 Thank you so much for that wonderful introduction. As you said, my name is Adam Slavny. I guess when I wrote that, I was the second year. I'm now a third year graduate student in Hemakurinidasa's group. And yeah, so today I wanted to talk to you about bismuth-based double porovskites, specifically in the context of porovskite foldable tanks. If you saw Mike McGee's talk yesterday, you've already gotten a bit of an introduction to this. Here I'm plotting. I'm showing a chart produced by the National Renewable Energy Laboratory. And oh, what happened there? Great. And so it's just charting the rise of solar cell efficiency over time for a variety of different technologies. And since this chart is a little busy, let me just blow up the region that we care about. Porovskites are the red circles with the yellow filling. And you can see that since they started keeping track of this in 2013, the efficiency has just soared. They're now up above 20%. They're competitive with several commercial technologies, including cadmium telluride. So as you can imagine, this has sparked a lot of interest in the research community. There's probably several dozen groups around the world working on this. My group is a synthetic inorganic chemistry group. And so when all of this porovskite work started to happen, we got very interested. And the reason we got very interested is actually due to the porovskite structure. So briefly, let me just tell you what that is. I'm showing you an example of it here. It consists of a metal halide octahedra connected to other metal halide octahedra to form a three-dimensional network. And since this network has an overall negative charge, you need a counterbalancing cation to sit in that cavity. And this gives you an overall structure, overall formula of ABX3, where A is the cation that sits in the cavity. It's one plus. B is the cation that sits in the center of the octahedra, and that's a two-plus metal. And then X is the halides. And from a chemist's perspective, what's really exciting about this structure is its flexibility. You have a huge amount of ability to change the A site, to change the B site, and even to change the halide site. And you can still get the porovskite structure. I'm just showing you a few examples here. And the porovskite foldable take community has started to realize this until about last year, pretty much every porovskite solar cell was made with the structure I'm showing you here, methyl ammonium lead iodide. But now the real state of the art of the industry is where you take methyl ammonium iodide and you get rid of it and you use formimidinium or cesium or some combination of, and this helps impart better stability. So it's clear that we can use this sort of substitutional strategy to access new materials properties in this field. However, so the A site has gotten a lot of attention. The B site, I would argue, has gotten far less attention. But it's no less important. And that's mostly because of the element that sits in the B site. All porovskite solar cells, high performing porovskite solar cells have lead at the B site. And this is a pretty significant problem because lead is toxic. This was in the news earlier this year in Flint, Michigan where acidic water from the Flint River leached lead into the water supply and ended up poisoning a lot of the town folk. I only bring this up because humans have been living around lead and working with lead for a long, long time. And our experience tells us, and we know now that the only level of lead exposure that's not detrimental to human health is zero, none. This is a bigger problem, I would argue, for porovskite solar cells than it is for some older solar cell technologies. Because unlike, say, in a silicon cell where you do have some lead, the lead that's in that cell is metallic lead in the soldering and the contacts. Whereas in porovskites, the lead is ionic lead. And ionic lead is significantly soluble in water. In fact, you can get 620 milligrams of lead iodide into water, which is a big deal from a toxicity perspective. I don't want to oversell this. I do think that lead porovskites do have a future as a commercial technology. But what it does mean is that at every step of the solar cell panel life cycle, we need to have systems in place that limit lead exposure. And importantly, those systems must be completely fail safe. The thing you don't want to have happen is you have, say, solar panels, porovskite solar panels on your roof and somebody throws a ball up there or you have a hail storm and it cracks the glass. Then the next time it rains, not only do your solar panels no longer work, but you potentially have lead on your roof, lead in your drinking water. So this is the problem that I wanted to look at. We want to reduce the toxicity of porovskite solar cells by removing the lead. But at the same time, we need to preserve several of the properties that make lead porovskites so effective as foldable tag materials. So those are things like the moderately sized band gap of 1.6 EV and the ability to deposit high quality thin films from solution. The problem with this is that there's not a huge consensus yet among the community about what it is about lead porovskites that makes them so effective. However, everybody can agree, seems to agree that the lead porovskite electronic structure is key. So let me just give you a brief introduction into that. Here's just a very simple schematic where we have the valence band of the material and the conduction band of the material separated by a band gap. And you can see that the valence band of the material is made up of mostly halide P orbitals mixed in with lead S orbitals. And the conduction band of the material is made up of pretty much empty lead P orbitals. And so it's clear just from this picture that lead is playing an incredibly important role in the band edges of this material. And so if we're looking for a replacement for lead porovskites, what we need to do is find another element that has the same electronic configuration as lead 2 plus. Specifically, we need to find an element that has a filled 6S2 orbital to make up the valence band and an empty 6P orbital to make up the conduction band. So I'm a chemist and usually when I'm confronted with a problem like this, I go straight to the periodic table. If you don't like where you are on the periodic table, it's very easy. All you do is you go up or down. And so this gives us things like germanium and tin, both of which form porovskite phases and people have made solar cells out of this. The problem with both of these is that they're very oxygen sensitive. You can make a decent solar cell in the glove box, but as soon as you pull that out of the glove box, it oxidizes right away to tin 4 or germanium 4 and your solar cell doesn't work anymore. We could also potentially go down. However, fluorovium is radioactive and there's perhaps 60 atoms of it that's been made ever. So we might have a bit of a scaling problem. So if we can't go up or down, the other option that we have is to go left and right. This gives us thallium or bismuth as options. Thallium, it turns out, is actually significantly more toxic than lead. So if we rule that one out, we have really one very obvious candidate, which is bismuth. This presents a problem of its own. If you recall