Chalcogenide compounds have emerged as a leading class of thin films for photocathodes due to their suitable band gap for solar energy absorption, high photo-absorption coefficients, and usability in the polycrystalline state. Although Cu-chalcogenide photocathodes such as CuInS2, CuGaSe2, CuGa3Se5, (Ag,Cu)GaSe2, and Cu(In,Ga)Se2, show excellent solar-to-hydrogen conversion efficiency, there are many difficulties in largescale production from the viewpoints of low throughput and material utilization because of the limited availably of elements (i.e., In, Ga and Se). It is therefore evident that more research is required to search for alternative inexpensive and earth-abundant materials.
Cu3BiS3 is one of the promising chalcogenide compounds for a photovoltaic device due to its ability to absorb visible light (Eg ≈ 1.4–1.7 eV), a high absorption coefficient (>105 cm-1) compared to that of CuInSe2 and Cu2ZnSnS4, and p-type conductivity with a carrier concentration of ~ 2×1016 cm-3. Moreover, it is made of cheap, non-toxic, earth-abundant elements and can be easily produced on a large scale.
We fabricated a Cu3BiS3 thin film by a simple electrodeposition technique on a molybdenum-coated glass substrate to utilize it as a photocathode for PEC hydrogen evolution. The Cu3BiS3 film was composed of dense crystallites with thicknesses of ca. 0.65 μm, and the optical bandgap was estimated to be ca. 1.6 eV. Photoelectrochemical characterization revealed that the Cu3BiS3 film was a p-type semiconductor with a flat band potential of ca. +0.1 V (vs Ag/AgCl at pH 6), which is suitable for water reduction but cannot be used for water oxidation. Deposition of an In2S3 buffer layer, by which effective contact between p-type Cu3BiS3 and n-type In2S3 was formed, resulted in a significant increase in the photocurrent and a large shift of the photocurrent onset to the positive region. H2 evolution under AM 1.5G simulated solar light was demonstrated using the Pt-In2S3/Cu3BiS3 electrode combined with a Pt electrode under an applied bias (0 V vs. RHE).