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phone yeah so I'm going to talk to you about something that's actually quite old but has only been recently really discovered the so called halide perovskites and how they're potentially promising to revolutionize what we're doing in terms of solar technologies and other opto electronic applications so if this works if this works ok just a little bit slow I guess all right so sorry about the color scheme looks a little bit crazy but basically so one of the big problems that's troubling the world today is is energy obviously as you know and this little circle here represents the global energy consumption essentially what the what the world uses which is about 16 terawatt-hours per year if you compare that then to the Reaser reserves that we have all right you compare that to the reserves that you have there you see that at least a number of these fossil fuel elements coal uranium oil we still have substantial reserves although if you compare it for example to natural gas you see that something on the order of only about 15 years possibly left over to sustain the maximum energy so obviously people have now turned to renewable energies and you can see the sort of scale that they're able to produce each year so some of these minor ones tidal geothermal and hydro basically are not comparable really to what we need to produce ocean thermal wind power which is actually one of the most productive they're actually has quite a lot of potential but obviously it doesn't work when the winds not blowing but if you compare this essentially to one of the sources that we have here that we're not really using which is the Sun and you'll see that essentially none of these technologies necessarily really needs to matter so all we need to do is capture all the small part of the light that actually is shining onto the earth and use that then to easily sustain the global energy demands and so just to look at what that might look like if you needed to capture all of this is actually an older slide where we're basically the efficiency of solar cells were actually quite low so at 8% if you covered in an area in the earth with the solar cells full of 8% efficiency how large the area would be needed to cover the entire earth or to cover the entire Earth's global energy you can see that these are large spots but technically compared to the landmass of the earth really not not anything particularly large and so if you compare that then to the current usage of land when you can see that there's a lot of land that's that's essentially not not usable glaciers make up 10% of land barren land so desert sand and salt flats or rocky domains about 19% 27% actually is used for livestock so that means meat and dairy production or for for raising food for growing cattle for example forests 26% that are not really used fresh water itself owns only a very very small part and the built-up area that we have currently that's all towns cities roads airports anything like that infrastructure only makes up about a 1% of the total Earth's landmass and so just to point out so essentially farming itself what the gland that we're using for farming takes up nearly about 1/3 of all land area that we have on the planet ok so then people came up with the idea great why don't we take some of this land that's it's not usable for example here there's a the north part of the Sahara Desert and try to cover that with solar cells so these are these squares represent the area that you need to cover nowadays now to be able to sustain the energy demands of of the earth and you can also see that essentially they would nicely fit into the into the desert without really anybody noticing that they're actually there they're two major problems of putting solar cell parks in the desert first of all it gets very hot there and solar cells don't work very well when it's hot and the main problem is sand so the sand gets blown about sometimes we've noticed it in here in Germany when it gets blown over but essentially would cover the solar cells and then and then render them nearly ineffective so that's why they basically gave that idea okay so could this could we do this in Germany so where do we have the area to do this well basically you know that a number of years ago when we started up the renewable energy program and people started building up solar cells like crazy especially people in the south and Bavaria and so if you just take take a look at the the total roof space that we have available in all of Germany you could actually produce about 161 gigawatts of energy if you covered all of them with with solar cells and current yearly production in Germany is about 81 gigawatts and other the other forms of energy so essentially we have twice the amount of space necessary even to cover all of the energy demands just by putting solar cells on the roofs on top of Germany without even having to build up any solar parks okay so back to the history of solar cells they're actually not that old 1954 was the first working solar cell it's built by Bell Labs and this is one of the first ones where they installed it on top of a telephone pole and essentially they powered a telephone call using this this solar cell back then the efficiency was only about six percent nowadays obviously we have advanced quite a lot so in the labs the best silicon solar cells get up to twenty six point six percent these are only really tiny ones and not necessarily useful for anything and typically commercial cells are on the order of about fifteen to twenty percent so since then since this first solar cell was built up and there's been a lot of solar built up of the entire world Germany is one of the main leaders in that also China now is picking up quite substantially not they're realizing the pollutions not really so cool and so currently we have about 400 gigawatts of energy from solar panels installed over the entire world which is actually quite remarkable okay so the solar cell industry currently is dominated by silicon most of it most of it is gone there and essentially it's all all it is is driven by price all people care about is how expensive is energy so essentially the key number that they're looking to is the dollars per watt and you can see that essentially it started up basically and this is in the 70s when they when they started bringing out the first commercial solar cells around about $100 per watt and it's gone down significantly and now if you compare that essentially with the installed power you can obviously see an inverse trend so the cheaper it becomes the more powers installed and so at the end of 