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Cite This: ACS Energy Lett. 2019, 4, 879−887 http://pubs.acs.org/journal/aelccp

Perovskite Stories from Around the World he surge of interest in ■ Matt Beard, National Renewable Energy (PVs) in recent years has led to multifaceted research Laboratory, Golden, Colorado, USA; matt.beard@ opportunities. Although metal halide perovskites were nrel.gov; @mbthz fiT rst 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 Matt Beard and Ye Yang at NREL (photo credit: Dennis Schroeder) defect-driven processes. Researchers around the world are now We were challenged to investigate the optical properties and riding the third wave of perovskites. spectroscopy of perovskite films and crystals by Joey Luther and ACS Energy Letters asked a few of our authors to share their Kai Zhu. At first, we were skeptical that we could add something motivations for pursuing perovskite research. Their quotes useful but decided to study hot-carrier effects. Early on in our (shown alphabetically, below) show their exciting and investigation, Art Nozik (hot-carrier extraordinaire) walked by a interesting experiences during initial days and how they led to computer where one of our carrier-cooling plots was displayed “ ’ ’ success. 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. ■ Osman Bakr, King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, ■ Juan Bisquert, Universitat Jaume I, Institute of Saudi Arabia; [email protected] Advanced Materials, Castello, Spain; bisquert@ uji.es Downloaded via UNIV DEGLI STUDI DI PERUGIA on March 22, 2019 at 14:12:20 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

Nam-Gyu Park, Hui-Seon Kim, and Juan Bisquert in Korea, 2013 fi One of the most striking aspects of working with ABX The rst person who spoke to me about the power of hybrid 3 perovskites for photovoltaic applications was Tom Miyasaka. hybrid perovskites is their surprisingly intuitive , We were duly waiting for our planes in the airport in 2007 after which has made working with them exciting and satisfying. he had just presented the new perovskite-sensitized “quantum ’ Many of our students discoveries on crystallization and dot” in a conference in Saint Gallen. I was not very compositional tuning of these materials were a result of “Friday impressed but should have paid more attention. The next time I night”-type experiments. However, the field has reached a stage heard about it was in 2012 at some Conference in Asia, and it where a new kind of intuition is required for the metal (B) site was Seigo Ito who mentioned something about a new solar cell one different from those developed in the early days for the organic cation (A) and halide (X) sitesin order to Received: March 7, 2019 achieve breakthrough progress with lead-free perovskite Accepted: March 7, 2019 compositions.

