http://pubs.acs.org/journal/aelccp
A Conversation with Henry Snaith
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HS: So, I’d like to profess that it was all predetermined and looked obvious that perovskites should be very good, but that would be an entire lie. It really was a serendipitous sort of series of events let us say. I had a Japanese−U.K. grant with Takurou Murakami from Toin University, who was a close colleague of Tom Miyasaka who had done the perovskite work. And I visited Japan and saw Tom Miyasaka. He gave a presentation of some of his perovskite work, and at the time, I was trying to use quantum dots, lead sulfide quantum dots, as sensitizers for dyesensitized solar cells. I had personally synthesized these and tried to stick them on the solar cells and they’re basically a long, tedious process. In comparison, Tom Miyasaka’s route with the perovskites was just to mix salts in a solution, deposit them on a substrate, and spin coat, and then there they were. They worked. So, in discussing with Takurou Murakami what we should do, we decided it would be a good idea to try to look at the perovskites as absorbers in what were then solid-state dyesensitized solar cells, swapping out a dye for a perovskite absorber. We did not imagine they would work so well. In fact, we were just hoping they would work and maybe a bit better than a few percent efficiency so we could do some nice investigations. On our first set of devices that we made back in Oxford, we had over 6% efficiency straightaway, which was already better than our best solid-state dye cells. That was really the start of it. EL: Thanks. And in this respect, I mean I think a breakthrough that you contributed to the field was your 2012 Science paper (note from EL, DOI: 10.1126/science.1228604, cited more than 3250 times) on aluminum oxide-supported perovskite solar cells, which clearly showed that one did not need a mesoporous TiO2 scaffold to realize high-efficiency solid-state devices. Can you tell us how you decided to investigate the behavior of perovskites on an insulating oxide scaffold? HS: So, at the time, we were investigating the perovskite in porous TiO2, the cells composed of perovskite-infiltrating porous TiO2, and what we noticed was that if we compared the dye-sensitized solar cells to a solar cell using perovskites, our charge extraction was much faster in the perovskite cells. We pulsed the solar cells with light, and we measured the time it took for the current to decay, and it decayed much faster in short-circuit conditions in perovskite solar cells, by about an order of magnitude. So, we postulated that we may be getting some contribution to charge transport occurring though the perovskite phase. So, in the first batch of devices we made with porous alumina, we had about 60 different solar cells, 58 were with porous TiO2 and we did 2 devices on porous alumina, simply to then take and study the electronic properties and the charge extraction. Luckily, Mike Lee, the student, also measured them under the solar simulator, and on our first batch of
rof. Henry Snaith (Figure 1) is the Group Leader of The Photovoltaic and Optoelectronic Device Group, Department of Physics at Oxford University. His research is at the forefront of perovskite solar cell research. Nature named him as one of the top 10 people making scientific breakthroughs in 2013. His current research interests include semiconductor physics, energy materials, and thin film photovoltaics. His keen interest in perovskite solar cells continues to encourage young researchers to advance renewable energy efforts across the globe. A Q&A with Prof. Snaith is summarized here.
Figure 1. Left panel: From left to right: Filippo De Angelis, Henry Snaith, and Annamaria Petrozza (2016 PSCO organizers) wearing the ACS Energy Letters conference t-shirt. (Reprinted with permission from the American Chemical Society, DOI: 10.1021/ acsenergylett.7b00217.) Right panel: Henry Snaith at the Perovskite Solar Cells and Optoelectronic conference (PSCO) held in Oxford in September 2017. (Photo courtesy of F. De Angelis).
ACS Energy Letters (EL): So, I am here with Henry Snaith in Oxford, and we take this opportunity to ask him a few questions about perovskite solar cells. Henry, how did you become interested in energy research prior to engaging in perovskite solar cell research? Henry Snaith (HS): I am a physicist, and after I finished my undergraduate degree, I was trying to decide what area of research I wanted to go into for a Ph.D., and I was interested in doing something that would have a positive impact on the planet and energy seemed to be one of the key areas that needed significant activity. My choice as a physicist was between nuclear fusion, wind, or photovoltaics. And photovoltaics seemed like the most sensible option, the most likely option to be our future power source. So, I wanted to put my efforts toward that goal. EL: OK, thank you. What motivated you to initiate the lead halide perovskite research at Oxford? Can you briefly describe the events leading to the initial success of achieving highefficiency solar cells? © XXXX American Chemical Society
Received: October 9, 2017 Accepted: October 9, 2017 2552
DOI: 10.1021/acsenergylett.7b00979 ACS Energy Lett. 2017, 2, 2552−2554
Energy Focus
Cite This: ACS Energy Lett. 2017, 2, 2552-2554
ACS Energy Letters
Energy Focus
suddenly degrade. As much as we are doing a lot, a lot of people are doing a lot of environment stressing and really trying to accelerate the aging to get a prediction of how long the modules will last and confidence in that, but really, seeing them in the field for many years, 5 years toward 10 years, will really be important for giving complete confidence in the technology. So, I really see a first product being perovskite on silicon because that is the most prevalent technology in the moment with the silicon cells. And then once that is proven, all the other possibilities will become very feasible. But it is also very competitive in the PV industry. We saw in the conference a very nice talk from Martin Green showing the rate of progress, and also Christoph Baliff pointed out where further cost reductions in crystalline silicon will take place. So, we’ve got to imagine crystalline silicon modules at 20% efficiency at 25 cents/watt. So, for a new technology to set up its first manufacturing plant, which will be inherently small volume compared to where silicon is, and already come off the starting blocks competing at that sort of levels is going to be very, very hard. So this takes more scaling, and I think perovskites need to prove themselves first, and then it will be worth putting in the effort and the resources into scaling the thin film all-perovskite technology. EL: Thank you. So, as a final question, what advice can you give to young guys, young researchers who want to pursue a career in the energy field, in the energy research field? HS: My advice would be to do it if you are making a choice between trying to pursue a research career versus maybe a financial career. I’d say you need to be doing something you are passionate about and really enjoy. If you have an opportunity to do that, you should take it. In terms of advice, in terms of undertaking science, from my personal experience, most of the breakthroughs we’ve made with perovskites have been through being observant of results and not through what we preconceived. So, usually when we come up with a way we think we are going to make the solar cell better, it does not work. In fact, it is more often than not the control experiments that you really think should not work that actually turn out the very interesting results. So, my advice in terms of undertaking research would be to first be very observant and do not just ignore the weird results. They are usually the most interesting that lead to the best improvement. And to also, in terms of what you vary, vary things broadly because it is often at the edge of your distribution of parameters that you actually see the interesting things. EL: Thanks, Henry. It has been a great time to talk to you. HS: Thank you.
