Short Circuit Current in Solar PV Cells

[BeardedW0nder]

I'm reading about short circuit current in a solar PV cell and I am a little confused.

The short circuit current is defined as the current across the solar cell when there is zero potential difference across the cell. At this point the short circuit current is equal to the light current.

What I'm confused about is that there is an inherent potential difference built into the solar cell, right?

As holes and electrons diffuse across the P-N Junction of the solar cell a potential difference builds up, lending way to an electric field. This electric field is what pushes the electrons and holes in their respective directions once the material is struck by an incoming photon. This is motion of electrons and holes is the light current.

So there has to be a a potential difference across the cell for there to be any light current, correct? Does the short circuit current occur when there is a zero APPLIED potential difference?

Any help that could be offered would be great.

 

[OmCheeto]

...
The short circuit current is defined as the current across the solar cell when there is zero potential difference across the cell. ...

Can you provide a link to were you saw this definition. I've a lot of experience with solar cells, and this statement has me confused.

Thanks!

 

[BeardedW0nder]

Sorry I should have phrased that better

The short circuit current is the current PRODUCED by the solar cell when there is zero potential difference across the cell

Energy Systems Engineering Pg 256

http://dualibra.com/wp-content/uploads/2011/06/Energy-Systems.pdf

V= Voltage across Device
"At V= 0, the amount of current produced is the short circuit current"

 

[OmCheeto]

Sorry I should have phrased that better

The short circuit current is the current PRODUCED by the solar cell when there is zero potential difference across the cell

Energy Systems Engineering Pg 256

http://dualibra.com/wp-content/uploads/2011/06/Energy-Systems.pdf

V= Voltage across Device
"At V= 0, the amount of current produced is the short circuit current"

Wow! That is one high end paper. Much beyond my 7th grade maths & physics skill levels.

hmmm...

$$I_{D} = I_0 [ exp (\frac{eV}{mkT}) -1 ]$$

I_D = Dark current, can occur regardless of whether the PV cell is in sunlight or not. (wow. Sounds like it's violating Maxwell's demon!)

hmmm...

Is this statement the one you are basing your comment, "there is an inherent potential difference built into the solar cell, right?" upon?

Sounds a bit fishy to me. Let me check another source.

http://www.uccs.edu/~rtirado/PES_1600_SolarEnergy/fotovoltaic_effect.pdf
page 9
I_D which flows across the device under an applied voltage, or bias, V in the dark

That makes more sense. No "inherent potential difference". Phew!

$$V_{OC} = \frac{mkT}{e} ln [ \frac{I_L}{I_0} + 1 ]$$

m = 1 (a parameter whose value depends on the conditions of operation of the device)
e = 1.602E-19 J/V (elementary charge)
k = 1.38E−23 J/K (Boltzman Constant)
T = temperature (Kelvin)
I₀ = Saturation current (I₀ <<<<< I)
I_L = light current (Current generated by light) (?) (Ha! These high end solar people have a PV language I was never aware of.)
V_OC = Open circuit voltage

Anyways, as I said, this is now way over my head, so I can't help you. But thank you for bringing this up. The paper at uccs.edu answered some old questions I had.

Cheers!

 

[BeardedW0nder]

Wow! That is one high end paper. Much beyond my 7th grade maths & physics skill levels.

hmmm...

$$I_{D} = I_0 [ exp (\frac{eV}{mkT}) -1 ]$$

I_D = Dark current, can occur regardless of whether the PV cell is in sunlight or not. (wow. Sounds like it's violating Maxwell's demon!)

hmmm...

Is this statement the one you are basing your comment, "there is an inherent potential difference built into the solar cell, right?" upon?

Sounds a bit fishy to me. Let me check another source.
That makes more sense. No "inherent potential difference". Phew!

$$V_{OC} = \frac{mkT}{e} ln [ \frac{I_L}{I_0} + 1 ]$$

m = 1 (a parameter whose value depends on the conditions of operation of the device)
e = 1.602E-19 J/V (elementary charge)
k = 1.38E−23 J/K (Boltzman Constant)
T = temperature (Kelvin)
I₀ = Saturation current (I₀ <<<<< I)
I_L = light current (Current generated by light) (?) (Ha! These high end solar people have a PV language I was never aware of.)
V_OC = Open circuit voltage

Anyways, as I said, this is now way over my head, so I can't help you. But thank you for bringing this up. The paper at uccs.edu answered some old questions I had.

Cheers!

I don't think I'm really talking about the dark current.

There definitely is a built in potential difference. It's a inherent quality of a PN juncion...this video (although a bit corny) does a good job of explaining it.

My main points are at 1:30, 3:40, and 4:20 (4:20 they specifically talk about the built in potential)

Towards the end of the video he even draws a diagram that connects the P and N sides that don't touch each other with a resistor. If light is striking the solar cell then current will flow even when there is zero applied voltage to the cell (the short circuit current). At this point by your own listed equations the dark current is negligible, and the light current is equal to the dark current.

The thing that confuses me is that the short circuit current is defined as the current flowing when there is a zero potential difference across the cell, but there is a potential difference across the cell. If there wasn't then there would be no electric field and there would be no current.

I think that the author means that the short circuit current is the current that flows when there is zero applied voltage to the cell. Or is the potential difference so small that it becomes negligible at the ends of the cell?

I guess I'm just looking for someone with a bit better understanding of PV cells and PN junctions to let me know if I'm going in the right direction with this line of thinking. I understand what your saying about dark current, but that doesn't really answer my question.

 

[nsaspook]

Internal to the PV cell (The PN junction to substrate connection resistance Rs) there will not be a short and the current supplied to the external short to the PN junction outputs will behave as if the PV cell was a current source with a shunt diode and resistors.