Street, J. E. Northrup and A. Salleo, Phys. Rev. B 71, 165202 (2005).
9:45 AM - O1.4
Comparative Study of Charge Transport in High-mobility Microcrystalline Conjugated Polymers
Ni Zhao 1 , Jui-Fen Chang 1 , Marta Tello 1 , Henning Sirringhaus 1 , Iain McCulloch 2 , Martin Heeney 2 Show Abstract
1 Cavendish Laboratory, University of Cambridge, Cambridge United Kingdom, 2 , Merck Chemicals, Chilworth Science Park, Southampton United Kingdom
The performance of polymer field-effect transistors (FETs) crucially depends on the intermolecular interaction in the semiconductor polymer, as it dominates the charge conduction in the active layer. Recently a new class of semiconducting liquid-crystalline polymers, poly(2,5-bis(3-alkylthiophen-2-yl) thieno[3,2-b]thiophene) (PBTTT) , was reported with a considerably increased mobility (up to 0.6 cm2/Vs) as compared with the prototype microcrystalline conjugated polymer, poly(3-hexylthiophene) (P3HT). It is believed that such an increase of the mobility is due to the enhanced intermolecular interaction in the PBTTT thin films. However, microscopic charge transport studies that support the correlation between the microstructure and the mobility are still lacking. Here we present a comparative study on the nature of the charge transport species in the P3HT and PBTTT systems with the aim of understanding the difference of the charge carrier mobility from a molecular scale. It has been discovered recently that upon crystallizing from the melting phase a highly ordered film structure consisting of aligned long nanoribbons can be achieved in PBTTT films . These nanoribbons are arrays of π-π stacked extended chain lamellae with a width that approximates to single molecular length. We focus our studies on this morphology because, unlike the chain folding arrangement in P3HT nanoribbons, the PBTTT molecules are able to form a system with fewer folding-induced intrachain defects and enhanced intermolecular interaction. By using charge modulation spectroscopy we observe that the charge transfer (CT) transition in PBTTT films with the nanoribbons exhibits the signature of strong electronic coupling as a result of interchain polaron delocalization and narrower band width as compared to P3HT films. We also perform Scanning Kelvin probe microscopy to directly measure the local field-effect mobility in the PBTTT FETs without being affected by the contact resistance. This allows us to obtain a clear correlation between the microstructure and the charge conduction in the semiconducting polymer films.  Iain McCulloch, et. al., Nature Materials 5, 328, (2006).  Dean M. DeLongchamp, et. al., 2006 MRS spring meeting, M4.5
10:00 AM - **O1.5
The Development of pBTTT.
Iain McCulloch 1 , Clare Bailey 1 , Martin Heeney 1 , Maxim Shkunov 1 , David Sparrowe 1 , Steve Tierney 1 , Michael Chabinye 2 , R. Joseph Kline 3 Show Abstract
1 , Merck Chemicals, Southampton United Kingdom, 2 , Palo Alto Research Center, Palo Alto, California, United States, 3 , National Institute of Standards and Technology, Gaithersburg, Maryland, United States
The further development of organic semiconductors to be utilised in field-effect transistors (OFETs) for incorporation into lightweight, flexible electronic devices, requires high performing, solution processable materials. Polymeric semiconductors offer an attractive combination in terms of appropriate solution rheology for printing processes, mechanical flexibility for rollable processing and applications, but their electrical performance has been significantly lower than amorphous silicon, limiting their application. In this work, we report the design, synthesis and characterisation of a range of liquid crystalline pi conjugated thieno[3,2-b]thiophene co-polymers (pBTTT) exhibiting high charge carrier mobilities. A detailed study of the polymer thin film morphology by both AFM and XRD reveals an extremely ordered and oriented microstructure. Field effect transistors were fabricated, and their performance and lifetimes will be discussed.
10:30 AM - O1.6
Reliable Suzuki Chemistry For Functionalised Polythiophene Synthesis.
