1- Fill factor will be equal one for ideal solar cell with no losses due to series of shunt resistances, which means practical solar cell will always has a fill factor lower than one.

2- Fill factor means how percent from the theoretical power generated within the solar cell (Isc X Voc) can be  utilized as a real power (Pmax).

Power is defined as IxV. If you take a look at a cell's IV curve you'll see that at the start I has it's maximum value (Isc) and voltage is at 0. There is a slight drop in current while voltage increases, followed by a steep decrease in current and an increase in voltage. Voltage reaches it's maximum value (Voc) when current is at 0. Somewhere in between the product of IxV reaches the maximum value, which by definition is the maximum power point.  At that point you have your Impp (current and maximum power point) and Vmpp (Voltage at maximum power point).

The fill factor of a PV cell is the maximum power that a cell has at Impp and Vmpp  divided by the amount of power it could have if the current and the voltage were at their maximum value at the same time. The maximum voltage you can have in a cell is the Voc (open circuit voltage) and the maximum current you can have is the Isc(short circuit current).

Take a look at the pic and you'll see that that the fill factor is the area defined by ImppxVmpp divided by the area defined by IscxVoc, therefore it can never be more than 1

I agree with Amr. The series and shunt resistance of a solar cell result in reduced Pmax. In general, series resistance depends on the cell processing step while the shunt resistance depends on the crystallization step. In practice, my experience is that the fill factor is dominantly affected by series resistance. In other words, fill factor is always less than 1.

An important point to add to what has been said by the colleagues. Even if  the series resistance RS= zero and the Shunt Resistance Rsh is infinity, the fill factor will be still less than one because of the recombination of the photo generated electron hole pairs in the solar cell diode. So, in this case the fill factor will be affected by the diode ideality factor n. As n increases the curve will be less square and the fill factor decreases. The smallest value for the ideality factor is one.

Another important factor affecting the fill factor is the reverse saturation current of the solar cell diode. As the reverse saturation current decreases the squareness of the iv curve increases and  so does the fill factor.

wish you success

Thanks Abdelhalim for the reminder about ideality factor. However, since a small increase in ideality factor has a detrimental effect on the performance of the solar cell, a typical commercial solar cell has ideality factor very close to one. Solar cells with ideality factor larger than one often fail the final quality check at the end of the cell processing line and never find their way to the market. It should also be mentioned that the fill factor also depends on the temperature. Therefore, it is customary to work with normalized Voc. That is NVoc=Voc/n*Vth, where n is the ideality factor and Vth is the thermal voltage given by kT/q. In this way, one takes into account both the effects of ideality factor and temperature.

I agree with all. FF is never greater than 1. For ideal cell, FF is equal 1. FF is defined by (ImaxImin)/(IocVoc) which is less than 1. Zekry explains very well.

Thanks to all for the valuable suggestions... 

How can you say that for an ideal cell, the fill factor could equal 1? This would mean a square shape of the I–V characteristic, but that's not even true for an ideal diode. Please tell me what kind of ideal device you have in mind that could reasonably show a square characteristic.

Dr. Jan-Martin Wagner

I mean a theoretical device that can deliver both the short circuit current and the open circuit voltage at the same time.

Also, can you explain to us more about the fill factor

Your explanation of the fill factor is nearly perfect. You stated: "Fill factor means how percent from the theoretical power generated within the solar cell (Isc X Voc) can be utilized as a real power (Pmax)." I would just prefer to avoid speaking of "theoretical power" when referring to the product of Isc times Voc, because this is just an easy-to-obtain reference value, however with no real meaning -- precisely because there is no ideal characteristic that is square-shaped.

Yet even theoretically, an ideal device that delivers both Isc and Voc at the same time is not possible, since in general a voltage can only be built up by "sacrificing" some current. As Voc is the maximum possible voltage, it can only be obtained by using all possible current that way. The reason behind is the fundamental principle of charge conservation: A single charge cannot be kept inside the device (to build up voltage) and been taken out of it (to generate current) at the same time.

Of course the question remains whether there is a theoretically well-defined upper limit for the fill factor of an ideal solar cell. To approach this consider the following practical question: For what kind of I–V characteristic does the fill factor attain higher and higher values? The answer is easy: Reduce the relative share of the rouded part of the characteristic by having a high-as-possible Voc. Since Voc depends strongly on the type of solar cell (single absorber or tandem or triple structure etc.) and on the material and doping used, there is no general answer possible.

I recommend http://www.pveducation.org/pvcdrom/solar-cell-operation/fill-factor for further reading.

