Crystallographic Changes Mediated by Copper Migration from Hole Transporting Layer into CsPbBr₃Perovskite

Copper oxides, due to their low cost and high ambient stability, are promising candidates for use as a hole-transporting layer (HTL) in perovskite solar cells. However, it has been found that they can be very unstable in contact with organic–inorganic FAPbI₃perovskite, causing mutual chemical reactions and changing a photoactive to a nonphotoactive perovskite phase. To verify if the copper migration and the occurrence of chemical reactions are a matter of contact with a type of perovskite (organic–inorganic or fully inorganic) or the nature of copper oxides, in this work, the interface between cupric and cuprous oxides and a fully inorganic cesium lead bromide (CsPbBr₃) perovskite is investigated. It is found that CsPbBr₃is not robust against copper ion migration from HTL, similarly to organic–inorganic FAPbI₃; however, contrary to the previous results, they do not cause chemical reactions. Instead, crystallographic lattice shrinkage of the CsPbBr₃occurred along with the substitution of Pb²⁺by copper ions. The work explains the mechanism of this phenomenon and confirms the unreliability of copper(I) oxide and copper(II) oxide as HTLs in p-i-n perovskite solar cells based on both organic–inorganic and fully inorganic lead halide perovskites.