[Zakiir]

Hi,
 
  I'm doing a project on solar energy and was researching up how PV cells work. I wound up at www.howstuffworks.com and was reading through the info they have available there, i understood some of the material until they began describing the "electric field/diode" part. I understand that it is important that this field exists to give direction to the current but i dont really understand how it is formed.
I'll quote:
 "Do all the free electrons fill all the free holes? No. If they did, then the whole arrangement wouldn't be very useful. Right at the junction, however, they do mix and form a barrier, making it harder and harder for electrons on the N side to cross to the P side. Eventually, equilibrium is reached, and we have an electric field separating the two sides."
    ..that just makes...no sence to me...Okay we have the electrons moving from the Nside to the Pside, therefore making the Pside negative and the Nside positive (..er right?) ...and then i guess that makes it more difficult for the remaining electrons on the Nside to move to the Pside because the Pside has less "holes" (..getting more lost ) ...is that what creates the electric field? is that what the electric field is? Why cant all the electrons fill all the holes? Is it because the cell is designed to have an excess amount of electrons?
    ...yeah *stares at floor* ....im lost now...after they talk about the diode being able to push electrons from the Pside to the Nside ...i dont understand how that idea came to being because i dont think they explained why.(or maybe im blind)
Anyway, if anyone can help explain this to me or answer my questions it would be greatly appreciated.

Thanks alot
Zak

[Donaldson Tan]

does anybody (including staff) know? this isn't my area of expertise, but it has been remain unanswered for a long time.

[buckminsterfullerene]

the way that a photovoltaic cell works is by creating an electron current, the N side is silicon doped with 1 part per million of phosphorous, as you know phosphorous has five electrons.  This quality means that phosphorous is an electron donor, and will try to mimic the silicon that sorrounds it.  In the p side of the cell, the silicon is doped at one part per million of boron, an electron acceptor that has 3 electrons, i will too try to mimic the silicon atoms the sorrounds it, generally it will take an electron from the silicon creating a hole.

If i am not mistaken the silicon will attempt to contain 5 electrons from the phosphorous in its orbit, and when light hits the n-layer of the cell, it will knock this electron, since it is not supposed to be there in the first place.  the electron is knocked to the p-layer of the cell, and this will fill in the hole created by the boron on the opposing silicon atom.  but the boron is not supposed to have 4 electrons so the electon will eventually flow back into the n-layer.  The free electrons from the p-layer will move faster accross the "diode" (also called intrinsic layer) then the electrons in the n-layer, since there is pressure from extra electrons in the n-layer slowing the transfer of electron accross the intruinsic layer.

the diode (intrinsic layer) is a few atoms thick and its basically composed of a single silicon and hydrogen crystal.  So a solar cell is composed of a negative layer and a positive layer separated by the intrinsic layer.

The importance of the intrinsic layer is that the electrons flow through this layer.  this electron flow is what makes a solar cell work, its electricity.