 Okay, so Azima2 is our next speaker. Sorry, she was my student and I always call her Azima, but the name is Azima2. So our next speaker is Azima2. The floor is for you. Thank you, Garu. So this good morning and good afternoon. So I'll be talking about atomic and electronic structure of cesium-letraiodide surfaces. I'm currently with Alto University and in the physics department. So, cesium-letraiodide is a perovskite material and perovskite was discovered somewhere around 2010 and within a short time of its discovery it has the efficiency of perovskite has increased and currently it's almost close to the most efficient solar cell material which is the crystalline silicon and aside from its efficiency perovskite materials have strong light absorption and they also have the ability to transport both electrons and holes. They are very easy to fabricate at low temperatures and they are sustainable and then above all very affordable. However, despite all these properties of the perovskite material, its commercialization has not been realized yet. And this is due to stability issues. Mainly the perovskite material is not stable in ambient conditions and we also have problems with the transport layers. The most common transport layer which is currently known and which has also given the perovskite its current efficiency is the spiral ometage which is a whole transport material. But in this material is an organic compound and it's very unstable in UV environment. So in order to be able to achieve the commercialization of perovskite materials, we need to find ways to enhance the stability without compromising the efficiency of the material. So perovskite materials are materials with the ABX3 structure where you have the A site being occupied by an inorganic compound or an organic compound. Here mainly the inorganic compound that is used is cesium and then the most common organic compound that has been used so far is the metal ammonium. And then you have the B site being the the central atom within the octahedral and this is usually occupied by lead or tin. And then you have the X site which is occupied by pylite anions usually bromine chlorine or iodine in this structures we have iodine as the occupying the X sites. Now in a typical perovskite solar cell structure, you have the perovskite active layer being sandwiched between a p type and then an n type conductor with a transparent conducting oxide through which the sunlight is transported to the perovskite active layer. So in order to protect perovskite from ambient conditions, we would need to protect the layers the surfaces of the perovskite with a coating material. But as we see here, having a conductor as well as a coating material thickens the layer between the transparent conducting oxide and then the active layers of the perovskite. This can reduce the efficiency of the perovskite since the light rays have to travel through several layers before reaching the active layer of the perovskite. However, we can protect the perovskite without compromising the efficiency if we are able to have a coating that can also serve as a conducting material. So here you have almost the same thickness in there and then you have a p type conductor which also has the capabilities of protecting the perovskite from the ambient conditions. So in my previous work we set out to look for inorganic coating materials which have the ability to also serve as transport materials. So we assumed a perfect interface where the interactions between the perovskite and then the transport or coating material is perfect. So we didn't even consider what happens within the surface. And then we did a database search. So what we did is to take an already existing database, the airflow database. So at the time of this work, the entries of the airflow was almost 2 million. And then we set out some criteria to select potential materials that can serve as coating materials as well as conducting layers for the perovskite. So here we set out to have like six criteria. First of all, we needed to find materials that have wide bandgap and mainly because if you have a wide bandgap material, it serves as a window material for the perovskite. So in this case, we're looking at semiconductors and also insulators. And the entries of the airflow are all calculated with DFT. And as we all know, GGA has this under estimation of the bandgap. So we set our criteria to select materials with bandgap that's greater or equal to 1.5 EV. And we also restricted our search to binary and ternary compounds because we didn't want to have any higher dimensions that would have complicated structures for our constructions. Then we also had wanted materials that are abundant and non-toxic. I know here people are going to hold me with the fact that lead is already toxic but then it's in the perovskite. So we wanted to reduce the toxicity by making sure that our coating materials are not toxic. And we didn't want materials that were also interacting with water because as I said earlier on the perovskite disintegrates, mainly mostly the organic perovskite disintegrates when it gets into contact with water. And based on the lattice structure of the perovskite, we wanted surfaces that have like a cubic kind of structure. So we also restricted for appropriate lattice. And then we calculated the lattice mismatch between the selected coating materials with 12 different perovskite structures. So we had six inorganic perovskites and then six organic perovskites