http://pubs.acs.org/journal/aelccp
Perovskite Stories from Around the World
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ACS Energy Lett. Downloaded from pubs.acs.org by 95.85.71.234 on 03/21/19. For personal use only.
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he surge of interest in perovskite photovoltaics (PVs) in recent years has led to multifaceted research opportunities. Although metal halide perovskites were first discovered more than a century ago, their electronic and light-emitting properties became known only in the 1990s, notably through the work of researchers at IBM T. J. Watson Research Center. It was not until 10 years ago that the first paper on photoelectrochemical investigation of methylammonium lead halide was published. The solid-state PV devices reported in 2012 drew the attention of researchers who were active in the area of dye-sensitized solar cells, organic solar cells, and quantum dot solar cells. This led to the first wave of perovskite photovoltaic research, which mainly focused on boosting the efficiency of solar cells and addressing the issues related to reproducibility and stability. This upsurge was quickly followed by a second wave consisting of researchers who explored the synthesis of new perovskite materials, excited state dynamics, theoretical understanding of the mobility of charge carriers, and defect-driven processes. Researchers around the world are now riding the third wave of perovskites. ACS Energy Letters asked a few of our authors to share their motivations for pursuing perovskite research. Their quotes (shown alphabetically, below) show their exciting and interesting experiences during initial days and how they led to success.
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Matt Beard, National Renewable Energy Laboratory, Golden, Colorado, USA; matt.beard@ nrel.gov; @mbthz
Matt Beard and Ye Yang at NREL (photo credit: Dennis Schroeder)
We were challenged to investigate the optical properties and spectroscopy of perovskite films and crystals by Joey Luther and Kai Zhu. At first, we were skeptical that we could add something useful but decided to study hot-carrier effects. Early on in our investigation, Art Nozik (hot-carrier extraordinaire) walked by a computer where one of our carrier-cooling plots was displayed and exclaimed, “that’s the best behaved hot-carrier absorber I’ve seen”. That is when we realized that maybe there was something to all of the hype about perovskite’s fantastic properties.
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Osman Bakr, King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Saudi Arabia;
[email protected] Juan Bisquert, Universitat Jaume I, Institute of Advanced Materials, Castello, Spain; bisquert@ uji.es
Nam-Gyu Park, Hui-Seon Kim, and Juan Bisquert in Korea, 2013
The first person who spoke to me about the power of hybrid perovskites for photovoltaic applications was Tom Miyasaka. We were duly waiting for our planes in the airport in 2007 after he had just presented the new perovskite-sensitized “quantum dot” solar cell in a conference in Saint Gallen. I was not very impressed but should have paid more attention. The next time I heard about it was in 2012 at some Conference in Asia, and it was Seigo Ito who mentioned something about a new solar cell
One of the most striking aspects of working with ABX3 hybrid perovskites is their surprisingly intuitive chemistry, which has made working with them exciting and satisfying. Many of our students’ discoveries on crystallization and compositional tuning of these materials were a result of “Friday night”-type experiments. However, the field has reached a stage where a new kind of intuition is required for the metal (B) site one different from those developed in the early days for the organic cation (A) and halide (X) sitesin order to achieve breakthrough progress with lead-free perovskite compositions. © XXXX American Chemical Society
Received: March 7, 2019 Accepted: March 7, 2019
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tried in late September, 2012, Ray made a 3.2% TiO2/ CH3NH3PbI3/CuSCN/Au device, nearly besting my personal record Sb2S3 device on his first try! When we measured the devices for 10 min at short-circuit, we saw an approximately 50% decrease in photocurrent and a visible bleaching of the perovskite layer. This led us to switch from CuSCN to CuI, which initially had lower efficiencies of only about 2% but were much more stable. We struggled greatly just reproducing our own results because we did not understand the effects of atmospheric humidity at all in these early days. After working a few more months on the project, I had made a 6% efficient TiO2/CH3NH3PbI3/CuI/Au, device and we had the first perovskite paper with all inorganic contacts; it was still measured over 4% efficient after discovering it in the back of my drawer 1 year later!
involving Henry Snaith. He was clearly communicating a sense of importance and urgency. I remember well Seigo’s glance and emphasis, like a calmed auctioneer, suggesting the big value of the product with a contained smile, and everyone around taking about it very seriously, making metal notes, and requesting more information. Soon Nam-Gyu Park emerged as one of the leaders of the new field, and his student Hui-Seon Kim came in late 2012 to Castelló for some time to analyze operation of the device. I remember Michael Grätzel presenting 14% efficiency at HOPV Conference in Seville in May 2013. Clearly a race was starting and a lot of surprises lay ahead. Now in 2019, we still expect a few more.
