Material characterization for advanced upconverter systems
Upconversion is a promising way to enhance the efficiency of silicon solar cells, by also using photons with energies below the bandgap of silicon. Unfortunately, promising upconverting materials, such as Erbium-doped NaYF4 have only a narrow absorption range. More photons can be converted, when the upconverter is combined with a fluorescent material. The fluorescent material should absorb over a wide spectral range and emit in the absorption range of the upconverter. PbSe/PbS core-shell nanocrystal quantum dots are a promising fluorescence material for this application. In this paper we present an advanced upconverting system layout, which includes the combination of upconverter and fluorescent material, and the application of photonic structures for spectral management. We also present results from the characterization of the different materials needed for such an advanced upconverter system. An overall optical quantum efficiency of 9.4% was measured for a NaYF4: 20% Er3+ upconverter, and it is shown that the absorption and emission characteristics of the PbSe/PbS core-shell Nanocrystal Quantum dots embedded in polymer can be tuned into to the desired spectral region. We also show, that Si/SiC multilayer systems can be used as spectrally selective mirrors for the spectral management.