Lead-free NaSiI3 Halide Perovskite: Linking Photovoltaic Performance and Photocatalytic Activity Through First-Principles and SCAPS-1D Modeling

Ab-initio and device-scale simulations show that NaSiI3 is a thermodynamically stable, lead-free cubic halide perovskite with an optimized lattice constant of 5.75 Å, a negative formation energy, and a Goldschmidt tolerance factor of 0.98. Ab-initio molecular dynamics confirm thermal stability at 300 K and 400 K over 7 ps, and the elastic constants satisfy the Born stability criteria with the Pugh ratio, Poisson's ratio, and a negative Cauchy pressure indicate intrinsically brittle behavior with ionic-covalent bonding. Hybrid HSE calculations reveal a direct band gap of 1.56 eV with strong visible-light absorption, while Tauc-plot analysis gives an optical gap of 1.81 eV. The computed band edges straddle the water redox potentials over pH 0-8, yielding a theoretical solar-to-hydrogen conversion efficiency of 17.14%. Device-level SCAPS-1D simulations of an ITO/WS2/NaSiI3/CuSCN/Au cell, optimized over absorber thickness (0.1-1.0 μm), acceptor doping density (1015-1018 cm−3), and bulk/interfacial defect density, identify a best-performing configuration with a 1.0 μm-thick NaSiI3 layer, an acceptor doping density of 1018 cm−3, and bulk/interfacial defect densities of 1015 cm−3 and 1010 cm−2, respectively, delivering a power conversion efficiency of 26.59% (VOC = 1.21 V, JSC = 24.95 mA cm−2, FF = 87.95%) about 86% of the Shockley-Queisser limit for this band gap. These results establish NaSiI3 as a stable, lead-free halide perovskite with dual promise for photovoltaic and photocatalytic solar energy conversion.