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Like Tebow, these new solar cells are giving their "110 percent" week in and week out.  (Source: ESPN)
Gains to quantum efficiency could yield around a 35 percent gain in conversion efficiency, the key metric

Using quantum dots -- tiny nanometer scale semiconductor crystals -- researchers at the U.S. National Renewable Energy Laboratory have cracked an important physical barrier and achieved levels of performance long considered impossible for a solar cell.

I. Giving its 110 Percent

The special design used by the team utilized quantum dot nanocrystals in the 1-20 nm range.  The nanocrystals were composed of lead selenide treated with ethanedithol and hydrazine.

The photon-harvesting quantum dot-populated plane was sandwiched between a nanostructured zinc oxide layer and a thin gold electrode.  A top layer was formed using a transparent conductor.  

The overall design is in line with the "thin-film" methodology, which is currently rising in commercial production.  Thin film cells tend to rely on scarce (i.e. expensive on a per mass basis) resources, such as rare earth metals. However, they use so little of them -- given the low mass of the thin film -- that they are not significantly more expensive than existing polycrystalline silicon cells.  Generally, the only major extra cost to thin film is the initial cost of shifting the production technology.

The new NREL cell shatters the quantum efficiencies of previous designs, posting a peak external quantum efficiency of 114 ± 1% and a peak internal quantum efficiency of 130%.  

In order to understand these numbers and how any power efficiency device can be more than "100 percent" efficient, you must understand the meaning of quantum efficiency (QE), which is overall quite different, but related to conversion efficiency (which will never be over 100 percent -- or even close to 100 percent -- in traditional physics).

Thin film
The new cell is a thin film design. [Image Source: NREL]

Quantum efficiency is a measure of how many electrons come out of a cell for every photon that goes into the cell.  Traditional silicon solar cells can achieve near 100 percent quantum efficiency at around 600 nm, but drop to around 80 percent on either end of the 500-1000 nm range (visible light is 380 to 740 nm).  What this means is that the perfect "color" of light for silicon cells is orangish, while purple light can have a less than 45 percent conversion rate.  As white light (sunlight) is a mixture of different wavelengths, the lower quantum efficiency of certain parts of the spectrum leads to lower average quantum efficiency.

External efficiency directly uses the number of input photons and the number of output electrons from a device.  Internal efficiency, by contrast, uses theory to adjust these numbers to account for losses due to reflection and absorption.

We took the liberty of borrowing (Fair Use clause TITLE 17 > CHAPTER 1 > § 107) the charts for their 0.72 eV bandgap cell (their best-performing design) and comparing it to a traditional PC silicon cell, adding a helpful reference that shows what eVs roughly correspond to in the visible light range:

Solar Cell efficiency

Comparing the external quantum efficiencies of the new NREL design (top) and the PS silicon design (bottom) over the visible light range (middle bar), we see that the new cell is slightly less efficient in capturing red-end light, but is much more efficient in capturing blue-end light.

(The black line in bottom graph and the blue line in the top right graph are the internal QEs.)

Overall this could grant up to a 35 percent efficiency gain versus today's standard PS silicon cells, according to the paper's authors.

II. You "Cannot Change the Laws of Physics" -- So Pick a Better Law!

The better blue-range performance comes thanks to multiple exciton generation (MEG), a unique quantum effect, which like other oddball quantum effects, occurs at an extremely small scale.  In an MEG scenario, a single photon hits an atom, but rather than simply knocking off one electron via the formation of an "exciton" (an electron/hole pair), it puts multiple electrons into the flow.

MEG -- multiple exciton generation -- bends the traditional laws of physics.
[Image Source: Los Alamos Science & Tech Mag./U.S. Department of Energy's NNSA]

The exact quantum mechanics of this phenomena are being debated by physics.  Currently the three leading hypotheses are:
  1. Impact ionization -- the high energy exciton ("X") becomes a "multi"-X, decaying through a dense range of multi-X states.
  2. Eigenstate excitation -- a mixed "virtual" state consisting of multi-X and X (think superposition) is triggered by photon energetic absorption.
  3. Oscillatory decay -- photon absorption creates standard X, but in the special material X waffles back and forth, switching identity from X to multi-X and back, slowly dropping in energy, in the process.
Without MEG, no solar cell can have more than a 100 percent internal or external QE.  Hence no traditional solar cell has had greater than a 100 percent QE, even at its optimal part of the spectrum (e.g. orange light for silicon cells).  This means that the overall conversion efficiency (CE) of a traditional cell -- even if perfectly optimized -- would not exceed 32 percent.  Cumulatively this 100/32 (QE/CE) limit is named the Shockley-Queisser limit after its discoverers (S-Q Limit, for short).

As Scotty would say "you cannot change the laws of physics."  But sometimes you can have your cake and eat it to, if only you find the right quirk in complex and poorly understood physics of our universe.


