Multiple Energy Level Solar Cells
In multiple energy level solar cells, the mismatch between the incident energy of the solar
spectrum and a single band gap is accommodated by introducing additional energy levels such
that photons of different energies can be efficiently absorbed. Multiple energy level solar cells
can be implemented either as localized energy levels (first suggested as a quantum well solar
cell) or as continuous mini-bands (also called intermediate band for the first solar cell to suggest
this approach) (Marti and Luque 2003; Green 2004). Both cases, which are shown in Figure 23,
have a fundamental similarity in that the key issue is the generation of multiple light-generated
energy levels for electrons.
(a) (b)
Figure 23 (a) Intermediate band solar cell and (b) quantum well solar cellHot Carrier Solar Cells
Hot carrier solar cells utilize selective energy contacts to extract light generated hot carriers
(electrons and holes) from the semiconductor regions before they have thermalized with the
semiconductor lattice (i.e., converted their excess energy to heat) (Ross and Nozik 1982; Würfel
1997). This allows higher efficiency devices (up to a thermodynamic limit of 66% at one sun
intensity) by reducing the thermalization (heat) losses in
single-junction solar cells. To benefit from this
approach, the escape of the hot carriers through the
energy-selective contacts must be faster than the various
inelastic scattering processes that lead eventually to
thermalization to the lattice temperature as shown in
Figure 24. Specific materials, in particular, materials
with low dimensions such as quantum dots, show slowed
carrier cooling and thus hold the promise for realizing
such hot carrier solar cells. The obvious prerequisite is
reducing the cooling rate well below the hot carrier
collection rate and developing the energy-selective
contacts for hot carrier collection.
Figure 24 In hot carrier solar cells,
carriers are collected before they
thermalize.