Upconversion denotes the process of absorption of two or more photons followed by the emission of one photon with more energy than the individual energies of each of the previously absorbed photons. The manifold applications for upconverter materials range from bioimaging via theranostics to lasers. This work is concerned with upconversion in materials based on rare-earth ions in regard to the harvesting of solar energy. Due to upconversion the otherwise unused sub-band-gap photons in the solar spectrum can be utilized to increase the efficiency of solar cells.This work adds substantial progress to model, make, and measure upconverter materials and upconverter solar cell devices. In particular, core-shell upconverter nanocrystals as well as microcrystalline upconverter materials were investigated concerning their upconversion quantum yield as a function various parameters. Furthermore, upconverter solar cell devices were characterized especially using the radiation from a solar simulator and from the sun in an outdoor experiment. Record values were reached for the efficiency enhancement in a silicon solar cell due to upconversion of sub-band-gap photons.