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David Cahen, Weizmann Institute of Science and Bar-Ilan University, Israel;
[email protected] ■
Songyuan Dai, North China Electric Power University, Beijing 102206, P. R. China; sydai@ ncepu.edu.cn
Rump Session for Organo-Metal Halide Perovskite-Based Solar Cells. 2013 MRS Fall Meeting, Boston, ©Materials Research Society (Reprinted with permission)
It is been called a formative experience. While, as originator of the idea, I’d love this to be so, even if it was not, it likely had some effect. During a meeting in Japan I learned about the then fresh off the press Lee et al. 2012 Science paper. Gary Hodes and I worked at that time to find not GaAs but GaInP on the cheap; the high voltage efficiency dazzled me, and upon return, we forewent good old (Cd,Zn)(S,Se,Te) for MAPbBr3. During the next year, I tried, not always successfully, to infect colleagues with “perovskitis”, arguing this was a potential game changer. At the same time, the MRS meetings were frustrating as they missed the change that I felt in the air. As MRS board member, I searched during an August 2013 board meeting to do something about it for the upcoming Fall meeting and called Dave Ginley, former MRS president. I do not remember all of the reasons why nothing could be done, but in the end, we convinced MRS, and the rest is history.
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Perovskite solar cells appeared as a hot topic at the 7th Aseanian Conference on Nanohybrid Solar Cells 2012 in Taiwan. Prof. Nam-Gyu Park, Prof. Henry Snaith, and Prof. Tom Miyasaka reported their exciting result of hybrid metal halide perovskites, which revealed a new era of solar cell research. I believed that all of the participates were convinced that PSCs would be the next breakthrough in solar cells. What happened afterward fully supported this optimistic expectation. Inspired by the pioneering works, my group published some subsequent papers on doping modification and hole transporting materials for PSCs soon after. Our current research are aimed at developing two-dimensional perovskites with high power conversion efficiency and moisture resistance. Through a combination of theoretical and experimental approaches, one of our recent works found that the properties of perovskite materials can be modified by adding different ammonium salts into FAPbI3. I have faith that PSCs will pave the way for cheap, stable, and highly efficient solar cell devices.
Jeff Christians, Hope College, Holland MI, USA;
[email protected]; @jac997
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I remember reading the landmark Scientific Reports paper soon after it appeared online in 2012, and almost immediately Prashant Kamat, my graduate advisor, and I had Raymond Fung, an undergraduate researcher working with us at the time, start working on perovskites. At the time I was making TiO2/Sb2S3/CuSCN/Au solar cells; we did not have spiro-OMeTAD in the lab so we took the same process/structure I was using for Sb2S3 solar cells and plugged in CH3NH3PbI3. In what I believe was the first batch of devices he
Filippo De Angelis, University of Perugia, and CNRISTM, Perugia, Italy; fi
[email protected] Filippo De Angelis (left) and Edoardo Mosconi (right) at the Computational Laboratory for Hybrid/ Organic Photovoltaics in Perugia
I first heard about lead-halide perovskites from Henry Snaith at a European project meeting in June 2012 (I must admit I had 880
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ACS Energy Letters missed Miyasaka’s 2009 paper), a few weeks after Kanatzidis published the paper on CsSnI3-based solid-state DSCs. Henry first showed us MAPbI3/TiO2 solar cells with ∼6% efficiency, which I considered with some curiosity. That was, however, just the preamble to showing us a ∼10% efficient MAPbI3/Al2O3 solar cell. This result left the small audience astonished, testifying the semiconducting nature of lead-halide perovskites and paving the way to a new type of device. Later that day, I called my wife (Simona Fantacci, a colleague also working in modeling DSCs), telling her that I had witnessed a scientific breakthrough. I started working on perovskites at CNR-ISTM, Perugia during the summer of 2012 with co-workers Anna Amat and Edoardo Mosconi. Meanwhile, the papers by Park and Grätzel and by Snaith and Miyasaka were published. We submitted our first paper, together with Nazeeruddin and Grätzel, to J. Phys. Chem. Lett., but the paper was transferred to J. Phys. Chem. C for lack of urgency. Our work, published in April 2013, is the first modeling study of mixed-halide lead perovskites, and it now has more than 600 citations.
