Photocell

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Photocell, length approx. 90 mm; the anode is a wire bail, the photocathode is formed by the glass bulb coated with metal on the inside

With a photocell or photocell , the intensity of light with a suitable wavelength can be measured. It consists of two electrodes in an evacuated glass flask and, in a broader sense, counts to the electron tubes . The incident light accelerates electrons out of the photocathode due to the external photoelectric effect . If these electrons hit the anode, they are diverted and can be measured as a photocurrent. Otherwise, they will be attracted to the cathode again.

The photocell was invented in 1893 by Hans Geitel and Julius Elster . It has largely been replaced by optical semiconductor sensors. For the measurement of very low light intensities , the photocell has been further developed by integrating a secondary electron multiplier into a photomultiplier (PMT).

Demarcation : Photoreceivers built from semiconductors are among the semiconductor detectors . B. photodiodes , photoresistors or solar cells - these are not referred to as photocells.

construction

Circuit diagram of a photocell

A photocell consists of two electrodes in a mostly airless glass case. The two electrodes differ in structure and arrangement:

  • The cathode consists of a metal from which electrons can be released by light if the energy of the light is sufficiently high (external photoelectric effect ). For this reason it is also called the photocathode . The work function can for. B. by coating with cesium , or a cesium compound to make the photocell sensitive to longer wavelengths of the visible spectrum.
  • The anode is usually a wire ring that should not be hit by light. The anode is supposed to collect the electrons released from the cathode. So that they can no longer leave the wire ring if it is struck by light, it is made of a metal with a particularly high work function such as copper .
  • When a high current is required, the glass vessel can be filled with diluted gas. Impact ionization can then lead to an avalanche effect, which noticeably increases the measurable current.

Operation with suction voltage

Fig. 3: Current-voltage characteristic of a photocell. Dashed: saturation currents for 3 different light intensities

If a voltage is applied between anode and cathode, whereby the positive pole of the external voltage source is connected to the anode and the negative pole to the cathode, the electrons released by the light are accelerated towards the anode and an electric current (photocurrent) of a few microamps.

  • At a low voltage of a few volts, the photocurrent is roughly proportional to the applied voltage. At low voltages, the electric field strength between the cathode and anode is not sufficient to suck all the electrons emerging from the cathode through the anode and thus to contribute to the photocurrent. The others "fall" back onto the cathode. Cause: When an electron leaves the cathode, the cathode is positively charged. Opposite electric charges attract each other.
  • At higher voltages the photocurrent increases up to a limit value, one speaks of saturation . Then all the electrons that are released from the photocathode by the light are sucked off the anode. If the applied voltage is increased further above about 100 V, the current does not increase any further. This setting is chosen, for example, with the photomultiplier when you want to detect extremely low light intensities and no electron may be lost.

Operation with counter voltage

see main article Photoelectric effect # External photoelectric effect

If no voltage source is connected to the photocell and it is illuminated with light of sufficiently high frequency (and thus energy), a low, hardly resilient voltage of about one volt develops between the anode and cathode during exposure. The photocell works as a power source because some of the electrons released from the photocathode land on the anode and can no longer return to the cathode. The anode is therefore charged negatively and the cathode positively. This photo voltage increases with the frequency of the incident light.

This operating mode is only selected if the external photoelectric effect is to be demonstrated, for the explanation of which Albert Einstein received the Nobel Prize.

swell

  1. ^ Kurt Jäger, Friedrich Heilbronner: Lexicon of electrical engineers . 2nd Edition. VDE-Verlag, 2010, ISBN 978-3-8007-2903-6 , p. 117 .
  2. ^ Dieter Meschede: Optics, light and laser . Vieweg + Teubner, 2008, ISBN 978-3-8351-0143-2 , p. 392 ( limited preview in Google Book search).
  3. ↑ Dieter Geschke, Physikalisches Praktikum, Vieweg + Teubner, p. 257.

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