At the time of writing (2010) the best silicon cells have a conversion efficiency of 25% under optimum conditions and the theoretical maximum efficiency of 31%.
For practical purposes it seems reasonable to assume a conversion efficiency of 16% for commercially available cells in a typical production environment.
The incident light creates a number of moderately energetic electrons, which are collected to appear at the cell's terminals as electric current, and somewhat larger numbers of extremely energetic free electrons, which recombine with the silicon in about a picosecond. This is too fast for them to be harnessed to generate electricity, so this energy is converted into heat and lost.
- New Scientist, 26 June, 2010
In 2001 Arthur Nozic at the National Renewable Energy Laboratory in Golden, Colorado suggested that the high energy "hot" electrons could be trapped in quantum dots. The gaps between energy levels in quantum dots are much more widely spaced than those in a bulk semiconductor, such as the silicon wafers that form the solar cells. This makes it more difficult for the electrons to jump between levels, slowing the rate at which the electrons lose energy by a factor of 1000 and allowing them to be harnessed as additional output. Doing this would raise the theoretical maximum efficiency to 66%.
This "hot" electron capture was demonstrated experimentally in 2010 by Xiaoyang Zhu at the University of Texas, Austin. His team used lead selenide quantum dots on a titanium dioxide wafer and were able to detect changes in the wafer's optical properties that were due to electron capture. This system is purely proof of concept: it only captures "hot" electrons and does not produce electricity.