In situ growth of monocrystal p-CuGaO₂ nanosheet as a hole transfer layer in a photoelectrode for solar hydrogen production

Hole transporting layers are very important for solar energy conversion devices, such as perovskite solar cells, organic photovoltaic devices and solar water splitting cells. Recently, CuGaO₂ as a hole transporting layer has attracted intense interest due to its high hole mobility and wide band gap. However, there is still a challenge to prepare a semitransparent CuGaO₂ film. It is desirable to develop a facile method to prepare a CuGaO₂ film with good electronic contact and semi-transparency for application in solar energy conversion devices. Herein, for the first time, a one-step hydrothermal method is developed to directly grow semi-transparency p-CuGaO₂ films on FTO substrates. CuGaO₂ single crystals with much larger grain sizes are obtained, which are important for the applications as hole transporting layers. We find that Br^- in the precursor solution plays a key role in obtaining a semitransparent CuGaO₂ film. The effects of Br^- on the growth of CuGaO₂ are also investigated in details. As an example, a CuGaO₂ film is used as a hole transfer layer to improve photoelectrochemical performance of a CuInS₂ photocathode for solar water splitting.