To continue the path of cost reduction in photovoltaics the efficiency of silicon solar cells must be increased. With higher efficiencies more kWh can be produced from the same amount of silicon, which is the dominating cost factor at present. Fundamental loss mechanisms limit the maximum achievable efficiency: around 20% of the incident power is lost, because photons with energies below the band-gap are transmitted. Upconversion of two low energy photons into one usable photon reduces these losses. In this project we will realize upconversion with the help of nanostructures and nanotechnoloy-based materials and show a significant improvement in solar cell efficiency. The combination of upconverting Er-based phosphors with PbSe/PbS core shell quantum dots increases the spectral range of light that is upconverted. The quantum dots will be incorporated into a fluorescent concentrator to achieve concentration within the upconverting device. Both the increased photon flux due to a wider spectral collection and the additional geometric concentration will increase upconversion efficiency because of its nonlinear characteristic. Optical nanostructures shall serve as selectively reflective structures that avoid unwanted parasitic absorption. The development of very efficient quantum dots and suitable host materials, the optimization of the upconverter and the fabrication of photonic structures are main objectives. Additionally, solar cells and system designs will be optimized, to make the best use of upconverted photons. A thorough understanding of the underlying principles is critical for the success, so gaining knowledge about nanostructures and materials is a major goal. The big advantage of this concept is that the solar cells remain fairly unchanged. The proposed concept opens a technology path for an evolutionary development of silicon solar cell technology to efficiencies towards 30%, starting from the solid base of today s established silicon technology.