© XXXX American Chemical Society 879 DOI: 10.1021/acsenergylett.9b00512 ACS Energy Lett. 2019, 4, 879−887 ACS Energy Letters Energy Focus involving Henry Snaith. He was clearly communicating a sense tried in late September, 2012, Ray made a 3.2% TiO2/ ’ of importance and urgency. I remember well Seigo sglanceand CH3NH3PbI3/CuSCN/Au device, nearly besting my personal fi emphasis, like a calmed auctioneer, suggesting the big value of the record Sb2S3 device on his rst try! When we measured the product with a contained smile, and everyone around taking about devices for 10 min at short-circuit, we saw an approximately 50% it very seriously, making metal notes, and requesting more decrease in photocurrent and a visible bleaching of the information. Soon Nam-Gyu Park emerged as one of the leaders of perovskite layer. This led us to switch from CuSCN to CuI, the new field, and his student Hui-Seon Kim came in late 2012 to which initially had lower efficiencies of only about 2% but were Castellóforsometimetoanalyzeoperationofthedevice. much more stable. We struggled greatly just reproducing our I remember Michael Gratzel̈ presenting 14% efficiency at HOPV own results because we did not understand the effects of Conference in Seville in May 2013. Clearly a race was starting and atmospheric humidity at all in these early days. After working a a lot of surprises lay ahead. Now in 2019, we still expect a few more. few more months on the project, I had made a 6% efficient fi TiO2/CH3NH3PbI3/CuI/Au, device and we had the rst ■ David Cahen, Weizmann Institute of Science and perovskite paper with all inorganic contacts; it was still measured Bar-Ilan University, Israel; [email protected] over 4% efficient after discovering it in the back of my drawer 1 year later! ■ 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; Perovskite solar cells appeared as a hot topic at the 7th Aseanian the high voltage efficiency dazzled me, and upon return, we forewent Conference on Nanohybrid Solar Cells 2012 in Taiwan. Prof. Nam-Gyu Park, Prof. Henry Snaith, and Prof. Tom Miyasaka good old (Cd,Zn)(S,Se,Te) for MAPbBr3. During the next year, I tried, not always successfully, to infect colleagues with “perovskitis”, reported their exciting result of hybrid metal halide perovskites, arguing this was a potential game changer. At the same time, the which revealed a new era of solar cell research. I believed that all MRS meetings were frustrating as they missed the change that I felt of the participates were convinced that PSCs would be the next in the air. As MRS board member, I searched during an August 2013 breakthrough in solar cells. What happened afterward fully board meeting to do something about it for the upcoming Fall supported this optimistic expectation. Inspired by the pioneering works, my group published some meeting and called Dave Ginley, former MRS president. I do not fi remember all of the reasons why nothing could be done, but in the subsequent papers on doping modi cation and hole trans- end, we convinced MRS, and the rest is history. porting materials for PSCs soon after. Our current research are aimed at developing two-dimensional perovskites with high ffi ■ Jeff Christians, Hope College, Holland MI, USA; power conversion e ciency and moisture resistance. Through a [email protected]; @jac997 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. ■ Filippo De Angelis, University of Perugia, and CNR- ISTM, Perugia, Italy; fi[email protected]

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 under- graduate researcher working with us at the time, start working on perovskites. At the time I was making TiO2/Sb2S3/CuSCN/Au Filippo De Angelis (left) and Edoardo Mosconi (right) at the Computational Laboratory for Hybrid/ solarcells;wedidnothavespiro-OMeTADinthelabsowetookthe Organic Photovoltaics in Perugia fi same process/structure I was using for Sb2S3 solar cells and plugged I rst heard about lead-halide perovskites from Henry Snaith at a fi in CH3NH3PbI3.InwhatIbelievewasthe rst batch of devices he European project meeting in June 2012 (I must admit I had

880 DOI: 10.1021/acsenergylett.9b00512 ACS Energy Lett. 2019, 4, 879−887 ACS Energy Letters Energy Focus missed Miyasaka’s 2009 paper), a few weeks after Kanatzidis published the paper on CsSnI3-based solid-state DSCs. Henry fi ∼ ffi rst showed us MAPbI3/TiO2 solar cells with 6% e ciency, which I considered with some curiosity. That was, however, just ∼ ffi thepreambletoshowingusa 10% e cient MAPbI3/Al2O3 solar cell. This result left the small audience astonished, testifying the semiconducting 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 fi ̈ QD solar cells were presented (that time the thin lm perovskite Edoardo Mosconi. Meanwhile, the papers by Park and Gratzel was named perovskite QDs) that delivered an efficiency of around and by Snaith and Miyasaka were published. 6%. Those days, this efficiency was considered to be high for We submitted our first paper, together with Nazeeruddin and fi ̈ QDs solar cells. This was the rst time I heard about the metal Gratzel, to J. Phys. Chem. Lett., but the paper was transferred to halide perovskite and its ability to function as a light harvester. J. Phys. Chem. C for lack of urgency. Our work, published in April fi fi Immediately I had the idea to use it as a thin lm in a solar cell 2013, is the rst modeling study of mixed-halide lead with the same architecture that we used for QD-based solar cells, perovskites, and it now has more than 600 citations. meaning that the metal contact is directly touching the perovskite film, what we called a hole conductor free perovskite ■ Eric Wei-Guang Diau, National Chiao Tung solar cell. This work resulted in one of the first publication in this University, Hsinchu 30010, Taiwan; diau@mail. field (J. Am. Chem. Soc. 2012, 134 (42), 17396−17399). For a nctu.edu.tw 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. ■ , , Oxford, UK; laura.herz@.ox.ac.uk

