Photocurrent

The attainable photocurrent under a given illumination intensity depends on the relative depth of light penetration in the semiconductor, the diffusion length of the 

FIGURE 1.19. Creation and movement of electronic carriers at the semiconductor/electrolyte interface under illumination. 

FIGURE 1.21. Absorption coefficient of light in silicon. After Muller and Kamins. ( 1977, Reprinted by permission of John Wiley Sons, Inc.) 

In the case of strong light absorption so that a x (Fig. 1.20a), the region for photo hole generation is within the depletion layer. The photocurrent is independent of the potential and represents the maximum attainable. On the other hand, when light absorption is weak (Fig. 1.20b), the photocurrent is proportional to x + L, and thus depends on the potential which determines the width of the space charge layer. 

Photocurrent can generally be considered to consist of two parts, that due to generation in the depletion layer, idi, and that from generation in the bulk, 4  

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The combination of electrochemistry and photochemistry is a fonn of dual-activation process. Evidence for a photochemical effect in addition to an electrochemical one is nonnally seen m the fonn of photocurrent, which is extra current that flows in the presence of light [, 89 and 90]. In photoelectrochemistry, light is absorbed into the electrode (typically a semiconductor) and this can induce changes in the electrode s conduction properties, thus altering its electrochemical activity. Alternatively, the light is absorbed in solution by electroactive molecules or their reduced/oxidized products inducing photochemical reactions or modifications of the electrode reaction. In the latter case electrochemical cells (RDE or chaimel-flow cells) are constmcted to allow irradiation of the electrode area with UV/VIS light to excite species involved in electrochemical processes and thus promote fiirther reactions. 

Modestov A D, Zhou G-D, Ge FI-FI and Loo B FI 1995 A study by voltammetry and the photocurrent response method of copper electrode behavior in acidic and alkaline solutions containing chloride ions J. Electroanal. Chem. 380 63-8... 

Figure C3.2.4. Plot of the log of photocurrent against number of methyl units in a alkylsilane based monolayer self-assembled on a n silicon electrode. The electrode is immersed in a solution witli an electron donor. Best fits of experimental data collected at different light intensities ( ) 0.3 mW cm ( ) 0.05 mW cm. From [10].

Membranes and Osmosis. Membranes based on PEI can be used for the dehydration of organic solvents such as 2-propanol, methyl ethyl ketone, and toluene (451), and for concentrating seawater (452—454). On exposure to ultrasound waves, aqueous PEI salt solutions and brominated poly(2,6-dimethylphenylene oxide) form stable emulsions from which it is possible to cast membranes in which submicrometer capsules of the salt solution ate embedded (455). The rate of release of the salt solution can be altered by surface—active substances. In membranes, PEI can act as a proton source in the generation of a photocurrent (456). The formation of a PEI coating on ion-exchange membranes modifies the transport properties and results in permanent selectivity of the membrane (457). The electrochemical testing of salts (458) is another possible appHcation of PEI. 

The ordered columnar arrangement of the hexapentyloxytriphenylene molecules provides good overlap of the -electrons of the triphenylene moieties along the director axis. This results in efficient hole transport in the mesophase. The hole photocurrent shows nondispersive transport with a high mobihtyup to 1 X 10 cm /Vs (24). 

Fig. 8. The photodiode detector (a) band model where the photon generates electron—hole pairs that are separated by the built-in potential setting up a photocurrent (b) physical model for a planar diode. The passivation is typically Si02 for Si diodes, an In oxide for InSb diodes, and CdTe for HgCdTe...

Designing tandem cells is complex. For example, each cell must transmit efficiently the insufficiently energetic photons so that the contacts on the backs of the upper cells are transparent to these photons and therefore caimot be made of the usual bulk metal layers. Unless the cells in a stack can be fabricated monolithically, ie, together on the same substrate, different external load circuits must be provided for each cell. The thicknesses and band gaps of individual cells in the stack must be adjusted so that the photocurrents in all cells are equal. Such an optimal adjustment is especially difficult because the power in different parts of the solar spectmm varies under ambient conditions. Despite these difficulties, there is potential for improvement in cell conversion efficiency from tandem cells. 

Band gaps in semiconductors can be investigated by other optical methods, such as photoluminescence, cathodoluminescence, photoluminescence excitation spectroscopy, absorption, spectral ellipsometry, photocurrent spectroscopy, and resonant Raman spectroscopy. Photoluminescence and cathodoluminescence involve an emission process and hence can be used to evaluate only features near the fundamental band gap. The other methods are related to the absorption process or its derivative (resonant Raman scattering). Most of these methods require cryogenic temperatures. 

A role is also played by the temperature and frequency dependence of the photocurrent, the variable surface sensitivity at various parts of the cathode and the vector effect of polarized radiation [40]. All the detectors discussed below are electronic components whose electrical properties vary on irradiation. The effects depend on external (photocells, photomultipliers) or internal photo effects (photoelements, photodiodes). 

Photomultipliers Secondary electron multipliers, usually known as photomultipliers, are evacuated photocells incorporating an amplifier. The electrons emitted from the cathode are multiplied by 8 to 14 secondary electrodes dynodes). A diagramatic representation for 9 dynodes is shown in Figure 18 [5]. Each electron impact results in the production of 2 to 4 and maximally 7 secondary electrons at each dynode. This results in an amplification of the photocurrent by a factor of 10 to 10. It is, however, still necessary to amplify the output of the photomultipher. 

It is a disadvantage of all photomultipliers that the photocurrent is not completely proportional to the strength of illumination. Further, the photocurrent must not exceed 10 A or the photomultiplier becomes fatigued. Daylight switches are incorporated into some scanners for this reason in order to prevent over-illumination of the detector when the sample chamber is opened. 

The latter mainly results from the thermal emission current. The dark current is apparent mainly in the long-wavelength range of the spectrum when the photocurrent is appropriately small [53, 54, 131]. It is relatively small for alloy cathodes (e.g. Sb-Cs cathodes), but not small enough to be negligible. 

Photodiodes produce an electric field as a result of pn transitions. On illumination a photocurrent flows that is strictly proportional to the radiation intensity. Photodiodes are sensitive and free from inertia. They are, thus, suitable for rapid measurement [1, 59] they have, therefore, been employed for the construction of diode array detectors. 

These Schottky energy barriers are measured in the presence of an electric field in the structure which is necessary to be able to collect the photocurrent. The photocurrent thresholds are not the zero electric field Schottky barriers because of the electric field in the polymer and the image chaise potential created when the electron leaves the metal. This effect results in a lowering of the Schottky energy barrier given by [34]... 

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. 

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