Achieving High‐Efficiency and Thick‐Film‐Insensitive Organic Solar Cells with a Functional Aid ^†

Comprehensive Summary Presently, few methods offer broad applicability for optimizing thick‐film organic solar cells (OSCs). Here, we identify and implement a functional aid, 4‐iodobiphenyl (IBP), in OSCs to optimize active layer morphology and significantly boost device efficiency. In the PM6:A4T‐16 system, the optimized volatility of IBP promotes acceptor recrystallization and ordered molecular alignment during thermal annealing, leading to the formation of large aggregates and a fibrous morphology that facilitates efficient charge transport. As a result, the power conversion efficiency (PCE) of IBP‐treated devices increases by 20% compared with those using conventional additives such as DIO and DIB. Notably, IBP exhibits broad compatibility, delivering substantial efficiency enhancements in PM6:BTP‐eC9 and PM6:D18:L8‐BO systems, with the latter reaching a high PCE of 20%. Furthermore, IBP enables thickness‐insensitive performance, maintaining a PCE of ~17% at an active layer thickness exceeding 360 nm. These results demonstrate IBP as a highly effective processing aid for multiple donor:acceptor pairs, offering a promising strategy for morphology engineering and advancing the fabrication of high‐performance OSCs.