Solar Cells in Forward/Reverse Bias?

In summary, according to the discussion, a solar cell is primarily used to generate electrical power, which it does when in forward-bias mode. Reverse bias operation completely defeats the purpose of a solar cell. Photodiodes operate in reverse bias for exactly the reason you're suggesting.
  • #1
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Considering this is for a condensed matter course, I'm assuming it'll belong here :)

I cannot, for the life of me, figure out why solar cells are operated in forward bias as that would counteract the electron-hole separation in the depletion zone. Wouldn't it make more sense to operate them in reverse bias so that the external voltage supports the separation and increases the size of the depletion zone allowing for more absorption in the first place?
 
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  • #2
My understanding of solar cells is more from an engineering perspective, maybe somebody else can comment in terms of electron-hole generation.

That being said, a solar cell is primarily (exclusively?) used to generate electrical power, which it does when in forward-bias mode. In reverse-bias mode it actually consumes power, which is provided by whatever power source is used for the reverse-bias. So reverse bias operation completely defeats the purpose of a solar cell.
 
  • #3
As far as I know, solar cells operate with no bias. Both forward bias or reverse bias would defeat the purpose. They have an external load, and the built in field alone separates the electrons and holes.

Photodiodes operate in reverse bias for exactly the reason you're suggesting. They can detect the maximum number of photons that way. In that case it's perfectly acceptable to supply power.
 
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  • #4
johng23 said:
As far as I know, solar cells operate with no bias. Both forward bias or reverse bias would defeat the purpose.
Yeah, I said it wrong. No applied bias, though the polarity of the generated voltage is the same as for a forward bias.
 
  • #5
That's the part that confuses me. In zero bias, minority carriers are swept across the junction, so the current (holes) flows from the p-side, through the load, to the n-side. Forward bias should allow electrons to flow from n to p if the depletion region is decreased enough, so the direction of the current would switch.

Or is the reason that the polarity is the same as forward bias only because you don't switch the direction of the current until you exceed Voc? But in that case, the polarity of the voltage produced should be the same in reverse bias, zero bias, and forward bias up to Voc.

[PLAIN]http://www.coronene.com/blog/wp-content/uploads/2009/08/Picture-2.png
 
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  • #6
According to the plot you posted, the fourth quadrant is the only one in which power is generated. In the other two (first and third) the solar cell is consuming power.

Above Voc, of course, the solar cell stops working as a generator and start to consume power. You would need an external power source to work in this region.
 
  • #7
The fact that solar cells are operated in forward bias only means that the voltage at the terminals of the solar cell correspond to a forward bias. It does not mean that you need an external power source to bias the solar cell.
 
  • #8
That's just semantics. The point is, the current flows the same way whether you are in reverse bias (not in breakdown), or forward bias (below Voc). Correct? So doesn't that mean the polarity is the same?
 
  • #9
Think about the polarity of the voltage and the current across a resistor, and across a battery.

In the resistor, current flows from the positive terminal to the negative terminal. In the battery, current exits from the positive terminal an goes towards the negative terminal. In the first case, the resistor is consuming power, in the last case, the battery is generating power.

It is the same in the solar cell. The current flows the same way whether you are in reverse or forward bias (below Voc), but in one case you are extracting power from the solar cell and in the other the solar cell is consuming power.

In normal operation, with a solar cell connected to a passive load such as a resistor, you will not excess Voc no matter how intense is the light in the solar cell. You will be working always in the fourth quadrant.
 
  • #10
johng23 said:
That's just semantics. The point is, the current flows the same way whether you are in reverse bias (not in breakdown), or forward bias (below Voc). Correct? So doesn't that mean the polarity is the same?
The direction of the current is the same, but the polarity of the voltage changes. Thus, one goes from generating power (i·v is negative) to consuming power (i·v is positive). As Pete99 said, you want to be in the 4th quadrant of the i-v curve you posted.

BTW thanks for posting that graph, it helps clarify the discussion.
 

1. What is the difference between forward and reverse bias in solar cells?

Forward bias refers to the condition where the positive terminal of a power source is connected to the p-type semiconductor layer and the negative terminal is connected to the n-type semiconductor layer of a solar cell. This allows for the flow of current and the conversion of solar energy into electrical energy. Reverse bias, on the other hand, is when the positive terminal of the power source is connected to the n-type layer and the negative terminal is connected to the p-type layer. In this state, the flow of current is restricted, and the solar cell does not generate electricity.

2. How do solar cells behave in forward bias?

In forward bias, the solar cell operates as an energy converter, converting solar energy into electrical energy. As light strikes the cell, it knocks electrons loose from the atoms in the semiconductor material. These free electrons are attracted to the positive terminal, creating a flow of current. This current can then be harnessed for various applications, such as powering electronic devices or feeding into the grid.

3. What is the effect of reverse bias on the performance of solar cells?

When a solar cell is in reverse bias, it does not generate electricity. In fact, applying reverse bias for extended periods can actually damage the solar cell, reducing its efficiency and lifespan. This is because the flow of current in reverse bias can cause a buildup of heat, which can degrade the materials in the solar cell over time.

4. Can solar cells in reverse bias be used for any purpose?

While solar cells in reverse bias are not suitable for generating electricity, they can be used for other purposes. For example, reverse bias can be used to measure the performance and characteristics of a solar cell, such as its open-circuit voltage and short-circuit current. It can also be used for testing and troubleshooting purposes in the development of new solar cell technologies.

5. How can the forward/reverse bias behavior of solar cells be controlled?

The forward/reverse bias behavior of solar cells can be controlled through the use of external components, such as diodes and resistors. These components can be wired in series with the solar cells to regulate the flow of current and maintain the desired bias state. Additionally, the materials and design of the solar cell itself can also affect its forward/reverse bias behavior, so careful consideration is given to these factors during the manufacturing process.

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