Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session J22: Organic Electronics, Photonics and Magnetics: Theory |
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Sponsoring Units: DMP DPOLY Chair: Zhiqiang Li, University of California, San Diego Room: Morial Convention Center 214 |
Tuesday, March 11, 2008 11:15AM - 11:27AM |
J22.00001: Dependence of Mobility on Density of Gap States in Organics by GAMEaS - Gate Modulated Activation Energy Spectroscopy Woo-young So, David Lang, Arthur Ramirez We develop a spectroscopic method for determining the density of states (DOS) in the energy gap - GAte Modulated activation Energy Spectroscopy (GAMEaS), We also report the relationship of these gap states to the mobility of organic field-effect-transistors (FETs). We find that the field-effect mobility is parameterized by two factors: (1) the free-carrier mobility and (2) the ratio of the free carrier density to the total carrier density induced by the gate bias. We show that the highest mobility FETs have shallow exponential band tails of localized states with characteristic slope of 1/kT at 300K. Most remarkably, state-of-the-art crystalline FETs fabricated from rubrene, pentacene, and tetracene all have a very high free-carrier mobility, up to 200cm2/Vsec at 300K, with the somewhat lower effective mobilities dominated by localized gap states. This strongly suggests that further improvements in device performance could be possible with enhanced material quality. [Preview Abstract] |
Tuesday, March 11, 2008 11:27AM - 11:39AM |
J22.00002: Charge mobility of discotic mesophases of polyaromatic hydrocarbons: a multiscale quantum/classical study Denis Andrienko, Valentina Marcon, Kurt Kremer, James Kirkpatrick, Jenny Nelson Discotic liquid crystals form columnar phases, where the molecules stack on top of each other and the columns arrange in a regular lattice. The self-organization into stacks results in the one-dimensional charge transport along the columns. Using atomistic molecular dynamics (MD) simulations we study columnar discotic phases formed by various polyaromatic hydrocarbons. Combining Kinetic Monte Carlo and MD trajectories a correlation between the material morphology and charge mobility is then established. We are able to reproduce the trends and magnitudes of mobilities as measured by pulse-radiolysis time-resolved microwave conductivity technique. [Preview Abstract] |
Tuesday, March 11, 2008 11:39AM - 11:51AM |
J22.00003: Optical spectra and exchange-correlation effects in molecular crystals Na Sai, Murilo L. Tiago, James R. Chelikowsky, Fernando A. Reboredo We report first-principles GW-Bethe Salpeter Equation and Quantum Monte Carlo calculations of the optical and electronic properties of molecular and crystalline rubrene (C$_{42}$H$_{28}$). We predict the formation of intermolecular, charge-transfer spin-singlet excitons with energies in close agreement with the observed yellow-green photoluminescence in rubrene microcrystals. In contrast, spin-triplet excitons are localized and intramolecular with a predicted phosphorescence at the red end of the optical spectrum. We find that the exchange energy plays a fundamental role in raising the energy of intramolecular spin-singlet excitons above the intermolecular ones. Exciton binding energies are predicted to bearound 0.5~eV (spin singlet) to 1~eV (spin triplet). The calculated electronic gap is 2.8~eV. The theoretical absorption spectrum agrees very well with recent ellipsometry data. [Preview Abstract] |
Tuesday, March 11, 2008 11:51AM - 12:03PM |
J22.00004: Monte Carlo Simulation of Carrier Dynamics in an Organic Field Effect Transistor Dharmendar Reddy Palle, Leonard Register, Ananth Dodabalapur We demonstrate here for the first time a self consistent Monte Carlo Simulation [MCS] of carrier dynamics under high charge densities in an Organic Field Effect Transistor (OFET). Given the stochastic nature of carrier transport in organic semiconductors, MCS is ideally suited for simulating the carrier dynamics in these systems. Previous work on MCS of carrier transport in organic devices has been limited to two terminal device configurations with low carrier densities. Also, a modified Miller Abrahams hopping rate is introduced to account for the anisotropy of hopping