To mitigate the global warming issue caused by the current heavy dependence on fossil fuels, the researchers propose hydrogen as an alternative energy carrier, which can be produced from water splitting in a photoelectrochemical (PEC) cell. In the PEC cell, sunlight can be absorbed and stored in the form of chemical energy (i.e. hydrogen) by using a semiconductor photoelectrode. To achieve an efficient photocatalytic electrode, the semiconductor material has to satisfy several requirements such as high visible-light absorbing ability, effective charge separation, and fast catalytic reaction on its surface. However, a single semiconductor material often fails to have poor overall efficiency. Therefore, combination of two or more semiconductor junctions are suggested to compensate the other material’s drawbacks.

For example, WO₃/BiVO₄ heterojunction is one of the successful combinations in which WO₃ play a role in electron transfer and BiVO₄ increases the visible light absorption to increase the photoanodic water splitting performance. The charge separation properties at the WO₃ and BiVO₄ interface are characterized by measuring the photocurrent in the absence/presence of hole scavengers, light absorption spectroscopy, and intensity-modulated photocurrent spectroscopy (IMPS). The morphology and the thickness of the mesoporous WO₃ layer were controlled to monitor to understand the charge separation efficiency at the interface. Our study can provide insights to heterojunction design.

Meanwhile, a p-type chalcopyrite Cu(In,Ga)(Se,S)₂ (CIGS) semiconductor was prepared by a solution precursor processed method and used as a photocathode for hydrogen evolution reaction, and charge separation properties on its surface was studied by electrochemical impedance spectroscopy measurement. By controlling the surface states and introducing the passivation layer of ZnS, the photocurrent with the CIGS thin film on the transparent FTO substrate was significantly improved. Also, CIGS on FTO substrates was useful to compare PEC performances under forward and backward illumination conditions. Since slow electron transfer in CIGS photocathode is believed to limit PEC performance, further decrease of the defect structure of CIGS film is desirable for efficient solar-to-hydrogen conversion.