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P. G. Le Comber and J. Mort (Academic Press, New York, 1973). Chap. 8, 15. ... [23] J. Mort, G. Pfister, and G. M. Sessler, Electronic Properties of Polymers, eds.

CHINESE JOURNAL OF PHYSICS

VOL. 42, NO. 4-I

AUGUST 2004

DC Conduction in Fe3+ Poly(9-vinylcarbazole) Doped Films A. A. El Tayyan1 and A. Khogali1, 2 1

Physics Department, Al Azhar University, Gaza, Gaza strip, Palestine 2 Physics Department, University of Khartoum, Khartoum, Sudan (Received October 20, 2003)

DC conduction of poly(9-vinylcarbazole) (PVK) films doped with Fe3+ was studied as a function of dopant concentration and film thickness. Analysis of the current-electric field dependence showed that the conduction was bulk limited, obeying the Poole-Frenkel mechanism. Also, the thermal activation energy εac of the DC conductivity σ was typical of electronic conduction (∼0.627 eV). PACS numbers: 73.61.-r, 73.61.Ph

I. INTRODUCTION

Organic conducting polymers have a large variety of properties and structures. This opens the door for specific applications. The range of applications of these materials includes the fields of thin film technology, electroluminescence, synthetic metals, non-linear optics, as well as others. Considerable interest has been focused on electrical conduction in polymers in the last decades, in order to understand the nature of their charge transport [1–6]. Various conduction mechanisms have been suggested. Among these are the Schottky emission [7– 11], tunneling [7, 11], space charge-limited [7, 12], intrinsic conduction [7], and Poole-Frenkel conduction mechanisms [7,13–15]. Poly(N-vinylcarbazole) (PVK) is a prototype pendant group polymer. Its electronic properties, for instance the capability to transport optical excitations or charge carriers, are all controlled by the dopants rather than the main chain [16]. Most research so far conducted on PVK has been focused on various photoconductivity measurements and the fabrication of light emitting diodes [17–20]. Measurements of the DC electrical conductivity of doped PVK are very rare, and much work needs to be done on this material in order to get insight into its various electrical conduction properties, such as the conduction mechanism and trapping, and to reveal the role of dopant molecules. In this paper we discuss the results of our work on DC conduction in Fe3+ doped PVK films.

II. EXPERIMENT

II-1. A. Sample preparation Poly(9-vinylcarbazole) (Aldrich Chemical Co., USA) was first dissolved in tetrahydrofuran (THF). Then the desired weight concentrations of (anhydrous) ferric chloride were

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c 2004 THE PHYSICAL SOCIETY

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FIG. 1: The experimental set up arrangement used for the DC conductivity measurements: (1) modulated DC pulse at 10 Hz, (2) sample, (3) lock-in amplifier, (a) current input, (b) reference frequency input.

added to the solution. A homogeneous mixture was then obtained by thoroughly stirring this mixture. The films were then prepared by casting this solution onto smooth and clean aluminum electrodes 1.2 cm in diameter. Then the solvent, THF, was allowed to evaporate slowly for several hours at room temperature. Finally, the samples were heated in an oven at 80 ◦ C for twenty hours to remove any residual solvent. The films’ thicknesses were determined by capacitance measurements using a TE 2700 Universal Bridge. X-ray structural analysis on doped and undoped samples, using a Siemens D500 diffractometer with Cu target, revealed two broad peaks characteristic of amorphous substances, and showed that the structure was not influenced by the dopants. II-2. Experimental procedure The films were arranged in sandwich type configurations (Al-film-Al) using a special cell in which two soft springs were used to press two copper leads to the electrodes. The measurements were carried out at room temperature under normal atmospheric pressure. The input DC voltage was modulated at 10 Hz and the output current signals were processed through a Stanford SR530 lock-in amplifier. Before taking measurements, each sample was oven heated at 80 ◦ C for one hour or more to remove absorbed moisture. In order to remove the residual charges, an electric potential of about 25 volts was applied to the sample for 45 minutes. Fig. 1 shows the experimental setup used for these measurements.

III. RESULTS AND DISCUSSION

III-1. Current-electric field dependence Figures 2 through 6 show that the plots of Ln I against E 1/2 exhibit a linear relationship. At low applied fields the majority of the plots show slight deviations from this linear behavior. This deviation is, perhaps, due to the accumulation of space charges near the electrodes. The general trend of the slopes of the lines shows a decrease as the thickness

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FIG. 2: Plots of the natural logarithm of the current versus the square root of the applied electric field for films doped with 1.0% FeCl3 at various thicknesses.

FIG. 3: Plots of the natural logarithm of the current versus the square root of the applied electric field for films doped with 2.5% FeCl3 at various thicknesses.

of the film increases (see Table I). Thus, we conclude that the current flowing through the sample decreases as the film thickness increases. III-2. Conduction mechanism The current-electric field dependence exhibited by the plots in Fig. 2 through Fig. 6 was found to obey the following relation, I ∝ exp(eβE 1/2 /kT ) ,

(1)

where E is the applied electric field, e is the electronic charge, β is a constant characteristic of the conduction mechanism, k is Boltzmann’s constant, and T is the absolute temperature. The linear behavior of the Ln I versus E 1/2 plots points to an electronic-type conduction due to either the Schottky emission mechanism or the Poole-Frenkel mechanism.

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FIG. 4: Plots of the natural logarithm of the current versus the square root of the applied electric field for films doped with 3.5% FeCl3 at various thicknesses.

