Environmental Engineering Reference
In-Depth Information
Figure 5.3
Four main experimentally observed groups of open circuit
voltages appearing in CIGS-based solar cells.
The observation of the results summarised in Table 5.1 shows
that the
V
oc
values measured tend to appear in several groups.
These experimentally observed groups of
V
oc
values are graphically
shown in Fig. 5.3 and labelled as A, B, C, and D. The most striking
observation is that there are four different reported
V
oc
values in
reference [1]. The open circuit voltage is a function of the potential
barrier height (
φ
b
) of the device, as given by Eq. 1.16.
V
oc
=
n
e
ln
J
sc
kT
φ
b
+
(5.1)
AT
2
Thedifferentgroupsof
V
oc
areobservedduetodiscretevariation
in barrier heights, which depends on materials and processing
conditions.Thisidentificationisakeytobetterunderstandingofthe
physics behind the device structure. It must be stressed that for the
identification of these discrete
V
oc
values, only the results of high-
e
ciency CIGS solar cells should be considered. In the case of low-
e
ciency CIGS solar cells, these groups merge together, making it
even more di
cult to identify any pattern.
In all of the above cases, whether it is a homo-junction or a
hetero-junction, each situation should produce only one group of
V
oc
value for high-quality devices. How we experimentally observe
several groups of
V
oc
values is an unanswerable question based on
conventionalhomo-andhetero-junctiondevices.Thiswillbefurther
discussed in section 5.7 using an improved concept for this device.