2017 sorry those still in German didn't get all of it we're down at a price then for the low sells of something about a third of a dollar per watt so it's very very competitive and as you can see then the installed power is shot up because now with will government aid and so it's been more attractive and so it's VIP and can install more and more worldwide okay so typical solar cells rely on semiconductors such as silicon or gallium arsenide cadmium telluride just to explain briefly how a solar cell works in case you're not familiar with that or it's been too long since you've done solid-state physics so essentially in the semiconductor we have a so-called bandgap or electronic states don't exist and that separates them the valence band which is completely full electrons and the conduction band which is top where there is essentially all empty states and now if we bring in a photon that has an energy that can lift up the electron from the valence band into the conduction band we then have obviously one one state that's filled in the conduction band and in the valence band one it's empty and now these then can move along because essentially they can move through it and that essentially creates charge transport that's our solar cell works ok so I said we need we need a photon that has enough energy if we have a photon that doesn't have enough energy to bridge the gap Bob because it's just gonna travel through the through the solar cell not be absorbed so there's no absorption in that case if we have a photon that has an energy that's far above it it's gonna bring the electrons into a state that's far above and the charge carriers then gonna relax down to the edge of the bandgap of the valence band and then still be able to do charge reduction okay so where is this so the bandgap is critical so obviously where is it in in silicon the bandgap in silicon is about 1.1 electron volts that's far in the and the in or deep into the near infrared region and oops sorry I missed that up a little bit and if you compare that with the the spectrum of light that's coming from the Sun you can see essentially this is the the am 1.5 spectrum so essentially the amount of light that actually reaches Earth's surface there's a lot absorbed by by by water and by it by clouds and and and other thing I like use that we have in and in the atmosphere you can see this is only the visible part so this is all that we can see 780 nanometers so the band gap of silicon should sit right about sorry does work a little bit right about here and so essentially can absorb all of this light that's down here whereas all this light up here is wasted can't be absorbed okay so what about the structure of a solar cells so this is essentially what a solar cell looks like typically it's built up of a PN Junction and what happens is that we come in with light from the top the photon is absorbed where are we here photons absorbed we create an electron hole pair the electron hole pair is then separated at the PN Junction that we have so this interface here between the the dark blue and the light blue and once we separate them then they drift to the electrodes because we have an electric field in there so the electrons are the top electrode and the hole to the bottom electrode and that essentially creates current okay so as I said that the long wavelength light passes through the solar cell completely so if it sits below the bandgap then it can't be absorbed and if it's if it's slightly about the bandgap and essentially it takes a while for it actually to be absorbed and so that's why typically for a solar cell you need to have an absorber layer it's about 200 microns thick so that's about four times the thickness of a human hair it doesn't actually sound very thick all right there other bloss mechanisms that go into this for example we have top electrodes you can see there's a metal eyelet finger electrode obviously metal can reflect light so we can lose a lot of light in this case here as well and that brings me to one of the most important plots that you have for for solar cells essentially a couple of gentlemen noon chuckling quasi looked at how much of the light can you actually use that's coming into a solar cell how much of it can actually be produced and that's what's shown in this graph essentially looking at the the present' of the incident light energy versus the bandgap of the solar cell material that you have there and if you look at the different the different regimes where you have there all of these here so as your increase in the bandgap of the energy more and more these photons can't be absorbed and so they're all gonna be wasted any photon that has energy that's that's larger than the bandgap essentially still only produces one electron hole pair and all the other energy that's that goes above that is also wasted because the charge carries just relax down to the bandages so that all of this area's already wasted then and as I said then there we have tomp contact reflection we have interface issues and so those add up to the other losses and so out of this entire entire square this is the only usable electric power that you actually have from a solar cell for a single Junction which maximizes out to about 33% if you have an optimal band gap of about 1.3 electron volt so about 900 nanometers so also easily into the NIR and so the near infrared region okay so let's look at some of these materials that we have out there so silicon crystalline silicon is this this red one here it's about 1100 nanometers something that goes out here and what we're showing here is the absorption coefficient so how how thickened material needs to be to actually absorb absorb the light and you can see that there are a lot of materials that that have different bang gaps so they're shifted possibly further to that to the to the blue parts of the visible part of the spectrum but you can see quite quickly that they have absorption coefficients that are much much larger than silicon that means they absorb much much more intensity and it's actually done in a logarithmic plot so essentially one of these is n is a factor of 10 and so you can see quite quite easily that that soon then these materials absorb a hundred times as much as much energy as as much light as silicon does also another problem with silicon is that the bandgap is not optimal I said one point three is actually optimal for that silicon slightly off that and so you can see this is where silicon sits so this is the that that top curve from the black curve that I showed you before essentially limiting it and you can see silicon is not exactly at the optimum point but close