Pergamon Solid State Communications, Vol. 105, No. 4, 225-228. 1998 pp. 0 1997 Elsevier Science Ltd

Printed in Great Britain. All rights reserved 0038-1098/98 $19.CO+.oO

PII: s0038-1098(97)1010s-3

CONDUCTIVITY IN THE I*-DOPED PBMPV CONDUCTING POLYMERS

C.H. Lee,” D.K. Oh,” Cheol Eui Lee,“,* J.I. Jinb and S.-J. Chungb

‘Department of Physics, Korea University, Seoul 136-701, Korea bDepartment of Chemistry, Korea University, Seoul 136-701, Korea

(Received 11 August 1997; accepted 2 October 1997 by T. Tsuzuki)

A series of I*-doped poly[2-buthoxy-5methoxy- 1,4_phenylenevinylene] (PBMPV) conducting polymers were synthesized and systematically studied by means of electrical conductivity measurements. The conductivity vs degree of doping as well as in situ and aging measurements reveal a peculiar behavior attributable to the dopant kinetics. 0 1997 Elsevier Science Ltd

Keywords: A. polymers, D. electronic transport.

Unusually high electrical conductivities have been reported

for poly[2-buthoxy-5-methoxy-1,4-phenylenevinylene] (PBMPV) polymers doped with FeCls or I2 [l]. PBMPV, whose structure is shown in Fig. 1, is a derivative of the conjugated PPV (polyphenylenevinylene) polymer. It can be easily doped to controllable degrees of doping and provides a unique opportunity to systematically study the doping as well as conductivity dependence

of various properties including the dopant kinetics

and charge/spin dynamics. Thus, samples of PBMPV iodine-doped with various degrees were prepared and this work reports results of the electrical conductivity

measurements, which provide an insight on the

doping process and the dopant kinetics in the doped samples.

The PBMPV conducting polymer samples were prepared by thermal elimination of polyelectrolyte precursor polymer films and iodine doping and the iodine concentration was determined as previously described [l]. While the doped PBMPV samples are known to have inherent inhomogeneities, relatively

thick good quality samples were obtained according to visual inspection.

The electrical conductivity was measured by the standard four-in-line-probe method for samples prepared by various doping time, or equivalently, degrees of doping. An in situ conductivity measurement was made

* Corresponding author: E-mail: rscel@kuccnx.korea. ackr

for a sample in the process of I*-doping and a study of aging effects on the conductivity was also made on one of the highly conducting samples over a long time span. It should be pointed out that aging study can provide us with a new insight into the chemistry and physics of the dopant. To our knowledge, this is the first investigation ever reported on how and why aging of I*-doped PPV derivatives would change their electrical conductivities.

Figure 2 shows the electrical conductivity for a series of samples with various degrees of Iz-doping. Rapid

increases of the d.c. conductivity are noticed, especially around the doping levels of 10-31;/RU (repeating unit) and 10-lI;/RU. We first assumed that all the dopants are in I3_ state, although there are many reports that a small amount of dopants is in 1; state [2-71 especially for

polyacetylene. This can be explained by the effective bond percolation model applicable for systems in which the charge conduction mechanism is described by variable range hopping [8, 91. This model defines a percolation threshold at the critical doping degree where infinite percolation clusters are formed with infinite correlation lengths. Accordingly, the d.c. electrical conductivity can increase rapidly at the percolation threshold due to the formation of infinite percolation clusters. In bulk samples with three- dimensional network structures such as PBMPV, the d.c. conductivity can increase rapidly at the percolation threshold only when the infinite percolation clusters are formed, because charge carriers diffuse principally perpendicular to the polymer chains. The average electrical conductivity in those structures is determined

225

226 12-DOPED PBMPV CONDUCTING POLYMERS Vol. 105, No. 4

Fig. 1. Structure of repeating unit for PBMPV.

by the interchain diffusion rather than the fast diffusion along the main chain [IO].

The fact that iodine doped PBMPV shows two percolation thresholds in Fig. 2 can be attributed to the PBMPV structure consisting of the main chain and the two side chain groups, buthoxy (OC4H9) and methoxy (OCH3) substituents, attached to the benzene ring as shown in Fig. 1, which may dictate the directions in which the charge carriers diffuse as the doping proceeds. A doping kinetics study was made to confirm the behavior as shown in Fig. 2 by performing an in situ conductivity measurement in the process of the doping. Assuming that initially the doping time is proportional to the degree of doping, the conductivity as a function of the doping time as represented in Fig. 3 indeed exhibits a behavior quite similar to that shown in Fig. 2, including the two apparent thresholds between 0.1 and 1 min. Thus our conductivity vs doping level data can be explained by the model of three-dimensional anisotropic thresholds [ 111 and the temperature dependence of samples with high enough conductivities was well fitted with a(T) = a0 exp [-(To/T)“] with cx = l/3, which is consistent with that model.