The main point here to understand the geometric meaning of Fill Factor. Graphically, the FF is a measure of the "squareness" of the solar cell and is also the area of the largest rectangle which will fit in the IV curve. As FF is a measure of the "squareness" of the IV curve, a solar cell with a higher voltage has a larger possible FF since the "rounded" portion of the IV curve takes up less area [ref: http://www.pveducation.org/pvcdrom/solar-cell-operation/fill-factor]. As you may read above explanation, according analytical geometry rules it is impossible that Fill factor value of a an any solar cell can be greater than 1. Because mathematical current-voltage relationship can only be explained with "Shockley equation". As far as I know after the Shockley no one did not find better equation to explain current-voltage relationship of solar cells.

I agree with all the correspondents, that the FF can't exceed unity in a 'conventional solar cell' . HOWEVER, I find this is a very interesting question, and one I myself have never asked. So, thinking more laterally, and what it would imply, I don't believe it would be impossible to make a solar cell with a FF greater than 1: what you would have to do is to have a cell with a negative output impedance; the output voltage would go UP as you increased the current and the actual maximum power you drew from the cell at its peak power point could be greater than Voc x Isc. I agree no cell I know of does this - but it might be as possible to make one as it would be as useless to have it (it's efficiency would DROP as you reduced the power drawn, and the internal processing to create the negative impedance would be remarkable to say the least)! But it is certainly possible to make generators with negative output impedances - and they are very hard to stabilise, as power engineers know only too well! And a solar cell is a sort of generator.

Dear Tony thank you for your useful discussion, but at that point new question mark raised in my brain. Can you explain negative resistance using "Shockley equation". As far as I guess, having negative resistance device can not fit well with  Shockley diode equation. Therefore, I think... it is compulsory that you must have new diode current-voltage equation model for these kind of devices.

Dear Tony, please explain how this idea (to obtain a FF larger than 1 by a negative output impedance) is in line with thermodynamics. I think you simply cannot circumvent energy conservation.

There may be a slight misunderstanding! Of course no conventional solar cell will possess negative output impedance - that  would be indeed thermodynamically impossible as Dr Wagner points out. I am being - for a change - rather theoretical, saying that any generator which possesses a negative output impedance could possess a fill factor greater than unity. That is a generalised statement. I apply it to a solar cell because it is general, not because any existing solar cell would operate like that. However, it may be possible to construct a solar cell with some sort of built in voltage regulator or other similar mechanism, and such a cell, if it's parameters were set incorrectly, could demonstrate negative output impedance.

But, really, I don't think it is very practical suggestion, and definitely not something we should worry about!

The hypothesis of Tony is theoretically correct. There is some two terminal devices showing an S-curve I-V characteristics such as metal insulator silicon diodes in darkness. They have a  a portion of its  i-v curve that has a negative differential resistance.

If the super position principle is applicable and on illuminating such devices their i-v curves shifts down by the  the shortcircuit current Isc as the normal solar cell diode and the voltage axis cuts the i-v curve in the negative resistance region, then the open circuit voltage Voc will be smaller than the maximum power point  voltage Vm. The maximum power point current Im will be  smaller than the short circuit current . However, under certain circumstances Im Vm will be greater than Voc Isc.  However, this dos not mean a higher conversion efficiency since the open circuit voltage maybe smaller than that for conventional diodes following Shockley equation. I mean one can get a higher form factor but the overall efficiency may  not improved. 

An other point is to consider is that under open circuit where the operating point of the device reside inside the negative differential resistance region, the device is unstable and has tendency to oscillate which means it can oscillate.

Such nonconventional devices do not attracted any research till now. May it desreve more investigations to assess the feasibility of such devices as photvoltaic devices.

Thank you Tommy

Thanks Sir..for your valuable answer.

Abdelhalim,  your comment is very interesting. Maybe this could be a way to make solar cells generate alternating current directly so you don't have to use a separate inverter! But I'm not a material scientist so I'll leave the construction of such a device to the experts . . . Bulkesh, please don't waste your time on this - yet.

Thanks to all for interesting and valuable answers.

Dear powerful and respected Bulkesh Siwach,

Fill factor of a solar panel is always less than 1 which can not be greater than one. It is obtained from the formular

Fill factor= (I mp * V mp)/ (I sc * Voc)= Pm/ (I sc * Voc)

Where

I mp is the current output at maximum power.

V mp is the current output at maximum power.

Pm maximum power output of the solar panel

I sc is the short circuit current zero resistance.

V oc is the open circuit voltage at infinity resistance.

Always Isc is greater the I m and Voc is greater than V m. So the fill factor would be less than 1.

Best regards

Kifilideen.