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Solar cell

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Electronman

New Member
Hello there,

I have information about semiconductors and do know what's doping. Besides I do know how a diode does work at forward and reverse bias.
I do know how a battery causes the depletion region narrow in forward bias or thicker in reverse bias.
Today I read 2 papers about how solar cells do work.
Suppose the light exists and the load is connected to the cell. Now several electrons from the N-side go to the P-side via the load, right?
My main problem is how the cell does work after all free electrons from the N-side go the P-side??
I am not sure but I guess maybe electrons are able to reach to the N-side again after they went to the P-side via the depletion region?
If so, how they do that?? And if no then why the cell generates a for ever voltage when the light exists?

Thanks a lot
 

Electronman

New Member
Thanks,

But after reading 2 papers in the field yet I do not know how the electrons and hole can be restored after they go to the other part (i.e P and N side)???
 

Electronman

New Member
Sorry but it is vague to me yet,
Why you told about applying potential difference (voltage) on the solar cell?
Let me to be clear about my problem:
When you connect a load to a solar cell at the presence of the light the electrons from the N side start to go to the P side through the load, but if something which I do not know what it is does not happen then the whole electrons from the N side go to the P side and fill the whole holes and the current stops but because this does not happen (i.e the load sees current forever till the light exists), so maybe the electrons are able to go to the N side after they went to the P side via the barrier region right? If so, How they are able to pass through that region?? If no, please tell me the reality why this cycle exists forever?
 

dknguyen

Well-Known Member
Most Helpful Member
I was just talking about diodes and PN junctions in general.

Sorry but it is vague to me yet,
When you connect a load to a solar cell at the presence of the light the electrons from the N side start to go to the P side through the load
THis is incorrect. Electrons automatically move from the N-side to the P-side. This happens with or without light because of the electric field produced by the charge imbalance between the positive ions on the P side and negative ions on the N-side. What light does is make the electrons stolen by the P-side from the N-side, return to the N-side. But the PN junction acts like a diode so the electrons can't directly cross over from P-side back to N-side- they have to take a different path (through the load).

But it sounds like you have a few things wrong so I am going to start from scratch...we'll start with a diode first and then turn it onto a solar cell:

DIODE:
1. We two separate chunks of P-doped and N-doped silicon. The P atoms have an excess electrons that are weakly held and the N atoms have an extra holes. The atoms on both sides are neutrally charged at this point because the extra hole balances out the charge of the P atom and the extra weakly held electron balances out the charge of the N atom.
2. Now we take push the two chunks of material together so that they physically touch. Because they touch, electrons can now move back and forth between the P and N silicon.
3. Kinetic energy due to temperature will scatter these weakly held electrons to move around randomly. Some will cross over into the P-side where they will find the extra holes of the P atoms and combine with it. Every time this happens a P-side atom loses the extra hole that kept it's charge neutral and becomes a negative ion, and a N-side atom loses the extra electron that kept it's charge neutral and becomes a positive ion. (THAT'S RIGHT, the extra electrons on the N-side combining with the extra holes on the P-side leaves POSITIVE IONS in the N-side material and NEGATIVE IONS in the P-side.)
4. A electric field is set up between the positive ions in the N-type material and the negative ions in the P-type material which resists the flow of extra electrons from N-side to P-side. As more electrons and holes combine, more ions are formed in the P and N sides and eventually the electric field is strong enough so no more extra electrons can cross over from N-side to the P-side to combine.
5. THe region where these ions are formed are in the area near the PN junction and is called the depletion region. It basically makes it so once enough electrons have cross over from N to P, no more current can flow.
6. But If a potential difference is applied in the right direction, electrons are given to the positive ions on the N-side to make them neutrally charged, and the electrons on negative ions of the P-side are taken away to make them neutrally charged, then the electrons will continue to randomly scatter, crossing over from N-side to P-side to recombine.

SOLAR CELL:
8. Make sure how you understand a regular diode PN junction works before moving on. THat's the hard part. IT's super easy to understand the extra step of a solar cell if you understand that.
9. When light comes in, it knocks free the electrons from the N-side that cross over to recombine with extra holes on the P-side. Because of the ions in the depletion region form an electric field that prevents electrons from flowing straight back to the N-side, the electrons must take another path to return to the N-side. THey take the path through the load. This is what causes the current flow.
10. Once the electrons travel through the load and return to the N-side
11. THe process repeats.
 
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Electronman

New Member
In the last paper which I read several minutes ago, they told that when a photon knocks the atoms in the P side an Electron will be free!!!
P side has immensity of holes so how it is able to have free electrons due to the photon knockings??! I think just the N side has immensity of electrons which can be freed by photons so what's wrong here???!!
Beside they told that freed electrons in the P side can simply go to the N side via the junction barrier! How they are able to do so while there is a neutral region there???!
 

Electronman

New Member
Why do freed electrons in the P side can simply go to the N side via the junction barrier! How they are able to do so while there is a neutral region there???! If the answer is Electric field how it does so???
 

dknguyen

Well-Known Member
Most Helpful Member
In the last paper which I read several minutes ago, they told that when a photon knocks the atoms in the P side an Electron will be free!!!
P side has immensity of holes so how it is able to have free electrons due to the photon knockings??! I think just the N side has immensity of electrons which can be freed by photons so what's wrong here???!!
Beside they told that freed electrons in the P side can simply go to the N side via the junction barrier! How they are able to do so while there is a neutral region there???!

DId you even read my response? It's the first thing I explained in BOTH my replies.

Why do freed electrons in the P side can simply go to the N side via the junction barrier! How they are able to do so while there is a neutral region there???! If the answer is Electric field how it does so???
They don't. You need to understand diodes first. The reason that everything seems backwards is because you skipped studying diodes and started reading about solar cells right away. A solar cell is just a special type of diode. The step from diode to solar cell is very very easy. It's understanding the diode part that is hard. If you don't understand diodes, you won't understand where all the electrons and holes are to begin with and everything will seem backwards.

Read the first half of the explanation in my last reply again, and read this:
 
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