Structural, optical and photoelectrical properties of Cu2O films electrodeposited at different pH
Cu2O films were electrodeposited on FTO and ZnO thin film substrates under different electrolyte pH. The Cu2O films on FTO substrates have a preferred orientation along the (111) planes for the match between the growing Cu2O film and the FTO substrate at the interface, whereas a transition of the preferred orientation from (100) to (111) occurs for the films on ZnO thin film substrates as electrolyte pH increases. All the Cu2O films show very similar photoluminescence (PL) spectra characteristics with a green emission band, which is related to the inter-band transitions, and a red broad emission band, which is assigned to the defects. The relative weak PL spectra for the films on ZnO thin film substrates can be ascribed to the energy and charge transfer from Cu2O to ZnO. Under light illumination, the Cu2O films on ZnO thin film substrates show the photovoltaic effect, which indicates the formation of Cu2O/ZnO heterojunctions. The photoresponse of the heterojunctions at zero bias voltage shows high sensitivity to visible light illumination, with excellent stability and reproducibility. The photocurrent is larger for the heterojunctions of the Cu2O film deposited at the pH 10.5, which is attributed to the good crystalline quality of the Cu2O film and strong built-in electric field of the ZnO/Cu2O interface.