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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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RE: The break through needed
By inighthawki on 12/20/2011 7:48:23 PM , Rating: 4
Nonsense, that problem an be solved with more efficiency plus a capacitor bank to store extra energy produced that isn't used. Then it's just a matter of not using more than you can make.

RE: The break through needed
By cpeter38 on 12/20/2011 8:23:09 PM , Rating: 5
You would need an incredible volume of capacitors to store a night's worth of energy from even the smallest of solar farms. The only* currently viable large scale energy storage is pumped hydro. However, pumped hydro is politically difficult.

*(Flywheel & molten salt storage work, but they need significant development to bring down the costs)

RE: The break through needed
By FredEx on 12/20/2011 9:24:35 PM , Rating: 2
I'm confused. I know people doing solar and are off the grid. They don't go dark at night. They are not using any super secret high tech storage or something cost prohibitive. These are not people with money to burn. Should I explain to them they are doing something not yet possible?

RE: The break through needed
By MrTeal on 12/20/2011 9:31:15 PM , Rating: 5
Batteries != Capacitors

RE: The break through needed
By Fritzr on 12/21/2011 12:23:37 AM , Rating: 2
These and other power storage methods can be further supplemented by Backup_power_generation.

A system with "backup electric storage" draws down power_storage when the generator is offline

Note: "generator" in this usage is the device or devices that put electricity onto the user's grid including, but not limited to, solar power cells, fuel cells, national power grid, backup generator in the garage and other sources of power that can be used to put charge into the power_storage.

next statement is?

RE: The break through needed
By EricMartello on 12/21/2011 2:29:06 AM , Rating: 1
You're a moron. Pizza is a food and so is sushi, but they are not the same thing.

Capacitors store electricity in an electrical field; batteries rely on a chemical reaction. A hydro-electric system relies on gravity and a large source of water.

All three of the above methods store electricity but are not necessarily viable short term methods. Capacitors charge and discharge quickly. Batteries charge slowly and inefficiently; you'd have a constant high-current power draw on the system while they are charging during the day, resulting in an electrical "overhead"...the hydroelectric thing is just lol...not even practical on small or medium scales.

Improved solar panels would be ideal as SUPPLEMENTAL power for people on the grid. As far as being an off-grid primary power source...I'd say possible but not practical.

RE: The break through needed
By piroroadkill on 12/21/2011 4:52:15 AM , Rating: 2
Actually, in the UK, we have a pumped-storage hydroelectric plant to help in times of demand.

Infact, it seems there are a bunch in the US too.

RE: The break through needed
By Kurz on 12/21/2011 7:47:16 AM , Rating: 2
He said Small and Medium Scales... What you linked are Large and Massive scales intended for large cities.

RE: The break through needed
By tng on 12/21/2011 1:12:41 PM , Rating: 2
Improved solar panels would be ideal as SUPPLEMENTAL power for people on the grid.
Really even at night? That is where this started, what happens at night? Batteries, capacitors, Hydro storage schemes, etc....

If you want to comment stay on topic and remember that solar power at night is a oxymoron.

RE: The break through needed
By EricMartello on 12/21/2011 4:09:24 PM , Rating: 2
Improved solar panels would be ideal as SUPPLEMENTAL power for people on the grid. As far as being an off-grid primary power source...I'd say possible but not practical.

Really even at night? That is where this started, what happens at night? Batteries, capacitors, Hydro storage schemes, etc....

If you want to comment stay on topic and remember that solar power at night is a oxymoron.

Did you even read what you quoted? SUPPLEMENTAL power for people on the grid means that at night, the people would be getting their power FROM THE GRID and whenever there is sun shining, they get most or all of their power from their panels.


If you want to comment, try reading the posts you reply to and actually understand what they're saying before making an asinine remark.

RE: The break through needed
By tng on 12/21/2011 4:43:55 PM , Rating: 2
Of course you didn't get the point, but from someone who is prone to calling people moron and asinine, I would expect no less.

RE: The break through needed
By EricMartello on 12/21/2011 10:14:19 PM , Rating: 2
Of course you didn't get the point, but from someone who is prone to calling people moron and asinine, I would expect no less.

It isn't me who isn't getting the point. Let's see why you are also a moron:


Mr.Teal Said:
Batteries != Capacitors

This is true, to which idiot #1 replied:


Fritzr Said:

next statement is?

To which I posted my initial reply citing the moronity of this post, which attempts to lump together vastly DIFFERENT methods of storing power as if they are each equally viable and appropriate for any situation.

