
Japanese company develops a solar cell with record-breaking efficiency - Jaruzel
https://arstechnica.co.uk/science/2017/03/japanese-company-develops-a-solar-cell-with-record-breaking-26-efficiency/
======
jartelt
There seems to be a lot of confusion about efficiency. For a single junction
solar cell (like the type of solar cell talked about in the article) the
theoretical efficiency is 34% (under 1 sun illumination). The best a silicon
device can do is ~29% because silicon has a slightly lower than ideal bandgap.
Single junction solar cells made out of GaAs have gotten to ~31% efficiency
because GaAs has an ideal bandgap. However, GaAs is super expensive. Achieving
26% efficiency when the theoretical efficiency for silicon is 29% is super
impressive. Engineers have basically squeezed every ounce of potential real
world performance out of single junction silicon solar cells.

------
inetknght
[https://news.ycombinator.com/item?id=13939040](https://news.ycombinator.com/item?id=13939040)
~34%

[https://news.ycombinator.com/item?id=13938690](https://news.ycombinator.com/item?id=13938690)
~40%

[https://news.ycombinator.com/item?id=13938953](https://news.ycombinator.com/item?id=13938953)
46%

[https://news.ycombinator.com/item?id=13939181](https://news.ycombinator.com/item?id=13939181)
~86.8%

[https://news.ycombinator.com/item?id=13939399](https://news.ycombinator.com/item?id=13939399)
95%

One... two... three... four... five theoretical maximums in comments.

Something tells me that there's a lot more research to be done (or knowledge
to be learned, at the very least) available here than we realize.

~~~
syntaxing
Admittedly, there still a lot of room for solar panel research for the
upcoming years which is pretty exciting!

The discrepancies is often due to fact we're defining "solar panels" quite
different for each application and also the definition of input energy is
different.

A great example is engines. The theoretical (Carnot) efficiency is limited by
the heat difference between the source and the sink. You can burn fuel at a
higher temperature and pressure which will give you great efficiency, but it's
not realistically possibly due to physical constraints (usually the steel that
houses the explosions). That's why steel engines are often said to have a
maximum fuel efficiency of 37%.

This same idea applies to solar panel but it's even more convoluted. What is
input energy? Is it all the energy from the sun or is it only the energy
within the energy band that the panel can "see"? What is efficiency? The
efficiency for different energy band is different for each solar panel as
well. Worst of all, this efficiency changes for different temperature and is
effected by hysteresis! By the time you store the energy, you also lose
efficiency depending on the internal resistance of the energy storage (That's
why MPPT exist and fun fact of the day, from what I know, there is still no
reliable analytical method to account for this. Most MPPT trackers are done
empirically).

Sorry for the long rant, but yeah, solar panels are confusing and there is a
surprising about of research that still needs to be done!

~~~
nojvek
I heard about carnot energy but didn't realize the calculation was this
simple. Suppose we say we get a insulated material that is string enough to
handle high temperatures. We can beat the 37% right?

~~~
syntaxing
It's a partial yes. You have to worry about pressure as well. You can burn
fuel at a higher temperature by igniting the gas at a higher pressure. But
there are certain constraints due to the fuel. If the pressure is too high,
the fuel will spontaneous combust. That's why there are different octane
rating of fuels and why higher octane fuel gives you better fuel efficiency
(given your engine has some sort of valve timing like VTEC). This is also why
Diesel engines tend to have better mileage than gasoline engines. Even though
gasoline engines will perform better at the same pressure as Diesel, there
just no way to bring the gasoline to the same pressure that the Diesel can
handle.

When and how you combust the fuel can give you different fuel efficiencies as
well. For instance, the Brayton cycle can achieve easily over 50% fuel
efficiency. But in order to achieve this, you need a special housing (jet
engines uses this cycle) to be able to compress the fuel to the specific
pressure and also handle the pressure. Also, special fuel is required. These
two things makes it not really practical for the standard consumer.

On a tangent, Internal combustion engines is probably one of the funnest and
hardest class I took in graduate school...

------
nicholas73
This is what I understand about efficiency after working with photodetectors
(not used as solar cells, but similar in principle).

1\. The sun emits a wide spectrum, with a long tail in the infrared.

2\. Materials are only photoelectric at certain wavelengths.

3\. The sensitivity to each wavelength depends upon design which is ultimately
is related to the penetration depth of light in the material (longer
wavelengths travel longer distances before reacting with the crystal
structure).

4\. Silicon is the most common and cheapest material, but its sensitivity
rolls off sharply towards UV and infrared. Other materials are orders of
magnitude more expensive. Not just in manufacturing, but the materials
themselves.

5\. To capture a wide range of spectrum you need a very long depletion region
(kind of neutral area between p-n junctions). This has its own design
tradeoffs.

So the end result is that the real world efficiency limit can have many
reported figures, from theoretical energy physics to practical material,
design, and manufacturing limits. The biggest contributor to loss is that
silicon is simply not able to capture much more than the visible spectrum.
Next is that even with silicon there is a sensitivity curve (efficiency of
converting photons to current per wavelength), and then designing so that you
can capture the current (or else it would just recombine in the crystal).

This is why gain in solar seems to be slow. There are tremendous limitations,
and I have my doubts that there will be overcome any time soon.

But, there is plenty of sunlight, so I would say the real challenge is energy
storage. If sunlight can be stored cheaply and efficiently, then what we have
is good enough.

