16 February 2010
Mitsubishi Electric Corporation announced today that it has achieved a very high photoelectric conversion efficiency of 14.8% in a 5mm x 5mm thin-film silicon photovoltaic (PV) cell.
Photoelectric conversion efficiency is the rate at which sunlight energy is converted into electric current, with higher rates meaning more output. 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.
At present, crystalline silicon is used commonly for PV cells. Due to their relatively high photoelectric conversion efficiency, crystalline silicon PV modules are widely used in applications with limited surfaces, such as on the roofs of residential houses. The price of silicon wafers can fluctuate greatly, however, due to changes in market demand.
Thin-film silicon PV cells are garnering attention because they use just 1% the amount of silicon material required for crystalline silicon PV cells, which helps to save resources as well as reduce costs. Although thin-film silicon PV cells are lower in photoelectric conversion efficiency than crystalline silicon PV cells, their lower product costs offer benefits for midsized and large industrial PV systems, such as those used in factories, electric power utilities and municipalities. In addition to expected growth in these fields, there is great upside potential in other fields if their efficiency can be improved in the coming years.
Multi-junction layers offer an efficient way of raising conversion efficiency in thin-film silicon PV cells because each layer absorbs different wavelengths of sunlight. It is extremely difficult, however, to adjust the characteristics of each layer in the multi-junction structure, so most thin-film silicon PV cells today are only single or double layered. Mitsubishi Electric, however, has met a technological breakthrough to achieve 14.8% 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 further continue its research and development with aims to raise the photoelectric conversion efficiency of its thin-film PV cells by improving cell structure, materials, processing and other factors, aiming to develop advanced PV systems that contribute to sustainable, low-carbon societies.