Environmental Engineering Reference
In-Depth Information
front electrode and the metal layer on the substrate forms as the back electrode. A
range of metals has been applied as back electrodes, for example, Ag, Al/Cr, and
Cu/Ti [
116
-
118
]. The choice of metal for back electrodes can allow tuning of the
work function of the electrode allowing higher voltage extraction and therefore
higher PCE. This is observed in small devices when Ag grid is deposited by
evaporation which in turn allows PEDOT:PSS layer to be made very thin, ulti-
mately allowing superior light transmission into the device. When Ag grid is
deposited by ambient processing technique such as screen printing, a higher
thickness of PEDOT:PSS because the solvents from screen printing formulation of
Ag ink is observed to diffuse through the underlying PEDOT:PSS layer into the
photoactive layer, thereby destroying it. Consequently, the higher thickness of
PEDOT:PSS leads to poor transmission to the PAL and thereby very poor current.
In early reports on R2R produced screen-printed grids, a transmission as low as
30 % is reported [
116
-
118
].
The most recent advancement in ITO-free R2R produced modules were made
in bottom illuminated inverted structures. A vacuum-free all R2R-processed PSCs
employing high conductive PEDOT:PSS/metal grid as a front electrode and
PEDOT:PSS/metal grid as the back electrode in an inverted structure was recently
demonstrated [
115
]. Metal grids were printed by three R2R methods: R2R thermal
imprinting of embedded grids, R2R inkjet printing, and R2R flexographic printing.
R2R flexographic-printed and R2R-embedded grids delivered similar albeit
unprecedented performance for fully R2R processed vacuum-free large-area ITO-
free cells (active area 6 cm
2
) under ambient conditions with devices based on flexo
grids having a PCE: 1.82 %, J
sc
: 7.1 mA cm
-2
, V
oc
:5.1 V, FF: 51.2 %, and
embedded grids with PCE: 1.92 %, J
sc
: 7.06 mA cm
-2
, V
oc
: 0.50 V, FF: 54.6 %.
The raised topography and relatively poor conductivities in the R2R inkjet-printed
silver grids resulted in significantly lower PCE attributed to the lower FF and J
sc
caused as a result of shunt paths: PCE 0.75 %, J
sc
: 4.27 mA cm
-2
, V
oc
: 0.50 V,
and FF: 35.1 %. Among all these techniques, flexographic printing emerged to be
the favorable low-cost technique presenting no topography issues and the need for
multiple R2R steps as required in the embedded grids. Flexographic printing has
been recently adopted in producing large-area modules that are produced by all
R2R processing and have delivered PCE comparable to ITO-based devices. This is
discussed in the next section.
4 Roll-to-Roll Processing of ITO-Free Polymer Solar Cell
Modules
To date, almost all polymer solar cells reported in the literature are prepared using
a combination of two laboratory techniques: spin coating and metal evaporation.
However, neither of these techniques can be expected to share a future with PSCs,
where only processes that rely on flexible substrates and the absence of vacuum
steps are expected.
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