Revealing the Thermal‐Degradation Pathways of Organic Solar Cells Incorporating Elastomers With Varying Hard Segments

ABSTRACT Although the incorporation of thermoplastic elastomers markedly improves the stretchability of organic solar cells (OSCs), thermal instability remains a major limitation for their practical application. Here, we investigate how the hard‐segment content of styrene‐ethylene‐butylene‐styrene (SEBS) elastomers influences the thermal stability of OSCs based on PM6:L8‐BO blends. Incorporating SEBS into the active layer leads to two distinct morphological evolution pathways during thermal aging. SEBS with a low proportion of hard segments obviously loses modulus under thermal stress, promoting aggregation and crystallite growth of non‐fullerene acceptors in the active layer blends. Consequently, dynamic energetic disorder increases, charge mobility decreases, and device performance rapidly deteriorates. In contrast, active layer blends with SEBS containing a higher proportion of styrene segments are more resistant to thermally induced morphological evolution and enable much better thermal stability at elevated temperatures. This work clarifies the role of elastomer in the thermal stability of OSCs, highlights the critical trade‐off in elastomer selection between mechanical stretchability and thermal stability, and provides important design guidelines for developing intrinsically stretchable OSCs with balanced mechanical compliance and long‐term thermal stability.