Magnetoresistance and Corbino resistance of iodine doped-polyacetylene

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Solid State Communications, Vol.45,No.]|, pp.933-935, Printed in Great Britain.

0038-1098/83/110933-03503.00/0

1983.

Pergamon Press Ltd.

MAGNETORESISTANCE AND CORBINO RESISTANCE OF IODINE DOPED-POLYACETYLENE W. RSss, A. Philipp, and K. Seeger Institut fiir FestkSrperphysik der Universit£t Wien und Ludwig-Boltzmann-Institut fGr FestkSrperphysik 1060 Wien, Austria K. Ehinger, K. Menke, and S. Roth Max-Planck-Institut fGr FestkSrperforschung 7000 Stuttgart, W. Germany Received: December 28, 1982, by M. Cardona

The collinear magneto- and coaxial Corbino resistance of iodine doped polyacetylene have been measured at 4.2 K for doping levels from 7% to 30%. From a comparison of the two sets of data, the "carrier mobility" has been determined. It ranges between 85 and 180 cm2/Vsec, falling with increasing iodine concentration.

Introduction

The films were doped by exposing them to iodine vapour and subsequent pumping in high vacuum. Two sets of samples were prepared - Vienna samples: kept at -30oc for three days during doping and pumped for 15 minutes afterwards and Stuttgart samples: doped at room temperature and pumped for 15 hours. The molar iodine concentration was determined after pumping from the weight uptake. By the Vienna method up to 30 atomic per cent of iodine concentration can be obtained whereas the Stuttgart procedure never yields more than 20%, During the longtime pumping in Stuttgart about I/3 of the originally incorporated iodine is removed (weight loss). Table I gives a compilation of the samples investigated in this work. The room temperature conductivity of all samples is plotted versus iodine concentration in Fig. I.

Polyacetylene, (CH) x, one of the simplest conjugated organic polymers, has recently attracted considerable attention. The electrical conductivity can be varied by doping over 13 orders of magnitude, ranging from insulating up to metallic values I . Many technical applications have been discussed, for example lowprice solar cells and light-weight electrochemical energy storage devices (batteries) 2,3. To describe the unusual electrical properties, novel theoretical concepts have been introduced such as soliton conduction. Yet very little is known on the actual mechanism of conductivity. The present paper reports magnetoresistance and Corbino resistance measurements of iodine-doped polyacetylene at liquid helium temperature. From these data an attempt is made to deduce the carrier mobility, which is of eminent importance both for technical applications of polyacetylene and for theoretical considerations.

Table I y

Doping Method

Room Temp. Conductivity

Mobility (cm2/Vsec)

(slcm) Sample Preparation 0.072 0.085 0.107 O.12 0.13 0.15h 0.186 0.23 0.30

Films of (CH) were synthesized using the catalytic polymer~sation described by Shirakawa et al.4. The polymerisation was carried out at -80°C leading primarily to the cis modification of polyacetylene. No specific thermal treatment was used to convert the samples into the trans modification, hut iodine is known as a catalyst for the cis-to-trans conversion in organic chemistry, so that it may be assumed that doping has transformed the samples. This was, however, not explicitly checked on our samples and recent Raman studies5 raise some doubt as to the efficiency of iodine-induced conversion.

Table I:

933

Stuttgart Stuttgart Stuttgart Vienna Vienna Stuttgart Stuttgart Vienna Vienna

26 31 h5 25 35 109 162 270 500

180 125 165 85 120 -

Compilation of samples investigated in this work (y is the doping concentration).

IODINE D O P E D - P O L Y A C E T Y L E N E

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In principle the Hall mobility can be determined from the Hall voltage and from the conventional magnetoresistance, if geometrical factors are properly taken into account. Since polyacetylene films have a very complicated microstructure and look more like a fibrillar fleece than a bulk solid in scanning electronmicrographs 7, it is felt that "geometrical factors" are very difficult to estimate in these samples and it is hoped that some of the uncertainties. might drop out if the mobility is deduced from (I). Nevertheless, the obtained values should be used with caution. Collinear four-point contacts or annular electrodes were painted on doped polyacetylene films using gold paste. The samples were cooled in a liquid helium cryostat and the magnetoresistance was measured using a 7 Tesla superconducting magnet and a 30 Hz lock-in technique.

