Semiconductor quantum dots may be used in so - called third - generation solar cells
that have the potential to greatly increase the photon conversion efficiency via two
effects: (1) the production of mul tiple excitons from a single photon of sufficient energy
and (2) the formation of intermediate bands in the bandgap that use sub - bandgap
photons to form separable electron–hole pairs. This is possible because quantization of
energy levels in quantum dots produces the following effects: enhanced Auger proc -
esses and Coulomb coupling between charge carriers; elimination of the requirement to
conserve crystal momentum; slowed hot electron–hole pair (exciton) cooling; multiple
exciton generation; and forma tion of minibands (delocalized electronic states) in quantum
dot arrays. For exciton multi plication, very high quantum yields of 300–700% for exciton
formation in PbSe, PbS, PbTe, and CdSe quantum dots have been reported at photon
energies about 4–8 times the HOMO–LUMO transition energy (quantum dot bandgap),
respectively, indi cating the formation of 3–7 excitons/photon, depending upon the photon
energy. For intermediate - band solar cells, quantum dots are used to create the intermediate
bands from the con fined electron states in the conduction band. By means of the
intermediate band, it is possible to absorb below - bandgap energy photons. This is
predicted to produce solar cells with enhanced photocurrent without voltage degradation.