Progress of Cu2O/ZnO oxide heterojunction solar cells

Recent progress of low cost Cu2O/ZnO hetero-junction solar cells is reviewed in this paper. The Cu2O used as an absorbing layer in photovoltaic cells is a direct bandgap semiconductor, exhibiting natural p-type conductivity. The source material of Cu2O-based solar cells is abundant and environmentally friendly. The main device structure of Cu2O/ZnO solar cells presents a planar and nano-wire/nano-rod configuration. The nanostructured Cu2O architecture conduces to charge collection in the device. The planar Cu2O absorbing layer with large grain size, achieved through the thermal oxidation of Cu sheets, exhibits high quality of the Cu2O/ZnO solar cells. The interface buffer layer (like i-ZnO, a-ZTO and Ga2O3) and back surface field (BSF, such as p+-Cu2O) can effectively improve energy band alignment match and increase carrier transport. The Cu2O paired with a 10-nm-thick Ga2O3 layer provides a nearly ideal conduction band offset and thus reduces the interface recombination. The Ga2O3 is a highly suitable buffer layer for enhancing the Voc (Voc value reaches 1.2 V) and conversion efficiency of Cu2O-based solar cells. The p+-Cu2O like N-doped Cu2O (Cu2O:N) and Na-doped Cu2O (Cu2O:Na) can reduce back-contact resistance and create an electron-reflecting back surface field in the Cu2O based solar cells. When a p-type Cu2O: Na acts as an absorbing layer and a zinc-germanium-oxide (Zn1-xGex-O) thin film is used as an n-type layer (buffer), Cu2O hetero-junction solar cell with the device structure MgF2/Al-doped ZnO (ZnO:Al)/Zn0.38Ge0.62-O/Cu2O:Na shows an efficiency of 8.1%. The oxide hetero-junction solar cells have a great potential application in the future photovoltaic field.