Highly Efficient and Robust Ternary All‐Polymer Solar Cells Achieved by Electro‐Active Polymer Compatibilizers

All‐polymer solar cells (all‐PSCs), using polymerized non‐fullerene acceptors (PNFAs), have shown promise in improving device stabilities compared to small‐molecular acceptor‐based PSCs. However, low mixing entropy between polymer donors ( P D s) and PNFAs hampers the development of optimized blend morphology. Herein, this study develops efficient conjugated polymers that serve as interfacial compatibilizers between host P D and PNFA. Ternary all‐polymer blends containing the compatibilizer demonstrate improved blend morphology with strengthened interfaces, resulting in better photovoltaic properties and thermal/mechanical stabilities. In detail, the power conversion efficiency (PCE) increases from 15.4 to 17.1% upon the addition of the compatibilizer. Moreover, the devices based on the ternary blend enable good thermal stability, retaining 90% of the initial PCE after 96 h at 125 °C. Additionally, the mechanical properties are improved; the cohesive fracture energy ( G c ) of 2.6 J m −2 and crack onset strain (COS) of 20.4% of the ternary blend outperform those of the binary blend ( G c = 1.1 J m −2 and COS = 16.5%). Resultingly, the stretchable PSCs based on the ternary blend exhibit an excellent PCE of 13.7% and stretchability with a strain at PCE 80% of 35%.