Barrier-layer cell

Most modem instruments rely on photoelectric transducers, detection devices that convert photons into an electrical signal. Photoelectric transducers have a surface that can absorb radiant energy. The absorbed energy either causes the emission of electrons, resulting in a photocurrent, or moves electrons into the conduction band of a solid semiconductor, resulting in an increase in conductivity. There are several common forms of these detectors including barrier layer cells, photomultiplier tubes (PMTs), and semiconductor detectors. 

---

Barrier-layer cells 658 Baryta see Barium hydroxide Bases. Bronsted-Lowry theory of, 21 dissociation constants of, (T) 833 hard, 54 ionisation of, 21... 

The detector in a spectrometer must produce a signal related to the intensity of the radiation falling on it. For instruments operating in the visible region a photovoltaic or barrier-layer cell is the simplest and cheapest available. Current produced when radiation falls on a layer of a semiconductor material, e.g. selenium, sandwiched between two metallic electrodes, is proportional to the power of the incident radiation and can be monitored by a galvanometer. Barrier layer cells are robust and are often used in portable instruments but they are not very sensitive and tend to be unstable during extended use. 

Figure 2.25 Barrier layer cell. Radiation absorbed by the semiconductor, often selenium, causes electrons to be released and a small current flows which can be measured using a microammeter.

Barrier-layer cells and photomultipher tubes have both been used for photodetection in atomic absorption spectroscopy. The use of barrier-layer cells of course is limited by their sensitivity and the diflBculty encountered in amplifying their output. They will suflBce where determination of the alkali elements is desired only. For most other work photomultiplier tubes are necessary. These are available for a broad spectral... 

It is used for decolorising glass and in photoelectric cells. The electrical conductance of Se in the metallic form is increased as much as 200 times by light. Another application is in the iron-selenium barrier-layer cell which generates a current when illuminated. This system also operates as a rectifier because current flows more readily from iron to selenium than in the opposite direction when an external potential is applied. 

Photodetectors are devices that convert light into an electric signal that is proportional to the number of photons striking its photosensitive surface. The photomultipfier tube is a commonly used photodetector for measuring light intensity in the UV and visible regions of the spectrum. Photodiodes are solid-state devices that are also used in modern instruments. In older instruments, barrier layer cells (also known as photovoltaic cells) were used as photodetectors, because they were rugged and less expensive. ... 

Barrier-layer cells Phototubes Photomultiplier tubes... 

Three types are used. (I) Barrier-layer cells. These are satisfactory only for simple filter instruments. (2) Vacuum phototubes. These tubes require an external power supply, unlike barrier-layer cells, and their output is usually amplified before measurement. (3) Photomultiplier tubes are easily the most satisfactory detectors for use in flame photometry. The photocurrent is amplified inside the tube in such a way that much lower light levels can be detected and measured accurately than is possible with vacuum phototubes with amplifiers. A stable source of high voltage up to perhaps 2000 volts is required to operate the photomultiplier tubes, but these tubes are almost universally used in high-performance instruments and are essential if the advantages of using narrow band width are to be obtained. 

FIGURE 7-26 Schematic of a typical barrier-layer cell. 

In a barrier layer cell, also called a photovoltaic cell, a current is generated at the interface of a metal and a semiconductor when radiation is absorbed. For example, silver is coated onto a semiconductor such as selenium (see Figure 5.17) that is joined to a strong metal base, such as iron. To manufacture these cells, the selenium is placed in a container and the air pressure reduced to a... 

A photodetector such as a barrier layer cell or a photomultiplier tube. 

---