to it and some of these other materials that sit at different points along with their band gaps are actually very very close to the actual maximum efficiency that's that's achievable okay so if any of you have seen this talk on solar cells you've seen this graph this is one thing that the research scientists making solar cells like to boast with and essentially it's the best research cell efficiencies that are out there they're produced that have been certified versus time started out from 1976 I think there was the first year where they started at these different technologies and all the different colors and and and icons represent different technologies and so silicon is this one here is a simple silicon single crystal junction you can see that it's been gradually increasing from 12 13% slowly up to a value of 26.6% nowadays so it's been steadily improving over the last 40 years or so but one of the materials that's now gotten is excited or it got in the field excited is the halide perovskites which somehow this is shifted I don't know how this happened okay anyway I tried to increase it nowadays is actually this this yellow and red curve that's sitting out here so it started out and around about 200 2011 the first certified solar cell and you can see that the it's been increasing much much faster than all of these other technologies as quickly quickly overtaking some now is up to a value about 24.2% this is only within about seven years that it's actually been able to do this essentially it's been able to improve as much as silicon has in 40 years in only about a fifth of okay alright so what is Piroska well prostate actually is calcium titanate so it's a single mineral the chemical structure to calcium titanium trioxide and was discovered in 1839 by a German Gustav Louisa and named after a sponsors benefactor count count four parofsky in 1839 and so that's why that was called perovskite but basically nowadays all the minerals were the same crystal structure of calcium titanate or dub perovskites so they have this sort of a bx3 crystal structure okay so I said new materials an old dog 1839 was discovered 1892 was actually the first first scientific research paper interesting also by a German by dr. Bell's on this then then it took nearly nearly 80 years or a little over 80 years until until actually anything really happened with that number someone again another German dataviva actually managed to understand the crystal structure was using using x-ray diffraction to figure out how this materials actually composed and and built up to understand how the compositions actually changed the crystal structure in there and then in the 90s that was the first instance where it was actually used for for anything opto electronic there was a group by Sherry Kagan and David Mitzi you use it for field effect transistors but he actually says nowadays he he doesn't know why he ever caught and he never came up with the idea of using it for solar cells so he's kind of disappointed about that 2009 was in the first demonstration of a solar cell using a so called dye sensitized solar cell with a profs gate essentially is just an absorber material to pass the the the electrons and holes then onto it to charge transport layer four percent efficiency the problem with the solar cells the electrolyte that they needed to transfer the electrons and holes was made up of an aqueous solution and profs guys tend to agree degrade in water so the solar cell lived for about 30 seconds before it died so that's why it took about two or three years them to for people to figure out that this was actually a super material because it degraded quite quickly and they replaced this aqueous solution with a fully solid-state solar cells in 2012 them was was a breakthrough when they actually made two we're able to do this in that the efficiency jumped up to 12% row two quickly within only three years of development it had 20% and so currently nowadays we're at a value the end of 24.2% as I said earlier alright so the crystal structure I said a bx3 I'm just gonna run through this in detail that basically we're sort of confined in in what we what we can use as materials because we have to preserve charge neutrality so the oxide materials that I'm not going to get into are used for all sorts of things like piezoelectric devices for for nano positioners for example but the case that we're interested in the halides and halides have a single negative charge so essentially we need to balance them with these a and B cations and so we're left only with the combination of the a being a single positive and the B being a double positive nevertheless all of these materials can actually be used to make these halide perovskites or can and have been used to make the hell out profs guides although essentially the another thing we need to look into is this the size of the the ion so obviously you can see this crystal structure they can't be arbitrarily large these this a cation that's sitting here in the octahedral void obviously can't be large enough otherwise it won't won't fit in there and so we're left up with a certain other restrictions there to factors that essentially just take into the account the radii of these materials the Goldschmidt factor in the octahedral factor that basically give us a range of Tolerance factors for where these peroxides can exist and so there's essentially just plotting than these two factors versus each other with the predicted range where all of these materials are stable and you can see it's only more or less of a very arbitrary number as there's some materials that sit in this region that profs gates shouldn't exist that do and the other way round we also have materials that that should exist that don't so it's sort of a very arbitrary science and in which of these materials actually work in form stable profs gates okay so the materials that have gotten the world really interested now actually don't only contain single ions but contain organic molecules that are sitting here these actually sit here in the in octahedral voids in this ace basis of this methyl ammonium and form ammonium these are about basically the only real molecules that I can actually fit in these spaces anything large slightly larger than that wouldn't fit in there you can see again here fitting on the tolerance factor is octahedral factor so these different combinations fit nicely into there with did the different halides the chloride the bromide and the iodine and the different the different he said I am sitting in there okay so now that we know the composition the structure we need to figure out what how this how the