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

tel-00916300, version 1 - 10 Dec 2013
been observed in Si nanocrystals in SRSO and in p-Si, which are promising for
Si-based PV [Timmerman 11].
Hot Carrier Solar Cells : This is another approach to overcome the thermalization
losses. Hot carrier solar cells (HCSC) attempt to minimise the loss by
extracting the hot carriers at elevated energies in a narrow energy range. The underlying
concept is to decrease the cooling rate of photoexcited carrier and to allow
the collection of hot carriers [Ross 82, Wurfel 97, Konig 10]. HCSC is necessarily
constituted of a hot carrier absorber layer where the carrier cooling is delayed and
an energy selective carrier extraction through contact layer. This selective energy
contact is usually made up of small quantum dots with high resonant state energy
for resonant tunneling. Experiments using double barrier electron-tunneling structures,
with a single layer of Si QDs have been reported [Jiang 06b]. However, the
problem in decreasing the carrier cooling rate remains as an obstacle to the ecient
demonstrations of HCSC.
Up-Down conversion : The process in which at least two sub-bandgap photons
are absorbed to emit one above bandgap photon is called as up conversion.
The process of absorbing at least twice the energy of semiconductor bandgap and
emitting two photons is called as down conversion. An up converter layer can increase
the current by absorbing the below bandgap photons which are generally
not absorbed. A down converter layer can increase the current by converting photons
in the ultraviolet range into a large number of photons in the visible range.
Nanostructures of Si are ecient light emitters and hence can be used as a photoluminescent
down shifter (PDS) layer, through which high energy photons are
absorbed to emit low energy photons. The principles are similar to that of IBSC
and MEG with slight modications in the congurations. The possibility of using
porous Si, Si nanocrystals and rare-earth doped nanostructures for these purposes
have been reported [Wang 92, Trupke 02a, Svrcek 04, Timmerman 08, Miritello 10].
1.5 Si Quantum Dots (QDs) in Photovoltaics
Third generation PV approach takes advantage of the quantum connement
eect and it can be seen from the previous section that Si QDs can be utilised
in all the approaches. Si QDs have a wide range of applicability in a solar cell
device due to their optical and/or electrical properties. Among those, the most
important applicability of Si QDs includes the active layer in a simple p-n junction
[Cho 08, Hong 10, Hong 11] or intrinsic layer in p-i-n junction device, selective energy
contacts, photoluminescent down shifter layers, and a tandem cell's active layers
in all the cascaded p-n junctions or p-i-n junctions. In this context, it becomes im-
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