Studying the Fundamental Mechanics behind Cesium Lead Iodide Perovskite Phase Degradation Due to Surface Defects

Currently, the solar cell market is dominated by silicon based solar cells. However, over the last decade, photovoltaic cells made using halide perovskite absorbers have shown rapid and consistent increases in efficiency. This, coupled with their improving stability and inexpensive manufacturing, has brought much attention to the material as a potential competitor to silicon as both a semiconductor more generally and a solar cell technology. There is a desire to improve thermal stability of halide perovskites, which have the general formula ABX₃, by using cesium lead iodide (CsPbI₃); however, crystal phase stability for CsPbI₃ based perovskites is poor. This phase stability is easily seen visually as the perovskite film will transform from a dark brown/black perovskite phase to a visibly transparent clear non-perovskite phase. One suspected factor influencing the metastability of the perovskite phase in CsPbI₃ is the surface defect concentration, specifically iodide vacancies. During high temperature thermal annealing, defects form on the crystal surface where, it is hypothesized, they become nucleation sites for the non-perovskite phase to begin the phase transformation. In this study, CsPbI₃ perovskite film solutions were made and post-treated with increasing concentrations of iodide by the addition of CsI and CdI₂ to reduce the concentration of surface iodide vacancies. Two distinct film formation methods were used to fabricate CsPbI₃ to understand the generalizability of the results. In the first method, a stoichiometric mixture of CsI and PbI₂ was dissolved in dimethylformamide and deposited by spin-coating. In the second method, a mixed dimethylformamide/dimethyl sulfoxide solvent system was used and an antisolvent, methyl acetate, was used during spin-coating to improve film formation. The phase transformation between the perovskite and non-perovskite phases was monitored at constant temperature and humidity using a home-built system to control film conditions and monitor absorbance. From this, we find that reducing iodide defect concentrations using both CsI and CdI₂ treatments directly leads to improved phase stability, providing strong evidence that iodide vacancies are nucleation sites for non-perovskite phase formation in CsPbI₃ perovskites. This insight has important implications for materials design for phase stable halide perovskite materials.