when I was introducing the porovskite structure, I said you need a two plus metal to sit in the B site of the porovskite structure. Bismuth is a three plus metal. And so instead of forming a porovskite, you more often form these lower dimensional compounds, non-porovskite structures, which aren't as ideal for foldable tachs. So we need some sort of strategy here to compensate for the extra charge on the bismuth. And so what we chose to do is we took the porovskite structure and then we expanded it into what's called a double porovskite structure. And all I've really done here is I've split the B site between two different elements. So now on one half of the B sites, we can put the bismuth three plus cation. And then on the other half of the B sites, we can put the sum one plus cation. And so that'll allow us to maintain charge balance in material overall. The material that actually ended up working out for this is cesium silver bismuth bromide where we're using silver as that one plus cation on the B site. It's very, very easy to make this material. All you do is you take cesium bromide, silver bromide, bismuth bromide, dissolve them in hydrobromic acid. And then when you cool that down, you get out these beautiful red crystals of your double porovskite material. We published this in February of this year. Another group came out with a similar report right after that. And since then, there's been a lot of other people following us finding new porovskite phases, new double porovskite phases. And so this is starting to take off. But to evaluate cesium silver bismuth bromide as a photovoltaic material, the first thing you wanna do, of course, is to look at the absorption spectrum, which is what I'm showing you here. And I want to draw your attention to the region down around two electron volts. And you can see that if you squint, that the material is starting to absorb some of the light, but it's not absorbing very strongly in that region. So this is indicative of an indirect band gap. So whereas in a direct band gap, we have nearly a step function, we have a very strong absorption right near the band edge. In an indirect band gap, we have only weak absorption near the band edge. And that's due to the fact that these bands are offset. We can figure out what the band gap is. It ends up being 1.95 EV. This is a little bit higher than what we were shooting for. We were shooting for around 1.6 or lower. However, this still has a lot of use potentially as the top absorber in a tandem solar cell, as Mike talked about yesterday. If you pair it with silicon, it has around the right band gap energy there. I also should point out that indirect band gap materials, the fact that this material has an indirect band gap does not disqualify it from acting as a absorbing material. So silicon is a great example of this. And so what it means though, is that we need a thicker layer to absorb all the light that's available to us. And what becomes important in that case is the carrier lifetime. You need a long carrier lifetime. So that's what we looked at here using time-resolved photo luminescence. We're watching the intensity of the photo luminescence decay over time. And that can give us a sense of how long the carriers are sticking around. Usually this takes the form of an exponential function, which would look like this. Our material does not look like that. In fact, it takes three different exponential terms to find a good result here. However, the most important part about this is the fact that we're seeing a very long lifetime in the tail. So we have a 616 nanosecond lifetime in the tail. This means that at least some of the carriers are sticking around for a significant amount of time. And we have the possibility of grabbing those carriers out as current. Briefly, I just want to say a little bit also about material stability. Methyl ammonium lead iodide has a known problem with moisture. You can see that in the powder pattern, it is decaying over time as we expose it to 55% humidity. We've looked at that for this material as well. And we see in the same conditions, no change in the powder pattern, no change in the peaks and under both the same humidity conditions and also under exposure to light. We've looked at this in terms of heat as well. Again, I'm showing you methyl ammonium lead iodide and methyl ammonium lead broide decaying after three days under 60 C, which is a very reasonable temperature for a solar cell to reach on a sunny day. We can heat this, my material a lot hotter up to 100 C for the same period of time. And again, we see no change. So with that, I would just like to conclude. We've, I've made a new class of materials called double perovskite materials. The way we did this is we took out the lead and we replaced it with a combination of silver one plus and business three plus. This is actually the first business bromide perovskite ever reported and it has low toxicity because we've gotten rid of the lead as well as improved moisture and heat resistance and long carrier lifetimes. There are some things we'd still like to work on here. The band gap is a little too high for a single junction solar cell and the indirect band gap, it would be more convenient if we could find a way to get to a direct band gap. However, I think the most important point here is that double perovskites greatly expand the scope of halide perovskites before we were really stuck only with two plus metals in the B site. And now we have the option to think about one plus through four plus metals, which essentially gives us access to the entire periodic table. With that, I would like to thank my group, the Krunidasa group, as well as my collaborators here in material science, Tehu and Professor Aaron Lindberg. Thank you so much to GSEP for funding. It would not have been possible to do it without them. I'll be happy to take any questions at this time. So is the indirect band gap a function of the fact that you have the double perovskites and do those other double perovskites also have indirect band gaps or what are their limitations? So it's not necessarily the case that all double perovskites will have indirect band gaps and it's mostly the fact that actually the presence of silver is what we think is causing the form of the indirect band gap due to some sort of complicated orbital interactions. So people have made, for example, people made the Thallium-Vismith double perovskite with Thallium-1 plus and that has a direct band gap just essentially very similar to the lead perovskites. So yes, it's not out of the question that double perovskites could have direct band gaps. Time for one more question. I think there's one over here. It looks like everyone's using bismuth for the double perovskites. Do the, like if you go up and down from bismuth, do those not crystallize the same way? Is there some reason you can't use the other ones or is it just, we haven't gotten there yet? Mostly the second one. We haven't gotten there yet. If you go too far up, you hit arsenic which would probably not be a great replacement for lead. But people have looked at antimony and that's definitely something that people are looking at and I don't think there's any reason that you couldn't make an antimony double perovskite. Okay. Thank you, Adam. Thank you. Thank you Adam. One more time.