But when the layer is hit by light, the energy of the photons liberates electrons and allows current to pass through! These newly freed electrons are what neutralizes the positive charge coating the drum to form the latent image. In the dark, the photoconductive layer on the drum acts as an insulator, resisting the flow of electrons from one atom to another. What makes elements like selenium so cool is that they can conduct electricity in some cases, but not in others. This layer is made out of a semiconductor such as selenium, germanium or silicon. A drum is basically a metal roller covered by a layer of photoconductive material. The photoreceptor drum (or, in some photocopiers, belt) is the heart of the system.
