What is Fill Factor?

Fill Factor (FF) quantifies how "square" a solar cell's I-V curve is. A perfect cell would have FF = 1.0; real cells have resistive and recombination losses that round the curve knee, reducing FF.

Formula

FF = P_max ÷ (Voc × Isc)

Example: Voc = 0.685 V, Isc = 10.2 A, Pmax = 5.76 W → FF = 5.76 ÷ (0.685 × 10.2) = 0.824.

What reduces Fill Factor

• Series resistance (Rs): Poor contact metallisation — compresses the curve
• Shunt resistance (Rsh): Defects providing leakage paths — rounds the knee
• Recombination: Higher bulk or surface recombination

Technology comparison

• PERC cells: FF 0.80–0.82
• TOPCon cells: FF 0.82–0.84
• HJT cells: FF 0.78–0.82 (higher Voc compensates)