Optimized Crystal Framework by Asymmetric Core Isomerization in Selenium‐Substituted Acceptor for Efficient Binary Organic Solar Cells

Abstract Both the regional isomerization and selenium‐substitution of the small molecular acceptors (SMAs) play significant roles in developing efficient organic solar cells (OSCs), while their synergistic effects remain elusive. Herein, we developed three isomeric SMAs (S‐ C SeF, A‐ I SeF, and A‐ O SeF) via subtly manipulating the mono‐selenium substituted position (central, inner, or outer) and type of heteroaromatic ring on the central core by synergistic strategies for efficient OSCs, respectively. Crystallography of asymmetric A‐ O SeF presents a closer intermolecular π–π stacking and more ordered 3‐dimensional network packing and efficient charge‐hopping pathways. With the successive out‐shift of the mono‐selenium substituted position, the neat films give a slightly wider band gap and gradually higher crystallinity and electron mobility. The PM1 : A‐ O SeF afford favourable fibrous phase separation morphology with more ordered molecular packing and efficient charge transportation compared to the other two counterparts. Consequently, the A‐ O SeF‐based devices achieve a champion efficiency of 18.5 %, which represents the record value for the reported selenium‐containing SMAs in binary OSCs. Our developed precise molecular engineering of the position and type of selenium‐based heteroaromatic ring of SMAs provides a promising synergistic approach to optimizing crystal stacking and boosting top‐ranked selenium‐containing SMAs‐based OSCs.