Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session X41: Focus Session: Electronic Structure and Applications to Energy Conversion II |
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Sponsoring Units: DCP Chair: Troy van Voorhis, Massachusetts Institute of Technology Room: A115/117 |
Thursday, March 24, 2011 2:30PM - 3:06PM |
X41.00001: Fundamental understanding and computational design of thin-film photovoltaics materials Invited Speaker: The search for abundant and clean energy sources has placed photovoltaics at the focus of research over a variety of disciplines spanning physics, chemistry and materials science. However, the quest for more cost-efficient photovoltaics is challenged by limitations in efficiency of charge excitation and collection in the materials and their interfaces. We will present our recent \textit{ab initio} calculations aimed at understanding important microscopic mechanisms in solar photovoltaic materials. Our goal is to predict accurately key properties that govern the efficiency in these materials, including structural and electronic effects, interfacial charge separation, electron and hole traps, excited state phenomena, band level alignment, and binding energies. Examples of our work in the areas of organic and other thin-film photovoltaics will be presented. We use these examples to illustrate how accurate electronic structure approaches can improve our understanding and lead to more efficient materials. [Preview Abstract] |
Thursday, March 24, 2011 3:06PM - 3:18PM |
X41.00002: Effect of environment and long range behaviour of exchange functional on polaron formation in $\pi$-conjugated polymers Iffat Nayyar, Enrique Batista, Sergei Tretiak, Avadh Saxena, Darryl Smith, Richard Martin Organic conjugated polymers find a variety of applications in devices such as solar cells, light emitting diodes and lasers. An accurate understanding of the role of nonlinear excitations as polarons in charge carrier transport is critical to improve the efficiency of these devices. PPV and MEH-PPV are the candidates of choice for the extensive experimental data and relative simplicity compared to other polymers. This motivated us to perform a density functional theory study to describe the charge defects in these systems. We emphasize on the role of surrounding dielectric medium and the amount of long range orbital exchange in the density functional to predict the polaron localization in agreement with experiment. The particle-hole symmetry observed in trans-geometries is broken by introducing certain cis defects. [Preview Abstract] |
Thursday, March 24, 2011 3:18PM - 3:30PM |
X41.00003: Photo-induced modulation in the dipole moment of a donor-acceptor pair of organic molecules Yoshiyuki Miyamoto, Mina Yoon, Matthias Scheffler We have investigated the photo-induced electron dynamics in donor-acceptor pairs of organic molecules. Specifically we will discuss TTF and TCNQ molecules and study their electron dynamics under illumination by means of time-dependent density functional theory within the local-density approximation. In their stable molecular structure, we find that these molecules align in parallel and show maximum optical oscillator strength with an optical polarization parallel to their molecular axis. Without illumination, a dipole moment from TTF to TCNQ directs perpendicular to the molecular axis. This dipole-moment is further increased upon illumination with an optical polarization parallel to the molecular axis at resonant excitation energies of 2.00 eV and 3.55 eV. The light-induced increase of the dipole moment, which reflects the separation of electron and hole pair, is caused by the internal electric field between these molecules. Therefore, these molecules may have a high potential as building blocks of future organic photovoltaic devices. [Preview Abstract] |
Thursday, March 24, 2011 3:30PM - 4:06PM |
X41.00004: Exciton transport and dissociation at organic interfaces Invited Speaker: This paper focuses on modeling studies of exciton transport and dissociation at organic interfaces and includes three parts: 1) Experiments have shown that the values of exciton diffusion length $L_{D}$ in conjugated polymers (CPs) are rather low, in the range of 5-10 nm, apparently regardless of their chemical structure and solid-state packing. In contrast, larger $L_{D}$ values have been reported in molecular materials that are chemically more well-defined than CPs. Here we demonstrate that energetic disorder alone reduces the exciton diffusion length more than one order of magnitude, from values typically encountered in molecules ($>$50nm) to values actually measured in CPs ($<$10nm). 2) A number of organic crystals show anisotropic excitonic couplings, with weak interlayer interactions between molecules that are more strongly coupled within the layers. The resulting energy carriers are intra-layer 2D excitons that diffuse along the interlayer direction. We model this analytically for infinite layers and using quantum-chemical calculations of the electronic couplings for anthracene clusters. We show that the exciton hopping rates and diffusion lengths depend in a subtle manner on the size and shape of the interacting aggregates, temperature and the presence of energetic disorder. 3) The electronic structure at organic/organic interfaces plays a key role, among others, in defining the quantum efficiency of organic-based photovoltaic cells. Here, we perform quantum-chemical and microelectrostatic calculations on molecular aggregates of various sizes and shapes to characterize the interfacial dipole moment at pentacene/C60 heterojunctions. The results show that the interfacial dipole mostly originates in polarization effects due to the asymmetry in the multipolar expansion of the electronic density distribution between the interacting molecules. We will discuss how the quadrupoles on the pentacene molecules produce direct electrostatic interactions with charge carriers and how these interactions in turn affect the energy landscape around the interface and therefore also the energy barrier for exciton dissociation into free carriers. [Preview Abstract] |