the electronic and the opto electronic properties come to be and essentially what we see is they're mainly governed by the inorganic framework so the energetic bands I talked a little bit earlier about the conduction band and the valence band they're essentially given by the electronic orbitals of the lead and the iodide itself and what makes the profs gets really interesting is that if you exchange now the halides which are in different rows in the periodic table you can get a very efficient bandgap tuning just by tuning the composition of these so essentially what you're doing is you're shifting the position of this bottom valence band with respect to the to the conduction band and so your being your your you're able to tune your bandgap from somewhere in the NIR out to nearly the you altra visible ultraviolet range and so now if you're looking at these are actual solar cells in the absorption spectra of these and and varying the composition from from purely iodide to purely bromide you can see a very nice tuning capability throughout a very very large range of the of the perovskite alright so I said that the absorption improv Scott is about ten to a hundred times as high as in silicon so it's actually alright not enough for prof Scott itself but for other metals but prof. Scott is also very very efficient at absorbing and that means that the absorber layer can be a lot thinner in the peroxide itself and so if you look at the cross section this is taken in scanning electron microscopy this cross section of a solar cell essentially you have an absorber layer of the profs gate now that's only about 300 nanometers thick so again in fact in fact about a factor of a thousand thinner than from silicon so that means you need a lot less material and you can also come up with interesting applications and I'll get into a little bit later all right so basically just to build up the solar cell again we have the profs given mid all we need electron transfers and hole transporters they basically make sure that the electrons go into one direction the holes go into the other direction we have top and bottom contacts and that bender creates our current and our voltage in there all right so that's what a final profs good absorber layer looks like made out of methylammonium lead iodide so it's you can see it's a very deep brown so it's absorbing most of light that comes through the visible spectrum and that's what the fine marrascaud solar cell looks like alright what are the advantages of props get solar cells well first of all you can shift the bandgap to the optimal position so you get a higher theoretical maximum efficiency so about 33% with respect to silicon which can only achieve 29 you can get a lot thinner layers and the materials that you need for that are abundant and cheap and this thins of this is a lot easier than to do them for silicon and more very important that you have tunable band gaps all right so why is a thin layer actually interesting so basically now you don't need to have a very rigid substrate to be able to hold all of this low but you can actually start to use flexible substrates that means you can do roll-to-roll printing where essentially you have a polymer substrate that basically gets printed on just like people do with with newspapers if they are still printed nowadays and so basically that makes the process a lot faster and a lot cheaper you can make things like mobile chargers where basically you can roll them up and then unfold them then to charge anything up any devices up in the field they're not bulky anymore you could think about using wearable electronics it's especially something in the military in the US is very interested in having their soldiers then basically with electricity and the deserts for any of their devices that they need there or solar pilot vehicles obviously you don't want anything heavy and bulky sitting on top of your of your car then basically that you always have to lug around and move and so I think last year one of these these planes actually broke a record for the amount of duration that it could fly around the earth 'litham something like about three months it was in the air continuously without having to recharge alright another an invitation is net advantages that you can integrate these into building architectures so for example you can create semi-transparent windows to absorb some of the lights so to use a lot of this area then not only the roofs but now all those also the windows firfer for for producing electricity the problem or one of the problems as I showed you before the the absorber layer it's brown and potentially ugly so not necessarily anybody everybody wants to have these code in their windows and so one of the ideas that somebody came up with is actually producing color neutral windows and the way that they did this was instead of making a thin a thin film of Piroska to continuous film they actually developed islands and through this sort of island formation there what they were able to do is to change the absorption from from deep to basically just only partial and so the absorption spectrum itself is nearly flat throughout the entire visible range so instead of having a dark brown you essentially just have a sort of gray obviously it reduces the efficiency to only about five to ten percent but if you think about the areas that you have on buildings for for windows especially skyscrapers or something like that you've huge areas available and so that we can more than compensate the loss and efficiency another idea is to use so-called smart windows so there's a material so a profs got here where a material can be switched between an absorbing and a non absorbing state and the important thing here was that this was essentially done through by a phase transition so the transition between two different configurations of this crystal so the the normal parofsky ID form that you can see and this is a so-called zero D State and this is actually achievable only through heating through from from sunlight so this peroxide itself can absorb light it goes into then absorbed absorptive state and then essentially can be used as a smart window for example during the day if it's too bright then dim that didn't the light that's that's coming from the inside so sort of like a pair of photochromic sunglasses all right that was that was thin layers additional thing that I want to look at is the tunable band gaps because now you can if you have gaps to noble you can begin to think about tandem solar cells so tandem solar cells I told you the the idea essentially that the in the single Junction solar cell we can only really use