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QD solar cells were presented (that time the thin film perovskite was named perovskite QDs) that delivered an efficiency of around 6%. Those days, this efficiency was considered to be high for QDs solar cells. This was the first time I heard about the metal halide perovskite and its ability to function as a light harvester. Immediately I had the idea to use it as a thin film in a solar cell with the same architecture that we used for QD-based solar cells, meaning that the metal contact is directly touching the perovskite film, what we called a hole conductor f ree perovskite solar cell. This work resulted in one of the first publication in this field (J. Am. Chem. Soc. 2012, 134 (42), 17396−17399). For a PV-QDs researcher, this was an exciting moment because until then we had struggled to establish high-performance QD solar cells. Following that, I established my own laboratory at the Hebrew University concentrating on perovskite thin films and nanostructures from a device and fundamental point of view.
Eric Wei-Guang Diau, National Chiao Tung University, Hsinchu 30010, Taiwan; diau@mail. nctu.edu.tw
■ In October of 2012, I attended an ECS conference held in Honolulu, Hawaii (PRiME 2012). During that meeting, I met an old friend, Prof. Nam Gyu Park, who gave an impressive presentation for all-solid-state perovskite solar cells (PSCs) attaining efficiency approaching 10%. I had a chance to discuss with Prof. Park and was aware of another team reporting 10.9% using mesoporous Al2O3 instead of TiO2 in an n-type (regular) device structure. After that trip, PSCs became our major research topics. It was quite difficult to make a good cell with high efficiency due to poor film formation, until we realized the two methods, antisolvent and solvent annealing. However, it was like a tsunami striking the field, and researchers working on either dye-sensitized solar cells (DSSCs) or organic photovoltaics (OPVs) jumped into this battle to chase the efficiency. Until today, two issues are still challenging for PSCs, stability and lead-free. We therefore have focused on the development of stable lead-free PSCs since 2016. In 2018, we finally worked out a recipe for a tin-based lead-free PSC to attain efficiency of 9.6%, which is close to what Park and Grätzel reported for a MAPbI3 cell 6 years ago.
Laura Herz, University of Oxford, Oxford, UK;
[email protected] When my group first investigated MAPbI3 in 2014, we were struck by just how much the solution-processed material seemed to resemble classical inorganic semiconductors. Transient conductivity measurements showed high charge-carrier mobility, and its ratio with the bimolecular recombination rate constant fell short of the value expected from simple Langevin Theory by 5 orders of magnitude! This fortunate asset underpins the long (>microns) diffusion lengths observed in these materials and permits the thin film (planar heterostructure) architecture commonly used for perovskite photovoltaic cells today. We now know that charge recombination in these materials is best understood in terms of a classical semiconductor band structure picture, similar to GaAs, explaining the deviations from Langevin Theory. The big surprise find in many ways has been the discovery of a group of semiconductors whose defect chemistry is so benign that near-intrinsic behavior can be observed even when simple, low-energy processing routes are employed. On a lighter note, I would like to share with you a crosssectional image (well, photograph) of a “metal halide perovskite
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Lioz Etgar, The Hebrew University of Jerusalem, Israel;
[email protected]; https://lioz.etgar. huji.ac.il During my last month at EPFL, on August 2012, when I conducted my experiments on quantum dot (QD)-based solar cells, I came across the Nanoscale paper published in 2011, where perovskite 881
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Material Research Society Spring Conference when Henry Snaith presented the only one topic in this conference on perovskite solar cells. We started the research on perovskites at the end of 2013 because a postdoc Qingfeng Dong was desperate with his project on organic solar cells, and fortunately, he could synthesize methylammonium iodide, a precursor that could not be purchased anywhere at that time. In about 2 weeks, we made p−i−n planar structure perovskite solar cells with efficiency of 15%. We were thrilled and knew nothing could prevent us to be crazy about this material. One can easily get bored by watching the efficiency chart every day. Nevertheless, this material kept giving us surprises almost every day, including many new applications, such as X-ray detectors, which may be the first real application, and many new sciences. These materials are a library, and you can find almost everything in the textbook on condensed matter physics, but they are beyond any textbooks.
solar cell” fabricated at my group’s Christmas party last year. The architecture: FTO (plastic ruler), C60 (blue icing), MAPbI3 (ginger bread), spiro-OMeTAD (yellow icing), aluminum foil (silver). It is not known if it worked, but I have my doubts.
