Environmental Engineering Reference
In-Depth Information
diagrams formulated are with respect to the Fermi level, which is
pinned at oneof theexperimentally observed nativedefect levels.
Depending on the fine details of the growth of CIGS material,
processing steps, and metallisation, the Fermi level will be pinned
at one of the four experimentally observed defect levels. The
φ
b
is,
therefore, defined by the Fermi-level pinning position. When the
Fermi level is pinned at E
4
level, the best solar cell is formed with
p-CIGS. If the Fermi-level pinning is at the E
1
level, the device will
have the least band bending and, hence, poor performance of the
device. Depending on the pinning position, discrete
V
oc
values will
beobserved.
In Fig. 5.5, the bandgap has been increased towards the front
of the solar cell due to the addition of S. Also, the ODC layer with
a
∼
1.30 eV bandgap is naturally helping for the e
cient creation,
separation, and collection of charge carriers. Since the naturally
occurring ODC layer is In-rich and n-type in electrical conduction,
the Fermi level is automatically pulled closer to the conduction
band minimum. The experimentally observed E
4
defect level may
be due to these Cu deficiencies in the vicinity of the p-CIGS surface.
It is interesting to compare the experimentally observed potential
barrier heights for good solar cells based on CIGS material. These
barrier heights are usually in the region of 1.14 eV. The Fermi-level
pinning at the E
4
level created by the ODC layer produces
∼
1.03 eV
Schottky barrier (see Fig. 5.4) and the rest is provided by the effect
of the MIS contact used. The naturally occurring ODC layer has been
very helpful in producing high e
ciency solar cells fabricated on
CIGS.
The three layers (n-CdS, i-ZnO, and n-ZnO:Al) are an interesting
combination.TheyformacompoundI-layerinordertoformanMIS-
type solar cell structure. This increases the effective
φ
b
and, hence,
an improved
V
oc
value when compared to a device with MS contact.
However, it should be noted that the thickness of this compound
I-layer (
∼
300 nm) is much larger than that of conventional MIS
structures. This is still acceptable, since both n-CdS and n-ZnO:Al
are two semiconducting layers with comparatively large electrical
conduction. The insulating layer, i-ZnO, sandwiched between the
two semiconductors will have a thinner layer due to interface
interactions on both sides.
