Synergistic Enhancement of Efficiency and Mechanical Robustness in Flexible Organic Solar Cells via Solvent‐Assisted Aggregation Reconstitution of the Buried Donor Layer

Abstract The advancement of flexible organic solar cells (f‐OSCs) has long‐term been hindered by an intrinsic trade‐off between power conversion efficiency (PCE) and mechanical robustness, which is attributed to the severe aggregation of the active layer with significant mechanical fragility. Via combining layer‐by‐layer (LBL) fabrication process and solvent‐assisted aggregation reconstitution (SAAR) approach, 2‐bromothiophene (2Br‐Th) and bromobenzene (BrB) are systematically compared as processing solvents for regulating D18 donor layer. Owing to the asymmetric configuration with differentiated electrostatic potential distribution, 2Br‐Th demonstrates a strengthened electrostatic interaction with D18, which coherently suppresses excessive self‐assembly while maintaining an ordered molecular framework critical for efficient charge transport. More importantly, the improved dispersibility of the D18 donor layer after SAAR treatment greatly facilitates the interdiffusion of L8‐BO molecules into the donor matrix, which alleviates local stress concentration during deformation and simultaneously enhances exciton dissociation and charge transport dynamics. As a result, the optimized D18:L8‐BO achieved PCEs of 20.0% in rigid and 18.3% in flexible devices, along with a high crack‐onset strain (COS) value of 13.2% with enhanced mechanical stability. To the best of the knowledge, this represents the first demonstration of f‐OSCs concurrently achieving both high efficiency (PCE > 18%) and mechanical resilience (COS > 10%).