First-principles study of lead-free Na2TmAgCl6 and Na2TmCuCl6 double halide perovskites for photovoltaic and thermoelectric applications.

This study thoroughly investigates the lead-free halide double perovskites Na2TmAeCl6 (Ae = Ag, Cu) using first-principles density functional theory (DFT) within the CASTEP framework. The formation energies are -3.987 eV for Na2TmAgCl6 and -3.453 eV for Na2TmCuCl6. Additionally, phonon spectra without imaginary frequencies confirm their thermodynamic and dynamic stability. Both compounds adopt a cubic Fm-3m structure with tolerance factors of 0.76 and 0.83, placing them within the stable perovskite range. The elastic constants meet the Born-Huang stability criteria, and Pugh's ratios of 3.48 for Ag and 3.23 for Cu indicate they are ductile. Moreover, Debye temperatures of 124.48 K for Ag and 91.53 K for Cu suggest good lattice thermal stability. Electronic structure analysis shows Na2TmAgCl6 has a direct bandgap of 2.51 eV, while Na2TmCuCl6 features a direct bandgap of 2.32 eV. Their optical properties reveal high absorption coefficients of approximately 16 × 104 cm-1 in the UV region. The calculated SLME efficiencies are 11 % for the Ag compound and 16.5 % for the Cu compound, with ZT values of 0.91 and 1.20, respectively, indicating strong thermoelectric potential. These findings suggest that both Na2TmAgCl6 and Na2TmCuCl6 are promising candidates for advanced optoelectronic, photovoltaic, and thermoelectric applications.