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What is going on when a solar cell goes into reverse bias?

by ecvolt
Tags: bias, cell, reverse, solar
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ecvolt
#19
Dec7-12, 02:26 PM
P: 23
So we have two currents light generated current and diode current that are involved in the circuit ?Is the light current generated by the photons and is the diode current derived from voltage drop over the solar cell which is acting like a diode
K^2
#20
Dec7-12, 02:30 PM
Sci Advisor
P: 2,470
Correct. Keep in mind, though, that this is a description from equivalent circuit, where the current source and the diode are treated as two separate elements.

In reality, both processes are happening within the same diode that makes up the photovoltaic cell. That means that you won't really have two separate currents flowing two separate paths, but rather a net current that looks like the difference of the two.
ecvolt
#21
Dec7-12, 02:37 PM
P: 23
OK I am getting there thanks to you!!!!! Could i contact you with my personal e mail address?
ecvolt
#22
Dec7-12, 02:56 PM
P: 23
OK if we have a closed circuit of solar cells in series with a inverter in the circuit would the inverter constitute the load in the circuit ?
ecvolt
#23
Dec7-12, 03:05 PM
P: 23
I sent you a private message but it came back and said your box is full thanks ecvolt
K^2
#24
Dec7-12, 04:28 PM
Sci Advisor
P: 2,470
Yes, inverter will act as a load, but its effective resistance will depend on what you connect to it.
ecvolt
#25
Dec7-12, 05:10 PM
P: 23
would you happen to know were a electrical contractor such as myself could hire a on-line tutor for the subject we are talking about thanks ecvolt
ecvolt
#26
Dec7-12, 05:31 PM
P: 23
does the current flow in the diode have to do with the minority carriers through the pn depletion zone
ecvolt
#27
Dec7-12, 10:08 PM
P: 23
question#1 If cell number ten is shaded than Il would be zero for that cell is that correct ? question #2 What would be the Id for cell number ten ? question #3 Also if Il is zero for cell ten than Il would be zero for the other nine cells is that correct?Lets assume the cells are in a closed circuit with th 100 ohm load
K^2
#28
Dec8-12, 12:23 AM
Sci Advisor
P: 2,470
Quote Quote by ecvolt View Post
does the current flow in the diode have to do with the minority carriers through the pn depletion zone
No. The diode current occurs when it's in forward bias, which happens if the voltage across the cell is high enough. I mean, realistically, yeah, you have to deal with minority carriers, etc. But you can do a very good estimate by ignoring all of that. You can assume the diode to be perfect, with absolutely no current flowing unless you have sufficient forward bias.

question#1 If cell number ten is shaded than Il would be zero for that cell is that correct ? question #2 What would be the Id for cell number ten ? question #3 Also if Il is zero for cell ten than Il would be zero for the other nine cells is that correct?Lets assume the cells are in a closed circuit with th 100 ohm load
1) Yes.
2) In the same situation and with other cells illuminated? Nothing. The diode will be reverse biased, and so no current will flow through it.
3) No. The net current through each cell will have to be zero. But the IL depends only on the amount of light a cell receives. If no current can flow, then ID must be equal to IL so that net current is zero. Like I said, you can think of it as a relief valve.


