Why does gray selenium conduct electricity better when exposed to light?

Gray selenium, a metalloid in the oxygen group, exhibits a property known as photoconductivity. Under normal conditions, gray selenium acts as a semiconductor, meaning it has relatively few free electrons available to conduct electricity because most are bound within the valence band of its atomic lattice.12

When the material is exposed to light, it absorbs photons that provide sufficient energy to excite these relatively loosely held electrons. This energy allows the electrons to jump across the band gap—a forbidden energy region—into the conduction band. Once in the conduction band, these electrons are free to migrate, which significantly increases the material's electrical conductivity. This process is highly efficient in gray selenium, where illumination can increase conductivity by more than 1,000-fold.12