(Invited) Development of High-Efficiency Transparent Cu2O Top Cells for Tandem Photovoltaics with Efficiency Exceeding 30%

Solar cells with high power generation efficiency are expected to be used in mobility applications that require high power output in a limited footprint, such as electric vehicles (EVs) and high-altitude platform stations (HAPS), as well as in current products for housing and industrial applications. A tandem structure is an effective option for high-efficiency solar cells, and various combinations of solar cells have been proposed as the top and bottom cells, including InGaP/GaAs and perovskite/Si. Our proposed tandem solar cell consists of a transparent cuprous oxide (Cu2O) top cell and a crystalline Si bottom cell and is a promising candidate for the next generation of tandem solar cells because of its combination of high efficiency and low cost. High tandem efficiency of 30% or higher is required for various advanced applications such as mobility. Cu2O has a wide bandgap of 2.1 eV, and its spectral sensitivity complements that of crystalline Si, so high bottom-cell efficiency can be achieved. By using Cu2O in the top cell, the Si bottom cell can be expected to have an efficiency of 20% due to long-wavelength light transmitted through Cu2O, and by generating an efficiency of 10% in the Cu2O top cell, a tandem efficiency exceeding 30% is within sight. Cu2O is a low-cost material because it consists of the abundant elements copper and oxygen. Also, it can be formed on inexpensive glass substrates by low-cost manufacturing processes such as sputtering. We have established a technology to deposit high-quality Cu2O thin films as a single phase on transparent electrodes by reactive sputtering. By precisely adjusting the oxygen gas flow rate and substrate temperature, Cu and CuO present as different phases could be removed and a metastable Cu2O phase selectively deposited. A top cell using this low-defect Cu2O as the optical absorption layer has achieved an efficiency of 8.4% [1]. In this paper, we report on a Cu2O top cell that exhibits the world's highest efficiency of 10.5%, which was achieved by improving the short-circuit current density (Jsc) and the open-circuit voltage (Voc). By increasing the size of the Cu2O top cell by a factor of about 3, the area ratio of the dead area at the cell edge to the power-generating area was reduced. We expect that our Cu2O film has a long carrier diffusion length, and thus when the cell area is small, many carriers diffuse to the cell edge face and recombine. By contrast, a larger cell area increases the number of carriers that can contribute to power generation without carriers reaching the cell edge face. Device simulations showed that there were many interfacial defects between p-type Cu2O and n-type Ga2O3, which reduced Voc. Therefore, a unique passivation layer was introduced at the pn interface. This suppressed the interfacial defects and increased Voc by about 0.1 V compared with the conventional cell. The measured efficiency of 10.5% for the Cu2O top cell exceeds our milestone target of 10% for top-cell efficiency, which is required to achieve tandem efficiency of 30%. This work is based on results obtained from a project commissioned by the New Energy and Industrial Technology Development Organization (NEDO). [1] S. Shibasaki, Y. Honishi, N. Nakagawa, M. Yamazaki, Y. Mizuno, Y. Nishida, K. Sugimoto, and K. Yamamoto, "Highly transparent Cu2O absorbing layer for thin film solar cells," Appl. Phys. Lett. 119, 242102 (2021).