Lead-free perovskite solar cells
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Beilstein Talks14 / 34
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Transcript: English(auto-generated)
00:05
Excellent. Okay, so good afternoon, everybody, for those in Europe. Today I'd like to show what I do for research, like in solar cell, and I try to give a general introduction trying
00:22
to straighten it as broad as possible, but also go into the detail of what we do in the lab on a daily basis in Berlin. So the title is Lead-Free Perovskite Solar Cell, and I'm going to explain why I'm trying to highlight this lead-free condition for
00:40
this new potential technology. When we talk about perovskite, or a lot of perovskite, we have in mind, we have to have in mind this crystal structure. So it's like a crystalline material which is made by a combination of these three elements, so A, B, and X, which are the lattice position forming this structure. This material is attracting enormous
01:07
interest for photovoltaics because it's performing incredibly well, and in essence, in only 10 years of research, has been already proven to achieve the world potential behind
01:22
its capability to work in a solar cell. So that's there are two challenges that I'm passionate about. One is the stability of the material in the working condition of the device. Another one is the toxicity of lead. That's why the lead-free that I lie at the beginning of my talk. And that's what I'm going to talk today. When we think about perovskite solar cell,
01:45
there's actually a plethora of different device structure that we can have. So we can group them in three main examples. So the so-called NIP or PIN, which essentially differ from the sequence of stacks. So this perovskite in the middle of the device is the dominant layer
02:04
of the system. And then you have two contact materials on top and the bottom. If the material on the bottom is an n-type contact for extracting electron, then you end up with an NIP structure. So if you flipped it, you have the p-type on the bottom, you have the PIN structure. And this is how it looks like in SCM cross-section. So this is a real device
02:24
with thickness about a micron, where the dominant layer in the middle, about half a micron, is the perovskite polycrystalline film. So as I mentioned, there are extremely interesting potential technology. Because if you look at this graph, which is quite crowded, where all
02:45
the existing PV technology have been reported, with a record efficiency measured all the time, you can see that the perovskite entered 2013 in this chart, with the first device published by EPFL around 13%, 14% certified efficiency. But they rapidly went up, and now they are
03:07
approaching 26%, which essentially was the theoretical limit, the practical limit, actually, for this material system. You can go, of course, higher than that. But to do that, you need to employ multiple materials, so combining the potential of different materials.
03:23
Let's say the perovskite can compete with the highest efficient device on a single junction, and they can be implemented in multi-junction approaches to go towards the highest possible efficiency. When we go into the lab and work with this material, we have to face
03:41
with a huge challenge, which is the variability of the performance and efficiency. Here, I'm reporting an example where we compare two different contact materials, one which is a self-assembly monolayer, another one which is a nickel oxide inorganic contact. And you can see the device efficiency, the PCs, the power conversion efficiency of the device,
04:03
is spread over a large range of values. And also, there's a significant number, like half of the device that we prepared actually ended up with some issue that did not allow us to measure them to extract data. So since we use a very basic
04:26
processing procedure on the lab scale, we face this problem on the lab scale. Of course, when we go on a scale-up system, you rely on machines to be more reproducible. This is not a problem anymore. But I'm introducing this to you, because to explain the distribution
04:45
of the data, you will see in the next slide, and to understand where they're coming from. So to solve this problem of the data variability, we use this high throughput approach to study the stability. So since data is spread at a large range of value,
05:03
so we need to have statistically robust analysis of the device. And that's why we realized these huge setups, which can hold hundreds of device and run them under operating condition to see how they behave in simulated working conditions, or illumination and applied
05:23
voltage bias. And of course, we analyze this data. We start to collect some trends, observe some trends over 1,000 device that we measure over the last three to five years. And the first interesting information, in my opinion, that we extract is that we can identify
05:45
for characteristic decay in efficiency over time. So here, you have the power conversion efficiency function of the aging time. So it's only 150 hours of accelerating indoor testing condition, which simulates a few months of real working condition outdoor.
06:03
And you can see that this full cluster of trends, one where there's an initial rise in efficiency with almost a linear decay, then there's an exponential slow decay, a faster exponential, and an extremely fast one. And interesting, the device which better perform
06:25
in terms of efficiency, but also stability, are device that belongs to cluster one. Then where the device belongs to cluster two. And then the worst performing device are those who are cluster four and to the cluster three to some extent. So why this is interesting?