As for tutoring, check at local universities. There are usually graduate students who need money and are happy to tutor. Electrical engineering grad student is probably your best bet. But you might be able to find somebody in physics as well.
ecvolt
#29
Dec8-12, 09:21 AM
P: 23
OK then #1 each of the ten individual cells will have there own Il an ID? #2 When the ten cells are connected in series the cell with the least amount of iradiance will determine the amount of current flow in the CIRCUIT? #3 If cell number ten is completely shaded and cells 1 through 9 are not and all cells are connected in a series string that would be the equivalent of connecting diodes 1 through 9 in a forward bias{anode to cathode } BUT cell number ten would be the equivalent of a diode in reverse bias so cell number 10 would be the equivalent { we don't physically change it}connecting the cathode of cell nine to the cathode of cell 10?#4 Due to the reverse bias created by cell number 10 the current will stop flowing in the circuit because cell number ten acts as a one way valve blocking the flow? Thank you
ecvolt
#30
Dec8-12, 11:37 AM
P: 23
Thank you again for hanging in there with me on this subject!!!It was extremely hard for me to find some one to talk to me about this subject so i am very gratefully for your time and Patience. One concept I am trying to grasp is What is creating the voltage on the solar cell to forward bias the cell???????? I understand if you connect a battery (which is a voltage source) to a diode you can create either a forward bias ( battery positive to diode anode and battery negative to diode cathode) or a reverse bias (Battery negative to diode anode and battery positive to diode cathode) But what I don't understand is a photo cell is a Current source NOT a voltage source.How is the voltage established across the solar cell??When we say solar cell are we relay talking about the voltage across the PN junctions depletion zone which was established when we introduced the doping of impurities into the silicon solar cell.Were is the voltage coming from since we are not putting a battery to the solar cell Thank you
nsaspook
#31
Dec8-12, 12:16 PM
P: 643
A (non-perfect) current source driving any load with a greater than zero resistance will have a voltage. It's simple ohms law for circuits.

http://www.eere.energy.gov/basics/re...onductors.html

To separate electrical charges, crystalline silicon cells must have a built-in electric field. Light shining on crystalline silicon may free electrons within the crystal lattice, but for these electrons to do useful work—such as provide electricity to a light bulb—they must be separated and directed into an electrical circuit.
ecvolt
#32
Dec8-12, 12:34 PM
P: 23
Thank you.can you tell me if the questions #1 through# 4 in post #29 are true or not.Also I believe the force field across the depletion zone which is free of charged carriers is were the voltage of the solar cell is established thanks ecvolt
ecvolt
#33
Dec8-12, 01:40 PM
P: 23
Hello K^2 can you also please comment on posts #29 ,#30,#32 thanks
K^2
#34
Dec8-12, 04:10 PM
Sci Advisor
P: 2,470
Picture a capacitor. Imagine that every time a photon comes in, you take an electron from one plate, and move it to another. Always in the same direction. Can you see that electrons being moved that way give you a current? That's your IL. Can you see how the charges displaced that way will give you a voltage across said capacitor? It's kind of like that fora cell.

However, if you don't connect such a cell to anything, as more and more electrons are moved across, and none of them have a way to return, the voltage will grow without a bound. A real solar cell is a diode, which will act as a relief valve. Once the voltage across the cell gets too high, it allows electrons to flow back through it until the voltage is normalized. That return current is the ID.

If you shade one of the cells in a closed circuit, it will not have IL to build up the voltage. In fact, it will not be able to carry charges across at all. So the charges trying to go around the circuit and return to the other 9 cells will bunch up on cell 10 and create the potential difference in the wrong direction, reverse-biasing the diode. All 10 are still connected anode to cathode. It's the opposite sign of voltage differential that will cause cell 10 to have reverse bias. The diode doesn't flip.

For example, if your 9 cells have voltage of 0.5V across each, and you closed the circuit across some load with cell 10 being shaded, voltage across cell 10 will be -4.5V, because the voltages going around a closed loop in a circuit have to add up to 0V. Since there is no current through the load, there is no voltage across it. On the other hand, when all 10 cells receive enough light to drive a current, you get 0.5V across each cell, and -5V across the load.

And again, the least shaded cell only limits the current if the load does not. See the first example I showed with 17mA and 12mA. The 12mA would be the limiting current, but because the entire circuit only takes up 10mA, it doesn't matter. If the total current through your load is 10mA, then total current through each of the cells is 10mA, so as long as each IL is higher, it doesn't matter which cells are more or less shaded. If one or more of the cells doesn't have IL high enough to drive 10mA across, the voltage across these cells will drop, and the total current will be limited by the least IL.

And yes, of course, each cell will have its own IL and ID.
ecvolt
#35
Dec8-12, 05:12 PM
P: 23
K^thank you ever so much!!! I just ordered a book called Applied Photovoltaics third edition by Stewart R Wenham.I am gone to read it front to back in hopes to understand the subject of how the solar cell works. YOU HAVE GIVEN ME A VERY GOOD EDUCATION ON THIS SUBJECT!! I am sure I will have some more questions in the future and perhaps you would look at them THANK YOU ecvolt


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