The cells are made of purified, doped silicon. That means very pure crystalline silicon modified with tiny additions of boron and phosphorus. A chemist would consider them "pure" silicon -- they are more than 99.99% silicon. About 95% of solar module manufacturing uses silicon cells. The remaining ~5% is split between thin film cadmium telluride modules and an even smaller volume of copper indium gallium diselenide (CIGS) thin film modules.

GaAs cells are far too expensive for terrestrial use.