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.