What is fill factor?
Fill factor is a measure of a solar cell’s quality. We look at the I-V (current-voltage) behavior of a solar cell in operation. The current reaches a maximum for short circuit condition while the voltage reaches a maximum for open circuit condition. If we draw lines extending up from the V_oc and I_sc, we get the box B in the diagram. Since power is what we want to get out of a solar cell, we need the product of V and I. The maximum power point (MPP) is the point on the curve in which the I and V product is a maximum. The MPP corresponds to a V_mpp and I_mpp. If we draw lines extending from the MPP V and I, we get another box A. The fill factor (FF) tells us the ratio of the areas of the two boxes. Looking at I-V characteristics, this translates to:
I_sc and V_oc are easy to measure, but the MPP is a little trickier. The max power at the MPP can be obtained through software. The FF is essentially a ratio of the maximum operating power to the theoretical maximum. The larger the fill factor (closer to 1), the better. Of course, focusing too heavily on increasing the fill factor may result in sacrificing optical absorption and inducing some extra losses. There is always a trade-off between improving one parameter and consequently worsening others. One method of checking resistive losses is taking the slope from the origin to the MPP. The slope is equal to -1/R, such that R is the total resistance seen by the solar cell. It should be noted that using a Suns-Voc curve to generate a pseudo I-V curve can be used to compare with a measured I-V curve and find series resistance.