Environmental Engineering Reference
In-Depth Information
transparent conductor directly adapted from other optoelectronic applications in
general and inorganic solar cells in particular. It has high conductivity (sheet
resistance of 10-20 X!
-1
) and transmission ([80 %) in the visible region of the
electromagnetic spectrum. However, the processing of ITO requires high prepa-
ration temperatures and vacuum-based highly energy-intensive and throughput
limiting deposition techniques such as sputtering while all other components in
PSCs can be coated and printed in ambient conditions. As a result, the cost
footprint of ITO in PSCs is much larger than in inorganic solar cells. In fact, cost
analyses [
3
,
5
,
6
] suggest that ITO accounts for [50 % of the total cost of a PSC
module. Life cycle analyses of R2R-produced ITO-based PSC modules reveal that
ITO on PET substrate accounts for *90 % of the total energy (embodied energy
and direct process energy) imposed by all input raw materials [
1
]. Furthermore, the
brittle nature of ITO is not conducive for application on flexible substrates that are
prone to bending and flexing and may inevitably lead to deterioration in the
performance of PSCs. Lastly, the scarcity of indium resources in the world and its
high demand from the display industry has created large cost fluctuations and
future supply concerns. All these factors substantiate the need for finding a cost-
effective alternative to ITO that ideally involves no challenges with scarcity and
can be processed in a vacuum-free environment. Such an alternative would sig-
nificantly
improve
the
environmental
and
commercial
feasibility
of
PSC
technology.
Following a brief overview on PSCs, this chapter lays out the different alter-
natives of ITO that are widely researched. Each alternative is introduced along
with the current development in their application in PSCs. Particular emphasis is
laid on the implications of the processing choices of fabrication of ITO alternatives
on the solar cell performance and suitability for low-cost upscaling. At the end of
the chapter, a subsection is presented with recent progress on ITO-free roll-to-roll
processed PSC modules.
2 Polymer Solar Cells: A Brief Overview
2.1 Device Structure
A polymer solar cell comprises of a thin film of the photoactive material sand-
wiched between two electrodes of different work functions. One of the electrodes
is a transparent conductor from where light is permitted to the photoactive layer.
Two different architectures are commonly used: a normal structure and an inverted
structure (Fig.
3
). In normal structure, holes are collected at the front electrode—
the transparent conductor—while electrons are collected at the back electrode
which is a lower work-function metal (usually Al) than the front electrode. In
inverted structure, the reverse process takes place, that is, the electrons are col-
lected by the transparent conductor and holes are collected by the back electrode
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