Light-emitting diodes, commonly called LEDs, are potentially more energy efficient than traditional lighting sources, such as incandescent bulbs, that waste about 90% of the input energy as heat. However, widespread use of LEDs is limited by a controversial and unexplained decrease in efficiency at high, practical currents; this decrease is known as LED efficiency droop. Researchers at the Center for Energy Efficient Materials identified the cause of this drop off -- a major first step in increasing the diodes' efficiency and accelerating the universal replacement of our usual light sources with LED technology.
Typical incandescent light bulbs operate by heating a metal filament and emitting a continuous spectrum of radiation, only a fraction of which is useful for lighting, most of it being lost as heat. An LED operates by a very different process. An LED is made of a semiconductor, rather than a metal. In a semiconductor, electrons are located in different energy levels called bands. These energy bands are separated by empty levels or bandgaps. When an electron falls across the bandgap, it can release its energy as light, directly converting electricity into light. The colors of light emitted can be controlled so that mostly useful, visible light, is emitted, unlike a traditional light bulb. Additionally, LEDs do not need to heat up to emit light like an incandescent bulb, further resulting in higher efficiency.
LEDs operate well at low current densities, when only a small number of electrons are injected, corresponding to a low light, or dim output. When the power is increased to make the LEDs bright enough to be directly competitive with incandescent bulbs, the LEDs' efficiency droops. To retain a high efficiency while giving enough light intensity, LEDs have to be grouped while operating each at low current, hence the high price of LED lamps compared to other lamp types. The reason for droop has long been an unresolved and controversial issue, hindering more focused research on solving the problem.
The Center's team, with the CNRS-Ecole Polytechnique in France, solved this longstanding mystery. It all comes down to that electron falling across the bandgap and losing its energy. Sometimes, the energy released by the falling electron does not become a photon, but, instead, is transferred as kinetic energy to another electron that might be nearby. This process is called Auger recombination and can cause most of the energy of the excited electrons to eventually be wasted as heat, that same old problem as with incandescent lights.
The researchers performed an experiment where they measured the energy spectrum of electrons emitted from an LED into vacuum. The energy of some of the ejected electrons showed that they were generated as energetic Auger electrons inside the LED. From this, they determined where the missing energy caused by droop was going. These results suggest a clear path for further research towards more efficient, cost-effective LED technology to widely replace current residential and commercial lighting.