a small part of the light because either the photons have to have not enough energy and aren't absorbed or they have too much energy and all of this is lost due to thermal ization but you can actually overcome that with this strategy that if you instead of using a single absorber layer you actually start to use multiple absorber layers that are tuned to different parts of the solar spectrum because now they can absorb a different amount of light and transfer a different amount of this energy actually into into a usable electric energy and so now if you look at the actual efficiency of the theoretical maximum efficiency so for a single Junction as that was 33 percent as you increase the amount of junctions you see using only a second absorber material already at about 50 percent of the light and then it increases on until technically if you have infinity junctions stacked on top of each other you could absorb eighty seventy percent of all the light obviously it's not quite feasible to actually do this alright so has then done it has been and so in using only pure of perovskites because you can you can tune the band gap of these prostitutes you can actually achieve set them up to the optimal configuration so what's what's shown here is is is a theoretical calculation of the band gap energy of the bottom cell versus the top cell and in the color coded is the the maximum efficiencies that you can achieve so if forty six percent is the top in red zero and and dark blue and you can see there's sort of a relatively large area here that you can tune the band gaps within so about one point one point two one point seven I think is the optimal value so these are Omni on the border of the NIR region and you can very nicely replace part of these these components and Piroska to basically tune at tune the absorption throughout the entire necessary wavelength range to get this there are two different configurations I'm not going to go into too much detail on this essentially two terminal and four terminal contacts that can be done or essentially this is a the the four terminal is essentially two separate solar cells that are just stacked on top of each other to work independently of each other it's a lot easier to produce although it has more layers and is more expensive so it's not the optimal and that's why people are trying to go towards the two terminal variant of these solar cells and so there's again another cross section of the one of these tandem profs guide solar cells with the top layer absorbing at one point to the bottom at one point eight and you can again see here to the scale bar is five hundred nanometers so this entire thing this is very very thin and efficiencies they showed there are still below what you can actually get in single Junction perovskite so you can see that while theoretically it's quite easy to do it's very very difficult to actually match all these energies to get all of these layers nicely consistent and and sitting on top of each other so it's definitely something that people are working on right now alright but possibly easier to achieve and and quicker to commercialize or profs guide silicon hybrids I mean silicon's already been established for forty years and was working quite efficiently and so the idea is why not make use of this highly efficient silicon PV technology and just put a second per off sky on top to absorb some of the light that the silicon can't uses as efficiently and so that's also observe possible in the two and four terminal geometry so having the Piroska to absorb lights only some of the passes through into the silicon then at the bottom and that's again another one of these plots just looking at whether for which energy you need so essentially because this band gap of silicon is fixed now you have to that that sets them the optimal range of the profs guide so about one point seven the slightly shifted from the from the other case but it is doable and so now now nowadays there's a company that sits in Oxford founded by Henry Smith Oxford PV they're called they bought up one of the failed German solar cell companies that has a production system in button book I think it is and basically now they're in are they're already industrial producing these these silicon solar cells so these are it says 16 by 16 should be a little bit larger it's about I think six by six by six inches that's about the right size that are 20 to 22 percent efficient so actually very very efficient silicon solar cells and what they're doing out just putting a profs guide solar cell on top of that there to make the make the joint join Perales get silicon hybrid solar cell or technically they should be able to reach 30% efficiency sorry for spelling your cross section again of this the cell so this is only the top of the silicon layer then you have a little layer in between to passivated and also for extracting charges and then the profs got sitting on top of it and then a transparent conductive electrode sitting on the top and this is what one of their cells looks like and they currently hold the record for a tandem solar cell which is at 28% and I listened to a talk by their CTO in April and he said that by the end of the year they expect to be selling these things commercially already so it's already quite efficient and and essentially the cost of adding this prostate layer on top is really not a non-factor with respect to the overall cost of the total module of these systems alright so obviously one one way of getting around our energy problems is by getting alternative light sources in hand but a second second method is by enhancing energy efficiency so cartoon looking at it essentially an old light bulb we hardly use anymore replaced then with with the CTE bulbs that we had there and nowadays basically all that's installed anymore and as LEDs and so profs guides itself ideally suited for light emission basically we talked about that before sewing that as you tune that the band gap you can tune the tune the absorption but you can also tune the emission of these so they can emit but anywhere between the blue and the red and actually if you go into making nano crystals out of these it turns out that they're extremely efficient in how well they produce energy so essentially if they absorb a UV photon and then near with yields of nearly a hundred percent convert this then into a photon of a different energy in this case and so obviously you can mix this up now to it to make very nice LEDs okay what does an LED and an LED is essentially just a solar cell that you drive in Reverse so technically if you actually you can use the solar cell and put the voltage on