Environmental Engineering Reference
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Fig. 5 Current-voltage
curve of a solar cell in dark
and under illumination
3 Alternatives to ITO
Indium tin oxide is a ubiquitous material in most optoelectronic devices and as
such the efforts in finding an alternative to ITO is exhaustive. In general, the
alternatives of ITO could be categorized into four broad material groups: (1)
nanomaterials; (2) polymers; (3) metals; and (4) metal oxides. Nanomaterials can
further be classified into carbon nanotubes, graphene, metal nanowires, and metal
nanogrids. These material groups are not mutually exclusive and are often used in
some combination with each other. Particularly, Poly(3,4-ethylenedioxythio-
phene):poly(styrenesulfonate) or PEDOT:PSS often used in combination with
many of these alternatives. Figure 6 summaries material categories that are
investigated as alternatives to ITO.
When evaluating the efficacy of transparent conductors for application in solar
cells, two property parameters are of prime importance: optical transparency and
sheet resistance (R sh ). However, these parameters are often in competition with
one another and an improvement in one often requires a sacrifice in the other.
Commercially available ITO usually has an R sh of 10 X! -1 and a visible
transmittance of [80 %. Such a property of ITO is used as a benchmark against
which alternatives are being developed and assessed. A figure of merit relationship
that enables property comparison between different transparent conductors is the
ratio between DC conductivity (r) and absorptivity (a) which is given by [ 12 ];
1
t R sh
ln ð 1 A Þ
t
r
a ¼
ð 2 Þ
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