LED efficiency puzzle solved by theorists

Apr 19, 2011

Researchers at the University of California, Santa Barbara, say they've figured out the cause of a problem that's made light-emitting diodes (LEDs) impractical for general lighting purposes. Their work will help engineers develop a new generation of high-performance, energy-efficient lighting that could replace incandescent and fluorescent bulbs.

"Identifying the root cause of the problem is an indispensable first step toward devising solutions," says Chris Van de Walle, a professor in the Materials Department at UC Santa Barbara who heads the research group that carried out the work.

Van de Walle and his colleagues are working to improve the performance of nitride-based LEDs, which are efficient, non-toxic and long-lasting light sources. They investigated a phenomenon referred to as "droop"?the drop in efficiency that occurs in these LEDs when they're operating at the high powers required to illuminate a room. The cause of this decline has been the subject of considerable debate, but the UC Santa Barbara researchers say they've figured out the mechanism responsible for the effect by performing quantum-mechanical calculations.

LED droop, they conclude, can be attributed to Auger recombination, a process that occurs in semiconductors, in which three charge-carriers interact without giving off light. The researchers also discovered that indirect Auger effects, which involve a scattering mechanism, are significant?a finding that accounts for the discrepancy between the observed degree of droop and that predicted by other theoretical studies, which only accounted for direct Auger processes.

In nitride LEDs, "These indirect processes form the dominant contribution to the Auger recombination rate," says Emmanouil Kioupakis, a postdoctoral researcher at UC Santa Barbara and lead author of a paper published online April 19 in . The other authors are Van de Walle, Patrick Rinke, now with the Fritz Haber Institute in Germany, and Kris Delaney, a project scientist at UC Santa Barbara.

droop can't be eliminated because Auger effects are intrinsic, but it could be minimized, the researchers say, by using thicker quantum wells in LEDs or growing devices along non-polar or semi-polar growth directions in order to keep carrier density low.

"With now established as the culprit, we can focus on creative approaches to suppress or circumvent this loss mechanism," Van de Walle says.

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that_guy
4.5 / 5 (2) Apr 19, 2011
Dear scientists. Never say never. You just aren't thinking creatively enough if you can't find a way to circumvent the auger effects.

Just kidding. I am stoked tho that they found part of the problem to making practical led lighting.

Now they just need to get to the real problem. Seriously, that tiny little dot that is actually the LED is super bright for it's size. If you could make the light emitting part of the LED the size of a lightbulb, it would turn you blind instantly. Lets figure out a practical way to cheaply pack a hundred of these suckers on a single wafter 1 inch by 1 inch...
Jayman
1 / 5 (1) Apr 19, 2011
Let's crack cold-fusion first. Efficiency would not matter with unlimited, renewable, clean energy.
Similar to writing code in the 90s. Nobody bothers with using Assembly language to fine-tune their code anymore with the advent of super-fast CPUs - except in systems programming.
TabulaMentis
1 / 5 (1) Apr 20, 2011
Now they just need to get to the real problem. Seriously, that tiny little dot that is actually the LED is super bright for it's size. If you could make the light emitting part of the LED the size of a lightbulb, it would turn you blind instantly. Lets figure out a practical way to cheaply pack a hundred of these suckers on a single wafter 1 inch by 1 inch...
You had me with you until you said "a hundred of these suckers on a single wafter 1 inch by 1 inch."
Why not just have a LED the size of a regular light bulb with only one 'BIG' diode. The LED light bulb would be made of solid clear or colored non-breakable recyclable material, so it will not break. Call the light bulb a SuperLED light bulb.
bugmenot23
not rated yet Apr 20, 2011
Let's crack cold-fusion first. Efficiency would not matter with unlimited, renewable, clean energy.


Cold Fusion has been cracked. Check out Rossi's E-Cat http://nextbigfuture.com/search/label/cold%20fusion

As to efficiency.. It will always matter. Twice as efficient means twice as long before you need to recharge your portable devices.
eachus
5 / 5 (1) Apr 20, 2011
Similar to writing code in the 90s. Nobody bothers with using Assembly language to fine-tune their code anymore with the advent of super-fast CPUs - except in systems programming.


