New Solar Cell Gives Its "110 Percent" in Efficiency
December 20, 2011 6:23 PM
Like Tebow, these new solar cells are giving their "110 percent" week in and week out.
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
be over 100 percent -- or even
to 100 percent -- in traditional physics).
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:
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:
-- the high energy exciton ("X") becomes a "multi"-X, decaying through a dense range of multi-X states.
-- a mixed "virtual" state consisting of multi-X and X (think superposition) is triggered by photon energetic absorption.
-- 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
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
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 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
[abstract] in the peer-reviewed journal
, 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.
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
"If a man really wants to make a million dollars, the best way would be to start his own religion." -- Scientology founder L. Ron. Hubbard
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