Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session S1: Focus Session: Charge & Energy Transfer III |
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Sponsoring Units: DCP Chair: Martin Wolf, Fritz-Haber-Institute Room: 103/105 |
Thursday, March 6, 2014 8:00AM - 8:12AM |
S1.00001: First-principles calculation of photo-induced electron transfer rate constants in phthalocyanine-C$_{60}$ organic photovoltaic materials: Beyond Marcus theory Myeong H. Lee, Barry D. Dunietz, Eitan Geva Classical Marcus theory is commonly adopted in solvent-mediated charge transfer (CT) process to obtain the CT rate constant, but it can become questionable when the intramolecular vibrational modes dominate the CT process as in OPV devices because Marcus theory treats these modes classically and therefore nuclear tunneling is not accounted for. We present a computational scheme to obtain the electron transfer rate constant beyond classical Marcus theory. Within this approach, the nuclear vibrational modes are treated quantum-mechanically and a short-time approximation is avoided. Ab initio calculations are used to obtain the basic parameters needed for calculating the electron transfer rate constant. We apply our methodology to phthalocyanine(H$_2$PC)-C$_{60}$ organic photovoltaic system where one C$_{60}$ acceptor and one or two H$_2$PC donors are included to model the donor-acceptor interface configuration. We obtain the electron transfer and recombination rate constants for all accessible charge transfer (CT) states, from which the CT exciton dynamics is determined by employing a master equation. The role of higher lying excited states in CT exciton dynamics is discussed. [Preview Abstract] |
Thursday, March 6, 2014 8:12AM - 8:24AM |
S1.00002: Exciton dissociation at phthalocyanine-C$_{60}$ interfaces S.W. Robey, G.J. Dutton Exciton dissociation and charge transfer processes occurring within 10's of nanometers of donor-acceptor interfaces are critical for the performance of organic photovoltaic (OPV) structures. We investigated fundamental issues of exciton dissociation near prototypical donor-acceptor interface using time-resolved two-photon photoemission (TR-2PPE). Phthalocyanine (Pc)-C$_{60}$ interfaces with known structures were formed using organic molecular beam epitaxy. Pc $\pi \to \pi $* (Q-band) transitions were created by a sub-picosecond pump pulse, producing a population of singlet (S$_{1})$ Pc excitons. The dynamics of this population were then probed via photoemission by a time-delayed UV pulse. For CuPc$\backslash $C$_{60}$ interfaces, the dynamics for excitons created far from the interface were modeled with a combination of vibrational or intraband relaxation plus intersystem crossing (ISC) to triplet levels. Relaxation leads predominantly to triplet (T$_{1})$ exciton levels on timescales of $\approx $ 1-2 ps. The decay dynamics of S$_{1}$ excitons excited in the CuPc layer adjacent to C$_{60}$ were increased due to the additional channel leading to exciton dissociation, occurring with a rate of $\approx $ 7 x 10 $^{12}$ sec$^{-1}$. However, excitons that relax to T$_{1}$ levels at the interface dissociate with a rate $\approx $ 500 to 1000 times slower, providing a picture of the energy dependence of exciton dissociation at this interface. The dependence of exciton dissociation versus Pc thickness at analogous H$_{2}$Pc $\backslash $C$_{60}$ interfaces will also be presented. The results indicate that, for this interface, exciton dissociation is much faster for the interfacial layer with dissociation from the 2$^{\mathrm{nd}}$, and subsequent layers of H$_{2}$Pc, reduced by at least a factor of 10. [Preview Abstract] |
Thursday, March 6, 2014 8:24AM - 8:36AM |
S1.00003: Passive exciton gating via directed energy transfer in organic photovoltaic cells S. Matthew Menke, Tyler K. Mullenbach, Russell J. Holmes We present a Kinetic Monte Carlo method to model exciton diffusion across non-reflective, non-quenching, exciton permeable interfaces where standard analytical and numerical solutions to the exciton diffusion equation are not available. The combination of energy transfer rates and separately measured natural lifetimes allows for the modeling of exciton diffusion across a wide range of inhomogeneous landscapes. This model is successfully applied to quantitatively account for the photocurrent enhancements present in dilute donor organic photovoltaic (OPV) devices incorporating the archetypical electron donor boron subphthalocyanine chloride (SubPc), the wide energy gap host material p-bis(triphenylsilyl)benzene (UGH2), and fullerene C60. Furthermore, this modeling is extended to provide an optimization route for advanced, cascade OPVs where excitons are transferred between layers before reaching the acceptor material. Analysis of exciton motion in these architectures also reveals how inhomogeneous energy transfer landscapes can lead to directed exciton motion, thus deviating from the typical diffusive or sub-diffusive behavior. The implications for directed exciton motion are then discussed in terms of passive exciton gating towards the ultimate goal of finely controlling the migration of energy in these devices as well as the broader field of organic optoelectronic devices. [Preview Abstract] |
