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.