FIG. 5: Plots of the natural logarithm of the current versus the square root of the applied electric field for films doped with 4.5% FeCl3 at various thicknesses.

FIG. 6: Plots of the natural logarithm of the current versus the square root of the applied electric field for films doped with 5.5% FeCl3 at various thicknesses.

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DC CONDUCTION IN Fe3+ . . .

TABLE I: Experimental values of β and slopes of lines for FeCl3 doped thicknesses. Thickness βPF µm (V1/2 m1/2 )×10−5 1.0% FeCl3 doped PVK 84.9 9.85 88.6 7.27 90.9 10.05 92.2 9.94 97.5 9.94

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PVK films at different slope ×10−3 3.81 2.81 3.89 3.85 3.85

2.5% FeCl3 doped PVK

64.9 66.0 67.0 68.0 78.9

12.93 16.62 12.00 11.46 10.20

5.00 6.43 4.64 4.43 3.94

3.5% FeCl3 doped PVK

56.0 58.7 60.0 65.5 74.8 100.0

10.34 8.62 12.37 12.37 11.31 16.01

4.00 3.33 4.78 4.78 4.38 6.19

4.5% FeCl3 doped PVK

54.3 55.2 56.7 58.8 61.6 74.8

12.93 15.70 13.54 8.36 9.48 9.13

5.00 6.07 5.24 3.24 3.67 3.53

5.5% FeCl3 doped PVK

59.5 61.0 64.0 68.0 76.8

9.94 10.94 9.70 6.31 4.76

3.85 4.23 3.75 2.44 1.84

The expression for the current density according to Schottky emission is "    3 1/2 # e E eφ 1 RS , JS = RT 2 exp − exp kT kT 4πεε0

(2)

where R is the Richardson constant, T is the absolute temperature, φRS is the barrier

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height at the metal-polymer interface in the absence of a field, e is the electronic charge, k is Boltzman’s constant, and εε0 is the dielectric constant. The expression for the current density in the case of the Poole-Frenkel mechanism is "    3 1/2 # 1 eφPF e E , (3) JPF = cE exp − exp kT kT πεε0 where c is a constant, and eφPF is the depth of the potential well. In order to differentiate between these two mechanisms, the theoretical values of β were calculated for the Poole-Frenkel and Schottky mechanisms βPF = (e/πεε0 )1/2 = 4.38 × 10−5 , βPF = 2.19 × 10−5 , βRS = 2

(4) (5)

where the dielectric constant ε of the PVK is taken equal to 3.0 [21], and the experimental values of β were calculated from the slopes of the LnI versus E 1/2 plots (see Table I). These values of β increase, in most cases, as the thickness of the films decreases. They are also generally larger than the theoretical values βPF and βRS . Thus, the comparison between the theoretical and the experimental values of β doesn’t reveal which mechanism is involved in conduction. The distinction between these two mechanisms depends on the pre-exponential factors [1, 22] of Eq. (2) and Eq. (3), namely   eφRS 2 , (6) JOS = RT exp − kT in the case of the Schottky mechanism and   eφPF JOPF = cE exp − , kT

(7)

in the case of the Poole-Frenkel mechanism. Therefore, if one uses an asymmetric structure of electrodes, i.e. electrodes of different work functions, the current will be very asymmetrical when one reverses the polarities in the case of the Schottky mechanism. On the other hand, the current remains practically unchanged when one reverses the polarities, in the case of the Poole-Frenkel mechanism, which does not depend on the potential barriers between the electrodes and the bulk of the solid. We found that the variation of the current with the square root of the applied electric field for Al-film-Au sandwich structure is independent of the polarities (see Fig. 7). This indicates that the Poole-Frenkel mechanism governs the conduction in these films. III-3. Activation energy measurement The dependence of the DC conductivity σ on temperature is depicted in Fig. 8. It shows that σ is given by:  ε  ac σ = σ0 exp − , (8) kT

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FIG. 7: The variation of the current with the square root of the applied electric field for an Alfilm-Au sandwich structure for a 68.0 µm film doped with 2.5% FeCl3 . Filled symbols, Au+ ; open symbols, Al+ .

FIG. 8: Temperature dependence of the DC conductivity. Film thickness is 68.0 µm.

where σ0 is the maximum conductivity, and εac is the activation energy. The εac for the Fe3+ doped sample is found to 0.627 eV, which is typical of electronic conduction [23]. This provides another support for our previous conclusion. III-4. Electrical conductivity-concentration dependence The dependence of the DC electrical conductivity on the Fe3+ concentration is illustrated in Fig. 9. It is clear from this figure that σ decreases with an increasing Fe3+ concentration.

IV. CONCLUSIONS

(i) Conduction in Fe3+ doped PVK films obeys the Poole-Frenkel mechanism.

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FIG. 9: Fe3+ concentration dependence of the DC conductivity.

(ii) Conduction decreases with increasing dopant concentrations and film thickness. (iii) The activation energy for DC conduction is about 0.627 eV, which is typical of electronic conduction [23].

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[21] A. K. Jonscher and A. A. Ansari, Phil. Mag. 23, 205 (1971). [22] H. Carchano and M. Valentine, Thin Solid Films 30, 335 (1975). [23] J. Mort, G. Pfister, and G. M. Sessler, Electronic Properties of Polymers, eds. J. Mort and G. Pfister (Wiley, New York, 1982).