Environmental Engineering Reference
In-Depth Information
∼
1.20-1.45 eV. Most research groups report the CIGS layers used in
the solar cell devices as p-type materials.
5.2.3
Ordered Defect Compound Layer
The natural formation of a Cu-deficient (In-rich) surface layer on
CIGS material has been observed by many researchers [20-22],
and reports agree in general, with a few exceptions. The reported
thickness of this ordered defect compound (ODC) layer is
∼
15
nm, and this surface layer is n-type in electrical conduction with
a bandgap of
1.30 eV. Since this has been observed in all CIGS
materials grown using different techniques, this must be due to a
natural occurrence taking place near the surface. This surface layer
can play a considerable role in device performance and, therefore,
device designs must consider this well-known natural phenomenon
and use it to thedevice's advantage.
Both In (melting point
=
156
.
6
◦
C) and Ga (melting point
=
29
.
8
◦
C) metals are solids at room temperature, but when brought
together, they readily form an In-Ga eutectic, which is a liquid at
room temperature. Perhaps for this reason, the CIGS alloy can be
formedeasilybytheselenisationofasoftalloywithinashortperiod
oftime.Forthesamereason,andotheryet-unknownthermodynam-
ical properties, Cu atoms diffuse into the crystal, away from the free
surface, forming this experimentally observedODC layer.
∼
5.2.4
Latest Developments in Materials Growth
Publications between 2004 and 2007 on the electrodeposition of
CIS [23] and CIGS [24, 25] reported a controlled way of growing
p
+
,p,i,n,andn
+
material layers for the first time. This electrical
conduction type variation can be achieved by controlling the
chemical composition of the material layers. These results are
summarisedinsection3.4.3inchapter3.InCISmaterial,Cu(Group-
I element) richness provides p-type electrical conduction and In
(Group-III element) richness provides n-type electrical conduction.
In the case of CIGS material, although Ga is a group-III element
very similar to In, addition of Ga makes the material more p-
type. This is purely an experimental observation from the recent
