Chloride in Lead Chloride-Derived Organo-Metal Halides for Perovskite-Absorber Solar Cells

Organo-metal halide perovskites are an intriguing class of materials that have recently been explored for their potential in solar energy conversion. Within a very short period of intensive research, highly efficient solar cell devices have been demonstrated. One of the heavily debated questions in this new field of research concerns the role of chlorine in solution-processed samples utilizing lead chloride and 3 equiv of methylammonium iodide to prepare the perovskite samples. We utilized a combination of X-ray photoelectron spectroscopy, X-ray fluorescence, and X-ray diffraction to probe the amount of chlorine in samples before and during annealing. As-deposited samples, before annealing, consist of a crystalline precursor phase containing excess methylammonium and halide. We used in situ techniques to study the crystallization of MAPbI3 from this crystalline precursor phase. Excess methylammonium and chloride evaporate during annealing, forming highly crystalline MAPbI3. However, even after prolonged annealing times, chlorine can be detected in the films in X-ray fluorescence measurements.