The fill factor is defined as the ratio of the maximum power (Vmp x Jmp) divided by the short-circuit current (Isc) and open-circuit voltage (Voc) in light current density - voltage (J-V) characteristics of solar cells.

Under illumination the fourth quadrant of the light I-V is the region of interest and the figures of merit for the device are: 1. The open circuit voltage (Voc) - the maximum voltage obtainable at the load under open-circuit conditions 2. The short circuit current density (Jsc) - the maximum current through the load under short-circuit conditions 3. The fill factor (FF), is thus defined as (VmpJmp) / (VocJsc), where Jmp and Vmp represent the current density and voltage at the maximum power point.

The Fill factor is illustrated in the image below, which plots both the current density and the power as a funciton of voltage.

--Jmpearce 20:24, Apr 17, 2005 (UTC)