Enhanced photoelectrochemical properties of Z-scheme ZnO/p-n Cu₂O PV-PEC cells

We have tailored a photovoltaic-photoelectrochemical (PV-PEC) cell by integrating p-n Cu₂O thin films and ZnO nanorods (NRs) on conductive oxide substrates for water splitting. The combined PV-PEC cell can benefit not only from the high oxidation potential of ZnO NRs but also from the visible range light absorption of Cu₂O photovoltaics. Differently from previous methods to fabricate p-n Cu₂O junctions, we employed the type transition from p-to n-Cu₂O by a simple solution treatment and then p-Cu₂O was coated on n-Cu₂O to smoothen the p-n junction interface. We for the first time produced p-n homojunction Cu₂O layers using the identical electrodeposition process. After Au thin film coating on n-Cu₂O and p-n Cu₂O layers by sputtering high-density ZnO NRs were grown by the hydrothermal method to construct the Z-scheme band structures. By comparing the photoresponses of ZnO NRs/Au/n-Cu₂O and ZnO NRs/Au/p-n Cu₂O, we could elucidate the effect of the built-in potential on PEC properties by p-n Cu₂O. ZnO NRs/Au/p-n Cu₂O showed the onset potential of −0.330 V which is more cathodic by 0.167 V than ZnO NRs/Au/n-Cu₂O. Also, its photocurrent was 0.206 mA/cm² under 0.2 V vs SCE which is 237% higher than that of ZnO NRs/Au/n-Cu₂O. These enhancements are attributed to the internal electric field by the p-n junction and the efficient charge separation and migration by utilizing both the p-n junction and the Z-scheme.