Selectively Modulating the Donor and Acceptor Aggregation Behaviors Through Solid Additive Isomerization Engineering for Organic Solar Cells Exceeding 20% Efficiency

Regulating the film morphology is essential for achieving high‐performance organic solar cells (OSCs). Given the distinct features of donor and acceptor materials, designing solid additives to selectively control the aggregation behaviors of them represents a key strategy for further development of OSCs. Herein, a series of bithiophene isomers solid additives (2,2‐TT, 2,3‐TT, and 3,3‐TT) are designed for film morphology regulation. Systematic analysis reveals that the addition of 2,2‐TT significantly enhances the π–π stacking of L8‐BO, yet exerts a relatively limited effect on PM6 aggregation. Conversely, the incorporation of 3,3‐TT not only facilitates the formation of compact and highly ordered molecular packing of L8‐BO but also substantially promote the denser π–π stacking of PM6. Consequently, the PM6:L8‐BO‐based OSCs with 3,3‐TT treatment achieve a higher efficiency of 19.34%, benefitting from the balanced charge mobility and longer carrier lifetime. More impressively, the D18:L8‐BO‐based devices treated with 3,3‐TT deliver a remarkable efficiency of 20.32%, with an excellent fill factor of 82.47%, which ranks among the best values reported so far. This work highlights the critical role of the solid additive isomerization strategy in selectively regulating the aggregation characteristics of donor and acceptor materials, thereby facilitating the further advancement of OSCs field.