Discerning morphological evolution under thermal stress in polymerized small molecular acceptor-based all polymer solar cells

Despite significant advances in the power conversion efficiency (PCE) of polymerized small molecular acceptor (PSMA)-based all-polymer solar cells (all-PSCs), morphological evolution within the bulk heterojunction under prolonged thermal stress remains a key challenge to achieving robust device performance. In this study, we systematically investigate the thermal degradation pathways of PSMA-based all-PSCs by employing five representative polymer donors (PDs) alongside the prototypical PSMA, PY-IT. The resulting PY-IT-based all-PSC model systems demonstrate relatively high thermal stability, retaining approximately 90% of their initial PCE after thermal aging of the active layers at 80 °C for 250 hours. However, a notable decline in photocurrent and fill factor remains unavoidable. To elucidate the underlying causes of this efficiency degradation, we analyze specific morphological changes in both the PDs and PSMA during thermal aging, focusing on features such as molecular aggregation, fibril width, crystallinity, and vertical phase distribution. Our findings reveal that severe thermal disaggregation of polymer donors is the primary driving force behind vertical phase imbalance, which plays a critical role in the performance loss of PSMA-based all-PSCs under thermal stress. This study provides valuable insights into the internal morphological evolution governing long-term thermal stability and highlights the importance of designing polymer donors with improved resistance to thermal disaggregation for the development of efficient and stable all-PSCs.