Temperature‐Dependent Aggregation Behavior Regulation for Non‐halogenated Solvent Processed Organic Solar Cells With Approximately 20% Efficiency

ABSTRACT The nanoscale morphology of the active layer constitutes a critical determinant for the performance of organic solar cells (OSCs). This work demonstrates a green solvent‐processing strategy to precisely modulate the temperature‐dependent aggregation behavior of polymeric donors in the D18:L8‐BO system. We systematically investigated thin‐film formation and device characteristics under three distinct processing temperatures (100°C, 130°C, and 150°C). Active layers processed at 100°C exhibited excessive phase separation domains due to detrimental molecular pre‐aggregation, limiting the power conversion efficiency (PCE) to 18.38%. In contrast, processing at 130°C optimally suppressed unfavorable aggregation while promoting the development of well‐balanced fibrillar networks with characteristic 20–40 nm domain sizes, consequently elevating the PCE to 19.31%. Further increasing the temperature to 150°C reduced molecular ordering and degraded the fibrillar morphology, leading to a diminished efficiency of 18.94%. Building upon the fundamental insights derived from this optimized bulk heterojunction (BHJ) morphology, we subsequently constructed planar heterojunction (PHJ) devices via sequential deposition. The resulting distinct vertical phase distribution yielded a champion PCE of 19.92%. This study elucidates the profound influence of thermal processing conditions on the microstructural evolution of active layers, establishing precise temperature control as an industrially compatible yet crucial parameter for achieving high‐performance, green‐solvent‐processed OSCs.