In general, heterojunctions, like electronic devices, are non-linear elements, which means that their OCA cannot be described by the usual Ohm's law. When illuminated from the side of the n-type semiconductor, the radiation is absorbed in the p-type semiconductor and creates an electron-hole pair. The short-circuit current and open-circuit voltage are the maximum current and voltage provided by the p-n junction, so the power of the p-n junction at these points is zero.[4,6,7] The FF parameter, called the fill factor, characterizes the maximum power of the p-n heterophotocell and graphically characterizes the squareness of the VAC heterojunction. In VAC, it is determined by the area of the rectangle: