Earth-abundant trigonal BaCu2Sn(SexS1−x)4 (x = 0–0.55) thin films with tunable band gaps for solar water splitting

Abstract

Photoelectrochemical (PEC) solar devices can convert abundant solar energy directly into storable, clean hydrogen fuel. Here, earth-abundant thin-film solid solutions, BaCu₂Sn(Se_xS_1−x)₄ (x = 0–0.55), demonstrated promise as low-cost absorber materials in efficient PEC solar devices. These p-type compounds crystallize in a trigonal structure and exhibit tunable band gaps of 1.67–2.05 eV for the composition x = Se/(Se + S). The photocathode based on the pure sulfide (x = 0) shows a photocurrent of ∼5 mA cm⁻² saturated at −0.35 V vs. reversible hydrogen electrode (RHE) from a neutral electrolyte. Adding selenium decreases alloy band gaps and extends the absorption range of the solar spectrum, which increases the saturated photocurrent to ∼11 mA cm⁻² at −0.55 V vs. RHE.