Electrons can be raised to the empty conduction band by either doping or optical excitation. Both effects are involved in the photovoltaics (PV) of a solar cell. Phosphorous “impurities” can be implanted in the forbidden gap of crystalline silicon. The boron atom has one electron less than silicon on its outer shell. In the silicon crystal, this deficiency is compensated by an electron jumping up from its valence band, leaving behind a movable hole. The Bloch’s theorem explains that the discrete energy levels of the electrons in a single atom become broadened into bands in solid matter through interaction. The electron–hole pairs created by the light are separated by the voltage drop in the barrier layer and become the PV current. In 2015, the global spending on information technology devices reached $725 billion, almost 10 times more than the global PV market that year.