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 key role in charge generation, recombination, and transport, thus 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. Herein, guided by molecular electrostatic potential distributions, we have established a connection between the chemical structure and bulk morphology. We show 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. 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. Fine-tuning the bulk morphology can give balanced charge generation and recombination, which is crucial for further boosting the efficiency of the OPV cells.
Keywords: bulk morphology; domain purity; electrostatic interaction; non-radiative energy loss; organic photovoltaic cells.
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