Hello, and welcome to the Physics World Weekly
podcast. I'm Hamish Johnston.
In this episode,
I'm exploring how we can improve the sustainability
of solar cells
with 4 researchers
at the UK's
University of Oxford.
They've coauthored
the roadmap on established
and emerging
photovoltaics
for sustainable
energy conversion.
First though, we'd like to thank Pfeiffer Vacuum
and Fab Solutions
for supporting this podcast.
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and gas abatement systems.
Under its umbrella,
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clean solutions.
The sustainable production of solar cells is a
significant step towards clean energy generation.
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including coating,
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The use of vacuum technology
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In 2020,
the UK's Henry Royce Institute
teamed up with the Institute of Physics and
the Institute For Manufacturing
to release a series of roadmaps
that focused on the roles that materials will
play in the UK's transition to clean and
sustainable
sources of energy.
One roadmap charted the future of materials
for photovoltaics,
also known as solar cells, and it received
lots of positive feedback from the photovoltaic
community.
Today,
4 years on,
86
photovoltaic
experts from around the world have joined forces
to update and expand on that success.
To produce the blockbuster
roadmap on established and emerging photovoltaics
for sustainable
energy conversion.
And that's published in the Journal of Physics
Energy.
I'm at the University of Oxford to talk
with 4 of those experts.
They are the inorganic
chemist,
Robert Hoy,
the physicists,
Nikita Noel
and Pascal
Kuyenberg,
and the materials scientist,
Sebastian Bonilla.
Hi
everyone. Welcome to the podcast.
Hello. Hello.
So Robert, the first question is for you.
Can you explain what sustainability
means in terms of photovoltaic
production
and use? Sure. So by sustainability,
the main thing we mean is, how the
electricity is produced. So for a solar cell,
it harvests sunlight or harvests any form of
lighting, produces,
green electricity without producing any greenhouse gases.
So and also right now, photovoltaic's,
you know, very cheap compared to other sources
of energy,
and,
in many parts of the world, they're the
cheapest source of green electricity.
And you can use that for, you know,
powering homes, for
recharging electric vehicles.
So that's one, you know, one way in
which we mean sustainability.
But also we're thinking about the life cycle
of a solar cell. We're thinking about how
they're produced
and how they're used and what happens to
them at the end of life.
So in terms of production, we're thinking about
what sort of elements go into these solar
cells, and we're thinking about how can we
make solar cells such that we're not make
we're not relying on expensive or scarce elements.
And we're also thinking about, what the materials
are that are going into these solar cells.
Do you want to avoid any materials that
could harm the environment by deploying these technologies?
And we're also thinking about what the CO2
footprint is.
And right now that's quite low. So the,
you know, CO2 payback time for silicon solar
cells is quite very short, about 3 years.
And for some thin film based technologies, which
have a much lower CO2 footprint, the payback
time for that is, you know, even less
than a year. So these are very sustainable
technologies right now. And of course at the
end at the end of the life, we're
thinking about what happens to these once they're,
they finish their useful life lifespan.
And currently, it's the the cheapest way the
cheapest thing to do right now is just
to,
throw away a silicon solar cell and to
buy a new one.
But we're thinking about, you know, other ways
to,
recycle
and reuse some of these scarce elements which
are being utilized in solar cells. So that's
what we're thinking about with sustainability.
So Sebastian, much of your research focuses on
silicon solar cells
which currently dominate the market.
What are the sustainability
challenges that are specific to silicon and how
can they be overcome?
So, when we talk about silicon solar modules,
we have 2 big things that, that that
sort of are interconnected.
The first one, Robert already touched a little
bit on which is to do with the
raw materials that are used to make solar
panels.
So
the,
the context to all of this is that
making silicon solar modules is extremely, extremely cheap.
I mean, a solar module today sells for
less than 10¢,
of US dollar per watt, which is one
of the lowest,
sources of electricity everywhere.
So the,
the forecast to this is simply that we're
going to continue to make much more silicon
solar modules and install them everywhere.
When we get to making billions of solar
modules per year,
we're talking about
so such a large area that the materials
that we use to make those solar modules
start to become very constrained.
So, we we we have issues with some
of the materials that we use. For example,
we use a lot of silver and if
we continue to expand these manufacturing of solar
modules then we're going to essentially use almost
the whole of the silver supply in the
world, which essentially we cannot do.
We also use bismuth for soldering, we use
indium for transparent conductors.
So those materials really are some of the
keys that we need to that we need
to address
in the future.
Now that's the big problem, let's say.
Then in terms of the one second problem
a little bit smaller has to do with
recycling and circularity.
