Comprehensive investigation of the structural, mechanical, optical, and thermoelectric properties and photovoltaic performance of lead-free novel Li2PdCl6 and Na2PdCl6 using DFT and SCAPS-1D simulations

Abstract

This study presents a detailed first-principles investigation of lead-free double perovskites Li₂PdCl₆ and Na₂PdCl₆ to evaluate their suitability for high-efficiency solar cell applications. Using density functional theory (DFT) and SCAPS-1D simulations, we examined their structural, electronic, optical, mechanical, thermoelectric, and photovoltaic properties, along with population and phonon analyses. Both materials are structurally stable, with Na₂PdCl₆ exhibiting slightly greater stability based on Goldschmidt tolerance factors and formation energies. Direct band gaps are calculated as 1.195 eV (Li₂PdCl₆) and 1.110 eV (Na₂PdCl₆) using GGA-PBE, and 2.225 eV and 2.133 eV with HSE06, placing them close to the optimal range for single-junction solar cells. PDOS and charge density analyses confirm strong bonding and atomic-level contributions. Optical results reveal excellent visible light absorption and low reflectivity. Mechanical studies indicate that Li₂PdCl₆ is more ductile, while Na₂PdCl₆ is stiffer yet brittle; anisotropy assessments further confirm directional mechanical reliability. Phonon dispersion confirms dynamic stability without imaginary frequencies. Thermoelectric analysis highlights favorable heat capacity and Debye behavior. Population analysis supports stable ionic bonding and charge localization. Photovoltaic performance was evaluated using SCAPS-1D by systematically varying absorber layer thickness, defect densities, shallow acceptor concentration, operating temperature, and series and shunt resistances, along with analyzing J–V characteristics and quantum efficiency (QE) responses. Na₂PdCl₆-based devices exhibit superior performance, achieving a PCE of 25.55%, a J_SC of 42.55 mA cm⁻², an FF of 79.16%, and a V_OC of 0.758 V. In comparison, Li₂PdCl₆ attains a PCE of 23.06%, a J_SC of 38.12 mA cm⁻², an FF of 76.97%, and a V_OC of 0.786 V. Na₂PdCl₆ also exhibits better defect tolerance and thermal stability. Overall, Na₂PdCl₆ stands out as a thermally stable, defect-tolerant, and eco-friendly absorber candidate for next-generation perovskite solar cells, with Li₂PdCl₆ serving as a promising alternative.