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Anita Ho-Baillie, University of New South Wales, Australia:
[email protected] ■
Prashant Kamat, University of Notre Dame, Indiana, USA;
[email protected]; @KamatlabND
12% 16m2 monolithic perovskite cell certified in 2016. The cell is still working!
I first came across metal halide perovskites when Prof. Martin Green at the School of Photovoltaics at UNSW mentioned this material and asked me if I would be interested in combining it with silicon for tandem solar cells. I then started reading every paper I could find on perovskite solar cells (only 83 papers in 2013), but I soon found out I couldn't keep up (>200 in early 2014!). Perovskites are versatile, and a relatively easy material to work with. Who would have thought that a simple spin coating and a relatively low temperature anneal would deliver cells with efficiency > 10%! The surprises we get in terms of material properties and device performance continue to amaze me. It has created a vibrant research community and has captured the imaginations of many Ph.D. students around the world, many of whom I am very proud of, including those who work day and night trying to break our own records. I still remember the stress involved in getting the cells certified knowing perovskites do require a lot of care given their unpredictable stability! I look forward to seeing where perovskites will be going in the future. The early history of the development of this technology was published in ACS Energy Lett. (https:// pubs.acs.org/doi/10.1021/acsenergylett.7b00137).
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C&EN issue on February 24, 2014 highlighting the first story on “Tapping Solar Power with Perovskites”, featuring Joseph Manser, a graduate student from our laboratory on the cover
During the Fall MRS meeting (2014) I was leading a panel discussion in the Symposium on Perovskite-Based and Related Novel Material Solar Cells. The room was packed with nearly 200 participants. A question came from a young researcher: “Everyone claims high efficiency of their solar cells. Could this community suggest a protocol to produce solar cells in a reproducible way and develop procedures to measure solar cell efficiency correctly?” At that time, everyone was eager to claim high efficiency using fast J−V scans, charging, etc., and there was some confusion on the validity of these efficiency claims. Critical questions from young researchers, such as this, helped us share the different groups’ procedures and shape the field in an unprecedented way. This discussion at the MRS meeting was summarized by the organizers in the J. Phys. Chem. Lett. article “Perovskite Solar Cells: Do We Know What We Do Not Know?” (https://pubs.acs.org/doi/pdfplus/10.1021/jz502726b), providing a collection of early developments in the field.
Jinsong Huang, University of North Carolina at Chapel Hill, North Carolina, USA;
[email protected] ■
Mercouri Kanatzidis, Northwestern University, Illinois, USA;
[email protected] We started on halide perovskites in 2008. Our goal was to achieve reproducible strong photoluminescence for the 960 nm emission line of CsSnI3 and MASnI3. It turned out to be much more complicated than we had expected and full of surprises, but ultimately we succeeded. This problem pulled us deep into the throes of perovskite science. We synthesized all possible 3D
I first came to perovskite solar cells when I organized a symposium on Organic and Hybrid Solar Cells at the 2013 882
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perovskites with tin and lead with Cs, methylammonium, and formamidinium. We figured out key synthesis procedures and solved all of the crystal structures (more than 20 3D perovskites all described in our 2013 Inorg. Chem. paper). By 2011 we learned enough about perovskite chemistry to use CsSnI3/SnF2 as a thin layer of hole transport layer in a dye-sensitized solar cell in collaboration with my colleague R. P. H (Bob) Chang. Between the last 6 months of 2011 and first couple of 2012, we increased cell efficiency from 1 to 10%. This was the first perovskite solar cell ever to be reported that employed a solid continuous layer of perovskite, and this layer functioned not only as a hole transporter but, to our surprise, also as a light absorber. Our Nature paper appeared in May of 2012. We had seen the Miyasaka JACS paper in 2009 but somehow ignored it. We did not think anything of it because it used discrete and disconnect nanocrystals of MAPbI3 attached to mesoTiO2 and the solar cell had a liquid phase. It looked a lot like a classical Ru(bipy)3-style cell but was unstable because the MAPbI3 nanocrystals dissolved quickly into the solution. In late 2012 going into 2013, we had observed X-ray and γ-ray detection from CsPbBr3. Perhaps, in 2022 there will be another 10 year anniversary to again reminisce about the early days.
