The 70th Japan Society of Applied Physics Spring Meeting 2023
Highlighted Presentations Press Release
April 18, 2023

Development and Prospect of Cu2O/Si Tandem Solar Cells

Toward Carbon Neutrality
Development of high-efficiency Cu2O/Si tandem solar cellToshiba Corporate Research & Development Center
Atsushi Wada

Summary• Developed “Transparent Cuprous oxide (Cu2O) Solar Cells” that achieved a world record
power conversion efficiency of 9.5%.
• The power conversion efficiency of the Cu2O/Si tandem solar cell, which consisted of atransparent Cu2O solar cell and a high-efficiency silicon (Si) solar cell stack, wasestimated to be 28.5%.
• Cu2O/Si tandem solar cells in an electric vehicle (EV) are expected to have a range ofabout 37 km per day.
A research group of Atsushi Wada and colleagues at Toshiba Corporation’s Research & Development Center has
developed a “transparent cuprous oxide (Cu2O) solar cell” that achieved a world record power conversion efficiency of
9.5% (Toshiba Corporate Research & Development Center survey, as of February 2023). The results contribute to theresearch and development of “tandem solar cells,” in which two solar cells are stacked to generate power and increase
the overall power conversion efficiency, an area of focus for the company. The “Cu2O/Si tandem solar cell” consists of ahigh-efficiency silicon (Si) solar cell with a power conversion efficiency of 25% stacked on the transparent Cu2O solar cell
developed by the company. It is estimated to have an overall power conversion efficiency of 28.5%. This power
conversion efficiency is expected to provide a daily cruising distance of approximately 37 km when mounted on anelectric vehicle (EV). The company has begun to develop larger transparent Cu2O solar cells for efficient mass
production of tandem solar cells. Moving toward practical use in FY2025, the company plans to gradually increase thecell size and eventually establish a cell manufacturing technology to produce cells having the same size as commerciallyavailable Si solar cells.

DetailTandem solar cells that realize the "best of both worlds
The widespread social use of renewable energy must be promoted to achieve carbon neutrality.
Solar cells represent a promising power-generating technology. The Ministry of Economy, Trade,and Industry (METI) announced in its “Green Growth Strategy Accompanying Carbon Neutrality in2050” a goal of increasing the share of renewable energy in Japan’s power generation to 50–60%
by 2050 (*1). This growth strategy advocates the shift to renewable energy as the main source ofelectricity and the electrification of transportation. These results are expected to open new
avenues for the use of solar cells.
Research and development of “tandem solar cells,” which realize high efficiency, low cost, and highreliability by overlapping two solar cells and combining the “best of two worlds,” is currently heatingup. Tandem solar cells may also be useful in mobility systems such as electric vehicles, where theinstallation space is limited.
Prior studies on tandem solar cells include those using III-V solar cells (*2), such as gallium
arsenide semiconductors (GaAs), and those using perovskite systems. However, GaAs
semiconductors are hundreds to thousands of times more expensive to produce than Si solar cells.
Moreover, perovskite systems have reliability issues compared with Si solar cells, which have an
output guarantee period of 20 years or more.
The company’s transparent Cu2O solar cells are characterized by their high potential for reliability
while offering significant cost reductions. Transparent Cu2O solar cells absorb short-wavelength
light, such as green, blue, and ultraviolet light, and transmit long-wavelength light, such as red andnear-infrared light. Thus, a Cu2O/Si tandem solar cell is formed by combining a Si solar cell that
efficiently generates power with long-wavelength light in the bottom cell and a highly transparent
Cu2O solar cell in the top cell. Therefore, it is possible to realize highly efficient power conversion
over a wide wavelength range, from short to long wavelengths (Figure 1).
The company aims to develop an efficient mass-production technology with a target power
conversion efficiency of 30% and establish a manufacturing process for Cu2O solar cells having
the same size as Si solar cells.
Cu2O/Si tandem solar cells with 28.5% power conversion efficiency
The research team succeeded in improving the power conversion efficiency of transparent Cu2O
solar cells to a world-record efficiency of 9.5%, a 1.1% increase over the 8.4% efficiency
announced in December 2021. One of the reasons for the higher efficiency is the suppression ofcarrier recombination at the Cu2O generating layer wall, which causes a decrease in the power
conversion efficiency.
The research team found that increasing the cell size effectively suppressed carrier recombinationand expanded the cell area from the conventional 3 mm square to 12 mm × 3 mm. As a result, thenumber of photocarriers recombining at the cell wall was reduced, resulting in an approximately
10% increase in photocurrent and an increase in power conversion efficiency to 9.5%. The
property of Cu2O, in which an increase in cell size leads to an increase in power conversion
efficiency, is advantageous for larger Cu2O solar cells.

The power conversion efficiency of the Cu2O/Si tandem solar cell developed in this study wasestimated to be 28.5%, where transparent Cu2O with a power conversion efficiency of 9.5% wasapplied to the top cell and a 25%-efficient Si solar cell was applied to the bottom cell. In addition,
the company introduced large sputtering equipment capable of depositing films on large-area
substrates and produced a prototype 40 mm square cell with a power conversion area
approximately 180 times larger than that of a 3 mm square cell (Figure 2). The company is looking
toward mass production in the future.
Achieves a range that eliminates the need for home-recharging facilities for short-distancedrivers.
The company also details the trial calculation of the case in which the developed Cu2O/Si tandem
solar cell was installed in an EV (Figure 3). Assuming the Cu2O/Si tandem solar cell has a power
conversion efficiency of 28.5%, a target efficiency of 30%, a theoretical maximum efficiency of
42.3%, and a vehicle installation area of 3.33 m2, the daily driving distance without recharging isestimated to be approximately 37 km (power conversion efficiency), 39 km (target efficiency), and55 km (theoretical maximum efficiency) (*3). In other words, according to estimates, an EV
equipped with Cu2O/Si tandem solar cells can travel more than 30 km without recharging. Thisrange covers the entire daily driving distance of a short-distance user. Therefore, this eliminates
the need to install recharge facilities at home for short-distance drivers. For long-distance users,
this range reduces the amount and frequency of recharging at the recharging facilities. In otherwords, our calculations show that the Cu2O/Si tandem solar cell can be used as a realistic powersource for EVs.
Cu2O/Si tandem solar cells are expected to contribute to the main source of renewable energy andelectrification of transportation in the Green Growth Strategy Accompanying Carbon Neutrality in2050. The company is conducting joint research with Toshiba Energy Systems Corporation andaims for practical use in FY2025.

Annotation*1 https://www.meti.go.jp/press/2020/12/20201225012/20201225012.html*2 Group III-V solar cells: Compound semiconductor solar cells composed of group III elements such as gallium (Ga),indium (In), and aluminum (Al) and group V elements such as phosphorus (P) and arsenic (As).
*3 EV electricity costs are based on the assumed value of 12.5 km/kWh in 2030 used in the NEDO calculations.

Figures

Figure 1 Schematic of Cu2O/Si tandem solar cell (4 terminals) (courtesy of Toshiba)

Figure 2 Prototype of 40 mm square transparent Cu2O solar cell (courtesy ofToshiba)

Figure 3 Image of a Cu2O/Si tandem solar cell installed in an EV (courtesy ofToshiba)