“1+2” Alloy‐Like Strategy: Restricting Molecular Diffusion Enables Highly Thermally‐Stable and Efficient Organic Solar Cells

Abstract Achieving concurrent enhancement in both power conversion efficiency (PCE) and long‐term durability of organic solar cells (OSCs) is challenging for their commercial applications. Despite the critical roles of small molecule acceptors in achieving high photovoltaic performance, they suffer from inherent diffusion tendencies under operational conditions due to weak intermolecular interaction and small‐sized structure, commonly leading to significant device degradation. Herein, a “1+2” alloy‐like strategy consisting of monomer L8‐BO and dimer D‐Y3F in the ternary active blend is developed, which effectively realizes molecular diffusion restriction and morphology stabilization while achieving a remarkable PCE of 19.70%. The PM6:L8‐BO:D‐Y3F system demonstrates exceptional thermal stability, retaining over 85% of its initial PCE after 200 h of thermal aging at 85 °C. This alloy‐like approach synergistically enhances device performance and stability through multiple mechanisms: 1) controlling pre‐aggregation behavior and optimizing the nano‐micromorphology to promote exciton dissociation and charge transport; 2) suppressing severe molecular diffusion by D‐Y3F‐induced robust network with enhanced π–π interaction and molecular entanglement in the alloy‐like composite; 3) regulating relatively higher glass transition temperature to overcome the diffusion‐driven morphological evolution such as aggregation and phase separation. The work provides a promising approach for realizing high‐performance OSCs while effectively mitigating diffusion‐induced device degradation.