06:42
This is interesting because in 150 hours of indoor accelerated testing, we can somehow predict already how the device behave on the long run. And this is extremely important because when you talk about solar cell, you have to think on a time span of 20, 25 years or even more. And of course, you cannot wait for 25 years to get some information
07:02
about the stability of the system. You have to figure it out how to accelerate this degradation and how to project the result on the long run. And that's a possibility to do that. So these are the information that we extract. So we group the device in terms of maximum efficiency that they perform. So how do they convert the light into electricity? And then the stability.
07:25
So you can see there's a trend here. So the device that performs poorly in terms of efficiency below 10%, they're also the device which degrade the most. So this is the median at the mean value. They are actually higher, meaning that they lose 50%,
07:41
nearly 50% of the efficiency after 150 hours. But if you go to the highest efficient device, you have the device can lose down to 20% of the efficiency after 150 hours in average. Of course, we have device that achieved zero degradation under 50 hours and device which
08:01
degrade the most. But statistically speaking, there's a clear linear trend going to the highest efficiency. So what does this tell us? It tells us what this graph shows. There's a very nice correlation between stability and efficiency. The most stable device are also, statistically speaking, the most stable one. So going for high performance
08:23
is, in this case, means also going for the most reliable system. That's good news. And this is the most recent result that we get in terms of efficiency and stability. So these here are the current voltage characteristic devices. The standard measurement is used for extracting the power conversion efficiency of the system.
08:45
And we have a particular device configuration which gives us up to more than 24% record efficiency. But what is really interesting here that this efficiency is retained nearly with very little degradation after 1,000 hours. So we have a loss here
09:03
below 5% in 1,000 hours. Just to get you an idea where we are, so a benchmark silicon solar cell lose in average 1% in efficiency every year. One year of working condition outdoor are equivalent to roughly 1,000 hours of continuous illumination and voltage bias.
09:23
So basically, this experiment here simulates what the device will experience in roughly one year of working condition. And measuring degradation below 5% means that we are not that far from the stability of benchmark technology on the market. So I think we are still missing a factor
09:45
of 2 or 3 to get an equivalent stable system to the silicon. So I think we are well on track to see this technology on the market rather soon. But what is another interesting and important test to do to evaluate the stability of the
10:06
system, the material, is the temperature stability. The device is, of course, exposed to temperature variation because the natural occurrence of the seasons and the day, night changes in temperature. But also, the device experiences extreme temperature variation because it gets heated
10:24
up during the function. So if the device is like if you leave the car under the sun during the summer, it becomes extremely hot. That's exactly what the solar cell does. So under the sunlight, it can become extremely hot. So the temperature inside the system
10:43
can reach up to 60 or 70 degrees during the day. And that can drop down during the night to whatever temperature you reach outside. But that's an extremely stressful condition that we have to face when we implement a new material
11:01
into the technology. And with the perovskite, we used to have a little problem there. So here is the power conversion efficiency as functional temperature between minus 60 and plus 80, which is the standard testing range of temperature suggested to evaluate the stability of the system versus temperature. And we see that there is a rather significant variability,
11:24
especially when you go to low temperature. But when you stabilize the system by implementing a new material that we'll show in a second, we experience extremely stable output. So the device becomes extremely stable to temperature variation. And over several cycles,
11:45
this shows a significant improvement into the stability. So again, another good news for being more optimistic towards the delivery of a technology based on perovskite. So what we did to make this material stable in temperature, we implement a polymer
12:03
into the structure. As I showed before, the perovskite is made by a polycrystalline thin film of thickness around alpha micron to one micron. So this film is made by a lot of crystals or grains which are packed together to form this compact film.
12:24
The packing of these grains is fundamental for the stability, because especially for thermal stress, they can cause cracking or friction which degrade the material at the interface between these grains. So if you make these films less brittle, so more soft by introducing an organic
12:41
polymer, you might end up with a significant improvement in stability. That's what we experienced by using this particular polymer into the film. This polymer also has some other side interesting positive effects, which I'm not going to go into detail, but for those interesting, you can look into this publication which is now out in Science. Sorry for not putting the
13:01
reference, it was really like a few days ago it's been published. So this polymer has a series of positive effects not only on the stability but also on the efficiency. That's why we managed to make extremely high efficient and stable device. Now, let's go ahead. So I mentioned that we need accelerated the testing of the aging of
13:23
the system to predict the outdoor lifetime performance. And one complication here arise from the fact that perovskite have a very peculiar behavior. So they show when exposed to light, and applied voltage, they should change in performance on the timescale of a few hours.
