Charge Generation in Organic Solar Cells: Interplay of Quantum Dynamics, Decoherence, and Recombination

The dynamics of photoinduced charge generation is studied for donor acceptor (D-A) organic interfaces, with focus on the interplay of quantum dynamics, decoherence effects, and recombination. A coarse-grained molecular envelope function model is developed to enable the investigation of large scale D-A heterojunctions, taking into account morphology and molecular orientation as well as the underlying quantum nature of the system. Simulations show that, upon photoexcitation, Frenkel excitons delocalize over several molecules in <300 fs. At the interface, they dissociate without dwelling in intermediate charge transfer states, evincing that exciton motion and dissociation cannot be describe by point particle models. Moreover, as decoherence suppresses the excitonic quantum coherence length, it also decreases the geminate recombination rate. Although ultrafast coherent charge separation is more efficient at early times and, particularly, for excitons created at the interface, diffusion becomes important for excitons created far away from the D-A interface. In this case, decoherence provides a slower but steadier diffusion migration that protects the exciton from geminate recombination. We discuss the balance between charge dissociation and transport in OPV devices and photosynthesis.