Dedoping

FIGURE 18. Normalized conductivity vs. time of a FeCl3-doped P3HT film at 110 in laboratory 

The dedoping process, as studied by following the conductivity at constant and elevated temperature, can be described with an activation energy [87-89]. The activation energy Ea is determined using the temperature dependence of the time constant of the conductivity decay, x, (die Arrhenius law) 

Comparable results have been published by Hagiwara et al. [90] for the thermal dedoping of CIO4 doped poly(3,4-dimethoxythiophene). Although the given data do not 

FIGURE 19. The dedoping time constant t =k = a/(dc/dt) of conductivity a, of poly(3-octylthiophene) doped with hydrous ferrichloride, as a function of inverse temperature, 1/T. The measurement has been performed by changing the temperature stepwise upwards and downwards, repeatedly, measuring the time constant at each temperature. The thick line is fitted to a subset of points measured consecutively. It corresponds to an activation energy E, of 14500 K R. The first points in the measurement (labelled start ) deviate from the general trend. [89] 

The observed instability of doped P3ATs is a serious problem in practical applications. In the case of ferrichloride, which so far is the most stable dopant in room temperature, the dedoping time constant is shortened from 10. . 100 years at room temperature in dry atmosphere to minutes at 180 °C. The iodine doped P3ATs have a less severe temperature dependence, but the room temperature value of the dedoping time constant is already much shorter, of the order of one day at 1 S/cm for sample thicknesses of 0.1 mm. Wang et.al. [87,88] also notice that FeCls doped samples is much more stable against thermal dedoping than PF doped samples. 

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ZB Zhang, M Fujiki, M Motonaga, and CE McKenna, Control of chiral ordering in aggregated poly 3-(S)-[2-methylbutyl]thiophene by a doping-dedoping process, J. Am. Chem. Soc., 125 7878-7881, 2003. 

Since the ionization potential of thiophene is relatively high, the electric fields required for its anodic polymerization are rather steep (= 20V vs SCE). In addition, the simplest supporting electrolyte for this operation is Li BE- and deposition of Li at the cathode (usually Pt) is also energetically unfavorable. Recently, Druy (13) reported that substitution of 2,2 -bithiophene for thiophene gave better quality films, probably due to the lower ionization potential of the dimer relative to thiophene. An additional improvement consisted in replacing the Pt counter electrode by A1 (9). Spectroscopy revealed that dedoped PT films produced with the above improvements were indistinguishable in quality from the chemically coupled PT. 

With these excellent films on hand we were able to do highly sophisticated experiments of in situ doping and dedoping while performing another measurement such as electronic spectroscopy (9). The results of such experiments showed that charges, in PT, are stored as dications a finding that parallels observations on poly-(pyrrole (15). 

Contrary to PT, the fully dedoped PITN is blue-black and is a semiconductor an observation which is in agreement with the small energy gap of this new polymer. 

Table 4.3 Optimum electropolymerization potentials of monomers 35-37, p-doping and n-doping potential ranges (V) and photophysical data for the corresponding dedoped polymers.

The role of the carrier density in M-I transitions is shown for an oriented sulfuric acid-polyparaphenylenevinylene (PPV-H2SO4) sample. The optical anisotropy of this oriented PPV sample, from dichroic ratio measurements at 1520 cm 1, is nearly 50 [15]. The value of pr continuously increases upon reducing the carrier density by systematically dedoping the sample, as shown in Fig. 3.4. However, it is difficult to locate the M I transition from the a vs. T plot alone. Instead, the W = d(lnmetallic regime with a weak negative TCR, then W shows a positive temperature coefficient at low temperatures. Moreover, this ensures that there is a finite conductivity as T —> 0. As pr increases, W(T) gradually moves from positive (metallic) to negative... 

Redox cycled polymers, which electrochemically doped and dedoped, are used for energy storage (batteries) and when one wants to have a property tuned by a potential. For instance, in the case of smart windows the color or absorbance of a glass window can be controlled electrically. 

PPy film is blue-violet in doped (oxidized) stet. Electrochemical reduction yields the yellow-green undoped form. The schematic of the doping/dedoping process can be given as... 

Fig. 9.34 Correlation of electrical conductivity and charge carrier mobility for a doped conjugated polymer thermally dedoped device and o separately doped devices. The fits, thick and thin lines, are described in the text. Reproduced with permission of the American Institute of Physics from Jamett et al. (1995).

A polyaniline film prepared by doping an emeral-dine base with optically active CSA showed a CD spectrum. Even after dedoping, the film exhibited CD bands which were different in pattern from those of the original dedoped film, suggesting that a chiral conformation such as a helix remains in the polymer chain. The dedoped film exhibited chiral recognition ability toward phenylalanine.235... 

For a polyanUine film, the light absorption measurements were conducted after the film was exposed to HCl and NH3 vapors, respectively, as shown in Fig. 14 [19]. The difference in the spectra indicated that HCl and NH3 vapors induced a different band structure and conformation of the polymer. Therefore, the optical property of the film changed when the film switched from one state (doped by HCl) to another (dedoped by NH3). The refractive index measurement by ellipsometry showed that the refractive index changed from 2.43 (doped by HCl) to 1.95 (dedoped by NH3). 

Polypyrrole is one of a series of heterocyclic polymers which has attracted much attention due to its characteristic electric and electronic properties. However, there are some problems relating to the physical and material properties associated with its structure. The fundamental structural formulae shown in Fig. 16.5 have been generally proposed for the structures of dedoped and doped polypyrroles, where the aromatic form corresponds to the dedoped state and the quinoid form corresponds to the doped state [9-11]. However, the actual structure appears to be more complicated. At present the exact structure is not known because the polymer is amorphous and insoluble. Consequently, various structures have been proposed for polypyrrole [10]. 

NMR spectroscopy will provide a simpler spectral pattern, when compared with NMR spectroscopy, because a given N resonance line may correspond to a given structure. Therefore, the structure of doped and dedoped N-labeled polypyrrole films can be successfully studied by high resolution solid-state NMR [14, 15]. Doped and dedoped samples were prepared by electrochemical polymerization [16] using 20-30% N-labeled pyrrole. To obtain a dedoped sample, the electrodes were inverted after the doping experiment and the same voltage applied. 

As shown in this table, the relative intensities of peaks a and increase from 5.5 to 8.0% and 28.7 to 30.2%, respectively, on going from sample (b) doped to sample (c) dedoped. However, the relative intensity of peak y decreases from 53.7 to 49.4% by dedoping. Hence, the relative intensities of peaks a and /3 increase with a reduction in conductivity, but peak y decreases. In addition, the relative intensity of peak 8 does not change with the increase in conductivity. When the N CP/MAS experiment is performed using a contact time of 100 jls, the intensities of the peaks a and )3 are relatively enhanced as shown in Fig. 16.6(b), and the chemical shifts and halfwidths of the observed shoulder peaks are determined accurately. Furthermore, the difference of the intensity enhancement between peaks a, and peaks y, 8 shows the difference of the magnetic environments, i.e, a difference in T h (contact relaxation time between N and H) values between N and H and in Tip, between peaks a, j8 and y, 5. 

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