Disentangling Thermal from Electronic Contributions in the Spectral Response of Photoexcited Perovskite Materials

Abstract

Disentangling electronic and thermal effects in photoexcited 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 the visible to mid-to-deep-ultraviolet (UV) ranges on MAPbBr 3 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 an understanding of the ultrafast responses of the UV bands and the underlying mechanisms governing them. Our investigation reveals that the photoinduced 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 photoinduced transients possess a significant thermal origin and cannot be attributed solely to electronic effects. Following photoexcitation, as carriers (electrons and holes) transfer their energy to the 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 the lattice in photoexcited perovskite materials and provide insights into the limited utilization efficiency of photogenerated charge carriers.

Band Gap Renormalization at Different Symmetry Points in Perovskites

“…These transitions are annotated in the absorption spectrum that was derived from spectroscopic ellipsometry measurements on a single crystal ( Figure 1 b), and they are identified with the help of the calculated band structure diagram in Figure 1 c. Specifically, transitions 1 and 3 at the R and M points, respectively, arise from VB1 → CB1 transitions. 43 The signature of BGR in a transient spectrum is a derivative-like line shape with a positive low-energy side and a negative high-energy side ( Figure S1 ). It is worth noting that as our continuum probe covers the 3.3–4.3 eV range, this hinders the detection of BER at transitions 2 and 4, and we therefore focus our study on transition 3 for the higher-energy edge.…”