Very high efficiency LEDs can harness the fundamental thermodynamics of light to generate new possibilities for optical cooling in the form of thermophotonic (TPX) heat pumps. Such optical refrigerators and heat pumps could outradiate thermal energy along with light, either to open space or towards a photovoltaic cell further recycling the optical energy to increase the efficiency of the refrigeration. In contrast to existing heat pumps and refrigerators these optical refrigerators could allow compact, noiseless and robust cooling solutions with capabilities outperforming the presently prevailing solutions based on mechanical vapor-compression systems or thermoelectric devics. While promising, the concept requires new solutions to optimize the performance of light emitting devices to reach the regime where cooling becomes possible. This requires (1) quantum efficiency of the light emitter to be near unity, (2) operating the LEDs at voltages that are smaller than the voltage corresponding to the average photon energy of the emitted photons (i.e. typically voltages < Eg/q) and (3) new thermal insulation or light extraction solutions.
Figure 1: Schematic illustrations of the fundamental thermodynamics of thermophotonic systems. Under ideal conditions the TPX system can operate at very high system efficiencies over a broad range of temperatures and material bandgaps.
Recent studies have produced several results indicating that it is possible to fabricate LEDs with a quantum efficiency that is, in principle, sufficient to access the operating regime allowing thermophotonic cooling.