Yash
The I-V characteristics of a solar cell define its performance and efficiency.
Solar cell absorbs the sunlight and converts it into electricity.
Structure of a solar cell
A solar cell is made of two types of semiconductors:
- P-type
- N-type
Silicon is a semiconductor device that has 4 electrons in its outermost shell.
A P-type semiconductor is made by doping tri-valent impurity such as boron into the silicon.
It has one electron less than required by the silicon to complete the octet.
Hence an electron vacancy or a hole is created.
Now, it becomes an acceptor of electrons.
The N-Type semiconductor is made by doping pentavalent impurity such as phosphorus.
Phosphorous has 5 electrons in its outermost shell.
4 electrons are involved in making bonds with the silicon.
The one-electron left is not involved in the bonding.
Hence, it is free to move inside the structure.
The resultant becomes more in electron concentration. When the P-type and the N-type are combined.
The holes (p-type) and the electrons (n-type) near the junction start diffusing with each other.
There reached a stage when no further diffusion takes place.
This becomes the depletion region.
The p-type side where initially holes were present become negatively charged electrons.
And the n-type side near the junction where electrons were present now contains holes.
This creates an internal electric field that prevents the electrons from the n-type to combine with the holes in the p-type.
Hence this diffusion process stops.
Now no more electrons and hole transfer take place from one place to the other.
The open-circuit voltage (Voc) of a solar cell
After the formation of the depletion region, the electrons need the energy to cross this band gap.
And this energy comes when the photons of light fall on the depletion region.
The electrons and the holes within the depletion region get excited and start accumulating at each end of the solar cell.
Electrons get accumulated on the n-type layer and the hole concentration increases at the p-type layer.
Due to this, the potential starts building up.
At the same time, the accumulation of electrons at one point starts building the repulsion force.
(When the same charge electrons accumulate at one place, the repulsive force starts building up).
Thereafter reaches a saturation point when the energy gained by the electron in the depletion region is balanced by the repulsive force built at the n-type layer.
At this time, the electron is bounced back to the depletion region.
And no more transfer of electron takes place.
This is the point when we see the maximum potential being developed at the ends of the solar cell.
And we call this open-circuit voltage.
Thereafter, no more increase in the sunlight intensity causes any change in the potential across the cell.
Affect of sunlight intensity on Voc
Well, I would that it depends on sunlight intensity to some extent.
When photons of light fall on the PN junction, the current start flowing and the potential starts building up.
But after reaching the saturation point, any further increase in the sunlight intensity does not have any effect on the Vօс of the solar cell.
Temperature dependence
However, the open-circuit voltage reduces with the increase in temperature.
The increase in the temperature causes the shrinking of the depletion region of the solar cell.
Causing electrons to gain more energy.
As a result, the potential developed is reduced.
The Short circuit current (Isc) of the solar cell
It is the maximum current that a solar cell can produce.
When the positive and the negative terminal of the cell are joined.
It has zero resistance.
When the resistance is zero.
All the electrons accumulated at the n-type layer, start flowing through the wire and recombine with the holes in the p-type layer.
There is no potential developed.
Hence the potential is zero across the cell.
You can see in the graph above that when I draw perpendiculars from Isc and Voc respectively.
I get a rectangular-shaped area.
This is the maximum ideal area under the I-V graph.
All the other values lie inside this area.
Dependence of Isc on the sunlight intensity
When more photons fall on the solar cell, more electrons are released per unit of time.
This is current and hence more current is produced and the short circuit current has a direct relationship with the sunlight intensity.
Plotting I-V characteristics of a solar cell
Thereafter, I place a very small resistance across the cell.
Now, the electrons feel small resistance and very electrons can’t re-combine the holes.
Therefore, a small potential is developed.
However, the current is still large but less than the short circuit current.
Let me show you diagrammatically.
After extrapolating the current and voltage values in the I-V graph.
I draw perpendiculars from these values in such a way that they intersect at one point.
We get a rectangular figure.
The area under this figure shows the power at the given voltage and current values.
Thereafter, I keep increasing the load.
I get different current and voltage values.
The area under these values gives me different power values.
Going like this there comes a set of voltage and current values when their product is the maximum.
This is called the maximum power.
- Vmp: It is the voltage when the power produced is the maximum.
- Imp: Similarly, the current at this point is called the current at the maximum power.
This is a value that we get to see at the back of the solar panel called the datasheet.
These values are found when the sunlight intensity is 1000 W/m² and the cell temperature is 25ᵒ Celsius.
By looking at this graph, we can find the performance of a solar cell by knowing the term fill factor.
It is the ratio of the product of Vmp and Imp to the product of Voc and Isc.
I can write mathematically as:
Fill factor = (Vmp x Imp)/(Voc x Isc)
It varies between 0.7 to 0.8 for most the solar cells or the solar panels.
A solar cell with a fill factor of 0.77 is more efficient than a solar cell having a fill factor of 0.75.
Read: The most efficient solar panels in the world
Parallel and Series connection
when solar cells are connected in parallel, the current produced is added.
While the Vmp remains the same.
Another hand, when the solar cells are connected in series, the voltage adds up and you get double of Vmp as that of the single cell.
And the Imp remains the same.
Key takeaways
- A solar cell converts sunlight into electricity
- Voc is the maximum voltage that a solar cell generates when its terminals are not connected to the load.
- Isc is the maximum current that a solar cell produces when its negative and positive terminals are connected.
- The Voc reduces with the increase in the temperature
- Isc has a direct relationship with the sunlight intensity. An increase in the sunlight intensity causes more electrons to flow and the flow of electrons per unit of time constitutes an electric current.
- The area under the I-V graph represents the power produced by the solar cell
- Vmp and Imp are the voltage and the current values when the power produced by the solar cell is the maximum. These values are taken under STC when sunlight is 1000 W/m² and the cell temperature is 25ᵒ Celsius.
- A solar cell with a high fill factor is more efficient than the one having a low value. Hence produces more current and improves the financial feasibility of the system.
- When cells are connected in series, the voltage adds up. When the cells are connected in parallel, the current adds up.
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