The I-V curve deviates from that of an ideal diode curve and the FF increases more than that of its initial value (almost doubles). Are such values of FF accurate? Do we need to make correction before using them to calculate efficiency? If yes, How?
I think you should check the value of FF. As degradation occurs, value of FF should decrease. By definition, FF= Vm*Im/Voc*Jsc, where Vm and Im are the voltage and current at maximum power point respectively. So if degradation occurs, the product of Vm*Im should decrease which will reduce the efficiency of solar cell. You can check the value of efficiency and see the effect of degradation on your solar cell. Sometimes, FF increases slightly as Voc and Jsc also decreases with degradation. However,in your case, degradation almost doubles the FF which is an anomalous result. Therefore, you should check your J-V or I-V curve or reproduce the J-V curve and others related data with standard procedure.
Best Regards,
Uday
If the FF is being limited by the series resistance of the cell, you can get an interesting effect, that as the cell ages its output voltage and current drop, but the resistive losses actually drop faster (I^2 R) so the FF appears to increase but only because the losses are less. Could this be happening to your cells? Look at the actual peak power your aged cells generate - they will definitely be a lot less than new ones. This increase in FF is an artifact of measurement which argues against using FF as a meaningful indicator of a cells usefulness in practice.
Roja!
Uday and Tony gave possible causes for the increase of the fill factor. I would like to notice that the FF can increase appreciably as in your case where the mainly the detraction of the output power of the cell after aging will be due to an appreciable decrease in the the short circuit current and the open circuit voltage.
My expectation is that your series resistance is highly nonlinear such that it decreases rapidly with the decrease of the level of the short circuit current which means it became appreciably smaller. As a symptom for that effect the solar cell I-V curve gets more square in shape than before the aging.
You can even display the i-v curves before and after the aging, you will find that the i-v after aging will be more square than that before aging.
Best wishes
I think that a fill factor which nearly doubles after degradation is nothing but an irrelevant number -- because it is meaningless.
Tony Maine pointed it out already: Look at the actual peak power reached by your aged cells. Just if it is NOT a lot less than that of new ones, only then it's worth to have a look at the fill factor (or at any other cell property) -- because only then there is no reason to throw such cells away immediately and start right over.
But let me come to another point: What do you mean by the following question: "Do we need to make correction before using them to calculate efficiency?" Please go more into detail about this aspect. (I just want to make sure everything goes the right way.)
It sounds very surprising to me that the FF can almost double. This either means the FF was from the beginning very low (
Dear Roja,
In case of by aging time, your spiro may be doped more and can improve your FF as a results. But having double improvement is hard to understand it.
Could you show your I-V curve? I thick there is something weird on your I-V curve I guess.
Thanks for the reply everyone. Yes Michiel, As I go on measuring my device in high humidity condition, the Jsc value decreases tremendously and my curve gets bumpy. My FF has increased from 50-55% to almost 81.5%. Randi, please find my initial and a few days old I-V below. After a week or 10 days things get even worse. Please let me know what I am doing wrong.
Looking at your J--V curves I'd say your FF has decreased, because after degradation (i) the high-voltage part is less steep and (ii) the mpp current differs more from the maximum photocurrent -- which after degradation is not the short-circuit current anymore but the one at about 0.4 volt.
Obviously, the latter effect (the short-circuit current not being the maximum photocurrent) is the most important problem to clarify. I remember that we have discussed here about a similar observation already (cf. https://www.researchgate.net/post/I-V_curve_strange_shape_of_a_PV_cell_whats_wrong_with_it); maybe you get some ideas from that.
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