Photoconductivity -.

The measurement of the photoconductivity of pure liquids or solutions is of fundamental interest since it yields information on the electronic energy levels in liquids. These studies are supplemented by investigations of the radiation-induced conductivity (Chapter 5) and of the charge transfer at interfaces (Chapter 6). From a practical point of view, photo-induced conductivity changes may be applied as laser-triggered fast high voltage switches, photon detectors, or monitors for chemical compounds. 

The conductivity of semiconducting materials depends on the number of free electrons in the conduction band and the number of holes in the valence band, according to Equation 18.13. Thermal energy associated with lattice vibrations can promote electron excitations in which free electrons and/or holes are created, as described in Section 18.6. Additional charge carriers may be generated as a consequence of photon-induced electron transitions in which light is absorbed the attendant increase in conductivity is called photoconductivity. Thus, when a specimen of a photoconductive material is illuminated, the conductivity increases. 

This phenomenon is used in photographic light meters. A photoinduced ciurent is measured, and its magnitude is a direct function of the intensity of the incident hght radiation, or the rate at which the photons of light strike the photoconductive material. Visible light radiation must induce electronic transitions in the photoconductive material cadmium sulfide is commonly used in light meters. 

Sunlight may be directly converted into electrical energy in solar cells, which also use semiconductors. The operation of these devices is, in a sense, the reverse of that for the light-emitting diode. A p-n junction is used in which photoexcited electrons and holes are drawn away from the junction, in opposite directions and become part of an external current, as illustrated in chapter-opening diagram(a). 

Important applications for semiconductor LEDs include digital clocks and illuminated watch displays, optical mice (computer input devices), and film scanners. Electronic remote controls (for televisions, 

DVD players, etc.) also use LEDs that emit an infrared beam this beam transmits coded signals that are picked up by detectors in the receiving devices. 

Indeed, for the activation energy of to be zero, the lattice relaxation energy should be equal to the trap depth, which is about 0.7-0.9 eV. Such a large lattice relaxation should give the defect transition a large Stokes shift, but none is observed. The optical and thermal transition energies have been measured for n-type a Si H and are shown in Fig. 

The energies of thermal emission, observed by DLTS and by optical absorption and luminescence agree to within 0.1 eV. The Stokes shift is therefore very small and unable to account for the capture cross-section within the multiphonon model. The mechanism of non-radiative capture at defects remains puzzling. 

Photoconductivity occurs when carriers are optically excited from non-conducting to conducting states. It is an indireet measure of the recombination and does not distinguish between radiative and non- 

When there is incomplete collection of charge, the primary photoconductivity is complicated by the presence of trapped space charge which distorts the electric field (see Section 10.1.2). Although the primary photoconductor is usually the best structure for a light detector, the recombination mechanisms are more commonly studied 

65) assumes that electrons and holes form a quasi-equilibrium with the band tail states. This is valid for electrons near room temperature, but only approximate for holes since dispersive hole transport indicates that a quasi-equilibrium is not fully established in the band tail. 

In the presence of an electric field, E, the Coulomb potential barrier in the downfield direction is lowered and escape from geminate recombination becomes easier. The quantum efficiency for charge generation is then given, to first order in the field, by  

There are other processes for charge generation these include two-photon absorption and single photon ionisation of singlet and triplet excitons. These processes are proportional to the square and higher powers of the light intensity and usually somewhat weaker than the processes described in the previous paragraph. 

Typical examples of photoconductive polymers (group (b)) are listed in Table 2.2. Concerning the field of conducting polymers, including photoconducting polymers, the reader is referred to various books and reviews [1-21]. 

At first, diarsenic triselenide was used as the photoconducting material. Poly(A -vinyl carbazole) is now used. This polymer absorbs ultraviolet light, producing an exciton, which ionizes in an electric field. Poly(vinyl carbazole) behaves as an insulator in visible light, but can, however, be sensitized with certain electron donors to form a charge transfer complex. 

McCrum, B. E. Read, and G. Williams, Anelastic and Dielectric Effects in Polymeric Solids, Wiley, London, 1967. 

Fukada, Piezoelectric dispersion in polymers. Prog. Polym. Sci. Jpn. 2, 329 (1971). 

Electrical Properties of Polymeric Materials, Plastics Institute, London, 1973. 

Dechema Monography, Vol. 72, Elektrostatische Aufladung, Verlag Chemie, Weinheim, 1974. M. W. Williams, The dependence of triboelectric charging of polymers on their chemical compositions,/. Macromol. Sci.—Rev. Macromol. Chem. C14, 251 (1976). 

