Effect of Oxide Intermediate Layers on Pyramidally Textured Cu2O/ZnO Solar Cells Prepared By Electrodeposition
Authors: Shinagawa Tsutomu , Chigane Masaya , Tani Junichi , Izaki Masanobu 
Published: February 2020

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.

References

1. Y. Ievskaya, R. L. Z. Hoye, A. Sadhanala, K. P. Musselman and J. L. MacManus-Driscoll, Sol. Energy Mater. Sol. Cells, 135, 43 (2015).

2. M. Izaki, T. Shinagawa, K. T. Mizuno, Y. Ida, M. Inaba and A. Tasaka, J. Phys. D: Appl. Phys., 40, 3326 (2007).

3. T. Shinagawa, K. Shibata, O. Shimomura, M. Chigane, R. Nomura and M. Izaki, J. Mater. Chem. C, 2, 2908 (2014).

4. M. Izaki, T. Ohta, M. Kondo, T. Takahashi, F. B. Mohamad, M. Zamzuri, J. Sasano, T. Shinagawa and T. Pauporté, ACS Appl. Mater. Interfaces, 6, 13461 (2014).