Mechanistically Studying the Degradation of Cesium Lead Iodide Perovskite Phases Due to Temperature and Humidity

Halide perovskites offer exciting potential as photovoltaic materials and simply as semiconductors. Perovskites’ ever-increasing efficiency, stability and lowering manufacturing costs is making them even more attractive as a competitor to silicon solar cells. The focus of this research was on halide perovskites which have the formula ABX₃, and more specifically on cesium lead iodide (CsPbI₃). A major drawback of CsPbI₃ is its limited stability in the perovskite crystal phase, the colored phase with desirable optoelectronic properties. The source of this instability is a size mismatch between the cesium cation and the lead iodide octahedra. Cesium is somewhat too small of a cation to stabilize the perovskite structure and in the presence of outside factors, such as increased humidity and temperature, the metastable perovskite structure changes to the thermodynamically favored, mostly colorless orthorhombic phase. Most work has centered on improving the stability of the perovskite phase whereas less has been done to mechanistically study the phase change process. In this study, a home-built system was constructed to analyze the absorbance of a CsPbI₃ film over time while controlling humidity and temperature. Absorbance data collected was converted to phase fraction data and the phase transformation was modeled by the JMAK model, x(t) = exp(–ktⁿ), where n is the growth coefficient, k is the rate constant and t is time. It is found that relative humidity increases the phase transformation rate exponentially, indicating a first order process; however, increasing temperature leads to a monotonic decrease in the rate constant. We hypothesize that the decrease in phase transformation rate with increasing temperature is the result of surface water desorption. Experiments are also consistent with the phase transformation rate being nucleation rate limited where individual grains transform very rapidly once a non-perovskite phase nucleates, but neighboring grains do not influence one another. Work continues to model the adsorption and desorption mechanism of water onto the surface of CsPbI₃ and the effect this has on the phase change by analyzing the effects of differing temperature and humidity.