Semiconductor/electrolyte junction

Figure 27. Minority charge carrier profiles near the semiconductor/electrolyte junction. calculated for a silicon interface at two different electrode potentials. Uf- -0.25 V and Uf= 5.0 V10 ((//= forward bias = t/ - Ufl>).

Following the same procedure, the kinetic constants have been determined for very different electrochemical conditions. When n-WSe2 electrodes are compared in contact with different redox systems it is, for example, found9 that no PMC peak is measured in the presence of 0.1 M KI, but a clear peak occurs in presence of 0.1 M K4[Fe(CN)6], which is known to be a less efficient electron donor for this electrode in liquid junction solar cells. When K4[Fe(CN)6] is replaced by K3[Fe(CN)6], its oxidized form, a large shoulder is found, indicating that minority carriers cannot react efficiently at the semiconductor/electrolyte junction (Fig. 31). 

The PMC transient-potential diagrams and the equations derived for PMC transients clearly show that bending of an energy band significantly influences the charge carrier lifetime in semiconductor/electrolyte junctions and that an accurate interpretation of the kinetic meaning of such transients is only possible when the band bending is known and controlled. 

How can such problems be counterbalanced Since a large capacitance of a semiconductor/electrolyte junction will not negatively affect the PMC transient measurement, a large area electrode (nanostructured materials) should be selected to decrease the effective excess charge carrier concentration (excess carriers per surface area) in the interface. PMC transient measurements have been performed at a sensitized nanostructured Ti02 liquidjunction solar cell.40 With a 10-ns laser pulse excitation, only the slow decay processes can be studied. The very fast rise time cannot be resolved, but this should be the aim of picosecond studies. Such experiments are being prepared in our laboratory, but using nanostructured... 

Lemasson P, Etcheberry A, Gautron J (1982) Analysis of photocurrents at the semiconductor-electrolyte junction. Electrochim Acta 27 607-614... 

Lemasson P, Boutry AE, Tiiboulet R (1984) The semiconductor-electrolyte junction Physical parameters determination by photocurrent measurement throughout the Cdi xZnxTe alloy series. J Appl Phys 55 592-594... 

Energy level diagram of a semiconductor-electrolyte junction and solid-state and electrochemical energy scales. 

The different types of quinones active in photosynthesis are being used as electron acceptors in solar cells. The compounds such as Fd and NADP could also be used as electron/proton acceptors in the photoelectrochemical cells. Several researchers have attempted the same approach with a combination of two or more solid-state junctions or semiconductor-electrolyte junctions using bulk materials and powders. Here, the semiconductors can be chosen to carry out either oxygen- or hydrogen-evolving photocatalysis based on the semiconductor electronic band structure. 

This section and the next are dedicated to the basics of the silicon-electrolyte contact with focus on the electrolyte side of the junction and the electrochemical reactions accompanying charge transfer. The current across a semiconductor-electrolyte junction may be limited by the mass transport in the electrolyte, by the kinetics of the chemical reaction at the interface, or by the charge supply from the electrode. The mass transport in the bulk of the electrolyte again depends on convection as well as diffusion. In a thin electrolyte layer of about a micrometer close to the electrode surface, diffusion becomes dominant The stoichiometry of the basic reactions at the silicon electrode will be presented first, followed by a detailed discussion of the reaction pathways as shown in Figs. 4.1-4.4. 

Photoelectrochemical effect involves production of a voltage and an electric current when light falls on a semiconductor electrode immersed in an electrolyte solution and connected to a counter electrode (Becquerel, 1839). Working with germanium electrodes, Brattain Garrett (1955) showed that the Becquerel effect was due to the formation of a semiconductor-electrolyte junction. The idea of using an illuminated... 

The existence of surface states in general can lead to a variety of nonidealities in the output parameters associated with semiconductor-electrolyte junctions. Figure 28.6 provides the current-potential response for a photo-electrochemical cell containing a cadmium ferrocyanide-modified n-CdS electrode in an aqueous ferri/ferrocyanide electrolyte. Although open-circuit and... 

Electronic energy levels localized at the surface of a semiconductor have frequently been used to explain.experimentally observed currents at semiconductor-electrolyte junctions.1 These surface or interface states are invoked when the observations are inconsistent with direct electron transfer between the conduction or valence band of the semiconductor and electronic states of an electrolyte in contact with the semiconductor. Charge carriers in the semiconductor bands are transferred to surface states that have energies within the bandgap of the semiconductor. From these states electrons can move isoenergetically across the interface to or from electrolyte states in accordance with the widely accepted view of electron transfer. 

By contrast, electrolyte states are much more limited in their distribution than metal conduction band states so that in many cases electron transfer through surface states may be the dominant process in semiconductor-electrolyte junctions. On the other hand, in contrast to vacuum and insulators, liquid electrolytes allow substantial interaction at the interface. Ionic currents flow, adsorption and desorption take place, solvent molecules fluctuate around ions and reactants and products diffuse to and from the surface. The reactions and kinetics of these processes must be considered in analyzing the behavior of surface states at the semiconductor-electrolyte junction. Thus, at the semiconductor-electrolyte junction, surface states can interact strongly with the electrolyte but from the point of view of the semiconductor the reaction of surface states with the semiconductor carriers should still be describable by equations 1 and 2. 

Therefore it is very important to complete the data obtained by (photo) electrochemical techniques with surface sensitive spectroscopic measurements. One promising possibility of gaining microscopic information on interfacial processes is the use of UHV surface science techniques. However due to the analysis requirements emersion of the samples from the electrolyte and transfer into UHV is necessary. During this procedure the semiconductor interface may change drastically. Alternatively the basic chemical and physical interactions of electrolyte components may be studied by adsorbing redox components on defined semiconductor surfaces thus simulating semiconductor/electrolyte junctions. 

Fig. 8.2. (a) Potential distribution across the semiconductor/electrolyte junction under depletion conditions, (b) Band bending corresponding to (a). 

Fig. 8.4. Profile of light intensity at the semiconductor electrolyte junction. W is the width of the depletion layer and L, is the hole diffusion length. The penetration depth of the light...

---