Two-Stage Ultraviolet Degradation of Perovskite Solar Cells Induced by the Oxygen Vacancy-Ti4+ States

The failure of perovskite solar cells (PSCs) under ultraviolet (UV) irradiation is a serious barrier of commercial utilization. Here, a two-stage degradation process of TiO2-based PSCs is discovered under continuous UV irradiation in an inert atmosphere. In the first decay stage, oxygen vacancy-Ti3+ (Ti3+-V-O) transform into active Ti4+-V-O trap states under UV excitation and cause photocarrier loss. Furthermore, Ti4+-V-O states can convert back into Ti3+-V-O states through oxidizing I-, which result in the accumulation of I-3(-). Sequentially, the rapid decomposition of perovskite accelerated by increasing I-3(-) replaces the photocarrier loss as the dominant mechanism leading to the second decay stage. Then, a universal method is proposed to improve the UV stability by blocking the transformation of Ti3+-V-O states, which can be realized by polyethyleneimine ethoxylated (PEIE) modified layer. The optimized devices remain similar to 75% of its initial efficiency (20.51%) under UV irradiation at 72 days, whereas the normal devices fail completely.