Improving Upconversion Efficiency by Photon Management in Self-Assembled Core/Shell Nanocrystal Films

We
present the synthesis of Er-doped NaYF4/NaLuF4 core/shell nanocrystal films, which
enable optical upconversion of photons from a wavelength of 1500 nm
into photons of a wavelength of 980 nm. Combining such films with
specially tailored Au nanostructures and a Au back-reflector, it is
demonstrated that the upconversion luminescence yield can be enhanced
up to 31 ± 3 times and that the performance of the upconverting
layer can be modeled well for a broad range of excitation conditions.
In particular, the analysis shows that the performance of the Au structures
corresponds to a solar concentration factor of 4.1 ± 0.6. The
relevance of the results for improving the performance of crystalline
silicon solar cells is discussed.