~~~
Zenst
I often wonder how good/cost effective optics would be, given the price of
glass is cheap and that could be used to focus a larger area upon a small,
more costly and yet efficient material.

But as you highlight, cost and with that a balance of cost/return is a huge
factor and whilst scale can help in many area's. It gets more detrimental with
more expensive materials.

~~~
nicholas73
I don't know the exact answer, but one should be able to calculate how
effective this method would be. You are knocking out electrons from a crystal.
Those electrons have to be replaced, so you are limited by the rate of
replacement. Also heat generation from the current.

My hand wavy gut feel is that optics wouldn't buy you that much. Silicon is
orders of magnitude cheaper. To give you an idea, an infrared detector made of
InGaAs about the size of a dime can already cost $5-10k. Solar cells of that
size are priced in ... dollars?

On the other hand, optics have been used for energy/storage in the form of a
heating crucible. I've heard that promoted by some experts.

------
syntaxing
To put things in perspective, the current theoretical maximum efficiency for
solar panels is about 34% [1]. There are a couple assumptions that are made to
derive this number so there are ways to exceed this maximum efficiency. Though
the assumptions made in the equations are fairly application to most solar
panels product (not the ones made in labs).

[1]
[https://en.wikipedia.org/wiki/Shockley%E2%80%93Queisser_limi...](https://en.wikipedia.org/wiki/Shockley%E2%80%93Queisser_limit)

~~~
Retric
There are already 46% efficient solar cells, the 33.7% limit is for a specific
type of solar cell (single p-n junction) which are cheaper to manufacture.

The maximum theoretical limit is ~86.8% for solar panels.

~~~
seiferteric
Is the maximum theoretical efficiency I often see stated just for type of
technology we use today (pn junctions)? Or is it a hard physical limit for any
light -> electricity system? Would it be possible that some new technology for
solar cells could be more efficient?

~~~
averagewall
I think the highest physical limit would be Carnot efficiency for a heat
engine driven by the temperature difference between the sun and the earth.
(6000K - 300 K) / 6000 K = 95%. There could be a different lower limit we hit
before that though.

~~~
seiferteric
That does make sense for an absolute upper limit, thanks.

------
cmrdporcupine
Around here it seems like solar installation companies are taking every
decrease in material costs and increase in efficiencies and eating it up by
increasing labour costs or profit margins.

It always works out to a bullshit calculation that "the system pays for itself
in 10 years" [if you ignore inflation, opportunity cost, and accept their
electricity rate projections and assume the gov't won't cut green electricity
subsidies]. That number never seems to change, it's always 10 years.

I'd love to get panels put on my house and shop, I have a lot of square
footage. But it seems like there's some very greedy middlemen between me and
the panels and I lack the skill to install myself.

~~~
walrus01
You're only considering grid tie when you say that a system won't really pay
for itself in ten years. For off grid solar there are a number of places
(example: very rural parts of ID and WA) where it might cost $40,000 to extend
code compliant grid electricity from the nearest road to a new bare land
property. And that is before you pay any monthly bills for ten years. Versus
spending $35,000 one time on a big ground mounted solar system, charge
controllers and a set of Tesla power wall 2 for battery storage.

