Surface Halide Vacancies Influence the Phase Transition in CsPbI3

Halide Perovskites are an emerging technology with promising potential for photovoltaic applications. The improvement of halide perovskite stability and efficiency in recent years has begun to make halide perovskites a potential competitor to traditional Silicon based photovoltaic technologies. However, despite these continued improvements, halide perovskites continue to lack long-term stability limiting their viability in large markets. This research was conducted using Cesium Lead Iodide (CsPbI₃). Cesium Lead Iodide is known to have severe stability issues, one of which is that this perovskite will undergo a change in state which can be visibly observed as the perovskite film will transition from a dark colored phase to a light, yellow phase impacting the materials ability to effectively act in a photovoltaic system. The Cesium atoms in this perovskite’s crystal structure are too small, ultimately allowing the structure to tip over causing this change in state. Additionally, when manufacturing halide perovskites, vacancies within the crystal structure. These vacancies can then act as a nucleation site for the phase change. One proposed method of limiting these crystal vacancies is through doping the perovskite with additional material This research had the primary focus of exploring the effects of doping halide perovskite films by doping Cesium Lead Iodide perovskite with Iodine using a solution of either Cadmium Iodide (CdI2) or Cesium Iodide (CsI) dissolved in Isopropyl alcohol. This research consisted of doping Cesium Lead Iodide perovskites with Iodine using the previously mentioned solution at varying concentrations. Trials with Cesium Iodide and Cadmium Iodide were done at concentrations ranging from 0.001 mmol/ml to 0.01 mmol/L. The effects of doping these perovskites were observed by measuring the change in phase fraction of the doped perovskites over time. Absorbance data was collected via spectroscopy and converted into phase fraction through the use of the JMAK model, x(t) = exp(–ktⁿ) where n is the growth coefficient, k is the rate constant and t are representative of time. Graphing the calculated phase change data against time displayed decreasing rates of change with increasing concentrations of doping material. It was then concluded that doping these perovskites slowed increased the materials stability suggesting that halogen doping increases halide perovskite stability.