
the localization of polarons is due to Coulombic attractions to their counter ions
(e.g., I
3
), which normally have very low mobilities. However, in order to achieve
high electrical conductivities, the polarons must be able to migrate along the
polymer chain. Figure 5.61 illustrates the migration mechanism of a polaron; in
order to migrate, a high conce ntration of the counter ions must be present to induce
the movement of the polaron into the Coulombic field of nearby ions. Accordingly,
high doping levels are required to achieve sufficient conductivity in conductive
polymers. This may be compared with inorganic-based semiconductors such as Si,
which require minute concentrations of dopants due to the pronounced mobility of
electrons/holes through the extended solid-state lattice.
If a second electron is abstracted from a section of the polymer that is already
oxidized, either a second independent polaron may be generated, or two polarons
may condense to form a bipolaron.
[99]
The two positive/negative charges of the
bipolaron are not independent, but rather move as a pair similar to the Cooper pair of
Table 5.6 (Continued)
Polymer Structure Dopants O
1
cm
1a
Polyisothianaphthene BF
4
, ClO
4
100
Poly(3-alkylthiophene)
BF
4
, ClO
4
100
Polyazulene
BF
4
, ClO
4
2.5
a
Maximum conductivity of doped polymers; the conductivity of pristine (undoped) polymers is ca.
0.001–0.1 O
1
cm
1
. The unit O
1
cm
1
is equivalent to S cm
1
.
b
The maximum anisotropic (stretch oriented) conductivity for polyacetylene: Naarmann, H.; Theophilou,
N. Synth. Met. 1987, 22,1.
c
Maximum conductivity of PEDOT is obtained by doping with poly(styrene sulfonate) – PSS. Accord-
ingly, PEDOT:PSS is the industry leader in transparent conductive polymer films, with ca. 80% light
transmission.
422 5 Polymeric Materials