events between molecules with different spatial orientation. The semiconductor is modeled as a polycrystalline region with a Gaussian density of states. Injection and transport have been modeled as hopping events with different hopping rates. Comparison of measured and simulated transient and steady state current voltage [I-V] characteristics of Pentacene FETs will be presented. The effect of various parameters such as interface barriers, grain boundaries, and temperature on simulated transient and steady state I-V characteristics of Pentacene FETs with channel lengths varying from a few hundred nano-meters to a micron based on Kinetic MCS will be discussed. [Preview Abstract] |
Tuesday, March 11, 2008 12:03PM - 12:15PM |
J22.00005: Dynamical Barrier to Impurity Trapping in Organic Semiconductors David H. Dunlap, Paul E. Parris, Stephan De Bievre We consider the trapping of a moving electron by a polar impurity in an uncompensated organic semiconductor when the multipole moment of the impurity is coupled to an intramolecular vibration, a dynamical generalization of the Vannikov-Novikov dipole trap model.[1] Due to the slow power-law dependence of the multipolar interaction, the vibrational coordinate shifts adiabatically with the approach of the charge carrier. The fast molecular motion can be decoupled from the electron's translational motion to first approximation by transforming to the polaron basis, leading to a polaron binding energy which serves to enhance the propensity for the moving charge to be captured. For an isolated impurity, however, the transformed Hamiltonian contains a repulsive ponderomotive term not described in conventional polaron theory. The repulsion can outweigh the attractive force at long range, presenting a barrier to trap formation. [1] S. V. Novikov and A. V. Vannikov, Chem. Phys. 169 (1993) 21-33. [Preview Abstract] |
Tuesday, March 11, 2008 12:15PM - 12:27PM |
J22.00006: Inverse Molecular Design in a Tight-Binding Framework Dequan Xiao, Weitao Yang, David Beratan The number of chemical species of modest molecular weight that can be accessed with known synthetic methods is astronomical. An open challenge is to explore this space in a manner that will enable the discovery of molecular species and materials that exhibit optimized properties. Recently, a strategy was developed to perform continuous optimization of molecular properties, the linear combination of atomic potentials (LCAP) approach.$^{1}$ Here, using a simple tight-binding (TB) implementation, we show that the LCAP strategy can successfully explore vast chemical libraries that are based on planar $\pi $-electron motifs. We show that LCAP property optimization of $\pi $-electron polarizabilities and hyperpolarizabilities is effective for libraries with 10$^{4}$ to 10$^{16}$ members. This approach finds optimal structures among 10$^{4}$ candidates with about 40 individual molecular property calculations. As such, for molecular candidates with strong structural similarity, the TB-LCAP approach may provide an effective means of identifying structures with optimal properties. \newline $^{1}$M. Wang, X. Hu, D. N. Beratan, and W. Yang, J. Am. Chem. Soc. \textbf{128}, 3228 (2006); S. Keinan, X. Hu, D. N. Beratan, and W. Yang, J. Phys. Chem. A \textbf{111}, 176 (2007). [Preview Abstract] |
Tuesday, March 11, 2008 12:27PM - 12:39PM |
J22.00007: Ab initio Evaluation of the Charge-Transfer Integrals and Band Structures of Phenanthroline-based Molecular Crystals H. Li, J.-L. Bredas, C. Lennartz Ab initio calculations are carried out to study the charge-transport properties of phenanthroline-based molecular crystals, BCP and Bphen. The charge-transfer integrals in the two crystalline structures and in a quasi-1D chain model for BCP are evaluated based on: (i) a tight-binding approximation for dimers; (ii) the band structures of the periodic systems. In these compounds, the LUMO/LUMO+1energies of the isolated molecules are very close, which results in the LUMO and LUMO+1orbitals both having significant contributions to the LUMO level in the dimer. In this case, the usual definition based on the electronic coupling between the two LUMO orbitals from each molecule cannot be applied to describe the charge-transfer characteristics in the dimer. A new definition of ``effective transfer integrals'' based on ``mixed states'' was proposed (H. Li, J.L. Bredas, and C. Lennartz, J. Chem. Phys. 126 (2007) 164704) to address this problem. Within the tight-binding approximation, the ``effective transfer integrals'' for both hole and electron transfers are found to be in very good agreement with the valence and conduction band dispersions obtained from plane-wave DFT calculations. We acknowledge many stimulating discussions with Veaceslav Coropceanu and Demetrio da Silva Filho. [Preview Abstract] |
