 Okay, Haiti says we're ready. I'm ready, so let's go. So, it's a pleasure to be here. I've spent the last few months putting together a proposal for NSF with members of the Science Circle and some of my colleagues. So, I've really put the bulk of this talk together in about the last week and a half. It's going to be a little more rough than some of the ones that I've given in the past. So, with that, let's take it away. I'm going to talk to you about some recent advances in perovskite solar cells. If you don't know what perovskites are, no problem, that's part of the talk. Let's see. So, oh yeah, going back here, you're going to see lots of my cats. Oh, there's a little bit of a lag in there. That's Ishtar and, of course, as a cat, she is a goddess. So, how does solar power work? Ideally, what we have is some sort of panel which harvests the free energy that just comes from the sky and then channels it into cell phones so we can see pictures of cats. That's the ideal of how solar power should work. Today you'll note that my talk is an odd juxtaposition of hand-drawn and computer-generated images. I really like the old school stuff, but the new school stuff also works really well for me. So, first, let's talk about how electrons interact with nuclei. It's always all about how positive and negative charges attract each other and what rules and boundaries there are on those attractions. So, with this scary little diagram, I'm showing you a potential energy well. You've seen these sorts of pictures before when you think about gravitational wells and black holes and things, but here I'm talking about the electromagnetic force. So, right at the bottom of the well, we can have a nucleus, a positive charge. And an electron, being a quantum thing, has to have some space in which to exist and it will have some energy. And essentially what will happen once all the boundary conditions are satisfied is that it will sit at specific energy levels in this well. So, this bottom level here, with the longest wavelength, think of a string vibrating. The math is pretty much the same as governing a string vibrating as governing the probability of where you're going to find the electron. You can have an electron down here at, let's call it the lowest energy primary harmonic and then the second harmonic would be where it's got roughly twice as much energy, but the boundary conditions are a little bit different because the shape of the well. And these give us three-dimensional regions where an electron could be the lowest energy and an electron might look like a sphere, next higher up, might look like a sphere that's been divided in two or a dumbbell type shape. All right, so it does look like a face. And that's my drawing, I'm sorry. How about this? This looks a little bit better. And I've actually recorded a little video. Let's put it up here. You're going to have to watch it on your own. Let's see. There we go, edit. I'm going to have to unlink those. There we go. I'm getting the bad sound. Do you really want to unlink? Yes, yes, yes. There we go. Okay, I was having trouble selecting. So, there we go. So, with that one, that panel, that'll give you a link to a video I recorded so it actually has an animation and forgive me for having my scary face in the corner of there. I'll just pop that up there. Okay, so don't expect that one to actually animate in second life. We didn't set it up to do that. But here's a mock-up or a metaphor for the photoelectric effect. Let's say that this yellow brick object here is a photon. And it's going to hit an electron that's sitting in a potential energy well at a particular surface. No, I don't really want you to watch it right now. It can be something you can do later. Because I've got a bunch of slides to just animate this quickly for us. So, let's have that electron be hit by that photon. Some energy transfer and the electron gets kicked out of that well. In the video, I try a couple of times and I don't have enough energy the first two times and the electron goes back and stays in the well. Okay, of course, interesting things like balls moving around and little holes interest my cats as well. But what can happen is that the electron, if it has sufficient energy, goes away. So that would be a metaphor for the photoelectric effect where a photon hits a metal and kicks an electron out completely. And it's an actual sharp transition between not having enough energy and having enough energy. If there's not enough energy, the electron goes back in its hole. Photoelectrons can be ejected by metals. Einstein's Nobel Prize was for explaining this effect. And there's a wonderful technique called photoelectron spectroscopy where we can compare the energy of the electrons that were ejected with the original energy of the photons used. And we can find the depth of the well, i.e. how far down in a well the potential energy well the electrons came from. And so we can actually probe the molecular orbital structure of a molecule through photoelectron spectroscopy. It's a wonderful technique. But what happens when your photons aren't energetic enough to pick an electron right out? Well, the energy will get wasted basically through collisions. It will