The Effect of Stoichiometry on the Stability of Inorganic Cesium Lead Mixed-Halide Perovskites Solar Cells

Metal halide perovskite solar cells that use the inorganic cation Cs have been shown to have better thermal stability than the organic cation containing counterparts, and CsPbI 2 Br has a more suitable (lower) band gap than CsPbIBr 2 as a photovoltaic energy harvesting material. However, increase in iodine content reduces structural stability due to the preference toward the non-perovskite orthorhombic phase when the film is exposed to air. In this work, the effect of varying stoichiometry of CsPbI 2 Br perovskite on film quality such as the grain size, presence of impurities and nature of impurity grains, photoluminescence, morphology, and elemental distribution are studied. Details on how to vary the stoichiometry during the dual source thermal evaporation process are reported. It is found that the air stability of CsPbI 2 Br film correlates with the CsBr-to-PbI 2 deposition rate ratio, in which the CsBr-rich CsPbI 2 Br is the most stable upon air exposure, while the stoichiometrically balanced CsPbI 2 Br perovskite film gives the best photovoltaic performance. The encapsulated device maintains 90% of the initial performance after 240 h damp and heat test at 85 °C and 85% relative humidity.