Temperature‐Dependent Excitonic Band Gap in Lead‐Free Bismuth Halide Low‐Dimensional Perovskite Single Crystals

In this study, the optical behavior of lead-free Bi-based low-dimensional perovskite single crystals (Cs3Bi2Cl9, Cs3Bi2Br9, Cs3Bi2I9, and MA3Bi2I9) is investigated by spectroscopic ellipsometry, supported by X-ray diffraction and density functional theory calculations. All materials exhibit a strong excitonic peak resulting from photogenerated electron–hole Coulomb interactions, whereas the threshold of continuous absorption is found at higher energies. The resonances of the excitonic and continuous bands, along with exciton binding energies, are extracted through Critical Point Analysis of the ellipsometric data over a wide temperature range (from −90 °C to 90 °C), revealing subtle variations in the optical characteristics for each single crystal. These materials can be applied in optoelectronics as photodetectors because of their high stability and lower toxicity compared to their Pb-based perovskites.