heat up the metal, the electron goes back into the original situation. And this is because in metals there are so many ways for the electron to transfer energy to other electrons, to the lattice, and the electrons are so mobile in metals that we can't really get energy easily from just shining light on a metal. We use semiconductors. In metals the electrons move way too easily. So we need a longer lived charge separation. And by charge separation I'm going to define that as the electron going somewhere and the hole that's left behind. Both of these, electron and the hole, allow for conductivity. Just like, I don't know if you remember those puzzles. They're panels of numbers, they're square. They have one panel missing so that you can move the numbers around and basically get them back in order. If you don't have a panel missing, none of the numbers can move. That is the value of having a hole because an electron can jump into a hole and then leave another hole behind. So semiconductors allow the electrons to move around but slowly enough to allow some control over what they do. And one of the points of this talk is that perovskites as semiconductors are a recent discovery. 10 years, maybe 15 years. But here's the thing, in the last 10 years of work on them their efficiency in solar cells is as good as 50 years of development on silicon. That's actually going to be one of the bottom lines of my talk. So going back to this scary little picture, going back to this scary little picture, let's think of the vibrations on the string. Let's just think of the electron as just having some sort of phase as it were. What if you have a row of atoms? A row of these wells that are overlapping just a little bit. Well, if there's an electron in each well then these electrons could each be vibrating in parallel or they could be vibrating in outer phase. We could set up interference patterns basically of all the possible combinations of their vibrations. Is that because of the advantages of perovskites themselves? I actually think it's because of the advantages of the perovskites. We pretty much do know how to make solar panels in the last 30 years or so and the perovskites aren't manufactured in the same way as the silicon ones are. So talking about strings being in phase or out of phase just talking about maybe four atoms. You could have the strings all vibrating in the same way and that might be a low energy combination or we could have two vibrating the same way and then two that are out of phase. So when these guys are at their maxima on the left these guys might be at their minima on the right. And essentially if we have I'm going to call them orbitals now because that's exactly what they are. If we have four orbitals on four atoms there's going to be four ways to arrange the phases. Not really a law of conservation probably is but whenever you have a molecule or structure of some kind the number of atomic orbitals that go in is always conserved, you always get the same amount if they are molecular orbitals or structure orbitals out at the end. So here's the thing. I basically started with one orbital on one well but if we have more and more overlap atoms then what happens is that the interference patterns that are built up end up giving you continuous bands of orbitals. So if I have some atom that's got an S that's a spherical orbital if I've got the same atom and it's got p orbitals those are the dumbbell shaped they tend to be aligned along x, y and z axes these things are essentially going to interfere with themselves and if I have a billion of these atoms say we are going to get a lot of orbitals that are closely spaced but there's going to be discrete regions, gaps where there is no overlap. So essentially what I've done here is shown you a full band at the bottom here and an empty band up at the top. Thank you for the drawings. I actually like doing the drawings. My postdoc advisor once told me that reviewers will appreciate a hand drawn image if it conveys a message and can be done in a minimal amount of time. Sometimes you don't need to spend days making the same image by computer. There's going to be some more cats around. So let's think about two different situations. Maybe the upper band and the lower band can overlap because maybe that original gap between S and P isn't so big. That's what we have here on the left. I call this a metal because this lower band and I've just kind of drawn it off set a little bit overlaps an energy with the upper band and that means that some of the electrons that are over here in the full band can kind of move over to this empty band and that provides space. It provides those gaps in the little square puzzles I was talking about. I'm sure they have a name. It allows the electrons to move around freely. Here's the thing. Going back here. Okay, back here. Okay, this is the slide I need. Each atom or each orbital does have a contribution to make. The electron can be anywhere in the structure. It can essentially tunnel from any atom to any other atom even one that might be a millimeter away. While that is unlikely it does mean that the electrons can move around and are delocalized throughout the whole structure as a metal. But