It should be noted that photocurrents usually observed in organic polymers are small, primarily because of the very low charge-carrier mobilities. The charge drift mobilities// (defined as the velocity per unit electric field) are typically s at the normally used 

Generation of charge carriers. The excitons are captured at donor or acceptor sites, the functional groups are polarized and separate charges are formed. These sites can be either inherent to the polymer structure or be present as additives or impurities. 

Separation of charge carriers. The electric field assists the separation of charges, electrons and holes. Some fraction of the hole-electron pairs, however, may undergo geminate recombination. 

Migration (transport) of charges. Either electrons or holes or both drift towards electrodes in the presence of an electric field. Random diffusion of carriers will result in zero current. 

Recombination. Coulombic forces will eventually cause recombination of free electrons and holes at recombination sites in the circuit. 

The photoconduction action spectra are usually similar to the absorption spectra. 

Illumination leads to two to three orders of magnitude higher conductivity if compared to dark conduction [13,181]. In presence of oxygen the conductivity increases another order of magnitude whereas nearly no influence of oxygen on the dark conductivity is found [181, 367]. 

The photoconductivity raises proportional to the square root of the illumination intensity [181]. 

If the long axes of the molecules are perpendicular to the applied field a square root dependence of the logarithm of the photocurrent with the applied field is found according to the expected Poole-Frenkel behavior, whereas molecules oriented with their long axes along the field exhibit a linear field dependence [13]. 

Handbook of Conducting Polymers (Eds. T. A. Skotheim, R. L. Elsenbaumer and J. R. Reynolds), Marcel Dekker, New York, 1998, 2nd edition. 

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Free-electron lasers have long enabled the generation of extremely intense, sub-picosecond TFlz pulses that have been used to characterize a wide variety of materials and ultrafast processes [43]. Due to their massive size and great expense, however, only a few research groups have been able to operate them. Other approaches to the generation of sub-picosecond TFlz pulses have therefore been sought, and one of the earliest and most successfid involved semiconducting materials. In a photoconductive semiconductor, carriers (for n-type material, electrons)... 

Selenium exhibits both photovoltaic action, where light is converted directly into electricity, and photoconductive action, where the electrical resistance decreases with increased illumination. These properties make selenium useful in the production of photocells and exposure meters for photographic use, as well as solar cells. Selenium is also able to convert a.c. electricity to d.c., and is extensively used in rectifiers. Below its melting point selenium is a p-type semiconductor and is finding many uses in electronic and solid-state applications. 

Mid- and near-infrared Nernst filament globar NaCl or KBr Grating interferometer Golay cell thermocouple bolometer pyroelectric photoconductive semiconductor... 

FURNACES,ELECTRIC - INDUCHON FURNACES] (Vol 12) -photoconductivity [PHOTOCONDUCTIVE POLYMERS] (Vol 18) for papermaking [PAPER] (Vol 18)... 

Photochemical technology Photoconductive polymers Photography Printing processes Radioactive tracers Radiopaques... 

Hg Cd Te is an example of a ternary detector, in which the value of x controls the cutoff wavelength. Photoconductive detectors are generally simpler to couple to low noise amplifiers photodiodes generally have lower power consumption because these have no external bias, and better high frequency performance (15,16). 

The polysdanes are normally electrical insulators, but on doping with AsF or SbF they exhibit electrical conductivity up to the levels of good semiconductors (qv) (98,124). Conductivities up to 0.5 (H-cm) have been measured. However, the doped polymers are sensitive to air and moisture thereby making them unattractive for practical use. In addition to semiconducting behavior, polysilanes exhibit photoconductivity and appear suitable for electrophotography (qv) (125—127). Polysdanes have also been found to exhibit nonlinear optical properties (94,128). 

Lead sulfide is used in photoconductive cells, infrared detectors, transistors, humidity sensors in rockets, catalysts for removing mercaptans from petroleum distillates, mirror coatings to limit reflectivity, high temperature solid-film lubricants, and in blue lead pigments (82). 

Lead Telluride. Lead teUuride [1314-91 -6] PbTe, forms white cubic crystals, mol wt 334.79, sp gr 8.16, and has a hardness of 3 on the Mohs scale. It is very slightly soluble in water, melts at 917°C, and is prepared by melting lead and tellurium together. Lead teUuride has semiconductive and photoconductive properties. It is used in pyrometry, in heat-sensing instmments such as bolometers and infrared spectroscopes (see Infrared technology AND RAMAN SPECTROSCOPY), and in thermoelectric elements to convert heat directly to electricity (33,34,83). Lead teUuride is also used in catalysts for oxygen reduction in fuel ceUs (qv) (84), as cathodes in primary batteries with lithium anodes (85), in electrical contacts for vacuum switches (86), in lead-ion selective electrodes (87), in tunable lasers (qv) (88), and in thermistors (89). 

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