Cost and efficiency also hugely impacts developing nation solar (most of rural
Africa) where no grid whatsoever currently exists.

~~~
ZeroGravitas
Interestingly, this applies on very small scales too. For certain street
furniture like parking meters etc. it can be cheaper to install solar and a
battery than to dig a cable.

~~~
walrus01
In terms of city government unionized labor salaries and equipment cost, or
hiring a contractor cost, it can be VERY expensive per meter to dig up an
existing concrete sidewalk/curb and road to run code compliant AC electrical
to a parking meter. Or something like a bus stop arrival time sign. Looking at
$20,000 per unit in some cases, or more. And then there's the question of
where do you get the AC power from in a built up downtown type area, you can't
just tap into the nearest building, it usually involves dealing with the
electrical grid for the streetlight system which was not designed to support
thousands of small accessory loads hanging off it.

------
candiodari
I'm not sure I like this. It seems that we're very clearly in the area of
diminishing returns when it comes to solar panels.

[https://upload.wikimedia.org/wikipedia/commons/4/42/Efficien...](https://upload.wikimedia.org/wikipedia/commons/4/42/Efficiency_chart_rev_05-2012.png)

The conclusion of that chart seems to be : concentrated solar power (even on
small scale) is unbeatable, and tops out (realistically) at ~40% efficiency.

So now we should probably work on very, very cheap lenses if we want to
advance solar panels, and quick ways to make small, very cheap panels.

~~~
Arnt
Why?

AFAICT, there are three ways to double the output of a solar cell: Double the
area, double the efficiency, and move the cell to somewhere with twice as
bright sunlight and install a cable. That one option presents intractable
problems doesn't seem to matter very much when either of the two others are
available.

~~~
petra
Area isn't unlimited in the residential market, place is fixed and
installation is expensive.

~~~
taneq
That's why you need to install the cable.

------
jacquesm
The highest efficiency for solar cells is at 46% already (multi-junction,
concentrating). So this is not record breaking per se but record breaking for
this particular set-up.

Also: The best way to look at solar panel breakthroughs is to look at what
actually makes it to market. If I had a dollar for every solar cell
breakthrough that eventually did not make it to market I'd be fairly wealthy.

~~~
itp
I think your comment is the first use of "breakthrough," to be fair. The
article title and body refer to it as "breaking" the record. You are correct
about the (vastly more expensive) multi-junction concentrating cells having
higher efficiency, of course.

You're also correct that not all improvements make it to market. Without
research and continuing efforts to improve, nothing makes it to market. What's
the point of your second paragraph? The article makes it _very_ clear that
this is not in production, and speaks specifically to concerns about whether
the process is suitable for industrialization.

~~~
jacquesm
The point is that you see announcements like these at a rate of 4 to 5 every
year and hardly any of those ever make it to market. I've been active in the
renewable energy-scene off-and-on for two decades and I really can't count how
many times there have been announcements like the one here that did not go
anywhere. It makes me quite skeptical of announcements like these, they do not
really convey much in terms of information other than to create an optimism
about solar panel efficiency gains that is usually met with disappointment
when someone actually costs out a system and realizes that over approximately
a decade we tend to see gains on the order of 1 to 2%.

Hype simply doesn't help, at best this is 10 years away, worst you'll never
hear from it again.

~~~
philipkglass
This paper is closer to industrialization than most and I expect production on
scale in less than 10 years. It uses process steps that are already operating
on an industrial scale. It's achieved on an industrial area wafer (compare
with many academic records where cells are _tiny_ , less than 10 cm^2). It's
achieved on an industrial thickness wafer (some records use thinner wafers
that are closer to the theoretical optimum, but those thinner wafers are too
fragile to process industrially with current technology). It's just combining
features of two cell concepts that are already manufactured on an industrial
scale (interdigitated back contact cells and heterojunction cells). The core
patents on heterojunction silicon cells expired just a few years ago so I
expect more vanilla heterojunction and heterojunction-plus-other manufacturing
going forward.

~~~
jacquesm
Well, I certainly hope you are right and I'm wrong. Still, the devil will be
in the details and even with expired patents multi-junction cells are simply
more expensive to produce. Keep in mind they only have to be ~24% more
expensive to make to wipe out any efficiency gains and that's not a whole lot
of margin. If this succeeds you might see a fraction of that passed on to the
consumer and the cost of the bare panels long ago ceased to be the dominant
factor in a regular (domestic) installation.

For large areas it might work out to be beneficiary earlier.

