Kesterite Cu2ZnSnS4-based photoelectrochemical water reduction with high photocurrent density employing all-environmental benign materials

A novel architecture of a Cu2ZnSnS4 (CZTS)-based heterojunction photocathode is proposed to enhance its photoelectrochemical (PEC) water reduction performance as well as to substitute the toxic CdS buffer layer as the state-of-the-art buffer layer in CZTS PEC device. The CZTS photoactive layers on Mo-coated soda-lime glass substrates were prepared first via spin-coating of the DMSO-based CZTS precursor, followed by subsequent sulfurization at 620 °C for two hours. The CZTS precursor used in this contribution contains dimethyl sulfoxide solvent, salts of Cu, Zn and Sn as well as the sulfur source of thiourea. Subsequent sputter depositions of ZnO, Zn(O,S), Nb-doped TiO2, and Pt overlayers were carried out on the as-prepared CZTS photoactive layer. These overlayers were directly sputtered from their respective sputter targets, simplifying and speeding up the overall heterojunction photocathode preparation through a single sputtering run. All layers encompassing the heterojunction photocathode were structurally characterized by SEM, FT-Raman, Grazing-Incidence XRD and FT-IR. XPS analysis was additionally performed to quantify the composition of the CZTS layer. A Kelvin Probe system and Ambient Pressure Photoemission Spectroscopy (APS) was also employed to study the band alignment between CZTS and Zn(O,S) layer. This contribution demonstrates and discusses the PEC device photocurrent density enhancement through the fabrication of various photocathode architectures utilizing a CZTS as an absorber layer, with ZnO, Nb-doped TiO2 and Pt as the overlayers. The CZTS photoresponse in the photoelectrochemical (PEC) water reduction increases with an implementation of sputtered Zn(O,S) overlayer, indicated by an increase in PEC photocurrent density. Further significant photoresponse improvement is achieved by the use of ZnO, Nb-doped TiO2 and Pt overlayers. The maximum PEC photocurrent density was delivered by the photocathode with CZTS/Zn(O,S)/Nb:TiO2/Pt architecture in 0.2 M Na2SO4 electrolyte, yielding ~16 mA/cm² at 0 VRHE and pH 10. It was also found that the presence of a ZnO layer in the heterojunction photocathode has a drawback, i.e. it reduces the photocurrent density, probably due to an additional interface in the photocathode multilayer structure. To the best of our knowledge, this contribution represents the highest photocurrent density achieved for a Cd-free CZTS PEC device.