Environmental Engineering Reference
In-Depth Information
Comprehensive work on electrical contacts to n-CdTe bulk
crystals with over 20 different contacting materials show that
Fermi-level pinning is a strong mechanism in these interfaces and
determined by the history of material growth and the nature of
surface[7].Chemicaletchingcouldmodifythissituationtoacertain
extent. This work has also shown that Sb produced the noise-free
and most stable electrical contacts to form Schottky barriers to
n-CdTe surfaces. Furthermore, the best I-V curves with the highest
RF (
10
9
) and the lowest ideality factor (
n
) are observed when the
CdTe surfaces are produced with Cd richness [10] and the metal
contacts are formed with Sb.
All the experimentally observed Fermi-level pinning positions,
as measured by different techniques, are summarised in Table 4.1
and schematically shown in Fig. 4.6. The agreement of the locations
determined by four different techniques for defects in CdTe is
conclusiveand impressive.
∼
Table 4.1 Discrete Fermi-level pinning positions measured using I-V and
ballistic electron emission microscopy (BEEM) techniques and deep levels
observedusingPLanddeep-leveltransientspectroscopy(DLTS)techniques
for n-CdTe bulk crystals [7]
φ
φ
(
b
)
IV
(eV)
PL
(
E
c
-
E
t
)eV
DLTS (
E
c
-
E
t
) eV
BEEM (
b
)eV
±
±
0.40
0.04
—
0.42
0.03
—
±
±
0.65
0.02
—
0.65
0.03
—
±
±
±
0.73
0.02
0.715
0.005
0.74
0.03
—
±
±
±
0.96
0.04
0.965
0.005
0.95
0.03
0.96
±
±
±
1.18
0.02
1.240
0.005
1.18
0.04
—
4.3.4
Similar Observations on Thin-Film CdS/CdTe Solar
Cells
Current-voltage (I-V) measurements of a large number of thin-
film solar cells (glass/CG/CdS/CdTe/metal) fabricated using elec-
trodeposited CdTe showed discrete barrier heights in agreement
with results obtained for bulk n-CdTe/metal Schottky diodes. These
results are summarised in Fig. 4.7(a). The
V
oc
values measured
underilluminationconditionsalsofollowsimilardiscretevalues,in-
dicating Fermi-level pinning at different levels for different batches
