Situ Electron Transport Measurements

The huge literature on the electronic conductivity of dry conducting polymer samples will not be considered here because it has limited relevance to their electrochemistry. On the other hand, in situ methods, in which the polymer is immersed in an electrolyte solution under potential control, provide valuable insights into electron transport during electrochemical processes. It should be noted that in situ and dry conductivities of conducting polymers are not directly comparable, since concentration polarization can reduce the conductivity of electrolyte-wetted films considerably.139 Thus in situ conductivities reported for polypyrrole,140,141 poly thiophene,37 and poly aniline37 are orders of magnitude lower than dry conductivities.15 

In situ electron transport measurements on conducting polymers are commonly made by using a pair of parallel-band electrodes bridged by the polymer [Fig. 9(A)].141142 Other dual-electrode techniques in which the polymer film is sandwiched between two electrodes [Fig. 9(B)],139,140 rotating-disk voltammetry [Fig. 9(C)],60,143 impedance spectroscopy,144,145 chronoamperometry,146 and chronopotentiometry147 have also been used. 

Rotating-disk voltammetry has a practical range of about 10 9 to 10 4 S cm-1 and is particularly useful for investigating the conductivity of very 

Impedance spectroscopy is best suited for the measurement of electronic conductivities in the range 10 -7to 10 2S cm 1.145 In principle, it is perhaps the best method for this range, but it is often difficult to interpret impedance data for conducting polymer films. The charge-transfer resistance can make measurements of bulk film resistances inaccurate,145 and it is often difficult to distinguish between the film s ionic and electronic resistances.144 This is even more of a problem with chronoamperometry146 and chronopotentiometry,147 so that these methods are best avoided. 

The electronic conductivity of a conducting polymer can vary by more than 10 orders of magnitude with changing potential. For lightly p-doped materials, the conductivity generally increases exponentially with increasing potential (see Fig. 11). Slopes of 60-130 mV decade-1 are 

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Figure 9. Schematic diagrams of (A) parallel-band electrode,141 142 (B) sandwiched electrode,139 140 and (C) rotating-disk voltammetry60 143 methods for making in situ electron transport measurements on polymer films.

The ammoxidation of substituted toluenes over differently prepared (NH4>2(V0)3(P207)2-and V0(P03)2-phases as well as over (VO)2P207 has been studied by catalytic and in situ-ESR measurements. For effective catalytic performance at least two structural properties were found to be essential i) Closely neighbouring centres must be exposed at the surface which enable the simultaneous adsorption and conversion of the substrate and ii) the catalyst structure must contain building blocks of exchange-coupled ions e. g. in the form of chains or layers which support the electron transport during the redox process. 

After a brief discussion of fundamentals of charge transport mechanisms, this chapter summarizes and discusses the most significant results obtained by using different junctions and in particular LAJs. In order to facilitate a systematic discussion, we make a functional distinction between non-active and active junctions we will refer to active junctions as those aimed at changing the electrical response by means of an external stimulus acting in situ to modify the molecular electronic structure non-active junctions are those used to measure and compare the electrical properties inherent to the different electronic structure of incorporated molecules, without any modification induced by an external signal. 

The monopolar devices with predominantly hole current were constructed by placing a thin Alq layer between NPB layers, which act as hole-transport and electron-blocking layers. The nearly monopolar characteristic of such devices was confirmed by observing only weak electroluminescence, which also precluded using it to evaluate the extent of operational degradation. On the other hand, the differences in absorption spectra of Alq and NPB permitted the separate photoexcitation of the thin Alq layer with 442 nm Hg line. In situ measurements revealed continuously decreasing photoluminescence... 

In situ conductivity measurements [52, 53, 164, 189, 195-202], luminescence techniques [203], different spectroscopies in combination with electrochemical methods [35-39, 82, 114, 115, 153-156, 204-208], and surface plasmon resonance [209] have also supplied valuable information about the ionic and electronic charge-transfer and charge transport processes. Microscopies have given new insights into the structure and morphology of surface polymer films, as well as their changes with the experimental conditions [151-159, 210-218]. The successful... 

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