photoelectric device[¦fōd·ō·i′lek·trik di¦vīs]
(Russian, fotoelement), an electron device in which an electromotive force (emf) called a photo-emf or an electric current called a photoelectric current is generated as a result of the absorption of the energy of optical radiation that is incident on the device. In English, a distinction is made between two types of photoelectric devices: photoelectric tubes, or phototubes, and photoelectric cells, or photocells. Phototubes are vacuum-tube devices that operate on the basis of photoemission; photocells are solid-state devices that operate on the basis of the internal photoelectric effect of photo-emf generation. The Russian term fotoelement encompasses both phototubes and photocells.
A typical phototube (Figure 1,a) is a two-electrode vacuum-tube device that contains a photocathode and an anode, or electron collector. The electrodes are placed in an evacuated or gas-filled envelope made of glass or quartz. A luminous flux that is incident on the photocathode causes photoemission from the cathode’s surface; when the phototube’s circuit is closed, a photoelectric current that is proportional to the luminous flux flows in the circuit. In gas-filled phototubes, the photoelectric current is amplified as a result of the ionization of the gas and the occurrence of a non-self-sustaining avalanche gas discharge. The most widely used phototubes have cesium antimonide or cesium oxide-silver photocathodes.
A photocell is a semiconductor device with a homogeneous p-n junction (Figure 1,b), a semiconductor heterojunction, or a metal-semiconductor contact. The absorption of optical radiation in photocells causes an increase in the number of free carriers in the semiconductor. The electric field at the junction or contact spatially separates the charge carriers; for example, in a p-n-type photocell, the electrons accumulate in the n-region and the holes, in the p-region. Consequently, a photo-emf is generated between the layers. When the external circuit of a photocell is closed through a load, an electric current begins to flow. Photocells are made of such materials as Se, GaAs, CdS, Ge, or Si.
Photoelectric devices are usually employed as radiation detectors or optical detectors; in this case, photocells are often photo-diodes. Photocells are also used for the direct conversion of the energy of solar radiation to electric energy in solar batteries and photovoltaic converters.
The main parameters and characteristics of photoelectric devices are the luminous sensitivity, the spectral response, the voltage-current characteristic, and the conversion efficiency. The luminous sensitivity is the ratio of the photoelectric current to the luminous flux producing the current at the rated anode voltage (for phototubes) or when the output terminals are short-circuited (for photocells). As a rule, the luminous sensitivity is determined by using standard light sources, such as an incandescent lamp with a reproducible filament color temperature, which is usually equal to 2840°K. For example, the luminous sensitivity is about 150 microamperes per lumen (μA/lm) for a phototube with a cesium antimonide cathode, 600–700 μA/lm for a selenium photocell, and 3 × 10–4 μA/lm for a germanium photocell.
The spectral response gives the optical wavelength range in which a given photoelectric device is sensitive. For example, this wavelength range is 0.2–0.7 micrometers (μm) for a phototube with a cesium antimonide cathode, 0.4–1.1 μm for a silicon photocell, and 0.5–2.0 μm for a germanium photocell.
The current-voltage characteristic shows the relationship between the photoelectric current and the voltage across a given photoelectric device with a constant luminous flux and makes it possible to determine the best operating conditions for the device. For example, phototubes are operated in the saturation region, where the photoelectric current remains practically unchanged as the voltage is increased. For an optimum load, the photoelectric current generated by a silicon photocell illuminated by an incandescent lamp may be as high as several tens of milliamperes per square centimeter of illuminated area, and the photo-emf may be as high as several hundred millivolts.
The conversion efficiency (also applicable to solar cells) is the ratio of the electric power generated by a given device at a nominal load and the incident luminous power. For the best devices, the conversion efficiency may be as high as 15–18 percent.
Photoelectric devices are used in automation and telemechanics, photometry, measurement technology, and metrology, as well as in optical, astrophysical, and space research. They are also employed in motion-picture and photographic technology and in facsimile communication. The use of photocells in the power-supply systems of spacecraft, marine and river navigation equipment, and radio sets is relatively new and very promising.
REFERENCESRyvkin, S. M. Fotoelektricheskie iavleniia v poluprovodnikakh. Moscow, 1963.
Fotoelektronnyepribory. Moscow, 1965.
Vasil’ev, A. M., and A. P. Landsman. Poluprovodnikovye fotopreobrazovateli. Moscow, 1971.
M. M. KOLTUN