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.