Effect of Oxide Intermediate Layers on Pyramidally Textured Cu2O/ZnO Solar Cells Prepared By Electrodeposition

Abstract

Thin-film oxide solar cells composed of a pn heterojunction of p-type cuprous oxide (Cu2O) and n-type zinc oxide (ZnO) have attracted increasing attention as a promising candidate for a low-cost and environment-friendly photovoltaic device because the main constituent elements of copper, zinc, and oxygen are earth abundant and non toxic. In this solar cells, p-type Cu2O with a bandgap energy of 2.1 eV acts as a solar light absorber, and exhibits a theoretical conversion efficiency of ca. 18%. To date, many efforts have been devoted to develop the fabrication methods and improve the photovoltaic conversion efficiency of Cu2O/ZnO-based solar cells.1 We have reported that p-Cu2O/n-ZnO heterojunction can be formed using only electrodeposition in aqueous solutions (< 80°C), where no vacuum process and no post annealing were employed.2 This preparation method, therefore, potentially offers a simple and large scalable route to more low-cost and environment-friendly solar cells. To improve the conversion efficiency, we recently prepared pyramidally textured ZnO films by electrodeposition.3 Pyramidally textures are a well-known structure that exhibits a large light-scattering effect, making efficient solar-light absorption possible. Indeed, the obtained pyramidally textured ZnO films showed a high haze value of 52% in the visible region, and pyramidally textured Cu2O/ZnO solar cells showed a 1.3 times higher short circuit current density than that using a traditional planar ZnO.3 As another approach to improve the conversion efficiency, we demonstrated that electrodeposited highly resistive ZnO thin layers inserted between the Cu2O/ZnO interface can act as an effective intermediate layer.4 Oxide intermediate layers with a wide bandgap energy, such as gallium oxide (GaO x ) and zinc-magnesium oxide (ZnMgO x ), have also been reported to act as an effective interface buffer layer.1 In the present study, these oxide intermediate layers have been inserted between the pyramidally textured Cu2O/ZnO interface to further improve the conversion efficiency. The oxide intermediate layers were prepared using solution processes including electrodeposition and spin-coating techniques, and the photovoltaic performances were evaluated. In one case, for example, a ~1.7 times higher conversion efficiency was achieved by using a solution-processed GaO x intermediate layer, where both the short circuit current density and open circuit voltage increased significantly. Acknowledgement: This work was supported by JSPS KAKENHI, JAPAN.