Anion-Engineered CIGS: Unlocking Defect-Passivated Surfaces for High-Yield CO2 Photoreduction via Vapor-Phase Sulfurization

Abstract In this work, a surface anion engineering strategy was presented to boost photocatalytic CO₂ reduction on Cu(In,Ga)Se₂ (CIGS) thin films. A 30 min sulfur‑vapor anneal at 450 °C substitutes sulfur for selenium only within the top ≈ 100 nm, yielding an S/Se ratio of ≈ 0.28 while leaving the 1.2 µm bulk and Cu(I)/In(III)/Ga(III) oxidation states intact. Zero‑shift‑corrected X‑ray diffraction showed reflection‑specific shifts of 0.11–0.16°, corresponding to a 0.24% lattice contraction. Ultraviolet photoelectron spectroscopy registers a 0.10 eV bandgap widening and a work‑function rise from 4.6 to 4.9 eV. Room‑temperature photoluminescence exhibits a 13 meV blue shift and a 42% drop in integrated intensity, while time‑resolved PL reveals the average carrier lifetime doubling from 26 to 49 ns, evidencing defect passivation rather than increased non‑radiative loss. Electrochemical impedance spectroscopy shows that the charge‑transfer resistance decreases from 120 to 45 Ω, and linear sweep voltammetry records a cathodic photocurrent density of − 0.33 mA cm⁻ 2 at − 0.4 V vs Ag/AgCl (–0.18 mA cm⁻ 2 pristine). Under AM 1.5 G, CH₄ and CH₃OH formation rates rise to 3.3 and 3.2 µmol g⁻ 1 h⁻ 1 , respectively. These results establish vapor‑phase sulfurization as a rapid, iso‑volume route to defect‑passivated, high‑performance CIGS photocathodes for solar‑fuel production.