Simon Higgins 1 , Iain Liversedge 1 , Iain McCulloch 2 , Mark Giles 2 , Martin Heeney 2 Show Abstract
1 Chemistry, University of Liverpool, Liverpool United Kingdom, 2 , Merck Chemicals Ltd., Southampton United Kingdom
Regioregular polyalkylthiophenes and their derivatives have been widely-studied as polymeric organic semiconductors. Synthetic chemists strive to improve the already impressive field effect mobilies of these materials, while increasing their stability to ambient conditions and their purity. We recently described the application of functionalised polyalkylthiophene derivatives in biosensing (see M. Fouzi et al., Chem. Comm. 2004, 2314, and S.J. Higgins et al., Mater. Res. Soc. Symp. Proc. 871E, Warrendale, PA, 2005, I1.3). Additionally, functionalised polyalkylthiophene derivatives designed for improved intermolecular interaction (e.g. via H-bonding) might show improved transistor performance (higher field effect mobility). Therefore, in spite of the success of the conventional Grignard and organozinc-based routes to the parent polyalkylthiophenes, it is important to devise new chemistry, applicable to the synthesis of functionalised derivatives.We have therefore investigated Pd complexes of bulky, electron-rich phosphines as catalysts for the polymerisation of halothiophene boronate ester derivatives via Suzuki coupling. With the best catalysts, we can obtain >98% regioregular polyalkylthiophenes with useful molecular weights (MW 17000). Importantly, we have also found that it is possible to synthesise the key AB-type monomers by Ir-catalysed boronation of 2-halo-3-alkylthiophenes. This is significant because it means that we can synthesise the monomer without recourse to strong organolithium bases, making the route truly functional group-tolerant.
10:45 AM - O1.7
Non-conjugated Polymer Hosts with High Triplet Energies
Lan Deng 1 , Biwu Ma 2 , Jean Frechet 1 2 Show Abstract
1 , Department of Chemsitry, University of California, Berkeley, California, United States, 2 , Material Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California, United States
Incorporating carbazole units into polymers to make high triplet energy host materials is essential for efficient solution-processible OLEDs. When carbazole units are used as building blocks in the main chain of the conjugated polymers, the resulting polymer hosts typically have triplet energies around 2.5-2.6 eV. Host materials with higher triplet energies are difficult to obtain using this approach due to the decrease of triplet energy with the increase of conjugation length. We hereby report a different way to incorporate carbazole units into polymers through side-chains. Using the new approach, the triplet energies of the polymers can remain the same as that of their small molecular building blocks. Polymers with triplet energies as high as 3.0 eV have been obtained. OLEDs have been fabricated, showing external quantum efficiencies of 4-5 %.
11:30 AM - **O1.8
Synthetic And Processing Strategies To High Performance Organic Thin-Film Transistors.
Antonio Facchetti 1 Show Abstract
1 , Northwestern University, Evanston, Illinois, United States
The general design and synthesis of new rylenes and oligo/thiophenes functionalized with a variety of phenacyl, alkylcarbonyl, and perfluoroalkylcarbonyl is presented. These organic semiconductors exhibit low-lying LUMOs allowing efficient electron injection/transport under ambient atmosphere. Organic thin-film transistors (OTFTs) fabricated via conventional methods exhibit electron mobilities as high as 2 cm^2 V^-1 s^-1 for vapor-deposited films and 0.3 cm^2 V^-1 s^-1 for solution-cast films, with current modulation as high as 10^8. Furthermore, unconventional processing approaches to addressing the realization of well-defined supramolecular architectures with precise, nanometer-level control of bulk electronic properties are described. The ultimate goal is the realization of inexpensive electronic circuits employing unconventional materials classes and simple fabrication techniques.
12:00 PM - O1.9
In-plane Molecular Alignment in Thin Films of Pentacene Grown by Solution Casting and Performance of Thin Film Transistors.
Takashi Minakata 1 , Yutaka Natsume 1 , Yuji Yoshida 2 , Yasukiyo Ueda 3 Show Abstract
1 Central R&D Laboratories, Asahi-KASEI Corporation, Fuji-shi, Shizuoka-ken, Japan, 2 , National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki, Japan, 3 , Kobe University, Kobe, Hyogo, Japan
12:15 PM - O1.10
High Field-Effect Mobilities for Diblock Copolymers of rr-P3HT
Genevieve Sauve 1 , Richard McCullough 1 Show Abstract
1 Chemistry, Carnegie Mellon University, Pittsburgh, Pennsylvania, United States
One important challenge in the field of organic field-effect transistors (OFET) is to better understand the relationship between structure and transport properties of conjugated polymers. Much work has focused on regioregular poly(3-hexylthiophene) (rr-P3HT), because of its excellent electrical properties and ease of synthesis. To further tune the structure and properties of rr-P3HT, we have synthesized several well-defined diblock copolymers of rr-P3HT, where the second block is a coil polymer. Here we present surprising results of high mobilities for our diblock copolymers of rr-P3HT and poly(methylacrylate) (PMA), despite the presence of various amounts of insulating PMA. This was observed when the silicon dioxide dielectric layer was treated with octyltrichlorosilane. In contrast, mobilities went down with PMA content when the silicon dioxide was not treated. It is therefore possible to control the self-assembly of these block copolymers at the dielectric layer-semiconductor interface by using different SiO2 surface treatments. These results also demonstrate the promise of using block copolymers as organic semiconductors in plastic electronics.
