Environmental Engineering Reference
In-Depth Information
heat treatments, and chemical etching. When the Fermi level is
pinned at the E
5
level, it produces the largest Schottky barrier at
the metal/CdTe interface, creating an excellent band bending or the
highest internal electric field across the device. This will produce
a good solar cell due to the existence of a strong internal electric
field for charge carrier separation and collection. If the conditions
are favourable to pin the Fermi level at E
1
, the band bending will be
at a minimum and, therefore, the internal electric field will be weak.
Consequently, this will form a very poor solar cell. Since there are
five possible Fermi-level pinning positions, the resultant solar cells
will vary from laboratory to laboratory and, sometimes, from batch
to batch within the same laboratory. This effect can show severe
problems in reproducibility and yield during a solar cell production
process. In such a situation, when the I-V curves are measured, for
example between
+
1.0 and
−
1.0 V, the Fermi level will be swept
acrossthefivedefectlevels.Forexample,intheforward-biassweep,
E
1
-E
5
levels will be below the Fermi level for a certain period of the
measurement time and, therefore, the defects will trap electrons —
Fig. 9.3(b). In the reverse-bias sweep, all the defect levels are above
the Fermi level and, therefore, the defects will de-trap electrons
during this period. The observations indicate that these traps are
veryslowtorespondbutsensitivetoelectrical,thermal,andlighting
stresses applied to the device. Figure 9.3(b) shows the open circuit
voltage developed when the device is illuminated, and this is
identical to the forward biasing of the device under dark conditions
by
V
=
V
oc
. Therefore, the I-V characteristics of the devices can
vary even after exposure to light, depending on the concentration
of defects at the interface. If the trapping during illumination is
considerable, the Fermi-level pinning position can move upwards,
showing a lower
V
oc
when measured for the secondtime.
The effects of multi-defect levels at the metal/CdTe interface of
CdS/CdTe solar cells are demonstrated by the data presented in
Table 9.1.
When solar cell-1 was measured for the first time, the
V
oc
was
610 mV and the
J
sc
was 32 mAcm
-
2
. When the I-V measurements
were repeated consecutively at intervals of a few minutes, the
V
oc
reduceddrasticallyandfinallysettledat280mV,indicatingtheshift
of Fermi-level pinning position in the direction E
5
to E
1
, reducing
