Strategic Inner/Outer Side-Chain Tuning for High-Efficiency Green-Solvent-Processed Organic Solar Cells
The rapid development of halogen-free solvent-processed organic solar cells (OSCs) has been enabled by side-chain modification on small molecular acceptors, yet the structure-property relationship between inner/outer chain lengths and device performance remains unclear. This study systematically investigates five non-fullerene acceptors (NFAs) with varied side-chain positions and architectures, clarifying the effects of inner versus outer modifications on energy level distribution, film morphology, and carrier dynamics. Notably, longer alkyl chains are not always superior; excessive solubility reduces molecular packing order. The optimized PM6:BTP-TO12 blend achieves a power conversion efficiency (PCE) of 18.2%. Furthermore, ternary OSCs incorporating BTP-TO12 as a guest material reach a remarkable PCE of 19.5%, enhancing the performance of L8-BO-based devices processed with green solvents. This improvement is attributed to the low energy loss and well-controlled aggregation behavior of BTP-TO12 in environmentally friendly toluene. These findings establish a design guideline for side-chain engineering in green-solvent-processed OSCs, achieving state-of-the-art performance and advancing scalable, eco-compatible photovoltaic technologies.