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

tel-00916300, version 1 - 10 Dec 2013
in the depletion zone the photocarriers dissociate due to an internal electric eld
and are sent to n-region or p-region accordingly contributing to a directly generated
photocurrent. Thus the photocurrent of a solar cell is a sum of the three components:
photocurrent due to diusion of holes, photocurrent due to the diusion of electrons
and directly generated photocurrent in the depletion region. Thus it can be seen
that the diusion of the minority carriers plays a vital role in the photocurrent of the
device. In c-Si wafers, the minority carrier diusion lengths, L diff are suciently
high to transport the minority carriers by diusion to the junction. But, c-Si being
an indirect semiconductor requires 100 µm thick layer to absorb 90% of light whereas
1mm of GaAs is sucient to achieve the same level of absorption being a direct
semiconductor.
Conventionally the band structure of a semiconductor is represented by the E-k
diagram (Fig. 1.5), where E is the energy of an electron or a hole at the band
edge with wave vector k in the rst Brillouin zone. The dierence between the
highest point in the valence band and the lowest point in the conduction band gives
the bandgap (E g ) of the material. When these two points do not lie at the same
position of k as in the case of Si, the semiconductor is said to possess an indirect
bandgap.
Figure 1.5: Energy vs. momentum (E-k diagram) of Direct and Indirect bandgap semiconductors
[Coa 05]. ILLUSTRATION: JOHN MACNEILL.
Bulk c-Si is an indirect bandgap semiconductor (E g = 1.1 eV), and hence the
recombination of electron in the conduction band with the hole in the valence band
requires the participation of a third particle, which is the phonon (lattice vibrations)
with a wavevector equal to the initial conduction band state. The probability of
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