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Need help to understand the photovoltiac effect please

  1. Mar 20, 2006 #1
    in the following text i will wright how a photovoltiac cell works based on my knowledge so please read trough it and correct me.

    when a photon hits a solar cell made of 2 layers of n-doped and p-doped silicon a electron in the p-doped silicon recieves the energy from the photon and jumps into the n-doped silicon because of the energy bands. so when the electrons jump into the n-doped silicon that layer gat an excess of negative charges and turns into a negative charge while the p-doped silicon gets an excess of holes so it turns positive. as the electric field created by the semiconductor stops em from directly passing to each other you can connect the sides in an other way so the electrons will flow and you can use that by connecting it to an engine or something.

    is the valance band the p-doped silicon and the conduction band the n-doped?

    why does an electron in the valance band recieve the energy and not in the conduction?

    where exactly does the p-n junction come in here?

    and why/how is the energy field preventing electrons from n-type to p-type to pass created?
  2. jcsd
  3. Mar 20, 2006 #2


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    You have a lot of misconceptions here.

    To start with, as you know, a basic photovoltaic cell is just a large, flat p-n junction. A p-n junction is formed when you slap a block of p-type semiconductor against a block of n-type semiconductor.

    The p-type semiconductor originally has an excess of holes, while the n-type has an excess of electrons. When you put them together, some of the holes jump over to the n-type, while some of the electrons jump over to the p-type. This process can't go on forever, because this movement of charge builds up an electric field between the p-type and n-type semiconductors.

    Any charge carriers (electrons or holes) that happen to be in the region around the junction are swept across the junction by the field. This region is therefore normally devoid of any charge carriers.

    If you shine a light on the junction, however, the photons will promote electrons (or holes) from the valence band to the conduction band. These free carriers are then immediately swept across the junction by the electric field. They form a current, which can be used to charge batteries or run motors.

    No. Both pieces of semiconductor, both p- and n-type, have valence and conduction bands. The "valence band" refers to a range of possible energies, as does the "conduction band." Electrons (and holes) normally exist with energies in the valence band, until they interact with photons. The gain energy from the interaction, and then have energies in the conduction band.

    Photons can give energy to electrons (or holes) and promote them from the valence band to the conduction band. Soon afterwards, those charge carriers are swept across the junction.

    There are far, far more electrons in the valence band than in the conduction band, so its much, much more likely for the photon to hit an electron in the valence band.

    The entire solar cell is nothing but one large p-n junction.


    It's not an "energy field," it's an electric field. Electrons (and holes) are charged particles and feel forces when subjected to electric fields. The electric field presents a barrier (think of it like a hill) which prevents the excess electrons in the n-type from flowing freely over to the p-type and combining with its excess holes. Electrons with enough energy (like those stimulated by photons) have enough energy to "roll up the hill," while most electrons do not have enough.

    - Warren
  4. Mar 21, 2006 #3


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    ok, so the flow of electrons is achieved because the way with the motor is the only way they can get back to the p-type side because of the electric field?
    Last edited: Mar 21, 2006
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