That's fundamentally what has been done here.  MEG was first theorized by NREL researcher Arthur J. Nozik, Ph.D back in 2001, and was later confirmed to work in quantum dots, thanks to their special scale.  This method is also known as "hot carrier generation".  Using this quantum effect, later proved in the laboratory, the S-Q performance barrier could be shattered.

A useful property of quantum dots, is that their size determines their band gap, and hence the efficiency.  Thus building the "perfect" MEG cell is simply a matter of picking the right size dots.  As the bandgap tends to decrease as the quantum dot size and efficiency increase, the trick is to pick a quantum dot that is as big as possible, without losing the quantum effects.

Quantum dots
Quantum dots don't just look pretty, they have some handy physics quirks too!
[Image Source: Elec-Intro]

Quantum dots also generate electron/hole pairs easier, with room temperature being enough excite (generate electricity) in some quantum dot materials.

The most recent paper was published [abstract] in the peer-reviewed journal Science, with Matthew C. Beard taking the distinction of senior author and Octavi E. Semonin the distinction of being first author.  Professor Novik was listed second to last, after four additional NREL colleagues.

III. Third Generation Solar Cells -- Finally a Solar Tech. Worth Investing In

"First" and "second" generation solar cells use various bulk semiconductors such as silicon, cadmium telluride, or copper indium gallium (di)selenide, which are then mixed with third, fourth, and fifth column (in the periodic table) elements to improve performance.

Ideally quantum dot cells could be combined with these traditional thin-film semiconductor cell designs, or applied using a mixture of nanocrystalline quantum dots optimized for different wavelengths.   Either methodology could yield an optimized "third" generation (aka. next generation) design.  Such a cell would enjoy the best of both worlds -- silicon cells' excellent red range performance, along with quantum dots excellent performance on the higher end (blue) of the visible light spectrum.
Quantum Dot mixture
One approach to make a third generation ultra-efficient cell is to use a mixture of wavelength optimized quantum dots.
[Image Source: Los Alamos Science & Tech Mag./U.S. Department of Energy's NNSA]

While quantum dots are generally thought to be amenable to thin film cell "roll-to-roll" printing processes, the precise methods to do this on a mass production scale still have to be ironed out.  Furthermore, the quantum dot cells measured in this study exhibited a pretty low 4.5 percent efficiency.  While that sounds quite bad, it’s largely a result of the lower amount of quantum dots used in the absorbing layer.

If quantum dot deposition techniques can be refined, the aforementioned "third" generation mixed cell could be finally realized.  If somebody is going to do that, it will probably be Professor Nozik's team at the NREL.  After all, they're who first discovered how to play the grand MEG prank on the laws of physics in the first place.

With these third generation solar cells, the technology may finally have the legs under it to compete with cheaper power generation methods (e.g. carbon-based fuels and nuclear energy).  That's not only good news for mankind's terrestrial future; it's good news for future interstellar travellers, who will likely rely heavily on a mixture of solar and nuclear (fusion) energy.

Sources: Science Magazine, NREL

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Umm... One thing...
By MrBlastman on 12/20/2011 8:59:35 PM , Rating: 2
interstellar travellers, who will likely rely heavily on a mixture of solar and nuclear (fusion) energy.

Solar Power is useless for interstellar travel. Perhaps it will work for interplanetary travel (at least within the inner solar system and to a lesser extent the outer), but once you pass the termination shock/helopause/bow shock, it becomes exceedingly less useful, especially when a starship is concerned. I'd go so far to say that once you reach this point, it is basically useless and actually far before that.

As far as fusion... we can't even pull off nuclear fusion on our own planet yet so we're quite a ways off from putting it in a starship. I'd say for now, fission will reign supreme for the time being.

Otherwise, interesting article. :)

RE: Umm... One thing...
By JasonMick on 12/20/2011 10:17:09 PM , Rating: 2
Solar Power is useless for interstellar travel. Perhaps it will work for interplanetary travel (at least within the inner solar system and to a lesser extent the outer), but once you pass the termination shock/helopause/bow shock, it becomes exceedingly less useful, especially when a starship is concerned. I'd go so far to say that once you reach this point, it is basically useless and actually far before that.

As far as fusion... we can't even pull off nuclear fusion on our own planet yet so we're quite a ways off from putting it in a starship. I'd say for now, fission will reign supreme for the time being.

Otherwise, interesting article. :)

I was more referring to the colonization process, once inside the heliosphere of other solar systems, not as in powering the trip in between. ;)

I think the trip there will be heavily powered by fusion, but once you get in the system, solar will come in handy. It also could be very handy, say in creating automated 'bots to harvest minerals from the asteroid field of our system or other systems.

In theory some asteroids should contain the necessary minerals to make say quantum dots or other cell designs, so you could in theory have a self-replicating mining task force. When they finish mining, you can just slag them all and use the metal and semiconductor to make a space outpost or something!

Just don't program them with a survival instinct, or mankind could be in trouble...