O2: Materials Physics and Characterization I
Tuesday PM, April 10, 2007
Room 2002 (Moscone West)
2:30 PM - **O2.1
Spectroelectrochemistry Studies of the Charging and Discharging of Single Conjugated-Polymer Nanoparticles.
Paul Barbara 1 , Rodrigo Palacios 1 , Allen Bard 1 , Fu-Ren Fan 1 Show Abstract
1 Department of Chemistry and Biochemistry, Center for Nano and Molecular Science and Technology, University of Texas, Austin, Texas, United States
The unique ability of single molecule spectroelectrochemisry, SMS-EC, to unravel complex electrochemical process in heterogeneous media is used to study the oxidation of nanoparticles of the practically important conjugated polymer poly (9,9-dioctylfluorene-co-benzothiadiazole) (F8BT). Two main processes have been observed, an irreversible chemical reaction on the surface of the oxidized F8BT nanoparticles, and a reversible hole-injection charging process. The latter occurs primarily by initial injection of shallow (untrapped) holes, but soon after the injection, the holes become deeply trapped. Good agreement between experimental data and simulations strongly supports the presence of deep traps in the studied nanoparticles and highlights the ability of SMS-EC to study deep traps in organic materials at the nanoscale.
3:00 PM - O2.2
On the Validity of Continuum Drift-Diffusion Device Models for Organic Light Emitting Diodes.
B. Ramachandhran 1 , M. Uijttewaal 2 , M. Bouhassoune 1 , P. Bobbert 1 , R. De Groot 2 , G. De Wijs 2 , Reinder Coehoorn 3 4 Show Abstract
1 Polymer Physics Group, Department of Applied Physics, Eindhoven University of Technology, Eindhoven Netherlands, 2 IMM, Radboud University, Nijmegen Netherlands, 3 , Philips Research Laboratories, Eindhoven Netherlands, 4 Molecular Materials and Nanosystems Group, Department of Applied Physics, Eindhoven University of Technology, Eindhoven Netherlands
It has recently been demonstrated by Pasveer et al. that the energetic disorder of amorphous polymer organic semiconductors used in organic light emitting devices (OLEDs) can give rise to a strong carrier concentration dependence of the mobility, and that this can have a significant effect on the current density (J) versus voltage (V) curves of polymer-based hole-only devices . In ref. 1, the mobility was obtained from a numerically exact Master-Equation (ME) approach, assuming a Gaussian density of states (DOS). For obtaining the J(V) curves, a continuum solution of the coupled transport and Poisson equations was used, assuming ohmic contacts and neglecting diffusion. However, one might question under which conditions such a continuum approach is valid. In actual OLEDs, large carrier concentration gradients can occur near external and internal interfaces, and the current density can have a strongly filamentary nature [2,3]. The conductivity is then a non-local property, so that the concepts of ‘mobility’ and ‘diffusion coefficient’ could no longer be useful. In this contribution, we present the results of a numerical investigation of the validity of the continuum approach, by making a comparison with the results from ME calculations for complete metal/organic/metal devices. We have varied the (uncorrelated) Gaussian disorder (width σ), the layer thicknesses (20 to 100 nm), and the interface conditions. For the continuum drift-diffusion calculations, the compact expressions for the mobility given in  were used, with the diffusion coefficients as obtained using the generalized Einstein equation. We confirm the highly filamentary nature of the current density and present a quantification of the current density distribution. We also present a 3D visualization of the non-uniform current density. Even for highly disordered systems (σ = 6kBT) and for layer thicknesses of only 20 nm, for which the current is shown to be strongly filamentary, the continuum model is found to be surprisingly well obeyed. We have investigated the effect of injection barriers (and image potentials), by making use of two types of models: (i) by assuming a fixed carrier concentration at the outermost planes of the organic layer (as in ), (ii) by self-consistently calculating all carrier concentrations in all layers (beyond ). A critical comparison is given