Electrodeposited p-Cu2O Films – Role of Redox-Active Compounds Under Photoelectrochemical Operation Revisited

Abstract

The p-type semiconducting copper oxides CuO and Cu₂O are of interest for the conversion of solar energy due to their medium-wide bandgap and the position of their conduction band, allowing for reductive processes in junctions with electrolytes under irradiation. In this work, on Cu₂O, the efficiency of several such processes in competition with self-reduction is critically reviewed and experimentally studied. Up to 2000 nm thick films were obtained via potentiostatic electrodeposition on fluorine-doped tin oxide on glass from alkaline solutions of CuSO₄ using lactic acid as a complexant. The films consisted of a dense arrangement of crystallites as seen by scanning electron microscopy and were of phase pure Cu₂O as shown by X-ray diffraction (XRD). The films were specular, with an absorption coefficient of 50,000 cm⁻¹ at 480 nm and a direct bandgap of 2.5 eV. In junctions with aqueous electrolytes, the material was found to be p-type. Under electrical bias, cathodic and photocathodic currents passed and increased dramatically when reducible redox compounds were added. The influence of various redox couples (O₂, H₂O₂, and methylviologen [MV, 1,1'-dimethyl-4,4'-bipyridinium]) and their concentration in the electrolyte on the stability of the electrodes was studied. Long-time experiments showed that to avoid degradation of the electrodes, the use of oxygen-saturated solutions was mandatory when no other redox couple was added. H₂O₂-containing electrolytes gave rise to constant photocurrents and no alteration of the electrodes was found by XRD. MV yielded cathodic photocurrents. Reoxidation of its reduced form by dissolved oxygen was necessary in order to hinder dimerization or further reduction to MV⁰ and association of the latter to MV⁰_n, producing a whitish layer on top of the electrodes which led to their inactivation.