Environmental Engineering Reference
In-Depth Information
Bhattacharya
et al.
, 1985 [28], and Galloway
et al.
, 1995 [29]).
This method creates a peak of EBIC at the junction area where the
internal electric field is maximum. However, previous reports have
shown peaks appearing at the CdS/CdTe interface, in the middle
region of the CdTe layer, and at the metal/CdTe interface and,
therefore, provide inconclusive EBIC results. In fact, if the device is
fully depleted, the EBIC peak could appear at any place where the
material quality is high and, therefore, the charge carrier collection
is more e
cient. The EBIC results reported in the past could be
explained using both models for this device structure.
4.5.4
Observation of Discrete Barrier Heights and V
oc
Values
Figure 4.7 summarised the experimentally observed potential
barrierheight(
φ
b
)valuesanddiscretesetof
V
oc
valuesforthin-film
CdS/CdTe solar cells. These observations arouse curiosity about the
device structure. If the device is a simple p-n junction, it provides
only one
φ
b
depending on the bandgaps of the two materials.
Observationsoffivedifferentbarrierheightsanddiscretesetsof
V
oc
values for different batches of the same solar cell structure provide
the main reason to move away from the p-n junction model. Fermi-
levelpinningpositionsobservedatCdTe/metalinterfaces(asshown
in Fig. 4.6), the exact match between these Fermi-level positions
with experimentally measured
φ
b
values, and the presence of five
different sets of
V
oc
values strongly supported the formation of new
model for this device structure.
4.5.5
A Thin-Film CdTe Solar Cell Device Without a CdS
Layer
There exists one report in the literature on the comparison of
CdTe solar cells with and without a CdS layer. Das and Morris [24,
25] established the electrodeposition of CdTe and fabricated thin-
film solar cells under similar conditions, one set with CdS and
the other set without CdS. This work produced solar cells with
e
ciencies around 10%, but the device without a CdS layer showed
comparatively better performance. These researchers must have
had substrate preparation conditions right for CdTe to grow on CG
