Exciton-Dissociation and Charge-Recombination Processes in Pentacene/C60 Solar Cells: Theoretical Insight into the Impact of Interface Geometry

The exciton-dissociation and charge-recombination processes in organic solar cells based on pentacene/C-60 heterojunctions are investigated by means of quantum-mechanical calculations. The electronic couplings and the rates of exciton dissociation and charge recombination have been evaluated for several geometrical configurations of the pentacene/C-60 complex, which are relevant to bilayer and bulk heterojunctions. The results suggest that, irrespective of the actual pentacene-fullerene orientation, both pentacene-based and C-60-based excitons are able to dissociate efficiently. Also, in the case of parallel configurations of the molecules at the pentacene/C-60 interface, the decay of the lowest charge-transfer state to the ground state is calculated to be very fast; as a result, it can compete with the dissociation process into mobile charge carriers. Since parallel configurations are expected to be found more frequently in bulk heterojunctions than in bilayer heterojunctions, the performance of pentacene/C-60 bulk-heterojunction solar cells is likely to be more affected by charge recombination than that of bilayer devices.