When I-V measurements are performed on a solar cell, usually the light is illuminated and electrical excitation(voltage) is also applied. However, when deployed on the field, solar cells are subjected only to optical excitation and electrical bias is not applied. Are the two situations exactly the same? If not , how do we justify using electrical excitation in the lab along with illumination?
Dear Sardahi and Agrwal,
Your discussion about measuring the i-v curves of an illuminated solar cells is fruitful and attracted me.The question of Ananth is interesting and deserve effort to answer it.
The solar cell is two terminal device which converts the solar radiation into electricity.
As a source of electrical power it has a specific i-v characteristics. All of us know the shape of the i-v curve of a solar cell. It is a diode i-v curve shifted by a negative photocurrent such that it lies in three quadrants. The question now how can we measure such i-v curve. There are different methods.
We can use constant voltage source across the terminals of the solar cell. In this case the load line is vertical and intersects the i-v curve in one point which is the operating point. This means that the current will adjust itself in the solar cell. When the applied voltage is such that the solar cell is reverse biased, the third quadrant is measured. When the applied voltage, one acquires the first and the fourth quadrant. In the fourth quadrant where the solar cell out sources electrical power the electrical source connected to the solar cell is said to be an active load as it sink electrical energy getting out of the solar cell.
Only in the 4th quadrant we can use passive loads, ohmic resistances to measure the i-v curve of the solar cell since it is active. In the other two quadrant it is not possible.
In conclusion, active sources can act as loads while ohmic resistances can not be used as active sources.The resulting i-v curve will be the same irrespective of the load used.
My best wishes to you.
Electrical biasing dictates potential drop in external circuit. Whenever solar cell operate in the field there is an external load across the cell where potential drop occurs. In that sense both situations seems same. Also if we look at the characteristics of the cell obtained using the rheostat or variable potential source both gives same results (till open circuit potential don't know whether true beyond that point, may not be true beyond that point). Fundamentally I don't see any difference in two situation (till Voc) however a deep insight is required to say whether both situations are same or different. I will look into the situation and will let you know if there is any difference.
The potential drop across the solar cell when connected to an external circuit is a part fo the energy that is incident in the form of light whereas the energy supplied by biasing a solar cell is in addition to that of the incident light, isn't it?
It is not like that. You see the electrostatic of solar cell. Refer to http://pveducation.org/
What we do with the biasing of solar cell after illumination is oppose of flow of current in the direction of electric field. So we are putting a potential and looking what is happening at that much external potential drop across the cell. The overall potential you can get through a solar cell after illumination is open circuit voltage. You apply a opposing bias say Va, then Voc-Va is the potential drop inside the cell and Va is the drop externally as you have applied that bias. If externally applied bias is more than open circuit potential then actually we are injecting more carriers in the cell as compared to which could come out of the cell because of photogeneration. This situation causes dissipation of power inside the cell and we get no power out of cell while we dissipate some power. So the operating point of cell is always between 0V-Voc. Mostly solar cell operating point is maximum power point which is roughly 80% of open circuit voltage (Voc). For more details about the maximum power point you may refer to site I mentioned above. This site has very basic knowledge of pn junction solar cell. Hope this will help you.
Sumanshu
@Ananth,
I don't have much idea about the equipment specification as my work is mostly theory oriented. However I will let you know after checking the specification of power supply in lab. For a rough I-V characteristic a potentiometer is good to go with, but near the Voc current falls rapidly, so one doesn't get much points using potentiometer. You may need a power supply which can give you much point near Voc and should be able to measure current and voltage at very high resolution.
Dear Sardahi and Agrwal,
Your discussion about measuring the i-v curves of an illuminated solar cells is fruitful and attracted me.The question of Ananth is interesting and deserve effort to answer it.
The solar cell is two terminal device which converts the solar radiation into electricity.
As a source of electrical power it has a specific i-v characteristics. All of us know the shape of the i-v curve of a solar cell. It is a diode i-v curve shifted by a negative photocurrent such that it lies in three quadrants. The question now how can we measure such i-v curve. There are different methods.
We can use constant voltage source across the terminals of the solar cell. In this case the load line is vertical and intersects the i-v curve in one point which is the operating point. This means that the current will adjust itself in the solar cell. When the applied voltage is such that the solar cell is reverse biased, the third quadrant is measured. When the applied voltage, one acquires the first and the fourth quadrant. In the fourth quadrant where the solar cell out sources electrical power the electrical source connected to the solar cell is said to be an active load as it sink electrical energy getting out of the solar cell.
Only in the 4th quadrant we can use passive loads, ohmic resistances to measure the i-v curve of the solar cell since it is active. In the other two quadrant it is not possible.
In conclusion, active sources can act as loads while ohmic resistances can not be used as active sources.The resulting i-v curve will be the same irrespective of the load used.
My best wishes to you.
3 conditions in PV device
1. Unbiased- Unknown Vbi in the device ( A constant Fermi )
2. Biased (Dark)- Lowers the barrier- resulting in charge injection--> electron easily flow from n to p followed by recombination of that electron at the p side either at the contact or at the bulk leading to an electron ejection at the p-side.
*The direction of current (Id) is from P to N
3. Biased (Illumination) -
we are actually reducing the Vbi with applied bias and simultaneously measuring the current,the separation of photo generated carriers takes place till it the drift field gets low (flat band). During the reduction of the barrier, some electrons hops from n-to p also take place. Thats why they used to add J Dark with J light to represent J total (superposition).In this the Direction of current flow is from N to P. The IV shifts down or up when illumination.
With out the applied bias we can not find out the exact operating point (Pmax- Max.V*I) of the device before installing in the field.
.
Bias+ illumination_will help you in finding the Maximum power point (Vocx Isc) which the device can be operated.
It is just the convention that is being used. When we talk about the solar cell we mean it is being operated in illuminated condition and we have light characteristics. We don't want to play with -ve current values while talking about solar cell. So it is represented in the first quadrant. But it is more correct and better understandable to a layman if represented in 4th quadrant.
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