Fig. 2 presents the eollinear magnetoresistance and the Corbino resistance of some selected samples. In the insert the eollinear magnetoresistance ~p/Q_ at 7 T is plotted versus the • od~ne concentratzon. A strong dependence on the doping level is evident: the higher the iodine concentration the lower the value of the u

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Collinear magnetoresistance and Corbino resistance for selected Stuttgart samples at 4.2 K with the iodine concentration as parameter The insert shows the eollinear magnetoresistanee Ap/p ° at 7 Tesla as a function of the iodine concentration. (o Stuttgart samples, Vienna samples).

magnetoresistance, partially due to the increase of a negative contribution which has been observed already by Kwak er al. and by Gould et al. 9 in highly AsF~ doped (CH) . The magnitude of this negative ~ffect depends strongly on sample treatment such as compaction in a pellet press. In highly compacted samples the effect even changes sign. A more detailed investigation of this beha~6our will be published in a separate paper In Fig. 3 we have plotted the difference between the Corbino resistance and the eollinear magnetoresistanee. It is seen that AR/R - Ap/p 2 o o indeed follows a B law as suggested by (I) for

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Results and Discussion

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Conventional magnetoresistanee measurements usually are carried out using collinear fourpoint contacts. The Corbino geometry uses a coaxial contact arrangement instead 6. From the difference between the Corblno resistance ~R/R ° and the collinear magnetoresistance ~P/0o the Hall mobility PH can be determined 6

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Conductivity o e Stuttgart (e) and Vienna (~) samples as a function of iodine concentration at room temperature. The solid line serves as a guide to the eye.

Magnetoresistance Measurements

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Fig. 3:

Difference between Corbino resistance and magnetoresistance as a function of the magnetic field squared, (see Eq. I) at 4.2 K for different iodine doping levels (S ... Stuttgart samples, V ... Vienna samples).

50

Vol. 45, No. I I

IODINE DOPED-POLYACETYLENE

small Ap/p . From fits to these plots the mobi• O . llty has been determlned. The obtained values are represented in Table I. We find it remarkable that the two sets of samples have a very similar overall behaviour in spite of independent sample preparation in different laboratories and in spite of using different doping procedures. In some details, however, the samples show unlike behaviour. The insert in Fig. I shows that the magnetoresistance reaches higher values in the Vienna samples than in the Stuttgart samples for the same iodine concentration. This aspect is also reflected in the higher mobility values in Table I. On the other hand the conductivity of both sets of samples falls pretty well on one smooth curve up to the highest iodine concentrations of 30% (Fig. l). Consequently there is no simple correlation between conductivity and mobility. This deviation might be due to different fractions of electrically active iodine or a different 13/15 ratio 11 , depending on the pumping time after doping, or to differences in the morphology of the polymer.

935

measurements in polydiacetylene, also a conjugated polymer, have been interpreted to yield B ~ 2 x 105cm2/Vsec and in naphtalene = 400 cm2/Vsec has been reported13. When the carrier mobility of polyacetylene is estimated from the simple conductivity formula = n e W, values of ~ ~ I cm2/Vsec are obtained, if n ~s assumed equal to the doping concentration . The difference between this value and the estimate from our Corbino data might be interpreted by assuming that not all but only a few per cents of the doping molecules create mobile charge carriers or that the macroscopic o is not an intrinsic value of polyacetylene, but that it is limited by fluc~4 tuation induced tunneling between the fibrils . On the other hand, Hall effect data 15, if analyzed in a one-band model, would yield a carrier concentration much higher than that inferred from the doping level which might be due to the fact that a homogeneous Hall field cannot be built up due to the fibrous morphology of (CH) . Apparently, a consistent picture of .X . . electrlcal transport properties In polyacetylene cannot yet be established but our data can be interpreted as an indication that the intrafibrillar mobility is much higher than the interfibrillar mobility of carriers.

Table I shows that in any case the Hall mobility decreases with increasing iodine concentration. This might be due to scattering between the charge carriers, or more likely, to increasing disorder in polyacetylene as the dopant is intercalated.

Acknowledgements

The mobility values so obtained range from 85 to 180 cm2/Vsec at 4.2 K. These values are very high when compared to other organic polymers, where usually ~ ~ 10-~cm2/Vsec has been found. On the other hand, recent photoconductivity

We want to thank Prof. K. Dransfeld for valuable discussions. The support of this work by Stiftung Volkswagenwerk and Fonds zur FSrderung der wissenschaftlichen Forschung in 0sterreich is greatfully acknowledged.

References I.

2.

3.

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