Quantum dynamical studies of ultrafast charge separation in nanostructured organic polymer materials: Effects of vibronic interactions and molecular packing

Abstract

We review recent work employing high-dimensional quantum dynamical techniques to study ultrafast charge separation in functional organic materials, in view of understanding the key microscopic factors that lead to efficient charge generation in photovoltaics applications. As highlighted by recent experiments, these processes can be guided by quantum coherence, despite the presence of static and dynamic disorder. The present approach combines first-principles parametrized lattice Hamiltonians, based on Time-Dependent Density Functional Theory (TDDFT) and/or high-level electronic structure calculations, with accurate quantum dynamics simulations using the Multi-Configuration Time-Dependent Hartree (MCTDH) method. This contribution specifically addresses the mechanism of charge generation in (i) regioregular oligothiophene-fullerene aggregates, and (ii) highly ordered oligothiophene-perylene diimide co-oligomer assemblies. These studies highlight that chemical design of donor-acceptor combinations needs to account for the effects of electronic delocalization and the modified energetics due to molecular packing, as well as multiple transfer pathways and internal conversion channels induced by vibronic interactions.excitonic states, molecular packing, organic photovoltaics, quantum dynamics, ultrafast charge separation | I N TR ODU C TI ONIn organic donor-acceptor (DA) materials developed for photovoltaics applications, the break-up of photogenerated excitons at DA heterojunction interfaces initiates the irreversible separation of charge carriers. [1,2] Over the past few years, it has become increasingly clear that the efficiency of this process depends on a complex interplay of different factors, such that design principles based on the engineering of molecular structure and DA band offsets are not necessarily sufficient. Indeed, the photochemical processes inherent to the molecular donor and acceptor materials necessitate a molecular-level analysis that may deviate from the basic picture derived from semiconductor physics. This is underscored by an increasing body of evidence from time-resolved spectroscopies [3][4][5][6][7] showing that the elementary energy and charge transfer (CT) processes in typical photovoltaic materials are often of quantum coherent character. Conventional kinetic descriptions are therefore not apt to render a faithful picture of these elementary steps, and may only be valid on longer time scales where hopping type transport dynamics sets in.Even long-range electron-hole separation under the effect of a Coulomb barrier may not necessarily obey the slow, thermally activated dynamics described by the Onsager-Braun model. [8] Indeed, recent time-resolved experiments, for example, for prototypical blends of poly-3-hexylthiophene (P3HT) and the fullerene derivative [6,6]-phenyl-C 61 butyric acid methyl ester (PCBM) and related systems, report ultrafast longrange charge separation, especially in materials exhibiting regioregular morphologies. [9][10][11][12][13][14][15] To explain these ob...

Toward Predicting Efficiency of Organic Solar Cells via Machine Learning and Improved Descriptors

“…[23,24] Therefore, the accurate simulating of OPVs requires high-level theoretical methods in quantum chemistry, quantum dynamics and statistical mechanics, and in recent years there have been substantial progress for the theoretical understanding of many microscopic processes such as charge transport, [25] exciton dissociation, [26,27] singlet fission, [28][29][30] etc. [23,24] Therefore, the accurate simulating of OPVs requires high-level theoretical methods in quantum chemistry, quantum dynamics and statistical mechanics, and in recent years there have been substantial progress for the theoretical understanding of many microscopic processes such as charge transport, [25] exciton dissociation, [26,27] singlet fission, [28][29][30] etc.…”

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include the strong electron-electron interactions and strong electron-phonon couplings as well as the complicated donor/ acceptor (D/A) interface morphology, which are fundamentally different from inorganic semiconductors. [23,24] Therefore, the accurate simulating of OPVs requires high-level theoretical methods in quantum chemistry, quantum dynamics and statistical mechanics, and in recent years there have been substantial progress for the theoretical understanding of many microscopic processes such as charge transport, [25] exciton dissociation, [26,27] singlet fission, [28][29][30] etc. These methods are useful for few benchmark systems but cannot be used to explore the chemical space, i.e., screen a large number of candidate materials.The predictive power of a theoretical model by high-throughput virtual screening has been explored in the recent years.

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Non-Markovian Quantum-Classical Ratchet for Ultrafast Long-Range Electron-Hole Separation in Condensed Phases

“…The time evolution of the reduced density matrix ρ mn (t) = m|ρ(t)|n for the above Hamiltonians and spectral density can be solved in a numerically accurate fashion [24] through the use of the so-called hierarchical equations of motion approach [27]. This approach adequately describes the small polaron formation process with finite timescales, which has not been well captured in the approach using the multiconfiguration time-dependent Hartree method [14,28,29]. For numerical calculations, the Coulomb binding energy is fixed so that e 2 /4πε 0 ε r = 0.30 eV.…”

Vibronic coupling models for donor-acceptor aggregates using an effective-mode scheme: Application to mixed Frenkel and charge-transfer excitons in oligothiophene aggregates

“…Over the past few years, it has become increasingly clear that electron-phonon coupling plays a key role in the photophysics of semiconducting polymers, 16,17 and conventional rate theories turn out insufficient to describe the primary electronic conversion and transport steps. 18 Hence, a quantum dynamical treatment is in order, comprising a large number of electronic states and vibrational modes.…”