So there have been loads of reports
sort of stating how difficult it's going to
be to recycle solar modules,
But the reality of it is that it's
not as difficult as people as people tend
to think. A solar module is around 80
to 90% glass
and then maybe 10%
aluminum
and then another 10% of plastic and the
silicon solar cell
itself. So in in terms of the silicon
that goes into it, it's not a huge
amount. Glass, we know how to recycle. Aluminum,
we know how to recycle. So we know
that there is going to be a big
sort of waste stream of solar panels coming
out in the next 10 to 20 years.
But we're I think we're well prepared to
to deal with that. So so I would
say the the really big problem for scalability
is is material,
use.
I see. And I'm guessing that because silicon
is such a ubiquitous
material,
that
the the the challenges that we face with
silicon solar cells are similar to computer chips
and and other
silicon based technologies. Or are there specific challenges
associated with silicon solar cells?
So it's a little bit different than semiconductor
the the microchip semiconductors
in in the sense that
making a silicon solar cell is far easier
than making a microchip.
So a big part of the cost of
making a Silicon solar cell is just the
price of Silicon,
right? So extracting and processing Silicon is a
very energy intensive process and so this,
this costs quite a bit of money. Whereas
when you're making a microchip,
you essentially take a very big lump of
silicon
and then you pass it through around
100 of different processes and what you end
up at the end is is an extremely
expensive,
microchip.
Right? And and so in in that sense,
they're they're slightly different. For silicon solar cells,
we think a lot about where the material
comes from, what is the energy that is
being used to make it into into a
solar cell. Whereas for microchips, we primarily think
about,
can we make all of those really complicated
processes,
in a in a scalable way and in
different parts of the world so that we
can have a sustainable or safe,
supply chain for microchips. So that's that's different
in a sense. Yeah. So Nikita, your expertise
lies in the development of photovoltaics
based on perovskite
materials.
And you'd be happy to know that at
the Institute of Physics Publishing, where where I
work, we have a a meeting room called
perovskite.
So so what are perovskites,
Nikkita? And how are they being used to
develop more sustainable devices?
So one of the things with perovskites that's
actually really interesting is that I feel like
in order to not make,
you know, quantum materials people really upset, we
need to talk about what specifically which type
of perovskite we're talking about here. So type
of perovskite we're talking about here. So perovskite
is basically just a crystal structure. So anything
that crystallizes in an ABX 3 crystal structure
can be called a perovskite.
And one of the fun facts that we
like to bandy around is that it is
actually the structure of the most common mineral
on Earth, which is bridgemanite.
But
when we're talking about photovoltaics,
we're talking about an entirely man made class
of perovskites,
specifically the lead halide perovskites.
So if we're thinking about this ABX 3
crystal structure on the a sites, we can
put
small organic cations. We can put inorganic cations
like cesium.
And for PV applications, we're usually talking about
lead or tin on the B site and
on the X site we have halides.
So in terms of how these materials are
being used in terms of sustainable photovoltaics,
like Robert mentioned before, there are a 1000000
different definitions of sustainability.
And if we think about the elements that
we're putting into perovskites,
all of these are earth abundant elements.
So just by virtue of that, we, we
don't have a problem on that front.
There are potential issues that may crop up
with CGM, for example.
But one of the things that people like
to talk about here is that, well, actually
we don't have that much cesium.
It's not that we don't have that much
cesium. We don't mind a lot of cesium
at the moment.
And when we think about these things, we
have to think about supply and demand. The
reason why we don't mind a lot of
it is because there isn't a big demand
for it at the moment. If parasites were
to become a thing, there might be a
big demand for it on the market. And
then we don't necessarily have to deal with
our problem
with regards to lead. I know
a lot of people might, you know, have
their hackles raised a little bit about this,
but really
the fraction of lead that we use, because
this is a thin film technology,
the amount of lead that actually goes into
that absorbing layer is minuscule.
And there is a lot of research being
done in terms of how we actually
deal with potential lead leakage issues and what
the bioavailability
of that lead would be if a panel
were to break, for example.
And if you think about
really how much carbon is generated
in producing these things, if we compare it
to something like Silicon, for example,
originally, when you're making a silicon solar panel,
and Sebastian
mentioned this earlier,
you do have a lot of carbon emissions
generated there and of course that payback time
is still fairly low. With perovskites it's even
lower. And one of the beauties of this
particular material is that you don't just have
to have, let's say a single junction perovskite
solar cell because of the fact that it's
actually really easy to process.
What perovskites can very readily do is hold
hands with other technologies.
And for example, have a perovskite silicone tandem
where these either of these technologies might be
around 26, 25% on their own, but together
currently over 34%.
