DFT assessment on the future prospects of inorganic lead-free halide double perovskites Cs2ABI6 (AB: GeZn, SnBe) for energy conversion technologies

Abstract

In this study, we employed density functional theory (DFT) calculations to investigate the structural, elastic, electronic, optical, and thermoelectric properties of Cs2ABI6 (AB: GeZn, SnBe) halide double perovskites (HDPs). We employed the full-potential linear augmented plane-wave (FP-LAPW) method, incorporating the generalized gradient approximation (GGA) and Tran-Blaha modified Becke-Johnson (TB-mBJ) approach for the exchange-correlation potential. We found that the HDPs are stable in a cubic structure (space group Fm-3m), as indicated by phase stability analysis, formation energies, tolerance factor, and elastic constants. The compounds exhibits ductile behavior, as assessed by Poisson's and Pugh's ratios. The electronic band structures of Cs2GeZnI6 and Cs2SnBeI6 exhibit indirect band gaps (X-L) of 1.124 eV and 1.551 eV, respectively, as calculated using the TB-mBJ approximation. Optical spectra were evaluated over the 0–13 eV energy range, including the dielectric functions, extinction coefficient, electron energy loss, refractive index, optical conductivity, reflectivity, and absorption coefficient. Additionally, we calculated thermoelectric parameters across a range of chemical potentials and temperatures to assess their suitability for thermoelectric applications. Our results suggest that these compounds are highly promising candidates for both optoelectronic and thermoelectric devices.