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Michael McGehee, University of Colorado, USA;
[email protected] In 2011 my research group was trying hard to find defecttolerant high-bandgap solar cells that could be cost-effectively deposited on top of silicon to make high-performance tandems. I will never forget our excitement when Michael Grätzel presented his early results on perovskite solar cells at a group meeting for the Center for Advanced Molecular Photovoltaics. We quickly knew that perovskites had the properties we needed. The next event that really stands out in my memory was Henry Snaith’s truly extraordinary lecture at the European Materials Research Society meeting. The organizers somehow knew that Henry was going to show something special and let him speak for 45 min. He showed that the titania scaffold was not necessary and that perovskites should be thought of as semiconductors with exceptional defect tolerance, not just titania sensitizers. He explained that the perovskites could be thermally evaporated, pretended to do a drum roll, and then told us that the evaporated perovskite cells had 15% power conversion efficiency. I immediately told several members of my group that they needed to completely stop working on their old projects and start doing everything it would take to make tandems with perovskites. I am glad I did. Thanks to those who discovered these materials.
Hemamala Karunadasa, Stanford University, California, USA;
[email protected] ■
I first encountered halide perovskites in 2005 when I read David Mitzi’s papers as a second-year graduate student. I was looking for a topic for a research proposal, which was required for the Ph.D. qualifying exam at UC Berkeley. As a graduate student in Jeff Long’s lab I was synthesizing molecules in solution. But as an undergraduate, I studied oxide perovskites with Robert Cava. So, I loved the fact that lattices, similar to those that are forged in a furnace at 1000°, could form in solution using the methods more typically employed by molecular chemists. I remember thinking then that if I ever were lucky enough to have my own research lab I would study halide perovskites. Of course, many of my initial ideas evolved as my group learned more about these materials. One of our few original goals that was realized without much change was the demonstration in 2014 that 2D perovskites can serve as solar absorbers.
David Mitzi, Duke University, North Carolina, USA;
[email protected] IBM team circa 1999 that focused on hybrid perovskite devices: Mike Prikas, Konstantinos Chondroudis, Cherie Kagan, and myself (left to right in photo). The tool behind us is the home-built single-source thermal ablation tool that we used for the perovskites. (Photo courtesy of David Mitzi)
I recall my excitement on first learning about organic−inorganic perovskites as I was completing my Ph.D. in 1990, which focused on copper-oxide-based superconductors. I was particularly intrigued by the analogy of the copper oxide perovskites to modulation-doped semiconductorsi.e., with the Cu−O sheets serving as the active superconducting layers, separated by modulation layers (e.g., Bi−O in Bi2Sr2CaCu2O8+δ) that facilitate tailoring electronic character while minimizing disorder within the active 883
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ACS Energy Letters layer. My interest in the halide-based perovskites related to the question of whether an organic layer (with essentially unlimited chemical flexibility) might replace the inorganic modulation layers in perovskite superconductors and related electronic systems. The exciting work that I came across on 2-D magnetism in the perovskite copper halides by researchers such as Roger Willett and co-workers hinted that this might be possible, and I wondered whether these halide systems might become semiconducting, metallic, or perhaps even superconducting. After spending several frustrating years unsuccessfully trying to dope/ tailor copper halide perovskites, I switched to the Sn-based systems, leading to our first paper in this area (Nature 1994, 369, 467−469), describing a family of tunable hybrid semiconductors. Subsequently, in the late 1990s, our interest migrated to using these systems in organic−inorganic electronic devices, notably LEDs and transistors (e.g., IBM J. Res. Dev. 2001, 45, 29−45; see the photo of my team at the time). One project goal involved designing systems with unique/useful interaction among hybrid components, still an area of contemporary interest for photovoltaic and other prospective application.
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transporting materials. However, we were disappointed with nonreproducible efficiencies. Our striking finding was the nonstoichiometric ratio of PbI2 to methylammonium iodide (an excess of PbI2) that significantly improves perovskite crystal size, efficiency, and reproducibility and modifies the interface between the perovskite and electron transporting layer. To further enhance the stability of perovskite solar cells, we developed layerby-layer growth of 3-dimensional and 2-dimensional perovskites yielding over 22.5% certified efficiency.