It is well known that doped conducting polymers have significant time dependences, generally showing

Fig. 2. Conductivity as a function of the I*-doping degree.

.

/ a : I

i()-’ :

0.01 Oil 1 lb

Time (mi?utc)

Fig. 3. Conductivity as a function of the doping time.

degradation in ambient conditions believed to arise from chemical reactions such as oxidation. Aging experiments in other conducting polymers have shown exponential or stretched-exponential time dependences attributed to a first-order kinetics of the oxidation mechanism [12]. An aging experiment was done by measuring the con- ductivity as a function of time in ambient conditions in order to investigate the dopant kinetics, which is believed to play a crucial role in the conductivity of the PBMPV conducting polymers. Figure 4 shows the result of an aging experiment performed on one of the highly con- ducting samples, which showed metallic behavior, i.e. negative temperature coefficients of resistivity near room temperature. In fact, similar patterns as that shown in Fig. 4 were repeatedly observed in samples of various conductivities.

While the decrease in conductivity from 125 S cm-’ for the initial 48 h could be described by a stretched- exponential form as in Fig. 5, it was followed by an anomalous increase to a value 145 S cm-‘. which is even

140 -

p 130- 2 5 .* ‘*O .2 2 IIO- 2 s loo-

Q”, 0 loo 200 300 400 5

Time (hour)

0

Fig. 4. Conductivity as a function of the aging time.

Vol. 105, No. 4 IrDGPED PBMPV CONDUCTING POLYMERS 227

130

,120 E g

0110 .L ti s E 0 ‘O”

90 1 5 10 15 20 25

Time (hour)

Fig. 5. Initial decrease of conductivity fitted to the stretched-exponential time dependence of exp [(t,&) “1.

Fig. 6. Anomalous increase of conductivity fitted to the time dependence of rln.

higher than the initial conductivity, during the 48-60 h span. The initial decrease in conductivity upon aging can be ascribed to reactions of the sample with moisture and oxygen in the measurement cell, but the anomalous increase afterwards is rather surprising that is going to be discussed about below. After the anomalous increase, the conductivity showed a steady decrease, linear in time. This linear decrease of the conductivity is a novel phenomenon which has not previously been reported and at present there exists no model that can describe it quantitatively, although it appears to arise from dopant kinetics.

The most striking result being reported in this work is the anomalous increase with time of the conductivity even in air, indicative of a mechanism by which the overall conductivity can increase for a certain period of time. While a complete understanding of the aging pattern in our I*-doped PBMPV may not be reached at this point, we suggest the following model that seems reasonably plausible in view of the doping process in this system. Especially in fully doped samples such as that used for results shown in Fig. 4, the doping will be inhomogeneous and there will be excessively doped regions where the dopants coexist in the forms of I; and 1s. After the doping is finished dopants in the form of I; can decompose into 17 and 12 and the resultant I2 molecules can diffise by chemical potential differences or thermal energy and recombine to self-dope other regions of the sample. The processes can be represented as

1; = I-j- + 12, (1)

a12 + 12) =213 +12. (2)

As a consequence, the doping will become more homogeneous and the conductivity can increase until

140-

60, . * . 1 _ I . I . 1 . I . 46 46 50 52 54 56 56

Time (hour) 3

the doping and dedoping (or degradation) processes reach an equilibrium, decreasing thereafter as the contribution by the dedoping process becomes greater. Supporting our arguments, the region of the anomalous increase of the conductivity in Fig. 6 shows a time dependence of tin, characteristic of random diffusion processes [13]. Following the aging process by ‘2?-Mijssbauer, Raman and X-ray photoelectron spectroscopies (XPS) may provide us with better experimental supports for our supposition [2-71.

Acknowledgements-This work was supported by the Korea Science and Engineering Foundation (961-0210-056-2) and Special Fund for University Research Institute through the Advanced Materials Chemistry Research Center at Korea University. C.H.L. wishes to acknowledge a postdoctoral fellowship supported by the KOSEF.

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