the wrong way around and you actually get light out of that you won't be able to see anything obviously because it's silicon and so it emits in the in the infrared but it's essentially the same principle it's just that you know you're taking electrons and holes and injecting them so you're going the wrong way around because you have a current they they they are able to overcome these energies and then they reach the middle of the Prostate layer in this case that and recombine the electron falls down to the conduction end into the valence band and produces a photon so the first profs got leds were demonstrated in 2015 these are only very very small ones but but emitting in the red and green a different case where you can show now you can actually do two mixed cases so basically now you can tune this between it throughout the entire visible range and efficiency is at least in the red and green are already about twenty eight twenty five percent which is a very good for for solid-state lighting applications okay so question our white light LED is possible it's first of all what you need to know is what are we looking for if we have white light so obviously technically white light is or this is this is a daylight spectrum you can see it's a relatively flat spectrum that's coming from from the Sun more or less but if you're absorbing or if you're if you're looking at your eyes essentially we don't absorb light equally but we have basically three different components we have three photoreceptors that that are sensitive to two individual wavelengths that absorb in the blue the green the red and their absorption that makes up all these different colors that we can actually see in our eye so essentially all we need to do is tune the tune the emission to these different wavelengths and then we're able to get an LED that that looks as white as possible so basically getting a bread a blue and a green LED together to make a white light LED one of the big problems though is there's essentially no efficient material that actually produces green LEDs and this is a huge problem this is also known as the the big green gap as defined here so so the nitride LEDs are basically what drove the white light LED production itself phosphide LEDs other materials in the red are very efficient as well but there's no no semiconductor material currently that can actually overcome this and this is where profs guides then actually come in typically so how people get around this is they make a blue LED so a nitride based LED have a yellow absorber and emitter on top of that that absorbs some of this blue light and confined or emits it and basically what you're doing is you're tuning the amount of blue light that's still coming through with the yellow light that's produced by this this phosphor and that gives them a different color spectra that we can see there and that's what we're calling them then warm white and and cool whites or daylight or all these different different types of white scales essentially all we're doing there's in the blue to yellow ratio and that changes them the apparent light of the whiteness of this this light that we're getting out okay so what about for Piroska LEDs I mean you have different strategies that go out there so again you can emulate the same strategy that we have currently so far where you have a blue LED and you bring on the yellow emitter so this is a new burner where a process known as self trapped exit ons actually creates a very very broad emission state and in the yellow and that produces essentially white light so that's that's feasible with with profs guides you can also then tune the profs gets to be red and green emitters absorbing the blue light to get to get white light out of there there are also people that have have made LEDs there so essentially this is the light from the blue LED that's on the back and then the green and the red perovskites add up to make white light going further basically producing something in the UV so that essentially all of the emitters are now per Ops guys so you have a blue and a green and a red perovskite adding up to make white light as well as you do in a TV a typical but ideally what woman would like is to have a blue a green and a red LED that are driven directly by electricity or electric current and don't actually have to absorb any light and then re-emit it again because that is obviously not and not always 100% efficient so that's essentially what what a simulated spectrum of such such a thing with actually look like okay so there's still a couple of limitations the prof skits not all perfect there are a lot of things that they're currently impede widespread commercialization so one of the big problems especially in Europe is that it contains lead at least most of the efficient profs cuts contain lead and European Union's is not like lead at all so that's one of the big problems for putting into TVs are putting two solar cells although if you actually covered the entire land mass with profs guide solar cells and have them leak out completely into into the ground you wouldn't be able to detect any of the lead behind the background contamination that you have led anyway nevertheless the European doesn't let Union isn't like lead so what people are trying to do to replace lead is good to look at the periodic table okay LED sits here this is this yellow component over there why don't we stay in the same period use tin or germanium because they're essentially chemically more or less the same the problem is the stability because tin and germanium like to form four plus states instead of two plus States and the actors then traps then for electrons and holes so essentially once they're created they then tend to get lost and not be able to use for for electricity and so an alternative strategy that people have come up with is okay so we knew we looked at a tin and germanium for staying in charge neutrality but why not go one to the left and one to the right and combine those together because one is a two plus one is a four plus and so essentially you come up with a three plus case so using tantalum and bismuth essentially that's where people have started developed these so-called double profs cuts because essentially the crystal structures double is large one of the big problems there is businesses actually even more toxic than lead is but there also there are also other strategies that people then have gone into the turn into the transition metals looking to silver for example think one of them is a cesium silver and something else that they're using in there so a lot of a lot of material space then to possibly overcome this