Environmental Engineering Reference
In-Depth Information
Solution-based processing is an appealing alternative to vacuum-based depo-
sition approaches. The straightforward comparative advantages of solution meth-
ods include atmospheric pressure processing, which requires significantly lower
capital equipment costs, suitability for large-area and flexible substrates, higher
throughput, and the combination of more efficient materials usage and lower
temperature processing. These approaches can also be readily adapted for simul-
taneously patterning the materials while coating or printing, which eliminates the
need for additional processing steps.
An enormous palette of film-forming techniques has been investigated for
processing one or more of the layers in a PSC, which usually is comprised of five
or six layers. Some of these film-forming techniques investigated are slot die
coating, gravure coating, knife-over-edge coating, offset coating, spray coating,
and printing techniques such as inkjet printing, pad printing, and screen printing.
A complete review on roll-to-roll processing can be found elsewhere [
39
,
119
].
Recently, fully functional large area highly flexible ITO-free modules, reaching
a total area of 180 cm
2
were reported [
120
]. These modules were fabricated on a
60 lm thick barrier substrate on which a flexographic printed silver grid in
combination with printed hcPEDOT: PSS formed the transparent electrode. Such a
composite front electrode exhibits a transmittance of [60 % in the visible region
and printed silver shows a sheet resistance of [1 X!
-1
. The complete device
stack comprised of Ag grid/hcPEDOT:PSS/ZnO/P3HT:PCBM/PEDOT:PSS/Ag is
known as the IOne process. The R2R processing carried out in the preparation of
IOne stack is shown in Fig.
18
. Such devices were found to be highly scalable;
single cells of 6 cm
2
demonstrated a FF of 51 % while up-scaled modules with an
area of 180 cm
2
had a FF of 55 % (Fig.
19
). These modules were subjected to
several accelerated lifetime testing conditions and were found to be rather stable
for more than 1,000 h under different operational and storage conditions [
120
],
thereby attesting to [1 year of lifetime.
The IOne process has been further adopted in the processing of large arrays of
serially interconnected modules, all accomplished in the printing and coating pro-
cessing [
122
]. Such a structure can ideally be used to produce infinitely interconnected
solar cell modules on an infinitely long flexible substrate and be readily installed for
power generation. Such a concept has been demonstrated on an 80 m foil comprising
of 16,000 serially connected cells delivering a voltage of 8.12 KV under outdoor
conditions. Characterization under overcast conditions at 138 Wm
-2
showed a PCE
of 1.6 %, V
oc
:874V,I
sc
:5.49mAcm
-2
,andFF: 50 % on an area of 11 meters.
5 Summary and Future Outlook
Polymer solar cells present an attractive technology by the sole virtue of its low-
cost processing potential. Research efforts in low-cost processing until now have
shown that very low energy payback time (0.30 years) can be obtained with PSCs
which inevitably would allow the technology can be within the economical reach
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