Achieving High‐Performance Organic Solar Cells by Tailoring Polymer Packing Behavior and Crystallinity

Comprehensive Summary This study successfully designed and synthesized two wide‐bandgap donor‐acceptor (D‐A) copolymers, XY‐1 and XY‐2 with newly synthesized, low‐cost ester‐substituted benzotrithiophene as the acceptor unit, difluoro‐substituted thiophene or bithiophene as the donor unit, respectively. XY‐1 exhibits more rotational sites and poorer molecular planarity compared to XY‐2, allowing the polymer backbone to rotate to some extent. These molecular properties resulted in not only appropriate phase separation size during the blending film formation process, but also optimal crystallinity, favorable molecular orientation, and excellent transport properties. In contrast, XY‐2 exhibits conformational rigidity stemming from intramolecular F···S noncovalent interactions, resulting in detrimental phase segregation and compromised charge transport efficiency. Consequently, XY‐1:L8‐BO‐based organic solar cells (OSCs) achieved an outstanding power conversion efficiency of 16.04 %, outperforming the XY‐2‐based OSCs. This work emphasizes the significance of regulating the stacking orientation and crystallinity of polymers to improve device performance, thereby offering valuable guidance for the future development of organic photovoltaics.