A couple of years ago I made a video looking at the material called perovskite
that was being hailed as a transformational game changer in the world of solar photovoltaics.
It still is being hailed as a transformational game changer actually. The only thing is though,
no one's actually managed to produce a perovskite solar PV cell with a commercially
viable operational lifespan. Until now. A new scientific research paper published in June
2022 claims to have hit that crucial milestone thanks to a new additional process and an age
testing method that brings a much more detailed level of understanding about the
stresses and strains that the perovskite structure experiences over long-duration
use. So I reckon it's about time we took another look at this potentially revolutionary technology.
Hello and welcome to just have a think. Perovskite exists in nature as a mineral made from calcium
titanium and oxygen arranged in a crystalline cube and diamond-like structure with titanium atoms at
the corners, oxygen atoms at the midpoints of the edges, and a calcium atom in the centre.
It was first discovered in the Ural mountains in the early 19th century by a geologist called
Lev Perovski, hence the name. Since then scientists have developed a whole class of
materials called Perovskite Structures. These things have the same crystalline cube and
diamond-like shape as naturally occurring perovskite but they can be synthesized
from a fairly wide variety of commonly available and relatively cheap chemicals. The scientists
refer to them as ABX-3 structures with A and B acting as cations and X acting as an anion.
To understand why perovskites are causing so much excitement in the solar PV world, we need to take
a quick look at how photovoltaic panels work. Most panels today are made up of a semiconductor,
usually silicon, which acts as a solar energy absorber. The semiconductor is sandwiched between
two electrodes, one positive and one negative. The top section of silicon is infused or doped
with an element like phosphorus and the bottom section is doped with something like boron.
When photons hit the silicon layer they knock negatively charged electrons free
leaving a positively charged hole behind. The two doped materials act like electrodes
on either side of a battery to facilitate the movement of the negatively charged electrons
towards the more positively doped material and the positively charged holes towards the
more negatively doped material. The electrons are then free to move out of the photovoltaic sandwich
and into an electrical circuit. The efficiency of a silicon solar cell drops off a lot if there
are defects or blemishes in the material so the cells have to be subjected to extremely
high temperatures to get the defects out and of course that's very energy hungry and expensive.
And for reasons that are a bit outside the scope of this video, the photons hitting the electrons
in the middle silicon layer need a bit of extra help from things called phonons to provide enough
energy to free the electrons and help them jump up to the top conductive layer. Getting randomly hit
by a photon and a phonon is much less likely than just getting hit by one or the other on its own so
to increase the chances of that happening the middle layer has to be made relatively thick,
which again adds significantly to the cost of manufacture. Perovskites don't need that extra
phonon to liberate the electrons from their middle layer which means they can be manufactured as very
thin films using a technique known as solutions processing. They can also absorb a wider range of
wavelengths of light than silicon which means more of the sunlight hitting the panel can be
converted into electricity. Their structures are also more tolerant of defects than silicon. That
eliminates the need for the high cost high energy machinery that silicon cell production requires,
and it also means about 20 times less material is needed for each cell, which in turn means a
smaller environmental footprint from production. An awful lot of work has been carried out over
the past decade by academic institutions like the National Renewable Energy Laboratory in the U.S,
Oxford PV in the UK, City University in Hong Kong, and the Federal Institute of Technology
in Lausanne, Switzerland to improve lab test cell efficiency from about three percent 10 years ago
right up to 29% today, so on the face of it perovskites are a potentially very attractive
option indeed. The trouble is they've so far proven to be frustratingly fragile
often degrading after only a few minutes when subjected to intense light and heat.
But now a team at Princeton University reckon they produced a device that's not only
highly durable but that hits the industry standard for conversion efficiency as well.
