Temperature and pressure induced structural transitions of lead iodide perovskites

During the past fourteen years, conventional lead iodide perovskites, APbI3 [A = Cs+, methylammonium (CH3NH3+) and formamidinium (NH2CHNH2+)], have emerged as the forerunner materials for optoelectronic applications, including photovoltaics. However, their photoactive phases suffer from structural instabilities – including phase transitions under extrinsic factors such as heat, light, pressure etc. They even show chemical instability in water, oxygen and humid air. Consequently, their optical and electronic properties show marked changes under device working conditions, which is one of the major obstacles in long-term use. At the same time, phase transition offers plenty of opportunities for modulating structures to physical properties. Hence, there is a need for a comprehensive understanding of the structural instabilities and phase transitions of APbI3 perovskites. This perspective focuses on three conventional lead iodide perovskites, CsPbI3, MAPbI3 and FAPbI3, which are among the most widely studied materials in this family. We critically review the experimentally known structures as a function of temperature or pressure. We describe crystal structures of these three perovskites as well as highlight the reason for their instabilities, mechanism of structural transitions, and fundamental insights into their optoelectronic properties. In addition to temperature- and pressure-driven structural changes, we highlight structural changes due to chemical instability towards water and oxygen. We focus mainly on the structures reported based on X-ray, synchrotron, and neutron diffraction data. We conclude the perspective by discussing current challenges in these exciting materials, possible future opportunities to improve them and our thoughts on new directions.