A further so-called ‘third generation’ approach aims to reduce the emission of luminescence radiation which, in the Shockley-Queisser limit, balances the incoming and outgoing photon fluxes to establish a steady state, preventing the exit of photons in a particular range of wavelengths and emission angles by placing
a photonic crystal on top. By appropriately designed photonic crystals , the emission of luminescence photons from the excited absorber is suppressed. As a consequence, the photon field in the absorber is enhanced, which might lead to subsequent reabsorption of luminescence photons and thus to a higher photoexcited state of the absorber. Figure 6.10 exemplifies the effect of suppressing luminescence photons from a dye-doped fluorescence collector and from a c-Si absorber by means of suitably designed photonic crystals .
photon energy hwg (eV)
Fig. 6.11 Schematic spectral solar energy flux and cutoff energy eg to represent the portion of solar energy absorbed (shaded area) and converted subsequently into chemical energy of excited species such as electron-hole pairs. Note that the excess photon energy remains largely unused