Design and characterization of ZnO/NiO and ZnO/CuO heterostructures for high-performance solar cell applications

Metal-oxide-based heterojunction solar cells have received burgeoning research interests owing to the prospect of high efficiency, low cost, and environmental benignity. Of all the systems, ZnO heterojunctions with either NiO or CuO have received much interest for next generation photovoltaic devices. This review summarizes the material properties, fabrication methods, interface characteristics, and photovoltaic performances of ZnO/NiO and ZnO/CuO heterojunctions in a comprehensive comparison manner. ZnO, as a wide-bandgap n-type semiconducting and high electron-conductivity material, with properties including optical transparency and chemical stability, has become a good candidate for the ETL layer. Because of its large band gap and high work function, NiO can efficiently extract holes and enhance the stability of devices. These merits make ZnO/NiO hetero-junctions particularly suitable for transparent and UV-active solar cells. By comparison, CuO with a small bandgap and high absorption coefficient in the visible region can also lead to efficient light utilization of ZnO/CuO heterojunctions through type-II band alignment at the expense of becoming more susceptible to interfacial defects and environmental deterioration. In this review, we present a critical overview of physical, chemical, and green-synthesis routes, band-alignment engineering strategies, interface-defect passivation methods, and degradation mechanisms affecting the two systems. This work, which discusses the tradeoffs among optical transparency, absorption efficiency, stability and interfacial recombination inform practical design guidelines for future advances in optimizing ZnO-based heterojunction solar cell devices for next generation of scalable and environmentally friendly photovoltaic technologies.