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 presenta- tion 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. fi It was quite difficult to make a good cell with high efficiency due to When my group rst investigated MAPbI3 in 2014, we were poor film formation, until we realized the two methods, antisolvent struck by just how much the solution-processed material seemed and solvent annealing. However, it was like a tsunami striking the to resemble classical inorganic semiconductors. Transient field, and researchers working on either dye-sensitized solar cells conductivity measurements showed high charge-carrier mobi- (DSSCs) or organic photovoltaics (OPVs) jumped into this battle lity, and its ratio with the bimolecular recombination rate to chase the efficiency. Until today, two issues are still challenging constant fell short of the value expected from simple Langevin for PSCs, stability and lead-free. We therefore have focused on the Theory by 5 orders of magnitude! This fortunate asset underpins the long (>microns) diffusion lengths observed in these development of stable lead-free PSCs since 2016. In 2018, we fi finally worked out a recipe for a tin-based lead-free PSC to attain materials and permits the thin lm (planar heterostructure) architecture commonly used for perovskite photovoltaic cells efficiency of 9.6%, which is close to what Park and Gratzel̈ reported today. We now know that charge recombination in these materials for a MAPbI cell 6 years ago. 3 is best understood in terms of a classical semiconductor band Lioz Etgar, The Hebrew University of Jerusalem, structure picture, similar to GaAs, explaining the deviations from ■ Langevin Theory. The big surprise find in many ways has been the Israel; [email protected]; https://lioz.etgar. discovery of a group of semiconductors whose defect chemistry is huji.ac.il so benign that near-intrinsic behavior can be observed even when During my last month at EPFL, on August 2012, when I conducted simple, low-energy processing routes are employed. my experiments on quantum dot (QD)-based solar cells, I came On a lighter note, I would like to share with you a cross- across the Nanoscale paper published in 2011, where perovskite sectional image (well, photograph) of a “metal halide perovskite

881 DOI: 10.1021/acsenergylett.9b00512 ACS Energy Lett. 2019, 4, 879−887 ACS Energy Letters Energy Focus solar cell” fabricated at my group’s Christmas party last year. The Material Research Society Spring Conference when Henry architecture: FTO (plastic ruler), C60 (blue icing), MAPbI3 Snaith presented the only one topic in this conference on (ginger bread), spiro-OMeTAD (yellow icing), aluminum foil perovskite solar cells. We started the research on perovskites at (silver). It is not known if it worked, but I have my doubts. 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 ■ Anita Ho-Baillie, University of New South Wales, the first real application, and many new sciences. These materials Australia: [email protected] are a library, and you can find almost everything in the textbook on condensed matter physics, but they are beyond any textbooks. ■ 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 Icouldfind 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%! C&EN issue on February 24, 2014 highlighting the first story on “Tapping Solar Power with Perovskites”, The surprises we get in terms of material properties and device featuring Joseph Manser, a graduate student from our laboratory on the cover performance continue to amaze me. It has created a vibrant During the Fall MRS meeting (2014) I was leading a panel research community and has captured the imaginations of many discussion in the Symposium on Perovskite-Based and Related Ph.D. students around the world, many of whom I am very proud Novel Material Solar Cells. The room was packed with nearly of, including those who work day and night trying to break our own 200 participants. A question came from a young researcher: records. I still remember the stress involved in getting the cells “Everyone claims high efficiency of their solar cells. Could this certified knowing perovskites do require a lot of care given their community suggest a protocol to produce solar cells in a unpredictable stability! I look forward to seeing where perovskites reproducible way and develop procedures to measure solar cell will be going in the future. The early history of the development of efficiency correctly?” At that time, everyone was eager to claim this technology was published in ACS Energy Lett. (https:// high efficiency using fast J−V scans, charging, etc., and there was pubs.acs.org/doi/10.1021/acsenergylett.7b00137). some confusion on the validity of these efficiency claims. Critical questions from young researchers, such as this, helped us share ■ Jinsong Huang, University of North Carolina at the different groups’ procedures and shape the field in an Chapel Hill, North Carolina, USA; [email protected] 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), pro- viding a collection of early developments in the field. ■ Mercouri Kanatzidis, , 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 I first came to perovskite solar cells when I organized a ultimately we succeeded. This problem pulled us deep into the symposium on Organic and Hybrid Solar Cells at the 2013 throes of perovskite science. We synthesized all possible 3D