Then you chimed in with a statement that evidences a total lack of reading comprehension...


TNG Said:
Really even at night? That is where this started, what happens at night? Batteries, capacitors, Hydro storage schemes, etc....

Ignoring the fact that I DO NOT SUPPORT USING BATTERIES or other forms of storage as a crutch to allow solar power to act as the "primary" power source for electricity. Get it?

The point is that supplementing grid power with solar power is viable today. Trying to live "off the grid" relying SOLELY on solar power is as dumb as these last few comments.

RE: The break through needed
By tng on 12/22/2011 11:47:12 AM , Rating: 2
Rude, just rude. Nice that you know how to call names and all, but it is easy to do when you are online I guess.

Again you missed MY point, but it is easier than thinking.

RE: The break through needed
By EricMartello on 12/23/2011 12:19:51 AM , Rating: 2
You had no point, brah, so nothing to miss. Solar power requires the sun and nobody in this thread suggested that one could generate electricity relying on solar panels at night. Why are you still talking? Just like there was no point to your original reply, your follow-up comments are just as pointless.

RE: The break through needed
By testerguy on 12/26/2011 1:47:48 PM , Rating: 2
You're a moron. Pizza is a food and so is sushi, but they are not the same thing.

But they are both food.

So if I said:

Pizza = Food
Sushi = Food

...I would be absolutely correct. Just as he was, since he never claimed they were all the same, but simply pointed out that they are storage mechanisms - and in this context, they are all potential solutions to the problem.

Batteries to store energy are in widespread use in a whole variety of domestic solar power solutions. The 'overhead' argument is nonsense - the excess power generated during the day is used to charge specifically designed batteries, which are automatically switched to during the night - it works, it's practical, and it means all-day power from Solar.

RE: The break through needed
By Solandri on 12/21/2011 4:27:24 AM , Rating: 3
To put it more nicely than the others:

- Capacitors store a small amount of power for a short time very efficiently (close to 100% if you use the power quickly).
- Batteries store moderate amounts of power for a long time moderately efficiently (80%-95% for slow charges).
- Pumped hydro stores large amounts of power for an indefinitely long time at poor efficiency (60% or less).

Unless your power demands at night and during bad weather are very low, you either need huge banks of batteries, and/or a huge amount of excess solar capacity for pumped storage.

To give you an idea of the scale we're talking about, one of the solar proponents here suggested solar-powered street lights which wouldn't need to be on the grid. They could get their electricity from a solar panel on top of them, store it in a battery, and use that to run at night.

Assuming a 400 Watt sodium street lamp (they go down to less than 100 W, but the ones on the 20+ ft tall posts are 400 W), and 12 hours of daylight, 12 hours of night (I won't even get into Winter), and leaving the street light off for 1 hour of dawn and 1 hour of dusk, you need 400 W * 10 hours = 4 kWh of electricity.

Assume the battery charger is 95% efficient (85%-90% is more likely but I'm trying to be nice), and 20% efficient panels (commercial grade panels are about 16% efficient right now), having the panels tilted at the optimum angle, and in the desert Southwest. Under ideal conditions, such a panel will average about 28.9 W/m^2 for a day, or about 57.8 W/m^2 for the 12 hours of daylight. That's 693 Wh/m^2 for 12 hours of daylight.

You'd need 5.8 m^2 of panels just to collect enough electricity to power this single street light through the night. So each street lamp would need to have a 10'x6' panel sitting on top of them. And from all the assumptions I made, that's a very conservative lowball estimate.

Solar is great if you have lots of surface area to put the panels, very low power requirements, and are off the grid. But for any substantial power load, it quickly consumes huge quantities of land.

RE: The break through needed
By JediJeb on 12/22/2011 6:28:52 PM , Rating: 2
How would this work out if you replaced the 400W bulb with a more efficient array of LEDs?

RE: The break through needed
By inighthawki on 12/21/2011 6:50:17 PM , Rating: 2
I have to apologize for my poor use of terminology, I shouldn't have said capacitor bank, since it's not what I meant. What I did mean was some method of energy storage (batteries, capacitors, etc) to store the energy.

Didn't really mean to start a small argument, I was more or less trying to point out that it's certainly possible to rely on other existing technologies to outlast the night.

"We basically took a look at this situation and said, this is bullshit." -- Newegg Chief Legal Officer Lee Cheng's take on patent troll Soverain

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