Josef Michl (Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado, U.S.A., and Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic, Prague, Czech Republic)
Efficient utilization of solar energy is currently considered to be the key to mankind's sustainable energy future, both through the generation of electricity and the production of fuels. Increasing the yield of useful energy that can be obtained from solar radiation without increasing the cost of the conversion is an important challenge made complicated by the polychromaticity of solar radiation, i.e. the fact that it contains photons from the infrared to the ultraviolet (all colors). Two of the possible paths toward increased solar cell efficiency are solar photon upconversion and downconversion, based on the use of materials capable of tailoring photon energy from values that are inconveniently low or high, respectively, to an optimal value. We shall illustrate some of the current scientific effort in photon downconversion by describing the use of first principles for the design of materials suitable for singlet fission, a process in which the energy of a single photon is used to produce two excited molecules, each of which can in principle provide an electron to one electrode and take an electron from another electrode, producing electrical current.