ON THE ELECTRICAL CONDUCTIVITY AND PHOTOCONDUCTIVITY OF 9-VC POLYMERS

S. A. ZAIDMAN, N. Ya. FILIPPOV, D. I. SVIRYAKIN, V. P. LOPATINSKII,
E. E. SIROTKINA, V. I. BERZIN

(Presented by the scientific seminar of the Research Institute of Electronic Introscopy)

In order to study the physical properties of polyvinylcarbazole polymers, an investigation was carried out of their electrical conductivity and photoconductivity.

The substances used for the investigation were obtained by telomerization of 9-vinylcarbazole /9=VC/ in various solvents and in the presence of various initiators. Depending on the preparation conditions, the telomers have different end groups and different degrees of polymerization \((n)\), which are calculated from the end groups. The structural formulas of the telomers studied are given in Table 1.

Table 1

where

\[ R = \begin{matrix} & \mathrm{CH} & & \mathrm{CH} \\ \mathrm{CH} & \diagup & & \diagup & & \mathrm{CH} \\ \mathrm{CH} & & \text{carbazole ring} & & \mathrm{CH} \\ & \mathrm{CH} & \mathrm{N}—\mathrm{CH} \end{matrix} \]

The samples studied were prepared in the form of pressed pellets with metallic electrodes applied to the end faces.

The change in electrical conductivity was measured under direct current at atmospheric pressure. Depending on the structure of the end group, the resistance of samples obtained from telomers in the presence of ...

in the presence of carbon tetrachloride. As can be seen from Fig. 1, the samples of structure III have the highest dark conductivity, and those of structure II the lowest. The conductivity of the polymers also depends on the chain-transfer agent. Thus, for example, polymers with chlorine-containing end groups, in comparison with bromine-containing ones (structure IV), possess better conductivity.

Fig. 1. Current–voltage characteristics for 9-VC with different end groups.

The change in electrical conductivity with temperature for the samples studied is exponential in character and obeys the Arrhenius equation [1]

\[ \sigma=\sigma_0 e^{-\frac{\varepsilon_0}{kT}} =\sigma_0 e^{-\frac{\Delta E}{2kT}}, \]

where \(\sigma\) is the specific electrical conductivity, \(\Omega^{-1}\cdot \mathrm{cm}^{-1}\);

\(\varepsilon_0\) is the activation energy of conductivity, eV;

\(\Delta E\) is the width of the forbidden band in an intrinsic semiconductor, eV;

\(k\) is Boltzmann’s constant;

\(T\) is the absolute temperature, \(K^\circ\).

The magnitude of the thermal activation energy for structure II, determined from the slope of the straight line in Fig. 2, is 0.94 eV. Up to temperatures of 40–50°C, the electrical conductivity depends only weakly on temperature, which corresponds to the horizontal segment of the straight line in Fig. 2. Apparently, this is connected with the fact that the generation and displacement of carriers within the polyconjugated region require almost no activation energy, while the increase in current with increasing temperature is due to a hopping mechanism—

Fig. 2. Temperature dependence of the electrical conductivity for 9-VC, structure II.

the conduction band, in which energetic barriers between polyconjugated regions must be overcome.

Noteworthy is the fact that the high-temperature slopes of the straight lines for the structures studied coincide within the experimental error and vary from 0.85 to 1.1 eV.

A study of the temperature dependence of the Seebeck coefficient showed that, as the temperature increases, the magnitude of the thermoelectromotive force decreases. This can probably be explained by a phonon mechanism of carrier scattering in the low-temperature region and by an increase in the role of acoustic lattice vibrations in the high-temperature region.

Preliminary data indicate that the samples studied have hole-type conductivity.

In studying the photoelectric properties of polymers of 9-vinylcarbazole, the main attention was paid to the ratio of the photocurrent to the dark current at specified illuminations and values of the applied external voltage. The photoconductivity was investigated in the visible region of the spectrum.

Of all the structures in Table 1, the structure of type II proved to be the most sensitive to the visible region of the spectrum (Fig. 3). For the best samples, the photocurrent increased by one and a half orders of magnitude compared with the dark current.

Table 2 presents experimental results reflecting the dependence of the ratio of the photocurrent to the dark current on the degree of polymerization, which is in direct correspondence with the percentage chlorine content in the polymer of structure II.

The chlorine content in the polymer of structure II was regulated by means of

Fig. 3. Dependence of the dark current (1) and photocurrent (2) on voltage for the 9-VK structure II: 1 — dark current–voltage characteristic; 2 — current–voltage characteristic at \(\Phi = 200\) lx.

Table 2

changes in the concentration of the carbon tetrachloride solution — \(S\), and the degree of polymerization by a change in the number of monomers — \(M\).

It follows from the table that there exists an optimal degree of polymerization at which the maximum ratio of photocurrent to dark current is observed. For this same structure II, the kinetics of the photocurrent (Fig. 4) and the lux–ampere characteristic (Fig. 5) were investigated.

As can be seen from Fig. 4, the time required for the photocurrent to reach its stationary value after the light is switched on varies within the range \((25 \div 30)\) sec,

which indicates the high inertia of such photoresistances.

From the lux-ampere characteristic (Fig. 5) it is seen that in the initial portion of the curve the photocurrent depends nonlinearly on the illuminance, and subsequently increases linearly with increasing power of the incident light flux.

Fig. 4. Kinetics of establishment of the photocurrent for structure II.

Fig. 5. Lux-ampere characteristic for the PVC structure II.

REFERENCES

1. Organic Semiconductors. Edited by V. A. Kargin. Moscow, “Nauka,” 1968.