Conduction, ambipolar

Key words Mixed ionic electronic conductors, ionic conductivity, electronic conductivity, ambipolar diffusion, I-V relations, defect distributions, applications of mixed conductors. 

Figure 24. Models illustrating the source of chemical capacitance for thin film mixed conducting electrodes, (a) Oxygen reduction/oxidation is limited by absorption/de-sorption at the gas-exposed surface, (b) Oxygen reduction/ oxidation is limited by ambipolar diffusion of 0 through the mixed conducting film. The characteristic time constant for these two physical situations is different (as shown) but involves the same chemical capacitance Cl, as explained in the text.

HOMO of DBTTF and the LUMO of TCNQ, respectively (Fig. 4b). Source and drain electrodes are several organic metals of the TTF TCNQ type having different chemical potentials predicted using Fig. 4c which is the same as Fig. 2a. For the electrodes whose chemical potentials are set within the conduction band of the channel material, FET exhibited n-type behavior (A in Fig. 4d). When the chemical potentials of organic metals are allocated within or near the valence band of the channel, p-type behaviors were observed (E, F in Fig. 4d). When the chemical potentials of the electrodes are within the gap of the channel, FET exhibited ambipolar-type behavior (B-D in Fig. 4d). Since the channel material is the alternating CT solid, the drain current is not excellent and a Mott type insulator of DA type or almost neutral CT solid having segregated stacks is much preferable in this context. 

From the theory of ambipolar conduction (see, e.g., Ref.56) it follows immediately that given an oxygen potential difference A/i0 between the lhs (u(]) and the right-hand side, an oxygen flux which is of the form... 

As the oxygen partial pressure ratio, and hence A/u0, is known, the ambipolar conductivity is readily determined from the flux. This knowledge can be further used to calculate the partial conductivities, and by knowing Ef from the transient (i.e., by also evaluating the delay time231) to derive the thermodynamic factor (i.e., the chemical capacitance). 

Langmuir-Blodgett (LB) technique has been also used for the preparation of Pc-based OFET, as it allows the fine control of both the structure and the thickness of the film at the molecular level [226,227], OFET devices based on amphiphilic tris(phthalocyaninato) rare earth, triple-decker complexes have been prepared by LB technique, showing good OFET performances [228], More recently, ambipolar transport has also been realized in OFET devices through a combination of holeconducting CuPc and n-conducting Cgo fullerene, in which the asymmetry of the... 

The photoreceptor of electrophotography is a charge depletion device in which replenishing carriers from the electrodes must be completely prohibited. Since the a-Si H is a typical ambipolar material in which both carriers, i.e., electrons and holes, are allowed to move, great care must be paid to the contact with either the conductive electrode or the free surface... 

The physical meaning of the ambipolar conductivity relates to the correlated transport of several charge carriers, when an internal -> electrical field in the system impedes the migration of species with a higher - mobility, but enhances transfer of less mobile species. 

The phenomenon of ambipolar conduction is not limited to chemical potential gradients only, and may occur in systems with several driving forces (e.g., chemical-potential and temperature gradients in combination with external electrical field). However, this phenomenon is always related to conjugate transport of several charge carriers. 

The quantity of ambipolar conductivity is widely used for the analysis of -> electrolytic permeability of -> solid electrolytes, caused by the presence of electronic conductivity. Other important cases include transient behavior of electrochemical cells and ion-conducting solids, dense ceramic membranes for gas separation, reduction/ oxidation of metals, and kinetic demixing phenomena [iv]. In most practical cases, however, the ambipo-... 

See also - ambipolar conductivity, -> diffusion determination in solids, - Wagner factor, - insertion electrodes, -> batteries. 

The equilibrium conditions in electrochemical systems are usually expressed in terms of electrochemical potentials. For non-equilibrium systems, the gradient of chemical and/or electrochemical potential is a driving force for flux of particles i. See also Wagner equation, - Wagner factor and ambipolar conductivity, -+ On-sager relations. 

Similar approaches are used for most steady-state measurement techniques developed for mixed ionic-electronic conductors (see -> conductors and -> conducting solids). These include the measurements of concentration-cell - electromotive force, experiments with ion- or electron-blocking electrodes, determination of - electrolytic permeability, and various combined techniques [ii-vii]. In all cases, the results may be affected by electrode polarization this influence should be avoided optimizing experimental procedures and/or taken into account via appropriate modeling. See also -> Wagner equation, -> Hebb-Wagner method, and -> ambipolar conductivity. 

As for the permeability measurements, most techniques based on the analysis of transient behavior of a mixed conducting material [iii, iv, vii, viii] make it possible to determine the ambipolar diffusion coefficients (- ambipolar conductivity). The transient methods analyze the kinetics of weight relaxation (gravimetry), composition (e.g. coulometric -> titration), or electrical response (e.g. conductivity -> relaxation or potential step techniques) after a definite change in the - chemical potential of a component or/and an -> electrical potential difference between electrodes. In selected cases, the use of blocking electrodes is possible, with the limitations similar to steady-state methods. See also - relaxation techniques. 

Electrolytic permeability — of ion-conducting - solid materials is the transport of neutral potentialdetermining component(s) under a -> chemical potential gradient due to the presence of bulk electronic conductivity in the material, or a parameter describing this transport. As the flux of ions is charge-compensated by a simultaneous flux of electronic charge carriers, -> steady-state permeation can be achieved without external circuitry. The transport processes can be quantitatively described in terms of -> ambipolar conductivity. 

The overall permeation rate of a material is determined by both ambipolar conductivity in the bulk and interfacial exchange kinetics. For -> solid electrolytes where the electron - transference numbers are low (see -> electrolytic domain), the ambipolar diffusion and permeability are often limited by electronic transport. 

Concepts of local equilibrium and charged particle motion under - electrochemical potential gradients, and the description of high-temperature -> corrosion processes, - ambipolar conductivity, and diffusion-controlled reactions (see also -> chemical potential, -> Wagner equation, -> Wagner factor, and - Wagner enhancement factor). 

The chemical diffusion coefficient includes, as we know from the formal treatment in Section VI..3iv., both an effective ambipolar conductivity and an effective ambipolar concentration. The latter parameter is determined by the thermodynamic factor which is large for the components but close to unity for the defects. 

TqVjUo (with crs =aeonalon/cr being the ambipolar conductivity, see Section NlA.ii). Owing to its constancy the flux can be recast as (1/L) Jaod o r lus die growth rate measured by the thickness... 

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