Photovoltaics (PV) is the technology that generates electrical power from mainly semiconductors, recently from organic and other materials, when they are illuminated by photons. PV power generation technology is one of the most promising renewable energy technologies because of possibility of solving environmental problems and limited sources for energy. Table 1 shows sustainable potentials of renewable energy sources presented by N. S. Lewis (2005). From the 1.2x105 TW (the Solar constant is 1.76x105 TW) of solar energy that strikes the earth’s surface, a practical sitting-constrained terrestrial global solar power potential value is about 600 TW. Thus, for a 10% efficient solar farm, at least 60 TW of power could be supplied from terrestrial solar energy resources. Therefore, solar energy is the only renewable energy resource that has enough terrestrial energy potential to satisfy a 20 TW or more carbon-free supply.