Two-stage ultraviolet degradation of perovskite solar cells induced by the oxygen vacancy-Ti4+ states

Summary 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+-VO) transform into active Ti4+-VO trap states under UV excitation and cause photocarrier loss. Furthermore, Ti4+-VO states can convert back into Ti3+-VO states through oxidizing I-, which result in the accumulation of I3-. Sequentially, the rapid decomposition of perovskite accelerated by increasing I3- replace 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+-VO states, which can be realized by Polyethyleneimine ethoxylated (PEIE) modified layer. The optimized devices remain ∼75% of its initial efficiency (20.51%) under UV irradiation at 72 days, while the normal devices fail completely.