Kinetic Morphology Evolution of Non-Fullerene Bulk Heterojunction Blends: the Role of Solvent Additives in Thermal Stability

The thermal stability of bulk heterojunction (BHJ) active layers is a critical factor in determining the long-term performance of non-fullerene organic solar cells (OSCs). In this study, the kinetic morphology of PffBT4T-2OD: ITIC BHJs was systematically investigated under varying annealing temperatures. Pristine PffBT4T-2OD films demonstrated excellent thermal stability, while ITIC thin films exhibited diffusive interfaces and morphological changes when annealed above 120°C. For PffBT4T-2OD: ITIC BHJs, elevated temperatures at 120°C induced the migration of ITIC clusters toward the air interface, accompanied by the formation of a PffBT4T-2OD-rich layer near the substrate. To address this thermal instability, we incorporated the 1,8-diiodooctane (DIO) as a solvent additive into the BHJs. The addition of DIO significantly enhanced the intermixing of donor and acceptor materials, resulting in a uniform morphology of the active layer and improved thermal stability up to 120°C. These findings confirm that solvent additives effectively mitigate thermal-induced phase separation and prolong the stability of BHJs.