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[dlnW/dT]
The negative slope of the lnW = f(lnT), where W = -T is the reduced activation energy,
as it is shown in Figure 2, reveals that the samples are in the insulating state, for which hopping transport is
described by the general Mott Variable Range Hopping (VRH) formula [4]
σ = σ 0
⎡ ( )
exp - T
0
/ T α ⎤
⎣ ⎦
The exponent α decreases with thermal aging, from the value 1 at the beginning of the thermal treatment, to a
value of about α =1/2 at the end of this process, as it is shown in Figure 3.
(1)
1,2
1,1
1,0
0,9
α
0,8
0,7
0,6
0,5
0,4
0 10 20 30 40 50 60
t (h)
Figure 3. The exponent α in Equation (1) as a function of the different thermal treatment times for the same
PEDOT:PSS sample, as in Figure 1.
Percolation theory in conducting polymer networks showed that the value of the exponent α
systematically decreases from about α ≈ 1 to 0.25 upon increasing the volume fraction of the conducting polymer
into the insulating matrix to the percolation threshold [5,6]. A value of α = 0.5 indicates a variable range hopping
process in one dimension, revealing a much better alignment of the polymer chains at the end of the thermal
treatment. This seems to contradict the fact that the conductivity declines with thermal treatment. However, Fig.4
shows that the peak corresponding to PSS - -Na bond is decreasing after heat treatment, which indicates that there is
an elimination of the PSS-Na salt. Apart from UPS, SEM pictures indicate a decrease of the size of the conductive
grains with increasing treatment time.
XPS (MgKα) S2p
as-received
PSS
XPS (MgKα) S2p
after thermal treatment
PSS
XPS Intensity (a. u.)
PEDOT
PSS - (Na + , H + )
PSS 0
PSS - (Na + , H + )
PEDOT
PSS 0
162 164 166 168 170 172162 164 166 168 170 172
Binding Energy (eV)
Figure 4. S(2p)-XPS peaks from the “as received” sample(a) and after heat treatment(b).
It seems reasonable to assume that a decreasing of the grain size at constant separation has as a result the
decrease of conductivity, as the number of the potential barriers increases [7, 8].
[1] Choulis S.A., Choong V.-E., Patwardhan A., Mathai M.K., So F., Adv. Fun. Materials, 16, 1075, (2006).
[2] de Kok M.M., Buechel M., Vulto S.I.E., van de Weijer P., Meulenkamp E.A., de Winter S.H.P.M., Mank
A.J.G., Vorstenbosch H.J.M., Weijtens C.H.L. van Elsbergen V., Phys. Stat. Sol. (a) 201 (2004) 1342.
[3] Crispin X., Marciniak S., Osikowicz W., Zotti G., Denier van der Gon A.W., Louwet F., Fahlman M.,
Groenendaal L., de Schryver F., Salaneck W.R., J. Polym. Sci. Part B: Polym. Phys. 41 (2003) 2561.
[4] Kohlman R.S., Joo J., Min Y.G., MacDiarmid A.G., A.J. Epstein, Phys. Rev. Lett. 77 (1996) 2766.
[5] Reghu M., Yoon C.O., Yang C.Y., Moses D., Smith P., Heeger A.J., Cao Y., Phys. Rev. B 50 (1994) 13931.
[6] Duvail J.L., Rétho P., Garreau S., Louarn G., Godon C., Demoustier-Champaghe S., Synth. Met. 131, (2002)
123.
[7] Zhou Y., Yuan Y., Lian J., Zhang J., Pang H., Cao L., Zhou X., Chem. Phys. Lett. 427, (2006) 394.
[8] Timpanaro S., Kemerink M., Touwslager F.J., de Kok M.M., Schrader S., Chem. Phys. Lett. 394, (2004) 339.
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