Deciphering the Role of Side‐Chain Engineering and Solvent Vapor Annealing for Binary All‐Small‐Molecule Organic Solar Cells

Abstract Fibrous interpenetrating network structure morphology is extremely crucial for all‐small‐molecule organic solar cells (ASM‐OSCs) in achieving high power conversion efficiency (PCE). Rational molecular design and suitable posttreatment to the film are feasible methods to accomplish this goal. Herein, two small molecule donors, namely T4 and T6, with different substituents on their selenophene conjugated units, alkyl for T4 while trialkylsilyl for T6, are developed. Both as cast devices obtain poor PCEs (≈4.5%) when blending these two donors with N3 due to the oversize phase separation. Satisfactorily, the PCEs are dramatically increased after CS₂ annealing, which mainly originates from the favorable reorganization of donor and acceptor in the active layer, ultimately improving the phase separation and vertical electronic properties. As a result, the device based on trialkylsilyl‐substituted T6 acquires a remarkable PCE of 16.03%, much higher than that of the blends of alkyl‐substituted T4 and N3 (12.61%). The enhanced PCE of the T6‐based device is attributed to the deeper HOMO energy levels, more obvious fibrous interpenetrating networks, and stronger molecular interaction between T6 and N3, as compared with T4‐based ones. This study indicates that precise molecular design and the proper posttreatment process can be a brilliant approach for realizing highly efficient ASM‐OSCs.