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Nitin P. Padture, Brown University, Rhode Island, USA;
[email protected] Iván Mora-Seró, University Jaume I, Castelló de la Plana, Spain;
[email protected]; @IvanMoraSero
Prof. Nitin Padture (left) with Dr. Yuanyuan “Alvin” Zhou
I had just moved to Brown University in early 2012 and wanted to branch out in a new area of materials research, having spent most of my career researching advanced structural ceramics and functional nanomaterials. So, I wrote a proposal to the National Science Foundation on some new ideas in the materials science of halide perovskites for solar cells, without having any background in halide perovskites or solar cells! Astonishingly, the proposal was funded, just as the perovskite solar cells (PSCs) field was taking off. I was fortunate to hire the talented Yuanyuan “Alvin” Zhou as a fresh Ph.D. student, who is now Asst. Prof. (Res.) at Brown, to work on that project in 2012, and it has continued to be an amazing ride since then. We, as bona fide materials scientists, could relate to halideperovskite synthesis/processing, microstructures, grain boundaries, and related phenomena and are proud to bring new thinking to the field from a different angle. The solar cells community has been most welcoming toward me as a total newcomer to the field, and it is appreciating the important contributions from my research group and collaborators, for which I am very grateful. Still, much remains to be done as we keep our eyes on the prizewidespread commercialization of PSCs.
The first time I heard about perovskite was with the seminal work of Prof. Miyasaka in 2009, and I also followed the paper from Prof. Park in 2011 as at that time I was working on quantum dot sensitized solar cells. However, to be honest, I did not give them too much attention because they presented even more stability problems than the standard quantum dots used in sensitized solar cells at that time. After the publication of the allsolid perovskite cells in 2012, I started to work with perovskites quite soon, at the end of that year, with the visit of Hui Seon Kim, first author of the paper on this issue from Prof. Park and Prof. Grätzel, to our laboratory to characterize these cells with impedance. A priori, I believed it would be routine work due to my experience with quantum dot sensitized devices but soon began to bring surprises. Where was the chemical capacitance, the key parameter in sensitized cells? What was the capacitance observed at low frequencies? These were just my first surprises of the many that this field has given me.
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Nam-Gyu Park, Sungkyunkwan University, Suwon 16419, Korea;
[email protected] When I attended NanoEuro 2007, held in St-Gallen, Switzerland, I listened to Prof. Miyasaka’s talk on perovskite-sensitized solar cells with power conversion efficiency (PCE) as low as around 2%. Such a low efficiency could not have enough power to attract attention from the audience. However, I was very interested in his work because the terminology “perovskite” was familiar to me thanks to my research background on oxide perovskites during M.S. and Ph.D. studies at Seoul National University.
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Mohammad K. Nazeeruddin, Swiss Federal Institute of Technology (EPFL), Lausanne, Switzerland; mdkhaja.nazeeruddin@epfl.ch Coming from a dye-sensitized solar cells background, seeing over 1 V open-circuit potential in perovskite solar cells was striking, and we started exploring compositional engineering of perovskite and design and development of electron and hole 884
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So, I started work on perovskite solar cells after coming back from Switzerland. In 2009, I joined Sungkyunkwan University (SKKU), and there, I achieved PCEs up to 6.5% by discovering the importance of the precursor solution concentration, which was published in Nanoscale in 2011. We then considered solid hole conductors in place of liquid electrolyte because of the instability of the perovskite in polar liquid. While we were searching for an effective methodology, we eventually found a method that a thinner TiO2 film would be better because the light penetration depth is inversely proportional to the absorption coefficient (the absorption coefficient of MAPbI3 was 1 order of magnitude higher than that of the Ru-based sensitizer, as reported in Nanoscale in 2011). So, we eventually achieved solid-state perovskite solar cells demonstrating a PCE of 9.7% by decreasing the film thickness to 0.6 μm and 500 h stability without encapsulation, which was published in Scientific Reports on August 21, 2012.
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research group slowly engaged in working on these nanocrystals. The stunning moment was the use of preformed oleylammonium ions in the reaction flask, and its concentration was observed to tune the intensity of Mn d−d emission in CsPbCl3 nanocrystals (Angew. Chem., Int. Ed. 2017, 56, 8746−8750). When we applied the same trick for CsPbBr3, the size of the cubes was tuned (ACS Energy Lett. 2018, 3, 329−334). Further, this could also help stabilize the α-CsPbI3 phase, even obtaining a near-unity absolute PLQY for all CsPbX3 perovskites, colloidally prepare doped layered perovskites, and, importantly, support thermal annealing where these nanocrystals sustain even more than 5 h of annealing at 250 °C without any phase transformation (J. Phys. Chem. Lett. 2018, 9, 6599−6604). The beauty of this salt is supplying adequate halides at high temperature during the formation of perovskites and at the same time helping to passivate the nanocrystals.