that's only if there's some vacant orbitals to move in. Most of the time or a lot of the time you have these non-overlapping bands of orbitals and you've got a gap in between them. And here's the thing. Thermal energy can sometimes give the electrons enough oomph to jump into the empty band. And once they're in the empty band those electrons can move around and they leave behind hole in the original lower band and they can move around as well. I don't. Quantum mechanic effects mess up our control more than they do. Oh, I'm simplifying things so much. The quantum mechanical stuff is terrible. I've actually got a little bit from Roald Hoffman who got a Nobel Prize for doing quantum mechanical stuff calculations and chemistry. I've got a few slides from him later on. They're just called sliding puzzles. Okay, I'm going to go with sliding puzzles. So bandgap. Let's see. So for metals I think a property of a metal is that conductivity decreases as you increase the temperature and that's explained by atomic vibrations kind of blocking the electrons from being able to get to where they maybe want to go if there's a voltage applied. Semiconductors, on the other hand, the more thermal energy there is the more promotion of electrons into the upper band there is and that means that you have more conductivity. So metals and semiconductors can be differentiated from each other by measuring conductivity as a function of temperature. Here's the thing. Technically, there's no such thing as an insulator. There's only a semiconductor with a huge bandgap and it may be that you would vaporize an insulator before you get a single electron jumping its bandgap but technically once that electron has jumped then you've got more conductivity so it would be classed as a semiconductor. That's just a tiny thing. So what I've drawn here are called intrinsic semiconductors where you can have a promotion of electrons and they allow for conductivity. Bandgaps much greater than KT. Yes. Yes. Glass, quartz, things like that. Very good insulators, technically. But if we're going to be correct with the terminology it's still a semiconductor. So the semiconductor we're really familiar with is silicon. And believe it or not, silicon has the same structure as diamond and I think I have a silicon up here somewhere. There we go. So I've drawn two representations of silicon. And one of them I've just represented the silicon atoms by tetrahedra. So we have this little chunk of silicon. You could call it a little chunk of a diamond if you want. Let's see if I get out of that. That may just appear in front of you. And essentially that shows you how this block of atoms might form an ordered array of crystal that could be used for making devices. Let me move that one out of the way. I actually like looking at atoms better. That one can go away. I have to find the other one now. Up we go, click and right down there. So believe it or not this is the same structure from the same data. I think this data comes from 1963 or was collected in 1963. It's actual crystal structure data done by X-ray methods. Each of these spheres represents a silicon atom. You can see that each sphere or some of these spheres look as if they're at the center of a tetrahedron. This is just a chunk of the structure. Each silicon atom is at the center of a tetrahedron atom. I don't really know what happens at the boundaries of the crystal. It's probably got oxygens and hydrogens and things like that. But inside the crystal every silicon atom has the same environment and it's exactly the same way of putting together the atoms as it is for diamond. That always blows my mind. Silicon and diamond have the same structure. But why do you think of silicon as a semiconductor and diamond as an insulator? That's a good question. One cool thing about this structure since it's the same data as the other structure is that let's see if I can edit this. I can spend just a tiny bit of time moving these structures. The ball and stick model actually fits inside the other model. It fits inside there perfectly if I can get them arranged quickly enough so that it doesn't look like I'm just fiddling around. There it is. Inside those tetrahedra that those ball and stick models fit quite nicely. Alrighty. Next slide. Let's just talk about diamond and silicon and if we're going to reduce down diamond and silicon as far as they can go having a carbon in a tetrahedral environment that's simple think of carbon sitting inside a tetrahedron of hydrogen atoms or methane. One of the tools that I teach this stuff to my classes is this little guy. I'm going to move this thing up front. Because the orbitals are all based on Cartesian coordinates by N.T. and tetrahedral is hard to visualize but the tetrahedron can fit inside a cube quite nicely. If you have a cube and you have two corners on the top and the opposite two corners on the bottom and you join them you've got a tetrahedron. I use this tool as a way of helping my students visualize what happens when they have tetrahedral environments and how they can relate that to what X, Y and Z are actually doing. Let's see. I'm going to move you down. Okay, so thinking about tetrahedra what can they do? Thinking