Charge transport parameters

If a material containing a density, n, of mobile charge carriers, each carrying a charge Q, is situated in an electric field E, the charge carriers experience a force 

If the drift velocity of the charges is proportional to the force acting on them, then 

For materials for which nQu is constant at constant temperature, this is a statement of Ohm s law  

The resistivity p, like the conductivity, is a material property and the two are related by 

In practice it is often the conductive or resistive characteristics of a specimen of uniform section A and length / which are relevant. The resistance R, conductance G and specimen dimensions are related as follows  

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The three experiments do not only introduce decisive mass and charge transport parameters, they also permit their determination. Some points relevant in this context will be investigated in the following. (Note that electrochemical measurement techniques are covered by Part II.1) At the end of this section we will have seen that—close to equilibrium—not only all the D s and the k s can be expressed as the inverse of a product of generalized resistances and capacitances, but that these elements can be implemented into a generalized equivalent circuit with the help of which one can study the response of a material on electrical and/or chemical driving forces. 

Table 3. Selection of charge transport parameters from some pure charge-transporting components of organic photorefractive materials.

The Dublin group has also reported a three-color electrochromic metallopoly mer 6 based on a ruthenium phenolate complex bound to poly(4-vinylpyridine). The reversible color changes (wine red to green) are associated with the Ru(II) oxidation, whereas the mixed redox state produces a red-orange. The charge transport parameters of the polymer were thoroughly characterized [27,28]. 

Valeev, E.F., Coropceanu, V., da Silva Eilho, D.A., Salman, S., and Bredas, J.L., Effect of electronic polarization on charge-transport parameters in molecular organic semiconductors, J. Am. Chem. Soc., 128, 9882, 2006. 

Sancho-Garcia, J.C., Horowitz, G., Bredas, J.L., and ComU, J., Effect of an external electric field on the charge transport parameters in organic molecular semiconductors,... 

TABLE 5 Approximate Values for the Charge Transport Parameter in Electrochemical Situations... 

The various terms in the equation are as follows i is the current, n is the number of electrons transferred, F is the Faraday, A is the electrode area, C is the concentration, D is the diffusion coefficient, and t is the time. Thus the technique may be used to estimate, among other things, the charge transport parameter. The reader interested in the solution of the Fick s law equation using the Laplace transformation should consult Ref. 6. A closely related technique is chronocoulometry, in which the excitation function is still the potential pulse, but instead of monitoring the current, the integrated charge is monitored as a function of time. This entails less error as a cumulative measurement is made. The equation for chronocoulometry is... 

The charge transport parameters can of course be estimated by examining the short-time-period region of the chronoamperometric... 

Table 5.4 Summary of charge transport parameters obtained for BiPhenyl-BiThiophene based compounds...

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