  Modeling the charge-generation process is highly important to understand
device physics and optimize power conversion efficiency of bulk-heterojunction
(BHJ) organic solar cells (OSCs). Free carriers are generated by both ultrafast
exciton delocalization and slow exciton diffusion and dissociation at the
heterojunction interface. In this work, we developed a systematic numerical
simulation to describe the charge-generation process by a modified
drift-diffusion model. The transport, recombination, and collection of free
carriers are incorporated to fully capture the device response. The theoretical
results match well with the state-of-the-art high-performance organic solar
cells. It is demonstrated that the increase of exciton delocalization ratio
reduces the energy loss in the exciton diffusion-dissociation process, and
thus, significantly improves the device efficiency especially for the
short-circuit current. By changing the exciton delocalization ratio, OSC
performances are comprehensively investigated under the conditions of
short-circuit and open-circuit. Particularly, bulk recombination dependent fill
factor saturation is unveiled and understood. As a fundamental electrical
analysis of the delocalization mechanism, our work is important to understand
and optimize the high-performance OSCs.