about carbon, carbon's got an S, carbon's got its three P's and a tetrahedral arrangement of hydrogens will end up having an orbital and then three other orbitals that are at the same energy. It ends up that these three on the hydrogens can combine with those three on the carbons and the single ones do the same and you end up getting a set of low orbitals and a set of high orbitals and I've just really sketched out how these orbitals are, it's not quantitative or anything. Carbon is four from the left of the periodic table which means it's got four electrons available for forming bonds. Hydrogen only has one but there are four of them and there are four electrons. You might see, I've drawn one, two, three, four, five, six, seven, eight little arrows each representing an electron in the lower set of orbitals. So these guys end up being from constructive interference. It's basically where the electrons can sit in between the nuclei and have plus minus plus measurements that contribute to overall attraction. Up here these are from destructive interference and if they were occupied they would result in desabilization of the tetrahedron. Hey, silicon has the same thing going on. It's qualitatively the same but the original gap is smaller so the gap between the filled and the empty or the silane as it's called is smaller. What does that mean? Okay, so I've kind of drawn a better picture of less hand-drawn picture I suppose of where we get some band structure from. If you look at these, they're called bonding and anti-bonding orbitals, the stuff that's right in the middle of each diagram, I've reproduced it here on the left of each of these diagrams and as you get more and more and more carbon atoms arranged in tetrahedra, these anti-bonding guys are going to form a band as we discussed earlier. The bonding ones are going to form a band as we discussed earlier. The bottom band is completely full, packed like herring in a can. There's no room for the slide puzzle thing to happen and for any electrons to move. The band gap is giant, so very few electrons can make their way into this upper orbital. So, carbon when it's in the diamond structure you think of it as an insulator. Dilucan, on the other hand has a smaller gap since the electrons are further away from the nucleus in silicon to begin with. They are less differentiated in energy. So, you get a smaller gap and this gap is small enough so that some electrons can penetrate their way into the upper set, making silicon an intrinsic semiconductor. It's not very conductive though and there's ways that we can improve conductivity. So, let's say that we've got silicon here, we've got our filled band, we've got an empty band and let's randomly take some of the silicon atoms and change them to phosphorus. I don't know, maybe one in a billion or one in a million, we change to phosphorus. Phosphorus has one more electron than silicon does and those electrons will form their own tiny little band in the gap. So, the new band gap is much smaller than the overall old band gap was. That means that these uppermost electrons don't need nearly as much energy to get up into this empty band, this band where conductivity can happen. You can significantly increase the amount of conductivity that you have in silicon by doping. The amount of doping basically determines how much. If you have one part per thousand, then you can get conductivity similar to actual metals. Usually it's like one part per billion or one part per million. This actually leads to a nice point about solar energy. If you want solar energy you need conductive silicon. So silicon that isn't quite as pure as what is used in the electronics industry is what you need to make solar cells. One thing that can happen as well we could put aluminum atoms in. Aluminum has one less electron than silicon does and so our band gap actually also decreases but the conductivity is due to the holes that are left in the lower band. So when you have more electrons and you have more negative charge, we call it p-doping, when we have fewer electrons or positive charge we call it p-doping. The beautiful things about n-doping and p-doping for n-doping the electrons are mobile in the lower band. For p-doping the electrons are mobile in the lower band. So what happens when you put two materials together, one is n-doped one is p-doped well in the p-doped the electrons travel at a lower energy in the n-doped they travel at a higher energy. So if you want the electrons to move let's say from right to left in this diagram where the two substances come into contact the electrons have to get energy from somewhere and a convenient source might be a photon so a photon could take an electron give it enough energy and have it hop up into this upper level and essentially that's the basis of our solar power having electrons hop up from the lower energy holes into the high energy conduction band and then have the substance not just decide oh well the electrons just going to roll back and I'm going to make heat having it easier for the high energy electrons to actually escape into a wire and do some work and then come back to another wire into the p-type semiconductor that's the trick so in our