Ferroelectric Properties of Perovskite Thin Films and Their Implications for Solar Energy Conversion

Whether or not methylammonium lead iodide (MAPbI3) is a ferroelectric semiconductor has caused controversy in the literature, fueled by many misunderstandings and imprecise definitions. Correlating recent literature reports and generic crystal properties with the authors' experimental evidence, the authors show that MAPbI3 thin‐films are indeed semiconducting ferroelectrics and exhibit spontaneous polarization upon transition from the cubic high‐temperature phase to the tetragonal phase at room temperature. The polarization is predominantly oriented in‐plane and is organized in characteristic domains as probed with piezoresponse force microscopy. Drift‐diffusion simulations based on experimental patterns of polarized domains indicate a reduction of the Shockley–Read–Hall recombination of charge carriers within the perovskite grains due to the ferroelectric built‐in field and allow reproduction of the electrical solar cell properties. Recent advances in probing polar domains in methylammonium lead iodide thin films and their implications for perovskite solar cells are reviewed. The fundamental crystal properties and the formation of domains with predominant in‐plane polarization, as monitored with piezoresponse force microscopy, provide evidence of the semiconducting ferroelectric nature of methylammonium lead iodide thin films.