Exploiting photonic structures to improve the efficiency of upconversion by field enhancement and a modification of the local density of photonic states

Upconversion of infrared photons has been established as a concept to mitigate transmission losses in solar cells. Higher upconversion efficiencies, however, are necessary, to enable a profitable application of upconverting components. In this paper, we examine the embedding of upconverting material into a photonic structure to increase upconversion efficiencies. The structure influences the upconversion efficiency in two ways: First, higher local field intensities increase the absorption and the population of higher excited states. Second, the local density of photonic states is modified by the structure. Thus, the probabilities of desired transitions can be increased. This work covers the design of a suitable model structure and its examination via various simulation methods, most importantly finite-difference time-domain simulations, combined with an upconverter rate-equation model. An increase of the upconversion efficiency by a factor of about 3.6 at an excitation irradiance of 200 Wm-2 is determined.