Even in the context of a rapidly developing global renewable energy market I think we can fairly
safely say that the reduction in the cost of solar photovoltaics over the last decade or so has been
nothing short of astonishing. Solar panels are now more than 80 percent less expensive than they were
in 2010. That's largely due to the mind-boggling volume of panels being produced in China,
but the solar PV panel industry is growing quickly in several other countries around the world as
well. So we know how to make them and we know how to make them in massive quantities and with
remarkable reliability and longevity. The only slight wrinkle in an otherwise flawless resume is
the fact that the pesky laws of physics are making it devilishly difficult to achieve a real world
commercially available panel with a sunlight to electricity conversion efficiency rate
greater than about 22 or 23 percent. And in theory those laws of physics will only ever
allow an absolute maximum conversion efficiency of about 30 percent. Which is a shame. It's all
to do with the different wavelengths of light and which wavelengths can actually be used by
the photon absorption layer to make electrons move out into an external circuit. And of course being
a sciency thing the phenomenon has a sciency name. It's called the Shockley- Queisser limit, named
after the two clever chaps who first worked it out back in 1961. Of course lots of research teams are
working on possible solutions to overcome this irritating limitation. You've probably heard of
perovskite for example which we looked at in this video and you may also know about the technology
called multi-junction cells. And doubtless in the fullness of time one or both of these ideas
may get worked up into a genuine commercially viable product. But now there's another bunch of
new kids on the block to challenge the existing research work going on around the world they're
a UK firm called Cambridge Photon Technology, and they reckon they found a way to take full
advantage of the whole spectrum of light that our sun provides. So how do they do that then?
Hello and welcome to Just Have a Think. A typical solar panel is made up of a transparent conductive
oxide, or TCO, layer on top of a semiconductor, typically silicon. The top section of silicon is
infused, or doped, with another element like phosphorus which gives it a negative charge,
which is why phosphorus doping is also called n-type doping. The bottom section of silicon is
doped with something like boron which gives it a positive charge, which is why boron doping is
referred to as p-type doping. When a photon hits the silicon layer it creates something called an
exciton which is essentially a negatively charged electron and a positively charged electron hole
which are released from the silicon but connected to each other by an electrostatic charge. In very
basic terms the negatively charged electron is attracted to the p-type layer and the positively
charged hole moves up to the n-type layer. The two doped layers act like electrodes to facilitate the
movement of electrons out of the photovoltaic sandwich and into an electrical circuit.
Now that limitation I highlighted a moment ago comes from the fact that silicon
works best with photons in the red and near infrared portion of the spectrum.
The wavelengths further out in the spectrum like far infrared and microwaves and radio waves don't
carry enough energy to kick the electrons out of their position and get a current flowing.
At the other end of the spectrum you've got the shorter wavelength photons which carry far more
energy, in fact far more energy than the silicon absorber can deal with, so any energy that doesn't
kick an electron out of place is simply wasted as excess heat, which as I mentioned earlier
is a shame. William Shockley and Hans Joachim Queisser tried just about every combination of
wavelengths you could think of and once they crunched the numbers from their experiments
they did indeed find that the maximum efficiency from a PV cell set up with these n-p layers
was limited to about 30 percent. The insight that Cambridge Photon Technology say they've
come up with is a way to convert those wasted high energy photons into lower energy photons
that the silicon absorber can handle. In this article, published in the online journal nature,
the company's head of business development, David Wilson, explains how it works. The
technique uses a phenomenon called singlet exciton fission. It was originally developed
by a physicist called Akshay Rao at the University of Cambridge in the UK, and although it's an
extremely smart piece of lateral thinking it's actually a remarkably simple idea in principle.
What Rao and his team have created is a film made from an organic polymer called Pentacene, which
is studded with lead selenide quantum dots. These things are tiny light emitting clumps of inorganic
material. The longer wavelength light at the red end of the spectrum is able to pass straight
through the Pentacine film and gets absorbed by the silicon semiconductor in the normal way
but the shorter wavelength blue and green photons get absorbed by the film which converts them into
pairs of excitons and for reasons that would probably melt my brain if I tried to understand
them the quantum dots then absorb the pairs of higher energy excitons and emit lower energy red
or infrared photons back out again, which is quite handy because when the film is placed on top of a
silicon solar cell the longer wavelength light from the quantum dots shines onto the silicon
where it can then be absorbed in the normal way. So you're effectively getting more usable photons
out of the available sunlight, which results in an increase in electrical current.
Actually Rao has calculated that his method could increase the potential
conversion efficiency of solar cells up to about 35 percent. He hasn't actually achieved that yet
so we're still in the ubiquitous territory of "promising new breakthrough technology",
and yes I know quite a few of you good folks get a bit frustrated when I talk about new concepts
that haven't yet been built at commercial scale, but you hopefully know my view by
now. I think these ideas should be given airtime so that interested parties can get involved and,
who knows, maybe even accelerate their development to market. And in fact
David Wilson is hopeful that by the end of 2022 Cambridge Photon Technology will have a prototype
that converts about 31 percent of sunlight into electricity. The company's Chief Executive,
Claudio Marinelli explains that the plan from the outset was to look to commercialize this work,
so the researchers approached real-world solar panel manufacturers to understand what they
needed and how this new technology could make a difference within existing production parameters.
I mentioned other technologies like perovskite and tandem cells or multi-junction cells earlier.
Those techniques can also capture shorter wavelength photons and they can be combined
together with silicon cells to make a kind of hybrid combo that can produce more electricity,
but that means making two devices that need to work seamlessly together while producing different
currents, which can be quite complicated. Plus it adds new equipment and an extra step into
the manufacturing process, which drives up the cost Cambridge photon technology reckon they can
avoid all that by using this simple non-toxic film material that requires no electrical
connections at all and that adds very little complication to an existing solar PV design.
The finished film could be sold to solar panel manufacturers to apply directly onto
their PV modules during production. Or the film could be supplied straight to the companies
that make either the vinyl acetate layer that encapsulates the silicon or the glass panels
that cover the solar cells. The PV panel manufacturers could then just assemble the
already treated components into the final device. The company currently employs about a dozen people
and has raised 1.4 million dollars in working capital. They also have a number of research
grants and access to researchers and facilities at the University of Cambridge. With that level
of investment and expertise in place they aim to have a market-ready product available by 2025.
As David Wilson puts it "it's really clear that there's a fairly urgent need and this technology
if it works as promised will go a long way to meeting that need". I know there are lots of
you out there with a great deal of knowledge and experience in this field so if that's you and
you can share some insights or if you just want to voice an opinion on solar PV technology in general
then why not dive down to the comment section below and leave your thoughts there.
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