Typical layout of a Cu(In,Ga)(S,Se)2-based solar cell

Researchers: Dr Michael V. Yakushev, Professor Robert W. Martin

The solar cell industry is rapidly growing in Europe and America by 20% an year. Only in the UK in 5 years the solar cell market is expected to reach the level of 4.3 billion pounds. CuInSe2, CuGaSe2 and CuInS2 are the most promising semiconductors with chalcopyrite structure used in the absorber layer of thin-film solar cells. Chalcopyrite-based solar cells are becoming leading technologies for solar energy generators being champions in terms of efficiency (which is about 20%) among thin-film devices. One of their mysterious features is super high tolerance to any radiation. Their life-time in outer space was found to be at least 50 times as long as that of amorphous silicon solar cells. In fact, irradiation with quite high doses of MeV protons and electrons improves their performance. The material seems to repair itself at room temperature.

Despite such achievements very little is known about these materials in comparison with Si or binary compounds. The progress of Cu(In,Ga)(S,Se)2-based technologies has mostly been attained using scientific intuition rather than knowledge-based design: technology first and scientific explanation later. To make the design of solar cells more scientific we have to learn more about the defects in the chalcopyrites. The most reliable way of collecting information on the defects is to use optical spectroscopy. The most informative features in the optical spectra are excitons: sharp peaks associated with electron-hole complexes. The higher the quality of semiconductor the sharper the peaks. At the moment we are certainly the world leading group in terms of the number of resolved excitonic peaks in the optical spectra of the chalcopyrites. Analysing free and bound excitons we try to understand the electronic properties of the chalcopyrites, identify the nature of the defects and learn more on their chemistry. Very important information about the defects we plan to gain applying magnetic fields and pressure. We also try to modify the properties of the chalcopyrites by incorporating hydrogen, which was found to passivate certain defects improving the quality of the material, and other atomic species.