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

tel-00916300, version 1 - 10 Dec 2013
Approach Objective Principle Eciencies Expected/
Achieved
Tandem Cells
Intermediate band
gap solar cells
(IBSC), Multiband
solar cells
QD solar cell
(Multiple Exciton
Generation-MEG);
hot carrier solar
cells
Up-Down
conversion
Utilise a
wide range
of solar
spectrum
Absorb
energies
lesser than
bandgap
Prevent
semiconductor
thermalization
losses of
high energy
photons
Prevent
losses of
sub-bandgap
and above
bandgap
photons
Stacking of multiple
bandgaps in the solar cell
Impurity Photovoltaic
Eect. Placing an
intermediate band in the
forbidden energy gap of
semiconductor
Electron-Hole pair
multiplication in QDs or
extract the carrier before
it cools down
Up-conversion: Absorb
two or more sub-bandgap
photons and emit one
above bandgap photon
Down conversion: Absorb
above bandgap photons
and emit several low
energy photons
Innite stack of bandgaps
(Expected)-66% and
Using three stacks
(Achieved)-40.7%
Intermediate band
located at 0.36eV below
the conduction band or
above the valence
band-Expected-54%
[Quan 11]
Theoretical-(IBSC)63%
(multiband)87%
[Luque 97, Green 02]
Two excitons per photon
at a threshold of
2E g (Expected) 42- 44%
[Hanna 06, Nozik 08]
Up-conversion: About
48% under 1Sun
(estimated) Down
conversion: About
38-40% (optimum
eciency for a bandgap
of solar cell around 1.5
eV)[Trupke 02a]
Table 1.1: Third Generation Photovoltaics- Objectives and Approaches.
Multiple Exciton Generation (MEG) : Also called as carrier multiplication,
it refers to the generation of multiple electron-hole pairs from the absorption of a
single photon [Ellingson 05]. In this process, an electron or a hole with kinetic
energy greater than the semiconductor bandgap produces one or more additional
exciton pairs. This is achieved either by applying an electric eld or by absorbing
a photon with energy at least twice that of semiconductor bandgap energy. In
QDs since there is a formation of discrete electronic states, the cooling rates of hot
carriers can be decreased [Nozik 08] and Auger processes are enhanced due to the
decrease in distance between the excitons. These aid the production of multiple
excitons in QDs as compared to the bulk semiconductors. In relation with the
present scenario, MEG has been reported in dierent semiconducting QDs such as
PbSe, PbS, PbTe, CdSe, InAs and Si [Beard 07]. Very recently, highly ecient
carrier multiplication and enhancement in the luminescence quantum yield have
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