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

occurance of this three body event is lower than a direct electron-hole recombination
in direct bandgap material. The total conservation of energy and momentum of
phonon and exciton, leads to the low absorption and emission behaviors in indirect
semiconductor materials. This explains the requirement of a thick c-Si as the active
layer for ecient absorption as compared to direct bandgap materials. Therefore,
reducing the material thickness by exploring the possibilities to tailor its bandgap,
would be a possible solution to lower the production costs which is the goal of a
second generation PV cell.
1.3.2 Amorphous Silicon - Advantages and Drawbacks
tel-00916300, version 1 - 10 Dec 2013
Amorphous Si (a-Si) was identied as a material relevant for solar cells as early
as 1969 [Chittick 69] and a systematic study was reported by 1972 [Spear 72]. By
1976, the rst solar cell based on a-Si was reported by Carlson [Carlson 76]. In
its native form, a-Si is not a material suitable for PV due to the disorder in the
material with a high density of dangling bonds (>10 19 cm −3 ), unless passivated
with hydrogen. It was shown that in a-Si with 5-20 at% hydrogen, the dangling
bond density reduces to 10 15 -10 16 cm −3 [Street 78, Chopra 04]. Among the various
reasons for broadening of bandtails in a-Si, the short range order in the material
and/or defects can be considered important. The bandtails and the dangling bonds
causes relaxation in the transition rules such that it behaves as a direct bandgap
semiconductor with a bandgap about 1.7 eV [Wronski 02, Pieters 08].
Even after hydrogen incorporation, a-Si:H still possesses neutral silicon dangling
bonds. Their associated electronic levels in the bandgap act as deep traps and
recombination centers which are observed in photoluminescence [Street 78], optical
absorption [Jackson 82] or transient photoconductivity [Street 82] and follow various
recombination dynamics as illustrated (Fig. 1.6).
Depending on the alloying process, the bandgap of a-Si can also be tailored
between 1.4 eV and 1.8 eV, which seems promising towards reducing the material
requirements. On the other hand, the light induced degradation, popularly known as
the Staebler Wronski eect (SWE) challenged a-Si:H supremacy despite its attractive
features, over c-Si [Staebler 77]. The new additional recombination centers and the
dangling bonds that appear on illumination aect the electronic characteristics such
as the lifetime of carriers. Though the SWE decreases the eciency of a-Si based
solar cells upon illumination, this process of defect formation is reversible when a-Si
sample is annealed above 150°C and the cell's eciency stabilizes at a value below
the initial eciency [Staebler 80]. But, even without the SWE, the initial eciency
of a-Si:H is lower (9-15%) than c-Si (14-23%).
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