semiconductor industry diodes, leds they're all based on this p-n junction where you've got these two types of materials in contact with each other you simply apply a voltage maybe you have a voltage that will allow the current to want to go from left to right i.e. the flow of electrons from left to right all these electrons can then just tumble down into the lower energy band and release some of that energy as heat for certain substances as the electrons tumble down into the lower band they can release a photon and then you have an led so these p-n junctions can serve as harvesters of energy or releaseers of energy depending on what you do with them so I was a touch lazy I actually got some figures from the American chemical societies service where you actually look up a paper and then you can click on the option export this figure as a powerpoint slide so I did that and here's a mock-up of a solar cell and in this particular one we've got the perovskite instead of the silicon as the magic ingredient a photon would come in excite an electron from the lower band into the upper band and then we arrange matters so the electron can diffuse into some other material and then go into a third material FTO stands for fluorinated tin oxide it happens to be transparent so it's actually a great substance to make transparent electrodes out of there we'll get to what's a perovskite in a second the electron can move around and then back into another transparent electrode which for some reason has carbon attached to it I don't really know why you need a transparent electrode if you're just going to cover it with carbon and then some other material that has a conduction band that matches the valence band of it could be perovskite but this could also work exactly the same for a silicon solar cell nice point about I got some annotation here this guy matches the copper pyrocyanate CUSEN matches the perovskite and it turns out doesn't leach atoms into the perovskite to poison its function the titanium dioxide has a band that matches the upper band and doesn't leach atoms to poison the perovskite these are issues of more engineering than the actual chemistry but if you want a solar cell to last for years like 25 years on top of your house you actually have to make it so that the slow diffusion of atoms doesn't screw things up Oh, fluorinated tin oxide FTO is simply a transparent electrode that just blows my mind because whenever I think of electrodes I think of platinum and I can't really see through platinum here we get to what perovskites are the actual mineral perovskite is calcium titanium oxide C-A-T-I-O3 but these days anything that has a structure similar to the perovskite title mineral is called a perovskite so basically anything that's got an A with BX3 point about each X is that each X connects two of the B's we form an extended lattice with roughly cube shaped vacant spaces let's see okay so here's here's another slide I got from the ACS from journal physical chemistry letters this one is more about teaching physical chemistry this is an actual perovskite structure so what we can see is that these B's form a cube and each B is connected to the next B by whatever atom X is and A sits in the middle of the cube okay I actually like the picture on the left more than the picture on the right picture on the right is one of the types of models that we use to make to assemble 3D models in hands-on experiments in the lab and you know I think second life does such a better job of allowing people to visualize something like this in a more hands-on way than even this physical model alright so I built the same thing up here somewhere there it is let's bring it down down we come okay so this I built from crystal structure data connected in 1925 I think on one of these panels behind you if you click on it you'll actually get a link to the original article or at least a reference and essentially the blue atoms represent titanium the red atoms represent oxygen so each titanium atom is in the center of an octahedron okay so an octahedron actually has six vertices basically at 1 and minus 1 on x, y and z axes and these octahedra share vertices throughout the structure the green basically represents calcium so it's a very cube oriented type of structure it's actually very reminiscent I'm going to raise that up again move it back up maybe not terribly high if you can use your cameras to move around the structure to kind of see it in its full three-dimensional glory that would be wonderful it's very reminiscent of the blue structure I made a Prussian blue model for an earlier talk I don't want that to come in just the audience so let's move it over here so in the Prussian blue model come on down it looks very much like a cage the Prussian blue model is a little bit different each of the vertices represents an iron atom and each of the crossbars represents a C and an N a cyanide but for the purpose of this talk we basically have kind of a cubic structure and it would be based on the AX3s and inside each of these cubes there lives an A whatever that is it has a positive charge and BX3, whatever it is ends up having a negative charge and when we put those units together we get this cage