Quantum Simulations of Charge Separation at a Model Donor-Acceptor Interface: Role of Delocalization and Local Packing

Abstract

We investigate the electronic dynamics of a model organic photovoltaic (OPV) system consisting of polyphenylene vinylene (PPV) oligomers and a [6,6]-phenyl C61-butyric acid methylester (PCBM) blend using a mixed molecular mechanics/quantum mechanics (MM/QM) approach. Using a heuristic model that connects energy gap fluctuations to the average electronic couplings and decoherence times, we provide and estimate of the state-to-state internal conversion rates within the manifold of the lowest few electronic excitations. We show that the electronic dynamics of the OPV are dramatically altered by varying the positions of the molecules simulated at the interface. The lowest few excited states of the model interface rapidly mix allowing low frequency C-C out of plain torsions to modulate the potential energy surface such that the system can sample both intermolecular charge-transfer and charge-separated electronic configurations on sub 100 fs time scales. Our simulations support an emerging picture of carrier generation in OPV systems in which interfacial electronic states can rapidly decay into charge-separated and current producing states via coupling to vibronic degrees of freedom.

Mixed Quantum Classical Simulations of Charge-Transfer Dynamics in a Model Light-Harvesting Complex. II. Transient Vibrational Analysis

“…A growing number of studies recognize vibrational dynamics as a key controlling factor in the electron transfer process of molecular systems. − Identifying the exact nature of the electron–nuclear motion is no trivial feat. In condensed phases, vibrational specificity is rapidly lost due to relaxation into intermolecular modes. , In previous works, our heuristic approach indicated that pervasive low energy modes (often of energies ≈ kT ) modulate electronic states at donor/acceptor heterojunction interfaces. , Response to photoexcitation and thermal fluctuation changes low-lying electronic excited states between localized excitonic to charge separated in character.…”

“…8,9 In previous works, our heuristic approach indicated that pervasive low energy modes (often of energies ≈ kT) modulate electronic states at donor/acceptor heterojunction interfaces. 10,11 Response to photoexcitation and thermal fluctuation changes low-lying electronic excited states between localized excitonic to charge separated in character.…”