Achieving Uniform Phase Structure for Layer‐by‐Layer Processed Binary Organic Solar Cells with 20.2% Efficiency

Abstract Layer‐by‐layer (LBL) deposition has become a facile and promising method to fabricate highly efficient organic solar cells (OSCs). However, characterization and optimization of 3D morphology remain a grand challenge for LBL‐processed active layers, and their correlation with photovoltaic properties of OSC devices is not clear to date. Here, to address this issue, the morphology and its formation mechanisms of LBL‐processed active layer based on the classical D18/L8‐BO blend were investigated systematically. Intriguingly, a unique 3D nanomorphology is achieved and uncovered within the LBL processed active layer, which highlights a highly uniform and “zigzag”‐shaped phase structure formed by the intersection of donor and acceptor aggregates along horizontal direction in the middle‐depth layer, rarely found in bulk heterojunction (BHJ) films processed by blend casting. Our results revealed that solid additive DBM with a twisted conformation plays a crucial role in achieving the uniform phase structure of LBL‐deposited active layer. Consequently, the characteristic 3D morphology of LBL‐processed device significantly improves short‐circuit current and fill factor, enabling an impressive PCE of 20.2% in such binary device. Therefore, this work unambiguously demonstrates a unique 3D nanomorphology within LBL active layer, unveils the morphology manipulation mechanism, and their correlation with optoelectronic properties of OSCs.