Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session G33: Focus Session: Organic Electronics and Photonics - Theoretical Photophysics and Excited State Dynamics |
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Sponsoring Units: DMP Chair: Richard Lunt, Michigan State University Room: 341 |
Tuesday, March 19, 2013 11:15AM - 11:27AM |
G33.00001: Polaritons in Organic Microcavities: The Effect of Phonons on the Dicke Model Justyna Cwik, Jonathan Keeling We study the effect of vibrational excitations on the condensation of polaritons. Recently, a lot of attention has been focused on microcavities based on organic semiconducting materials since, unlike their inorganic counterpart, they provide a suitable environment for the formation of a room temperature Bose-Einstein condensate. In order to model such materials we add terms to the usual Dicke Hamiltonian to account for the coupling of each two-level system to vibrational excitations (phonons). A mean field treatment, at zero temperature, gives us insights into the phase diagram of the Hamiltonian. In particular, we discuss the origin of the first order phase transition between two superradiant states which occurs as the coupling between the phonons and two-level systems is varied. An extension of the mean field treatment leads to the discussion of the equilibrium luminescence spectrum in the presence of phonons. We also present the way in which these results are modified at finite temperature. [Preview Abstract] |
Tuesday, March 19, 2013 11:27AM - 11:39AM |
G33.00002: Estimating the Magnitude of Exciton Delocalization in Regioregular P3HT through Computational Modeling and Transient Absorption Spectroscopy Michael Heiber, Ali Dhinojwala Exciton delocalization has been shown to have a potentially strong impact on the performance of organic solar cells. However, very few attempts have been made to estimate the magnitude of exciton delocalization in common semiconducting polymers. We show how the magnitude of exciton delocalization can be extracted from two types of femtosecond transient absorption spectroscopy experiments using computational modeling tools. By fitting exciton delocalization models to previously published experimental data, we extract two separate estimates of the magnitude of exciton delocalization in regioregular poly(3-hexylthiophene) (P3HT). First, fitting exciton-exciton annihilation behavior in pristine P3HT films leads to an estimation of the exciton delocalization radius of 1.6$\pm$0.25 nm. Second, dynamic Monte Carlo modeling of the exciton dissociation dynamics for a P3HT:PCBM blend film results in a second approximation of the exciton delocalization radius of 1.9$\pm$0.6 nm. These estimates are significantly smaller than previously published values and provide strong evidence for less delocalization than used in previous device models. [Preview Abstract] |
Tuesday, March 19, 2013 11:39AM - 11:51AM |
G33.00003: Influence of \textit{trans} and \textit{cis } defects on the localization of charged excitations in $\pi $-conjugated organic polymers Iffat Nayyar, Enrique Batista, Sergei Tretiak, Avadh Saxena, Darryl Smith, Richard Martin Optoelectronic devices with $\pi $-conjugated polymers are in demand for use in light-emitting diodes (LED), solar cells and lasers. A recent study predicted differences in the response of the hyperfine field by polaronic species in organic LEDs. The improved fluorescence exhibited by different isomeric forms of PPV derivatives in these devices motivated us to investigate the influence of various conformational defects of \textit{trans} and \textit{cis} nature on the energetics and localization of positive (P$^{\mathrm{+}})$ and negative (P$^{\mathrm{-}})$ polarons using density functional theory. We observe the P$^{\mathrm{+}}$ and P$^{\mathrm{-}}$ states are highly sensitive on the structural conformation and atomic charge distributions. The P$^{\mathrm{-}}$ state is observed to be more localized than P$^{\mathrm{+\thinspace }}$in consistent with recent experiments when the polarization effects are included. These defects not only break the particle-hole symmetry but demonstrate higher charge-carrier mobilities for holes than electrons. This helps in tuning the charge-transport and photo-physical properties of organic materials by understanding their structure-property correlations for technological innovations. [Preview Abstract] |
Tuesday, March 19, 2013 11:51AM - 12:03PM |
G33.00004: The role of exciton diffusion in the Forster-type energy transfer in hybrid organic-inorganic nanocomposites Burak Guzelturk, Pedro Ludwig Hernandez Martinez, Donus Tuncel, Hilmi Volkan Demir The role of exciton diffusion in the Forster-type energy transfer in hybrid organic-inorganic nanocomposite is essential for devices applications. To understand the underlying interplay between the exciton transfer and exciton diffusion, we investigate the temperature dependent nonradiative energy transfer (NRET) in polymer-quantum dot (QDs) nanocomposites at high and low QD loading levels. For the low QD loading, the diffusion coefficient (D) is estimated to be greater than 1000 nm2/ns and the diffusion length (LD) is approximately 13 nm at room temperature. However, significant modifications of D and LD are observed in the case of high QD loading, where D is estimated to be 150 nm2/ns and LD is smaller than 5 nm. This suppression is attributed to the increased rates of NRET from the polymer to the QDs, with a smaller effective donor-acceptor separation at high QD loadings. In summary, the exciton diffusion plays a critical role in the resulting exciton dynamics of such polymer-QD nanocomposites, and the experimental evidence and supporting theoretical model suggest that the exciton diffusion is weak at the high loading levels when the exciton transfer dominates. [Preview Abstract] |
