
Breaking max efficiency of solar cells by activating 2 electrons with 1 photon - dirtyaura
http://news.mit.edu/2019/increase-solar-cell-output-photon-2-electron-0703
======
eloff
This is incredible. I mean it's a long way to seeing this in affordable
silicon solar cells, but still amazing to me that the materials engineering
can accomplish things like this.

It leads to roughly a twenty percent boost in theoretical max efficiency. If
that can be translated to a twenty percent actual improvement, that's huge.

~~~
toomuchtodo
We're already below 2 cents/kwh utility scale cost for solar and utility scale
battery storage [1]. We're rapidly approaching the point where the solar is
"almost free" and you're just going to pay for the battery storage (until that
cost is rapidly driven down as well).

[1] [https://www.utilitydive.com/news/los-angeles-solicits-
record...](https://www.utilitydive.com/news/los-angeles-solicits-record-solar-
storage-deal-at-199713-cents-kwh/558018/) (Los Angeles solicits record solar +
storage deal at 1.997/1.3-cents kWh)

~~~
HereBeBeasties
There's nothing wrong with solar per-se, but it's just not sufficiently power
dense if you're talking national scale power usage.
[https://www.finder.com/uk/solar-power-
potential](https://www.finder.com/uk/solar-power-potential) seems to think
that countries like the UK would need more than 10% of their area covering
with solar panels to cover their energy needs. I think you'd need to be
talking about an order of magnitude improvement in efficiency to really start
making it look really feasible as a significant way to change our fossil fuel
usage, no?

~~~
llukas
Methodology is flawed. If you switch from burning oil in ice engines to
electric you use at least 50% total energy less.

So they overestimate area needed.

~~~
gtirloni
What's that due to? Less energy lost to dissipation?

~~~
Robotbeat
For the same kind of reason solar panels only access about 15-20% of the
energy that hits them.

~~~
Robotbeat
I was serious. Solar cells are like solid state heat engines in that they’re
fundamentally limited by Carnot Efficiency (which is theoretically high for
solar cells as the effective temperature of sunlight is high). If we’re going
to use “primary energy” and not useful electrical or mechanical energy, then
we’d need to include the energy of sunlight falling on the solar panels, which
is about 5 times the useful electrical energy produced.

And just like actual heat engines, solar cells are improving. The best cells
have achieved 47% efficiency in the lab, comparable to a high performance
combined cycle natural gas plant (note that thermal power plants often cheat a
bit by listing the efficiency in terms of the low heating value of a fuel, ie
the heat produced not counting condensing the water vapor out).

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geoalchimista
"2 electrons with 1 photon" \--- that is how photosynthesis works in nature.
Great thinking!

~~~
snaky
Photosynthesis is less efficient than solar panels. According to the Wikipedia
page on photosynthetic efficiency, typical plants have a radiant energy to
chemical energy conversion efficiency between 0.1% and 2%.

~~~
geoalchimista
It's true but photosynthesis has two stages. The light reaction stage is quite
efficient. Chlorophyll a is slightly more efficient than commercial solar
panels. The dark reaction stage, which assimilates CO2 to produce
carbohydrates, is the limiting step that drives the efficiency down.

~~~
anticensor
Non-light sensitive parts of photosynthesis has to be inefficient because
otherwise aerobic respiration would be inefficient.

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mycall
Interesting how "through a process called singlet exciton fission" and other
article "General Formula for Bi-Aspheric Singlet Lens Design Free of Spherical
Aberration" [1] both deal with Singlets. Odd coincidence of the day.

[1]
[https://www.osapublishing.org/ao/abstract.cfm?uri=ao-57-31-9...](https://www.osapublishing.org/ao/abstract.cfm?uri=ao-57-31-9341)

~~~
albutr
FYI they are totally different "singlets", a singlet exciton refers to the
spin state of the particle (as opposed to a triplet exciton, which has a
different total spin). A singlet lens is just a lens with a single simple
component.

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dharma1
This is great - but isn’t hafnium pretty rare?

~~~
kragen
Kind of, but not enough to matter for this application anytime in the next
several years. The USGS report at
[https://www.usgs.gov/centers/nmic/zirconium-and-hafnium-
stat...](https://www.usgs.gov/centers/nmic/zirconium-and-hafnium-statistics-
and-information) says the US imported 36 tonnes of hafnium last year. If a
three-atom-thick layer of hafnium is 0.6 nanometers and weighs 13.3 g/cc,
those 36 tonnes would cover 4500 km², enough to produce about 700 GWp of solar
energy. The US installed 10.6 GWp of photovoltaic generation capacity last
year, which would amount to one seventieth of its hafnium imports. (China
installed 45 GWp.)

It _is_ relevant once photovoltaic energy becomes the main power source for
the humans, because the materials used in current photovoltaic cells are
dramatically more abundant than hafnium.

