Phonon-Assisted Reversible Thermochromism in a Lead-Free Antimony-Based Cs3Sb2Br9 Perovskite

ABSTRACT: Lead-free antimony-based perovskites have emerged as promising photovoltaic materials due to their outstanding stability, lower toxicity, and easy processability. Recently, researchers have showed their successful implication in indoor energy applications, thus prompting their possible applications in buliding integrated device applications. Here, we investigate temperature-dependent band gap engineering leading to reversible thermochromism (TCM) in all-inorganic antimony-based Cs3Sb2Br9 single crystals (SCs). The 2D Cs3Sb2Br9 single crystals exhibited a color change from yellow to deep orange at elevated temperatures and returned to the original yellow when they were cooled to room temperature. Temperature-dependent UV-vis absorption spectroscopy and a differential thermal analysis (DTA) confirmed the optical band-edge modulation with no structural phase transition. Higher Urbach energy values at higher temperatures reveal that the shift in band edge is coming primarily from an extension in the indirect band edge, thus confirming that high-energy phonons are responsible for the reduced indirect band gap. X-ray diffraction (XRD) spectra were also obtained for varying temperatures, which confirmed the lattice expansion in 2D Cs3Sb2Br9 SCs. Additionally, Raman spectroscopy confirmed the enhancement in symmetric stretching at elevated temperatures. Our work studied the band gap modulation in 2D Cs3Sb2Br9 SCs with temperature and validates that the presence of high-energy phonons at elevated temperature results in an elongated Urbach tail and thus leads to the reversible TCM phenomena.