13:45
So timescale compatible with the day-night cycling. That means that the device in the real working condition will always run in this transition regime. It will never reach a real steady state condition. So we have to be carefully evaluating this transition rather than going to
14:06
look at how device behave on a steady state. And to do that, we need to essentially include into our stability testing this day-night cycle. That's how we realize that we essentially start to test device in outdoor condition in Berlin. These are perovskite encapsulated on the roof
14:25
of our institute. And we start to monitor the system and we do observe that the device actually when run in the real working condition is rather different from when was running in accelerated
14:40
continuous illumination or light. And so we had to take into account this by implementing this on-off of the light, so this cycling. And here what we realize. So this is the same device when measured under continuous illumination, so this line here in this case, or this one here.
15:01
And these are the device when tested under cycle illumination, so light on and light off for an equal amount of hours, like here is probably about 12. And you can see that depending on the device, depending on the material that we use in the device, we might have like a good agreement or not between the indoor and outdoor testing. That's a problem.
15:23
There's a problem because that means that we cannot rely blindly to the accelerated indoor continuous illumination to predict outdoor lifetime. So we have to check how the indoor testing actually can mimic outdoor cycling, or we have to reproduce outdoor cycling in your accelerated indoor testing. So this is a little bit of complication to
15:44
extrapolate the lifetime condition for the device in 20, 30 years. But the good news is that we managed to get an extremely stable system, so here is like 3,000, now the experiments keep running, so past 3,000 hours. So we have
16:04
like 11 independent devices, an average result. Device cycled, like in outdoor testing, showed nearly zero degradation. So essentially, we're saying that there's absolutely no problem to produce a technology based on perovskite, which is going to be stable for 20 years.
16:26
So good news. Let's go to the real challenge. And what I am really passionate about is the lead-free condition. So why we want lead-free? So perovskite is, all the data that I show so far and the majority of the research activity going around, they are based on formulation
16:44
employed large quantity of lead. So we're talking about 30 to 50 percent weight in, of the material, which is lead. The question of lead is not by itself a problem, but it might become a problem if it becomes available to life, so it becomes bioavailable.
17:06
So, and this was the case from previous application of lead. For example, so here, I reported a few of them, where lead was heavily used in painting. Lead oxide was used in painting to make paints more bright and more stable. And then it was associated to poisoning in
17:26
children and then banned in the last century. As the most famous case of lead gasoline, lead-loaded gasoline, was, tetraethyl lead was used in gasoline as an anti-knocking agent, essentially was retarding the explosion of the fuel in the engine, so basically helped the
17:44
engine to run smoothly. But it was again causing severe health issue because it spread in the atmosphere, so inhaled by people, it was associated to poisoning or like long-term impact on human health. Lead, and then it was banned again at the end of the last century.
18:05
There's the most recent application of lead in electronics, which is not completely banned now, but it's heavily regulated, so the amount of lead which now we have in our electronics significantly lower than what we used to have 20 or 30 years ago. There's a much
18:22
more recent ban in lead for hunting on wetlands. So bullets made by lead are now forbidden for hunting wetlands because, spreaded on the ground, they may poison animals, and from animals they can enter the food chain and then arrive to human. Now we are really at the forefront of
18:45
this new era of lead-based perovskite. So perovskite is an incredible interesting material for a number of reasons, and I have no doubts it's going to be applied, not necessarily in photovoltaics, but for sure, as an amazing semiconductor,
19:03
it's going to find application in many optoelectronics. So there might be a scenario where we're going to produce tons of perovskite in the future, in the near future. So we have to ask ourselves whether or not this is going to cause a problem similar to what we experienced
19:21
already with the previous application of lead-based materials or chemicals. To answer this question, I have to look into the literature of lead, which is quite broad, and into a table, which I find very interesting, and there's a report in this slide. So we can very accurately estimate the amount of lead that we intake on a daily or weekly basis in this
19:48
case, in making some simple blood test or like test on the lead absorbed in the bones, and that's easily accessible kind of measurement. So we know precisely how the lead weekly intake
20:05
evolved over the human history, because we can check it from the bones. And that's the scenario. So I divided this scale of lead weekly intake, the unit is gram, so in three regions. So the region of milligrams, the region of micrograms, and the region of nanograms,
20:25
because in the milligram region, so if we have an intake of lead of milligrams per person per week, we experience immediate toxicity. So this is well known, well documented.
20:42
Before the industrial era, people used to experience an intake in the region of nanograms, so they were here 5,000, 3,000 years ago. But we ended up recently in this middle region, which is a gray area, where we know there exists some evidence of neurotoxicity and
21:06
neurotoxicity on the long run. So this chronic level of exposure to lead cause on the long run a clear impact on, can cause clear intellectual deficits, there's a paper from 2005 that showed this evidence, and they can cause DNA methylation.
