Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session T16: Focus Session: Organic Electronics and Photonics: Transport |
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Sponsoring Units: DMP DPOLY Chair: Leeor Kronik, The Weizmann Institute of Science Room: B115 |
Wednesday, March 17, 2010 2:30PM - 3:06PM |
T16.00001: Multiscale modelling of charge transport in organic electronic materials Invited Speaker: Charge transport in disordered organic semiconductors is controlled by a complex combination of phenomena that span a range of length and time scales. As a result, it is difficult to rationalize charge transport properties in terms of material parameters. Until now, efforts to improve charge mobilities in molecular semiconductors have proceeded largely by trial and error rather than through systematic design. However, recent developments have enabled the first predictive simulation studies of charge transport in disordered organic semiconductors. In this presentation we will show how a set of computational methods, namely molecular modelling methods to simulate molecular packing, quantum chemical calculations of charge transfer rates, and Monte Carlo simulations of charge transport can be used to reproduce experimental charge mobilities with few or no fitting parameters. Using case studies, we will show how such simulations can explain the relative values of electron and hole mobility and the effects of grain size, side chains and polymer molecular weight on charge mobility. Although currently applied to material systems of relatively high symmetry or well defined structure, this approach can be developed to address more complex systems such as multicomponent solids and conjugated polymers. [Preview Abstract] |
Wednesday, March 17, 2010 3:06PM - 3:18PM |
T16.00002: Temperature dependent charge transport in regioregular poly (3-hexylthiophene) crystalline nanoribbon field effect transistor Muhammad Islam, M. Arif, Lei Zhai, Saiful I. Khondaker In order to fabricate high performance devices with enhanced morphology for improved charge carrier transport, one (1D) and two dimensional (2D) crystalline nanostructures based on self organized P3HT in the form of nanowires, and nanofibres are creating significant interest for next generation optoelectronic devices. In this work we report the fabrication of organic field effect transistors (FETs) using low molecular weight poly (3-hexylthiophene) (P3HT) crystalline nanoribbons and study charge transport at low temperature. At room temperature the devices show p type behavior with maximum saturation mobility as high as $1\times 10^{-2}cm^2/Vs$ and current on/off ratios of $3\times 10^4$. The temperature dependent measurements show that the mobility decreases with decreasing temperature, indicating thermally activated hopping type transport. The threshold voltage shows a significant shift with temperature indicates that a large fraction of the mobile charges are trapped in the defect states. In addition we have measured resistance as a function of temperature which also indicates the hopping type behavior. [Preview Abstract] |
Wednesday, March 17, 2010 3:18PM - 3:30PM |
T16.00003: Exponential temperature dependence of conductance in metal-organic sandwiches Corneliu Colesniuc, Alexander Balatsky, Ivan Schuller An exponential temperature dependence of the conductance was found in the linear regime of the J-V curves in palladium-copper phthalocyanine (CuPc)-palladium trilayers. The devices were prepared in-situ by UHV organic molecular beam epitaxy using a floating shadow mask. DC electrical measurements were performed as a function of film thickness and temperature. The low voltage conductance increases exponentially with temperature for devices with thicknesses between 20 nm and 600 nm. The temperature dependence can be explained by assuming that: a) the carriers tunnel through rectangular barriers formed by small groups of molecules along parallel molecular stacks, b) molecular disorder and phonons produce fluctuations that modulate the barrier potential and c) self averaging over different configurations occurs within each conduction channel. Work supported by AFOSR and DOE [Preview Abstract] |
Wednesday, March 17, 2010 3:30PM - 3:42PM |
T16.00004: Master Equation Approach to the Simulation of Transport Properties in Materials with Structural and Site-Energy Disorder Avadh Saxena, Ivo Batistic, Enrique Batista, Richard Martin, Darryl Smith The study of diorder in crystalline materials assumes an underlying lattice. However, many materials such as conducting polymers and amorphous semiconductors do not in general have an underlying lattice. Therefore, instead of a lattice a network with disorder must be considered. To this end, we apply the master equation approach to a network in order to study transport properties of materials that exhibit both structural and site-energy disorder. The level of disorder is characterized by several parameters such as energy distribution function, spatial correlation function, fluctuations in the number of nearest neighbors or distance between them. We calculate mobility as a function of temperature and applied electric field for different levels of the disorder. The theory is also applicable to other materials such as organic semiconductors. [Preview Abstract] |
Wednesday, March 17, 2010 3:42PM - 3:54PM |