Tuesday, March 19, 2013 12:03PM - 12:15PM |
G33.00005: First-principles simulations of exciton diffusion in organic semiconductors Xu Zhang, Zi Li, Gang Lu Exciton diffusion is of great importance to the performance of organic optoelectronic devices, including organic photovoltaics and solid-state lighting. The ability to control exciton diffusion in organic semiconductors is crucial to the design of efficient optoelectronic devices. However, such ability can only be achieved through a fundamental understanding of exciton diffusion mechanism. We have proposed a first-principles based frame work that can predict exciton dynamics in organic semiconductors.The framework is based on time-dependent density functional theory to provide the energy and many-body wave functions of excitons. Nonadiabatic \textit{ab initio} molecular dynamics is used to calculate phonon-assisted transition rates between localized exciton states. Using Monte Carlo simulations, we determine exciton diffusion length, lifetime, diffusivity, and harvesting efficiency in poly(3-hexylthiophene) polymers at different temperatures, and the results agree very well with corresponding experimental values. We find that exciton diffusion is primarily determined by the density of states of low-energy excitons; a widely speculated diffusion mechanism has been confirmed and elucidatedby the simulations. Some general guidelines for designing more efficient organic solar cells can be gleaned from the simulation results [Preview Abstract] |
Tuesday, March 19, 2013 12:15PM - 12:27PM |
G33.00006: Relating Crystal Structure and the Charge-Transfer Nature of Excitons in Pentacene from First Principles Sahar Sharifzadeh, Pierre Darancet, Leeor Kronik, Jeffrey Neaton The nature of low energy optical excitations within pentacene has been the subject of many experimental and theoretical studies, with much disagreement as to the degree of their charge-transfer character. Here, we use many-body perturbation theory to study singlet excitons within different solid phases of pentacene and demonstrate that inter-molecular interactions lead to delocalized, charge-transfer-like excitations in the bulk crystalline phase. Using the Bethe-Salpeter two-particle correlation function, we demonstrate that the interplay between intermolecular hybridization, local exchange interactions, and attractive electron/hole interactions controls the nature of the exciton. Additionally, we explore simple models to understand and predict the nature of the excitonic wavefunction, in particular whether it has charge-transfer character. [Preview Abstract] |
Tuesday, March 19, 2013 12:27PM - 12:39PM |
G33.00007: Exploring the correlation between molecular conformation and optoelectronic properties of conjugated polymers : side-chain versus main-chain electron acceptors Yu-Chen Huang, Ching-I Huang Polythiophene derivatives have been shown among the most promising materials for solar cell application because of their high charge mobility and light absorption. In the mostly studied, a recombination process often occurs, which is mainly due to the fact that the mobility of hole is much lower than that of electron. Hence, research about conjugated polymers containing donor-accepter pairs (such as PT-TPD) becomes quite popular because these materials have narrow band-gaps. Interestingly, these experimental studies have indicated a much more complex correlation between the optoelectronic properties and molecular conformation for polymers with acceptor units on either main or side chain. However, the effects associated with the molecular packing on the resultant chain conformation behavior and thereafter the optoelectronic properties have not been systematically discussed. In order to clarify the effects of the molecular conformation as well as the optoelectronic properties, we employ molecular dynamics and quantum mechanical methods to examine PBTTPD molecules with acceptor unit (TPD) on either main or side chain [Preview Abstract] |
Tuesday, March 19, 2013 12:39PM - 12:51PM |
