Soft Lattice and Phase Stability of α‐FAPbI3

Since the certified power conversion efficiency (PCE) of perovskite solar cells (PSCs) has reached 26.1%, exactly equal to that of crystalline silicon solar cells, a strong demand for ensuring the long‐term stability of PSCs has arisen for commercialization. The intrinsic stability of the perovskite layer must be guaranteed as a top priority to ensure the whole device's stability. Recently, the state‐of‐the‐art PSCs, performing a high PCE, employ α‐FAPbI3 (FA = formamidinium) for the perovskite layer in spite of its metastable tendency to spontaneously transform into its photoinactive polymorph at PSC operating temperature. In this review paper, the intrinsic structural stability of α‐FAPbI3 soft lattice is understood from the thermodynamic point of view, with key parameters to restrain the undesirable phase transition. Besides, reported experimental results are closely examined to find fundamental origins, derive the enhanced phase stability in each experiment, and understand their role in maintaining the lattice structure from the collective perspective. The state‐of‐the‐art PSCs employ α‐FAPbI3 in spite of its metastable crystalline phase at RT. In this review, the intrinsic structural stability of α‐FAPbI3 is discussed from the collective perspective based on various experimental results which are closely examined to understand fundamental origins and their role in maintaining the lattice structure, particularly by assessing the contribution between entropy and enthalpy term.