Junked Si panels have Si crystals with a vanishingly low concentration of dopants, and metal foil on the surface easily dissolved off with acid. The crystals go into the crucible where the dopants will be refined out.

There are also CdTe panels, in relatively small proportion, even more easily reclaimed: dissolve the Te and Cd in acid, and refine that back to pure metal. Glass is glass.

Later there will be perovskite panels, with overwhelmingly less mass, so of correspondingly less moment.