~~~
dharma1
Thanks for putting it into context!

Generally I wonder about sustainability of rare earths (and other elements
with limited supply like helium). We build a lot of products that don't last
very long - some of it gets recycled, a lot of it doesn't. Wonder what humans
200 or 2000 years later will think of our consumption

~~~
kragen
Helium is a special case: when it leaks into the atmosphere, it eventually
escapes into space. So the humans really do _consume_ helium, just as the
humans consume fossil fuels. By contrast, all the lithium, phosphorus, carbon,
aluminum, iron, hafnium, zirconium, and erbium ever mined on Earth is still on
Earth, except for the extremely small amounts sent to Venus, Mars, the Moon,
and interstellar space on spacecraft, or transmuted to other elements in
nuclear reactors. It is not consumed, just moved around a bit. _Where is it_?

Well, it varies by element.

While some of the phosphorus makes up human bones, most of it is dissolved in
the ocean as phosphate, enormously less concentrated than the original
deposits of apatite and other minerals it was mined from. At some point the
humans might run out of easily accessible apatite deposits, since phosphorus
only makes up 0.1% of the Earth's crust. If we consider only the top 2 km of
the continental crust (29% of the surface) to be "easily accessible", that's
only 3.0 × 10⁸ km³ of rock, or about 7.1 × 10¹⁷ tonnes, of which about 7.1 ×
10¹⁴ is phosphorus. Since phosphorus is currently being mined at 153 million
tonnes per year, the humans are currently on track to deplete most of it in
only 4.6 billion years, considerably earlier than the Earth is on track to be
destroyed by the sun.

As for the carbon, it's mined for its chemical potential energy, which is lost
when it is burned; but the carbon remains on Earth. 870 billion tonnes of it
is in the atmosphere, of which about a third is due to the humans setting
rocks on fire; a comparable amount has been added to terrestrial biomass and
carbon in the oceans. The part in the atmosphere is relatively easy to
recover; existing processes have comparable cost to the cost of digging up
rocks. This is not being done at scale because it is "unprofitable".

More than half of the aluminum that has been mined on Earth is still in use.
The other half is mostly in landfills, where it is less concentrated than the
bauxite ores that are commercially mined, but less oxidized.

Most of the iron that has been mined is in landfills, but not only ores but
also slag heaps near foundries are a more concentrated form of iron.

Rarer metals like gold, hafnium, zirconium, and erbium are mostly in
landfills. They mostly do not readily form soluble ions to leach into the
ocean or groundwater, and they mostly only find their way into the air when
incinerated. Their concentrations in landfills are dramatically higher than
the concentrations in the ores they are mined from, and this is becoming a
"profitable" way for the humans to spend their lives before being killed by
the hazards of the landfills. Presumably automation will speed up this
process.

~~~
dharma1
This is really insightful. A lot of human “progress” and activity includes us
exporting growing amounts of high entropy waste, for a mostly unaccounted for,
deferred cost.

In most natural systems there is no waste - everything gets recycled. We
create a mess digging for elements around the planet, consuming the “value” of
the things we make from those elements (and large amounts of energy) in a way
which lasts a very short time, and disposing of the waste which then mostly
get spread around the planet in even greater mess. I find it difficult to see
how automation will easily solve our waste problem - it seems many orders of
magnitude easier to try to solve it _before_ human waste gets to a giant
garbled heap or gets spread around the atmosphere/oceans/groundwater/(soon low
earth orbit) where it’s difficult to retrieve, and seems to cause huge
problems to the natural world (and ultimately is) by being in places it
shouldn’t be.

