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Science Technology

Researchers Conquer "LED Droop" 113

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."
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Researchers Conquer "LED Droop"

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  • Re:Dumb question (Score:4, Informative)

    by IANAAC ( 692242 ) on Wednesday May 09, 2012 @06:40PM (#39948241)

    Why must a single LED provide all the light? Couldn't an array of, say, four LEDs, each equivalent to a 25W incandescent and using mirrors and/or lenses to even out the light distribution, get the same efficiency and substitute for a 100W bulb? Am I missing something obvious?

    That's exactly what is being done now with many of these "shed' lights. I purchased a couple of these that have 20 LEDs inside a casing that has a highly reflective back (they're attached to small solar panels) for my cabin, since our electricity is quite prone to outages from all the thunder/lightning storms we have in Northern Wisconsin. Each one is enough to illuminate a 10x12 room on their own. I can read comfortably with just this light from pretty mch anywhere in the room.

    They're not the prettiest lights, but I built a wood/translucent plastic shade, to make them at least a bit better looking. They also come with their own remote control switch so you can turn them on/off as you would any other sconce or ceiling light.

    It's only a matter of time before some decent designs start coming out for these things.

  • by Osgeld ( 1900440 ) on Wednesday May 09, 2012 @07:37PM (#39948739)

    thats why your 1+ watt LED's usually come attached to a chunk of metal (unless you got them from china, then its metal painted plastic)

  • Re:Dumb question (Score:5, Informative)

    by IANAAC ( 692242 ) on Wednesday May 09, 2012 @08:08PM (#39948989)
    You joke, but actually, each light array has a small 6x6 inch panel that your could mount either outside or hang in a window (the power cord from panel to battery pack is 16 ft. long). They provide enough energy to store in the enclosed small battery packs to last about 12 hours a charge. It's really not a bad solution to the problem.

    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.

  • by slew ( 2918 ) on Wednesday May 09, 2012 @08:16PM (#39949061)

    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).

  • by wrook ( 134116 ) on Wednesday May 09, 2012 @09:05PM (#39949399) Homepage

    CFLs burn out quickly if you cycle them. Once you turn them on, they shouldn't be turned off for 20 minutes. This makes them less than ideal for some locations (like the bathroom, hallway, etc). I currently have CFLs as the main lighting in areas like the living room, but LEDs in other areas. LEDs are expensive, but it's not like I'll starve if I spend a couple of hundred dollars on lights. Prices are dropping fast (at least here in Japan). It wouldn't surprise me if the cost per lumen approaches CFLs soon.

    I've never been one to dislike CFLs. Personally, I like the color of "daylight" bulbls *much* better than incandescent. But I must say that I like my LEDs better than the CFLs. The biggest issue is that the lumens don't drop off as quickly through use. They also come to full brightness more quickly (basically instantly). I will probably switch over completely in the next couple of years.

  • by tlhIngan ( 30335 ) <> on Thursday May 10, 2012 @01:05AM (#39950705)

    That's daft. With LEDs if you want more light, you simply use more LEDs. They are not bulbs, they're bloody diodes! Overdriving LEDs results in earth deaths, this has been known for 40+ years, keeping them within tolerances will ensure they'll last forever, or as near it in human terms.

    Problem is, driving more LEDs is tricky. Clusters wired in series a la Christmas lights die if one of the LEDs die (see Lights of America LED bulbs). Wiring them in parallel, you need to balance the current so one LED isn't being overdriven while the other is being starved for current.

    A proper LED bulb like philips often have a driver circuit per LED (when you're dealing with 5W LEDs, it's not a bad idea), but the downside is adding LEDs means adding a lot of cost in driver circuits.

  • by sunspot42 ( 455706 ) on Thursday May 10, 2012 @01:37AM (#39950861)

    >I just bought a 30 pack of 40 watt Incadescent bulbs for better lighting and environmental efficiency - No Mercury.

    Unless coal is used to generate some - or worse most - of the electricity where you live, in which case powering those incandescent bulbs will release far more mercury into the environment than an equivalent number of CFLs would.

    Worse, the mercury that comes from burning coal isn't elemental mercury, as you'd find in a CFL. Which means it's far more easily absorbed by living things like us.

No problem is so large it can't be fit in somewhere.