• Probing errors primarily cause problem with FF but can also cause Voc and Jsc errors.

Cell testing uses a four point probe to contact the cell. A current and voltage probe on top of the cell and a current and voltage probe on the bottom of the cell. The most common arrangement is to have the metal of the block act as the rear current probe and then to have a voltage pin through it. For the top contacts it is usually insufficient to have a single voltage and current probe thus a number of pairs are used.

Top side probes in a cell tester. At each end of the busbar there is a current voltage probe pair

Ideally the probes make good contact with the cell and the voltage and current probes are within close proximity but not touching each other. Spurious measurements occur if the probes don't make good contact with the cell or if the probes are spaced a distance apart. Depending on the internal arrangement of the measurement electronics, the apparent fill factor can rise above the theoretical maximum (0.85 for silicon solar cells at one sun illumination). The simple solution to contacting problems is to move the probes on the cell slightly and remeasure the cell. Large changes in FF when moving the probes are indicative of a contacting problem.