G33.00008: Band structure of polyethylene from many-body perturbation theory Ariel Biller, Sahar Sharifzadeh, Lior Segev, Sohrab Ismail-Beigi, Jeffrey B. Neaton, Leeor Kronik The electronic structure of polyethylene is an important benchmark and the infinite chain limit for the electronic properties of many molecules, monolayers, and oligomers. Therefore, the band structure of the ideal, one-dimensional polyethylene chain has been extensively researched, from both the experimental and the theoretical viewpoints. Despite this extensive effort, to the best of our knowledge agreement between theoretical calculations and the electronic structure obtained from photoelectron spectroscopy could only be obtained using artificial shifting and ``stretching'' of the computed data. Here, we present a quantitative quasi-particle band-structure for polyethylene using many-body perturbation theory. The approach is employed within the $G_{0}W_{0}$ approximation, based on a starting point calculated within the generalized gradient approximation to density functional theory. We compare our calculated band-structure to angle resolved photoemission spectroscopy measurements for various long saturated carbohydrates, demonstrate a much improved agreement with experiment, and discuss remaining discrepancies and their possible origins within both theory and experiment. [Preview Abstract] |
Tuesday, March 19, 2013 12:51PM - 1:03PM |
G33.00009: Ideal Energy-Level Alignment at the ZnO/P3HT Photovoltaic Interface Keian Noori, Feliciano Giustino Despite the significant progress made during the past decade, hybrid organic-inorganic photovoltaic devices comprising P3HT and ZnO still suffer from low short-circuit currents and moderate open-circuit voltages. These barriers call for a detailed examination of the atomic-scale physics underlying the energy-level alignment at the ZnO/P3HT interface, which is of critical importance if we are to understand what is the maximum ideal open-circuit voltage for this class of solar cell. Here we present the results of a first-principles study [1] on large model interfaces between ZnO and P3HT. Using a combination of density-functional theory (DFT) and post-DFT methods based on hybrid functionals, we analyze the atomic structure and energetics of the semiconductor/polymer interface, as well as the interfacial energy-level alignment. We explore the effect of charge transfer on the ideal open-circuit voltage and identify a failure in the standard electron affinity rule. We determine a maximum ideal open-circuit voltage of $\sim$2 V, which suggests that there is significant room for enhancing the performance of ZnO/P3HT solar cells by optimizing the interface at the nanoscale. \\[4pt] [1] K. Noori, F. Giustino, Adv. Funct. Mater. DOI:10.1002/adfm.201201478 (2012). [Preview Abstract] |
Tuesday, March 19, 2013 1:03PM - 1:15PM |
G33.00010: Interactions between linear organic chromophores: an improved line-dipole approximation Jean-Christophe Denis, Stefan Schumacher, Ian Galbraith Modelling accurately the interactions between chromophores is key for realistic simulations of the dynamics of exciton transfer and annihilation in organic semiconductor films. In the framework of F\"orster theory, it is required to calculate the interaction matrix elements for all relative orientations and separations of chromophores. Therefore a fast and robust approximation is necessary to simulate extended multi-chromophoric systems. From this perspective, using the line-dipole approximation is a very natural approach. However, by a comparative study of the dipole approximation with quantum chemistry (TD-DFT) we demonstrate that the usual line-dipole theory, while successful for short molecules, does not describe well the interactions of longer molecules, where separations are smaller than the interacting chromophores - a limit typically reached in polymer films. As an alternative, we propose an improved way of distributing the sub-dipole moments within a line. This approach remains simple enough to be used in large-scale calculations, while the agreement with the quantum chemistry is significantly improved for all relative orientations. [Preview Abstract] |
Tuesday, March 19, 2013 1:15PM - 1:27PM |
G33.00011: Identifying molecular features that maximize the second hyperpolarizability Christopher Burke, Timothy Atherton, Joseph Lesnefsky, Rolfe Petschek Designing materials with high nonlinear optical properties is of importance for a variety of applications ranging from optical switching to chemical sensing. A key figure of merit is the intrinsic molecular second hyperpolarizability $\gamma_{int}$, a dimensionless quantity which measures how close a molecule's second hyperpolarizability is to the theoretical maximum. By modeling a molecule as a one dimensional linear piecewise potential, $\gamma_{int}$ was optimized with respect to the shape of the potential. The number of parameters needed to describe the potential was varied. Searches were carried out for extrema in both the positive and negative directions, finding optimum potentials with $\gamma_{int}$ of 0.60 and -0.15. The optimum potentials possess parity symmetry and are specified by a very small number of parameters due to our simple and well chosen representation. Based on the shape of the optimized potentials, we use these results to suggest possible routes for synthesizing molecules with high $\gamma_{int}$. [Preview Abstract] |
Tuesday, March 19, 2013 1:27PM - 1:39PM |
