Vibration-mediated coherent mixing of exciton and polaron pair in a conjugated polymer

Organic solar cells have attracted considerable interest in recent years due to their potential as efficient and low-cost solar energy converters. For various conjugated polymers in solar cell applications, experimental studies have reported ultrafast charge dissociation of excitons at the interfaces between polymers and fullerenes on a sub-100 fs timescale [1]. It was suggested that vibronic coupling plays a central role in the excitonic dynamics of light harvesting [2], while time-dependent density functional theory suggested that strong electronic and vibronic couplings of organic photovoltaics may support ultrafast charge separation dynamics [3]. However, the functional relevance of the coherent electronic and vibronic couplings under noise and disorder and their experimental verification remain open questions. Here we theoretically investigate the polaron pair formation in a reference conjugated polymer for solar cell applications by means of 2D electronic spectroscopy. We show that experimentally observed polaron pair formation on a sub-100 fs timescale is governed by coherent electronic and vibronic couplings even in the presence of a high noise level at room temperature and a large energetic disorder of the conjugated polymer. We show that in contrast to an incoherent model, a coherent, vibronic model can reproduce the main features of experimental findings. Based on model parameters estimated from experimental data, we show that non-equilibrium vibrational motions of the C=C stretch mode of the conjugated polymer underpin ultrafast polaron pair formation and that this mechanism is robust against noise and disorder due to the strong electronic coupling and large Huang-Rhys factors of the present system.