Large-area thin-film synthesis of photoactive Cu₃PS₄ thiophosphate semiconductor with 0-14 pH stability range

Metal phosphosulfide materials have sparked growing interest due to their wide range of properties and applications. Despite a sizable presence in the bulk materials synthesis literature, reports of phosphosulfide thin films are extremely scarce. This may be due to the hazardous, volatile, and corrosive nature of many phosphorus and sulfur precursors, combined with the high sulfur chemical potential needed to incorporate large amounts of this volatile element in the (PS₄)^3- and (P₂S₆)^4- thiophosphate anions that are common in this material family. To overcome these limitations, we introduce directional-and-diffuse multi-anion reactive sputtering (DADMARS). DADMARS uniquely combines sputtered metal sources, reactive gas, and a thermally cracked evaporated nonmetal source with a high chemical potential to gain access to challenging multi-anion chemistries in thin-film form. In this study, we employ Cu, PH₃, and S _x as the sputtered, gaseous, and evaporated sources to deposit polycrystalline Cu₃PS₄ thin-film thiophosphate semiconductors with the narrowest X-ray diffraction peaks recorded for a phosphosulfide thin film. Single-phase Cu₃PS₄ can be grown over relatively large areas (8 × 8 cm² and extendable) at temperatures down to 375 °C, which is lower than for similar sulfide semiconductors CuInS₂ and Cu₂ZnSnS₄. This suggests potential compatibility with established device fabrication processes. Cu₃PS₄ thin films exhibit remarkable environmental, radiation, and chemical stability, with negligible etch rates in the 0-14 pH range. Thin-film Cu₃PS₄ is a p-type semiconductor with a bandgap of 2.3-2.5 eV, strong light absorption, and detectable photoluminescence at room temperature. This combination of stability and optoelectronic properties positions Cu₃PS₄ as a promising earth-abundant semiconductor for photoelectrochemical applications.