T16.00005: Unifying high- and low-temperture transport in organic semiconductors in large electric fields Jeff Worne, Douglas Natelson, John Anthony As organic semiconductors become more prevalent in solar cells and consumer electronics, developing a clear understanding of carrier transport within organic semiconductors becomes important. Recent work has shown that analyzing transport data over broad temperature ranges can be challenging and one must be careful in selecting a model to explain the data. We have selected two chemically unique organic semiconductors, poly(3-hexylthiophene) (P3HT) and triisopropylsilylethnyl (TIPS) pentacene, in order to make a general statement about bulk transport under large source-drain (10MV/m) and gate (400MV/m) biases from 4K to 300K. With these large carrier concentrations, we observe that charge carrier behavior evolves from Poole-Frenkel activation hopping at high temperatures to temperature-independent field emission hopping at low temperatures in both P3HT and TIPS-pentacene systems, fitting in well with the current understanding of high-temperature transport. This result illustrates the common behavior between these different systems and helps develop a unified picture of charge carrier transport within disordered organic semiconductors. [Preview Abstract] |
Wednesday, March 17, 2010 3:54PM - 4:06PM |
T16.00006: Microscopic insight into the hopping transport in disordered semiconducting polymers Nenad Vukmirovic, Lin-Wang Wang Charge carrier transport in disordered conjugated polymers has been modeled for decades using simple phenomenological models that assume certain density of electronic states and certain analytical expression for the hopping rates between the states. The density of states is therefore believed to be the crucial property of the polymer material when electronic transport is concerned. We show here that transport properties are also strongly dependent on the proper description of transition rates. We use our recently developed multiscale method [1] for the calculation of electronic transport where transition rates are modeled taking into account the interaction with all phonon modes. Our results indicate that the widely used Miller-Abrahams model of the hopping rates predicts different behavior than our detailed model: a) It gives significantly different dependence of the mobility on temperature. b) It predicts that the concept of effective temperature could be used to describe carrier heating in electric field, in contrast to the results of the detailed simulations. [1] N. Vukmirovic and L.-W. Wang, Nano Lett., in press (2009). [Preview Abstract] |
Wednesday, March 17, 2010 4:06PM - 4:18PM |
T16.00007: Charge and energy transports via poly-phenylacetylene based dendrimers Yongwoo Shin, Minghai Li, Xi Lin Poly-Phenylacetylene (PPA) is widely used in photoconductivity, photoluminescence, and light harvesting applications. In this work, we investigate the charge and exciton transport energetics and mechanisms in the PPA-based dendrimers using our recently developed adapted Su-Schrieffer-Heeger (SSH) model Hamiltonians and \textit{ab initio} Hartree-Fock (HF) calculations. We found both doping and photo-excitation lead to the formation of optical phonon dressed pi electron states, namely the self-localized polarons, in the energy gap. Independent from their origins, these polarons can be self-trapped at multiple lattice locations along the PPA chain, and migrate from one to the next with an activation barrier of $\sim $0.006 eV, slightly higher than the corresponding barrier found in \textit{trans}-polyacetylene. The PPA-based dendrimers can be constructed via the meta-positions of phenyl rings. In this case, we found the dendrimer junctions form attractive potential wells for both polarons and excitons, and the width and height of these junction potential wells can be controlled by the geometry of the dendrimers. [Preview Abstract] |
Wednesday, March 17, 2010 4:18PM - 4:30PM |
T16.00008: The trap DOS in small molecule organic semiconductors: A quantitative comparison of thin-film transistors with single crystals Wolfgang Kalb, Simon Haas, Kurt Pernstich, Thomas Mathis, Bertram Batlogg Our study shows that it is possible to reach one of the ultimate goals of organic electronics: organic field-effect transistors can be produced with trap densities as low as in the bulk of single crystals. Several analytical methods to calculate the spectral density of localized states in the band gap (trap DOS) from measured data were used to clarify, if the different methods lead to similar results. We then compared quantitatively trap DOS information from the literature, correcting for differences due to different calculation methods. In the bulk of single crystals the trap DOS is lower by several orders of magnitude than in thin films. The compilation of all data strongly suggests that structural defects at grain boundaries are the main cause of ``fast'' traps in TFT's made with vacuum-evaporated pentacene. For high-performance transistors made with small molecule semiconductors such as rubrene it is essential to reduce the dipolar disorder caused by water adsorbed on the gate dielectric. We will discuss to what degree band broadening due to the thermal fluctuations of the intermolecular transfer integral is reflected in the trap DOS very close ($<$0.15 eV) to the mobility edge. [Preview Abstract] |
Wednesday, March 17, 2010 4:30PM - 4:42PM |