882 DOI: 10.1021/acsenergylett.9b00512 ACS Energy Lett. 2019, 4, 879−887 ACS Energy Letters Energy Focus ■ Michael McGehee, University of Colorado, USA; [email protected]

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 fi learned enough about perovskite chemistry to use CsSnI /SnF In 2011 my research group was trying hard to nd defect- 3 2 ff as a thin layer of hole transport layer in a dye-sensitized solar cell tolerant high-bandgap solar cells that could be cost-e ectively in collaboration with my colleagueR.P.H(Bob)Chang.Between deposited on top of silicon to make high-performance tandems. ̈ the last 6 months of 2011 and first couple of 2012, we increased cell I will never forget our excitement when Michael Gratzel efficiency from 1 to 10%. This was the first ever presented his early results on perovskite solar cells at a group to be reported that employed a solid continuous layer of perovskite, meeting for the Center for Advanced Molecular Photovoltaics. and this layer functioned not only as a hole transporter but, to our We quickly knew that perovskites had the properties we needed. surprise, also as a light absorber. Our Nature paper appeared in The next event that really stands out in my memory was Henry ’ May of 2012. We had seen the Miyasaka JACS paper in 2009 but Snaith s truly extraordinary lecture at the European Materials somehow ignored it. We did not think anything of it because it used Research Society meeting. The organizers somehow knew that Henry was going to show something special and let him speak discrete and disconnect nanocrystals of MAPbI3 attached to meso- for 45 min. He showed that the titania scaffold was not necessary TiO2 and the solar cell had a liquid phase. It looked a lot like a and that perovskites should be thought of as semiconductors classical Ru(bipy)3-style cell but was unstable because the MAPbI3 nanocrystals dissolved quickly into the solution. In late 2012 with exceptional defect tolerance, not just titania sensitizers. He going into 2013, we had observed X-ray and γ-ray detection explained that the perovskites could be thermally evaporated, from CsPbBr . Perhaps, in 2022 there will be another 10 year pretended to do a drum roll, and then told us that the evaporated 3 ffi anniversary to again reminisce about the early days. perovskite cells had 15% power conversion e ciency. I immediately told several members of my group that they needed to completely ■ , , stop working on their old projects and start doing everything it California, USA; [email protected] would take to make tandems with perovskites. I am glad I did. Thanks to those who discovered these materials. ■ David Mitzi, Duke University, North Carolina, 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 . 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 So, I loved the fact that lattices, similar to those that are forged in thermal ablation tool that we used for the perovskites. (Photo courtesy of David Mitzi) a furnace at 1000°, could form in solution using the methods I recall my excitement on first learning about organic−inorganic more typically employed by molecular chemists. I remember perovskites as I was completing my Ph.D. in 1990, which focused thinking then that if I ever were lucky enough to have my own on copper-oxide-based superconductors. I was particularly intrigued research lab I would study halide perovskites. Of course, many of by the analogy of the copper oxide perovskites to modulation-doped my initial ideas evolved as my group learned more about these semiconductorsi.e., with the Cu−O sheets serving as the materials. One of our few original goals that was realized without active superconducting layers, separated by modulation layers − much change was the demonstration in 2014 that 2D perov- (e.g., Bi OinBi2Sr2CaCu2O8+δ) that facilitate tailoring elec- skites can serve as solar absorbers. tronic character while minimizing disorder within the active

