Metallic counterelectrode

The photopontential also approaches to zero when the semiconductor photoelectrode is short-circuited to a metal counterelectrode at which a fast reaction (injection of the majority carriers into the electrolyte) takes place. The corresponding photocurrent density is defined as a difference between the current densities under illumination, /light and in the dark, jDARK ... 

Similar photovoltaic cells can be made of semiconductor/liquid junctions. For example, the system could consist of an n-type semiconductor and an inert metal counterelectrode, in contact with an electrolyte solution containing a suitable reversible redox couple. At equilibrium, the electrochemical potential of the redox system in solution is aligned with the Fermi level of the semiconductor. Upon light excitation, the generated holes move toward the Si surface and are consumed for the oxidation of the red species. The charge transfer at the Si/electrolyte interface should account for the width of occupied states in the semiconductor and the range of the energy states in the redox system as represented in Fig. 1. 

The electronic excitation also promotes an electron to the conduction band, where it can function effectively as a reductant at a potenial governed by the position of the conduction band edge. In parallel fashion, reduced products can accumulate at a metal counterelectrode which has collected the photogenerated electrons from the conduction band. 

Next, photogeneration of electron-hole pairs leads to the formation of quasi-levels of minority and majority carriers, Fp and F , as shown in Fig. 12. Since, at the surface, Fp < Fs -/sl and F > Fs -/sl, illumination results in the acceleration of both forward and reverse reactions in a sulfide polysulfide couple. If the circuit is closed on an external load R, the anodic and cathodic reactions become separated the holes are transferred from the semiconductor photoanode to the solution, so that ions are oxidized to 82 , and the electrons are transferred through the external circuit to the metal counterelectrode (cathode) where they reduce S2 to The potential difference across a photocell is iphR, where iph is the photocurrent, and the power converted is equal to /phF. 

Fig. 2. Schematic illustration of the vacuum-tight, short path length, thin-layer spectro-electrochemical cell with a doubled platinum gauze working electrode. The side view shows how the cell assembly is positioned for acquiring spectral data. The top view shows the connection of the outer RC circuit. The symbols are as follows (a) thin-layer chamber, (b) doubled platinum gauze working electrode with edge eliminator, (c) expanded platinum metal counterelectrode, (d) platinum wire auxiliary reference electrode, (e) photo window, (f) reference point, the tip of the frit, (g) asbestos disk, (h) reference frit. CE, counterelectrode post WE, working electrode post RE, Ag/AgCI, KCI reference electrode ARE, auxiliary reference electrode post RE, reference electrode post. ...

The central problem in the study of ionic conduction is to discover the details of the atomic transport processes involved in the growth of films. This will be discussed in the first part of this review. The electronic conductivity is of considerable theoretical interest and is of great practical importance for microelectronic devices. We discuss the system in which a thin metal counterelectrode replaces the electrolyte solution in which the oxide was made. Thermionic and field assisted emission, tunneling processes, impurity band conduction, and space-charge limited currents, have to be considered. We shall draw on results for oxide films made by other processes, such as evaporation and thermally promoted reaction with oxygen. 

A discussion will be given of electronic currents through sandwich structures of the type tantalum (or other substrate metal)/oxide film/metal counterelectrode. The thickness of the oxide film has varied from 25 to 5000 A. The counterelectrode has usually been deposited on the oxide by evaporation, but pressure contacts, mercury droplets, and electroless plating have also been used. The behavior of the system metal/oxide/electrolytic solution is more difficult to interpret and little can be added to a previous article. Even with the simple metal/insulator/metal system there is disagreement about which mechanisms control the current under the various conditions of temperature, thickness, and field. However, recent work has clarified the picture with regard to the choice of mechanisms, and experimental results are beginning to accumulate. Some effects, such as the negative resistance, which has been observed with films which have been subjected to a preliminary breakdown, can be explained only very tentatively. 

If either an n-type photoanode or a p-type photocathode is used in conjimction with a metal counterelectrode and a reversible redox electrolyte (e.g., Fe(CN)6 ), we have the basis for a regenerative photoelectrochemical cell. Alternately both an n-type and a p-type semiconductor may be used in tandem in a twin-photoelectrode geometry for the cell, much like what plants do in photosynthesis (For example [13]). Note that in these case there is no net chemistry occurring in the electrolyte phase in response tophotoexcitation, i.e., what is photooxized (or photoreduced) at one terminal is re-reduced (or re-oxidized) back at the other. The result is conversion or transduction of photon energy to electrical energy. 

Fig. 15.2 Voltage profile of the Li[Lio.,7Mnoj8Nio.25] O2—carbon coated LiFeP04 in 50 50 wt% ratio built in this configuration with Li metal counterelectrode...

---