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How does a photocell work?

  1. Apr 9, 2005 #1

    I just have one question, How does a photocell work?
  2. jcsd
  3. Apr 9, 2005 #2
    Well, it works due to the photo-electric effect : the conversion of an EM-wave (light) into an electric current. It is this phenomenon that Einstein explained in terms of quantized energy (of the target atoms of the electrode, NOT the EM-wave) and earned him his Nobel Prize. It also suggested for the first time that EM-waves could be seen as a stream of particles called photons. Einstein did not invent the name photons and i must say that this effect did NOT prove the particle-behaviour of waves, it just suggests it. This is a common misconception.

    The incident light knocks lose conduction electrons from the target electrode. This electron will have enough energy to move to the surface of the electrode and it will then be 'ejected from' that electrode, yielding an electric current. The conduction electrons are bound to the constituting atoms of the electrode with less energy, so it is easier to knock them lose.

    These electrons are the valence (ie the outer electrons) electrons of the constituting atoms. When you put many atoms together, their wavefunctions (more specifically, their orbitals) will overlap and this yields a 'continuous' region over the atom-lattice, that can be occupied by these electrons. they are not localized to one atom but they can move 'freely' over the entire atomic-lattice.

  4. Apr 10, 2005 #3
    Thank yo marlon. :smile:
  5. Apr 10, 2005 #4


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    If photocells lose electrons in the process of use, they get old from too much light knocking off electrons. Correct? Because photcells lose more and more electrons the longer they are used, they get more postively charged he longer they're in use? Do photocells last long enough or have lasted longe enough to experiance these things?
    Last edited: Apr 10, 2005
  6. Apr 12, 2005 #5
    That is a good question, though i don't really know the exact answer. Clearly , you will need some kind of battery to 'refresh' the photo-cell.

    Perhaps somebody else will know more on this...Again good question though


    I think this is a question for ZapperZ :wink:
  7. Apr 12, 2005 #6


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    OK, ok.. I'll step in....

    The semiconductor is usually grounded, meaning it is being replenished with electrons that either were photoemitted, or the ones that got "conducted" away. If not, you will have charging effects on the cell and it will stop functioning, because the effective work function will just build up due to charge depletion.

  8. Apr 13, 2005 #7
    Ok, so , does this mean that a photo-cell reaches saturation ( at one point, the rise of the power of the bulb-light which lights the cell does not rise the intensity of the current produced by the photo-cell anymore ) because it is not perfectly grounded ?
    I worked with a device which was supposed to measure the power of various laser diodes using a photo-cell.It was very unpleasant that there was a maximum power that the device was able to measure.
  9. Apr 13, 2005 #8


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    If it is done continuously, yes, it is entirely possible. Or, you have a rather inefficient diode with very few charge carrier density. Or the bias voltage to detect photocurrent isn't large enough, etc, etc. It depends on the photocell, and what KIND it is.

    Either way, most manufacturers will indicate the working range of any photocell. And this is not just with respect to power, but also what freq. range, what temperature, etc...

  10. Apr 13, 2005 #9
    Thanks ZapperZ.
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