Chloride Incorporation Process in CH₃NH₃PbI₃–xClx Perovskites via Nanoscale Bandgap Maps

Abstract

CH₃NH₃PbI_3-xCl_x perovskites enable fabrication of highly efficient solar cells. In this work, the photothermal-induced resonance, is leveraged to measure the bandgap of CH₃NH₃PbI_3-xCl_x films obtained by a multicycle coating process that produces high efficiency solar cells. Because chloride ions modify the perovskite lattice, thereby widening the bandgap, measuring the bandgap locally yields the local chloride content. After a mild annealing the films consist of Cl-rich and Cl-poor phases that upon further annealing evolve into a homogeneous Cl-poorer phase, suggesting that methylammonium-chloride is progressively expelled from the film. Despite the small chloride content, CH₃NH₃PbI_3-xCl_x films show better thermal stability up to 140^°C with respect to CH₃NH₃PbI₃ films fabricated with the same methodology.