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

tel-00916300, version 1 - 10 Dec 2013
alternate fabrication techniques and by reducing the material requirements. This is
achieved by depositing thin lms (1-2 µm thick) of the semiconductor PV material
on glass, silicon or ceramic substrates. Though the costs are reduced to reach US$
1/W, the eciencies are lower than the rst generation solar cells. Cell eciencies of
12-20%, prototype module eciencies of 7-13% and production module eciencies of
9% have been reported from these cells [Internet 04]. The most successful materials
used in second generation solar cells are cadmium telluride (CdTe), copper indium
gallium selenide (CIGS), amorphous silicon and micromorph silicon (combination of
crystalline and amorphous Si).
The third generation PV cells aim at enhancing the performance of the
second generation thin lm technology in terms of eciency (targeting 30-60%) while
maintaining a low production cost (targeting < US$ 0.20/W). It aims at producing
future large scale aordable devices using dierent approaches like tandem cells
[Marti 96, Green 06], multiple exciton generation [Nozik 01], hot carrier solar cells
[Ross 82] etc. A more detailed discussion on the third generation PV will be dealt
in the forthcoming sections.
1.3 Silicon in photovoltaic industry
Traditionally, inorganic semiconductors are used as PV materials. A typical solar
cell contains one or more light-absorbing semiconductor layers that form a chargeseparating
junction (Fig. 1.3). This can be either a homojunction as in Silicon or a
heterojunction with other materials. An ideal solar cell material must be a readily
available, non-toxic, direct band-gap material with bandgaps between 1-1.7 eV in
addition to its high photovoltaic eciency and long term stability [Goetberger 02].
Besides, the solar cell material must also possess high absorption coecients of 10 4 -
10 5 cm −1 in the wavelengths between 350 - 1000 nm (1.24-3.54 eV), a high quantum
yield 2 for the excited carriers, a long diusion length and a low recombination velocity
[Poortmans 06]. The economical and ecological aspects such as the abundant
availability in earth's crust with low toxicity make Si to be the most important
choice as a PV material. The compatibility of Si materials with the already existing
CMOS microelectronic technology, their maturity in the industry, simple well established
processing techniques and mechanical strength become an added advantage
for their usage in the PV industries.
2 Quantum yield = Number of photons emitted/Number of photons absorbed
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