Prussian blue type of cat ooh it would have to be a blue cat a bright blue cat edit, I'm going to move Prussian blue out of the way it is really blue I mean it was discovered in the early 1700s Michael Faraday played with it and when we look up we go it's magic ok so essentially I covered that one things like science news discover other magazines have been covering perovskites recently perovskite is just a structural motif it's like paisley, paisley is a pattern perovskite is a pattern basically how how things are attached together so many materials exist with this pattern as I said before about a decade of progress has equaled where we are in 50 years of silicon solar cell development I usually look at chemistry or chemical and engineering news from the American chemical society usually picks up science stories a bit sooner science news or the other popular news magazines you know perovskite being used as solar cells to me that just blows my mind it's like saying oh my VCR can control satellites now I'm glad I didn't throw it out why perovskites so it's a great question one major advantage they have over silicon is that they are much easier to fabricate you don't need to get the purity nearly as you don't have to get the purity to exactly the same standard that you do for silicon and the purification of silicon has a really high carbon footprint and uses a lot of other toxic materials the perovskites that we use yes we are actually going to use lead however the amount of lead we use is very small in them and the overall process to fabricate the cells is so much easier that it counterbalances you know what the toxicity of the process to actually get the silicon and that's a great question which lasts longer if applied as a solar conductor I'm going to address that so in 2014 the perovskites had gone from about 6 to 8% efficiency up to 16 in December 2014 they were up to 20.1% and the best silicon cells have about 25% efficiency so basically that's news as of 2015 here we have our little picture fluorine dope tin electron that's the FTO titanium oxide so this is a graph showing what the substances actually looked like at the time very small micrograph there we gained some time yeah these guys are air sensitive the ones that have been using that have been used are a methyl ammonium lead iodide it has the perovskite structure but it really only lasted minutes on exposure to air this didn't sound very good at the beginning but over the last few years people have figured out what causes these to be sensitive so it turns out every one of those holes in the cage structure if it's the PBI3 stuff that we're using if you have a random absence of an iodide that is a place where oxygen can be turned into O2 minus which is then just choose up the rest of the structure the simple fix is simply just to coat the surface with sodium iodide or maybe methyl ammonium iodide just to coat some iodide on the surface to protect against that so another advance was methyl ammonium I've got a picture of that in a second changing the methyl ammonium to a mixture of this formamidinium that's hard to pronounce changing what is in the holes in the cage allows for increases in efficiency and stability this is again from 2014 and this was where this approach was first found as of early 2018 we're at 23% efficiency it's in the process of being commercialized the best ones actually now equal the silicon cells and the ones that were first investigated were these methyl ammonium we've got a carbon with 3 hydrogens we've got a nitrogen with 3 hydrogens the overall thing as a plus charge that lives inside the cages and that was replaced by this guy on amidinium but really just replaced by a mixture of these organic guys some rubidium and some cesium at this time it's a third PV it's a spin off from Oxford University their test unit they demonstrated recently was actually this is the current issue of CME news so that's this week the tested unit was at 25.2% very comparable to solar cells from silicon the units that they've made for their test assembly line 243 square centimeters they are at 24% efficiency and we've now got them running for quote thousands of hours at 60 degrees celsius and they actually withstand minus 40 up to 85 degrees celsius and they even withstand 85% humidity at 85 degrees celsius so these coatings are actually lasting very well this is a great point ironic that light is bad for solar power yeah it is I've always wanted the solar power light bulb it always seems ironic to me so this is commercially where we're at the Korovskites probably by 2020 there will be commercial manufacturers of these Oxford PV is looking to sell these commercially in 2020 you know until then and even past then these actually have the potential to have even more improvements in their efficiencies and there's other things that we can do there's no reason why you can't have a transparent electrode a thin layer of Korovskite sandwiched in between another transparent electrode to get the short wavelengths of light and the longer wavelengths of light can penetrate through to a regular silicon bottom cell to up the efficiency of these guys even have a design like this one where the electrodes