Photons strike and ionize semiconductor material on the solar panel, causing outer electrons in the atom to break free of their atomic bonds. Due to the semiconductor structure, the electrons are forced in one direction creating a flow of electrical current. Solar cells are not 100% efficient in crystalline solar cells, in part because only certain light within the spectrum can be absorbed. Some of the light spectrum is reflected, some is too weak to create electricity and some creates heat energy instead of electricity.