T16.00009: Polarization-resolved optical spectroscopy imaging of electronic states in crystalline organic semiconductors thin films Z. Pan, I. Cour, M. Sutton, R. L. Headrick, M. Furis We present linear dichroism imaging and polarization-resolved photoluminescence (PL) studies of small molecule organic semiconductor thin films with large crystalline grains. Solution processable, and metal-free phthalocyanine (Pc) films, deposited on different substrates using a pen-writing technique [1], exhibit large linear dichroism and optical activity at the HOMO-LUMO gap, that results from stacking of molecules in columns oriented in the plane of the substrate. The orientation of individual grains is unambiguously resolved through polarization spectroscopy and x-ray diffraction. The latter confirms the crystalline orthorhombic (Pc) phase of the deposited film. Photoluminescence from individual Pc grains is resonant to the lowest energy absorption feature in the Q-band and exhibits a large degree of linear polarization (50 $\%$), in contrast to the luminescence from Pc molecules dispersed in chloroform. This polarization is a direct consequence of the long range ordering of the electronic transition dipoles. [1] R. L. Headrick, S. Wo, F. Sansoz, and J. E. Anthony, Appl. Phys. Lett 92, 063302 (2008). [Preview Abstract] |
Wednesday, March 17, 2010 4:42PM - 4:54PM |
T16.00010: Percolation Processes in Scintillation of Organic Crystals Vincenzo Lordi, Sebastien Hamel, Leslie Carman, Stephen Payne, Natalia Zaitseva The use of organic crystals as scintillators that can discriminate between incident neutrons and gamma rays is gaining interest, particularly for applications in nonproliferation and diagnostics. Discrimination is achieved by measuring the delay of scintillation from neutrons, which arises from the predominant generation of triplet excited states that must diffuse and undergo a bimolecular Auger process to produce excited singlets before luminescence occurs. To understand this delayed scintillation process, we have studied triplet migration in two organic crystals: trans-stilbene (TS) and diphenylacetylene (DPAC). Both TS and DPAC show delayed neutron scintillation when pure, but the effect is quenched in DPAC if small amounts of TS impurity are present. We find that the first triplet excited state of TS is 0.3 eV lower than that of DPAC, creating a triplet trap in the impure DPAC crystals. Percolation theory is applied to predict a critical concentration of the mixed crystal to restore delayed scintillation, which compares favorably to experiments. We compute the triplet migration rate in different directions in the crystals and discuss the transport anisotropy in view of our percolation model. [Preview Abstract] |
Wednesday, March 17, 2010 4:54PM - 5:06PM |
T16.00011: Bottom-up modelling of charge transport in polymer semiconductors David Cheung, David McMahon, Alessandro Troisi By combining charge transport properties with ease of processing polymer semiconductors are attractive for large-area, low-cost applications, such as displays, or photovoltaics. Despite this interest, charge transport in polymers is poorly understood and, in particular, there is a gap between the phenomenological and microscopic descriptions of charge transport. In this presentation I will describe recent work that aims to bridge the divide between these levels of description. To explicitly consider both the nuclear and electronic degrees of freedom a combined molecular dynamics and quantum chemical study is performed on the archetypal polymer semiconductor, poly(3-hexylthiophene) (P3HT). MD simulations are used to generate configurations for quantum chemical calculations and to characterise the microstructure of P3HT$^1$. The charge carriers become localized at long-lived traps$^2$. The existence of activated transport is explained, and the trapped states are described with chemical detail. The charge transfer integrals, which are among the key parameters for charge transport models are calculated$^1$. \\ 1. D. L. Cheung, D. P. McMahon, and A. Troisi, J. Phys. Chem. B, 113, 9393 (2009) \\ 2. D. L. Cheung, D. P. McMahon, and A. Troisi, J. Amer. Chem. Soc., 131, 11179 (2009) [Preview Abstract] |
Wednesday, March 17, 2010 5:06PM - 5:18PM |
T16.00012: Vapor deposition polymerization of a conjugated n-type polyimide Alexander Papastrat, Shuai Zhu, Mitchell Anthamatten We are employing vapor deposition polymerization (VDP) to investigate the polymerization of two-component conjugated polyimides. A novel polyamic acid film containing 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTDA) and 4,4'-oxydianiline (ODA) subunits was prepared by VDP onto flat glass substrates. Understanding the relative rates of monomer adsorption, solid-state polymerization, and small molecule crystallization is important to establish control of film morphology and properties. Two true Knudsen sources were designed to co-deposit reactive monomer components. The sources emit according to a cosine flux distribution in vacuum, and are rate-controlled to vary monomer molar flux ratios. Polyimides deposited using reactive five-membered ring dianhydrides form homogeneous, crystalline-free films. On the other hand the six-membered ring NTDA monomer is much less reactive, for the NTDA-ODA pair and polymerization competes with monomer crystallization. The objective of this study is to examine the propensity of monomer components to either polymerize or crystallize. Formed crystals were studied using FT-IR, X-ray diffraction, and polarized optical microscopy. Identification of crystalline components, their nucleation rates, and their crystal habit will be discussed. [Preview Abstract] |
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