Orientation of Ferroelectric Domains and Disappearance upon Heating Methylammonium Lead Triiodide Perovskite from Tetragonal to Cubic Phase

We study the spontaneous polarization of the archetypal semiconducting halide perovskite methylammonium lead triiodide (CH₃NH₃PbI₃) that is currently being investigated for use in thin film solar cells and light-emitting diodes. Using both lateral and vertical piezoresponse force microscopy (PFM) to image polycrystalline thin films, we observed domains in the piezoresponse that reversibly appear and disappear below and above the tetragonal-to-cubic phase transition temperature. Importantly, we observe these domains to exhibit a piezoresponse that is predominantly in-plane for films with the (110) plane oriented parallel to the substrate, providing a measure of the polarization associated with specific crystal planes. We characterize the polarization and its temporal response using both local switching spectroscopy and time-dependent PFM spectra. These data show hysteresis loops with the polarization switching with bias but relaxing back on time scales of several minutes. Our results suggest the existence of ferroelectric behavior due to off-center displacement of the Pb²⁺ cation, although the local polarization response is complicated by the presence of local ionic and electronic conductivity. Understanding the nature of these domains paves the way for further optimization of optoelectronic devices using CH₃NH₃PbI₃ perovskite material