Up-and down-conversion as new means to improve solar cell efficiencies

Two new concepts for improved solar cell efficiencies involving the up-conversion of low-energy photons and the down-conversion of high-energy photons are discussed. We show that transmission- and thermalisation-losses, both responsible for reduced efficiencies of conventional single junction solar cells, can substantially be reduced with these concepts. Significantly enhanced limiting efficiencies for single junction solar cells in combination with up- or down-converters, the advantages and disadvantages of these new approaches and options for "built-in" up- and down-conversion are discussed.