The role of interface between electron transport layer and perovskite in halogen migration and stabilizing perovskite solar cells with Cs₄SnO₄

Although perovskite solar cells exhibit excellent power conversion efficiency, the poor stability toward electrical fields and the typically observed device hysteresis phenomenon severely restrict their applications. Ionic migration comes from the migration of halogen from lattice points to halogen vacancies. Considering the strong built-in potential located at the interface between perovskite and carrier transport layers, here we propose that it is the ionic migration at the interfacial depletion region that dominates device hysteresis. The most effective way to achieve hysteresis-free devices is to passivate ionic migration at the interface rather than within perovskite films. We utilized Cs₄SnO₄ as a modification layer, which could effectively suppress the defects at the interface and thus eliminate hysteresis, as well as shift the conduction band of perovskite/SnO₂ closer and decrease the band misalignment. In addition, the partial diffusion of Cs⁺ into (FAPbI₃)_0.85(MAPbBr₃)_0.15 is beneficial for phase stability toward long-term light illumination. Hence, we believe this paper provides a new understanding about the ionic migration process within perovskite solar cells, and Cs₄SnO₄ modification is an effective and also general method to overcome the device hysteresis issue. © The Royal Society of Chemistry 2018