Why do thermionic power converters work best at high temperatures?

Thermionic power converters function best at high temperatures because the rate of thermionic emission—the process by which electrons are liberated from the surface of a hot electrode—increases rapidly as the temperature of the emitter rises.1

This relationship is described by the Richardson–Dushman equation, which demonstrates that the emission current density is highly dependent on the absolute temperature of the emitter. Because the energy required to remove an electron from the surface (the electronic work function) acts as a barrier, higher thermal energy is necessary to ensure a sufficient proportion of electrons possess enough energy to escape the surface and flow toward the collector.12

Operating at high temperatures allows these devices to overcome this barrier effectively, facilitating the conversion of thermal energy directly into electrical energy. While high temperatures are essential for performance, they also necessitate the use of materials capable of operating reliably under such conditions, such as tungsten.1