Charge transfer semiconductors

In addition to yielding information about semiconductor charge-transfer dynamics, the fill factor parameterizes the efficiency with which the photo-electrochemical cell can be expected to convert optical energy to electricity. The practical value of a photoelectrochemical cell is usually evaluated by its maximum conversion efficiency. The energy conversion efficiency is defined as... 

The detector converts the intensity of the light reaching it to an electrical signal. It is by nature a single channel device. Two types of detector are used, either a photomultiplier tube or a semiconductor (charge transfer devices or silicon photodiodes). For both of which the sensitivity depends upon the wavelength. 

While these features greatly facilitate monitoring worker exposures, most instruments suffer from limitations associated with the types of sensing conponents enployed. For exeunple, the majority of the truly portable Instruments used for monitoring organic gases and vapors detect contaminants by catalytic combustion, semiconductor charge-transfer, or photo-ionization. 

For semiconductors, charge transfer takes place via either the bottom of the conduction band or the top of the valence band. In certain cases, charge transfer via surface states can also occur. Although Fig. 2.19 is drawn for the case of electron transfer to the valence band, which will only be significant if photogenerated holes are present, similar pictures can be made for other pathways, e.g., Dred Eq, Ec Do, or Ey Dox- An important conclusion from Fig. 2.19 is that the probability of electron transfer actually decreases if Fred is too far above Fy. This is markedly different from the behavior of metal electrodes, which show a continuous increase in current with applied potential. 

Gopidas KR, Bohorquez M, Kamat PV (1990) Photophysical and photochemical aspects of coupled semiconductors. Charge transfer processes in... 

The highly conductive class of soHds based on TTF—TCNQ have less than complete charge transfer (- 0.6 electrons/unit for TTF—TCNQ) and display metallic behavior above a certain temperature. However, these soHds undergo a metal-to-insulator transition and behave as organic semiconductors at lower temperatures. The change from a metallic to semiconducting state in these chain-like one-dimensional (ID) systems is a result of a Peieds instabihty. Although for tme one-dimensional systems this transition should take place at 0 Kelvin, interchain interactions lead to effective non-ID behavior and inhibit the onset of the transition (6). 

An alternative approach to stabilizing the metallic state involves p-type doping. For example, partial oxidation of neutral dithiadiazolyl radicals with iodine or bromine will remove some electrons from the half-filled level. Consistently, doping of biradical systems with halogens can lead to remarkable increases in conductivity and several iodine charge transfer salts exhibiting metallic behaviour at room temperature have been reported. However, these doped materials become semiconductors or even insulators at low temperatures. 

Figure 12. Energy diagram of a semiconductor/electrolyte interface showing photogeneration and loss mechanisms (via surface recombination and interfacial charge transfer for minority charge carriers). The surface concentration of minority...

Let us now investigate the case of a semiconductor with a relatively slow interfacial charge transfer. In this case the surface concentration of minority carriers is high and we can neglect the second term (which does not contain Ps). For higher values of electrode potential, the term L Qxp(-AUqfkT) can also be neglected. 

Figure 13. Numerically calculated PMC potential curves from transport equations (14)—(17) without simplifications for different interfacial reaction rate constants for minority carriers (holes in n-type semiconductor) (a) PMC peak in depletion region. Bulk lifetime 10" s, combined interfacial rate constants (sr = sr + kr) inserted in drawing. Dark points, calculation from analytical formula (18). (b) PMC peak in accumulation region. Bulk lifetime 10 5s. The combined interfacial charge-transfer and recombination rate ranges from 10 (1), 100 (2), 103 (3), 3 x 103 (4), 104 (5), 3 x 104 (6) to 106 (7) cm s"1. The flatband potential is indicated.

Experimental evidence with very different semiconductors has shown that at semiconductor interfaces where limited surface recombination and a modest interfacial charge-transfer rate for charge carriers generate a peak... 

With electrochemically studied semiconductor samples, the evaluation of t [relation (39)] would be more straightforward. AU could be increased in a well-defined way, so that the suppression of surface recombination could be expected. Provided the Debye length of the material is known, the interfacial charge-transfer rate and the surface recombination... 

T. Ioannides, and X.E. Verykios, Charge transfer in metal catalysts supported on Doped Ti02 A Theoretical approach based on metal-semiconductor contact theory, J. Catal. 161,560-569 (1996). 

Derivatized semiconductor photoelectrodes offer a way to design photosensitive interfaces for effecting virtually any redox process. Manipulation of interfacial charge transfer processes has been demonstrated using hydrolytically unstable redox... 

The photoelectrochemical properties of 283 colloids prepared by chemical solution growth [193] have been demonstrated by carrying out oxidation and reduction processes under visible light irradiation. Charged stabilizers such as Nation were found to provide an effective microenvironment for controlling charge transfer between the semiconductor colloid and the redox relay. 

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