Charge-selective-contact-dependent halide phase segregation in CsPbIBr2 perovskite solar cells and its correlation to device degradation

• Halide phase segregation of CsPbIBr 2 films on TiO 2 ETL and NiO x HTL is investigated. • Halide phase segregation is reversible for CsPbIBr 2 film on TiO 2 ETL, while it irreversible for the one on NiO x HTL. • Irreversible halide phase segregation induces inferior performance and stability of CsPbIBr 2 PSCs. • The obstacles caused by halide phase segregation could be overcome by optimizing charge transport layers. All-inorganic perovskite CsPbIBr 2 has emerged as one of promising candidates for perovskite solar cells (PSCs) due to its feasible thermal stability and relatively narrow bandgap. However, the current CsPbIBr 2 PSCs are generally faced with serious halide phase segregation issues. Herein, halide phase segregation phenomenon of CsPbIBr 2 films grown on the typical electron transporting layer (ETL) and hole transporting layer (HTL) of CsPbIBr 2 PSCs are studied. We show that such a phenomenon is reversible and can be fully recovered for CsPbIBr 2 film grown on TiO 2 ETL. However, a plenty of iodide ions are captured by surficial oxygen-vacancy defects of NiO x HTL, which hinders the recovery of halide phase segregation of CsPbIBr 2 film on it. Thus, halide phase segregation has a margin influence on photoelectric conversion properties and stability of CsPbIBr 2 film on TiO 2 ETL, while it significantly damages the photoelectric conversion characters and stability of CsPbIBr 2 film on NiO x HTL. Our work indicates that the obstacles caused by halide phase segregation of CsPbIBr 2 films could be overcome by optimizing the charge transporting layer, and it suggests a promising strategy to further improve the performance and stability of CsPbIBr 2 PSCs.