The Path to Perovskite on Silicon PV | Prof. Henry Snaith (2018)

[Music] how did you become one of the most influential scientists in parofsky like solar cell research and what have been the most notable achievements to date so far off sky solar cells are simply solar cells containing metal halide perovskites so I first became aware of metal halide perovskites in 2009 when visiting some colleagues and collaborators in Japan so the first paper are using metal halide frost coats in solace in in solar cells had just published and this was from the group of Tom Mia Sakura into an University and Tom gave a presentation and presented this work and it was it was very impressive the most impressive aspect of it was that he just took two salts led bromide and methyl ammonium bromide put them together in a solvent and spin coated a film and it crystallized immediately on during spin coating and formed a material at the same time back in Oxford I'd been playing around with them quantum dots LED sulphide nanoparticles and they were an extremely tedious synthesis process and it took a long time weeks in fact of work to make devices whereas this parofsky material appeared to just form instantly so that was something that I found quite interesting um I subsequently returned to the UK and managed to get a grant for a collaborative grant with one of my colleagues Taku Murakami and I sent a PhD student back to Japan to learn how to make these / off sky materials um but back in Oxford we managed to integrate them into what we were working on with solid state sally solid state at the time dye sensitized solar cells so the the device that Tom had developed on the Asarco developed was um based at your honor porous tio2 anode where the anode is sensitized typically with the dye but in in the case of what he'd done it used small nano crystals of these um Piroska materials and then it's infiltrated with an electrolyte so the big problem with this device was that the electrolyte dissolves the fur off spike crystals so the device lasted seconds and I think this is probably one of the reasons why that work wasn't instantly followed it looked like it was quite interesting but just led to unstable cells wears in contrast when you for this in state an organic whole conductor in into the device it was quite stable so we had films that would be stable for a thousand hours under sunlight for instance which was a massive step change um the big big shock if you like or the first shock was just our first batch of devices they were already six six percent efficiency at the time we are solid-state dye sensitized solar cells we'd sort of hit a plateau about five percent and for three to five years we couldn't get them any better so this was already more efficient on the first attempt and of course they rapidly stepped up so the first real surprises are there they work instantly usually when trying in new material if you can get one percent efficiency you're pretty happy um following on for that from that with the way we were playing around with the way in which we process and crystallize the parofsky material and what we were getting wasn't just small nano crystals but we were getting more of a poly crystalline material very very long range crystalline order and we we had a suspicion that we were also getting charged conduction in the per offs gate material so we made some test cells swapping out the porous tio2 which was an electron conductor with porous alumina and actually we tested these cells to see how they worked under the sunlight we weren't expecting highly functional solar cells because they were an insulator in place of one of the semiconductors but surprisingly they actually worked better than the cells without tio2 and we got a very big voltage step and improvement in voltage so this was the first real breakthrough if you like where we realized the parofsky material didn't just act as a light absorber but it also act as a charge conduction in the cell um followed following on from that work in in close succession we started investigating whether we really needed this porous alumina scaffold and we made it thinner and thinner and thinner and we got improved current improved device device efficiency at extracting charge and when we looked at a section of these devices they were they had a solid absorber layer of porous guide that was on top of a thin porous scaffold of alumina so this told us already that the solid parofsky material can conduct charge out of the device now that might not seem very surprising but at the time the paradigm was that you could either have a very expensive material that was a crystalline semiconductor low low defect density and that would work as a solid absorber material but if you went to cheap materials that were easy to process from solution at low temperature they had a lot of electronic disorder and they wouldn't work as a solid material to get them to work you had to blend and multiple materials typically one material it would conduct electrons of one which conduct holes and in the case of the dye sensitized solar cell the absorber sandwiched between the two you know rather complex mises structured or nano structured device and the the way of thinking was that materials either fell into this category of high disordered but needed nano structure or low disorder and would work in a solid crystalline film and the profs gates they see had the best of both worlds they worked as a solid crystalline film and they were very easy to process at low temperature so they were clearly in a different category if you like a league of their own um so moving on from that we demonstrated quite clearly that they could work as a solid thin film and in fact now the the embodiment of a parofsky solar cell is that you have on one side you have a charge selective material maybe an n-type organic material you have the per office kite absorber just a solid poly crystalline film between about a micron in thickness and then you have a p-type material there might be a an organic charge conductor or metal oxide and electrode on the top so it's a very simple sandwich structure device that we call a planar hetero Junction or a double hetero Junction so since those first few publications end of 2012 start of 2013 the research field of profs kite solar cells has