Critical length screening enables 19% efficiency in thick-film organic solar cells.

The commercialization of organic solar cells (OSCs) requires thick-film active layers, yet current thick-film-compatible acceptor selection based on zero-field mobility is unreliable due to methodological inconsistencies in experimental protocols, fitting models, and single-carrier device configurations. Existing literature indicates that the zero-field mobility in high-performance thick-film devices shows negligible differences compared to thin-film counterparts, thereby invalidating its significance as a selection criterion. This study introduces a protocol identifying critical length - an intrinsic property distinct from zero-field mobility - as the decisive factor for thick-film OSC performance. Comparative studies reveal that enlarged acceptor domains with high critical length yield increased hopping frequency, improved charge mobility and reduced field-dependent, collectively enhancing performance. Applying this criterion, we identify BTP-eC9 as a general acceptor, achieving 19.0% efficiency in thick-film D18:L8-BO:BTP-eC9 OSCs. This work not only demonstrates the fabrication of high-performance thick-film OSCs, but fundamentally advances material screening methodology specifically tailored for thick-film-compatible organic semiconductors.