BELECTRIC (Germany)

Light induced fullerene dimerization is controlled by both the fullerene and polymer morphology of organic solar cells.

ABSTRACTInterface physics is at the heart of organic photovoltaics (OPV). Here we reveal for the first time the actual charge generation mechanism in a low-band-gap polymer: fullerene blend as prototypical system for efficient OPV. We demonstrate that the photogenerated excitons dissociate into bound interfacial charge transfer states (CTS) and free charges in 20-50 fs, with a branching ratio that depends on the excess energy. Providing an excess energy, high energy singlet polymer states are excited, giving a direct hot electron transfer into the interfacial hot CTS* before internal energy dissipation occurs. This process ultimately leads to a higher fraction of free charges. Thanks to strong electronic coupling between high-energy-states and hot CTS, we demonstrate the opening of additional paths for charge generation that would otherwise be quenched by internal conversion to the lowest-lying states. Our results provide a new framework to understand charge generation in OPV system, suggesting that hot dissociation is a strategic option to enhance the photovoltaic conversion.