The fill factor of perovskite solar cells is considerably lower than that of GaAs and silicon, though they have similar open-circuit voltage (V oc ) deficits. To probe the fill factor loss, we applied the J sc -V oc characterization technique on three different types of perovskite solar cells. Based on the J sc -V oc measurements, we calculated the series resistance at maximum power point under different illumination—and thus injection— levels. We found that the series resistances of all three perovskite solar cells are illumination dependent, unlike silicon solar cells. Further analysis is underway to determine the origin of this phenomenon.