Multispectral Conversion

An absorber with a band gap eg, such as the optical threshold in semiconductors or the HOMO-LUMO gap in molecular absorbers, can be excited only by a specific regime of solar photons which are continuously distributed across the entire energy scale 0 < <i. Photons with energies < eg cannot be used, as they are not absorbed. Their companions with hrn > eg can in principle be absorbed in semiconductors, but their excess energy[102] Aeexc = hco — eg is mainly lost by rapid thermalization of the ‘hot’ carriers. As the probability for generation of more than one excited species (electron-hole pair or exciton) with one solar photon, even with equivalent high energy, is almost negligible in conventional bulk absorbers, a large fraction of solar photons essentially possess either too low or too high an energy to be exploited efficiently (see Fig. 6.11).

Updated: September 26, 2015 — 4:36 am