Photocathode Characteristics of a Spray-Deposited Cu₂ZnGeS₄ Thin Film for CO₂ Reduction in a CO₂-Saturated Aqueous Solution

A kesterite Cu₂ZnGeS₄ (CZGS) thin film was prepared on an Mo-coated glass substrate by spray deposition of an aqueous solution containing constituent elements followed by heat treatment in a sulfur atmosphere. The results of analysis of the ionization potential of the CZGS thin film by photoelectron spectroscopy (PES) and the external quantum efficiency (EQE) spectrum of a solar cell device based on the CZGS thin film showed that this compound had ca. 0.7 eV more negative conduction band minimum (CBM) than that of the well-studied kesterite Cu₂ZnSnS₄ (CZTS) but had almost the same energy level of the valence band maximum (VBM) as that of CZTS. Owing to the relatively shallow CBM, the CZGS film worked as a photocathode for photoelectrochemical (PEC) CO₂ reduction in a neutral aqueous solution (0.1 M KHCO₃) saturated with CO₂ by applying a bias potential lower than that for inducing the reaction under a dark condition, whereas the CZTS film could not work under the same condition. Enhancement of PEC CO₂ reduction activity by surface coverage of the CZGS thin film with a ZnS layer is also discussed on the basis of the electronic structure of the thus-formed CZGS-ZnS interface.