Thursday, March 6, 2014 8:36AM - 9:12AM |
S1.00004: Direct observation of ultrafast long-range charge separation at polymer:fullerene heterojunctions Invited Speaker: Carlos Silva In polymeric semiconductors, charge carriers are polarons, which means that the excess charge deforms the molecular structure of the polymer chain that hosts it. This effect results in distinctive signatures in the vibrational modes of the polymer. We probe polaron photogeneration dynamics at polymer:fullerene heterojunctions by monitoring its time-resolved resonance-Raman spectrum following ultrafast photoexcitation. We conclude that polarons emerge within 200\,fs, which is nearly two orders of magnitude faster than exciton localisation in the neat polymer film. Surprisingly, further vibrational evolution on $\la 50$-ps timescales is modest, indicating that the polymer conformation hosting nascent polarons is not significantly different from that in equilibrium. This suggests that charges are free from their mutual Coulomb potential, under which vibrational dynamics would report charge-pair relaxation. Our work addresses current debates on the photocarrier generation mechanism at organic semiconductor heterojunctions, and is, to our knowledge, the first direct probe of molecular conformation dynamics during this fundamentally important process in these materials. [Preview Abstract] |
Thursday, March 6, 2014 9:12AM - 9:24AM |
S1.00005: Exploring the Influence of Dynamic Disorder on Excitons in Solid Pentacene Zhiping Wang, Sahar Sharifzadeh, Peter Doak, Zhenfei Lu, Jeffrey Neaton A complete understanding of the spectroscopic and charge transport properties of organic semiconductors requires knowledge of the role of thermal fluctuations and dynamic disorder. We present a first-principles theoretical study aimed at understanding the degree to which dynamic disorder at room temperature results in energy level broadening and excited-state localization within bulk crystalline pentacene. Ab initio molecular dynamics simulations are well-equilibrated for 7-9 ps and tens of thousands of structural snapshots, taken at 0.5 fs intervals, provide input for many-body perturbation theory within the GW approximation and Bethe-Salpeter equation (BSE) approach. The GW-corrected density of states, including thousands of snapshots, indicates that thermal fluctuations significantly broaden the valence and conduction states by \textgreater 0.2 eV. Additionally, we investigate the nature and energy of the lowest energy singlet and triplet excitons, computed for a set of uncorrelated and energetically preferred structures. [Preview Abstract] |
Thursday, March 6, 2014 9:24AM - 9:36AM |
S1.00006: Singlet fission in reduced dimensions of crystals Paul Teichen, Joel Eaves In some molecular systems the decay of an initially excited singlet into two independent triplets, a process called singlet fission, is highly efficient. Organic crystals are among the most promising candidates for increasing yields in next-generation photovoltaics. Although excitons are known to exist in reduced dimensions of crystals the role of dimensionality in the entanglement of two triplets born out of singlet fission remains unclear. We develop a quantum lattice model for singlet fission to examine the role of quantum entanglement and exciton delocalization. [Preview Abstract] |
Thursday, March 6, 2014 9:36AM - 9:48AM |
S1.00007: Fission of Entangled Spins: An Electronic Structure Perspective Xintian Feng, Anatoliy Luzanov, Anna Krylov Electronic structure aspects of singlet fission process are discussed. Correlated adiabatic wave functions of the bright singlet and dark multiexciton states of tetracene and pentacene dimers are analyzed in terms of their character (excitonic, charge-resonance, multiexciton). At short interfragment separation (3.5-4.0 angstroms), both multiexcitonic and singly-excited singlet states have noticeable charge-resonance contributions that fall off quickly at longer distances. Non-adiabatic couplings between the states are discussed. The limitations of diabatic framework in the context of singlet fission are explained. Based on the Cauchy-Schwarz inequality, we propose using the norm of one-particle transition density matrix, $||\gamma||$, as a proxy for couplings. The analysis of $||\gamma||$ and state characters reveals that the couplings between the multiexciton and singly-excited states depend strongly on the weights of charge-resonance configurations in these states. To characterize energetics relevant to triplets separation step, we consider multiexciton binding energy (E$_b$) defined as the difference between the quintet and singlet multiexciton states. The effect of fragment orientation on the couplings and E$_b$ is analyzed. [Preview Abstract] |
Thursday, March 6, 2014 9:48AM - 10:24AM |
