Dedoping thermal

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).

Alternatively, undesirable side reactions may lead to persistent cation radicals. Due to these side reactions doping (p-type) of the organic semiconductor may occur, leading to a higher conductivity, but lower luminescence efficiency (photo and electroluminescence). However, by chemical or thermal (post-bake step at 180 °C) treatment, complete dedoping is possible and the luminescence efficiency is fully recovered. Additionally, in some cases the cation radical is able to attack the oxetane through nucleophilic reaction and ultimately start the same chain reaction as above [33]. 

Random co-polymers of methyl and octyl thiophenes were prepared electrochemically by Pei et al. [101] with the purpose of checking the thermal dedoping behaviour of such materials, based on the idea that long and short side chains simultaneously present should create additional space for the dopant molecules. It was found that the doping stability is intermediate between that of the parent polymers, and depends on the composition. Sfructural studies of the materials were not performed. 

Han et al. [64, 66] reported the synthesis of highly conductive and thermally stable self-doped mercaptopropanesulfonic-acid-substituted polyanilines by the concurrent reduction and substitution reaction between polyaniline and a nucleophile. These reactions were carried out on both electrochemically generated and free standing polyaniline films prepared from emeraldine base dissolved in N-methylpyrrolidinone. The electrochemically prepared films were dedoped with 5 % aqueous NaiCOs to convert them the into the emeraldine base form. The sulfonated polyaniline was prepared by reaction of a polyaniline emeraldine base film with 0.1 M 3-mercapto-l-propanesulfonic acid sodium salt in methanol under nitrogen at room temperature for approximately 14h [66]. A catalytic amount (0.01 M) of acetic acid was reported to accelerate the reaction. The resulting sulfonated polyaniline film was thoroughly rinsed with methanol, followed by 5 % aqueous NaiCOs to remove reactants. 

Chen et al. [24] studied the structure and effect of the side chain length on the doping level of self-doped poly(n-(3 -thienyl)alkanesulfonic acid)s with alkanes of carbon numbers 2, 6 and 10. They suggested that self-doping of poly(n-(3 -thienyl)alkanesulfonic acid)s is dependent on the side chain length (for details see Chapter 1 Section 1.4.3). In subsequent work, Chen et al. reported the irreversible thermal dedoping... 

In addition to positively impacting issues of dedoping of pH and thermally sensitive polymers such as polyaniline, self-doped forms enable the combination of p- and n-type conducting polymers without their spontaneous discharging by the concerted recombination of carriers and counterions. This has resulted in the formation of conjugated polymer p/n junctions, not possible with standard conjugated polymers. ... 

Additional annealing of doped polymers also accelerates the process of dedoping. Li and Wan (1999) have shown that temporal change in film conductivity of doped poly aniline at room temperature is increased strongly after thermal treatment of doped polymer at 7=150 °C. Results of this research are shown in Fig. 19.6. We need to recognize that the indicated effect limits the application of any thermal treatments during gas sensor fabrication. It was noted that the conductivity of PANICS A and PANI-p-TSA are the most stable below 200 °C, while the stability of PANI films doped with H SO and H PO are much better than the PANI-HCIO and PANI-HCl after thermal treatment at a high temperature (200 °C) (Li and Wan 1999). 

After chemical oxidation of the monomers with FeCla in chloroform under nitrogen for 20 hours, the products were recovered by quenching in methanol and dedoped with ammonium hydroxide. Unlike the methyl- and methoxy-substituted polymers prepared earlier [38], these copolymers are soluble in chloroform, methylene chloride, carbon tetrachloride and THF. In addition, they are thermally stable with an onset of decomposition around 400°C for the reduced material under nitrogen based on the TGA. But GPC measurements of molecular weight indicate an average DP of 6-10. The conductivity of the neutral polymers is... 

No simple reaction kinetic was found to describe the thermal dedoping process. It can be described using a master-plot technique. This master plot can be fitted with a Williams-Landell-Ferry type equation [491]. A reduction of the number... 

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... 

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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