Mixed-Isomers Strategy for Thermally Stable and High-Performance Thick-Film All-Small-Molecule Organic Solar Cells

All-small-molecule organic solar cells (all-SMOSCs) have emerged as promising candidates for next-generation photovoltaic technologies due to their advantages in chemical tunability, purification ease, and batch-to-batch consistency. Despite significant progress, their power conversion efficiencies (PCEs) still trail those of polymer-based solar cells (PSCs), primarily due to suboptimal charge management and morphology control. By introducing two structurally similar yet functionally complementary isomers, BTP-Br-γ and BTP-Br-δ, into a high-performing host system (MPhS-C2:BTP-eC9), herein we construct quaternary blends that exhibit superior morphological robustness, efficient charge transport, and suppressed energy loss. The optimized quaternary device delivers a record PCE of 19.06% (certified at 18.7%), representing the highest value reported for all-SMOSCs to date. Further investigations reveal that the quaternary devices also exhibit high thickness tolerance to device efficiency and thermal stability, owing to fast exciton dissociation, long carrier diffusion length, low recombination loss, and rapid charge extraction. The work offers a promising strategy to bridge the performance gap between all-SMOSCs and PSCs, paving the way for their practical deployment in high-efficiency organic photovoltaics.