aren't really separated by a gap so one of the nice things about the Korovskites is that they can simply be made quite easily they also serve as LED materials which are wonderful and you basically have to apply voltage to force the electrons to move and there if you're lucky you can get the electrons to jump down into the conduction band from the other conduction band as I talked about earlier so no talk from me would be complete without a few of these magic eye things I've got some wall-eyed stereograms I've got some rotating gifts if we can just bring these back forward okay let's see okay I can bring them all up and over as a unit there we go I'll leave these out I'll leave these out so you can see them I'll move these other things out of the way there we go yeah if you click on if you click on any one of those panels it should start and show you a rotating 3D gift the panels themselves have let's see the diamond structures on the left and porous skites at various magnifications are on the right they're the cross-eyed stereograms some people can actually see these if you just cross your eyes and make the images overlap and then just watch I have to be careful because I can get mesmerized by these things awesome so I've shown you a few of those I will put this presenter up somewhere in the mickem lab and I think group members will be able to click through it let's see I just had a few more slides about how do people approach this research well you know people approach it synthetically by making compounds yeah Arizona Australia we might have to worry about the heat I'm not sure if it gets above 85 degrees celsius in either of those areas although it could I suppose as these things absorb the energy so a couple of slides this is from a typical paper from recent work where they made the tin iodide the tin is a little less toxic than the lead and it does some of the same chemistry here you can see they have a different cation this thing on the left would sit within the cubic holes in the cages they made a couple of flavors of these things the second one I'm showing you here is fluorinated and they have a perovskite structure that's a little bit distorted this distorted structure actually slows down the movement of electrons a little bit better which actually is a beneficial thing and prevents energy wasting recombination of electrons and holes so there's the minor changes replacing a hydrogen by a fluorine doesn't really change the structure very much it kind of makes it touch more compact and you can see there's a gratuitous cat sketch there's addressing a concern about the number of cats in my talk earlier and you can kind of see that it's not perfectly cubic anymore the size of the cations that live in these cubic holes actually does play a role in just kind of changing the shape a little bit of those holes and again that's showing you how these species can pack plus another random cat yes and just at the end I talked a little bit about recent Roald Hoffman paper I saw Roald Hoffman a very interesting fellow he's a 1981 Nobel laureate in chemistry he's known for the theoretical chemistry any of you who took organic chemistry may have heard of the word word Hoffman rules these the Hoffman of word word Hoffman that explained things like the Diels Alder reaction that you would have seen in second year organic chemistry Nobel laureate also a playwright and a poet there's a lovely there's a lovely summary of his work on the wiki page which I can if I'm lucky if I'm lucky why am I not seeing this there we are I can cut and paste into text for you yeah I hate people I hate people who can do so many good things anyway so you know he talked about the theory of these a theoretical aspect here's a typical typical perovskite structure again maybe some of one of these pictures actually shows how it's put together better than and you know he was kind of looking at cesium lead bromide is one of the one of the that's in this paper and I gotta tell you the theory stuff ends up being way beyond me any aspects he defined several arrangements of how the lead and the bromide talk to each other and they're very they're very subtly different if you look at this guy right here and it's got a light color on the left dark color on the right the whole picture duplicated except this one is dark on the right I'm sorry dark on the left and light on the right and this basically shows different ways of combining orbitals to get to get the structure and all sorts of calculations to figure out what the band gaps are going to be as you morph from one structure into another and then putting all these together actually gives you some idea of what the band gaps are going to be interestingly you have to take into account relativistic effects when you have the electrons on lead like the s type electrons on lead when they fall down into that potential energy well toward the nucleus they're accelerated to relativistic speeds and that affects the mass of the electron and so that mass change actually does matter in terms of these calculations so in fact if you're looking at just gold itself you need the relativistic effects to be able to explain the yellow color of gold am I referring to hybrid