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

tel-00916300, version 1 - 10 Dec 2013
carrier tunnelling probability between the quantum dots can be increased due to
the lower bandgap of Si 3 N 4 (∼5 eV) as compared to SiO 2 (∼ 9 eV) [Green 05]. It
has been reported that the Si QDs form at a faster rate and at a lower annealing
temperature as compared to SRSO [Negro 06] and ecient photoluminescence have
been observed from SRSN lms [la Torre 06, Lelievre 07] which are reported to be
blueshifted when compared to Si QDs in SiO 2 (Fig. 1.13 from [Yang 04]). High
carrier mobility and relaxed interdot separation of 3-4 nm as compared to 1-2 nm in
SiO 2 has been estimated from Si QDs in Si 3 N 4 leading to better conduction property
of the device [Jiang 06a]. Under blue/green light it is reported that Si QDs in
silicon nitride show about 14 times enhanced absorption than bulk Si and a photocurrent
which is about 4 times higher than that of bulk Si solar cells [Kim 09].
The carrier mobility in silicon nitride containing QDs is attributed to direct tunnelling
process and is dependent on the Si QD density [Jiang 04]. Hence enhancing
the density of Si QDs in Si 3 N 4 has an added advantage of using this material in
the active layers of a solar cell. Besides, Si QDs embedded in Si 3 N 4 antireection
layer of solar cells can be used as luminescent converters to enhance photovoltaic
conversion in the blue region of the spectrum [Trupke 02a, Trupke 02b]. Apart from
such promising advantages of this material, the processes of luminescence mechanism
and defect formations is less understood than in the case of SiO 2 . The Si
QD size, and their surface passivation cannot be directly deduced from photoluminescence
because silicon nitride matrix itself exhibits a strong visible luminescence
which is attributed by several authors to nitrogen and silicon levels in the silicon
nitride bandgap [Deshpande 95, Gritsenko 99].
Figure 1.13: Comparison of luminescence peak positions between Si QDs embedded in
SiO 2 and SiN x (SRSN) matrices[Yang 04].
(c) Si QDs in SiC: SiC is another important successful material used in solar cells
in its amorphous thin lm form as p-type window layers and buer layers in p-i-n
congurations. Though very few works have been reported with Si QDs embedded
24