G33.00012: Photoexcitation and Photochemical Stability of Organic Photovoltaic Materials from First Principles Na Sai, Kevin Leung The development of high efficiency organic photovoltaics (OPV) has recently become enabled by the synthesis of new conjugated polymers with low band gap that allow light absorption over a broader range of the spectrum. Stability of these new polymers, a key requirement for commercialization, has not yet received sufficient attention. Here, we report first-principles theoretical modeling of photo-induced degradation of OPV polymers carried out using ab-initio density functional theory (DFT). We report photooxidation routes and reaction products for reactive species including superoxide oxygen anions and hydroxyl groups interacting with the standard workhorse OPV polymer, poly(3-hexyl-thiophene) (P3HT). We discuss theoretical issues and challenges affecting the modeling such reactions in OPV polymers. We also discuss the application of theoretical methods to low-band-gap polymers, and in particular, the effect of the chemical substitution on the photoexcitation properties of these new polymers. Sandia National Laboratories is a multiprogram laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Deparment of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. [Preview Abstract] |
Tuesday, March 19, 2013 1:39PM - 1:51PM |
G33.00013: Understanding the influence of solvent field and fluctuations on the stability of photo-induced charge-separated state in molecular triad D. Balamurugan, Adelia Aquino, Hans Lischka, Francis Dios, Lionel Flores, Margaret Cheung Molecular triad composed of fullerene, porphyrin, and carotene is an artificial analogue of natural photosynthetic system and is considered for applications in solar energy conversion because of its ability to produce long-lived photo-induced charge separated state. The goal of the present multiscale simulation is to understand how the stability of photo-induced charge-separated state in molecular triad is influenced by a polar organic solvent, namely tetrahydrofuran (THF). The multiscale approach is based on combined quantum, classical molecular dynamics, and statistical physics calculations. The quantum chemical calculations were performed on the triad using the second order algebraic diagrammatic perturbation and time-dependent density functional theory. Molecular dynamics simulations were performed on triad in a box of THF solvent with the replica exchange method. The two methods on different length and time scales are bridged through an important sampling technique. We have analyzed the free energy landscape, structural fluctuations, and the long- range electrostatic interactions between triad and solvent molecules. The results suggest that the polarity and re-organization of the solvent is critical in stabilization of charge-separated state in triad. [Preview Abstract] |
Tuesday, March 19, 2013 1:51PM - 2:03PM |
G33.00014: Quantum dynamics simulations of interfacial charge-transfer in organic dye-sensitized solar cells Luis G.C. Rego, R. da Silva, D.A. Hoff We describe a novel time-dependent quantum-mechanics/molecular-mechanics method for studying electron transfer in dye sensitized semiconductor interfaces, that takes into account the interacting electron-hole quantum dynamics, the underlying nuclear fluctuations and solvation dynamics. We provide a comprehensive investigation of the quantum dynamics, the electronic and the structural properties of prototypical D-$\pi$-A organic dyes sensitizing the TiO2 anatase surface, both in vacuum and solvated by liquid acetonitrile. The organic dyes are comprised of an electron donating moiety and an anchoring acceptor moiety, conjugated by thiophene bridges. Although interfacial electron transfer is very efficient, it is demonstrated that the coupling between the photoexcited electron and the hole delays the electron injection. Simulations demonstrate that the solvent screens the dye from the surface, narrowing the absorption peaks and delaying the electron injection. We have also studied several aspects that are relevant for the recombination process, such as the role played by surface defects and the interaction of redox species with the TiO2 surface, and the effect of additives. [Preview Abstract] |
Tuesday, March 19, 2013 2:03PM - 2:15PM |
G33.00015: First principles modeling of panchromatic dyes for solar cells applications. Rosa Di Felice, Arrigo Calzolari, Rui Dong, Marco Buongiorno Nardelli The state-of-the-art dye in Gr\"atzel solar cells, N719, exhibits a total solar-to-electric conversion efficiency of 11.2\%. However, it severely lacks absorption in the red and the near infrared regions of the electromagnetic spectrum, which represent more than 70\% of the solar radiation spectrum. Using calculations from first principles in the time-dependent domain, we have studied the electronic and optical response of a novel class of panchromatic sensitizers that can harvest solar energy efficiently across the visible and near infrared regions, which have been recently synthesized [A. El-Shafei, M. Hussain, A. Atiq, A. Islam, and L. Han, J. Mater. Chem. {\bf 22}, 24048 (2012)]. Our calculations show that, by tuning the properties of antenna groups, one can achieve a substantial improvement of the optical properties. [Preview Abstract] |
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