(Invited) Modification of P-N Heterointerface of Ternary CuGaSe2 Photochaodes for Efficient Water Reduction Under Sunlight Irradiation
Authors: Ikeda Shigeru , Ishizuka Shogo 
Published: December 2024

Photoelectrochemical (PEC) conversion of sunlight into a chemical energy of hydrogen (H2) through water splitting has been studied extensively to achieve production of storable and transportable energy sources from sunlight. Reduction of carbon dioxide (CO2) using water (H2O) as an electron source into a more reduced chemical species is also an important subject for utilization of solar energy because of its possible production of energy-rich carbonaceous compounds without the use of fossil resources. Among the various possible systems, a coupling of two different semiconductor electrodes termed a Z-scheme system has been studied as an attractive concept in recent years. For the photocathode side in the Z-scheme system, Cu-based selenides and sulfides and their mixed forms of selenosulfides crystallized in chalcopyrite and kesterite structures, which were originally used as photovoltaic materials, were shown to be effective upon modifications on their surfaces for inducing water reduction. A ternary chalcopyrite CuGaSe2 is one of the attractive materials for this application because of its wide bandgap energy (1.67 eV) for achieving a positive onset potential for water reduction. However, compared to other narrow-gap chalcopyrite- and kesterite-based systems such as CIGS (Cu(In,Ga)Se2, a marked improvement of PEC performance over the CuGaSe2-based photocathode has not been achieved. In this study, PEC activity for water reduction (H2 liberation) over a co-evaporated CuGaSe2 compact thin film modified with a CdS layer and Pt deposits under simulated sunlight (AM 1.5G) radiation was evaluated, specifically focusing on the impact of a Cu-deficient layer (CDL) loaded on the top part of the CuGaSe2 film. It was found that the intentional loading of the CDL with an appropriate thickness was effective for achieving a large current flow and relatively positive photocurrent onset. The half-cell solar-to-hydrogen efficiency (STH) reached 6.6% over the best photocathode used. Moreover, the highest photocurrent onset potential of more than 0.9 V vs. RHE was achieved over the photocathode based on the CuGaSe2 film having an extremely thick CDL (200 nm) with a relatively thick CdS layer (90 nm) due to efficient spatial separation of photogenerated carriers, though the thick CDL was also detrimental for efficient photoabsorption of CuGaSe2, resulting in appreciable decrease in photocurrent densities when compared to those obtained over photocathodes based on CuGaSe2 films covered with a relatively thin CDL. Furthermore, an appreciable increase in current density was observed by applying intentional doping of rubidium (Rb), leading to the best STH of 8%. Although further investigation is required, increase in porosity at the heterointerface induced by the Rh doping would contribute for the result. Therefore, regulation of properties at the p-n heterointerface should be important for designing chalcopyrite- photocathodes for further improvements of PEC parameters of water reduction. The concept would also be applicable for preparations of the other PEC devices aiming for water oxidation as well as CO2 reduction.