883 DOI: 10.1021/acsenergylett.9b00512 ACS Energy Lett. 2019, 4, 879−887 ACS Energy Letters Energy Focus 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 semi- transporting materials. However, we were disappointed with conducting, metallic, or perhaps even superconducting. After nonreproducible efficiencies. Our striking finding was the spending several frustrating years unsuccessfully trying to dope/ nonstoichiometric ratio of PbI2 to methylammonium iodide fi tailor copper halide perovskites, I switched to the Sn-based (an excess of PbI2) that signi cantly improves perovskite crystal systems, leading to our first paper in this area (Nature 1994, 369, size, efficiency, and reproducibility and modifies the interface 467−469), describing a family of tunable hybrid semiconductors. between the perovskite and electron transporting layer. To further Subsequently, in the late 1990s, our interest migrated to using enhance the stability of perovskite solar cells, we developed layer- these systems in organic−inorganic electronic devices, notably by-layer growth of 3-dimensional and 2-dimensional perovskites LEDs and transistors (e.g., IBM J. Res. Dev. 2001, 45,29−45; see yielding over 22.5% certified efficiency. the photo of my team at the time). One project goal involved designing systems with unique/useful interaction among hybrid ■ Nitin P. Padture, Brown University, Rhode Island, components, still an area of contemporary interest for photo- USA; [email protected] voltaic and other prospective application. ■ Ivań Mora-Sero,́ University Jaume I, Castellóde 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 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 The first time I heard about perovskite was with the seminal halide perovskites or solar cells! Astonishingly, the proposal was work of Prof. Miyasaka in 2009, and I also followed the paper funded, just as the perovskite solar cells (PSCs) field was taking off. from Prof. Park in 2011 as at that time I was working on I was fortunate to hire the talented Yuanyuan “Alvin” Zhou as a fresh quantum dot sensitized solar cells. However, to be honest, I did Ph.D.student,whoisnowAsst.Prof.(Res.)atBrown,toworkon not give them too much attention because they presented even that project in 2012, and it has continued to be an amazing ride since more stability problems than the standard quantum dots used in then. We, as bona fide materials scientists, could relate to halide- sensitized solar cells at that time. After the publication of the all- perovskite synthesis/processing, microstructures, grain boundaries, solid perovskite cells in 2012, I started to work with perovskites and related phenomena and are proud to bring new thinking to the quite soon, at the end of that year, with the visit of Hui Seon field from a different angle. The solar cells community has been Kim, first author of the paper on this issue from Prof. Park and most welcoming toward me as a total newcomer to the field, and it is Prof. Gratzel,̈ to our laboratory to characterize these cells with appreciating the important contributions from my research group impedance. A priori, I believed it would be routine work due to and collaborators, for which I am very grateful. Still, much remains to my experience with quantum dot sensitized devices but soon be done as we keep our eyes on the prizewidespread began to bring surprises. Where was the chemical capacitance, commercialization of PSCs. the key parameter in sensitized cells? What was the capacitance observed at low frequencies? These were just my first surprises of ■ Nam-Gyu Park, Sungkyunkwan University, Suwon the many that this field has given me. 16419, Korea; [email protected] When I attended NanoEuro 2007, held in St-Gallen, Switzer- ■ Mohammad K. Nazeeruddin, Swiss Federal land, I listened to Prof. Miyasaka’s talk on perovskite-sensitized Institute of Technology (EPFL), Lausanne, ffi fl solar cells with power conversion e ciency (PCE) as low as Switzerland; mdkhaja.nazeeruddin@ep .ch around 2%. Such a low efficiency could not have enough power Coming from a dye-sensitized solar cells background, seeing to attract attention from the audience. However, I was very over 1 V open-circuit potential in perovskite solar cells was interested in his work because the terminology “perovskite” was striking, and we started exploring compositional engineering of familiar to me thanks to my research background on oxide perov- perovskite and design and development of electron and hole skites during M.S. and Ph.D. studies at Seoul National University.