Not quite. There are always people who need to get another few percent efficiency out of their code.

I remember an article in the mid-eighties. A company had some inner loop code that they thought should be coded in assembly, not Ada. As part of the documentation for their waiver request (to use assembler any of a half-dozen high-level languages would have been okay) they wrote the code in a couple lines of Ada. Shock! It not only used fewer instructions than the hand-coded assembly, it executed 30% faster.

This was not an accident, Ada was designed to allow for a high level of optimization. But it has spread. ACT took over the maintenance of the GCC backend so that they could better support optimizations in the GNAT Ada front-end. Those features are now used by the C and C++ compilers also.
CHollman82
4 / 5 (4) Apr 20, 2011
I remember an article in the mid-eighties. A company had some inner loop code that they thought should be coded in assembly, not Ada. As part of the documentation for their waiver request (to use assembler any of a half-dozen high-level languages would have been okay) they wrote the code in a couple lines of Ada. Shock! It not only used fewer instructions than the hand-coded assembly, it executed 30% faster.

This was not an accident, Ada was designed to allow for a high level of optimization.


This is nonsense... Regardless of what optimizations the compiler makes it still ultimately becomes assembly code... so the "hand written" assembly they wrote that was less efficient than the assembly that the ADA compiler produced was just garbage.

They didn't know what they were doing, their "hand written" assembly was just bad... or you are misstating or misunderstanding the point of the article you are referencing.

FYI I am a firmware engineer, I write in a mix of ASM and C.
CHollman82
3 / 5 (2) Apr 20, 2011
Here is a small assembly routine I wrote today in fact, samples an ADC at 120ns reading values from 2 channels each sample and saving them to RAM buffers...

_Otdr_12mSampleCollect:

;Backup all registers we will be using to the stack
PUSH XAR4
PUSH XAR5
PUSH AL

;Move buffer addresses into registers
MOVL XAR4, #_AdcBufHigh
MOVL XAR5, #_AdcBufMed

;Move BufSize to AR6 (loop counter)
MOVW DP, #_BufLength
MOV AL, @_BufLength
MOVZ AR6, AL;

;Move DelayCount into AL
MOVW DP, #_DelayCount
MOV AL, @_DelayCount

;ADC start of conversion sequencer 1
MOVW DP, #_AdcRegs
OR @_AdcRegs+1, #0x2000

;Delay for DelayCount
RPT @AL || NOP

;Pulse laser with RTGATE
MOVW DP, #0x01C3
OR @53, #0x0800

MOVW DP, #_AdcRegs

L1:

;Move ADCRESULT0 to medium gain buffer
MOV AL, @_AdcRegs+8
MOV *XAR5++, AL

;Move ADCRESULT1 to high gain buffer
MOV AL, @_AdcRegs+9
MOV *XAR4++, AL

BANZ L1, AR6--

POP AL
POP XAR5
POP XAR4

LRETR
cryptopsii
5 / 5 (3) Apr 20, 2011
for example, in his routine he was able to save some time by not pushing AR6, a register only used here.
I'm also pretty sure a compiler would have used regular ram adresse instead of register xar4,xar5 which again, save time.

Compiler are usually much worst in smaller cpu
(or DSP in this case)

However,
x86 is a different game: benefit of hand-coded ASM is diminished because of the very complex branching/prediction path and cache memory scheme.