orbitals yes yes I was so the you know the details of all of this are way beyond this talk but you know we are actually looking at the theory of why we get certain band gaps one of the nice things about the perovskites is that the band gap is controllable over a wider range than what we can get for silicon and how might you prepare one of these solar cells if you have your electrodes titanium dioxide and the various materials you can just sandwich them together with binder clips at least in the lab for being able to teach people how these things work and in fact this journal of physical chemistry letters paper has a nice procedure for taking the fluorinated tin oxide glass applying a tin oxide a titanium oxide layer sorry titanium oxide occurs naturally in white paint to make it white you can apply a perovskite layer a copper psionate layer a carbon layer and then another glass layer and that ends up giving you a solar cell that lasts long enough for the experiment to take place because the early ones would just catch fire and exposure to air and in this kind of homemade lab electrode setup in the dark you can see 0 volts with some light shining there is a voltage yay yes eventually recycling these materials is going to be hazardous probably not much more hazardous than the silicon type materials with the various solder and various other components that are around I did see an article where the recycling for example if you use the lead the recycling was about issues would have to worry about the lead toxicity would be about 0.27% of the overall toxicity of the process for making and using these things and I think that was relative to the silicon solar cells so yeah anything we use is going to have a downside my last little story there's some lovely paper here from 2016 that says hey purity matters they used pure perovskites instead of just slop that was put together in situ and you can kind of see the film they made with these beautiful polarized pictures and again they have the indium tin oxide glass I don't know what petaut is I'm assuming it's something like the copper cyanide perovskite and this is a buckyball composite as the collector and you know basically they took various compounds you can see crystalline they take their crystalline pure materials and cast them and make films out of them essentially you can see the single crystal powders have some nice x-ray bands nice and sharp after you make a thin plate of them still nice sharp bands some of the weaker intensity bands aren't seen anymore that might just be a thickness issue and then let's see these bottom pictures basically absorbance versus wavelength so it's kind of showing you which wavelengths these things will absorb the black one seems to be I think it's the iodide one seems to be the one that gets most of the visible light how do these things compare with what people are usually doing so on the left it's the data for the pure materials it's on the right it's comparable data for stuff that just slop together and cast in situ you can see the difference in just the photographs in the top row pure materials give you lovely crystalline patterns the impure materials give you slop and the slop doesn't last nearly as long this bottom row row number F shows degradation over just a few days whereas E pure materials give you less degradation any interesting work being done with graphene and solar power not that I'm aware it's very conductive but yeah I'm not an expert in the field graphene is a great conductor and along certain directions yes so okay so here good time to conclude I think my time is over my time I always do this I'm sorry we have new tricks from a common structure we've come a long way in the last decade and there's probably a lot more progress that can be made particularly to stability efficiency and you never know we may find a perovskite structure that's based on totally non-toxic material because it's the structure it's like paisley I mean you can have paisley made from the any different color combinations floral there's just a lot of room in the periodic table to be able to explore the molecular structure so here we are some more resources I use jmo I use the crystal structure repositories I use blender to make my things I drew my cat I drew a cat and thanks to all my cats for their for their patience let's see and obviously thanks to the members and students of the science circle I always enjoy giving these talks thanks to my colleagues place where I work because this activity is recognized NSF for funding I always thank NSF and this little company for hosting some of my animated gifts and stuff on their website that's what I got for you we'll talk about dye sensitized solar cells sometime later yay alrighty tagline I agree rare earth elements for making semiconductors that is a problem they're not easily found in concentrations worth mining awesome awesome I will copy that browser I will copy that and open that and watch that later yeah I don't know much about thermal type panels cool well I'm going to have some coffee let's see physics today are very cool awesome I'm going to get some coffee and hang out for some of the for the taco chantel in about 10 minutes going off