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

[nsaspook]

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/renewable_energy/semiconductors.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]

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]

Hello K^2 can you also please comment on posts #29 ,#30,#32 thanks

 

[K^2]

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 I_L. 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 I_D.

If you shade one of the cells in a closed circuit, it will not have I_L 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 I_L is higher, it doesn't matter which cells are more or less shaded. If one or more of the cells doesn't have I_L high enough to drive 10mA across, the voltage across these cells will drop, and the total current will be limited by the least I_L.

And yes, of course, each cell will have its own I_L and I_D.

 

[ecvolt]

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