Defect-state engineering in Mn-doped SnS: Urbach tail-driven synergy of optical absorption and electrochemical charge transfer

Tin monosulfide (SnS) is a lead-free, earth-abundant semiconductor with promising optoelectronic and energy applications; however, its performance is constrained by weak photoluminescence and limited charge transport. Here, we demonstrate a defect-state engineering strategy through Mn doping in Sn₁₋ₓMnₓS (x = 0.0–0.6) to synergistically enhance its optical and electrochemical properties. The structural analysis shows that Mn incorporation causes lattice distortion, increases strain and dislocation density, and alters the local bonding environment. FTIR and Raman studies support the notion that there are increased Mn-S interactions and defect formation. Optical studies show that Mn doping reduces the bandgap and increases Urbach energy, resulting in increased tail-state formation, which is responsible for sub-bandgap absorption and red-NIR photoluminescence. Electrochemical measurements reveal improved charge-transfer kinetics, lower impedance, and higher Coulombic efficiency, implying that defect-tail states facilitate electron transport. The combined results show a direct relationship between lattice disorder, Urbach tail broadening, and charge-transfer enhancement. Sn₀.₄Mn₀.₆S is the ideal multifunctional semiconductor for optoelectronic and energy-storage applications. This study presents a novel approach to defect-state engineering in layered chalcogenides that combines structural distortion and dual optical-electrochemical functionality. • Mn doping in SnS (Sn₁₋ₓMnₓS, x = 0.0–0.6) induces lattice distortion, reducing crystallite size (47 → 33 nm), increasing strain, and enhancing dislocation density. • Vibrational shifts confirm stronger M–S bonding (570 → 602 cm⁻¹) and defect formation, directly linked to Urbach tail broadening. • Optical properties: bandgap narrows (1.20 → 1.08 eV), Urbach energy increases (0.32 → 0.45 eV), and PL emission red-shifts (680–770 nm) with highest intensity at x = 0.6. • Electrochemical performance: onset potential decreases (∼1.6 → 1.4 V), redox currents increase (∼210/−190 µA), and Coulombic efficiency improves to ∼90 %. • EIS results: charge-transfer resistance drops drastically (10 kΩ → 650 Ω), double-layer capacitance increases (∼40 µF cm⁻²), and conductivity improves (2.0 ×10⁻⁵ → 3.0 ×10⁻⁴ S cm⁻¹). • Novelty: Mn-induced defect-state engineering creates Urbach tail states that act as dual-function centers for sub-bandgap optical absorption and defect-assisted charge transfer , positioning Sn₀.₄Mn₀.₆S as an optimal multifunctional semiconductor.