absolutely exploded so to put it into context in 2012 there were four publications on prof sky solar cells 2013 there were about 60 in last year in 2017 there were about two and a half thousand this year there's probably going to be 5000 by the end of the year so it is just sort of going up exponentially now we were very very fortunate to be there right to the big and we've managed to stay in the front pack in the head in certain aspects of the research and that's that's really in terms of sort of impact of research obviously we've been lucky to be on this wave riding this wave and hopefully still riding this wave through to fruition so the the explosion of the field is the reason why we get so many citations for the work we do what's of the state of the upper Rob's got materials at the moment so so since since the starter or least since 2012 when the field took off the efficiency of the small lab base cells was just over 10% so our first publication we ten point nine percent efficiency now that's increased to over twenty three percent efficiency over that over the last five or six years if we compare that to what's the efficiency of existing photovoltaics silicon that's deployed today mainstream multi crystalline silicon is actually just under twenty three percent efficiency is twenty two percent efficiency so profs kites have surpassed the efficiency of the most widespread deployable PV so that puts them in a very strong position to look like a real contender for for utility-scale PV but of course Perot skites carry a lot more risk than silicon silicon has been in the field for a long time it's proven the stabilities known and we'd really come down to stability as a major component that we need to prove for the parofsky solar cells so in terms of what what's led to these improvements in efficiency a lot of it's been device structure engineering and controlling the crystallization of the profs kite film so making thick high-quality puros skates and that comes in part it comes from choosing the right components so we've made a transition from methylammonium led triiodide which was the workhorse of the first generation of pro skates through to a mixed cation mixed anion materials so typically a good quality profs kite might have form a MIDI neum and caesium at the a cation site led the metal site and they're not a mixture of iodine and bromine the halide site in the ABX 3 crystal structure there are many people exploring trying to put in multiple cations there's for instance the addition of rubidium even the addition of very small ions such as potassium and sodium actually if you go smaller than cesium in the a site the ion doesn't really fit into the prof sky structure so many of these things that are published with sort of quadruple Penta Penta mole hex hex hex cations they're probably not quite right though the profs kites probably made of four membered inium and caesium sometimes with methyl ammonium in there as well and the other ions are really doing something else they do have an impact there in the film they may be going interstitially they may be causing certain reactions on the surface of the crystal and the grain boundaries so that's one component has been tuning the composition the other component is just being controlling the crystallization working out good ways to process these parofsky materials in parallel in in the the peroxide absorber itself in pretty in principle is a pretty low defect density there aren't that many defects and the defects that exist aren't absolutely detrimental they don't kill the performance but they do limit the open circuit voltage so one way to improve the voltage is to passivate the crystal and in fact what passivation is really used in a very broad term because we don't really understand what's going on chemically and there's lots of routes of post-processing of the films that improve for instance the radiative efficiency the fraction of light re emitted from the Piroska material and and that we term passivation but we don't really know what's going on it could just be recrystallization of the surface there could be a whole host of things there so understanding that surface chemistry the exact chemistry at the grain boundaries and in the Buried interface is really important that's that's in part driven the performance improvements but there's still massive scope to make it better to go further so what you know we're currently at 23% efficiencies the record sells in labs actually there's nothing stopping that pushing up towards 30% efficiency in a single Junction cell and that will happen by better control of the surface properties in my view of the profs guide absorber coupled with that there's been a lot of tuning and optimization of the contact materials but what is quite surprising is that we haven't had to develop new contact materials we've just had to select materials that are already available either from the organic electronics field or from the UM from from the dye sensitized solar cell or thin film PV field as well we've used metal oxides organic semiconductors and they seem to make very good electronic contact to the profs gun so but tuning the properties of those materials for instance doping of the contact materials has all been part of getting the efficiency up and going up this ladder towards towards the shockley-queisser limit we saw that some way to go and quite a lot of headroom [Music] it's the current progress of multi Junction perovskite devices I mentioned the efficiency of a perovskite solar cell is 23% and either we could potentially get that up towards 30% efficiency but that does someone not familiar with photovoltaics that seems like a pretty low number it's like why we chucking away two-thirds of our power but actually there's there's fundamental limits to the conversion of sunlight to electricity and they relate to the bandgap of the semiconductor that you absorb the lighting so so if you have a very narrow band gap material this but this would mean that it absorbed all the light over the solar spectrum you generate a very high current but you wouldn't generate very much voltage if you a very wide bandgap semiconductor you can generate a lot of voltage but you're only absorbing a small fraction of the sunlight so if a semiconductor had a band gap of say three electron