Photonic structures for enhanced upconversion

Upconversion of low-energy photons presents a possibility to overcome the Shockley-Queisser efficiency limit for solar cells. In silicon 20% of the incident energy is lost due to transmission of these photons with energies below the band gap. Unfortunately, upconversion materials known today show pretty low absorption and quantum yields which are too low for this application. One possibility to boost the upconversion luminescence and even the quantum yield could be the embedding of the material in a suitable photonic structure environment. This influences the local irradiance onto the upconverter and the local density of states at the transition wavelengths. Thus, the radiative recombination from a specific energy level can be influenced. Hence, this approach has the potential to beneficially influence the upconversion quantum yield. For the buried grating structure shown here, a luminescence enhancement by a factor of 1.85 could be achieved, averaged over the grating.