Thursday, March 24, 2011 4:06PM - 4:18PM |
X41.00005: Correlating First-Principles Electronic Structure with Device Performance of Organic Photovoltaic Cells Eric B. Isaacs, Sahar Sharifzadeh, Biwu Ma, Jeffrey B. Neaton Organic photovoltaic cells (OPVs) are promising candidates for low-cost solar energy conversion. Here, we employ static and time-dependent density functional theory calculations to predict the excitation energy of the donor-acceptor charge transfer state (E$_{\textrm{CT}})$ at the interface between C$_{60}$ and several boron(subphthalocyanine)- and azadipyrromethene-based donor moieties, comparing to measured open-circuit voltage (V$_{\textrm{oc}})$ in bilayer heterojunction OPVs [1]. When E$_{\textrm{CT}}$ is approximated as the difference between the ionization potential and electron affinity of the isolated donor and acceptor molecules, respectively, we observe no apparent correlation between E$_{\textrm{CT}}$ and V$_{\textrm{oc}}$. Both bulk polarization and excitonic effects at the interface are found to influence the energetics significantly, the latter being strongly morphology dependent. We demonstrate that a linear relationship between V$_{\textrm{oc}}$ and E$_{\textrm{CT}}$ may be obtained once the interface morphology is considered. We acknowledge support from DOE, NSF-NCN, and NERSC. \\[4pt] [1] C. E. Mauldin \textit{et al.}, ACS Appl. Mater. Interfaces \textbf{2}, 2833 (2010). [Preview Abstract] |
Thursday, March 24, 2011 4:18PM - 4:30PM |
X41.00006: Photo-induced Charge Separation in Nanoscale Donor-Bridge-Acceptor Systems: Theory and Experiment Peter Doak, Pierre Darancet, Kasper Moth-Poulsen, Jesse Jenkins, Rachel Segalman, Don Tilley, Jeff Neaton Understanding and control of light-harvesting processes at the molecular-scale remains a fundamental challenge in solar energy conversion. Donor-bridge-acceptor molecules (DBAM), with atomically-defined interfaces made by a covalently bound bridge between donor and acceptor moieties, allow probing of excited states relevant to optical absorption and charge separation. In close collaboration with experiment, we use first-principles many-body perturbation theory, within the GW approximation and the Bethe-Salpeter equation approach, to compute excited states for six DBAMs. We compare with experiments, and quantitative agreement is obtained. Implications of our results for nanoscale light-harvesting are thoroughly discussed. Support: DOE via the Molecular Foundry and Helios SERC, and NSF via NCN. Computational support provided by NERSC. [Preview Abstract] |
Thursday, March 24, 2011 4:30PM - 5:06PM |
X41.00007: Exciton Scattering in Branched Conjugated Molecules: Towards Photoinduced Dynamics and Energy Transfer Invited Speaker: The exciton scattering (ES) approach attributes excited electronic states in quasi-one-dimensional (branched) conjugated molecules with perfect geometry to standing waves on the linear segments of a molecule formed by scattering of quantum quasi-particles (excitons). We extract their dispersion and frequency-dependent scattering matrices at termini, including donor/acceptor substitutions, joints, and branching centers from time-dependent density functional theory (TD-DFT) calculations, with applications to for conjugated phenylacetylene-based molecules. This allows electronic spectra for any structure of arbitrary size within the considered molecular family to be obtained with insignificant numerical effort. To extend the capability of the ES approach to treating photoinduced dynamics, including absorption and fluorescence lineshapes and energy transfer, the methodology should be modified to account for non-ideal molecular geometry. Geometry distortions break down translational symmetry of the linear segments, and excitations are not represented by perfect standing waves anymore. To overcome this difficulty we associate electronic excitations with the eigenstate of a quantum particle on an irregular lattice (graph), referred to as a tight-binding model. The morphology of the underlying lattice, together with the tight-binding parameters, can be identified by studying the topological and analytical properties of excitons at molecular termini, joints, and branching centers. The dependence of the tight-binding parameters on geometry distortions that controls effects of disorder and coupling to vibrational modes can be extracted from quantum chemical calculations by studying exciton scattering on localized geometry distortions, the latter considered as scattering centers. [Preview Abstract] |
Thursday, March 24, 2011 5:06PM - 5:18PM |
X41.00008: NA-ESMD modeling of photoinduced dynamics in conjugated molecules Tammie Nelson, Sebastian Fernandez-Alberti, Vladimir Chernyak, Adrian Roitberg, Sergei Tretiak The evolution of electronic excitations in optically active molecules can generally be defined by non-adiabatic (NA) dynamics. A number of fundamental and complex processes are associated with NA dynamics. To treat ultrafast excited state dynamics we have developed a non-adiabatic excited state molecular dynamics (NA-ESMD) framework incorporating quantum transitions. Our calculations combine the Collective Electronic Oscillator (CEO) package with the Tully's fewest switches algorithm for surface hopping, and the actual potential energy surfaces of the excited states are used. This method is applied to model the photoinduced dynamics of distyrylbenzene. Our analysis shows intricate details of vibronic relaxation and identifies specific slow and fast nuclear motions that are strongly coupled to the electronic degrees of freedom. Non-adiabatic relaxation of the highly excited mAg state is predicted to occur on a femtosecond timescale at room temperature and on a picosecond timescale at low temperature. [Preview Abstract] |
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