volts you'd only absorb the UV s spent part of the spectrum and you want to absorb the UV the visible and the infrared so this leads to a compromise where a bandgap for a single absorber material is the optimum bandgap somewhere between one to one and a half electron volts and that gives you a maximum of somewhere around thirty to thirty two percent efficiency so that you think well that's it okay we'll just work to make cells as efficient as we can but actually there is a way to make cells more efficient and of course if you look at the of the global efficiency chart that is published by NREL um there's solar cells which are 46 approaching 50% efficiency and they do that by not just having a single absorber but having multiple absorbers on top of each other so you you you absorb different parts of the solar spectrum in different cells and from the top cells that absorb the blue visible part of the spectrum they'll generate a high voltage and the cells behind will take out different sections of the solar spectrum generating a smaller and smaller voltage but by doing this we can we can extract more energy from the sunlight so this is a strategy for Perot skites that were starting to pursue it's looking very very promising so for instance in the in probably the simplest embodiment is to take a parofsky cell and stick it on top of silicon which is the existing PV material and if we do that we can improve the overall efficiency of that of that cell so the present world record for prof. Scott on silicon is twenty-seven point three percent efficiency which is much higher than the 23% for a single Junction Borowski cell and it's even higher than the all-time world record for crystalline silicon using a very advanced technology which is twenty six point seven percent so this is the point at which perovskite start to take the lead we've just passed all other possibilities were silicon and now they can go forward but it doesn't just stop at a tandem cell and in fact if we if we can estimate what sort of efficiency we can get if we put three layers on top of each other then we could push it up towards forty percent efficiency which is feasible and that could either be to per off sky layers on silicon or even learn all parofsky multi-junction cell so this is somewhere why I believe the field has to go and is going and it's somewhere where I think there'll be a lot of excitement and interest over the next few years [Music] Halcon Purab skite silicon multi-junction solar cells benefit from concentrator photovoltaics so as scientists we always try to think about what's the next step for improvement so going for instance from prostate single junctions to multi junctions the there's another way of improving efficiency and that's to concentrate sunlight or increase the the intensity of the sunlight so you don't get more sun hitting the earth but if you can concentrate the sunlight what you do is you generate a much higher voltage out of the solar cell in fact these multi-junction cells that are 40 percent efficiency get boosted up to 50 percent efficiency by concentration that's what's required so the natural next step for Paris Gates would be concentration high personal II did it haven't given this much consideration as a realistic prospects I'll have to give some credit to my postdoc some Zhi ping Wong and Qin Ching Lin who came to me one day to show me some of their results of intensity dependent measurements of Perot sky solar cells and I thought this is very interesting scientifically it's quite intriguing um but we ran some calculations and and worked out that Perot skite should really operate very well up to many tens of thousands of solar intensity times concentration from the sunlight but of course the practical challenge is we've been struggling and making big improvements with the stability under one Sun but how can we make it stable now under 10 or 100 Suns illumination but actually over the last year or so I've convinced myself that this is a very good idea so I'll try to convince you that maybe it's also quite a good idea so in principle we can we know fundamentally we can get higher efficiency out of concentrated sunlight and in fact what we've shown is with pure sky cells this can be achieved with with contemporary perovskite cells they can be 21 percent efficiency under full sunlight and we can raise that to approach 24 percent by concentrating the sunlight they're the cells from our lab so it can work and in fact we've seen many hundreds of hours stability under ten Suns illumination so they're progressing well and I wouldn't say stability is an absolute critical factor now and in fact if you think about a concentrating array provided you can thermally sync the solar cell and have the heat radiated away then in principle it can still operate at the same temperature as a flat plate technology would do depending on your concentrating optics and everything like that so what the opportunity here is and it won't be with single Junction profs cuts he'll be vote with with very efficient multi Junction Piroska it's the opportunity is not to go for say a thousand fold concentration or three five hundred fold concentration that's required for present-day multi-junction cells the reason why that's required is because they are just so expensive to make these these gallium arsenite and an multi-layer gallium indium phosphide cells with germanium they're they're made via epitaxial growth and they cost tens of thousands of dollars a square meter in contrast profs kites even cheaper than silicon to make and yet we think we'll get to the same sort of efficiencies so then then we can think about deploying intermediate or what's called low concentration factor on the order of ten to thirty fold concentration and deliver performance improvement and do that with very efficient cells the other thing that's riding in favor of concentrated photovoltaics for profs kites is that the trends in deployment are moving towards solar tracking so already presently deployed mainstream modules are put on a boom so they've single axis tracking now that could be combined with a low concentrator factor solar concentrator and there's even with existing technologies they're put on two axis trackers in some higher radiance areas so now the the more power you get out of a module the