Piezoreflectance of Metal Halide Films—A Sensitive Method for Studying Optical and Phase Transitions

A novel approach to investigate the optical properties of hybrid perovskites has been proposed to complement the commonly used ones. In this study a piezo-modulation spectroscopy was applied to investigate the optical transitions in well-known perovskite films. A series of methylammonium and formamidinium perovskite layers were prepared using spin-coating and drop-casting directly onto a piezoceramic substrate. The piezoreflectance signal, related to optical transitions, was successfully observed at room temperature. To explore the optical properties in MAPbI₃ in detail, measurements were conducted over a wide temperature range for layers prepared by both drop-casting and spin-coating techniques. Additionally, a series of spectra were measured over the same temperature range by photoluminescence, a reference method. The energies of optical transitions obtained from absorption-like (i.e., piezoreflectance) and emission-like (i.e., photoluminescence) experiments are in a good agreement. The applied piezoreflectance method demonstrated greater sensitivity to phase transitions in the standard MAPbI₃ perovskite compared to photoluminescence due to the intrinsic sensitivity of this method to structural phase transitions. Notably, piezoreflectance (and modulation spectroscopy techniques in general) is not sensitive to defect state-related transitions, which often appear in PL spectra and can make the interpretation of results difficult. A comparative analysis of these two measurement techniques revealed that utilizing piezo-modulation spectroscopy for exploring halide perovskite semiconductor properties provides valuable insights into the optical properties of MAPbI₃.