Higher efficiency, lower cost silicon based solar cells are provided by modifying the absorption coefficient of Silicon so that it strongly overlaps with the solar spectrum. In one embodiment this is achieved by co doping of the silicon with appropriate impurities. In another embodiment it is achieved by modifying the structure of silicon whereby a portion is converted into Silicon XII having the R8 structure.
STATEMENT OF GOVERNMENTAL SUPPORT
 The invention described and claimed herein was made in part utilizing funds supplied by the U.S. Department of Energy under Contract No. DE-AC02-05CH11231, and the National Science Foundation under grant number DMR07-05941. The government has certain rights in this invention.