Structural evolution, dielectric relaxation, and charge transport characteristics of formamidinium lead iodide (FAPbI3) perovskite

Recently, hybrid halide perovskite solar cells have drawn the researchers’ attention, owing to their high-power conversion efficiency. Among the different compounds, MAPbI3 (MA = Methylammonium CH3NH3) and FAPbI3 (FA = Formamidinium CH(NH2)2) are especially potential candidates since their bandgap suited the energy range of visible light. Here, the hybrid perovskite FAPbI3 is successfully synthesized via the inverse temperature crystallization method. Different characterization techniques such as variable-temperature structural analyses, differential scanning calorimetry (DSC), and impedance spectroscopic analysis show that FAPbI3 presents a phase transition at T = 300 K from the hexagonal to the cubic symmetry. Optical analysis of FAPbI3 shows the direct band-gap value of ∼1.4 eV. FAPbI3 is characterized by impedance spectroscopy technique performed in the 10−1–106 Hz frequency and 210–390 K temperature ranges. The alternating current (AC) conductivity is illustrated in terms of Jonncher's power law. The study on charge transportation within FAPbI3 suggests the presence of the NSPT (non-overlapping small polaron tunneling) and OLPT (overlapping large polaron tunneling) models in the hexagonal and cubic phases, respectively.