this toxicity issue all right another big problem that I said or that we have great lots of time left are so red and green profs cuts are quite efficient they actually produce extremely bright LEDs so I think 22 23 24 percent or something about the maximum one of the big problems that blue profs guides are much less efficient so this is essentially just to to dispersions of bluh parofsky nanocrystals in there and you can see the quantum yields are only about 6 and 22% so that means the the amount of photons absorbed by the light and they're actually producing blue photons so it's a lot a lot less efficient and so obviously if you want to have an all parofsky LED you'd like to have blue green and red coming together another big problems environmental stress so I told you about the first solar cell that was built into an aqueous environment and and disintegrated within about 30 seconds that's also a big problem for for solar cells because if you have a solar cell sitting anywhere it's going to heat up it's gonna cool it's gonna expand it's gonna contract and that then is is perfect for letting water in and that lets and the solar cells too great quite quickly you can see here the nice black absorber meaning essentially all the legends absorbed relatively quickly it becomes more and more yellow as this Piroska disintegrates back down to one of the original components that led iodide so that's that's that's one of the big problems that people are then looking into into encapsulation another problem that I'm not going to get into in too much detail is that I told you you can mix the the halide composition quite nicely to get individual emission colors in there and one of the problems though is that despite the profs kites being more or less semiconductor like in having a nice crystal structure is these ions don't actually like to sit necessarily the same spot that they're put in so if you put on an electric field which you need quite large in LEDs a couple of vaults in there what happens is that these ions then start to migrate to go to different areas so in this case where you've bromide and iodide they've built they form then regions that are bromide rich regions that are iodide rich and as you know from semiconductor physics then the electrons and holes always want to go to the energetic minimum and so they're going to go towards the iodide rich regions and so essentially what's going to happen is the color of your LED changes as you're running it which is obviously not ideal all right so some of the work that we've been doing on is as aim to attack some of these strategies so we've been looking at mainly looking at overcoming some of these blue issues and also looking at stability of these nano crystals and so what we've been doing is when when the first nano crystal is actually produced in 2014 we saw we wanted to understand how the synthesis mechanism worked and so essentially what you get if you we copied to one of the first paper that was out there produced these nice bulk nano cubes and think the scale bar is 200 nanometers this case you can see these nice cubes sitting in there and they were using long organic ligands to passivate the nano crystals and so what we found is that as we added these organic ligands instead if the crystals getting smaller and smaller what we were doing is we're transferring these cubes into quantum and fine nano plate it's very very thin nano platelets and to look at how this affected essentially what we're doing is starting out from these these bulk cubes and then adding a percentage of the the organic ligands in there and as you can see as as the contact goes goes larger and larger you can see the color of these actually shifts and from the TM images here you can see it's thinner and thinner and so essentially we're going down to two very very thin nano play that's and in this case actually of the hundred percent organic ligand what we're seeing there is actually a single mono layer which in this case is only about 0.6 nanometers thick so extremely thin layers out of here all right why is this in here again interesting okay sorry about that problem with the nano plate that's that we had in this case is a thickness control so if you look at the spectra in there you can see that they're not not nice ideal single emission spectra but they're actually quite large and bulky and that's that's due to the fact that we don't only have a single thickness in here but we have different thicknesses and each of these have a slightly different absorption and emission spectrum and so that leads to these these broad broad spectral that we're seeing there and also the efficiency in the blue is quite quite quite low and the stability on substrates was was quite weak and so there we replace them the organic organic ligand with with organic molecule with cesium and now we were able to control the precursor ratios a lot nicer as you can see here again we were able to do this this fine tuning of the colors but you can see the spectra are only single single Peaks aren't actually quite narrow in this case and if you look at electron microscopy images of the nano crystals in here you can see these are all nano platens with a single thickness of five monolayers in this case here you can see they're slightly thinner these all have three monolayers and essentially what we're able to do is control them now the thickness of these platelets buy it buy it buy it with a single mono layer position so from from two monolayers so just over a nanometer to a little large up two cubes and so can really nicely - now the emission color of these of these nano crystals again the quantum yields though suffer from that so the cubes that we're pretty able to produce have a quantum yield of about 95% you can see azimio thinner and thinner thinner they become worse and worse and what we found though is that we could actually produce or we can enhance the efficiency of these by adding a second step in the synthesis by basically adding an extra amount of lead bromide to this and so we're going from very very low values on the order of about five or six percent for these thin phase two valleys that are over fifty or sixty percent in the in the deep blue and what we found is that the enhancement factors that I've factored by which the PL quantum yield goes up is larger the thinner it is so we D Utley thought of the idea okay this has to have something to do with the surfaces that we have in there and so essentially what we're saying is that at the beginning we have a lot of platelets that are some platelets that absorb life and and make make photons there's a bright plates platelets we