Sequential Crystallization Engineering via a Dual‐Functional Thiophene Derivative Boosts Organic Solar Cell Efficiency to 20.5%

Abstract Controlling film‐formation kinetics is pivotal for optimizing active‐layer morphology and efficiency in bulk heterojunction organic solar cells (BHJ‐OSCs), yet sequential crystallization of donor and acceptor materials remains challenging due to their intertwined dynamics. Herein, a novel thiophene derivative, 1,2‐di([2,3′‐bithiophen]‐2′‐yl)ethyne (DBTE) is designed and synthesized to modulate crystallization dynamics during film formation. In situ spectroscopy reveals that DBTE exerts a dual role as a nucleator to promote donor aggregation while acting as a plasticizer to delay acceptor aggregation, which disrupts co‐crystallization in PM6:Y6 blends, triggering preferential ordering of donor (PM6) followed by retarded acceptor (Y6) growth. This kinetic decoupling mechanism significantly enhances the ordered molecular packing in the donor phase and establishes a favorable vertical composition gradient, collectively improving exciton dissociation and charge transport, leading to an impressive improvement of power conversion efficiency (PCE) from 17.6% to 18.9%. Notably, the strategy is further validated in the D18:L8‐BO system, yielding an outstanding PCE of 20.5% (certified as 20.2%). This work demonstrates that designing dual‐functional modulators to rationally control crystallization kinetics can effectively optimize blend morphology, providing a novel strategy for advancing the performance of high‐efficiency BHJ‐OSCs.