before reaching the p-n junction. These effects produce an upper limit on the efficiency of silicon solar cells, currently around 20% for common modules and up to 27.1% for the best laboratory cells (33.16% is the theoretical maximum efficiency for single band gap solar cells, see Shockley-Queisser limit.).

By far the biggest problem with the conventional approach is cost; solar cells require a relatively thick layer of doped silicon in order to have reasonable photon capture rates, and