In each bond, the silicon atom is sharing one of its four valence electrons and one of its neighbor's four valence electrons. As a result of the electron sharing, the atom's outer shell, which can have up to eight electrons, is full and the atom is at a stable state. The electrons are held in the valence band and cannot roam around from atom to atom. In order for the electrons to wander, they must gain energy and move to the conduction band. The energy difference between the conduction band and the valence band is called the band gap .
Different materials have different band gaps.
The gap also varies with temperature, decreasing slightly as temperature increases. The band gap determines the open circuit voltage of the solar cell. A material with a high band gap will have a higher open circuit voltage. Figure PVCM2 is a chart of band gap energies associated with various materials. Silicon has a band gap energy of 1.12 eV. The band gap energy for gallium arsenide is 1.43 eV .