Effect of Sodium Doping on the Phase Stability and Band Gap Energy of Tin-Based Perovskite Films

This study examines the effect of sodium (Na) doping on the phase stability and band gap energy of tin-based perovskite materials, specifically (MA_1-xNa_x)SnI₃ and (MA_1-xNa_x)SnICl₂ (x = 0, 0.1, and 0.15). The perovskite films were synthesized via solution casting at room temperature using dimethylformamide as the solvent and ascorbic acid as the reductant agent. Visual observations reveal that the 10% Na-doped perovskite films maintained their black color for up to 3 days, indicating enhanced phase stability. X-ray diffraction (XRD) analysis shows an improved crystallinity at 10 mol% Na doping, with sharper diffraction peaks, but also with a double perovskite structure due to Sn²⁺ oxidation to Sn⁴⁺. These changes are reflected in band gap energy values (E_g) up to 2.4 eV. While Na doping at 10 mol% enhanced crystallinity, it also leads to reduced stability and degraded optoelectronic performance due to structural distortions. These results highlight a trade-off between crystallinity and perovskite stability for higher sodium doping concentrations.