In some materials, the electrons are bound to the atomic nuclei and so are not free to move around but the energy required to set them free is low. In these materials, called semiconductors, the conductivity is low at low temperatures but as the temperature is increased the electrons gain more thermal energy and the conductivity increases. Silicon is an example of a semiconductors that can be used to create solar cells which become more conductive the more energy they receive from photons from the sun.