with different combinations of the halites. And then we ended up with 93 potential coating candidates for the perovskites. So the actual goal of this whole project is to model a perovskite material that is robust and also efficient with the ability to withstand harsh ambient conditions. But then in an ideal situation we know that the perovskite and the coating are not perfect. So there is this issue of the interface, what happens when you have another material on top of a different material. So to be able to achieve this we need to understand what happens at the interface. But then we cannot understand what happens at the interface if we do not understand the surface properties as well as the possible surface reconstructions of the perovskite. So the main subject of this talk is going to be based on the surface properties of cesium letriodide. So here we use density functional theory and also ab initio thermodynamics to study the surface of cesium letriodide. For the GFT patch we used the FHI Ames package and then we looked at the 2 by 2 supercell of the perovskites along the 001 plane. So we looked at the two different terminations. So if you look at this structure for instance, you can see that the top most layer is cesium is terminated by cesium and iodine. And then the next layer is terminated by lead and then iodine. So we looked at these two different terminations where we have cesium iodine termination and then we also have lead iodine terminations. And then we made our slab to be symmetric because we didn't want to have the interactions of adjacent slabs in the model that can induce some dipole interactions. And then we looked at the cubic and then the orthorhombic phase of the cesium letriodide. So here we have the alpha phase and then we have the gamma phase. And then we also, we use PBE sol in all our calculations because it gives a better estimation of the lattice constants. Then we also, aside from considering a symmetric slab, we also included dipole interaction corrections to avoid any dipole interactions in our calculations. So to cut that computational cost we fixed the, we fixed layers within the bulk and allowed just the symmetric endings of the material to relax. So as we've seen here. And then we looked at surfaces with missing and add atoms. So the surfaces with missing atoms will be represented by V subscript X and then the ones with added is I subscript X. And the X here are the constituent elements. And then we also considered the complexes within this element. So for the results of this calculation, I'm going to take you through the surface diagram analysis of various surface reconstructions and also the electronic properties of the clean and most stable surface reconstructions. So if we neglect finite temperature interactions, then we can find these, we can calculate the stability of a surface using the grand potential. So a special case of this is when you have the system interacting with its constituent elements in their stable states. So here we have the grand potential and then we have the energy that we calculate from DFT. And then we subtract the removed or added constituent elements by calculating their chemical potential. So this is the chemical potential of the high atom or compound. And then we have, so this is the DFT is calculated from DFT. And this is the most stable, the most stable structure of the constituent elements and then X is the number of added, the number of the elements within the structure. And then we have this parameter here, delta mu i. So what this delta mu i does is to find them, if in case you shift the value away from the most stable value of the mu, what happens? So this is more like a control for the calculation. So in a special case, you can calculate the formation energy by just subtracting the energy or the calculated from the surface you have modeled from the sum of the chemical potential of the elements that you have removed or added. And here we consider delta mu to be zero. So this is more like the state whereby the whatever element you're adding is rich. So delta mu equals zero. So here I have the surface phase diagrams from the alpha phase in this case. And then here what we did, this is a three dimensional problem. But what we did is to split it into two. So we consider a state where we vary the chemical potential of cesium and then the chemical potential of iodine. So here the delta mu Pb is zero. So from here we have this white line which we labeled as stable bulk. And as stated by Stefan in his presentation, the bulk is not in isolation from the surface. So in order to be able to find the most stable surface, you need to consider if the surface is stable within the bulk. So here you see that in regions where cesium is poor, you have the structures that are deficient in cesium. So all these surfaces are surfaces where we've removed cesium atoms. But then what is of interest here is the surfaces that intersect with the stable bulk. Those ones will be the most stable. So if you look at this side, for instance, even though the surface phase diagram shows all these surfaces, they are not the most stable surface. And then we have here we have a clean surface to also be stable and then we have all these surfaces that intersect the stable work to also be the most stable surfaces. Then we also considered a region where delta mu C is zero. And then here you see that as you move towards the lead rich region you have the surface that has more cesium atoms