Observation of interface modification of electrodeposited p-Cu2O thin films in an aqueous electrolyte

Abstract

Interface engineering via modification of semiconductor surfaces of junction devices is a powerful technique to improve the performance of devices. In addition, semiconductor material Cu2O has recently gained a considerable attention as a low cost semiconductor material suitable for developing thin film solar cells, water splitting in photoelectrochemical cells and gas sensors. Indeed, the possibility of relative band edge shifts of Cu2O with suitable interfacing materials will pave the way for interface engineering to improve the efficiency of those devices. In this study we have investigated this possibility of using electrodeposited p-type cuprous oxide thin films deposited using a lactate bath containing 3 M sodium lactate and 0.4 M CuSO4 at various pH values. These Cu2O films were used in a photolectrochemical cell to form semiconductor/electrolyte junctions in a 0.1 M sodium acetate aqueous solution and then to measure the flat band potential variations with the pH of the Cu2O film deposition baths. It was observed that pH value of the Cu2O film deposition bath is very sensitive to the flat band potential. This result gives a direct evidence that the surface of Cu2O film is modified at the Cu2O/electrolyte interface producing a relative band edge shift yielding the observed flat band shifts. We observed a general trend of flat band potential shift of about 350 mV in the positive direction, as the pH of the deposition bath was changed from 7 to 12.5. The observed shift in the flat band potential in the positive direction is very useful for the water splitting reaction because the valence band edge of Cu2O is shifted positively relative to the oxygen redox potential. Our observation of highest photoresponse for Cu2O thin films prepared at pH 13.5 is a direct evidence for the positive shift of the band edges. The observation of the interface modification of Cu2O in aqueous electrolyte may be further extended to other suitable interfaces for developing Cu2O based junction devices.