https://doi.org/10.1351/goldbook.08822
Ratio of the real maximum output power, \(P_{\rm{max}}\), that can be obtained with an actual solar cell to the product of the open-circuit potential, \(V_{\rm{oc}}\), and short-circuit photocurrent, \(I_{\rm{sc}}\): \[f_{\rm{FF}} = P_{\rm{max}}/V_{\rm{oc}}I_{\rm{sc}}\]
Note: The \(P_{\rm{max}}\) value is obtained from the cell volt-ampere characteristic as the maximum of the product \(V\!\cdot\!I = P\). Note that the product \(V_{\rm{oc}}\!\cdot\!I_{\rm{sc}}\) is not a theoretical maximum output power of the cell, since the product is zero at both these values as \(I = 0\) for the open circuit and \(V = 0\) for short circuit.