Environmental Engineering Reference
In-Depth Information
be in the low 10
15
cm
−
3
values [51]. By achieving these optimum
doping concentrations and bandgap engineering, a healthy device
structure can be produced for effective PV solar energy conversion
utilising minimum thickness of the material layers. Because of the
excellent optical absorption property of CIGS, only a
∼
3
μ
mthick
absorber layer is required for this solar cell.
5.8.2
Defect Level Identification and Engineering
As shown in Fig. 5.5 and 5.6, the most desirable position for Fermi-
level pinning is at the E
4
level. This provides the optimum band
bending required for charge carrier collection. Fortunately, many
device structures with CIGS show a
φ
b
of
∼
1.10 eV, indicating the
most commonpinning position is at the E
4
level.
The identification of other defect levels has paramount impor-
tance. Once identified, ways of passivation or removal of these
levels should be explored. The presence of the E
1
,E
2
,andE
3
levels deteriorates the device parameters either by pinning at these
undesirablelevels,causinglow
V
oc
,orthroughcontributionstoR&G
process, causing low
J
sc
values for high
V
oc
devices obtained with
Fermi-level pinningat the E
4
level.
From the accumulated knowledge to date, it seems that the E
4
level,closetotheconductionbandisrelatedtothen-typeODClayer.
Since this layer is Cu-deficient, and n-type in electrical conduction,
the E
4
defect level must be associated with Cu deficiencies (Cu
vacancies) or In richness. Fortunately, it seems that this natural
phenomenon is helping to produce a good device by pinning the
Fermi level at the most desirable native defect level of E
4
close to
the conduction bandminimum.
5.8.3
Growth of CIGS with Controlled Orientation
All CIGS layers used in thin-film solar cells are polycrystalline, and
XRD patterns show three major reflections, at (112), (220/204),
and (116/312). It has also been shown that controlling the
growth process leads to the production of materials with (112)-
or (220/204)-preferred orientations. The work reported in 2004
by Ott
et al.
[45] explains different defect structures using CL
