AFAIK photovoltaic cells with the electricity producing junction made of pure silicon do not exist. Don't know how much of non-silicon is there, but arsenic from GaAs cells is not something you want in your soil either.
As for WEEE, I hope most of the cells are returned and recycled or somehow properly disposed of, but not all will be collected, and there will be breakage, maybe even leakage of acid-rain digested cells. Also, even now, the collected e-waste is not really processed in a environmentally friendly way.
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.
