Research at Lawrence Berkeley National Laboratory (Berkeley Lab) has led to solar cells with record-breaking efficiency. Contrary to conventional scientific wisdom, it turns out that efficient solar cell materials are characterised by high photon emission instead of high photon absorption.
According to the researchers, external fluorescence is the key to approaching the theoretical maximum efficiency for conversion of sunlight into electricity. The maximum efficiency, called the Shockley-Queisser (S-Q) efficiency limit, is approximately 33.5% for a single p-n junction. An analysis by a member of the research team indicated that gallium arsenide is capable of approaching the SQ limit. Based on this work, Alta Devices Inc., a private company spun off by the researchers, has fabricate gallium arsenide solar cells that achieved a record conversion efficiency of 28.4%.
The most efficient solar cells in commercial use today are made from monocrystalline silicon wafers and typically achieve a conversion efficiency of around 23%. Although gallium arsenide is more expensive than silicon, it is much better at absorbing photons, so less material is needed for a solar cell with the same performance.
Past efforts to boost the conversion efficiency of solar cells focused on increasing photon absorption in the cell. Absorbed photons produce electrons in the cell, but some of these electrons decay before being extracted from the cell and release their energy. If this energy is released as heat, it reduces the solar cell’s power output, but if it is released as photons, the cell voltage is increased, improving its power output.
Image: Alta Devices, Inc.