# Why does the value of shunt resistor need to be very high for solar cell's efficiency?

#### PhysicsEngineer

Joined Jun 13, 2021
3
In the circuit equivalent of a solar cell, shunt resistor is described as "The irregular polycrystalline lattice grain boundaries that resist to the flow of electrical current in the silicon material."

If this explanation is correct, shouldn't it be "lower shunt resistance increases the current flowing".

However, the shunt resistor is connected in parallel to diodes. That means "higher the shunt resistor better the current output".

How and why so? I thought not having many grain boundaries is what makes the monocrystalline cells more efficient than polycrystalline solar cells?! https://www.scirp.org/html/7-6401007/fe85a7b6-645d-4341-8f35-dde69e519017.jpg #### PhysicsEngineer

Joined Jun 13, 2021
3

#### MrAl

Joined Jun 17, 2014
8,548
Isnt it obvlous? If you connect one load to a circuit you draw some current and therefore use up some of the available power, then if you connect a second load the second load draws some current and in the case of a non ideal voltage source also brings the voltage down. The voltage decrease also affects the first load so now the first load sees less power.
The first load above is like the actual load you would connect to your solar cell and the second load above is like the shunt resistance. The shunt resistance absorbs some of the power and pulls the voltage down making less power available for the actual load. A high shunt resistance will have less effect than a low shunt resistance.

• anniel747

#### PhysicsEngineer

Joined Jun 13, 2021
3
Isnt it obvlous? If you connect one load to a circuit you draw some current and therefore use up some of the available power, then if you connect a second load the second load draws some current and in the case of a non ideal voltage source also brings the voltage down. The voltage decrease also affects the first load so now the first load sees less power.
The first load above is like the actual load you would connect to your solar cell and the second load above is like the shunt resistance. The shunt resistance absorbs some of the power and pulls the voltage down making less power available for the actual load. A high shunt resistance will have less effect than a low shunt resistance.
They say that shunt resistance is the grain boundaries that resists the electron flow in the doped silicon material. This resistance should have been showed as a series resistance not parallel resistance. Since increase in the parallel resistance will increase the output current however, the grain boundaries=shunt resistance should lower the current output.

#### MrAl

Joined Jun 17, 2014
8,548
They say that shunt resistance is the grain boundaries that resists the electron flow in the doped silicon material. This resistance should have been showed as a series resistance not parallel resistance. Since increase in the parallel resistance will increase the output current however, the grain boundaries=shunt resistance should lower the current output.
Hi,

Who says that? Note i am not saying you are wrong, just wondering who said that.

It makes sense to me that there might be some series resistance involved, but there are a couple reasons why we may not be talking about the same thing.
For one, when we use the phrase "shunt resistance" it usually means some sort of parallel resistance.
Second, the lone sell behavior is often modeled as a constant current source with a given constant current level as the result of a given insolation level. With no shunt resistance the current available to the actual connected load would be maximum while with an added shunt resistance the current available to the actual connected load would be less than maximum so the effect of adding a shunt resistance would be to lessen the output current which should be able to model the imperfections in the cell itself.
Now this does not mean that the shunt physically models the cell, but it does seem to result in a decent behavioral model.

To get this effect with a series resistance we would need to model the cell as a voltage source which isnt usually the case, but i wont cross that possibility out entirely.

• anniel747

#### neonstrobe

Joined May 15, 2009
178
Grain boundaries in polysilicon cells create both a series resistance and a source of recombination which will act as a shunt load to any light-generated current. Both are voltage dependent (as are most diodes) but the series resistance can be approximated by an internal lumped value, calculated from say near maximum load conditions. The leakage effect of the recombination might be simulated with a parallel diode or lumped (hopefully highi) resistance for a given light condition. For each case the idea of a simple resistance is basically a simplification of this quite involved physics.

• nsaspook and anniel747

#### MrAl

Joined Jun 17, 2014
8,548
Grain boundaries in polysilicon cells create both a series resistance and a source of recombination which will act as a shunt load to any light-generated current. Both are voltage dependent (as are most diodes) but the series resistance can be approximated by an internal lumped value, calculated from say near maximum load conditions. The leakage effect of the recombination might be simulated with a parallel diode or lumped (hopefully highi) resistance for a given light condition. For each case the idea of a simple resistance is basically a simplification of this quite involved physics.
Ok very good, and that is a physical description of the cell which apparently you would like to use to create a physical model. The kind i was talking about is behavioral which is meant to be less complex yet provide similar results in a simulation.

• anniel747