8.4% Efficiency For Tandem Solar Top Cell

• Toshiba Corporation has announced achieving 8.4% efficiency for transparent Cu₂O solar top cell
• Toshiba said it was able to control impurities while fabricating the Cu₂O layer that helped achieve the efficiency
• It sees this development providing a boost for no-plug charging EVs and also advance other mobility applications
• With Toshiba Energy Systems & Solutions Corporation, by FY 2023, it plans to supply samples of large-scale cells of the 'same size as mass-produced silicon solar cells' for external evaluation
• By FY 2025, Toshiba targets to bring commercially available products to the market

Toshiba Corporation of Japan claims to have achieved the world's 'highest level' for a transparent cuprous oxide or Cu₂O solar top cell, reporting it as 8.4% in the 'lowest-impurity' thin film. This, it said, gives a boost to the development of no-plug charging electric vehicles (EV) that can run for 35 kms without any need for recharging.

According to the team, higher efficiency for this cell will also be helpful to advance other mobility applications as High Altitude Platform Stations (HAPS) that act as the telecom platform in the stratosphere.

It was back in 2019 when the Japanese company developed the 1^st transparent Cu₂O solar cell, reporting a tandem cell efficiency of 23.8% when the average power conversion efficiency (PCE) of silicon solar cells had reached 22%.

With the 8.4% efficiency reported, Toshiba engineers estimate overall PCE of 27.4% is possible with the transparent Cu₂O solar cell positioned on the top of a 25% silicon cell. It will be above 26.7%, which is the highest PCE reported for any standard silicon cell, the team added.

The researchers were able to reach 8.4% efficiency with precise control of CuO and Cu impurities while fabricating the Cu₂O layer. Impurities, occurring as a result of current reactive sputtering deposition method, are responsible for pulling down PCE and transparency.

"X-ray diffraction analysis allows us to detect and quantify the degree of the CuO and Cu, giving us data that helps us to identify the best deposition method to control the impurities to the lowest level," explained Fellow at Toshiba's Corporate Reseaarch & Development Center, Kazushige Yamamoto.

Toshiba focuses on transparent Cu₂O cells as these can be formed using naturally abundant materials and have a higher light transmittance. Current crop of tandem cells being researched have an overall efficiency of close to 30%, adds the team, but made with III-V materials as gallium arsenide and are quite expensive 'several hundred to several thousand times higher than for crystalline silicon solar cells', hence their application is restricted.

Going forward, Toshiba said it aims to improve efficiency further for the top Cu₂O cell to 10% with 80% of transmittance. By FY 2023, it plans to supply samples of large-scale cells of the 'same size as mass-produced silicon solar cells' for external evaluations, in cooperation with Toshiba Energy Systems & Solutions Corporation. Target is to complete manufacturing technologies for commercial products by FY 2025.

Scientific journal Applied Physics Letters published the research work for 8.4% efficiency by Toshiba.

In September 2021, the Japanese engineering and electronics company claimed highest PCE for large, polymer-film based perovskite solar module with 15.1% (see 15.1% Efficiency For Polymer Film Based Perovskite).