Polypyrrole Conductive Polymer Characteristics as an Optical Display Device.
A. S. AL-KABBI [++]
F. A. FARIS [*]
The redox dynamic of electrochemically prepared thin films of polypyrrole is studied. A semi-empirical formula to explain the redox behavior is formulated and it seems applicable to the reduction dynamic. The redox dynamic is dominated by a diffusion process, depending on the relation between the film thickness and the rise time ([t.sub.1/2]) of the reduction process. The diffusion coefficient, D, was measured to be 2 x [10.sup.-9] [cm.sup.2]/sec for perchloride ([[ClO.sup.-].sub.4] anion dopant and 1.1 x [10.sup.-9] [cm.sup.2]/sec for para-toluenesulfonate ([PTS.sup.-]) anion dopant. The polypyrrole films exhibited a color change during the oxidation and reduction processes. The electrooptical properties of these films are studied.
Recently, macromolecular electronic devices of organic materials have been exploited as active elements in several areas where conventional inorganic materials have limited properties. Conductive polymers have attracted much interest since their conductivity can be modified from insulator to metal range upon doping [1, 2]. In conjunction to this conductivity change the material changes color from brownishblack color in the oxidized state to a transparent yellow upon reduction. In addition, film blends may improve the mechanical and optical properties of these polymers, which may assist in producing self-supporting films [3, 4]. The capability of modifying the optical properties upon doping opens new industrial and medical horizons since they could be used in optically active devices such as optical switching, optical memory, display devices and sensors [5, 6]. The electrochromic behavior of thin films has been applied recently to investigate film surface roughness . Conductive polymer display devices diff er from the liquid crystal display in that the color change remains, even after the applied external field is removed.
The most important characteristics of these polymers for electrochromic applications are contrast (the difference in optical density between the oxidized and reduced film), frequency response (the speed of changing the redox state), and the number of times that the device is switched on and off. In a previous paper (8) we studied the influence of the film thickness on contrast and the rate of switching. In this paper we describe a new method to estimate the diffusion coefficient dependence on film thickness using the He-Ne laser beam reflectance technique.
Materials and Sample Preparation
Polypyrrole (PPy, [([C.sub.4][H.sub.5]N).sub.n]) films of various thickness and different dopant concentrations were prepared electrochemically on specular stainless-steel electrodes. The mixture was 0.1 mol of pyrrole and 0.1 mol of electrolyte (different types of electrolyte salts were used which were dissolved in distilled water).
Polymerization was carried out by applying a constant voltage of 2V. The current was registered as a function of polymerization time, and the thickness of the deposited film was estimated according to the amount of charge consumed during the process [8, 9].
In Situ Measurement
In order to study the oxidation and reduction reactions, a one-compartment cell of volume 8 [cm.sup.3] was constructed, as shown in Fig. 1. The working electrode was 1 X 5 [cm.sup.3] specular stainless-steel. The counter electrode was a mesh of platinum so that the kinetics of polymerization and the color change during electrochemical doping (oxidation) or undoping (reduction) can be recorded in situ. Figure 2 shows the change of the He-Ne laser beam reflectance of working (a) and reference (b) electrodes during electropolymerization. The results indicate that there is neither a change in electrolyte color nor a change in laser intensity stability during the polymerization process.
RESULTS AND DISCUSSION
Figure 2a indicates a linear relation for the rate of polymerization and hence to the thickness of the deposited sample. This can be attributed to radical cations coupling rather than a monomer diffusion process [9, 10]. The redox cycle was demonstrated by applying [plus or minus] 2V peak-to-peak square pulses to the cell. Two films were used in this study, the first was doped with [[ClO.sub.4].sup.-] anion (PPy/[[ClO.sub.4].sup.-]) and the other with [PTS.sup.-] anion (PPy/[PTS.sup.-]). It was found that the oxidation rate is faster than the reduction rate in both films. The unequivalence in the dynamics of oxidation and reduction is probably due to the influence of the electric field of the electrodes on the mechanism of ion insertion and removal, which is currently under investigation.
The relation between contrast and the film thickness at modulation voltage of [plus or minus] 2.5V is shown in Fig. 3. In thinner samples the peak contrast is relatively low due to small optical density, [alpha]d, where [alpha] is the absorption coefficient and d is the sample thickness. For thicker samples the contrast drops down as well due to lack of electroactive efficiency (which is attributed to the difficulty of the redox process]. There exists an optimum thickness that gives the highest contrast where in our case was about 900 nm.