Percent. So again, when you start thinking about
things like that, how much,
let's say what degree of carbon emissions you
have in order to make these devices.
When you start thinking about things like even
junction perovskites or perovskites, silicon tandems, for example,
you have a lot less carbon output
per the amount of power that you have
generated.
So in terms of perovskites being a sustainable
option for PV, I think we're in a
good spot. Okay. And and what about efficiency?
Is the reason you're interested in perovskites because
they they have the potential or perhaps they
are already more efficient than silicon solar cells?
So I would say on a lab scale
and this is one of the things that
makes perovskites really exciting.
So I started working on this material during
my PhD,
and our first ever devices were somewhere around
6%.
And then literature was at 3% power conversion
efficiency, which is effectively,
how well you actually can convert,
light into electricity.
And right now, we're over 26%
on a lab scale.
And like I said before, if we start
thinking about perovskite silicon tandems,
we have
exceeded the efficiency that either of these technologies
can get to, but on their own. So
it really is incredibly promising to have this
material or this class of materials that can
not only
provide high efficiency devices,
but provide power in a sustainable way. I
see.
So, Pascal, your research focuses on creating solar
cells using organic
semiconductors.
Can you give some examples of the materials
that you work with and how they could
be used to create sustainable
solar cells?
Sure. So semiconductors,
so we are working with organic
perovskites are semiconductors, silicon is a semiconductor. So
you do need a semiconductor to make a
solar cell. The organic, inorganic semiconductor
stems from is is is a reference to
organic chemistry. So we work with molecules
based on carbon and hydrogen with some heteroatoms
in them.
Broadly speaking,
these organic semiconductors come in 2 flavours,
that are related to a processing
technology.
So there's
solution processing, wet chemistry printing, and that uses,
polymers and then a a something called small
molecules, and then there's,
physical vapor deposition, so evaporating,
molecules that are in tendency a bit smaller.
But they're still both organic semiconductors. And one
example where such organic some semiconductors are already
used in our everyday life is the displays
in our smartphones and now more and more
in TVs as well. So we work with
this class of material.
And
then what they they have they're a bit
different from the, inorganic
semiconductors that we've heard about so far in
that, you can make them spectrally selective. So
you can
only absorb a certain part of the light.
That opens up a new application, the semi
transparency and color selectivity.
One thing people are thinking about at the
moment, for example, is, the combination of agricultural
products. So integrate them into greenhouses, only absorb
light that plants don't use. There's further applications
like indoor,
harvesting the indoor light to power the sensors
and transmitters for the Internet of Things.
And
And, we're talking a lot about sustainability
here as well.
Generally,
it's organic photovoltaics that have a great
footprint for. In terms of
sustainability,
it's very low embedded energy, that means low
embedded carbon.
It doesn't use any heavy metal, no scarce
elements.
So that's all great.
At the moment, it's made from petrochemicals.
And as the petrochemical
industry
decarbonizes,
that, the organic solar cells will also decarbonize.
In principle, you can make this from renewable
feedstock.
And maybe organic solar cells are actually a
good target, first target for this, this renewable
feedstocks of of, petrochemicals
as it is a high value product.
And and what about
robustness? Because, you know, when I think about
semiconductors
well, a semiconductor like silicon or a perovskite,
that's like a a rock, isn't it? You
could you could imagine that lasting forever. But
an organic
material,
does it have a shorter lifetime? Or is
it you have to be more precious with
it, protecting it from air or those sorts
of things? Right. So stability is is, an
important point for all all organ for all,
solar technologies, and it is certainly for organic
PV as well.
You intend to compare to silicon, which is
indeed a quite robust material. You have to,
do a better job at encapsulating it, so
it's shielding it from from oxygen and and
moisture.
But then,
companies have now the the certificates,
for the Skorp, the IEC
62115.
And that one certifies sort of sort of
the the industry standard for for stability.
I see. Okay.
So Robert, we're we're back to you. And,
you've coauthored a section of the road map
that looks at indoor
photovoltaics.
That's, something that Pascal's mentioned Yeah. Already.
What role will these play in creating a
sustainable future? I suppose what is an indoor
photovoltaic?
Sure. Maybe I'll I'll start by saying, you
know, one of the really great things about
photovoltaics is a whole wide range of things
you can use them for. I mean,
for most of us, we mostly just think
about using solar cells on rooftops, on solar
farms. And, you know, that's certainly the biggest
market. That's certainly the most common reason we
use solar cells, but there's wide range of
other things you can do with them. You
can integrate them to buildings to make more
sustainable building buildings. You can integrate them with,
satellites or cars.
