Disentangling Thermal from Electronic Contributions in The Spectral Response of Photo-Excited Perovskite Materials

Disentangling electronic and thermal effects in photo-excited perovskite materials is crucial for photovoltaic and optoelectronic applications, but remains a challenge due to their intertwined nature in both the time and energy domains. In this study, we employed temperature-dependent variable-angle spectroscopic ellipsometry, density functional theory calculations, and broadband transient absorption spectroscopy spanning from the visible to mid-to-deep-Ultraviolet (UV) ranges on MAPbBr₃ thin films. The use of deep-UV detection opens a new spectral window that enables the exploration of high-energy excitations at various symmetry points within the Brillouin zone, facilitating the understanding of the ultrafast responses of the UV bands and the underlying mechanisms governing them. Our investigation reveals that the photo-induced spectral features remarkably resemble those generated by pure lattice heating, and we disentangle the relative thermal and electronic contributions and their evolutions at different delay times using combinations of decay-associated spectra and temperature-induced differential absorption. The results demonstrate that the photo-induced transients possess a significant thermal origin and cannot be attributed solely to electronic effects. Following photo-excitation, as carriers (electrons and holes) transfer their energy to lattice, the thermal contribution increases from ~15% at 1 ps to ~55% at 500 ps, and subsequently decreases to ~35-50% at 1 ns. These findings elucidate the intricate energy exchange between charge carriers and lattice in photo-excited perovskite materials, and provide insights into the limited utilization efficiency of photo-generated charge carriers.