Photocopying with selenium

    The photoreceptive properties of selenium are responsible for its role in photocopiers: the technique of xerography developed rapidly in the latter half of the twentieth century. Amorphous selenium or As₂Se₃ (a better photoreceptor than Se) is deposited by a vaporization technique to provide a thin film (≈ 50 mm thick) on an Al drum which is then installed in a photocopier. At the start of a photocopying run, the Se or As₂Se₃ film is charged by a high-voltage corona discharge. Exposure of the Se film to light, with the image to be copied present in the light beam, creates a latent image which is produced in the form of regions of differing electrostatic potential. The image is developed using powdered toner which distributes itself over the ‘electrostatic image’. The latter is then transferred to paper (again electrostatically) and fixed by heat treatment.  An Se- or As₂Se₃-coated photoreceptor drum has a lifetime of ≈100 000 photocopies. Spent drums are recycled, with some of the main recycling units being in Canada, Japan, the Philippines and several European countries. Once the mainstay of the photocopying industry, Se is gradually being replaced by organic photoreceptors, which are preferable to selenium on both performance and environmental grounds.