Studying the Phase Change Mechanism in Cesium Lead Iodide Perovskites As It Relates to Temperature and Humidity

The need for renewable energy sources has never been higher than right now. Halide perovskites are a class of semiconductor with simple fabrication and high performance as solar cell absorbers. Although halide perovskites show high photovoltaic efficiency and have a tunable bandgap, some desirable materials undergo a phase change where they change from a photovoltaically active low bandgap phase, to a high bandgap, non-perovskite clear phase. One reason for this phase change is water (vapor) acting as a catalyst to lower the activation energy of the metastable perovskite phase, thus accelerating the phase change. In this work, we study the kinetics of this phase change to better understanding the mechanism causing this unfavorable process. Literature is consistent with higher humidities causing an accelerated phase change; however, a lack of consensus has been reached with regards to the dependence of temperature on this phase change, and details of rates and activation energies remain unclear. The goal of this research is to test how temperature and humidity both play a role in the phase transition. To do this, the phase change of the cesium lead iodide films was monitored in situ by absorption spectroscopy. Our data showed that an increase in temperature led to a decrease in the rate of phase change, which is opposite of typical Arrhenius behavior. The proposed explanation for this phenomenon is that water is desorbing off the film surface faster, causing less water interaction at the perovskite film surface. The collected data is being used to develop a more comprehensive model of the phase transformation process. Studying this phase change and the mechanism behind it could help to improve the long-term stability of perovskites and improve predictions of real-world lifetime from accelerated ageing protocols.