Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session F20: Focus Session: Organic Electronics and Photonics - Small Molecules |
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Sponsoring Units: DMP DPOLY Chair: Rodrigo Noriega, University of California, Berkeley Room: 405 |
Tuesday, March 4, 2014 8:00AM - 8:36AM |
F20.00001: POLYMER PHYSICS PRIZE BREAK |
Tuesday, March 4, 2014 8:36AM - 8:48AM |
F20.00002: Effect of mechanical deformation on the electrical properties of organic single crystals Marcos Reyes-Martinez, Alfred Crosby, Alejandro Briseno Despite efforts in the flexible electronics field, relatively little research quantified the effects of mechanical strain on the electrical properties of organic single crystals (OSCs) and their device performance in deformed geometries. Single crystals of organic semiconductors are ideal systems for the elucidation of these effects without having to account for imperfections, grain boundaries and other defects. The aim of this presentation is to bring new understanding of the effects of mechanical strain in charge transport phenomena on OSCs. First, the existence of a piezoresistive effect in rubrene crystals is demonstrated and experimentally quantified by the application of in-plane strain along its [010] axis. A piezoresistive coefficient approximately 50 is determined. Second, the effect of local mechanical deformation on the conductive channel is investigated in rubrene single-crystal field-effect transistors. A wrinkling instability is used as a technique to apply local strains of different magnitudes to the conducting channel of field-effect transistors. All devices maintain excellent transistor behavior, and small, reversible changes in performance are observed during wrinkling. This work provides useful knowledge for the effective application of organic semiconductors in strain intensive applications such as pressure sensors, electronic skins and strained-channel organic transistors. [Preview Abstract] |
Tuesday, March 4, 2014 8:48AM - 9:00AM |
F20.00003: Near Surface Structure of Organic Semiconductor Tetracene Single Crystal Yusuke Wakabayashi, Hazuki Morisaki, Tsuyoshi Kimura, Kazumoto Miwa, Takashi Koretsune, Jun Takeya Electric conduction in organic crystals is highly anisotropic because of the anisotropic molecular orbitals. Crystal structure governs the transfer through the overlap integral among the highest occupied (or lowest unoccupied) molecular orbitals. In case of organic devices, the place where electrons conduct is the interface. Therefore, the surface structure of organic single crystals is relevant. Surface relaxation of the structure of rubrene single crystal was firstly observed by means of surface x-ray diffraction a few years ago [1]. This time we performed similar measurement on tetracene single crystal, whose molecular shape has large similarity with rubrene while the crystal structure is very different. Tetracene single crystal was grown by the physical vapor transport method, and the surface x-ray diffraction experiments were performed at BL-3A and 4C of the Photon Factory, KEK, Japan. Obtained electron density profile shows a large structural deformation at the surface layer of tetracene. \\[4pt] [1] Y.Wakabayashi, J.Takeya and T.Kimura, Phys. Rev. Lett. 104, 066103 (2010). [Preview Abstract] |
Tuesday, March 4, 2014 9:00AM - 9:12AM |
F20.00004: Trap healing and ultra low-noise Hall effect at the surface of organic semiconductors Vitaly Podzorov Fundamental studies of intrinsic charge transport properties of organic semiconductors are often hindered by charge traps associated with static disorder present even in optimized single-crystal devices. Here, we report a novel method of surface functionalization using an inert non-conjugated polymer, perfluoropolyether (PFPE), deposited at the surface of organic molecular crystals, that results in accumulation of mobile holes and ``trap healing'' effect at the crystal/PFPE interface [1]. As a consequence, a remarkable ultra low-noise, trap-free conduction regime characterized by intrinsic mobility and transport anisotropy emerges in organic single crystals, and Hall effect measurements with unprecedented signal-to-noise ratio are demonstrated. This general method to convert trap-dominated organic semiconductors to intrinsic systems may enable the determination of intrinsic transport parameters with high accuracy and make Hall effect measurements in molecular crystals ubiquitous.\\[4pt] [1] B. Lee, Y. Chen, D. Fu, H. T. Yi, K. Czelen, H. Najafov, V. Podzorov, ``Trap healing and ultra low-noise Hall effect at the surface of organic semiconductors,'' Nature Mater. DOI 10.1038NMAT3781 (2013). [Preview Abstract] |
Tuesday, March 4, 2014 9:12AM - 9:48AM |
F20.00005: Effect of pressure on electronic charge coherence in organic semiconductor single crystals Invited Speaker: Jun Takeya Small molecular organic semiconductor crystals form interesting electronic systems of periodically arranged ``charge reservoirs'' whose mutual electronic coupling determines whether or not electronic states can be coherent over molecular distances. Recently, it turned out that band transport is realized in high-mobility organic semiconductor crystals though this situation is not common to all organic semiconductors. Series of Hall-effect measurements on different molecular crystal systems indicated that the extent of the charge coherence depends on molecular species at room temperature. In this presentation, we focus on the single-crystal molecular assembly of pentacene which does not exhibit full charge coherence at room temperature under atmospheric pressure. Hall coefficient, telling us the extent of the electronic coherence, is precisely measured for accumulated charge in pentacene single-crystal field-effect transistors at various temperatures with varied pressure. With the application of external pressure, the electronic coupling between pentacene molecules is continuously modified so that the extent of the intermolecular coherence grows with increasing pressure. [Preview Abstract] |
Tuesday, March 4, 2014 9:48AM - 10:00AM |
