there are several ways of doing this, but a common way is through light energy absorbance by the electrons leading to a charge disparity that drives electron movement. The electrons get excited to the higher energy of the diffuse cloud surrounding the alloy or ceramic or whatever particular material is present and become free to move beyond the confines of their original atom. This can create a negative charge that will be offset by a positive “hole” that also can migrate.
A lot of times the substance consists of layers that impart a direction to the flow (one side excites and the other receives the excited electrons).