larger arm you offset the balance of systems cost so as you move towards more powerful modules we will move towards to access tracking it will become the norm at some point at that point you've got no additional cost in the tracking to do concentrated PV and it's only then the optics and how you do that which may be prepare a bollock mirrors so I think ultimately looking through we will move to concentrator PV it also gives another order of magnitude in the challenge of the stability issue but we believe that that's going to be surmountable it will require new materials chemistry new interface design and everything else but it should be surmountable so that could be the path for the next generation beyond multi Junction crosscut cells [Music] what's the current status of standard lifetime measurements for profs quite solar cells we can get extremely excited by efficiency and and the remarkable performance of perovskites the ease of which they can be processed but of course we're all aware that no solar technology is of any worth unless it can last in the field silicon PV modules today will last for 25 years and that that's proven and it's known they'll last for 25 years because they have lasted for 25 years so we're in a situation with these parofsky materials where they work very well but we need to ensure that they're gonna last for 25 years and we don't have hindsight to know the tests were doing to stress them are the right tests that will show up failures in the field so this is really at the core of what we should be doing in in research looking within the community so there is a slight discrepancy within the community between methodology for stressing the parofsky solar cells how to report stability what measurements really mean this stuff is quite stable what measurements don't and I have the pleasure of recently working with um Peter Hackett from NREL and writing a paper on stability for perovskite solar cells and I learned a lot in the process about what's been done in the past with existing thin film and silicon cells and there's a lot we can learn from the existing industry and also with my role in Oxford photovoltaics I'm exposed to that industrial side and industrial demand and they're really complying to industry norms is is an absolutely essential component but it's not all that is something that has to be surpassed and then we also need to go further and find if there are failures that exist that we don't know about so that's we're doing lots of more advanced tests comparing laboratory tests to real on Sun testing is going to be an essential component going forward over the next few years one thing that's presently lacking in the scientific community has been able to compare results of stability from one lab and stability from the other there's a whole host of parameters they include the spectrum of light that you're subject in the the temperature whether it's under load or open circuit the the atmosphere whether they're encapsulated how they're encapsulated and a whole range of other parameters the real critical thing is that these are reported so it's very transparent as to exactly how a solar cells been stressed so you can make a comparison but what we would be better is if standards emerge that people had here too and we use them as the benchmark for yes this cells more stable in that cell and you can understand between laboratories where the chemistry's improve things not weather just the light to light comparison on the day was better or worse than their control so this is something we look forward to seeing happening and they're certainly on the European perspective there's quite a lot of motivation between different European funded projects to try to come up with some suggestions of standard lab practices but they will close as closely as possible adhere to the internet International Electrotechnical Commission standards on top of the stability stress is actually just measuring a per off sky solar cell is different to a normal cell we have some issue of mobile ions our high onyx PCs in these materials so again there's effort to try to standardize the measurement technique the the key factor is there is that we have to either measure a stabilized till steady state power output so you measured a fixed voltage over time and take to take the current until it stops changing and we call that a steady state power output measurement or max power point track where you track around the voltage until you stabilize the current they're an essential component for quantifying efficiency in these solar cells [Music] [Music] what are the main factors affecting the manufacturing cost of perovskite modules can you talk about the current status of inkjet printing for profs Clack modules historically there's been a lot of activity on printed solar cells and this originated or lots of the work was done on organic solar cells commercially there's been a lot of work done on thin film six there was a company nanosolar that raised you know many hundreds of millions to try to commercialize a printable six technology Mir Saleh also that ended up being taken over or bought by Hannah gee is is selling a printable sinks technology and there's there's this dream if you like that printable flexible peevish ad in able more than rigid base pv one of the motivating factors is cost the presumption is that reel-to-reel printing will be much less expensive than standard semiconductor processing but there we really have to look at what's the cost of existing processing of silicon PV cells and in fact the cost of equipment and manufacturing has dropped by orders of magnitude over the last decade an order of magnitude over the last decade so now the capital infrastructure for a silicon line say a gigawatt line is only a small fraction of the overall cost if you cost it in cents per watt it might be two to four cents per watt comes from the capital infrastructure when you depreciate it over seven years so looking at that it's not obvious that you're going to get a cost advantage from reel-to-reel printing there are many more challenges as well one of them is that every process has to be coinciding in the line at the same rate all you need longer routes to go through so there are more challenges for the for the printing rather than batch processing that means