A MM/QM Approach to Simulating Electron/Hole Separation Kinetics of Organic Polymers Bulk-Heterojunctions

We investigate the electronic dynamics of model organic photovoltaic (OPV) system consisting of polyphenylene vinylene (PPV) oligomers and [6,6]-phenyl C61-butyric acid methylester (PCBM) blend using a mixed molecular mechanics/quantum mechanics (MM/QM) approach. The π-MDX code is introduced and a methodology that allows the quantum treatment of the π systems of large clusters of molecules near the interface is developed. Using a heuristic model that connects energy gap fluctuations to the average electronic couplings and decoherence times, we provide an estimate of the state-to-state internal conversion rates within the manifold of the lowest few electronic excitations. We find that the lowest few excited states of a model interface are rapidly mixed by C=C bond fluctuations and low frequency torsional modes such that the system can sample both intermolecular charge-transfer and charge-separated electronic configurations on a time scale of 20fs. We show that the electronic dynamics of the OPV are dramatically altered by varying the positions of the molecules simulated at the interface. 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.