This paper describes the photovoltaic properties of a homojunction Cu₂O solar cells fabricated with a various impurity-doped epitaxially grown n-type Cu₂O thin-film by electrochemical deposited (ECD) method. The various impurity-doped n-type Cu₂O thin film layer was prepared on thermally oxidized polycrystalline p-type sodium-doped Cu₂O (p-Cu₂O:Na) sheet substrates under various deposition conditions using the following ECD process. Initially, an acetic acid and Copper (II) acetate aqueous solution was prepared with 0.08 M acetic acid, 0.02 M Copper (II) acetate and de-ionized water; then a 0.1-1.0×10^-5 M CuCl₂, InCl₃, CdCl₂, ZnCl₂ or MnCl₂ aqueous solution was added to the solution as a doping impurity; and after that, a 3 M LiOH, NaOH or KOH aqueous solution was added to adjust the pH. Next, a p-Cu₂O:Na sheet substrate was immersed in the above solution at a temperature of 60-90 ^oC; a Pt plate and the p-Cu₂O:Na sheet substrate were used as the anode and cathode electrodes, respectively, of the ECD. Finally, the deposition was carried out under various current densities of 0.1-2.0 mA/cm² for durations of 1-30 min. In addition, various n-type oxide semiconductor, such as Ga_XAl_1-X-O or Zn_1-XGe_X-O multicomponent oxide thin films and an Al-doped ZnO (AZO) thin film were deposited at room temperature using a pulsed laser deposition or magnetron sputtering methods as the n-type semiconductor layer and the transparent electrode, respectively. The solar cells were fabricated by forming an AZO/n-oxide semiconductor/various impurity-doped n-type Cu₂O/p-Cu₂O:Na structure on the front surface of the Cu₂O:Na sheets and an Au ohmic electrode on the back surface (the sheets function as the active layer as well as the substrate). The photovoltaic properties of the resulting solar cells were measured under AM1.5G solar illumination.

For example, the Cu₂O homojunction solar cells was fabricated by epitaxialiy growing a Cl-doped Cu₂O (Cu₂O:Cl) thin film on p-Cu₂O:Na (with a hole concentration of 10¹⁵-10¹⁹ cm^-3) sheets by ECD. Significantly enhanced open circuit voltage (V_OC) and conversion efficiency (η) were achieved in AZO/n-Cu₂O:Cl/p-Cu₂O:Na heterojunction solar cells fabricated by Cu₂O:Cl thin-film as the n-type layer by using ECD; these values were higher than those obtained in AZO/p-Cu₂O and AZO/n-ZnO/p-Cu₂O solar cells. The obtained enhanced Voc and η suggest that the Cu₂O:Cl thin-film n-type layer acts as an appropriate active layer. In addition, significant improvement of photovoltaic properties was achieved in heterojunction solar cells fabricated with a Ga_1-XAl_X-O or Zn_1-XGe_X-O multicomponent oxide thin film n-type semiconductor window layer prepared by optimizing the chemical composition (the Ga or Ge content). Note that the photovoltaic properties obtained in AZO/Ga_0.975Al_0.025-O or Zn_0.38Ge_0.62-O/n-Cu₂O:Cl/p-Cu₂O:Na (with a hole concentration on the order of 10¹⁵ cm^-3) heterojunction solar cells could be controlled by changing the Cl content doped into the Cu₂O:Cl thin film.