on top to be the most stable. So in all we can say that the surfaces with added fall cesium iodide are the most stable ones. If you look at the coverage area of the calculation here, then we look at the surface. You have five minutes. And the surface phase diagrams of the gamma phase. Again, we see a similar pattern as we saw in the alpha phase. And then the most stable surfaces are the same as what we saw in the alpha phase. And then also at delta mu C as we see the same kind of configurations. So I had to look at the band structures what is happening there. So here we looked at a bigger slab module, because we wanted to rule out quantum confinement. So this is a bigger slab where we didn't relax the system and then we computed the band structure, which is projected onto the bulk. And here we see that they are perfectly aligned, the clean surface is perfectly aligned to the bulk. But then after relaxation we realized that there are some surface states. So we have the edge of the conduction band being pulled into the gap, which shows a presence of surface states. But then with a gamma phase we don't see something like that. And then obviously the band gap of the gamma phase is wider. And then this is the brilliant zone that we use for calculation. Then we also looked at the band structures of the most stable surfaces. And again, we see surface states within the alpha phase. And then but then there are no surface states within the gamma phase. And again, we don't see any states within the gap for both cases. So in summary, we've been able to show that from our SVD analysis that the cesium iodide terminated surfaces are the most stable. And surfaces with cesium iodide and lead iodide are the most stable reconstructed surfaces. And this is because these surfaces do not induce any polarity within the structure. And then so there are no induced deep energy levels in the gap. And as I said, the gamma phase is more favorable because you don't see any surface states, the edges so there are no band edge perturbations. So for my current work, which would focus on now looking at now that we have most stable surface reconstructed models, it is time to look at the interface and then conclude the modeling of the structure. So here we combine DFT and then the machine learning program for the appropriate surfaces of the material of the perovskites and then the coating to see which ones are good. So here I have strontium, sorry strontium zyconia on a cesium letra iodide. And so we use this Bayesian optimization structure as such. This is a code that is developed in alto investing. So what this does is that it uses data that you have fed into the system, and then it will use Gaussian process regression, and then it selects the scans and then gives you the most stable surface with a lower energy level. So this gives you a landscape of the energy. As you can see, so this is the data position points here. And here we have the landscape. So this gives us the most stable configuration. And then this is the relaxed structure of strontium zyconia on a cesium letra iodide. And this is another example of zinc oxide on a cesium letra iodide. And this note I like to acknowledge the Finnish Academy of Science and Letters and then Academy of Finland for funding, and then also novel material discovery also for funding, and then CS Center for Scientific Computing for the computational time. And thank you very much to the organizers of this conference for giving me the opportunity to present my way. Thank you for your attention. Yeah. Thank you very much, Azima for the nice talk. The floor is open now for questions. Only you can start. Okay, thanks. Thanks a lot. Thank you Azima. It's good to see you. I had a two questions. The first one is in your surface reconstructions that you talked about. I guess you just looked at zero Kelvin optimizations. Yes, yes. Is it known experimentally, for example, how annealing the surface changes surface properties because, you know, temperature may play a very important role in the morphology of the system. So that was the first question. The second question is related to that, which is, what is the role of temperature in the band gaps in these types of systems? Okay, so temperature can really have a serious effect on this. And as I said, as part of the ambient conditions that are not conducive for perovskite is temperature. So that is the reason why we in the interface calculation would like to use wide band gap materials because they have the ability to kind of absorb more temperature. So that would help in protecting the perovskite. But then in this case we haven't considered temperature. So of course there would be some changes in the band gap. And that is why we have considered several reconstructions to see which ones are the most stable because, well, in experiments, unlike in modeling you can control what you put in and what you put out, right? But then in experiments it's really difficult to control such things. So yes, there could be an effect on the band gap. Okay, thanks a lot. I can continue the discussion on the, yeah, on the discussion, I mean on the discussion session. I would like to check with Hisham. Hisham, can you hear me please. I think his microphone is still not working for some reason. Okay, probably then the next speaker is Abdo El Fiki. Can you hear me please? Yeah, yeah, I hear you. Okay, so probably then we can we can go with your presentation. Okay, and then we will check Hisham later on.