Environmental Engineering Reference
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upon upscaling. The foray of graphene into large-scale processing will require
significant advancement in large-scale processing.
Currently, high quality graphene is either micromechanically cleaved or grown
by chemical vapor deposition; both of which are not low cost and large-scale
compatible. Defects are more prominent in graphene films processed by solution-
based methods such as liquid-phase cleaving with ultrasonication or by the
reduction of graphene oxide. Although these techniques provide lower cost
alternatives to processing of graphene, however, graphene produced by such
methods exhibit poor properties with R
sh
in the kX!
-1
range due to structural
defects and poor interlayer contact as a result of vigorous exfoliation and reduction
processes [
67
]. Several reviews on the properties and processing of graphene are
present elsewhere [
59
,
68
,
69
] and a recent review elaborates on the application of
graphene as electrodes in electrical and optical devices [
70
].
3.2 Transparent Conductor Oxides
Transparent conductor oxides (TCO) are semiconductor materials composed of
binary and ternary oxides containing one or two metallic elements. They have a
wide optical band gap of [3 eV making them optically transparent in the visible
range. The doping of intrinsic semiconductor oxides with metallic elements in a
non-stoichiometric composition results in increasing conductivity without
degrading their optical properties. There is a wide variety of transparent conductor
oxides such as ZnO, In
2
O
3
SnO
2
, CdO; ternary compounds like Zn
2
SnO
4
, ZnSnO
3
,
Zn
2
In
2
O
5
,Zn
3
In
2
O
6
,In
2
SnO
4
, CdSnO
3
; and multicomponent oxides such as Sn:
In
2
O
2
(ITO) and F:SnO
2
[
13
,
71
]. Of these, apart from ITO (Sn: In
2
O
3
), AZO and
GZO (Al-and Ga- doped ZnO, respectively) have optical transparency and con-
ductivity similar to ITO and therefore exhibit a figure of merit ratio r
=
a [ 1 X
-1
[
13
]. AZO is the best candidate for replacement of ITO because of its nontoxicity,
inexpensive materials, and low resistivity.
Transparent conductive materials can be prepared using a wide variety of thin
film deposition techniques, through physical vapor deposition methods such as
evaporation, magnetron sputtering, molecular beam epitaxy; and through chemical
vapor deposition (CVD) techniques such as high-temperature CVD, metal-organic
CVD (MOCVD), atomic layer deposition. Such methods are, however, not suitable
for high throughput production of PSCs. Liquid-based deposition methods are also
employed in the processing of TCO thin films, for example, sol-gel, and chemical
bath deposition. However, films produced by these methods have far inferior
conductivity. For example, the reported resistivity of sol gel-produced films ranges
from 7 9 10
-4
to 10 X cm whereas sputtered films have reported resistivity as low
as 1 9 10
-4
X cm [
72
]. The low resistivity is achieved by sintering at high
temperature under vacuum. Furthermore, TCO are brittle materials and hence
flexibility restricting. As such, it is unlikely that TCO will be explored for very
large-scale
high
throughput
production
of
PSCs.
They
are,
however,
more
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