DFT Study of Lead-Free Mixed-Halide Materials Cs₂X₂Y₂ (X, Y = F, Cl, Br, I) for Optoelectronic Applications

The intrinsic problems of toxicity and instability of lead solar cells have motivated extensive research intended to develop alternative materials for photovoltaic applications. First-principles calculations were performed in order to shed light on replacing the Pb²⁺ cation in lead- and mixed-halide materials with a formula of Cs₂X₂Y₂ (X, Y = F, Cl, Br, I). The calculated band gaps range from 0 eV to 3.188 eV, and the absorption coefficients from 6 to 16 × 10⁴ cm⁻¹. The band gap of Cs₂Cl₂I₂ is close to the Shockley-Queisser limit and its absorption coefficient is in the visible frequency range, which, in conjunction with its easy and economical synthesis process, qualify it as an alternative lead-free material for fabricating solar cells. It is suggested for further work that if the ratio of halogens is adjusted then the band gaps of Cs₂F₂Cl₂, Cs₂Cl₂I₂, Cs₂Br₂I₂, and Cs₂I₂F₂ may be tuned to the desired electronic band gap limit required to improve their optoelectronic performance.