I had an experiment using an old photocopier from Ricoh, model S-1. It works with a photosensitive paper or metal sheet. First, the sheet is charged by a corona discharge. The image is project on the paper surface. Next, two brushes distribute a charged metallic toner. Ok, the phenomena is perfectly clear for me. Except when the bias control (increases charging voltage) is set to the maximum setting. Instead of getting a positive image (black in source equals black in the photosensitive material), it gives a negative. I've researched a lot, and couldn't find any book about electrophotography neither xerography in my university library. I didn't find any evidence of a negative photosensitive material either. It appears to me it uses the photon energy to release electrons from valence band to the conduction one. The opposite phenomena would be absurd. So, why when the voltage is dramatically increased, the photosensitive material behaves as a negative one? My second doubt concerns a laser printer. Seems to me it uses a laser in order to remove charges from the photosensitive drum, where it should be black. The problem is the same, how does the printer make the charged toner adhere to neutral parts of the drum? I saw an explanation at wikipedia, which states that if the toner and the drum have like charges, it will adhere to neutral parts. I'm not very comfortable with this explanation because there's absolutely no force acting on the toner radial to the drum.