The fill factor, very commonly abbreviated as FF in solar energy technology is a measure of how closely a solar cell acts like an ideal source. To understand this fully, we have a brief look at an ideal source.
The Ideal Voltage Source
Open circuit Voltage Voc is the terminal voltage of a source when no current is drawn from the source. An ideal voltage source will maintain its terminal voltage constant at the Voc level for any value of current drawn from it. Thus, the VI diagram of an ideal voltage source will be a horizontal line at the level of V=Voc.
Maximum current and maximum Power Output
The maximum current out of terminals of a source is available when the terminals are connected with a zero ohm load i.e., a short circuit. This value of current is called the Short Circuit Current Isc of the source. When current output from an ideal source is increased, the power output (Voc I) increases linearly with current, and can go up to a maximum value of Voc Isc. Area under the lightly shaded blue rectangle represents this max power.
Maximum Power from the Solar Cell
Unfortunately, all practical sources have a built-in effect which reduces the terminal voltage when current is drawn. It is as if there is a series resistance inside the source. This resistance is called the equivalent series resistance or the internal series resistance. The solar cell also has this effect. The terminal voltage drops as the terminal current increases, and then drops much faster to zero volts. Now as we increase the current drawn from the cell the VI product increases almost linearly because the terminal voltage is almost constant. When the terminal voltage starts dropping faster than the increase in I, the product starts falling to zero. The highest point on this curve is the maximum power that the cell can deliver. The value of this maximum power actually available from the cell is represented by the area under the darkly shaded rectangle.
The Fill Factor
The dark shaded triangle fills only a part of the light shaded triangle. The ratio of their areas is called the fill factor and is equal to Vmp. Imp / Voc. Isc. The higher the fill factor, the better is the solar cell. Fill factors are normally about 80% for silicon cells. For more on Fill Factor read here : https://www.pveducation.org/pvcdrom/solar-cell-operation/fill-factor
The Role of the Series Resistance
Thus, we can see that without the series resistance, the VI product curve would be a triangle as shown in the diagram. The series resistance causes the triangle to become smooth curve due to increasing losses in terminal voltage as current increases. At any given current value, the power from a practical solar cell is less than what would be available from a constant voltage source.