If electrons can’t change levels, how can a photoconductor help them change one level to another?
In a metal, electrons can easily shift from one level to another empty level because the levels are close together in energy. In a full insulator, it’s very difficult for the electrons to shift from one level to an empty level because all of the empty levels are far above the filled levels in energy. In a photoconductor, the empty levels are modestly above the filled levels in energy, so a modest amount of energy is all that’s needed to shift an electron. This energy can be supplied by a particle or “photon” of light. An illuminated photoconductor conducts electricity.