High-efficiency heterojunction solar cells consisting of a nondoped Ga 2 O 3 thin film as an n-type semiconductor layer and a p-type Cu 2 O sheet as the active layer as well as the substrate, prepared by thermally oxidizing a Cu sheet, are demonstrated. The use of an n-type Ga 2 O 3 thin film can greatly improve the performance of n-Ga 2 O 3 /p-Cu 2 O heterojunction solar cells. The highest efficiency of 5.38% was obtained in an Al-doped ZnO/Ga 2 O 3 /Cu 2 O heterojunction solar cell fabricated with an n-Ga 2 O 3 thin-film layer prepared at room temperature with a thickness of 75 nm by a pulsed laser deposition method.

High conversion efficiencies were achieved in low cost n–p heterojunction oxide solar cells with an Al-doped ZnO (AZO)/non-doped ZnO (ZO)/Cu2O structure. This achievement was made possible by the formation of an n-ZO thin-film layer, prepared with an appropriate thickness by low damage deposition, on high quality Cu2O sheets produced by the thermal oxidization of copper sheets: n-ZO thin film optimal thickness ranges from 30 to 50 nm. Photovoltaic characteristics such as an open circuit voltage of 0.69 V, a fill factor of 0.55 and a conversion efficiency of 3.83% were attained under simulated AM1.5G solar illumination.

Efficiency enhancement was achieved in Cu2O-based heterojunction solar cells fabricated with a zinc–germanium-oxide (Zn1− x Ge x -O) thin film as the n-type window layer and a p-type Na-doped Cu2O (Cu2O:Na) sheet prepared by thermally oxidizing Cu sheets. The Ge content (x) dependence of the obtained photovoltaic properties of the heterojunction solar cells is mainly explained by the conduction band discontinuity that results from the electron affinity difference between Zn1− x Ge x -O and Cu2O:Na. The optimal value of x in Zn1− x Ge x -O thin films prepared by pulsed laser deposition was observed to be 0.62. An efficiency of 8.1% was obtained in a MgF2/Al-doped ZnO/Zn0.38Ge0.62-O/Cu2O:Na heterojunction solar cell.

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In this paper, we describe efforts to enhance the efficiency of Cu 2 O-based heterojunction solar cells fabricated with an aluminum-gallium-oxide (Al-Ga-O) thin film as the n-type layer and a p-type sodium (Na)-doped Cu 2 O (Cu 2 O:Na) sheet prepared by thermally oxidizing copper sheets. The optimal Al content [X; Al/(Ga + Al) atomic ratio] of an Al X -Ga 1%X -O thin-film n-type layer was found to be approximately 2.5 at. %. The optimized resistivity was approximately 15 Ω cm for n-type Al X -Ga 1%X -O/p-type Cu 2 O:Na heterojunction solar cells. A MgF 2 /AZO/Al 0.025 -Ga 0.975 -O/ Cu 2 O:Na heterojunction solar cell with 6.1% efficiency was fabricated using a 60-nm-thick n-type oxide thin-film layer and a 0.2-mm-thick Cu 2 O:Na sheet with the optimized resistivity.

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