From structure to performance: Phase-dependent optoelectronic, mechanical and SLME analysis of NaBI₃ (B = Si, Ge, Sn) via first-principles approach

In this work, density functional theory (DFT) calculations using the CASTEP were performed to comparatively investigate the structural, electronic, mechanical and optical properties of NaBI₃ (B = Ge, Sn, Si) in cubic and rhombohedral phases. In addition, a comprehensive investigation for Spectroscopic Limited Maximum Efficiency were carried out to analyze the theoretical photovoltaic efficiency of the reported materials. The exchange–correlation potential was treated using the PBE-GGA functional with norm-conserving pseudopotentials. The calculated electronic band structures showed phase-dependent variations, with band gap values ranging from 0.422 eV to 1.243 eV . The rhombohedral phases exhibited slight modification in band dispersion compared to the cubic phase. Density of states analysis indicates that the valence band maximum is mainly dominated by I–p states, while the conduction band minimum is primarily contributed by B-site cation states. Optical property calculations reveal efficient absorption coefficients in the visible region with peak values from (1.8 ×10⁵ –2.0 ×10⁵) cm⁻¹ . Electronic and optical properties were further calculated with HSE06 functional and compared GGA-PBE. The calculated mechanical properties revealed the mechanical stability of materials, with cubic NaGeI₃ material having isotropic nature, and rhombohedral materials slightly anisotropic nature. Phonon dispersion calculations revealed that cubic NaGeI₃ is dynamically stable, while cubic NaSiI₃ and NaSnI₃ exhibited soft modes. The rhombohedral phases showed dynamic stability with no imaginary phonon frequencies. The photovoltaic performance of materials was evaluated by computing the spectroscopic limited maximum efficiency (SLME), which predicted promising efficiency values from 10.7% to 22.5% for NaBI₃ (B = Si, Ge, Sn). The comparative results demonstrate that phase engineering significantly influences the optoelectronic performance of NaBI₃ compounds. This DFT study showed that NaBI₃ (B = Si, Ge, Sn) materials are potential candidates for efficient and environmentally friendly optoelectronic and renewable energy applications.