Electrode asymmetric

The simplest and most widely used model to explain the response of organic photovoltaic devices under illumination is a metal-insulaior-metal (MIM) tunnel diode [55] with asymmetrical work-function metal electrodes (see Fig. 15-10). In forward bias, holes from the high work-function metal and electrons from the low work-function metal are injected into the organic semiconductor thin film. Because of the asymmetry of the work-functions for the two different metals, forward bias currents are orders of magnitude larger than reverse bias currents at low voltages. The expansion of the current transport model described above to a carrier generation term was not taken into account until now. 

MIM or SIM [82-84] diodes to the PPV/A1 interface provides a good qualitative understanding of the device operation in terms of Schottky diodes for high impurity densities (typically 2> 1017 cm-3) and rigid band diodes for low impurity densities (typically<1017 cm-3). Figure 15-14a and b schematically show the two models for the different impurity concentrations. However, these models do not allow a quantitative description of the open circuit voltage or the spectral resolved photocurrent spectrum. The transport properties of single-layer polymer diodes with asymmetric metal electrodes are well described by the double-carrier current flow equation (Eq. (15.4)) where the holes show a field dependent mobility and the electrons of the holes show a temperature-dependent trap distribution. 

When the titration curve is symmetrical about the equivalence point the end point, defined by the maximum value of AE/AV, is identical with the true stoichiometrical equivalence point. A symmetrical titration curve is obtained when the indicator electrode is reversible and when in the titration reaction one mole or ion of the titrant reagent reacts with one mole or ion of the substance titrated. Asymmetrical titration curves result when the number of molecules or ions of the reagent and the substance titrated are unequal in the titration reaction, e.g. in the reaction... 

It should be kept in mind that all transport processes in electrolytes and electrodes have to be described in general by irreversible thermodynamics. The equations given above hold only in the case that asymmetric Onsager coefficients are negligible and the fluxes of different species are independent of each other. This should not be confused with chemical diffusion processes in which the interaction is caused by the formation of internal electric fields. Enhancements of the diffusion of ions in electrode materials by a factor of up to 70000 were observed in the case of LiiSb [15]. 

Very low asymmetric induction (e.e. 0.3-2.5%) was noted when unsymmetrical sulphides were electrochemically oxidized on an anode modified by treatment with (— )camphoric anhydride or (S)-phenylalanine methyl ester299. Much better results were obtained with the poly(L-valine) coated platinum electrodes300. For example, t-butyl phenyl sulphide was converted to the corresponding sulphoxide with e.e. as high as 93%, when electrode coated with polypyrrole and poly(L-valine) was used. 

For an unstable electrode system, the asymmetrical fluctuations first become unstable, then cascadelike transitions to the unstable state of the symmetrical fluctuations occur, if possible. As shown in Eqs. (42a) and (42b), when the amplitude factor becomes positive for certain wave numbers, the fluctuations become unstable, and the pits start to grow. When the amplification factor is negative for all wave numbers without exception, the growth of pits is depressed. From Eq. (43), the amplitude... 

At the potential beyond the critical pitting potential, the passive metal electrode system turns unstable. As mentioned before, the asymmetrical fluctuations arise from the electrostatic interaction between the electrode surface and solution particles in the double layer, so that the pitting current develops rapidly, and pits grow simultaneously. 

After the electrode potential is changed beyond the critical pitting potential, the fluctuations turn unstable through the critical state. At the same time, the reactions occurring at the surface yield new asymmetrical fluctuations in accordance with the potential difference. 

Electrode surface, and dipole potential difference or potential dependence, 15 Electrode systems, unstable, with asymmetrical fluctuations, 255 Electrode-electrite interface, microwave power and its effect on, 439 Electrogenerated films, storage capacity, 321... 

Flade potential, 247 Flame-annealed gold surfaces and the work of Kolb, 81 Flat band potential, 483 Fluctuations asymmetrical and unstable systems, 255 controlling progress in pitting, 299 in pitting dissolution, 251 and corrosion processes, 217 during dissolution, 252 at electrodes, theory, 281 during film breakdown, 233 and mathematical expressions thereof, 276... 

The study of optical isomers has shown a similar development. First it was shown that the reduction potentials of several meso and racemic isomers were different (Elving et al., 1965 Feokstistov, 1968 Zavada et al., 1963) and later, studies have been made of the ratio of dljmeso compound isolated from electrolyses which form products capable of showing optical activity. Thus the conformation of the products from the pinacolization of ketones, the reduction of double bonds, the reduction of onium ions and the oxidation of carboxylic acids have been reported by several workers (reviewed by Feokstistov, 1968). Unfortunately, in many of these studies the electrolysis conditions were not controlled and it is therefore too early to draw definite conclusions about the stereochemistry of electrode processes and the possibilities for asymmetric syntheses. 

From this discussion it is clear, that, independently of their redox properties, suitably modified electrodes offer themselves for the introduction of diastereo- or enantioselectivity into electrochemistry. Early reports of chiral inductions at modified electrodes include reactions at graphite and SnO surfaces derivatized with monolayers of (S)-(—)-phenylalanine. Asymmetric inductions at the chiral graphite electrode could, however, not be verified in other laboratories even after great efforts... 

Figure 13. (a) Experimental approach for simultaneous collection of potential and current noise, (b) Schematic for remotely controlled impedance and noise multichannel data collection system. (Reprinted from F. Mansfield, C. Chen, C. C. Lee, and H. Xiao, The Effect of Asymmetric Electrodes on the Analysis of Electrochemical Impedance and Noise Data, Corros. Sci. 38 (3) 497, Fig. 1. Copyright 1996 with permission of Elsevier Science.)...

The formation of new nuclei and of a fine-crystalline deposit will also be promoted when a high concentration of the metal ions undergoing discharge is maintained in the solution layer next to the electrode. Therefore, concentration polarization will have effects opposite those of activation polarization. Rather highly concentrated electrolyte solutions, vigorous stirring, and other means are employed to reduce concentration polarization. Sometimes, special electrolysis modes are employed for the same purposes currents that are intermittent, reversed (i.e., with periodic inverted, anodic pulses), or asymmetric (an ac component superimposed on the dc). 

When a droplet is deformed asymmetrically, the ratchet motions of the droplet can be induced as demonstrated on the vibrated gradient surface and on a saw-shaped electrode on which the wetting was changed by electrowetting [48]. 

We showed that the ratchet motions of a droplet were induced by very simple asymmetric guides [45]. Figure 16.7 shows a schematic drawing of the electrode causing the ratchet motions of a droplet. 

Figure 16.7 Schematic drawing of the asymmetric electrode pattern. The gold electrode was covered with a Fc-alkanethiol monolayer. The wetting of the gold electrode was switched from wetting to repulsive and vice versa by changing the electrochemical potential of the electrode.

In most systems the substrate electrodes are larger than the powered electrodes. This asymmetric configuration results in a negative dc self-bias voltage Vdc on the powered electrode. Without that, the difference in electrode areas would result in a net electron current per RF period [134, 169]. It has been shown that the ratio of the time-averaged potential drops for the sheaths at the grounded (V g) and the powered electrode (Vsp) are inversely proportional to a power of the ratio of the areas of the two electrodes (Ag, Ap) [134, 170-172] ... 

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