The transparent-type Cu2O solar cell (small-size cell measuring 25 x 25mm) (source: Toshiba)
[Click to enlarge image]

Toshiba Corp announced Jan 21, 2019, that it has succeeded in making a transparent cuprous oxide (Cu2O)-based solar cell.

By using a transparent-type Cu2O solar cell as a top cell and a crystalline silicon (Si) solar cell (which is currently widely used) as a bottom cell, it becomes possible to realize a high-efficiency tandem-type solar cell capable of converting lights of various wavelengths (from short to long wavelengths) into energy at a low cost.

A tandem-type solar cell consists of (1) an upper-layer transparent top cell that sunlight directly enters and (2) a lower-layer bottom cell. It is expected to realize a power generation efficiency higher than that of crystalline Si solar cells.

Currently, tandem-type solar cells using gallium arsenide semiconductor, etc have been developed, and power generation efficiencies of 30-40%, which are 50-100% higher than that of crystalline Si solar cells, have been reported. On the other hand, the production cost of tandem-type solar cells is several hundred to several thousand times higher than that of crystalline Si solar cells.

The newly-developed transparent Cu2O-based solar cell generates electricity by absorbing short-wavelength light and transmits 80% of long-wavelength light (600nm or longer). Its cost is expected to be reduced by using copper oxides, which are available in large quantities on the earth.

Also, because it generates power by absorbing light with a wavelength different from the wavelength of light absorbed by crystalline Si, it hardly inhibits the power generation of crystalline Si and, therefore, is expected to realize high-efficiency power generation.

Cu2O tends to produce impurity phases such as of copper oxide (CuO) and copper (Cu) and be mixed with them. This time, Toshiba applied its own film-forming method that meticulously controls the amount of oxygen in the process of forming a Cu2O thin film to prevent the generation of CuO and Cu in the thin film. As a result, the company realized the transparent Cu2O.

In a test of a prototyped tandem-type solar cell using the technology, the company confirmed that the crystalline Si solar cell (bottom cell) keeps about 80% of the output achieved when it is used alone.

Aiming to complete a low-cost tandem-type solar cell using a transparent Cu2O solar cell as a top cell in three years, Toshiba will continue the research and development targeted at an efficiency of 30-40%.