ASM is useful for newer instructions (SSEx, SIMD)
which may not be yet included in compiler.
CHollman82
1 / 5 (1) Apr 21, 2011
Had to give you a 5 for knowing your stuff :D
beelize54
not rated yet Apr 21, 2011
Let's crack cold-fusion first. Efficiency would not matter with unlimited, renewable, clean energy
We couldn't waste an energy even under the situation, when we would use cold fusion at large scales, because it would violate the thermal equilibrium of Earth. Currently the civilization produces about 1/500 of energy, which falls into Earth surface with sunlight via fossil fuel burning and additional 1/50 of energy increase comes from greenhouse effect. If we would increase the production of energy ten times with cold fusion, we would double the global warming even without any greenhouse gases production.
mohsinali
not rated yet Apr 22, 2011

We couldn't waste an energy even under the situation, when we would use cold fusion at large scales, because it would violate the thermal equilibrium of Earth

there are also a large number of alternatives at physicsinventions.com where i have written my own articles
PinkElephant
5 / 5 (1) Apr 22, 2011
Currently the civilization produces about 1/500 of energy, which falls into Earth surface with sunlight via fossil fuel burning
As of 2008, total world energy consumption was ~15 TW. Solar energy absorbed by land and oceans is ~89 PW. You're off by more than an order of magnitude. Of course, not all solar energy absorbed at the planet's surface is harvestable, but then not all energy used by the world comes from fossil fuels either.
and additional 1/50 of energy increase comes from greenhouse effect
Anthropogenic increase in greenhouse gases (all of them, not just CO2) so far contributes an extra ~2.7 W/m^2 to incident radiation (note that this is calculated for direct sunlight at the equator.) With Earth cross-section area of about 1.28e14 m^2, this amounts to about 350 TW. 89/0.35 = 254: you're off by more than a factor of 5.

Anyway, to equal current AGW forcing, global energy output would need to rise by a factor of 20, so in that respect you're close to the mark.
GSwift7
1 / 5 (1) Apr 22, 2011
Anthropogenic increase in greenhouse gases (all of them, not just CO2) so far contributes an extra ~2.7 W/m^2 to incident radiation


That's not the number I see in any of the major current sources.

From the IPCC 4th AR, summary for policymakers:

"The combined radiative forcing arising from increases in the major greenhouse gases is +2.3 W/m^2"

However, after accounting for the negative forcing from aersols and cloud albedo, they are saying numbers more like 1.6 W/m^2 for the net anthropogenic effect. I think I saw that figure in a recent paper where Mann was a co-author, but I can't find it now. Once you throw in aerosols the numbers get all messy because of different opinions between the different official groups there.
PinkElephant
not rated yet Apr 22, 2011
"The combined radiative forcing arising from increases in the major greenhouse gases is +2.3 W/m^2"
Note the word "major" (I said "all of them"). I've seen more recent figures of ~1.7 W/m2 for CO2 alone, and ~1 W/m2 for all the rest combined. But it's probably all within the error margins, anyway.
hey are saying numbers more like 1.6 W/m^2 for the net anthropogenic effect
True, I didn't account for other factors and feedbacks. However:
the negative forcing from aersols and cloud albedo
Recent studies (published a couple years after IPCC AR4) report that cloud feedback is actually slightly positive rather than negative:

http://www.scienc...1231.htm
http://physicswor...ws/39908

And, aerosols are short-lived in the atmosphere; we should expect them to drop over time (or at least not increase as quickly as GHGs) as the developing world catches up on clean air standards.
GSwift7
1 / 5 (1) Apr 25, 2011
To Pink:

The following does not refute what you said, but I think you will find it interesting.

According to an article from Pacific Northwest National Lab announced on Friday:

http://www.pnl.go...x?id=861

from the fourth paragraph:
Overall, most clouds have a net cooling effect


It's a very interesting study, as it looks at the combined effects of clouds and aerosols in three dimensions. Of course they are not looking at the day/night balance or convection. They are only looking at shading versus albedo, if I'm reading it correctly. Isn't it curious that they found puffy fair weather clouds can actually brighten the sky? Seems counterintuitive at first glance.
GSwift7
1 / 5 (1) Apr 25, 2011
The following presentation on the subject of clouds and aerosols from nasa/ncar is great too:

http://dsm.gsfc.n...ate2.pdf

They show the net cloud/aerosol effect as either + or - of about equal magnitude in either direction, within the limits of uncertainty.

I like that one bacause it's one of the most recent official references I could find.

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