Environmental Engineering Reference
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In some cases, PEDOT:PSS having transmission in the range 10-30 %
(350-600 nm) for a comparable R
sh
to ITO (10-20 X!
-1
) has been reported [
97
].
As a result, often PSC devices-based PEDOT: PSS delivered lower PCE than ITO-
based reference devices [
89
,
91
,
98
]. Subsequently, the use of highly conductive
(hc) PEDOT:PSS such as PH500 as transparent conductor has resulted in com-
parable performance to ITO-based control devices. A maximum PCE of 3.27 %,
V
oc
: 0.63 V, J
sc
: 9.7 mA cm
-2
, FF: 53.5 % have been reported [
99
].
In the inverted device configuration, a PCE of 3.08 %, V
oc
: 0.61 V; J
sc
:
9.1 mA cm
-2
; FF: 53.3 % is reported in solar cells based on hcPEDOT:PSS
transparent conductor and evaporated Ag counter electrode. Such a PCE, none-
theless, is lower than ITO reference devices having an average PCE: 4.20 %,
J
sc
: 10.25 mA/cm
2
, FF: 66.6 %, and V
oc
: 0.62 V. In such a device, the evaporated
Ag counter electrode, can also be replaced with PEDOT:PSS, therefore allowing
the fabrication of an all-solution-processed device [
100
]. Initially, such a device
showed poor rectification with a performance of PCE: 0.47 %, V
oc
: 0.31 V, J
sc
:
5.94 mA cm
-2
, and FF: 27.7 % [
100
]. However, later reports observed good
rectification with improved ZnO buffer films deposited by atomic layer deposition
[
101
]. With a new generation low-band gap polymer, a PCE of 2.69 % was
observed in a polymer-based solar cell processed with a roll-coated hcPEDOT:PSS
as the front electrode in combination with PEDOT:PSS/Ag as the back electrode
[
102
]. Nonetheless, all such devices had lower PCE than ITO-based reference
devices. However, PEDOT:PSS electrodes render improve mechanical flexibility
than ITO electrodes where PCE showed a 92 % retention in PEDOT:PSS-based
PSCs while only 50 % of the initial PCE was retained in ITO-based electrode after
300 bend cycle [
100
].
3.5 Metal grid/PEDOT:PSS Composite Electrode
Despite the development of highly conductive formulations of PEDOT: PSS, its
R
sh
values (10
2
-10
3
X!
-1
) still remains significantly higher than ITO (10-60 X
!
-1
) as a result of which stand-alone PEDOT:PSS front electrodes have yielded
lower PCE than ITO-based control devices. By physically reinforcing
PEDOT:PSS with metal grids, conductivity of PEDOT:PSS can be significantly
improved. Such a composite electrode when used in place of ITO in normal
structure solar cells is observed to cause a threefold decrease in series resistance
(R
s
) from [ 1kX in cells with only PEDOT:PSS as electrode to 400 X with
composite electrodes, ultimately resulting in a threefold increase in J
sc
[
98
]. The
challenge for using such a composite electrode is in the optimization required
between shadow losses due to the metal grids and the resistive losses due to the
resistance of the combined PEDOT:PSS/metal electrode. Depending on the R
sh
of
PEDOT:PSS films, completely different configurations of metal grid design are
required in the optimization of the cells [
103
]. Overall, the surface coverage of the
metal grids should be as small as possible so as to minimize loss of incoming
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