First-Principles and BoltzTraP approach to the optoelectronic design of CuMCl3 (M = Ge, Sn) halide perovskites for Future energy devices

Lead-free halide perovskites are emerging as sustainable alternatives to toxic Pb-based materials for energy conversion technologies. In this study, a comprehensive first-principles investigation combining density functional theory (DFT) and Boltzmann transport theory (BoltzTraP2) is employed to explore the structural, electronic, optical, and thermoelectric properties of novel Cu-based perovskites CuMCl3 (M = Ge, Sn). Both compounds crystallise in a stable cubic phase (Pm-3 m) with negative formation energies (−2.96 and − 2.87 eV/atom) and exhibit ductile mechanical behaviour. Electronic calculations reveal direct band gaps of 0.70 eV (CuGeCl3) and 0.91 eV (CuSnCl3), confirming their suitability for optoelectronic applications. CuGeCl3 exhibits a stronger optical response with a higher dielectric constant (ε1(0) = 5.12) and absorption, whereas CuSnCl3 displays lower reflectivity and a higher exciton binding energy (0.099 eV), making it suitable for LED and coating applications. Thermoelectric analysis identifies both materials as n-type semiconductors, with Seebeck coefficients around − 225 μV/K and maximum ZT values of 2.50 (CuGeCl3) and 2.99 (CuSnCl3) at 900 K, surpassing conventional thermoelectric benchmarks. These findings highlight CuGeCl3 as a promising optoelectronic absorber and CuSnCl3 as an efficient high-temperature thermoelectric material, offering multifunctional potential for next-generation energy and photonic devices.