Solar cells are made from semi-conductor materials like silicon and gallium. To make solar cells, the wafers of semiconductor materials containing impurities are arranged in such a way that when light falls on them, a potential difference is produced between the two regions of the semi-conductor wafers. This potential difference then produces electric current. The potential difference produced by a single solar cell of 4 square centimeter size is about 0.4 volt to 0.5 volt and generates a current of 60 milliamperes.