For a given thickness, as the modulation voltage increases, the rate of diffused anions increases. Consequently, the contrast increases too. However, by increasing the modulation voltage further the film appears to decompose and the electrolyte becomes contaminated with dusty particles. The decomposition process destroyed the film and the contrast decreases. It must be noted that in order to avoid film decomposition the device must be modulated by a voltage lower than the peak contrast voltage.
The reduction dynamics of Fig. 4 can be shown easily to fit an exponential time response of the form:
R(t) = [R.sub.o](1 - [e.sup.-t/[tau]]) (1)
where R(t) is the reflectance, [R.sub.o] is the peak reflectance, t is the device rise time which depends on the sample thickness and [tau] is the device response time.
In order to estimate the diffusion coefficient, D, of [ClO.sub.4] and PTS anions in the film during the reduction reaction, the time required for R(t) to reach 50% of its peak value, [R.sub.o]([t.sub.1/2]), is measured at different film thickness. Using Eq 1 above, the response time can be determined as:
[tau] = [t.sub.1/2] / Ln 2
Since d = [square route of]Dt, thus:
D = [d.sup.2]/t
Plotting t against [d.sup.2], as in Fig. 5, the diffusion coefficient can be determined from the slope. This was found to be 2 X [10.sup.-9] [cm.sup.2]/sec for the [ClO.sub.4].sup.-] anion and 1.1 x [10.sup.-9] [cm.sup.2]/sec for [PTS.sup.-] anion. The difference in diffusion coefficient can be attributed to the effect of ion size were the [ClO.sub.4].sup.-] has a smaller size than [PTS.sup.-] and hence its diffusion coefficient is larger. Since doping is achieved during polymerization, we believe that initial polymerization is better achieved with large anion (such as [PTS.sup.-] or [ClO.sub.4].sup.-] to create initially large sites inside the polymer followed by carrying out the redox cycles with smaller ions such as [Cl.sup.-].
In general, the efficiency of an electrooptical device is defined by the gain-bandwidth product (8), which is measured to be 1.7. This confirms that a faster device must be associated with a lower contrast and vice versa. The frequency response of the device can be determined by Lablace's transformation of Eq 1 from time domain to frequency domain to get:
R(f) = [R.sub.o]/[[1 + [(2[pi].f.[tau]).sup.2]].sup.1/2] (2)
where f is the frequency of modulation. Figure 6 shows the frequency response of two films, [PP.sub.y]/[ClO.sub.4].sup.-] and [PP.sub.y]/[PTS.sup.-], of thickness 900 nm and 1010 nm, respectively. These two film thicknesses correspond to the thickness of maximum contrast.
Although no comprehensive comparison is made here between the conductive polymer display and an LCD device, the frequency response of a commercially available single segment of liquid crystal video screen is taken as a standard. The conductive polymer display (CPD) shows a lower response than the LCD segment. However, the color change of the CPD remains, even after the applied external field is terminated.
The advantage of using the reflection mode of operation is that the optical density ad is doubled without having to increase the time of doping and depoping of the anion. This is true at any thickness and not restricted to the optimum thickness. The electrochromic characteristics of the device are very sensitive to the method of preparation as well as operation. The response time of the device depends on the capability of the anion to diffuse into the film. Therefore, the measurement of the diffusion coefficient is an important factor to estimate the switching speed of the device. The influence of the film thickness on contrast and response time is crucial. The maximum electrochemical contrast of the prepared films was found to be about 900 nm. In the range of the sample electroactivity, the gain-bandwidth product is constant, that is high-contrast devices are associated with slow response time and vice versa. There is a current interest in polymer disperse liquid crystal display (PDLCD) in order to eliminat e the need of polarizers and analyzers used in conventional LCD's and to develop a large and flexible display devices. Conductive polymers have the potential feature to hold the display state without the need for a permanent external field and can be manufactured in large scale. In addition, no polarizers and analyzers are needed for their function.
(#.) Department of Physics Aol AI-Bayt University Mafraq, Jordan
(++.) Department of Physics College of Science University of Basrah Iraq
(*.) Department of Physics Applied Science University Amman, Jordan
(1.) Correspondence: Ilkenbergstr. 19. D-28562 Suhlendorf. Germany.
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|Author:||AL-ATTAR, H. A.; AL-KABBI, A. S.; FARIS, F. A.|
|Publication:||Polymer Engineering and Science|
|Article Type:||Brief Article|
|Date:||Dec 1, 1999|
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