Copper-based kesterite thin films for photoelectrochemical water splitting

Copper kesterite Cu2ZnSnS4 is a promising photoabsorber material for solar cells and photoelectrochemical (PEC) water splitting. In this article, we will first review the crystallographic/energetic structures of Cu2ZnSnS4 in view of its applications to sunlight conversion devices. Then, historical progress in photovoltaic properties of Cu2ZnSnS4-based solar cells is introduced. Finally, studies on PEC H2 evolution over Cu2ZnSnS4-based photocathodes are reviewed in detail. For realizing efficient PEC H2 evolution, surface modifications with an n-type buffer layer (such as CdS) and a catalytic site (such as Pt nanoparticles) were found to be indispensable. Since these surface-modified photocathodes had poor resistances under an operating bias due to the occurrence of oxidative photocorrosion of the CdS layer and elimination of the Pt catalysts, coverage with a protection layer was required to improve the long-term durability. Moreover, partial or complete substitution of the constituent cations with some cations was proved to be effective for improving PEC properties. Although recent studies showed a rapid increase in PEC properties, there is room for further development of PEC properties by using effective combinations among surface protection(s), defect engineering(s), and band engineering(s).