A couple of years ago I made a video looking at a 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 Perovsky, 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 ABX3 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 dope 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 US, 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 3% 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 the team at Princeton University reckon they've 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 cesium lead iodide. chlorine between the active perovskite photon capture layer and whole transport layer.
This so-called 2D layer stabilized the interface between the perovskite and the whole 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 percent 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 band gap 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 Quizer 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 ABX3 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-Quasar 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 we'll 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 gained 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. A massive thank you as always to our fantastic Patreon supporters who keep these videos completely independent and ad free.
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