optimizing earth abundant cztsse solar absorbers via s/se alloying: a pathway to scalable and high efficiency photovoltaics for sustainable solar energy

Cu₂znsn(s,se)₄ (cztsse) thin-film solar cells offer an eco-friendly and earth-abundant alternative to conventional chalcogenides which still suffer from limited efficiencies (<13%) due to bandgap misalignment, defect-induced recombination, and poor carrier collection. this study investigates sulfur-to-selenium (s/se) alloying as a strategy for simultaneous bandgap tuning and charge transport optimization. a compositional gradient of five cztsse thin films (s/se = 1.0–0.0) was synthesized via rf co-sputtering followed by controlled selenization/sulfurization. structural, optical, and electrical properties were examined using xrd, raman, uv–vis–nir, hall effect, and j–v analysis under am1.5g illumination. the optical bandgap decreased nearly linearly from 1.50 to 1.00 ev with increasing se, improving infrared absorption. the intermediate alloy (s/se = 0.5, eg≈1.25 ev) exhibited the best crystallinity (55 nm), lowest urbach energy (36 mev), highest hole mobility (18.4 cm²/v·s), and minimal trap density (3.1×10¹⁶ cm⁻³), yielding a 9.8% efficiency—42% higher than the sulfide baseline. the characteristic features of these improvements include depressed band tailing, minimized recombination and improved carrier extraction. the results would offer a quantitative design of defect-controlled and scalable kesterite absorbers that support future generations of sustainable photovoltaic purposes.