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 makes them attractive as a competitor to silicon solar cells. The focus of this research was 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. 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 orthorhombic phase. It is suspected water acts as a catalyst to lower the activation energy of the metastable perovskite phase, thus accelerating the phase change. In this study, the absorbance of a CsPbI₃ film was analyzed over time while controlling humidity and temperature. Absorbance data collected was converted to phase fraction data and modeled by the JMAK model, x(t) = exp(–ktⁿ), where n is the growth coefficient and k is the rate constant. 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 due to increased surface water desorption. Experiments are consistent with the phase transformation rate being nucleation rate limited where individual grains transform very rapidly once a non-perovskite phase nucleates. SEM imaging shows growth across domain boundaries. The growth coefficient n was found to have little variance with an average value of 2.54. Holding this value constant, a global fit was applied to the phase fraction data to determine the activation energy of the phase transformation as well as the energy of adsorption for water to the perovskite surface. Work continues to model the adsorption and desorption mechanism of water onto the surface of CsPbI₃ to obtain more accurate activation energy and adsorption values. Further description of this process will help researchers design more durable materials to increase commercial use of halide perovskites.