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We improved the photovoltaic properties of Cu2O-based heterojunction solar cells using n-type oxide semiconductor thin films prepared by a sputtering apparatus with our newly developed multi-chamber system. We also obtained the highest efficiency (3.21%) in an AZO/p-Cu2O heterojunction solar cell prepared with optimized pre-sputtering conditions using our newly developed multi-chamber sputtering system. This value achieves the same or higher characteristics than AZO/Cu2O solar cells with a similar structure prepared by the pulse laser deposition method.

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This paper describes the effect of inserting a impurity-doped Cu2O thin-film buffer layer on the obtainable photovoltaic properties in heterojunction solar cells. The heterojunction solar cells were fabricated by preparing an n-type oxide semiconductor thin-film window layer on a Cu2O homojunction that was formed by epitaxially growing the doped Cu2O thin film buffer layer on p-type Cu2O sheets by electrochemical deposition (ECD). The doped Cu2O thin film buffer layer, incorporating various impurities, 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 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. Various n-type 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 as the n-type semiconductor layer and the transparent electrode, respectively. The solar cells were fabricated by forming an AZO/n-oxide semiconductor/doped 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. The Cu2O homojunction was formed by epitaxially growing a doped Cu2O 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/Ga0.98Al0.02-O/Cu2O:Mn/p-Cu2O:Na heterojunction solar cells fabricated by inserting a Mn-doped Cu2O (Cu2O:Mn) thin-film as the buffer layer with an appropriate thickness using ECD; these values were higher than those obtained in AZO/Cu2O:Mn/p-Cu2O and AZO/Ga0.98Al0.02-O/p-Cu2O solar cells. The obtained enhanced Voc and η suggest that the Cu2O:Mn thin-film buffer layer has excellent crystallinity as well as acts as an appropriate active layer. In addition, significant improvement of photovoltaic properties was achieved in heterojunction solar cells fabricated with a GaXAl1-X-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/Ga0.98Al0.02-O or Zn0.28Ge0.62-O/Cu2O:Mn/p-Cu2O:Na (with a hole concentration on the order of 1015 cm-3) heterojunction solar cells could be controlled by changing the Mn content doped into the Cu2O:Mn thin film. An efficiency over 8% was obtained in an AZO/ Zn0.28Ge0.62-O/Cu2O:Mn/p-Cu2O:Na heterojunction solar cell fabricated with a Cu2O:Mn thin film that was identified as an i-type semiconductor.