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Annamaria Petrozza, Italian Institute of Technology Center for Nano Science and Technology;
[email protected] Andrey Rogach, City University of Hong Kong, Hong Kong;
[email protected] Andrey Rogach (left) and Peter Reiss (right)
My personal encounter with perovskite photovoltaics has been rather short and goes back to January 2013, when my colleague Peter Reiss (CEA Grenoble) and myself submitted a project proposal on “Perovskite absorbers in nanostructured solar cells” for a joint French/Hong Kong funding. We received the reviewer comments telling us, among other quite positive points, that “...perovskite absorbers may have a challenge as to charge transfer due to their generally insulating naturecharge extraction from the absorber may be a problem...”, and “...very high purity of perovskite absorbers will be required”, as well as “...the proposed technology is already a bit late to make an impact on the market”. Due to those anticipated issues listed by the reviewers, the project has not been recommended for funding by the agency, thus preventing my personal involvement in perovskite photovoltaics, so far. I came back to the colloidal perovskite quantum dots in 2015, when we published our first paper on their exceptionally strong emission in Adv. Sci., which occurs last but not least due to the defect-tolerant nature of perovskites. This paper eventually became the best-cited research article of that journal.
On the first of November 2012, I received an email from Henry Snaith, the subject was “A magnificent system to look at...”. I must say that the following years were intense. The excitement of study something completely new (at least for me as I was coming from the world of organic semiconductors) was often coming together with that feeling of being a rabbit caught in the headlights. This field has given me the chance to learn old and new science. Looking back, I see that the community built good foundation, and I am sure more fun is to come!
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Narayan Pradhan, Indian Association for the Cultivation of Science, Kolkata, 700032 INDIA;
[email protected]; @npiacs While my students were keen to work on metal halide perovskites in early 2016, I was personally reluctant. However, when Mn-doped CsPbCl3 appeared, we were quite excited and started to make these nanocrystals. Within a couple of months, we were addicted with the beautiful science, and my entire 885
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of ferroelectricity, diversity of reported excitonic binding energies (spanning more than an order of magnitude of values), and puzzling changes in the photoluminescence under different environmental conditions are just a few of the issues that continue to tease us, demanding a better understanding.
Michael Saliba, Ecole Polytechnique Federale de Lausanne, Fribourg, Switzerland; miliba@gmail. com; @miliba01; @salibalab
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During my Ph.D. at Oxford University, I often managed to take some short notes of our weekly, every-Friday group meetings. In hindsight, one truly special meeting was on January 28, 2011, where perovskites were proposed as replacements for dyes in solidstate dye-sensitized solar cells. The idea was that the NH4+ group of the “perovskite dye” would attach itself to the oxygen of the TiO2 compact layer following the now famous JACS paper by Miyasaka et al. from 2009. One budding discussion was how this new dye would then stick to the mesoporous titania. And the rest is history. Little did I know at the time that this presentation would have such a dramatic impact. Many years later, hundreds of groups, including mine, work with great excitement on perovskite materials toward a sustainable energy future. What an unexpected development showing the beauty of science where a routine group meeting becomes the starting point for a new research direction.
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Henry Snaith, University of Oxford, Oxford, UK;
[email protected] Absent of any form of exaggeration, perovskites have been a lifechanging experience. The research environment post-2012, bears no resemblance to that before. The pace of research, the pace of scientific and technological advancement with these materials is like science on steroids (with the advantage that these steroids are legal). My first “exposure” to perovskites was in 2009, when I saw Miyasaka give a presentation of his work with Kojima in a small seminar held when I visited Toin University; at the time the Toin group was touting Kojima as “the JACS guy”. I thought perovskites looked intriguing. The first time I knew perovskites may really be something was in 2010, when Mike Lee made his first solid-state “perovskite-sensitized” solar cells; on the first shot, they beat our all-time lab record for solid-state dye-sensitized solar cells. My first “mind-bending” experience was in late 2011, when via a curiosity-driven experiment we investigated cells with the mesoporous TiO2 substituted for mesoporous Al2O3; Mike Lee sent me an e-mail at 11:30 pm with a J−V curve attached (10.8% efficiency), with the single message, “guess what the innovation was?”; I did not sleep much that night. My mind was finally broken when we discovered that a solid-perovskite absorber layer worked even better than a film infiltrating a mesoporous scaffold; this is the definition of a paradigm shift. At this point, I decided that what we perceive to be reality is simply transitory and best to stay agile and work hard while it is on the move. The rest, as they say, is history.
