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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