This chapter discusses electric power generation by solar cells as a technology for reducing global warming. Despite the large total incident solar energy on the earth, solar energy has a relatively low energy density. This implies that a large area is required to capture sufficient sunlight for practical applications. Typical solar cells essentially consist of a combination of n- and p-type semiconductors, which is called as p-n junction. The theoretical maximum photo voltage of solar cells is nearly equivalent to the difference of the electrochemical potential of the electron between two non-contacting semiconductors. Solar cells are expected to contribute to a reduction in global warming. The categorization of Si p-n junction solar cells is usually based on the crystallinity of silicon (Si) as single-crystal, polycrystal, and amorphous Si. The energy payback times for both polycrystal and amorphous Si solar cells were much shorter than the expected life of the solar-cell based power plant.