Why are humans so good/bad at creating a mess? Are we equipped with
skills/incentives to solve this conundrum, or will we continue to drown in our
mess?

~~~
kragen
The Earth doesn't recycle solar energy; it wastes ≈100% of it by radiating it
away into space. Over 99% of it never even gets photosynthesized, just being
absorbed as heat and then reradiated as infrared. Far less than 1% was ever
accumulated in fossil fuels, and of course accumulating it as heat at the
surface would be fatal. Nor does it recycle, for example, crustal iron; once
the iron finds its way into a subduction zone and melts into the mantle, it
gradually finds its way to the core, never to return. Even the fast CO₂ cycle
is noticeably leaky, as a substantial amount of CO₂ is taken up by marine
organisms that sink to the bottom and stay there, eventually being compacted
into limestone and remaining limestone for hundreds of millions of years (the
slow CO₂ cycle), until probably being liberated again by volcanism. So, I
think that for most definitions of "waste", it is false that in most natural
systems there is no waste.

It's true that recovering materials from the atmosphere and oceans is more
difficult than mining them. But most of the materials we're discussing don't
end up in the atmosphere and oceans; they end up in landfills. And recovering
them from landfills is, generally, much easier than mining them from natural
deposits.

Landfills are much more concentrated and heterogeneous deposits of "valuable"
elements than the ores from which they are mined; for example, a single
accidentally discarded catalytic converter may contain 5 grams of platinum-
group metals, which are reduced and relatively easy to recover; commonly mined
_natural_ ores of platinum-group metals (the Merensky, chromitite, and contact
types of deposits) typically contain 5 grams _per tonne of ore_ , mostly
oxidized and thus requiring not only froth flotation but also smelting. And
catalytic converter platinum-group elements are far from the only "valuable"
elements in landfills.

The trouble is that when the humans go digging around in landfills with their
bare hands, it reduces their life expectancy a lot, because of poisons,
injuries, and poor safety practices around refining. This is not a new problem
for mining — the Spanish silver mines were a death sentence for the enslaved
indigenous Americans sentenced to work in them — but the particular measures
needed to solve the problem are different for landfills than for other mining.
Automation will largely solve them. I suspect that the same ritual-pollution
taboos that cause the humans to deprecate garbage collectors, cannibals, and
undertakers are a major factor in the slow development of landfill mining.

A Kessler syndrome in LEO will be only a short-term problem, as the lifetime
of objects there is limited to decades, not even a single million years!, by
atmospheric drag, and the quantity of mass there is limited by launch "costs".
MEO Kessler syndrome would be a more serious problem.

Doing things creates messes. Lowering entropy locally — one definition of life
— raises entropy globally. Doing more things will create more messes. Doing
things intelligently can create safely contained messes.

------
Sniffnoy
So is two the most possible? Is it possible to activate three electrons with a
single photon?

~~~
sp332
Yeah but you'd have to start with very energetic photons. The article says
that you mint get two elections when a green or blue photon hits, so to get
three even theoretically you'd need to use ultraviolet optics photons. And
there just aren't enough of those in sunlight you make a noticeable difference
in efficiency.

~~~
perlgeek
Let me try to do the math here, please correct me if I'm wrong.

The band gap of Si in solar cells is about 1.1 ev. To excite three electrons,
a photon thus needs 3.3ev, or about 370nm.

The visible spectrum starts around 380nm or 390nm, depending on which source
you believe. So the triple photon frequency would be _very_ near UV.

According to this spectrum [0], there's a sharp drop in solar power around the
370nm.

It seems that with the 1.1ev band gap, exciting three electrons doesn't seem
to be worth it. But, if you could reduce it just a bit (by doping, most
likely), it might be worth considering.

Of course, reducing the band gap would likely lead to much worse performance
overall, so one would need a multi layer approach, which sounds pretty
complicated.

And then you need to pray that your glass cover doesn't absorb the UV light,
which seems to be a pretty close call[1].

[0]:
[https://en.wikipedia.org/wiki/Sunlight#/media/File:Solar_spe...](https://en.wikipedia.org/wiki/Sunlight#/media/File:Solar_spectrum_en.svg)

[1]:
[https://en.wikipedia.org/wiki/Soda%E2%80%93lime_glass#/media...](https://en.wikipedia.org/wiki/Soda%E2%80%93lime_glass#/media/File:Soda-
lime_glass,_typical_transmission_spectrum_\(2_mm_thickness\).svg)

~~~
pixl97
All that said, in space based operations your UV isnt going to be filtered and
may allow you to generate more power.

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georgeburdell
Pardon my ignorance but don't avalanche photodiodes, scintillators, phosphors,
etc. excite many electrons per 1 photon? Why is 2 electrons for 1 photon
noteworthy?

~~~
ambicapter
I assume those are powered.

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selimthegrim
[https://cen.acs.org/energy/solar-power/Supercharging-
silicon...](https://cen.acs.org/energy/solar-power/Supercharging-silicon-
solar-cell/97/web/2019/07) has some good takes from people in the field

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selimthegrim
I'm glad to see Baldo has moved on from useless experiments on hexacene.
Sincere congratulations to him and his group.

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m3kw9
I mean it all means bunk till a mass produced product is on the shelves. A lot
of these “breakthrus” fail to note edge cases like scalability, durability

~~~
rossdavidh
While true, it's worth noting that it's happened. It's rather like when
there's a theoretical breakthrough that might help to cure a type of cancer.
Will it result in an actual medical breakthrough that helps people? Usually
not. But, occasionally yes, and that's huge, so it's still worth noting. If
you don't get enough shots on goal, you never score.

