most are silicon. some high efficiency ones for special applications have been made using gallium arsenide, but these are too expensive for commercial or home use, also there is a potential problem...
there are three types ,they are photoconductive cell ,photoemissive cell and photovoltiac cell
Cadmium sulfide. Some are made from silicon. Gallium arsenide works well.
The advantage is that it is cheap to produce electricity with the sun. The disadvantage is the "CLOUDS, or no sun. No sun, no electricity.
A photo-electric cell responds to light by giving an electric output, it is not for illuminating anything