Researchers at the Japan-based Toshiba Corporation announced a method of layering a transparent solar cell over a standard silicon cell to develop an efficient, low cost and a reliable tandem solar cell that raises the solar module output.

Solar cells are also widely used to power electric vehicles (EVs). The new cell is expected to boost the development of EVs that are not dependent on plug-ins for charging and also advance other mobility applications.

These tandem cells position a solar cell over a standard silicon cell, generating power at different wavelengths, raising areal output, and potentially boosting the efficiency of solar modules.

In a project supported by Japan’s New Energy and Industry Technology Development Organization, the researchers developed a transparent cuprous oxide (Cu2O) solar cell with the highest power conversion efficiency (PCE) of 9.5%. The PCE was achieved by enlarging the cell and suppressing the carrier recombination in the edge of Cu2O generation later, which tends to degrade generation efficiency.

“We found that a larger cell size effectively suppresses photocarrier recombination,” said Kazushige Yamamoto from Toshiba’s Corporate Research & Development Centre. “Increasing the power generation area from the previous 3×3 sqmm to 10×3 sqmm produced a relative reduction in recombination in the edge of cell, and the resulting increase in photocurrent pushed the PCE to 9.5%.”

In addition to Toshiba’s transparent Cu2O cell, researchers are also working on two other tandem cell technologies: fabrication with gallium arsenide (GaAs) or other III (gallium, aluminum, indium)-V (arsenic, phosphorus, nitrogen) materials; and perovskite crystal thin films. The production costs for the former range from several hundred to several thousand times the cost of a single silicon cell, severely restricting application. The thin films still cannot deliver the necessary reliability and guaranteed output for the 20-years-and-more life of a silicon cell.

Fabrication with naturally abundant materials helps cut costs, and the material is strong and moisture-resilient, guaranteeing reliability. This further opens the way to high-level power generation efficiency because of the light transmittance qualities.

Tandem cells to power EVs 

Researchers have estimated that positioning the new Cu2O solar cell over a 25% PCE silicone cell realizes a Cu2O-Si tandem cell with a 28.5% PCE and surpasses 26.7% (the highest reported PCE for any standard silicon cell) and close to 29.1% (the highest reported PCE for any GaAs cell).

A one-time charge enabled the current Cu2O-Si tandem cell to power an EV for 37 kms. Further improvements in the solar cells towards the theoretical maximum efficiency of 42.3% are also expected to extend the range to a distance of up to 55 km. Both are recognized as practical distances for short trips without recharging and would reduce charging frequency for longer trips.

Toshiba targets a practical Cu2O-Si tandem cell with a 10% PCE Cu2O cell and an overall PCE of 30%. The new cell records solid progress towards that goal. The company is looking to mass production and is working to enlarge cell size. It has made a prototype with a power generation area of 40 sqmm and a PCE of around 8%.

While the efficiency in the larger cell remains unsatisfactory due to a less uniform layer, Toshiba says it will continue to refine thin film deposition technology for uniform deposition over a larger area.

Mercom had earlier reported that scientists at Helmholtz Zentrum Berlin claimed to have set a new conversion efficiency record of 29.80% in a tandem solar cell made of perovskite and silicon.