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

tel-00916300, version 1 - 10 Dec 2013
Figure 1.6: Electronic levels of a-Si:H and an illustration of few possible recombination
paths [Miroslav , Boehme 10].
Amorphous-Si materials have low carrier mobilities (m) and short carrier lifetimes
(t) thereby making the L diff very small (0.1-0.2 mm in a-Si vs. ∼ 100 µm in c-Si)
[Hegedus 97, Balber 98]. Therefore, a-Si based solar cells always use p-i-n structure
in contrast to the classical p-n diode structure used in c-Si solar cells. The width
of the depletion region is increased by placing an intrinsic layer (i) between the p
and n regions in order to prevent recombination of free carriers within the cell. The
electric eld in the i layer increases depending upon the voltage bias across the device
and increases the generated photocurrent. The high eld in the intrinsic region also
sweeps the diused photocarriers to the contacts before they recombine. Thus, the
photocurrent in a-Si p-i-n structures are largely dependent on eld assisted drift
rather than the minority carrier diusion processes. The main advantage of a-Si
as compared to c-Si lies in the incorporation of thin lm technology to a-Si, due
to better absorption arising from bandgap modications that leads to potentially
cheaper production. But, the low eciencies and poor electrical transport of a-Si
solar cells comes as their major limitation.
1.3.3 Alternate congurations of c-Si and a-Si for solar cells
In order to increase the eciency of a-Si based thin lm solar cells, various
congurations of a-Si have been investigated. The usage of a-Si in a multijunction
solar cell/tandem cell has been the most popular choice over the years. Dierent
junction devices with appropriately graded bandgap can be placed in a stack to form
a multijunction device. The top junction absorbs the higher energy photons and
12