Environmental Engineering Reference
In-Depth Information
5.7 Deeper Understanding of Mo/CIGS/CdS/i-ZnO/
n-ZnO:Al/Metal-Grid Solar Cells
The main clues for the new model are provided by the results
summarised in Table 5.3, Fig. 5.4 [39], and the discrete
V
oc
values observed for the same device structure (Fig. 5.3). A
comprehensive CIGS material characterisation [46], together with
I-V and C-V experiments carried out on metal/p-CIGS interfaces
[39], confirm strong Fermi-level pinning behaviour depending on
surfacetreatmentdifferencesandthepositionofthe2mmdiameter
metal contact fabricated on the CIGS layers (due to non-uniformity
of the material). If this is the case, the production of discrete
V
oc
values is expected since this parameter is a function of
φ
b
.
There is an excellent agreement of
φ
b
measurements, together
withindependentdefect-levelmeasurementsfromdifferentgroups,
using different techniques (Fig. 5.4 and Table 5.3).
The formation of a PV-active device structure depends on the
fine details of the materials growth and device processing. Careful
observation of the experimental results reported in the literature
shows the formation of two types of PV solar cells based on CIGS
material. These two types of devices are described below.
5.7.1
Type-I CIGS-Based Solar Cell
The Showa-Shell CIGS materials are produced by the deposition of
Cu, In, and Ga layers on glass/Mo substrates, and the subsequent
selenisation at 550
◦
C, followed by sulphidation at 600
◦
C. The
material produced is p-type in electrical conduction, and the full
characterisation results of this material were published in the year
2004 [46]. Using the p-type conduction of the CIGS material and
Fermi-levelpinningatdiscretelevels,thebanddiagramforthistype
ofdeviceispresentedinFig.5.5.The(a)n-typeODClayer,(b)n-type
CdSlayer,(c)i-typeZnOlayer,(d)n-typeAl-dopedZnOlayer,andthe
front metal contact are shown in the energy band diagram. The in-
creased bandgap of CIGS material towards the front of the solar cell
due to the introduction of S is also shown in the band diagram. The
formulation of this band diagram is based on the Fermi level pinned
atoneofthedefectlevels(E
4
),andthereferencelevelhasbeentaken
