Solar Cells in Forward/Reverse Bias?

DarkReality

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?

Redbelly98

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.

johng23

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.

Redbelly98

Yeah, I said it wrong. No applied bias, though the polarity of the generated voltage is the same as for a forward bias.

johng23

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.

Pete99

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.

Pete99

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.

johng23

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?

Pete99

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.

Redbelly98

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.

Phrak

http://en.wikipedia.org/wiki/Photodi...tovoltaic_mode