Open circuit electrode features

Indicator electrodes are used both for analytical purposes (in determining the concentrations of different substances from values of the open-circuit potential or from characteristic features of the polarization curves) and for the detection and quantitative characterization of various phenomena and processes (as electrochemical sensors or signal transducers). One variety of indicator electrode are the reference electrodes, which have stable and reproducible values of potential and thus can be used to measure the potentials of other electrodes. 

An alternative technique for the measurement of sc is to study the photopotential. The theory of this is discussed in some detail in Sect. 7, but the essential features of the measurement are shown in Fig. 16. After equilibration in the dark, when the potential of the electrode at open circuit becomes equal to the redox potential Vredox, the light is turned on and the electrode potential changes at open circuit in such a way that the bands become flat. There are many problems with this technique and it is considerably less reliable than a properly conducted a.c. experiment, but it may give a reasonably accurate picture if surface recombination is small (vide infra). Some results for p-GaAs in aqueous solution are shown in Fig. 17 and the S values derived are of the order 0.7, though the dispersion apparent in Fig. 17 makes a quantitative interpretation difficult. 

Electrode reactions can be studied in poorly conducting solutions, since there is no error due to the iR potential drop in the course of an open-circuit measurement. This feature may be particularly useful for studies in nonaqueoiis solutions and at low temperatures. Although this is fundamentally correct, there are practical limitations to its applicability. To give an extreme example, one cannot follow the open-circuit decay of potential over a range of 10 mV, if the iR potential during the pulse is, say, 10 V. 

However, when two subcells are stacked in series, the photocurrent is determined by the cell generating the smaller current. Therefore a current balance (or resistance match) is required between the front and back cell. Another approach is the so-called "stacking solar cell" in which two separate solar cells with transparent electrodes are fabricated and stacked together, with the front and back cell connected in parallel to give a larger short-circuit current. In this case, open-circuit voltage balance, which now becomes the key feature, can be fairly easily achieved. This will significantly increase the number of material systems applicable for efficient utilization of the solar spectrum. 

When the Pb—1.0 wt%Sn electrode is oxidized at +0.8 V (vs. Hg Hg2S04 electrode), the transient features a potential arrest on open circuit. After oxidation at higher potentials, a plateau appears at about +0.5 V on open circuit. This plateau has to be distinguished from the Pb02 plateau at 0.9 V after oxidation at 1.0 V. 

Luminous radiation (light) can produce changes in the open-circuit potentials and in the polarization characteristics of electrodes at constant potential the current may change (anodic or cathodic photocurrents appear), while at constant current, the electrode potential may change (photopotentials appear). It is an important special feature that electrochemical reactions may become possible that at the same potentials in the dark are thermodynamically prohibited, that is, associated with an increase in Gibbs energy under illumination, such reactions are possible because of the energy supplied from outside. 

In electrode kinetics, as empirically represented by Tafel s equation, a basic feature is the potential-dependence of the reaction rate (current-density). This effect arises in Gurney s representation in a fundamental and general way as the electric potential V, of the electrode metal is changed by AV relative to that of the solution (in practice, measured relative to the potential of a reference electrode at open-circuit), the effective value of the electron work function 4> of the metal is changed according to... 

Recently CMP of an interlevel dielectric SiO, deposited using evaporation techniques, was carried out for superconducting circuits. "CMP of the interlevel dielectric film allows the junction area to be greatly reduced, thereby increasing the speeds of operation of the circuits, by eliminating the need to open vias to make contact to the top electrode. ITie key to the success was in (a) the precise control to terminate the CMP within 200 nm after reaching the endpoint and (b) the ability to planarize a few hundred-micron-wide features. 

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