Effect of Bath pH on Interfacial Properties of Electrodeposited n-Cu2O Films

For developing semiconductor junction devices with low cost electrodeposited cuprous oxide (Cu2O), it will be very useful to investigate possibilities to improve the interface properties of Cu2O with suitable junction materials. We have investigated the effect of electrodeposition bath pH on the interfacial properties of n-Cu2O/electrolyte, n-Cu2O/Au, and n-Cu2O/p-Cu2O junctions by exploiting capacitance-voltage and photoresponse measurements. In addition, XRD and SEM measurements were also employed for thin film characterization. We have observed that for an increase in bath pH from 5.7 to 6.5 the resulted n-Cu2O films produced a shift in the extrapolated flat band potential at the aqueous electrolyte interface by 300mV. In addition, we have observed that extrapolated built-in potentials at n-Cu2O/Au and n-Cu2O/p-Cu2O interfaces are also affected by the bath pH. With the change in bath pH, the shift in the flat band potential at n-Cu2O/electrolyte interface is completely opposite to the built-in potential shift at n-Cu2O/Au interface. We explained the observation as a result of the change in net charge at the interfacial surface layers of the interfaces, due to the change in bath pH of the films. Evidence in support of the explanation was further established by the observation that the built-in potential at n-Cu2O/Au interface is increased due to the surface modification of n-Cu2O films with S2- ions. In addition, bath pH could be optimized in the fabrication process of Cu2O homojunctions to yield high photoactivity. In comparison with the best n-Cu2O/Au Schottky junction, we could fabricate a Cu2O homojunction with a 590% increase in V-oc and an 800% increase in I-sc. All the experimental evidences directed toward the idea that the relative band edge position of n-Cu2O at the interfaces is affected by the bath pH. The results reported in this investigation will be very useful in the applications of electrodeposited Cu2O films in photocataytic, solar cell, and other electronic devices.