Selenium cell
A selenium cell, also known as a selenium photo element , is a photodiode based on semiconducting selenium that generates an electrical voltage when illuminated. They were the first photo elements . They are rarely used today and have largely been replaced by silicon photodiodes in applications.
construction
Selenium cells consist of a nickel-plated iron base plate on which a polycrystalline selenium layer is applied. A thin layer of translucent cadmium oxide (CdO) is used as a cover .
function
A barrier layer forms between the selenium and a translucent CdO layer . Irradiation with light releases electrons which, due to the influence of the electric field, migrate towards the neutral (space charge-free) area of the CdO layer. The holes migrate into the neutral area of the selenium layer. This creates a tension between the two areas, which can be tapped between the base plate and the cover ring.
Historical applications
The sensitivity of selenium to light was discovered by the Briton Willoughby Smith in 1873 , followed by initial work in Great Britain by William Grylls Adams and others, and in 1883 in New York Charles Fritts built the first selenium cell with a very thin layer of gold instead of the translucent cadmium oxide used later. In contrast to a thin gold layer, cadmium oxide allows greater light transmission and the cell thus achieves a higher level of efficiency.
The selenium cell was used for decades in light meters for cameras , as no additional energy source (battery) was required for exposure measurement. They are mostly hidden behind a diffuser and can easily be distinguished from the photoresistors also used by their larger area. Because of their insufficient resistance to aging , they are no longer used today.
literature
- Gustav Eichhorn: Weather radio, picture radio, television . Springer Fachmedien, Wiesbaden 1926, ISBN 978-3-663-15752-6 , p. 31–35 (reprinted 2013).