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This requires to internally trap radiation by reflecting any lost radiation back, extending the time and probability that this energy can be reused, improving photon recycling. The optical design is important so that the only loss mechanism is photons exiting at the front surface.  The back mirror can be implemented by cheap and common material SiO_2 \cite{Zhao_2010} \cite{Andreani_2012}. The design and number of mirrors can be optimized according to any particular situation via simulation.  Different mirror geometries are illustrated in Figure .  While ultra-thin (<1µ) semiconductor layers are less effective at absorbing sunlight, they are cheaper from a material point view. If efficent light trapping techniques are used, the absortion efficency can be increased allowing for minimal loss. Intead of 1-2 µm thin films, 100nm could be adequete for a direct bangap material as CdTe.