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Researchers Conquer "LED Droop"
sciencehabit writes "Tiny and efficient, light-emitting diodes (LEDs) are supposed to be the bright future of illumination. But they perform best at only low power, enough for a flashlight or the screen of your cellphone. If you increase the current enough for them to light a room like an old-fashioned incandescent bulb, their vaunted efficiency nosedives. It's called LED droop, and it's a real drag on the industry. Now, researchers have found a way to build more efficient LEDs that get more kick from the same amount of current—especially in the hard-to-manufacture green and blue parts of the spectrum."
In any case, energy is energy, whether it's generated at a coal plant and then distributed or directly to a battery pack for later use.
My point was really that, while they're currentlly not the most attractive lighting, that won't always be the case - they can be made fashionable as well as usable.
Typically a "green" produced by GaN is fairly easy to manufacture and fairly efficient, but it is physically a very *hard* material. In contrast, the "blue-green" produced by InGaN (an alloy of a little bit of InN and base of GaN) isn't as efficient as it tends to have lots crystal defects and these defect cause brittle-ness and results in some electron-hole recombinations to be non-radiative (generating heat and not band-gap light emissions).
Regardless of this manufacturability issue, many white LEDs use an InGaN band-gap devices and create the "warmer" parts of the spectrum using phosphors. This makes most of the output light more blue-ish, but only the phosphor re-radiated (stoke's shifted) part in the warmer part of the spectrum where you pay the efficiency cost. For "cool" devices, less of the output is down-converted, so you have less efficiency loss. For "warmer" devices, more of the light is down converted and you pay for more conversion efficiency loss. Some warm devices actually have multiple LEDs (say a red, green, and blue), but color stability is generally hard to maintain over time and temperature, so these devices are generally less efficient and more expensive.
In any case, the effect that was described is that the currently "cheap" way of growing GaN base crystals for LEDs results in a polar orientation which is bad for high-current operation as it tends to generate a back field. This is described in more detail in this other site:
Most of the commercial GaN devices are grown along the [0001] direction, so-called “polar” or “c-plane” structures. However, there is an internal electric field perpendicular to the active regions in the c-plane devices as the c-axis is polar. This will result in band bending and a poor overlap of electron and hole wave-functions (the Quantum confined Stark effect, or QCSE), which reduces the radiative recombination efficiency and affects the device performance. In order to avoid (or reduce the effects of) the QCSE, GaN can be grown in “non-polar”, or “semi-polar”, orientations, in which there is no, or much less, internal polarization fields along the growth direction. In theory, this should increase the efficiency of light emitting structures. The high density of structural defects (such as basal plane stacking faults and partial dislocations) in heteroepitaxially grown non-polar and semi-polar GaN results in low internal quantum efficiency and output power of the devices, as reported in the literature.
Of course the answer is to just grow low-defect GaN in a non-polar or semi-polar orientation, but that's currently hard to do. These UCSB researchers aren't the only group working on this problem, but they apparently have done some cooperation with people doing actual manufacturing (Mitsubishi Chemical).