Charge transport processes in conducting polypyrrole/Prussian Blue bilayers
Introduction
Conducting polymers have attracted much attention in recent years because of the large number of possible applications of these materials in various electronic devices such as electrochromic displays (ECD), light emitting diodes (LED), field effect transistor (FET), chemical sensors, etc. [1], [2], [3], [4]. One of the extensively studied conducting polymer is polypyrrole (PPy) because of its good environmental stability and ease of synthesis. The feasibility of the use of PPy in ECD, Schottky diodes, photovoltaic cells, chemical sensors has been demonstrated in the past [5], [6], [7], [8]. However, the response characteristics have been limited to a narrow range of visible spectrum. There are a number of dyes which have been incorporated in polymers, both insulating as well as conducting, in order to enhance the optoelectronic properties. For example, phthalocyanine has been used in conducting polymer LEDs [9], [10]. In our earlier studies, we found that the Methylene Blue (MB) acts as a photosensitizer for improving the photosensitivity of conducting PPy [11]. The permanent red dye has been employed along with poly(methyl methacrylate) for non-linear optical response components [12]. These dyes and pigments are used as sensitizers for photoconductivity, photoelectrochemical reaction, etc. [13], [14].
Prussian Blue (PB) in particular, is an interesting pigment which exhibits electrochromism, redox catalytic activity, etc. [15], [16], [17], [18]. It is incorporated in polyaniline to increase the electrochromic response [19]. Composite films of PB and PPy have also been used for the optical determination of the pH [20] and for the detection of cytochrome-c [21]. In our earlier papers, we have demonstrated that the use of PB not only enhances the electrochromic response of the conducting PPy, but also extends the electrochromic response to wider region of the visible spectrum, thus, acting as a sensitizer [22], [23], also PB acts as a photosensitizer for improving the photosensitivity of conducting PPy [26], [27]. All solid state electrochromic display devices (ECD) were constructed by combining PB with other complementary electrochromic materials like WO3, conducting polymers, etc. [28], [29], [30], [31]. PB has also been used for sensitizing photocatalytic reactions [32], [33], [34]. However, there are very few reports on the exact charge transport process in such dye sensitized conducting polymers. The charge transport process in these materials plays an important role and it can even be the rate determining process for enhancing these effects. Hence, we have investigated the charge transport mechanism in the dye sensitized conducting PPy and the interesting results obtained are reported here.
Section snippets
Experimental
The PPy/PB multilayer photogalvanic cells were fabricated by the following method. Firstly, thin gold films were deposited on clean microscopic grade glass substrates. Conducting PPy films were then deposited on these by potentiostatic technique [22], [23] using single compartment electrolytic cell having platinum counter electrode, saturated calomel reference electrode (SCE), computerized potentiostat (EC2010) and aqueous sulfuric acid (0.1 M) in which the pyrrole monomer (0.1 M) was thoroughly
Results and discussion
The I–V characteristics of the PPy/PB photogalvanic cells in dark and under white light illumination are shown in Fig. 2. The curves (a)–(c) correspond to the PB thickness of 270, 285 and 315 nm, respectively (PPy film thickness is about 400 nm in all cases). It is interesting to observe that the I–V characteristics are highly non-linear both under illumination (L) as well as in dark (D). The dark current decreases and the photocurrent remains more or less the same with the increase in the PB
Summary and conclusions
These studies clearly bring out the importance of the charge transport mechanism in the sensitization process of PB in photocurrent and electrochromic response of conducting PPy. The non-linear space charge limited conduction appears to be the predominant process both in solid state as well as liquid electrolytic cells. This suggests that the photosensitization effects may be field dependent due to the formation of space charge in such materials. This could lead to certain value of the
Acknowledgements
Authors would like to thank Dr. B.K. Das, Executive Director, Center for Materials for Electronics Technology, for his keen interest and constant encouragement throughout the work.
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