Refrigeration is an intrinsic feature of light-emitting diodes, a fact that was recognized decades ago but has so far eluded direct experimental observation at practical power densities. The problem is insufficient external luminescence efficiency; for net cooling to occur, the losses in the device must be close to zero, and a sufficiently efficient LED has yet to materialize. We propose a possible structure for such an LED, and predict that with existing optoelectronic material quality and device processing, electroluminescent refrigeration is not only possible but is potentially more efficient than its solid-state alternatives, particularly at low temperature.
We derive the fundamental limits of energy harvesting from the outgoing thermal radiation from the ambient to the outer space. The derivations are based on a duality relation between thermal engines that harvest solar radiation and those that harvest outgoing thermal radiation. We also derive the ultimate limit for harvesting outgoing thermal radiation, analogous to the Landsberg limit for solar energy harvesting, and show that the ultimate limit far exceeds what was previously thought to be possible.
The new breakthrough in photovoltaics, exemplified by the slogan “A great solar cell has to be a great light-emitting diode (LED)”, has led to all the major new solar cell records, while also leading to extraordinary LED efficiency. As an LED becomes very efficient in converting its electrical input into light, the device cools as it operates because the photons carry away entropy as well as energy. If these photons are absorbed in a photovoltaic (PV) cell, the generated electricity can be used to provide part of the electrical input that drives the LED. Indeed, the LED/PV cell combination forms a new type of heat engine with light as the working fluid. The electroluminescent refrigerator requires only a small amount of external electricity to provide cooling, leading to a high coefficient of performance.
We present the theoretical performance of such a refrigerator, in which the cool side (LED) is radiatively coupled to the hot side (PV) across a vacuum gap. The coefficient of performance is maximized by using a highly luminescent material, such as GaAs, together with device structures that optimize extraction of the luminescence. We consider both a macroscopic vacuum gap and a sub-wavelength gap; the latter allows for evanescent coupling of photons between the devices, potentially providing a further enhancement to the efficiency of light extraction. Using device assumptions based on the current record-efficiency solar cells, we show that electroluminescent cooling can, in certain regimes of cooling power, achieve a higher coefficient of performance than thermoelectric cooling.
Recent work on electro-luminescent cooling has focused either on diodes at forward bias voltages just below the bandgap energy, where high cooling power density is possible, or voltages below the thermal voltage, where the effect is more tolerant to parasitic non-radiative recombination. Here we consider the possibilities for diodes designed to operate at intermediate voltages. Numerical calculations suggest that design for this regime may enable near- and mid-infrared devices capable of solid-state refrigeration with sufficient power density for some applications.
Experimental demonstration of net electro-luminescent cooling in a diode, or equivalently electroluminescence with wall-plug efficiency greater than unity, had eluded direct observation for more than five decades. We review experiments demonstrating light emission from a light-emitting diode in which the electron population is pumped by a combination of electrical work and heat.