21:25
So we are not in a safe area, but we are going in the right direction. So this was the level in the 80s, so approaching almost milligram, where it was at the peak of the lead gasoline consumption, but then as lead was removed from gasoline, things start to get
21:41
better and better, and we are moving towards 250 in this area. So we're getting better. The question is, is lead perovskite going to be a problem for this trend? Can it somehow retard or invert this positive trends that we activated since last few decades?
22:02
But that's the question that we need to answer to understand whether or not perovskite can be a problem. I don't have an answer, a definite answer here, I'm just writing the question that we have to figure out how to deal with that. So if you are interested in going deeper in the detail about lead use in different technology, I like to advertise some
22:27
my social activity. I have a Twitter account where I publish regularly this kind of subjects, like there's, for example, here a paper which shows, discuss about the possibility
22:40
that everybody born before 1996 may have a lower IQ index because exposure to lead gasoline. There's a very interesting video on YouTube available for free where it actually goes into detail of the story of the lead gasoline, how it was born, what was the
23:01
problem linked to it, and how it was complicated actually to ban since it was so much widespread. And then there is some curiosity about the use of lead in paint for a famous painter, which experienced some different level of lead poisoning during their career because they
23:21
used to be exposed to lead-loaded paints. There are some famous cases like Michelangelo, Caravaggio, Goya, where these are of course not proven, but there's some suggestion that they have been lead-poisoned because they use lead-loaded paint.
23:40
We did some additional work to figure out the risk of using lead perovskite. And one of the first step, important step, is to understand how much the lead perovskite is bioavailable. So what does it mean bioavailable? It means that an element is not toxic by itself. So we cannot define lead toxic by definition, but we have to go into detail of which chemicals
24:07
contains the lead, and if that chemical is bioavailable or not, then we can define that specific chemicals, toxic or not. And in the case of perovskite, so lead is contained in the perovskite, we test the bioavailability of the lead perovskite, and the way to test it
24:23
is like contaminating soil on the lab scale system and check the ability of some specific plants to uptake the lead from the soil. And here, without going into detail, because this is not my field, the message was that even though we have a background lead level into
24:41
soil in the range of 35 ppm, so each kilogram of soil worldwide contains, in average, 35 milligrams of lead, if we add only a little bit more, like in the range of 5 milligrams, that's enough to activate a huge uptake from the plant. So that means that the lead from perovskite
25:09
is way more bioavailable, and the simple reason behind that is just the lead perovskite is rather soluble in water compared to other lead contaminants which are not soluble. So the solubility in water is what makes the bioavailability of the element in this case,
25:23
of the compound in this case. To understand a little bit more where we can move in terms of numbers, so I published this schematic, and this paper here linked at the bottom, where essentially we correlate the fraction of the perovskite which might potentially reach
25:44
the food chain. So if we have like, we hypothesize production, animal production, and range of several tons of perovskite, we assume that this fraction can eventually leach out from the system
26:02
and arrive to the food. So we are from, we went down to per billion and below, so very tiny fraction. And then we have to take into account the fraction of the water population which gets exposed to that amount which we disperse in the environment. And to be safe,
26:22
we have to move in this area here. So we have to make sure that only a fraction in the range of part per billion, 10 to the minus 9, of our perovskite that we use in the future, potentially going to use into the future, is going to disperse into the environment and reach the food chain. So that's a very tiny fraction. So we need to really design
26:42
a production line and a recycling system very carefully to make sure that we are in this region here. But actually, we have a solution. And the solution is that we can start to push for entirely lead-free perovskite. It exists already in the research, at the research level,
27:02
and it's getting better and better. So here, I show you the power combustion efficiency of the solar cell made from lead-based perovskite. As I mentioned before, the first device we reported in 2013, and the efficiency went from 10 up to 26 now extremely fast. And at Almodzim in Berlin, we are using perovskite in tandem with silicon to reach
27:22
efficiency now above 30%. So extremely promising. TIN is started to be investigated in 2014. So I was in Oxford when I started working with TIN, and then we are systematically getting better. Now we have efficiency around 15%. So we are not there yet with the lead. So we're
27:42
still 10% missing compared to 26 benchmark lead-based perovskite, but still a lot of room to improve. Indeed, theoretically, TIN-based perovskite can outperform lead because they have a more ideal bang gap for a single-junction PV. And that the research seems suggesting that
28:02
there's still a lot of room for improvement with only 300 publications for TIN-based perovskite against 13,000 publications on lead-based perovskite. So there's room for improvement. We've done quite a lot of work on TIN where we are there. So we, it was a few years ago, so we have a 10% device, so like a state-of-the-art at the time,
28:24
and we managed to make this device working and relatively stable, so nothing too fancy to discuss here, except the fact that the stability of TIN was something that actually we realized immediately was extremely challenging compared to the lead. Why stability of TIN is challenging
28:41
is because the TIN into the perovskite tends to oxidize from the 2-plus to the 4-plus states, while the lead 2-plus is rather stable by itself. So there is actually more stable in 2-plus state than the 4-plus state. So the perovskite made by TIN might encounter a stability
29:02
issue because of this oxidation process. So we start to investigate heavily into this process, and we realized that there was a problem with the processing of the perovskite, which was causing the stability. It was linked to a solvent, which was like dimethyl sulfoxide, which was heavily used for processing lead perovskite. It was used to reproduce the process
29:22
for TIN. But we realized that there was an issue there, that we spent a lot of time trying to figure out how to replace. Obviously, we identified the mechanisms here, and then we're trying to figure out how to replace it. So we looked for all the possibility that we had, like all the solvent library to identify some solvent which can still solubilize the precursors to process the material, allow the crystallization, avoid the oxidation.
