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Transparent Cu2O solar cell achieves record 8.4 percent efficiency

Technology News |
By Nick Flaherty

Toshiba has developed a transparent Cu2O solar cell with a power conversion efficiency (PCE) of 8.4 percent, the world’s highest level for such a cell.

With the cell positioned as the top cell over a 25 percent PCE silicon cell as  a Cu2O-Si tandem cell, Toshiba estimates that an overall PCE of 27.4 percent can be achieved, notably above the 26.7 percent highest PCE reported for any standard silicon cell. 

The company is looking at using the low cost material for lightweight solar cells for electric vehicles. Under the test criteria defined by Japan’s New Energy and Industry Technology Development Organization (NEDO), Toshiba estimates that the Cu2O-Si tandem cell will carry an EV 35km without any need of recharging.

Toshiba also expects the new cell to boost development of power systems for High Altitude Platform Station (HAPS) stratospheric basestations.

The researchers are working with Toshiba Energy Systems & Solutions Corporation in the development of large-scale Cu2O solar cells that are the same size as mass-produced silicon solar cells. The two companies aim to supply samples for external evaluations by 2023, with a roadmap to completing manufacturing technologies for commercial products in 2025.

Tandem cells that achieve an overall efficiency close to 30% have been reported, far higher than any conventional crystalline silicon cell, but they are fabricated with III-V materials, such as gallium arsenide, with production costs several hundred to several thousand times higher than for crystalline silicon solar cells.

Toshiba has focused its R&D on transparent Cu2O cells. They are formed from naturally abundant materials, which lowers costs, and offer light transmittance that opens the way to excellent power generation efficiency. Toshiba developed the first transparent Cu2O solar cell in 2019, and demonstrated a tandem cell delivering 23.8 percent PCE in the same year, a time when the typical PCE of stand-alone silicon solar cells was 22 percent. 

The company’s latest advance in PCE is the result of precise control of CuO and Cu impurities during the fabrication of the cell’s Cu2O layer. These impurities are generated in the current reactive sputtering deposition method due to the nature of the Cu2O, and they are the major cause of diminished 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. Our targets are 10 percent PCE for the top Cu2O cell, and 80 percent of transmittance. This breakthrough brings us a step closer to those goals,” said Kazushige Yamamoto, Fellow at Toshiba’s Corporate Research & Development Centre.

Details on this technology were recently published in Applied Physics Letters. With support from NEDO, Toshiba will continue research to achieve the 10 percent PCE target for top Cu2O cells.

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