Photoconductive cell

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 sulfide occurs in nature as the mineral galena. Most lead comes from this ore. Additionally, lead sulfide has several industrial applications. It is used in infrared detectors transistors photoconductive cells high temperature lubricants and for glazing earthenware. It also is used as a catalyst in petroleum refining for removal of mercaptans from petroleum distillates. 

Several forms of selenium are known also. In addition to two red (monoclinic) and a grey (hexagonal) form (with melting point 217° C), there have been reported several forms of amorphous selenium (both red and black). The grey form (and presumably also the amorphous forms) of selenium consist of zig-zag chains of bound selenium atoms, but one of the red forms has been shown to contain Se8 rings. The grey form of selenium has become especially important in recent years the sharp increase in its electrical conductivity upon exposure to light makes it a valuable material for use in photoconductive cells. 

The three basic types are photoconductive, photovoltaic, and photo-emissive, and all are sensitive to both heat and light. The resistance of a photoconductive cell is lowered when it is illuminated and, over a small range, its response is linear. Cells containing lead sulfide, which is sensitive at wavelengths greater than 700 nm, and cadmium sulfide or selenide, with a sharp response maximum at 710 nm, have been used but may not give a stable response and are largely restricted to specialized applications in other fields. Silicon photodiodes and transistors are sensitive from 340 to 1200 nm with a peak at 900 nm. 

Thermal and photoconductive detectors are used to measure radiation intensities, but all have relatively slow responses and are subject to drift. The lead sulfide or telluride photoconductive cell has a response time of about 0.5 ms, but sensitivity decreases sharply above 2900 cm" for the sulfide and above 1700 cm- for the telluride. Thermal detectors are employed at longer wavelengths. The simplest of these is the thermocouple, which has a relatively slow response (about 60 ms), and several are usually linked to form a thermopile. Bolometers... 

Photoconductive cell A detector of electromagnetic radiation whose electrical conductivity increases with the intensity of radiation impinging on it. 

Selenium is widely dispersed. It is found, for example, in igneous rocks, volcanic sulfur deposits, hydrothermal deposits, and copper ores. Selenium is used in the electronics industry for the manufacture of rectifiers and photoconductivity cells. Selenium and its compounds are also used as additives in chromium-plating, glass, ceramics, pigment, rubber, photography, lubricants, pharmaceuticals, and organic substances. 

Pure PbS is a p-type semiconductor when S-rich, and an n-type when Pb-rich (the non-stoichiometric nature of solids is discussed in Section 27.2). It exhibits photoconductivity and has applications in photoconductive cells, transistors and photographic exposure meters. 

Beck Binomax stereo-microscope mounted over the DSC sample holder. The light detector was a Vickers CdS photoconductive cell which had a large light-sensitive area and a maximum response at 545 nm. The signal from the detector was amplified by a operational amplifier. Both DSC and light detector cell signals were recorded on a strip-chart recorder. 

A photoconductive cell is an important selective detector. Such cells show an increase in conductivity when illuminated with infrared light, and they have high sensitivity with fast response. These cells are used extensively in the spectral region... 

Apart from the liberation of electrons from atoms, other phenomena are also referred to as photoelectric effects. These are the photoconductive effect and the photovoltaic effect. In the photoconductive effect, an increase in the electrical conductivity of a semiconductor is caused by radiation as a result of the excitation of additional free charge carriers by the incident photons. Photoconductive cells, using such photosensitive materials as cadmium sulphide, are widely used as radiation detectors and light switches (e.g. to switch on street llghtiug). 

Near infrared detectors These are usually photoconductive cells which detect infrared radiation in the range 0.8 3.0 p. The sensing element is a semiconductor (germanium, lead sulphide, or lead tellurlde). Upon illumination with radiation of appropriate wavelength, the electrons of the semiconductor are raised to conduction bands. Tills causes a drop in electrical resistance. Consequently, if a small voltage is applied, a large Increase in current can be noted. The resistance of the system is such that the current may be amplified and finally indicated on a meter is recorded. 

FIGUeiS 3- ] 2 Two circuits for transducers whose conductance or resistance is the quantity of interest, (a) The output of the cell is a current proportional to the conductance of the electrolyte, (b) The ratio of ttie resistances of the photoconductive cells is proportional to the meter reading. 

FIQURE 7-27 Relative response of various types of photoelectric transducers (A-G) and heat transducers (H. I) A, photomultiplier tube B, CdS photoconductivity C, GaAs photovoltaic cell D, CdSe photoconductivity cell E, Se/SeO photovoltaic ceii F, siiicon photodiode G, PbS photoconductivity H, thermocouple /, Golay cell. (Adapted from R W. Druse, L. N. MoGlauchlih, and R. B. Quistan, Elements of Infrared Technology, pp. 424-25, New York Wiley, 1962. Reprinted by permission of John Wiley Sons Inc.)... 

Hamamatsu Photonics K.K. CdS Photoconductive Cells. Catalog No. KCDS 0001E03, 1999b. 

Detection of infra-red radiation is by means of its heating effect, and the thermo-couple, the bolometer and the Golay pneumatic cell are each used by different manufacturers. Photoconductive cells of the lead sulphide type, although having a speed of response superior to the heat-sensitive detectors, are not as yet sufficiently sensitive beyond about 7//. 

Near-infrared spectrophotometry did not become feasible for the ordinary laboratory until the lead sulfide photoconductive cell had been installed in commercial instruments. This did not occur until the early fifties. Near-infrared workers are still entitled to feel like pioneers. 

---