~~~
philipkglass
Heterojunction cells are an approach to reducing recombination losses near
contacts in single junction silicon cells. Sanyo (now Panasonic) developed
them in the 1990s and has shipped gigawatts of them. It's the same Panasonic
cell technology that SolarCity (now Tesla) is expected to use in their New
York factory. The vanilla heterojunction cell design is actually one of the
simplest high-efficiency cells in terms of process flow. The main drawbacks
are patents (until recently) and the limited heat tolerance of a-Si layers;
standard screen printed silver metallization, fired at high temperatures, does
not work for these cells.

[https://www.researchgate.net/profile/Stefaan_De_Wolf/publica...](https://www.researchgate.net/profile/Stefaan_De_Wolf/publication/239735318_High-
efficiency_Silicon_Heterojunction_Solar_Cells_A_Review/links/0deec51c2bde240fb8000000.pdf)

~~~
jacquesm
I spent some time reading that, thank you very much for the pointers. Looks
like you are right and this is in fact something that might just happen.
Here's to hoping that it does.

------
kolbe
What does this mean? Of the energy that all photons hitting the solar panel
represent, 26.3% of it is converted into electricity?

~~~
ihaveajob
For reference, in
[https://en.wikipedia.org/wiki/Photosynthetic_efficiency](https://en.wikipedia.org/wiki/Photosynthetic_efficiency)
there is mention that photosynthesis has a nominal efficiency of about 30%,
but in practice the absorbed energy stays in the 3-6% range, with sugar cane
being exceptionally efficient at 8%.

~~~
the8472
That 3-6% range is stored energy after the plant consumed some of the produced
glucose to maintain itself and grow.

Photovoltaics don't do that on their own, so you can't compare that to a solar
cell's nameplate efficiency.

Either you have to compare PV efficiency with gross glucose production or you
need to compare the full PV lifecycle (production, installation, maintenance
and recycling) with the harvestable sugar content.

------
jhallenworld
Rectennas are another interesting possibility. The theoretical conversion
efficiency is 44% according to this:

[http://ecee.colorado.edu/~moddel/QEL/Papers/Joshi13a.pdf](http://ecee.colorado.edu/~moddel/QEL/Papers/Joshi13a.pdf)

------
contingencies
Zooming out a little, is it not true to say that minor efficiency gains in
collection are still dwarfed in comparison to storage and current conversion
losses?

~~~
smdz
No - because 2.7% (or 0.7 efficiency difference) increase is not insignificant
when you scale up. As an analogy, think of what 0.7% tax saving would mean to
you annually.

In reality solar-cells and batteries are not at all cheap to install and
maintain. We can scale up the solar cells and collect more energy. However,
that needs more surface area to be covered with these panels. We still need
better ways to store the energy efficiently.

~~~
TillE
Storing huge amounts of energy is easy: use pumps to add potential energy to
water.

This isn't ultra efficient, but it's cheap and probably good enough. We have
lots of experience with hydroelectric power.

------
rick_cheese
Will be interesting to see manufacturing yields but also efficiency losses
over time, compared to cells made currently.

------
pizza
[https://en.wikipedia.org/wiki/Quantum_dot_solar_cell](https://en.wikipedia.org/wiki/Quantum_dot_solar_cell)

Could you tune/adjoin these to span a very wide band of wavelengths?

------
Ulti
Media is a few days lag behind academic publication on this one
[http://www.nature.com/articles/nenergy201732](http://www.nature.com/articles/nenergy201732)

------
stanislavb
Ah, that's interesting. I didn't know there were a theoretical limit of 29%

------
antouank
Isn't that a duplicate of
[https://news.ycombinator.com/item?id=13936480](https://news.ycombinator.com/item?id=13936480)
?

~~~
daenney
Yes. However:

> If a story has had significant attention in the last year or so, we kill
> reposts as duplicates. If not, a small number of reposts is ok.

This submission got traction, the other one was posted 12hrs ago and didn't
really get anywhere.

~~~
antouank
I didn't know that. Ok.