29:44
And starting from thousands of possible solvents, we ended up with this relatively small library of 15 solvents, which we start to optimize to to prepare our devices. And then we ended up finding the right combination of them,
30:03
mimicking what the DMSO was doing by replacing it with a molecule which is known as TBP, so tributyltpyridine. And we identified that the reason why the DMSO was so good working in processing perovskite was because there was a very strong interaction between the molecule and the tin iodide. But we can mimic this interaction by replacing lead with the TBP
30:27
without causing oxidation like the DMSO was doing. And we proved this concept with some theoretical support from colleagues, and experimental evidence that this interaction was actually even stronger with the DMSO. And then we proposed this mechanism that
30:45
pyridine-driven crystallization as an alternative to the DMSO, we proved that actually it's even better to inhibit the oxidation to prevent this stability issue. These are some basic measurements showing the defect density or the back downcharge density
31:02
due to the defects within the film, so I'm not going to go into detail on that, but just to say that we managed to prove the material is actually intrinsically better when we approved the DMSO. And then we managed to make a device, so it's still like far off the benchmark lead base, so we are like now about around 8-9% in efficiency
31:22
with this system. But what is really interesting here is that we have an extremely stable system. So this is what I showed before, the power combustion efficiency as function of time, so some hours of illumination, some hours of dark to mimic the day-night cycles. This is a typical response of a lead-based device. So we have like these transients that take
31:42
place all over the day. While we have tin, this is not transient, so the device seems like extremely stable. This is very preliminary data, but a very clear indication there's something intrinsic in tin which makes it actually more stable than lead. This was rather unexpected, but it's clear indication that as long as we fix the problem of tin oxidation, we might deal with
32:04
the system which is actually more stable than the original one. I just want to give a few more minutes to give you an outlook of where we're going for further improvement of the system and it's a working plan to achieve the efficiency of the lead-based perovskite without replacing
32:24
lead. So I show you this picture at the beginning of the perovskite case structure. B is our lead in the lead-based perovskite, or our tin in the tin-based perovskite. And usually people say, okay, we can replace B and then switching from lead to tin
32:42
is not a big deal. But in reality, there's a lot of differences in the material property when we replace lead with tin. And the first difference is that lead, like shown here, it allows the lead iodide to create perfect octahedra in the crystal structure of perovskite.
33:01
But when I replace the lead with tin, the tin iodide or octahedra are far from being symmetrically perfect. So they tend to have this distortion. And the reason is that in the tin, there's an expression of the lone pair. So essentially, the two electrons of the most external shell of energy of our element, they have some p-type character,
33:26
so they are not symmetrically distributed around the nuclei. And this causes the distortion of the octahedra. This distortion of the octahedra is reflected in the distortion of the lattice, tension in the lattice, which impacts the device performance. And we have to get rid of them if we want a highly performing device. That's what I'm trying to go.
33:44
This is shown in literature, but also in the lab, by some basic measurement like absorption or emission spectra in the system, which shows evidence of this tension in the lattice, which can relax over time when we use tin. And this is the explanation that the band gap
34:01
of the material changes over time because this distortion of the lattice can relax over time as a result of a reducing energy of the lattice. And also, the raw crystalline of the system becomes better and better as this distortion essentially gets relaxed.
34:23
So that's the way, in my opinion, to fill the gap with the lead. And I think we can get there. So we can have an extremely highly performing system and completely lead-free. Thank you for your attention. And I'm happy to take questions.