All presently available types of solar cells transmit light with energies below their band gaps, foregoing energy. An elegant way toward overcoming these subbandgap losses and using a larger fraction of the incident light is the re‐shaping of the solar spectrum by upconversion (UC) of photons. Recently, first results on solar cells augmented by either lanthanide-based UC or triplet-triplet-annihilation UC in organic chromophores were presented. Both of these UC strategies are characterized by a nonlinear response on the illumination density under conditions relevant to solar energy conversion, opening a route for increasing the UC yield by concentrating the light. While operation of the whole cell under concentrated sunlight is in most cases undesirable, application of micro-optical focusing of the transmitted light in the upconverting layer is a promising strategy. In the present work, a more than two-fold enhancement of the current gain by UC behind an amorphous silicon solar cell through optimization of the upconverter optical design is demonstrated, including employing a focusing microstructured back reflector. The experimental data is rationalized using a simple ray tracing modeling approach, highlighting a further enhancement potential of a microstructured UC unit.
