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

Introduction
State of the art
tel-00916300, version 1 - 10 Dec 2013
In the quest for new renewable sources of energy, the focus of research today is on
cost-eective photovoltaic technologies with higher levels of eciency. The second
generation thin lm solar cells were promising compared to the rst generation
of bulk solar cells in reducing the manufacturing costs. But, the eciencies are
lower since the thin lms in general cannot absorb eciently the solar spectrum
without light trapping mechanisms. The combination of the third generation solar
cell concepts and the second generation thin lm materials has been identied as
a solution to economically meet the global energy needs. The major targets of the
third generation photovoltaics (PV) are:
ˆ to reduce the cost from 1$/watt of second generation PV to 0.50-0.20 $/watt
by increasing the eciency and lowering the material need.
ˆ to overcome the power loss mechanisms in single bandgap solar cells using
approaches that eectively absorb photons below and above the bandgap of
the solar cell material.
Tandem cell is one of the third generation concepts that enables to absorb a wide
range of solar spectrum by stacking individual cells with dierent bandgaps. The
wide availability of the non-toxic Si, and its compatibility with the already existing
technologies make All-Si tandem cell an attractive approach. Silicon, being an
indirect semiconductor is not an ideal material in reducing the cost of solar cells
because it requires about 125 micron thickness to absorb 90% of solar radiation,
while in a direct bandgap material such as GaAs about 1 micron is sucient. The
discovery of quantum connement eect in Si nanostructures resolves this problem
arising from conventional c-Si solar cells, and therefore can be eciently incorporated
in a tandem cell. The bandgap engineering is achieved by stacking quantum wells
(QW) or quantum dots (QD) of Si with dielectric barriers which are also based on
Si-compounds such as SiO 2 , Si 3 N 4 or SiC. With increasing QD density, the eective
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