Atomistic view of interstitial occupation of small alkali cations in Perovskites and its impact on ion migration

Recent success in achieving highly stable Rb-containing organolead halide perovskites has indicated the possibility of incorporating small monovalent cations, which cannot fit in the lead-halide cage with an appropriate tolerance factor, into the perovskite lattice to form a stable “black” phase. Here, through a combined experimental and theoretical investigation on the incorporation of alkali cations (Rb+, K+, Na+ and Li+) in perovskite materials, we revealed unambiguously the size-dependent interstitial occupation of the alkali cations in the perovskite lattice. Interestingly, the density functional theory (DFT) calculations also predict the increased ion migration barriers in the lattice after interstitial occupation. To verify the theoretical prediction, we characterized the ion migration behavior via voltage-dependent electrical poling, temperature-dependent conductivity, hysteresis analysis of solar cells and time-dependent photoluminescence measurements. The results collectively point to the suppression of ion migration after lattice interstitial occupation by small alkali ions. The findings in this study shed light on new material designs to manipulate structural and ionic properties of multi-cation perovskite materials.