F20.00006: Controlling Conformations of Conjugated Polymers and Small Molecules: The Role of Nonbonded Interactions Kevin Kohlstedt, Nicholas Jackson, Brett Savoie, Lin Chen, Monica Olvera de la Cruz, George Schatz, Mark Ratner The chemical variety present in the organic electronics literature has motivated us to investigate potential nonbonding interactions often incorporated into conformational ``locking'' schemes. We have examined a variety of potential interactions, including oxygen-sulfur and nitrogen-sulfur, using accurate quantum-chemical wave function methods on a selection of high-performing conjugated polymers and small molecules. In addition, we evaluate a set of nonbonding interactions occurring between various heterocyclic and pendant atoms taken from a group of representative pi-conjugated molecules. From our survey, it is determined that while many nonbonding interactions possess weak binding capabilities, hydrogen bonding interactions, namely oxygen-hydrogen and nitrogen-hydrogen, are alone in inducing conformational control and enhanced planarity along a polymer or small molecule backbone at room temperature. [Preview Abstract] |
Tuesday, March 4, 2014 10:00AM - 10:12AM |
F20.00007: \textit{Ab Initio} Investigation of Conformal and Dipolar Effects on Subphthalocyanine Photovoltaic Properties Michael Waters, Guangsha Shi, Hossein Hashemi, Emmanouil Kioupakis, John Kieffer Boron subphthalocyanine chloride (B-SubPc-Cl) currently has the highest reported open circuit voltages of any organic photovoltaic donor coupled with C$_{60}$. In our density functional theory (DFT) investigations, we sought to understand the origins of this performance by substituting boron and chlorine with other trivalent and halogen elements, respectively. Substitution of the trivalent and halogen elements distorts the porphyrin ring and changes the molecular dipole moment. For the equilibrium conformation of each isolated molecule, time-dependent DFT was used to compute the optical absorption. Using DFT with added Van der Waals interactions, experimentally unknown crystal structures were predicted. The electronic and optical excitation energies of these crystal structures were calculated using the GW/Bethe-Salpeter equation method. We found that the optical absorption spectra are significantly affected by the strong exciton binding energies in these materials ($\sim$0.5 eV for B-SubPc-Cl). [Preview Abstract] |
Tuesday, March 4, 2014 10:12AM - 10:24AM |
F20.00008: Tunable Molecular Orientation of Organic Semiconductors in Vapor-Deposited Amorphous Solids Diane Walters, Shakeel Dalal, Mark Ediger Amorphous solids made by physical vapor deposition (PVD) of organic molecules have found increasing use in organic LEDs and photovoltaics. PVD is favored because it allows precise control of layer thickness and high material purity, however the impact of deposition conditions on the structure of amorphous solids has been largely uninvestigated. We have previously shown that solid films prepared by PVD can have drastically higher densities, moduli and thermal stability than are obtainable by cooling the liquid. Using a high-throughput characterization technique, we show that PVD is also able to impart significant molecular orientation into amorphous solids. We present work on several common molecules used in organic semiconducting devices including AlQ$_{3}$, NPB, TPD, CBP, DSA-Ph, and BSB-Cz. The molecular orientation depends systematically on the substrate temperature during deposition. At low temperatures there is a strong tendency to lie parallel to the substrate, while at higher temperatures there is a tendency to stand vertically on end. It is anticipated, and in some limited cases has been previously shown, that this orientation can significantly affect charge mobility and light out-coupling efficiency in devices. [Preview Abstract] |
Tuesday, March 4, 2014 10:24AM - 10:36AM |
F20.00009: The nanoscale morphology of new types of solar cells based on solution-processed small-molecules Nuradhika Herath, Valeria Lauter, Jim Browning Organic electronics have become promising alternatives for the today's energy demand, owing to their low cost fabrication processes, ability to performance under low light, and flexibility. Solution processed small molecule (SM)- fullerene based solar cell devices have been subjected to number of studies recently with significant progress of power conversion efficiency (PCE). The bulk hetero junction (BHJ) consisting SM-fullerene blend is the most critical part of the solar cell device as nano-to-meso-scale morphology of BHJ plays a significant role in the device performances and properties. In this study we investigate the morphological structure of a device constructed from solution processed SM-molecule $p-$DTS(FBTTh$_{2}$)$_{2}$ with fullerene PC$_{70}$BM BHJ blend using neutron reflectometry (NR). Here we present the scattering length density changes of PC$_{70}$BM concentration along the film depth and the history dependence of the BHJ device by taking the measurements as-cast as thermally annealed (150 $^{\circ}$C). [Preview Abstract] |
Tuesday, March 4, 2014 10:36AM - 10:48AM |
F20.00010: Photo-oxidation degradation mechanisms in P3HT for organic solar cells: Insights from first-principles simulations Kevin Leung, Na Sai, Judit Zador, Graeme Henkelman Photo-oxidation is one of the leading chemical degradation mechanisms in polymer solar cells. In this work, using hybrid density functional theory and periodic boundary condition, we investigate reaction pathways that may lead to the sulfur oxidation in poly(3-hexylthiophene)(P3HT) as a step toward breaking the macromolecule backbone. We calculate energy barriers for reactions of P3HT backbone with oxidizing radicals suggested by infrared spectroscopy (IR) and XPS studies. Our results strongly suggest that an attack of hydroxyl radical on sulfur as proposed in the literature is unlikely to be thermodynamically favored. On the other hand, a reaction between the alkylperoxyl radical and the polymer backbone may provide low barrier reaction pathways to photo-oxidation of conjugated polymers with side chains. Our work paves way for future studies using ab-initio calculations in a condensed phase setting to model complex chemical reactions relevant to photochemical stability of novel polymers.\\[4pt] Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL850. [Preview Abstract] |
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