Probing Photo-Induced Vibrational Kinetics in Perovskite Thin Films

Subwavelength perovskite thin films can only cause less than a l0% intensity reduction of the transmitted terahertz (THz) signal; this small dynamic range makes it difficult to track minor changes in the phonon modes. We propose a THz thin film total internal reflection (TF- TIR) spectroscopy technique to characterize photo-induced spectral changes of MAPbI3, MAPbI2.sBro.s, MAPbI2Brl and MAPbBr3. Our approach enhanced the THz responsivity of the perovskites thin films by a factor of 4 compared to THz transmission spectroscopy. The light induced the strength of the phonon modes at around 2 THz to increase after 5 min of illumination, corresponding to the enhanced stretching of the lead halide bond. After l0 min, the strength of the phonon modes at around 1 THz started to decrease, corresponding to the weakened bending of the lead halide bond. Blue shifts were observed on the resonant frequencies apart from MAPbBr3. By comparing the results of four perovskites, we conclude the perovskites that have higher iodide ratio are more unstable in the presence of light and moisture. With the support of X-ray diffraction measurements, we suggest that THz TF - TIR spectroscopy provides distinct information of vibrational kinetics of lead-halide bond in perovskites.