Homojunction Cu2O Solar Cells Fabricated with Various Impurity-Doped Epitaxially Grown n-Type Cu2O Thin Film By Electrochemical Deposition
Authors: Miyata Toshihiro , Minami Tadatsugu 
Published: February 2020

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 GaXAl1-X-O or Zn1-XGeX-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 (VOC) 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 Ga1-XAlX-O or Zn1-XGeX-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/Ga0.975Al0.025-O or Zn0.38Ge0.62-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.