Polycrystalline silicon (multicrystalline, multi-silicon, ribbon). Poly panels have lower silicon levels than mono panels. In general, that makes them less expensive to produce, but they're also slightly less efficient. However, their overall construction design can often make up for the efficiency loss, so they're also good for roofs. You can tell poly-silicon panels by their melange of silicon woven through thin rectangular conduit wires. Thin film (amorphous silicon, cadmium telluride, copper indium gallium (di)selenide) calls are inexpensive to make except right now, they're very inefficient, which means you'll see them in big solar farm projects with a lot of land, but not on your roof