Environmental Engineering Reference
In-Depth Information
Inordertoproducehigh-e
ciencysolarcells,theCdTematerials
should be grown with slight Cd richness, surfaces should be
modified to produce Cd richness, and hence the Fermi level should
be pinned closer to the valence band maximum at E
4
or E
5
.Since
there are five defect levels, the reproducibility, stability, and yield of
devices are going to suffer unless these defect levels are properly
understood and dealt with appropriately.
4.7 Summary
The large body of experimental evidence available in the literature
enabled the identification of the necessity of an alternative model
to describe and develop the glass/CG/CdS/CdTe/metal solar cell
further,beyondthelimitsofthep-njunctionmodel.Thisnewmodel
explains the device behaviour in terms of a combination of an
n-n hetero-junction and a large Schottky barrier at the CdTe/metal
interface. The materials growth, chemical, and heat treatments and
wet chemical etching provide the right condition for Fermi-level
pinning at one of five experimentally identified energy levels (0.40
±
±
±
±
±
0.02eV)at
themetal/CdTeinterface.Toproduceane
cientsolarcellstructure,
theFermilevelshouldbepinnedclosetothevalencebandmaximum
(i.e., at 1.18
±
0.02 eV below the conduction band minimum) and
other unwanteddefects should beeliminated by passivation.
The impact of various features of this model on the e
ciency
of the cell is considerable. The ability to obtain Fermi-level pinning
near the top of the valence band enables the production of a barrier
height of 1.18
±
0.02 eV. This enables a high open circuit voltage
to be achieved since
V
oc
is a function of the
φ
b
. Since this is an MS-
type rectifying contact, the introduction of a thin insulating layer
to form an MIS-type structure enables the further enhancement of
the
V
oc
and lifetime of the device. One of the main reasons for the
degradationofthisdeviceistheintermixingatCdTe/metalinterface,
but the insulating layer in this case acts as a reaction barrier to
improve the lifetimeof the device.
Since both window and absorber materials are n-type, doping
with n-type dopants (according to the new model) instead of p-
0.04,0.65
0.02,0.73
0.02,0.96
0.04,and1.18
