One of the world records, which Mitsubishi Electric has now renewed for the third consecutive year, is a 19.3 percent efficiency rating for photoelectric conversion of a practically-sized polycrystalline silicon PV cell of 100cm² or larger, with the PV cell measuring approximately 15cm x 15cm x 200μm. The rating is 0.2 points higher than the company's previous record of 19.1 percent.

The second world record, achieved with the same technologies in an ultrathin polycrystalline silicon PV cell measuring approximately 15cm x 15cm x 100μm, is an efficiency rating of 18.1 percent, a 0.7-point improvement over the company's previous record of 17.4 percent.

The conversion efficiency rates have been confirmed by the National Institute of Advanced Industrial Science and Technology, in Japan.

Mitsubishi Electric says it will be developing mass-production technology to deliver these high conversion rates in commercial PV modules. The company also aims to increase the output of its PV systems by combining this technology with PV inverters capable of high-efficiency conversion of DC current to AC.

The company has also achieved a high photoelectric conversion efficiency of 14.8 percent in a 5mm x 5mm thin-film silicon PV cell. The thin-film silicon PV cell developed by Mitsubishi Electric has a triple junction structure that utilizes a majority of the solar spectrum for higher efficiency.

Mitsubishi Electric, however, has met a technological breakthrough to achieve 14.8 percent photoelectric conversion efficiency, according to its own evaluation, by using a triple-junction configuration in which the first layer absorbs short wavelengths and the third layer absorbs long wavelengths, thereby enabling the use of a wide solar spectrum from visible light to infrared rays. Key technologies that help to make this possible include:

• Semiconductor materials that tune to a particular frequency of the spectrum
• High-quality film-deposition processing for each layer
• Texture fabrication applied to transparent electrodes for optimal confinement of sunlight.

Mitsubishi Electric intends to continue its research and development with the aim of raising the photoelectric conversion efficiency of its thin-film PV cells by improving cell structure, materials, processing and other factors.

- Paul Buckley
Power Management DesignLine Europe