884 DOI: 10.1021/acsenergylett.9b00512 ACS Energy Lett. 2019, 4, 879−887 ACS Energy Letters Energy Focus

So, I started work on perovskite solar cells after coming back from research group slowly engaged in working on these nanocrystals. Switzerland. In 2009, I joined Sungkyunkwan University (SKKU), The stunning moment was the use of preformed oleylammo- and there, I achieved PCEs up to 6.5% by discovering the nium ions in the reaction flask, and its concentration was − importance of the precursor solution concentration, which was observed to tune the intensity of Mn d d emission in CsPbCl3 published in Nanoscale in 2011. We then considered solid hole nanocrystals (Angew. Chem., Int. Ed. 2017, 56, 8746−8750). conductors in place of liquid electrolyte because of the instability of When we applied the same trick for CsPbBr3, the size of the the perovskite in polar liquid. While we were searching for an cubes was tuned (ACS Energy Lett. 2018, 3, 329−334). Further, effective methodology, we eventually found a method that a this could also help stabilize the α-CsPbI phase, even obtaining fi 3 thinner TiO2 lm would be better because the light penetration a near-unity absolute PLQY for all CsPbX3 perovskites, depth is inversely proportional to the absorption coefficient (the colloidally prepare doped layered perovskites, and, importantly, ffi absorption coe cient of MAPbI3 was 1 order of magnitude higher support thermal annealing where these nanocrystals sustain than that of the Ru-based sensitizer, as reported in Nanoscale in even more than 5 h of annealing at 250 °C without any phase 2011). So, we eventually achieved solid-state perovskite solar cells transformation (J. Phys. Chem. Lett. 2018, 9, 6599−6604). The demonstrating a PCE of 9.7% by decreasing the film thickness to beauty of this salt is supplying adequate halides at high tem- 0.6 μm and 500 h stability without encapsulation, which was perature during the formation of perovskites and at the same published in Scientific Reports on August 21, 2012. time helping to passivate the nanocrystals. ■ Annamaria Petrozza, Italian Institute of ■ Andrey Rogach, City University of Hong Kong, Technology Center for Nano Science and Hong Kong; [email protected] Technology; [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 fi On the rst of November 2012, I received an email from Henry Peter Reiss (CEA Grenoble) and myself submitted a project “ fi ” Snaith, the subject was A magni cent system to look at... . proposal on “Perovskite absorbers in nanostructured solar cells” I must say that the following years were intense. The excitement for a joint French/Hong Kong funding. We received the of study something completely new (at least for me as I was reviewer comments telling us, among other quite positive points, coming from the world of organic semiconductors) was often that “...perovskite absorbers may have a challenge as to charge coming together with that feeling of being a rabbit caught in the transfer due to their generally insulating naturecharge extrac- fi headlights. This eld has given me the chance to learn old and tion from the absorber may be a problem...”, and “...very high new science. Looking back, I see that the community built good purity of perovskite absorbers will be required”, as well as “...the foundation, and I am sure more fun is to come! proposed technology is already a bit late to make an impact on the market”. Due to those anticipated issues listed by the ■ Narayan Pradhan, Indian Association for the reviewers, the project has not been recommended for funding by Cultivation of Science, Kolkata, 700032 INDIA; the agency, thus preventing my personal involvement in perov- [email protected]; @npiacs skite photovoltaics, so far. I came back to the colloidal perovskite While my students were keen to work on metal halide quantum dots in 2015, when we published our first paper on perovskites in early 2016, I was personally reluctant. However, their exceptionally strong emission in Adv. Sci., which occurs last when Mn-doped CsPbCl3 appeared, we were quite excited and but not least due to the defect-tolerant nature of perovskites. started to make these nanocrystals. Within a couple of months, This paper eventually became the best-cited research article of we were addicted with the beautiful science, and my entire that journal.

885 DOI: 10.1021/acsenergylett.9b00512 ACS Energy Lett. 2019, 4, 879−887 ACS Energy Letters Energy Focus ■ Michael Saliba, Ecole Polytechnique Federale de of ferroelectricity, diversity of reported excitonic binding energies (spanning more than an order of magnitude of values), and Lausanne, Fribourg, Switzerland; miliba@gmail. ff com; @miliba01; @salibalab puzzling changes in the photoluminescence under di erent environmental conditions are just a few of the issues that continue to tease us, demanding a better understanding. ■ Henry Snaith, University of Oxford, Oxford, UK; [email protected]