So how have they done it? Well for the last couple of years they've been experimenting with layering
different materials on top of the perovskite that can protect its fragility while also optimizing
the amount of light that can be absorbed. The strongest performance was achieved by adding a 2D
layer made up of Caesium, Lead Iodide and Chlorine between the active perovskite photon capture layer
and hole transport layer. This so-called 2D layer stabilized the interface between the perovskite
and the hole transfer layer and suppressed unwanted ion migration into the HTL which was
previously contributing to rapid degradation. To establish just how well this kind of perovskite
cell would survive in a long-term real-world environment, the Princeton team developed
a very specific testing method which they believe should be seen as a prototype for standardizing
cell testing across the industry as perovskite enters the market in the coming years. Up until
recently long-term testing hasn't really been the main priority with perovskites because most
research teams have been preoccupied with simply trying to overcome the frailty of the material,
but as that work has continued and more durable examples have been created testing these designs
against one another in some sort of standardized way is becoming more important. And it'll be a
crucial requirement as perovskites start to be utilized in consumer-facing technologies.
The new testing method speeds up what happens naturally over many years of regular exposure
by subjecting the panels to high-intensity light and blasting it with heat. The researchers chose
four simulated aging temperatures ranging from a baseline temperature of a typical summer day right
up to more than 100 degrees Celsius. Then they used an extrapolation method to calculate how each
device would perform at room temperature over tens of thousands of hours of continuous illumination.
The results showed that the combination of materials in the latest iteration of their
perovskite cell would perform at more than 80% of its peak efficiency under continuous illumination
for at least five years at an average temperature of 35 degrees Celsius. Using standard conversion
metrics the team reckons that's equivalent to 30 years of outdoor operation in somewhere like
New Jersey where the Princeton campus is located. Perovskite solar cells probably won't completely
replace silicon cells in the coming decades but their relative cheapness, greater tunability
and the fact that they can be made as thin film layers, makes them a great complement to existing
silicon technology. It's all to do with finding ways to overcome something called the Shockley
Queisser limit which defines the maximum light to electricity conversion efficiency of a solar
cell and that in turn is all to do with finding ways to vary something called the band gap of the
photon capturing material, which in simple terms that I can get my head round basically means the
minimum energy required to excite an electron up to a state where it can jump out of its position
and into the conductive layer of the solar sandwich. A material with a very narrow band
gap would be able to absorb all the light over the solar spectrum and it would produce a very high
current. But the price you pay for that is that the potential difference or voltage across the gap
would be very small. A very wide bandgap semiconductor can generate a lot of voltage
but it's only able to absorb a small fraction of the light spectrum. So for example a semiconductor
with a band gap of three electron volts would only absorb the ultraviolet part of the spectrum.
In an ideal world you really want to be absorbing as much of the spectrum as possible to make use of
all that free energy. And when the physicists William Shockley and Hans Joachim Queisser
tested the myriad combinations back in 1961 they concluded that the optimal compromise band
gap for a single absorber material is somewhere between one and one and a half electron volts,
which translates to a maximum conversion efficiency of somewhere between 30 and 32 percent.
And it turns out that the band gap of silicon just happens to be 1.1 electron volts.
But inorganic perovskite band gaps can be increased or decreased depending on
the materials used in the ABX-3 structure - that's the tunability I just mentioned.
That means scientists can produce what they call tandem or multi-junction cells
that could potentially significantly exceed the Shockley Queisser limit.
Silicon- Perovskite tandems have already reached more than 29% efficiency in the laboratory and
the projection is that with a triple junction cell that number could push up towards 40%
or more, either as two perovskites on top of a silicon cell or if this new technology from
Princeton university proves to be successful it could be an all-perovskite combination.
Just like in so many other areas of renewable technology progress is rapidly accelerating in the
solar photovoltaic industry, leading us towards far more efficient and cheaper panels and films
that in the coming years will start to see all over the world, not just on the roofs of
buildings, but also in agricultural settings, on top of large bodies of water like reservoirs and
canals, and once perovskites gain commercial traction, on all sorts of non-uniform surfaces
that would previously have been considered to be economically unviable.
If you've got views on how you think our adoption of solar is likely to progress in the future,
or if you work in the industry and you can share some insights with us, then why not jump down
to the comments section below and leave your thoughts there. That's it for this week though.
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