Li dopant induces moisture sensitive phase degradation of an all-inorganic CsPbI2Br perovskite

All-inorganic halide perovskites without volatile components have great potential for long term thermal stability. However, the phase stability of all-inorganic perovskites is sensitive to moisture and has been seldom studied. In this work, the phase stability of alpha-CsPbI2Br was studied in different relative humidity conditions. The moisture resistance of alpha-CsPbI2Br can be significantly enhanced by using a polymer or organic molecule capping layer including Spiro-MeOTAD. Although an all-inorganic CsPbI2Br perovskite based device using the typical Li salt doped Spiro-MeOTAD layer could offer an efficiency up to 12.6%, it exhibited even worse moisture resistance than a bare perovskite film under the same ambient conditions. This unusual phenomenon is ascribed to the hygroscopic properties of Li-TFSI inducing moisture sensitive phase degradation. Therefore, it is desirable to decrease the amount of, or avoid the Li salt, as a dopant for stable all-inorganic perovskite solar cells with balanced stability and high performance.