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 solid- state dye-sensitized solar cells. The idea was that the NH + group of “ ” 4 the perovskite dye would attach itself to the oxygen of the TiO2 compact layer following the now famous JACS paper by Miyasaka Absent of any form of exaggeration, perovskites have been a life- et al. from 2009. One budding discussion was how this new dye changing experience. The research environment post-2012, bears wouldthensticktothemesoporoustitania.Andtherestishistory. no resemblance to that before. The pace of research, the pace of fi Little did I know at the time that this presentation would have scienti c and technological advancement with these materials is like science on steroids (with the advantage that these steroids are such a dramatic impact. Many years later, hundreds of groups, fi “ ” including mine, work with great excitement on perovskite legal). My rst exposure to perovskites was in 2009, when I saw materials toward a sustainable energy future. What an unexpected Miyasaka give a presentation of his work with Kojima in a small seminar held when I visited Toin University; at the time the Toin development showing the beauty of science where a routine group “ ” meeting becomes the starting point for a new research direction. group was touting Kojima as the JACS guy . I thought perov- skites looked intriguing. The first time I knew perovskites may fi ■ D. D. Sarma, Indian Institute of Science, Bengaluru really be something was in 2010, when Mike Lee made his rst solid-state “perovskite-sensitized” solar cells; on the first shot, they 560012, India; [email protected]; https://www. beat our all-time lab record for solid-state dye-sensitized solar cells. facebook.com/pages/D-D-Sarmas-Group/ My first “mind-bending” experience was in late 2011, when via a 567099976731144 curiosity-driven experiment we investigated cells with the mesoporous TiO2 substituted for mesoporous Al2O3;MikeLee 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. ■ Sam Stranks, , Cambridge, UK; [email protected]; @samstranks; Ashutosh Mohanty, D. D. Sarma, Sharada Govinda, and Bhushan Kore (left to right) @strankslab; www.strankslab.com It is difficult to pinpoint the time when we first became aware of the hybrid halide perovskites, but when we began to be inter- ested 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 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 Snaith Lab circa end 2013 stability, but devoted ourselves to attempting to understand its I fondly recall the buzz about the Snaith Lab when some of the basic properties, which seemed to be confusing with many directly first discoveries were being made during the “re-emergence” of conflicting claims in the literature. It appears that many of these perovskite solar cells in 2012/2013 (see Snaith Lab photo issues continue to remain controversial despite intense efforts of ca. 2013). At the time, we knew very little about these materials, many groups over several years. The mysterious presence/absence and so they were constantly generating surprisesnot just in

886 DOI: 10.1021/acsenergylett.9b00512 ACS Energy Lett. 2019, 4, 879−887 ACS Energy Letters Energy Focus device performance but also in their fundamental properties. optical, and electrical properties. I am delighted to see the focus I remember being blown away by their long lifetimes during of this issue on perovskites developed by ACS Energy Letters. many late nights in 2013 when we were performing some of the A multitude of outstanding researchers are actively working early measurements of their diffusion lengths. Many of us came on increasing the efficiencies of perovskite compounds and from organic or dye-sensitized solar cell backgrounds, and so a continue to raise our hopes of realizing a long-standing global solution-processed absorbing semiconductor with such long vision of a more sustainable, clean energy-only future. Inspired lifetimes and good device performance was an entirely new by this thought, I sketched this rudimentary image for this concept. I do genuinely think there are a number of surprises in Energy Focus piece, and I couldn’t help but include my “polar store from this materials family yet; let us see what the next bears”wife, Jodie, and two cubs, Jason and Justin...perovskites decade brings as we march quickly toward commercialization. from a personal perspective! Prashant V. Kamat, Editor-in-Chief, ACS Energy ■ Javier Vela, Iowa State University, Iowa, USA; Letters [email protected]; @vela_group Constance M. Biegel, Coordinating Editor, ACS Energy Letters University of Notre Dame, Notre Dame, Indiana 46556, ■ 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 perov- skites?” 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 shape- correlated photoluminescence at the single-particle level. Wonder- ing 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! ■ 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