Although the morphological instability of the bulk-heterojunction (BHJ) organic solar cells (OSCs) based on a small-molecule nonfullerene acceptor (SM-NFA) is regarded as an important issue for understanding thermal stability, interfacial deterioration between the chemically vulnerable SM-NFAs and an interfacial layer containing reactive species is also crucial. Herein, we identified that chemical degradation of SM-NFA on a zinc oxide (ZnO) interfacial layer under thermal stress is more critical by systematically examining three SM-NFAs that have different thermal transition temperature (T_g) values; as T_g decreased, the interfacial degradation of SM-NFAs increased. However, the introduction of a highly polar and volatile molecule, 5-methyl-1H-benzotriazole (M-BT), into the BHJ effectively passivates the defects of the ZnO surface by forming a self-assembled layer. The optimized target device exhibited excellent long-term thermal stability that retains above 85% of the initial efficiency after 1000 h (=T₈₅) at 85 °C under a N₂ atmosphere, while a short T₈₅ within 20 h was obtained in the control device.