Why must the emitter in a thermionic power converter be heated to a high temperature?

In a thermionic power converter, the emitter must be heated to a high temperature to facilitate thermionic emission, the process by which the device converts heat directly into electricity.1

Within the emitter material, mobile electrons are held by an energy barrier known as the electronic work function. To escape the surface of the metal, electrons must acquire sufficient energy to overcome this barrier. At lower temperatures, only a negligible fraction of electrons possess enough thermal energy to escape.2

As the temperature of the emitter increases, the number of electrons with enough kinetic energy to overcome the work function rises rapidly, as described by the Richardson–Dushman equation. By heating the emitter, the device provides the necessary thermal energy to liberate these electrons, allowing them to flow toward the collector and generate an electric current through an external load.12