Entropy-driven structural transition and kinetic trapping in formamidinium lead iodide perovskite

A challenge of hybrid perovskite solar cells is device instability, which calls for an understanding of the perovskite structural stability and phase transitions. Using neutron diffraction and first-principles calculations on formamidinium lead iodide (FAPbI$_{\mathrm{3}})$, we show that the entropy contribution to the Gibbs free energy caused by isotropic rotations of the FA$^{\mathrm{+}}$ cation plays a crucial role in the cubic-to-hexagonal structural phase transition. Furthermore, we observe that the cubic-to-hexagonal phase transition exhibits a large thermal hysteresis. Our first-principles calculations confirm the existence of a potential barrier between the cubic and hexagonal structures, which provides an explanation for the observed thermal hysteresis. By exploiting the potential barrier, we demonstrate kinetic trapping of the photovoltaic pseudo-cubic phase at low temperatures by thermal quenching $^{\mathrm{[1]}}$. [1] Chen \textit{et al. Sci. Adv.} 2016;2: e1601650 21 October 2016