Environmental Engineering Reference
In-Depth Information
(a) CIGS material can be grown by several different techniques,
and its bandgap value can be engineered between
∼
1.00 eV
and
∼
2.20 eV. Since five different elements are involved in this
alloy, the control of composition and, hence, uniformity and
reproducibility remain a challenging task.
(b) In a CIGS material sample, Cu atoms are driven away from the
freesurfacetowardsthebulkofthematerialduetoyetunknown
natural phenomenon. As a result an In-rich, n-type and, wider
bandgap material layer is formed at the surface. This is known
as the ODC layer among the PV community and naturally helps
in the formation of the device by pinning the Fermi level at E
4
,
close to the conduction bandminimum.
(c) Theworkreportedin2004and2007ontheelectrodepositionof
thin CIS and CIGS layers has shown that the predetermined p
+
,
p, i, n, or n
+
materials can be grown by controlling its chemical
composition. As solid-state physics principle guidelines, p-type
materials are produced when the material is rich in Cu (In-
deficient),andn-typematerialsareproducedwhenthematerial
is rich in In (Cu-deficient).
(d) The electrical contact work on CIGS material is almost non-
existent, but the work reported in 2005 on metal/p-CIGS shed
lightonunderstandingphysicsbehindthisdevice.StrongFermi-
level pinning is evident; therefore, the new model proposed for
the CdS/CdTe solar cell in the year 2002 is also applicable to
CIGS-based solar cells.
(e) Three different proposed models (based on n-CdS/p-CIGS
hetero-junction, n-ODC layer/p-CIGS hetero-junction, and n-
CIGS/p-CIGShomo-junction)arecurrentlyinuseforthisdevice,
indicating the complexity of the device structure. In fact, all
these three proposed ideas are embedded in the energy band
diagram shown in Fig. 5.6. In addition to the combination of
these three ideas, Fermi-level pinning at discrete positions and
an MIS-type structure at the front electrical contact have made
it even more complex to visualise andunderstand.
(f) There are two types of devices under development today. The
type-I is based on p-CIGS, and this is mainly due to an MIS-type
solar cell with four Fermi-level pinning possibilities. The type-
II is more e
cient in charge carrier creation, separation, and
