Impact of electrostatic interaction on bulk morphology in efficient donor-acceptor photovoltaic blends.

Bulk heterojunctions comprising mixed donor (D) and acceptor (A) materials have proven to be the most efficient device structures for organic photovoltaic (OPV) cells. The bulk morphology of such cells plays a critical role in charge generation, recombination, and transport, thereby determining the device performance. Although numerous studies have discussed the morphology-performance relationship of these cells, the method of designing OPV materials with the desired morphology remains unclear. In this work, guided by molecular electrostatic potential distributions, we have established a connection between the chemical structure and bulk morphology. Our findings demonstrate that the molecular orientation at the D-A interface and the domain purity in the blend can be effectively modulated by modifying the functional groups. In addition, enhancing the D-A interaction is beneficial for charge generation. However, the resulting low domain purity and increased charge transfer ratio in its hybridization with the local excitation states lead to severe charge recombination. Our work proposes a molecular design strategy that can finely-tune the bulk morphology to achieve balanced charge generation and recombination, which is crucial for further boosting the efficiency of the OPV cells.