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. In the figure, 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 .
It takes only a few picoseconds (1012 s) for the hot charge carriers to relax back to the edges of the electronic bands. The idea of hot carrier cells it to collect the charge carriers as long as they are still hot. Hence, an energy larger than the band gap energy can be utilised per excited charge carrier and the average bandgap utilisation would exceed the bandgap.
The fundamental challenge in HOT CARRIER SOLAR CELLS is to collect the hot carriers before they relax back to the edges of the electronic bands. Such a concept would require selective contacts, which only select electrons above a particular energy level in the conduction band and holes below a certain energy level in the valence band, respectively. At the moment the main challenge is to increase the lifetime of the hot charge carriers, such that they have time to move from the absorber material to the selective contacts.
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