Environmental Engineering Reference
In-Depth Information
Fig. 16 Chemical structure
of PEDOT:PSS
used polymeric salt for this application where PEDOT is a conjugated polymer in
its oxidized state carrying a positive charge and PSS is a polymer with deproto-
nated sulfonyl groups carrying a negative charge (Fig.
16
). PSS is added to EDOT
during polymerization as a charge-balancing counter ion and to improve the
inherently low solubility of PEDOT in aqueous medium. Several in-depth reviews
on PEDOT:PSS in general are present [
84
,
85
].
PEDOT:PSS was initially and still widely used in antistatic coating where R
sh
is
not as critical as when used in PSCs. In earlier reports on PEDOT:PSS as trans-
parent conductor films, a typical conductivity values 1-10 S cm
-1
(R
sh
of 10
5
X
!
-1
) was observed [
86
,
87
]. Such conductivity values were three orders of
magnitude lower than that of ITO ([4,000 S cm
-1
) at similar transmission (80 %).
Several approaches were subsequently employed to increase the inherently low
conductivity of PEDOT:PSS and many have been successful. Some of these
methods include the addition of high-boiling temperature polar compounds such as
diethylene glycol [
88
]; ethylene glycol [
89
,
90
]; sorbitol [
91
,
92
]; dimethylsulf-
oxide (DMSO) [
88
,
93
,
94
]; glycerol [
91
,
93
,
95
]; by different chemistry methods
such as controlling synthetic conditions, fundamental alteration of the polymer
back bone, and by functionalizing the backbone with substituent side groups [
87
].
Currently, highly conductive formulations of PEDOT:PSS are commercially
available, for example, from Heraeus with the latest generations Clevios
TM
PH500
and PH1000 having a conductivity of 300 and 850 S cm
-1
, respectively. This is in
stark contrast to previous generations such as H.C. Stark Baytron P variants having
R
sh
of 10
5
X!
-1
or 1-10 Scm
-1
.
Like TCO, the conductivity of PEDOT:PSS increases with increasing film
thickness reaching saturation at a finite film thickness [
85
,
96
]. However,
increasing thickness leads to decreasing optical transmittance. As a result, a trade
off between transmission and conductivity is required. Hence, particularly in
earlier reports prior to the commercial availability of highly conductive formu-
lations of PEDOT:PSS, such a tradeoff between transmission and conductivity
resulted in poor power conversion efficiencies of ITO-free PSCs and modules.
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