Thin film solar cells are the solar cells in which a thin film of gallium arsenide, cadmium sulfide, or other semiconductor material is evaporated on a thin, flexible metal or plastic substrate. These cells are attractive as power sources for spacecraft. The various thin-film technologies are developed to reduce the amount of light absorbing material required in creating a solar cell. Cadmium telluride is an efficient light-absorbing material for thin-film solar cells. A silicon thin film technology is also developed for building integrated photovoltaics (BIPV) in the form of semi-transparent solar cells, which can be applied as window glazing. This thin film solar cells save both materials and energy in the production of cells and modules.