RE: Umm... One thing...
By bh192012 on 12/21/2011 3:46:48 PM , Rating: 2
You may find redemption in beamed laser power originally collected by solar panels from the sun.

RE: Umm... One thing...
By TSS on 12/21/2011 4:46:03 PM , Rating: 2
IMO if we find a way to use fusion as a way of powering spaceships, for colonists it'll be far more likely they'll land and convert their spaceship into the first power plant on the surface. Then use that power to charge battery operated construction robots.

After all, their colonists. They aren't exactly counting on a return trip. And the planets we'll colonize will have more then enough resources so the colonists don't have to resort to mining asteroids for quite a while. Otherwise, why colonise at all and not just build a mining station in orbit?

RE: Umm... One thing...
By Jeffk464 on 12/20/2011 10:21:56 PM , Rating: 2
The best long term power source for long term use in space is nuclear. Look at the power source in the new rover going to mars.

RE: Umm... One thing...
By Arsynic on 12/21/2011 1:20:06 PM , Rating: 1
Bububub Fukushima bubububub...

RE: Umm... One thing...
By ClownPuncher on 12/21/2011 2:27:43 PM , Rating: 3
It's fitting that you didn't use a complete thought.

RE: Umm... One thing...
By geddarkstorm on 12/21/2011 2:51:28 PM , Rating: 3
Thank goodness there are no tsunamis in space.

RE: Umm... One thing...
By omnicronx on 12/21/2011 4:58:58 PM , Rating: 2
Lies.. the best long term power source is Tim Tebows throwing arm. I can take bathroom break in between the windup and the trow.

Maybe if he was not so busy giving 110%, he could release the ball quicker!!!

</end Tebow rant>

RE: Umm... One thing...
By axeman1957 on 12/21/11, Rating: -1
RE: Umm... One thing...
By themelon on 12/21/2011 11:25:29 AM , Rating: 5
They operate on fission, not fusion

BIG difference

RE: Umm... One thing...
By MrTeal on 12/21/2011 11:27:05 AM , Rating: 3
Unless the Navy has decided to let submarines set off a nuke behind them to rocket jump, you are very very wrong on this.

RE: Umm... One thing...
By MrBlastman on 12/21/2011 12:34:16 PM , Rating: 1
Are you illiterate?

I said FISSION, not FUSION. The Navy uses FISSION.

Here, let me help you:

Go subscribe to it. Practice your vocabulary. Then, when you are done, call these people:

And buy a whole set. Read them. Memorize them.

THEN, after you've done that... Go here:

Learn everything they have to offer. After you do that... and only after...

Go apply to a College of your choice and get a degree in a science. Then proceed to profit.

You sir, you fail at teh intarwebs.

Then again, maybe the Navy does use fission. They've got us all fooled I bet. Wait, I look up in the sky and see a bright star. Maybe not. Oh, but wait. Maybe they have! It isn't a Star, it is a giant LCD display! What?! No! It can't be!

Oh, but yes, yes it is! The Navy has managed to build a Dyson sphere around our whole star and are absorbing all the fusion-based energy it is outputting. Our Navy is teh masters of our Solar System! They singlehandedly have managed to propel us to a 2 on the Kardashev scale without us ever knowing!


Or maybe not. Maybe it is all just one brilliant fantasy. We might never know. The G-Men won't have it. Oh wait. Shh, I better be quiet. They're after me! *slinks off and hides in the shadows*

RE: Umm... One thing...
By MrBlastman on 12/21/2011 12:36:44 PM , Rating: 3
I singlehandedly failed at my own post. As such, in grand fashion, I bend my head between my legs and lick my own buttocks:

Then again, maybe the Navy does use fission.

should be:

Then again, maybe the Navy does use fusion.

Oh, and by the way, it is only bad on the first lick. Every subsequent lick thereafter tastes less and less worse as your taste buds slowly die off in utter disgust.

RE: Umm... One thing...
By muhahaaha on 12/21/2011 1:56:41 PM , Rating: 4
How many licks does it take to get to the Tootsie Roll center of your buttocks?

RE: Umm... One thing...
By MrBlastman on 12/21/2011 3:59:32 PM , Rating: 2
It depends on how much salad there is to toss. Remember, proper eating habits dictate you must first toss and eat your salad before you can have your Tootsie roll...

RE: Umm... One thing...
By prophet001 on 12/22/2011 9:18:17 AM , Rating: 2

Two things.

1. Tim Tebow rocks :D

2. We have created fusion on this planet in tokamak reactors for years. As well, universities in california are making headway on linear fusion colliders. The energy gained does not warrant the energy invested but still. We have done this. Oh, the hydrogen bomb is a fusion bomb too.

RE: Umm... One thing...
By MrBlastman on 12/22/2011 11:28:27 AM , Rating: 2
Correct, we have done fusion. The problem is, we can't sustain it with a net positive result--i.e. the sum of energy input into the process still exceeds the output. Until we can do this, fusion is not even remotely viable.

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