Regulating Donor Aggregation Structures via Main‐Chain Engineering to Enable High‐Performance and Mechanically Stable All‐Polymer Solar Cells

ABSTRACT Controlling the aggregation behavior of conjugated polymers is crucial for optimizing the morphology and performance of all‐polymer solar cells (all‐PSCs), yet achieving precise regulation remains a formidable challenge due to the complex self‐assembly of polymers in solution. Herein, we report a series of bithiophene imide‐based terpolymer donors (OH1 to OH4) with systematically increased thiophene π‐bridge content to modulate solution‐state aggregation. Increasing the π‐bridge content progressively enhances aggregation, leading to improved molecular ordering and a favorable fibrous network morphology in blend films. The optimized OH4:PY‐IT blend achieves a remarkable power conversion efficiency (PCE) of 19.25% in binary all‐PSCs—among the highest reported to date. Moreover, both intrinsically stretchable and flexible devices based on OH4:PY‐IT exhibit superior mechanical durability, retaining 85.3% of initial PCE after 200 deformation cycles under 10% strain and 80.1% after 7000 bending cycles, while achieving high PCEs of 14.47% and 15.63%, respectively. This work establishes π‐bridge main chain engineering as a powerful strategy to control polymer aggregation and morphology, providing key insights for developing efficient and mechanically robust all‐PSCs.