Do LEDs Cool During Low Voltage Operation?

There has been an ongoing hypothesis, since at least the 1950s, that light-emitting diodes (LEDs) operated with a forward bias voltage less than the bandgap-equivalent voltage should act as optical refrigerators. Since LEDs emit luminescence radiation approximately equal to the bandgap energy, application of a sub-bandgap-equivalent voltage supposedly means that the difference in the emitted photon energy and the applied bias must be made up by the net absorption of lattice phonons. As a result, it is surmised that cooling should occur. Here, we show that the voltage is simply the change in free energy, also known as the quasi-Fermi level separation, and we distinguish free energy from the electrical energy that is required to operate LEDs. Operation of LEDs with forward bias voltage less than the bandgap-equivalent voltage is normal, and this operating condition neither implies nor requires electroluminescence cooling or any other novel explanation.