Electronic structures, optical and acoustic phonons, and electronic and thermal conductivities of cesium ytterbium chloride perovskite crystal

The optoelectronic and transport properties of lead-free ytterbium (CsYbCl₃) perovskites have been characterized. The purpose of this study is to predict the electronic structure of the CsYbCl₃ perovskite crystal and propose guidelines for material design to improve its photovoltaic performance. The CsYbCl₃ crystal was prepared based on the cesium lead chloride crystal structure, structurally optimized, and the band structures and absorption characteristics predicted using first-principles calculations. The electronic structure consists of an occupied 4f orbital of the Yb²⁺ ion in the valence band state and a 5d orbital of the Yb²⁺ ion in the conduction band state, and it exhibits a direct transition band gap of 0.55 eV. Real (Re) and imaginary (Im) dielectric functions were found to be Re = 26.5 at 1.7 eV and Im = 32.2 at 2.9 eV. The absorption coefficient was widely distributed in the range of 163 – 1768 nm. From the intercept of photon energy with the slope of the Tauc plot, the band gap was found to be 0.55 eV. The acoustic phonon as lattice vibrations exhibited dynamic instability derived from the tilt of the octahedral structure. The temperature behavior of electrical conductivity decreased with increasing thermal conductivity. The hole conductivity was based on the combination of carrier diffusion with lattice vibration as acoustic phonons in the high-temperature region. As a novelty, CsYbCl₃ crystals, due to theoretical predictions, are expected to be applicable as photoactive materials operating at high temperatures for optoelectronic applications such as solar cells and fluorescent devices.