The concept of intermediate band solar cells (IB) tries to tackle the problem photons with energies below the bandgap cannot be utilised for current generation. in intermediate band cells energy levels are created artificially in the bandgap of the absorber material . As in conventional single-junction solar cells, photons with a sufficient energy can excite an electron from the valence band in to the conduction band. However, in difference to conventional semiconductors photons with energies below the bandgap can excite an electron from the valence band in to the inter- mediate band. A second low-energy photon is required to excite the electron from the intermediate band into the conduction band. therefore two photons with small energies can result in quasi- Fermi level splitting larger than the energy of each of these photons.
Various studies have been performed on how intermediate band cells can be realised, as for example summarised in Reference . In such solar cells, a layer with the intermediate states is placed in between pand n-layers. For example quantum dots can be used to realise the intermediate states. Further, various bulk materials are studied for realising the intermediate
states. One major problem of experimental IB cells is that the voltages are lower than the voltages of reference cells without IB structures. Yet, the efficiency loss due to the lower voltage cannot be overcome by subbandgap current density originating from the intermediate band solar cells transitions.
The idea of hot carrier solar cells is to reduce the energy losses due to relaxation and hence thermalisation. , this should be achieved by collect electron-hole pairs of high energy photons just after light excitation before they have a chance to relax back to the edges of the electronic bands. , the population of the charge carrier levels reflects the situation just after the excitation by the absorption of a photon. This distribution is not in thermal equilibrium as many electrons are excited into position further up in the conduction band and the holes are excited down to lower levels in the valence band. These charge carriers are called hot electrons and holes .