Tuesday, March 11, 2008 12:39PM - 12:51PM |
J22.00008: ABSTRACT WITHDRAWN |
Tuesday, March 11, 2008 12:51PM - 1:03PM |
J22.00009: Electron-phonon coupling in naphthalene crystal Roel S\'anchez-Carrera, Pavel Paramonov, Veaceslav Coropceanu, Jean-Luc Br\'edas We investigate the electron interactions with optical phonons in crystalline naphthalene. The lattice phonon modes were computed at both DFT and empirical force field levels. The electron-phonon couplings were evaluated by means of numerical differentiation; in this approach, the crystal geometry is distorted along normal modes and transfer integrals for several selected molecular pairs are then computed at each distorted geometry. We find that the DFT and force field results for phonon frequencies and electron-phonon couplings compare very well. Interestingly, several phonon modes are calculated to display significant quadratic electron-phonon coupling. In addition, we have also performed electronic band-structure calculations and derived the effective masses for both electrons and holes. [Preview Abstract] |
Tuesday, March 11, 2008 1:03PM - 1:15PM |
J22.00010: Solid state effects on the photophysics of $\pi$-conjugated polymer thin films Alok Shukla, Zhendong Wang, Sumit Mazumdar The photophysics of thin films of $\pi$-conjuated polymers (PCPs) are remarkably different from that of dilute solutions. The difference is generally ascribed to interchain interactions and disorder in films. Microscopic understanding of the consequences of interchain interactions has remained incomplete in spite of intensive investigations. We present a theory of the complete energy spectrum of interacting PCP chains that leads to correct qualitative, and perhaps even semiquantitative description of the photophysics of PCP films. Within our theory branching of photoexcitations occurs to excimers that occur both below and above the optical exciton. Emission, as well as ultrafast photoinduced absorption (PA) in films are from the lower excimer. The lowest energy PA at 0.35 - 0.4 eV corresponds to the allowed transition from the lower excimer to the lowest polaron-pair. We explain why the energies of the PA at $\sim$ 1.0 eV and higher are the same in films and solutions, even though the origins of these PAs are different. Finally, we give consistent explanations of the peculiarities associated with emission in films, including delayed emission, its quenching by electric field and the reappearance of the delated emission upon removal of the field.\footnote{Supported by NSF-DMR-0705163} [Preview Abstract] |
Tuesday, March 11, 2008 1:15PM - 1:27PM |
J22.00011: ABSTRACT WITHDRAWN |
Tuesday, March 11, 2008 1:27PM - 1:39PM |
J22.00012: Ab Initio Generated UPS of Electron Donors Reeshemah Allen, Tunna Baruah, Mark R. Pederson We have calculated the ultraviolet photoelectron spectra (UPS) data for a variety of molecules that have been suggested for donors for organic photovoltaic materials. The method employed here uses NRLMOL and the PBE-GGA density-functional based method for approximating energies of molecules within a constrained occupation approach. The UPS data is then simulated by calculating the energy difference between the N-electron ground state and a large set of self-consistently generated (N-1)-electron states. Incorporating the relevant absorption probabilities is also discussed. Comparison of calculated UPS spectra to experiment allows us to determine the conditions under which self-interaction corrections$^{1}$ to the energy functional are necessary for determining accurate excitation energies will also be discussed. 1. M. R. Pederson, R. A. Heaton, and C.C. Lin J. Chem. Phys. 82, 2688 (1985). [Preview Abstract] |
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