But you can also use them indoors. So
an indoor follow-upotake is just any follow-upotake device
used indoors under indoor illumination.
The reason why this has become so interesting
for a lot of people now is because
of the increasing importance of the Internet of
Things. And this is something that, Pascal mentioned.
The Internet of Things is, a lot of
people say, is one of the pillars of
our 4th industrial revolution.
Also, it's all about having many different devices
that can communicate with each other via the
Internet or via the cloud.
And this embeds we'd like to say this
embeds intelligence into infrastructure. What that means is
that infrastructure can become more responsive.
So if you think about a home, for
example, you could have just a very rigid
heating regime and be spending a lot of
money on heating your home, or you have
a more responsive, more adaptive heating regime depending
on what the temperature is outside, whether it's
there are any people in the room.
And this can be enabled by having these
wireless sensors that can detect where where the
people are.
Now the thing is the success of the
Internet of Things is becoming
a limitation as well because there's a, you
know, because the success was there's an exponential
increase in the number of devices part of
this ecosystem.
Right now, there's many billions of these devices.
Many people project that there could be trillions
of these devices in the future. So then
how do you power these autonomous devices? You
can't just plug them all into the wall
because they could be located in very hard
to reach places, for example, ceilings.
So right now, the most common way is
just to use a battery. But then if
you have a trillion Internet of Things devices,
you'd be throwing away a 100,000,000,000 batteries or
trying to recharge ahead of 100,000,000,000 batteries every
year. And if you think about the, drain
pump resources that will take and the waste
that will create, that's not really sustainable.
So the alternative, the more sustainable route to
power these Internet of Things devices is to
harvest the energy that we get for free
from our environment.
And if you think about look around this
room right now, you have this energy from
mechanical vibration when people walk. There's heat from
the radiators, so you can use something to
harvest that heat. But by far, the most
common source of energy is just from lighting,
from indoor lighting or from ambient lighting, from
windows, and you can harvest that energy through
photovoltaics.
And that's the idea.
And right now, the the fuel's going through,
you know, you know, like a rapid advance
right now because,
you know, for a long time the industry
has been using amorphous silicon. So if you
for anyone who's used a calculator with a
solar cell built in, that's amorphous silicon. That's
the most common commercial indoor photovoltaic device. But
it has very limited efficiencies, usually below 10%.
With some of the newer materials like lead
Taylor Perovskites or dicensed by solar cells, those
have now reached efficiencies beyond 30%
and in some cases beyond 40%. So that's
very exciting to have be able to produce
enough power to power more complex Internet of
Things devices.
And just as a, you know, very simple
example of of this application, so if you
go to the supermarket, go to the clothing
store, there's many paper tax around.
And, you know, for these companies to become
more sustainable, they can just get rid of
all these paper tags and replace them with
an electronic tag that displays the price. You
can just keep using the same,
tag for as they change their product,
and these can be powered using an indoor
PV device. You know, in the supermarket or
in the clothing store, you have very constant,
very reliable source of lighting. It can halter
energy through a follow-up tech that powers it.
And this is something that a lot of
companies commercializing new photovoltaic materials are thinking about.
I see. Yeah. Well, it sounds great. I
suppose unless
people think, oh, I better leave the lights
on to power to power my Internet and
things. Sure. Sure. You want to, use them
in synergy with an energy storage device.
So you have a PV device that has
the energy and recharges the battery
or instead, you can even just, replace the
battery altogether and use a supercapacitor.
That's more sustainable than a than a battery
is. I see. Okay. Well, that's great. Thanks
everyone. And I've got one last question
that is for everybody. So we'll we'll go
around the table with this one.
One of the conclusions of the roadmap is
that much more research will be needed before
solar cells can meet their full potential in
help helping us reach carbon neutrality.
What is one thing that you think scientists,
governments,
and indeed the public can do to ensure
that this happens? So, Nikita, do you wanna
go first?
So I think that's a loaded question,
and I don't think there is one thing
that's an answer.
I think
all of us have very different roles to
play,
in terms of being able to get to
net 0.
As academics,
I think we're doing everything that we can
do to
make the technologies
both viable and as cheap as we can.
But at the end of the day,
we still need a lot more funding for
research because there's a lot more that needs
to be done,
in terms of what the government can do.
They can invest in PV,
because to a certain degree,
we can do everything that we can do
in the lab, but but there needs to
be some sort of market drive and push
in order for these things to be deployed
at the rates that they need to be.
