Films minces à base de Si nanostructuré pour des cellules ...

and sophisticated anti-reection coatings in addition to its tandem or micromorph
congurations. This helps in ecient light absorption in all the wavelength range
(energy range) accessible to silicon and ensure that each absorbed photon leads to
a carrier quantum yield equal to 1 [Ginley 08]. The present thin lm based PV
technology is focussed towards exploring silicon nanostructures that oer promising
solutions to overcome the drawbacks of bulk c-Si and a-Si.
1.4 Advent of quantum eects in silicon nanostructures
tel-00916300, version 1 - 10 Dec 2013
The major obstacle to the performance, eciency and cost of Si-based solar
cells, as can be observed from the discussions in the previous sections, is the indirect
bandgap nature of bulk Si. Over 90% of the solar radiations that reaches the earth's
atmosphere contain photons with energies ranging from 0.5-3.5 eV and only those
photons which have energies equal to/higher than the bandgap of a semiconductor
are absorbed (Fig. 1.8a). The fundamental losses of the incident solar spectrum in
a solar cell are represented in gure 1.8b. Photons with energies below the semiconductor
bandgap are not absorbed while those with energies above the bandgap
are thermalized after being absorbed, (i.e. the created electrons and holes lose their
excess kinetic energy in the form of heat during relaxation (Fig. 1.8b)).
(a) AM 1.5 solar spectrum. Arrows indicate
the bandgaps of some semiconductors.
(b) Solar cell losses.
Figure 1.8: Incident solar spectrum adapted from [Brown 09] and schematic representation
of fundamental losses in a solar cell.
Such carriers, termed as hot electrons and hot holes, contribute signicantly in
limiting the conversion eciency of single bandgap cells based on bulk Si to the
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