The ability to systematically vary the flatband potential and the band edge positions of Cu 2 O by varying the deposition bath pH is considered important in the context of using Cu 2 O for photocatalytic water splitting. Capacitance-voltage (C-V) and dark and light current-voltage measurements were taken in order to investigate the electronic properties of electrodeposited Cu 2 O thin films grown in an acetate bath containing a 0.01 M Cupric Acetate having different bath pH values. The Mott-Schottky plots resulting from C-V measurements revealed that the flatband potential and doping density of the films strongly depended on the deposition bath pH. For pH values lower than 7.2, conductivity of Cu 2 O films remained n-type while it changed to p-type for higher pH values indicating that change of n-type to p-type character of the Cu 2 O films can be controlled by the bath pH. With increasing pH value, the grain size decreased, while crystal shapes transformed from truncated octahedral to cubic within the tested pH range. A well ordered cubic structure was observed in films deposited at pH 7.8. The rate of deposition decreased at higher pH values leading to thinner films indicating the effect of pH on the film thickness. Calculated band edge positions of conduction and valance bands of the Cu 2 O films were −4.19 eV and −6.29 eV respectively with respect to the vacuum level.

Capacitance-voltage (C-V) and modulated light-induced current-voltage measurements were employed to investigate the Cu 2 O/electrolyte junction of electrodeposited n-Cu 2 O thin films. The Mott-Schottky plots resulting from the C-V measurements revealed that the extrapolated flat-band potential of n-Cu 2 O films was strongly influenced by the pH of the bath where the films were grown. The flat-band potential change was 300 mV for a pH difference of 0.8 and showed that the surface chemistry at an n-Cu 2 O/aqueous electrolyte interface was strongly affected by the pH of the film deposition bath. In addition, current-potential measurements revealed that at the measured flat-band potential the photocurrent did not vanish for n-Cu 2 O films and the Fermi level at the interface was pinned due to the presence of electrically active surface states. Information on the presence of electrically active surface states and the shift in flat-band potential will be very useful for applications of n-Cu 2 O films in various devices.

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For developing semiconductor junction devices with low cost electrodeposited cuprous oxide (Cu 2 O), it will be very useful to investigate possibilities to improve the interface properties of Cu 2 O with suitable junction materials. We have investigated the effect of electrodeposition bath pH on the interfacial properties of n-Cu 2 O/electrolyte, n-Cu 2 O/Au, and n-Cu 2 O/p-Cu 2 O 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-Cu 2 O films produced a shift in the extrapolated flat band potential at the aqueous electrolyte interface by 300 mV. In addition, we have observed that extrapolated built-in potentials at n-Cu 2 O/Au and n-Cu 2 O/p-Cu 2 O interfaces are also affected by the bath pH. With the change in bath pH, the shift in the flat band potential at n-Cu 2 O/electrolyte interface is completely opposite to the builtin potential shift at n-Cu 2 O/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 builtin potential at n-Cu 2 O/Au interface is increased due to the surface modification of n-Cu 2 O films with S 2À ions. In addition, bath pH could be optimized in the fabrication process of Cu 2 O homojunctions to yield high photoactivity. In comparison with the best n-Cu 2 O/Au Schottky junction, we could fabricate a Cu 2 O 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-Cu 2 O at the interfaces is affected by the bath pH. The results reported in this investigation will be very useful in the applications of electrodeposited Cu 2 O films in photocataytic, solar cell, and other electronic devices.

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