I think one thing that is really
kind of specific to perovskites in in a
way is that most of these photovoltaic technologies
that have been around and have been around
for quite some time,
they have kind of slower stages of development,
which is one of the reasons why we
have these TRLs or technology readiness levels.
But perovskites are comparatively
new.
So let's say the real boom has been
2013,
2014 to now. So So it's really just
a little over a decade, and we're in
this really weird position where you have overlapping
TRLs.
So we're still in the lab doing very
fundamental research, trying to understand how these things
form, how they break, how we get the
stability to where it needs to be in
order for it to be out in the
field.
And then we have companies like Oxford PV,
who have just sold the 1st perovskite silicon
tandems.
So in terms of funding, it's not about
putting
all your eggs in one basket. We need
eggs in all of the baskets.
And in terms of what the public can
do is
really just be more aware,
invest, buy solar panels. And, you know, when
the conversations
about a solar farm being put out close
to your house, realize that it's for a
good cause.
Okay. Thanks.
And Sebastian, what about you?
Well, I I I sort of want to
double down on one point, Nikita made,
which is to do with the investment that
we need for research in solar.
The governments in the UK and Europe,
they they have to realize
that solar has the potential to become a
power generation industry of the similar size as
fossil fuels are today. We have this phenomenal
rise in the
expansion of solar and it can range from
everything from rooftops,
utility scale,
indoors,
agriculture,
and so on. So we really have a
phenomenal opportunity to exploit this this expanded industry,
and the UK is already needing in, in
a lot of the research that has to
do with solar.
So we we really need to be able
to capitalize on the on on those gains
and continue to drive forward,
that investment.
Maybe also support the the the the point
on the public.
We can all we can also go to
go to our energy bill and,
and check how much of our electricity comes
from renewables.
And if it's and if we think that
we can do more, then switch to a
provider that can give you more,
renewable electricity.
And, Pascal, what would you like to add?
Yeah. Just echoing what has been said before,
renewable energy is a huge growth market at
the moment. They will be the major power
source in the future. Solar is gonna have
a large or the largest part in that.
And as a government, as a country, you
probably want to own some of that technology.
You want to have some of that domestically.
That means you need sustained research input. Sometimes,
there's a tendency as soon as stuff gets
successful in the market, you kinda ramp down
the funding. But then you miss the sort
of fundamental
input into the sustained growth of technology over
decades
that goes lost then. And and finally, you
you need the talent. If you if you
develop technology
in a country, you need a con constant
stream of talent into that sector, and that's
what academic research certainly produces.
And the last word goes to our host
here at St. John's College,
at Oxford, Robert.
Yeah. I just want to absolutely support what,
Pascal just said, which is it's very important
we also invest in people.
So, you know, we have the UK is
probably one of the world leaders in terms
of follow-up test research across the entire pipeline.
But we need to, you know, have enough
investment,
in particular into, you know, the PhD students,
the students who will be developing
within our groups these new technologies.
And right now, that's something that's a little
bit lacking right now. There's, you know, there's
no specific polyp polyamide specifically for developing photovoltaic.
I think that really would be a missed
opportunity because if you look at this road
map, you know, of the 86 authors, the
majority were from the UK. This is capturing
the voice of the UK community and also
shows the depth and breadth of expertise in
the UK. And that's something that we should
be proud of as a country, and that
would that that's what we should be further
investing in.
That's great. Thanks. So I'd like to thank
everyone for for coming out on this beautiful
sunny day in Oxford.
Yeah. Thanks so much for sharing your thoughts
on sustainability
and solar cells. Wonderful. Thank you. Thank you.
That road map is published in the Journal
of Physics Energy,
and you can find it at IOP Science.
Just search for the title
road map on established and emerging
photovoltaics
for sustainable
energy conversion.
I'm afraid that's all the time we have
for this week's podcast, which is supported by
Pfeiffer Vacuum and Fab Solutions.
Thanks to Robert Hoye,
Nikita Noel,
Pascal Kuyenberg,
and Sebastian Bonilla
for joining me today.
And a special thanks to our producer, Fred
Ailes.
We'll be back again next week when we
will be looking at 2 important aspects of
space travel, the health effects of radiation
on astronauts,
and the need for an extraterrestrial
time standard to keep missions on schedule.
Thanks again to Pfeiffer Vacuum and Fab Solutions
for supporting this episode of the podcast.
Pfeiffer has been a globally established
and highly competent partner in the development of
sustainable
solar cells and renewable energy technologies
for many years.
The company provides all types of vacuum equipment,
including hybrid and magnetically
levitated turbo pumps,
leak detectors,
and analysis equipment,
as well as vacuum chambers and systems.
You can explore all its products at pfeiffer
vacuum.com.