Silicon is the most widely used material for commercially produced solar cells because it is so abundent to be usefull for makeing solar cell it must be refiend to 99.9999% purity.
Amorphous solids are materials which have very ramdomly placed atoms. The atoms do not form any crystalline structures, and also contain large amounts of structural and bonding defacts.
Polycrystalline Thin-Film is a discovery of the computer semiconductor industry. Cells made from this material does not require a large amount of semiconductor material. and is also easy to manufacture.
Gallium Arsenide is a product of gallium and arsenic. Gallium is a very rare byproduct of smelting metals, and arenic is a common but pionsons material.