devices we had a 10.8% efficient cell. One was 9.4, and one was 10.8. To put it into context, we had been working for about 5 years previously, squeezing the efficiency of the solid-state dye cell up from 5 to 6%, and then all of a sudden, we reached, what was at the time our end goal, our key target of getting to 10% efficiency, with the solid-state hybrid cell just like that. That was quite a surprise and a shock. EL: Thanks, thanks very much. So, moving to more recent research, you recently published a Perspective article (DOI: 10.1021/acsenergylett.6b00499) in ACS Energy Letters on the route toward lead-free perovskite solar cells, and this is among the most read articles in our journal, showing an immense interest in non-lead-based photovoltaics. What are the hurdles, in your opinion, in developing such new types of perovskites for energy conversion? HS: So, there are many, many hurdles. In essence, the relatively easy thing is to actually be able to synthesize new compounds or make new compounds and theorize new compounds, and we have been working quite closely with Feliciano Giustino from Oxford University on this. He is the coauthor on this review article. There is a lot of work, and there is a lot of work on trying to predict new materials. It is possible to make the materials, but getting them to work in solar cells is very, very challenging, and this could be for a number of reasons. It may be that this relative ease with which you get low levels of nonradiative recombination in the metal halide, lead halide perovskites is more unique than we first hoped. So, if we are looking for other materials, there are obviously other semiconductors that work very well as PV materials, CdTe, CIGS for instance. These are known materials, and if you just made powders of them and tried to cast them into films, you would get absolutely useless solar cells. You have to learn a root to passivate defects, treat surface states, and understand what defects responsible for charge recombination do exist in materials. So, one of the big challenges is we may find materials that could be found (to be) fantastic solar cell materials; they may not be exactly like the lead halide perovskites, but we need to somehow make a judgment of which materials we put a year, 2 years, 5 years of effort into trying to engineer to work and which do we put into the bin because they do not work straightway and then move onto another compound. So, it is very difficult. We do not have a simple set of measurements we can draw on a material to predict how its functionality will eventually be. EL: Alright. So, we heard at the conference there is a lot of impulse toward putting the perovskite solar cells on the market. So, do you see a competition between perovskites and other thin film photovoltaics, and will perovskite solar cells make their own way to the market as stand-alone or tandem devices? What’s your view on this? HS: My view is perovskite is a new technology that is completely unproven. So, the most assured way to actually make a product saleable is for it to deliver something that no other technology can deliver and at the appropriate cost. So there, maximizing the efficiency is absolutely paramount, and the approach of putting perovskite on silicon and making a tandem cell and, therefore, making silicon better is also very attractive because it can be sold as an enhanced or nextgeneration silicon product rather than a completely new perovskite technology. So I think the key is to get the technology proven in the market as soon as possible and then get it proven in the field so there will be long-term, many years of measurements on real modules showing that they do not
Filippo De Angelis, Senior Editor
Istituto di Scienze e Tecnologie Molecolari del CNR, 06123 Perugia, Italy
Prashant V. Kamat, Editor-in-Chief
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University of Notre Dame, Notre Dame, Indiana 46556, United States
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.7b00979. Audio of the conversation between Henry Snaith and Filippo De Angelis (ZIP) 2553
DOI: 10.1021/acsenergylett.7b00979 ACS Energy Lett. 2017, 2, 2552−2554
ACS Energy Letters
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Energy Focus
AUTHOR INFORMATION
ORCID
Filippo De Angelis: 0000-0003-3833-1975 Prashant V. Kamat: 0000-0002-2465-6819 Notes
Views expressed in this Energy Focus are those of the authors and not necessarily the views of the ACS. The authors declare no competing financial interest.
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DOI: 10.1021/acsenergylett.7b00979 ACS Energy Lett. 2017, 2, 2552−2554
