Sharp Corp. has claimed the highest solar cell conversion efficiency of 35.8 percent by using a triple-junction compound solar cell. The compound solar cell uses photo-absorption layers made from compounds consisting of two or more elements such as indium and gallium, as opposed to silicon-based solar cells which are the most common type of solar cell in use. Because of their high conversion efficiency, compound solar cells are used mainly on space satellites. Sharp has been advancing R&D on a triple-junction compound solar cell that achieves high conversion efficiency by stacking three photo-absorption layers since 2000.
To boost the efficiency of triple-junction compound solar cells, it is important to improve the crystallinity - the regularity of the atomic arrangement - in the top, middle and bottom photo-absorption layers. It is also crucial that the solar cell be composed of materials that can maximize the effective use of solar energy.
Conventionally, germanium (Ge) is used as the bottom layer due to its ease of manufacturing. However, in terms of performance, although Ge generates a large amount of current, the majority of the current is wasted, without being used effectively for electrical energy. The key to solving this problem was to form the bottom layer from indium gallium arsenide (InGaAs), a material with high light utilization efficiency. However, the process to make high-quality InGaAs with high crystallinity was difficult.
Sharp has now succeeded in forming an InGaAs layer with high crystallinity by using its proprietary technology for forming layers. As a result, the amount of wasted current has been minimized, and the conversion efficiency, which had been 31.5 percent in Sharp's previous cells, has been successfully increased to 35.8 percent.
Sharp achieved this breakthrough as part of a R&D initiative promoted by Japan's New Energy and Industrial Technology Development Organization on the theme of "R&D on Innovative Solar Cells".