D. D. Sarma, Indian Institute of Science, Bengaluru 560012, India;
[email protected]; https://www. facebook.com/pages/D-D-Sarmas-Group/ 567099976731144
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Ashutosh Mohanty, D. D. Sarma, Sharada Govinda, and Bhushan Kore (left to right)
It is difficult to pinpoint the time when we first became aware of the hybrid halide perovskites, but when we began to be interested in it, it was the most exciting time with everyday report of increasing efficiency, conflicting reports of the mechanisms involved, the sceptics’ voices becoming louder with accusing fingers pointing to its (in)stability, and growing confusion of its basic properties. Stories of MRS meetings being totally dominated by intense and heated discussion on this topic paralleled the massive response of the scientific community following the discovery of high Tc superconductivity in the late 1980s and in the field of graphene in later years; this made it clear that you can love it or you can hate it, but there is no way you can ignore this subject. I think that we made the early choice of staying away from the efforts of enhancing its useful properties, such as the efficiency and stability, but devoted ourselves to attempting to understand its basic properties, which seemed to be confusing with many directly conflicting claims in the literature. It appears that many of these issues continue to remain controversial despite intense efforts of many groups over several years. The mysterious presence/absence
Sam Stranks, University of Cambridge, Cambridge, UK;
[email protected]; @samstranks; @strankslab; www.strankslab.com
Snaith Lab circa end 2013
I fondly recall the buzz about the Snaith Lab when some of the first discoveries were being made during the “re-emergence” of perovskite solar cells in 2012/2013 (see Snaith Lab photo ca. 2013). At the time, we knew very little about these materials, and so they were constantly generating surprisesnot just in 886
DOI: 10.1021/acsenergylett.9b00512 ACS Energy Lett. 2019, 4, 879−887
Energy Focus
ACS Energy Letters
optical, and electrical properties. I am delighted to see the focus of this issue on perovskites developed by ACS Energy Letters. A multitude of outstanding researchers are actively working on increasing the efficiencies of perovskite compounds and continue to raise our hopes of realizing a long-standing global vision of a more sustainable, clean energy-only future. Inspired by this thought, I sketched this rudimentary image for this Energy Focus piece, and I couldn’t help but include my “polar bears”wife, Jodie, and two cubs, Jason and Justin...perovskites from a personal perspective!
device performance but also in their fundamental properties. I remember being blown away by their long lifetimes during many late nights in 2013 when we were performing some of the early measurements of their diffusion lengths. Many of us came from organic or dye-sensitized solar cell backgrounds, and so a solution-processed absorbing semiconductor with such long lifetimes and good device performance was an entirely new concept. I do genuinely think there are a number of surprises in store from this materials family yet; let us see what the next decade brings as we march quickly toward commercialization.
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Prashant V. Kamat, Editor-in-Chief, ACS Energy Letters Constance M. Biegel, Coordinating Editor, ACS Energy Letters
Javier Vela, Iowa State University, Iowa, USA;
[email protected]; @vela_group
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University of Notre Dame, Notre Dame, Indiana 46556, United States
AUTHOR INFORMATION
ORCID
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. In 2013, I visited Notre Dame during my “tenure” tour. After my talk, Prashant Kamat politely said: “Javier, your work on binary chalcogenides is interesting. When will you work on perovskites?” The only perovskites I had heard of were ternary oxides. He continued: “Everyone is talking about halide perovskites. They are the best big thing in photovoltaics. You should take a look.” Within weeks of returning to Ames, we had made some of the first nanocrystalline halide perovskites, which displayed shapecorrelated photoluminescence at the single-particle level. Wondering why mixed-ion perovskites gave better and more stable solar cells than single-composition perovskites, we introduced 207Pb solid-state nuclear magnetic resonance to probe the extent of alloying and phase segregation in these materials. Our studies revealed the spontaneous formation of nonstoichiometric nanodomains. We also pioneered the solvent-free synthesis of mixed-halide perovskites starting from the parent single-halide perovskites, leading to higher-purity materials compared to those made from solution. Talk about great advice!
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Jitesh Soares, Associate Publisher, American Chemical Society
Perovskite photovoltaics present scientists with a plethora of opportunities to selectively design and optimize its physical, 887
DOI: 10.1021/acsenergylett.9b00512 ACS Energy Lett. 2019, 4, 879−887
