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.