Strategic inner/outer side-chain tuning for high-efficiency green-solvent-processed organic solar cells

Though the rapid development of halogen-free solvent processed organic solar cells (OSCs) has been enabled by side chain modification on small molecular acceptors, the structure-property relationship on inner/outer chain lengths and device performance is unclear, which could inhibit further progress of this technology. In this study, five non-fullerene acceptors (NFAs) with different combinations of side chains are investigated through systematic variation of side-chain positions and architectures. The effects of inner versus outer side-chain modifications are clarified by a comprehensive study on energy level distribution, film morphology, and carrier dynamics. Notably, longer alkyl chains were not always superior, as excessive solubility was found to reduce molecular packing order. As a result, a power conversion efficiency (PCE) of 18.2% is achieved by PM6:BTP-TO12 blend. Furthermore, the optimized ternary OSCs incorporating BTP-TO12 achieved a remarkable PCE of 19.5%. The incorporation of BTP-TO12 as a guest material enhances the performance of L8-BO-based devices processed with green solvents, which represents state-of-the-art performance in the field. This improvement is attributed to the low energy loss and well-controlled aggregation behavior of BTP-TO12 in environmentally friendly processing solvents (toluene).