S1.00008: Exciton fission, dissociation and transport in organic conjugated materials: Modeling insights Invited Speaker: David Beljonne Electronic excited states in conjugated organic materials involve an admixture of localized (Frenkel-like) and charge-transfer (CT) excitations. We will first review some recent modeling work showing that this mixed Frenkel-CT character steers the Davydov splitting, mediates singlet fission and prompts triplet energy migration in oligoacenes crystals. Ultrafast and efficient charge separation occurs at interfaces between properly designed molecular donors and acceptors, despite the large electron-hole Coulomb binding energy. In a second part of the talk, we will describe the various mechanisms for such a dissociation process and assess them from atomistic simulations based on a combination of force-field, quantum-chemical and model Hamiltonian calculations. [Preview Abstract] |
Thursday, March 6, 2014 10:24AM - 10:36AM |
S1.00009: First Principles Calculations of Conformational and Electronic Properties of PTB7 Ram Bhatta, David Perry, Mesfin Tsige The thieno[3,4-b]thiophene-altbenzodithiophene copolymer (PTB7) is a promising electron donor in organic photovoltaic (OPV) devices with a power conversion efficiency (PCE) of about 9 percent. Further enhancement of the PCE is required for the practical realization and successful commercialization, which, in turn relies on the core understanding of structure-property relationships in OPV materials. Here, we present large-scale density functional calculations of the torsional and electronic properties of PTB7 oligomers. These first principles results include the chain length dependence of the torsional potential, the nearest neighbor torsional coupling, the band gap and the electronic conjugation length. PTB7 was found to have weaker nearest-neighbor torsional coupling, a lower band gap and a longer conjugation length compared to the other conjugated polymers like polythiophene and poly(3-alkylthiophene). These results help to explain the relative efficiency of OPV devices in which PTB7 is the electron donor. [Preview Abstract] |
Thursday, March 6, 2014 10:36AM - 10:48AM |
S1.00010: Understanding the Role of Orientational Heterogeneity on Photophysical Properties of Organic Polycrystalline Films Sahar Sharifzadeh, Cathy Wong, Hao Wu, Naomi Ginsberg, Leeor Kronik, Jeffrey Neaton Organic semiconductors are a highly tunable class of optically active materials that are promising as next-generation photovoltaics. Utilizing these materials for efficient solar energy conversion relies on an understanding of the connection between their excited-state electronic structure and their solid-state morphology. While many organic materials have varying degrees of disorder, crystalline films with long-range order provide an opportunity to understand many fundamental physical properties relevant to solar energy conversion. Here, we use a combined theoretical and experimental approach to investigate the nature of low-energy excitons and their dynamics within 6,13-bis(triisopropylsilylethynyl)-pentacene (TIPS-Pen) polycrystalline films. First-principles many-body perturbation theory and optical absorption spectroscopy on ordered domains reveal multiple low-energy absorption peaks that are composed of delocalized excitonic states. Further, we examine the nature of excitons in grains of different relative orientations and at grain boundaries, and discuss implications for their dynamics as measured by spatially-resolved transient absorption spectroscopy. This work was supported by DOE; computational resources provided by NERSC. [Preview Abstract] |
Thursday, March 6, 2014 10:48AM - 11:00AM |
S1.00011: Theoretical Modeling and Design of Organic Semiconductors with High Carrier Mobility Xiao Ma, Changgua Zhen, John Kieffer Charge transport in organic materials can be quite different from that in inorganic materials. The weak van der Waals interaction between organic molecules invalidates the band model used widely in inorganic materials. We have applied a multiscale hopping model based on Fermi's golden rule to study the carrier mobility in a pentacene single crystal structure. The pentacene single crystal adopts a herringbone stacking, which strongly limits the $\pi $-orbital overlap between neighboring molecules, resulting in poor charge carrier transfer and long-range mobility. To improve the charge transport performance of pentacene-related organic materials, we functionalize the pentacene with polyhedral oligomeric silsesquioxanes (POSS) cages to induce a parallel configuration. A higher theoretical carrier mobility is predicted based on using a combination of molecular dynamics, density functional theory calculations and kinetic Monte Carlo simulations. Accordingly, simulations constitute a cost-efficient means to derive design principle for materials with improved transport properties to be used in photovoltaic devices. [Preview Abstract] |
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