Environmental Engineering Reference
In-Depth Information
It is also noteworthy that Das and Morris, in 1993, produced a
better solar cell of 9.8% e
ciency for the ITO/SnO
2
/CdTe/metal
structure without the CdS layer than a solar cell of 8.8% e
ciency
for the ITO/SnO
2
/CdS/CdTe/metal device with the CdS layer [24,
25].ThisclearlyindicatesthattherectifyingcontactrequiredforPV
activity is at the metal/CdTe interface rather than at the CdS/CdTe
hetero-junction.Thisisequivalenttothesituationdescribedbycase
number5 in Table 4.2.
A large body of subsequent measurements on glass/CG/CdS/
CdTe/metal thin film solar cell structures fabricated using elec-
trodepositedmaterialsconfirmedtheFermi-levelpinningatdiscrete
levels — Fig. 4.7(a). It is interesting to note exactly the same barrier
heights observed for electrodeposited CdTe thin layers as observed
for metal contacts on bulk n-type CdTe (see Fig. 4.6). This indicates
that the defect levels (E
1
-E
5
) responsible are arriving from native
defects of CdTe and do not strongly depend on the method of
growth. These different barrier heights appear for different batches
of fabrication, and their respective
V
oc
values also tend to produce
values in discrete nature, as shown in Fig. 4.7(b). The
V
oc
values
follow respective barrier heights since this parameter is a function
of the barrier height (Eq. 1.16). In order to produce high e
ciency
solar cells, the Fermi level should be pinned at either 0.96
±
0.04 or
1.18
±
0.02eVlevelbelowtheconductionbandminimum.Thelatter
value is the most preferred value forbest devices.
4.6 Predictions for Further Development of CdS/CdTe
Solar Cells and Latest Observations
The experimental results available to date enabled the formulation
of this alternative model; hence, the following main steps should
allow further improvement of this solar cell structure to achieve
higher e
ciencies.
4.6.1
Doping of Window and Absorber Materials with
n-Dopants
The use of glass/CG substrates with lowest sheet resistance and
the selection of a suitable n-type window material with required
