16.4
hole pairs are separated before they can recombine such that one incident photons results
in more than one generated electron [129]. Just as for spectral downconversion, quantum
efficiencies exceeding 100% are theoretically possible when one incident photon creates
statistically more than one charge carrier. Figure 16.6 shows that EQEs exceeding 100%
can be achieved as demonstrated by Semonin et al. [128]. Here, the absorber layer consists
of PbSe quantum dots.
Figure 16.6: Multi-exciton generation using PbSe quantum dots with EQEs exceeding 100% [128]. From O. E.
Semonin, J. M. Luther, S. Choi, H.-Y. Chen, J. Gao, A. J. Nozik, and M. C. Beard, Science 334, 1530 (2011). Reprinted
with permission from AAAS.
Another way of performing downconversion is to utilize singlet fission, which for
example can be done using tetracene, a polycyclic aromatic hydrocarbon. In fact, singlet
fission is the equivalent to MEG in organic materials, where a high energy singlet excited
state fissions into two low energy triplet states on neighbouring molecules [130, 131].
Intermediate band solar cells
The concept of intermediate band solar cells (IB) tries to tackle the problem photons with
energies below the bandgap that cannot be utilized for current generation. As shown in
Figure 16.7 (a), in intermediate band cells energy levels are created artificially in the
bandgap of the absorber material [132]. As in conventional single-junction solar cells,
photons with sufficient energy can excite an electron from the valence band into 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
intermediate band. A second low energy photon is required to excite the electron from the
intermediate band into the conduction band. As illustrated in Figure 16.7 (b), the
absorption of two photons with energies smaller than the bandgap energy can therefore
result in quasi-Fermi level splitting exceeding the energy of each of these photons.
Various studies have been performed on how intermediate band cells can be realized,
hole pairs are separated before they can recombine such that one incident photons results
in more than one generated electron [129]. Just as for spectral downconversion, quantum
efficiencies exceeding 100% are theoretically possible when one incident photon creates
statistically more than one charge carrier. Figure 16.6 shows that EQEs exceeding 100%
can be achieved as demonstrated by Semonin et al. [128]. Here, the absorber layer consists
of PbSe quantum dots.
Figure 16.6: Multi-exciton generation using PbSe quantum dots with EQEs exceeding 100% [128]. From O. E.
Semonin, J. M. Luther, S. Choi, H.-Y. Chen, J. Gao, A. J. Nozik, and M. C. Beard, Science 334, 1530 (2011). Reprinted
with permission from AAAS.
Another way of performing downconversion is to utilize singlet fission, which for
example can be done using tetracene, a polycyclic aromatic hydrocarbon. In fact, singlet
fission is the equivalent to MEG in organic materials, where a high energy singlet excited
state fissions into two low energy triplet states on neighbouring molecules [130, 131].
Intermediate band solar cells
The concept of intermediate band solar cells (IB) tries to tackle the problem photons with
energies below the bandgap that cannot be utilized for current generation. As shown in
Figure 16.7 (a), in intermediate band cells energy levels are created artificially in the
bandgap of the absorber material [132]. As in conventional single-junction solar cells,
photons with sufficient energy can excite an electron from the valence band into 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
intermediate band. A second low energy photon is required to excite the electron from the
intermediate band into the conduction band. As illustrated in Figure 16.7 (b), the
absorption of two photons with energies smaller than the bandgap energy can therefore
result in quasi-Fermi level splitting exceeding the energy of each of these photons.
Various studies have been performed on how intermediate band cells can be realized,