Alloy [FA,Cs]PbI3 perovskite surfaces: The role of surface cesium composition in stability and tolerance to defect formation

Abstract

Halide-perovskite alloys that include cesium have achieved records of stability and efficiency in solar cells. Controlling the surface composition, defects, and electronic properties guarantees interface stability and improves performance. By using density functional theory and molecular dynamic simulations, we analyzed which surface compositions of the formamidinium (FA) and cesium (Cs) lead iodide perovskite FA1−xCsxPbI3 with 25 and 50% of Cs become more stable than pure perovskites. Structural and electronic properties and tolerance to defect formation were also evaluated. Surface energy calculations show that only the alloys with 25% Cs and FAI-enriched surfaces are more stable than pure FAPbI3 ones. The most stable alloy surface shows electronic energy levels similar to the FAPbI3 perovskite, suggesting that this alloy may also be efficient for charge transport in the cell. However, the presence of Cs on the alloy surface, although low, favors the formation of FAI vacancies, which is detrimental to the stability of the perovskite. These results suggest evaluating FA1−xCsxPbI3 alloys with small Cs compositions to mitigate the formation of defects or using a passivation scheme. This study delivers valuable information for efficiency device improvement from the perspective of interface stability.