it's gonna be a longer time till he gets to market but it doesn't mean that ultimately there isn't an opportunity and in fact I strongly believe there is an opportunity for reel-to-reel printing but there's lots of things we need to solve what one of the main opportunities is to have a laminator will foil for instance you can put in a building or you can imagine with the electrification of cars if you can have a laminate that gets laminated overall the car material that also produces very high power that could be a fantastic way of offsetting the requirement to charge every so often so there are applications as numerous applications for flexible lightweight solar cells but the key things we have to solve one is the solvent system we use most of the academic works still done with a mixture of DMF and DMSO and that DMF is not compatible with high-volume manufacturing we also need to control the viscosity and do more work on advanced imprinting techniques such as inkjet printing slot dye coating or spray coating these have to the these larger area coating techniques need to be developed most academic works done on spin coating that's not a methodology you'd use in manufacturing um one of the the bits of work we did a few years ago in this line is moving away from DMF to Osito nitrile now Osito nitriles not too perfect solvent you certainly wouldn't want to drink it but it is allowed in manufacturing and it's much better than DMF there we found that if we bubbled methylamine through a sita nitrile we could then use it to solvate the organic salts so we're now progressing this with this work and looking at other solvent systems that could be more compatible with printing but i think this is a a an approach that could lead to printable PV cells with very high quality of material another advantage if we think about deposition methodologies of course you can reel-to-reel via vapor in fact one of the leading possibly the leading organic PV companies is doing vapor deposited organic PV on reel-to-reel so that's also an option for reel-to-reel it also may be in batch processing we need to understand do we want to do vapor or solution processing so these these activities are still unsure if you look in the academic literature 95% of the work maybe 99% of the papers I haven't counted up or on solution process prostates 1% on vapor deposited that isn't because solution processes much better or certainly not fundamentally better than vapor deposited is simply there's more cap investment to get an evaporator and get it working than to buy a spin coater a whole group of researchers my groups 25 people we have three spin coders and they cost five thousand pounds each compared to an evaporator make us two hundred thousand pounds and that will service a two or three scientists so the rate and cost of getting into the field for evaporation is high but we will see more and more activity in this area more more groups are getting this capability and will see growth in this area of research over the next few years moving towards multi-junction cells I expect the vapor deposited cells to advance more quickly than the solution process [Music] what's the current status of commercial Purab squat modules as the research field advances there's more and more of the developments of being patent protected and this is also leading to the emergence of a number of companies trying to commercialize this so for instance there's a company called micro quanta in China that's trying to produce thin film for our sky modules as far as I'm aware on glass is a company in Poland called Sol technologies that strand do inkjet printed fur off skates on flexible substrates there's a there's a few companies in the US one called a hunt energy enterprises we don't really know what they're doing there quietly quite opaque um and there's another newly formed company called swift solar and they're working on flexible multi Junction rosco solar cells um the company closest to my heart is Oxford photovoltaics I'm co-founder and CSO of that company and they're we're focusing on the Perot skate on silicon technology and and we'd like to think of ourselves with Sony ah we certainly think of ourselves as the first mover in this technology and we're focusing on what we think is the most practical deployment strategy so I'll show you an example this um this looks to all intents and purposes like a silicon wafer but it's actually a Perot skate on silicon wafer so this is made in our in the Oxford PV facility in Brandenburg where we're scaling up to full size so this fortunately when talking about silicon cells is full size so the scaling challenge isn't that hard it's it's taken a lot of effort but we can now make very efficient cells of this six inch wafer wafer format and that's full scale so what we're doing now is trying to ramp up the volume towards pilot level manufacturing and then going beyond one of the key targets in entering the in essence the silicon PV industry is to match the stability of silicon there's no high D in a way and offering a short lifetime product for that in Oxford PV we've been laminating these cells and putting them into mini modules and stressing them through all the IEC environmental stress tests and they're showing very good progress not just to pass IEC but to go well beyond which is a requirement but the strategy of Oxford PV is also to license and work with joint development partners who are existing in the silicon manufacturing chain because that way the the existing value chain can be capitalized upon and instead of competing against the the silicon industry will actually enhance it so that's something for us kites could do they could literally be the next generation of silicon self is in the first instance ultimately we see many deployment strategies we see the flexible we see the thin film and especially the old Perot sky multi-junction coming through but my prediction would certainly be that the perovskite on silicon will be the first products in the market then beyond that we'll see the other technologies coming in [Music]

Channel: BULLAKI

Views: 44,556

Keywords: perovskite, solar cell, oxford university, oxford pv, renewable energy, clean energy, photovoltaics, silicon, science

Id: sbe9Z5oEs5o

Length: 33min 12sec (1992 seconds)

Published: Sun Nov 11 2018