have a lot of Dark One's that absorb light but basically the light is trapped it can't it can't come out of that again and what this this LED bromide is doing is essentially in these dark platelets we have a lot of unperfect imperfect crystals there and it's basically filling out these holes that we have in there filling out the crystal so that they then turn into nice fully passivated crystals and that's then turning a lot of these dark platelets and on again that they now are able to to emit light efficiently and so in a collaboration we had then with with a group but at Cambridge University we're able to produce a blue LED you can see in an extremely narrow spectrum so essentially all the light is focused into one small range that got us really excited if you ask me about the quantum efficiency of this device it's at that time it was the third best parofsky blue solar cell in the world with an e QE of 0.06 percent so nothing to write home about something you can definitely publish but nothing to go in there and there are a lot of issues that that are still on that sentence we figured out okay it's not only the quantum you'll do we need to go into that we need to take care of but we need to also look at the actual device structure so I'm not going to go into too much detail but one of the big problems here is that as we're injecting charge carriers there's a huge drop-off of the from the hole transporting layer into the profs guide where it's really really difficult to actually get these holes to go into the emitter layer and so they're essentially sitting right here at this interface and recombining non-radiative lee so we've been working on then adding additional layers into there to enhance the efficiency of these of these devices and that was a follow up paper the change that was accepted actually just a short while ago where now we are able to produce two different blue LEDs now with values approaching nearly zero point or nearly one percent eqe so a lot better than before again you can still see that are far far behind anything that the blue and the green peroxides are in alright one more time a couple of minutes okay I'm just gonna run through the end this is something that I'm really excited about that we've been starting up now with our new group essentially to look into the into the passivation of these profs kites and so now we start up with a new synthesis method then instead of adding organic ligands to the to the solution to passivate the crystals as they grow instead what we're doing is we're preforming a block copolymer micelles so these are these are polymers that consists of of two different parts that and thus have a different different effect in the in there or a surrounding ligand surrounding medium and so form these micelles and what we get to happens then we add the add the perovskite precursor to the solution they don't like to be out in the solvent but like to like to diffuse into the course of these micelles where they then form from Piroska nano crystals there and we can then take them together form really really nice monolayers on substrates and start to look at their actual properties so you can see here there's a TM image now of a feel of Piroska of these my solar and capsules nano crystals they're they're very small these are on the order of only about 4 nanometers yeah AFM studies show that they're they're very very homogeneous so we have a surface coverage here in this case of about 99.8% so you can make very very nice layers for that you can tune the emission energies throughout the full range of the visible spectrum so basically the same as you can do with their other nano crystals you do size tuning in them which is important then also for getting into quantum confinement effects but more importantly they're stable or more stable to water so if you look at just typical nano crystals there again they degrade relatively quickly these are just nano crystal mean without I knew this this polymer and so within a couple of days the total photo luminescence that's coming out of them vanishes down to zero in the case of our thin films basically we can have them sitting out in a sample and we can even put water on top of them and and and they don't change the spectrum at all and if you monitor the total photoluminescence intensity over over time you can see this here this black curve is essentially one of these nano crystals so it drops off this is them sitting out in air after about 13 days you can't see anything anymore in comparison this green curve is what we now have in these Piroska it's sitting out in air we've now reached over 400 days and they're still a mission they're still efficient at about 40% so a factor of about 40 longer and what's actually gotten is even more excited as this blue curve you can still see a drop up but essentially what you have to know is that this is a film of these profs cuts that's sitting in a glass of water and it's been sitting there for 75 80 days and it's still emits light afterwards so obviously it's not perfect but but it's a lot faster and a lot more stable than any of the prospects that we have nowadays all right and with that then I'd like to sum up briefly sorry about the colors here looks better on the screen so essentially Piroska at an old material that's been rediscovered and used for different different applications it bases on it's based on the extremely good optical properties including absorption and tunability the band gap because of this properties and the thin layers you can do is you can have lots of different applications ranging from solar cells to LEDs and also others that I didn't talk about obviously there's still some problems left toxicity a degradation there's still problems in the blue emission but interest is constantly growing so if you just look at this is a recently from Google Scholar the amount of amount of papers that are being published over years on the field of profs kites is increasing steadily so I think last year there's something on the order of about 10,000 papers that are produced on profs kites so even we can't keep up with everything that's being brought out there and so the future of profs kites I would say is quite bright with that I'd like to thank all the people that don't work on this so my former group that share for photonics not to electronics here at the LMU my new group which is now already twice the size of this this was an Oktoberfest last year so when we started up in August and collaborators that we have at the Tom and burpin Cambridge and then all of my son funding sources and I'd also like to thank you very much for your attention [Applause]