Mitigating Exciton Recombination Losses in Organic Solar Cells by Engineering Nonfullerene Molecular Crystallization Behavior

Although the advances in organic solar cells (OSCs) have been considerable, their efficiency is still limited by recombination losses. Photogenerated electrons and holes are generally bound as localized excitons in organic semiconductors. The transition from excitons into free charges requires diffusion and dissociation processes, in which parasitic recombination losses exist. Reducing these losses is necessary for highly efficient OSCs. The crystallization behavior of the active layers can influence the efficiency of exciton diffusion and dissociation. In this work, different additives are delicately designed to control the crystallization behavior. It is found that the crystallization quality of active layers can be improved by controlling the aggregation of nonfullerene acceptors. The π–π stacking of blend films becomes compact, meanwhile, the crystallization in the vertical direction is more uniform. These are beneficial to the diffusion and dissociation of excitons. As a consequence, recombination losses are reduced and power convention efficiencies (PCEs) are improved significantly. Meanwhile, the general applicability of the additive is demonstrated in various organic photovoltaic systems, in which a PCE of 19.3% is achieved in D18:BTP-eC9-4F OSCs. This work provides a facile strategy to reduce the recombination losses of excitons for efficient devices.