Q&A with Anvar Zakhidov on perovskites and their future
These solar batteries are quite large, are not very effective under diffused light and also cost quite a lot, although prices dropped significantly recently. Scientists all over the world who are actively searching for alternative solar-cell materials noticed the rather promising perovskites not so long ago.
Professor Anvar A. Zakhidov, Deputy Director of the UT Dallas Nano-Tech Institute, a leading expert on alternative energy, and head of initiative research project on “High-Capacity Polymer Tandem Photovoltaics on the Basis of Hybrid Perovskites” at the National University of Science and Technology MISIS (NUST MISIS), Moscow, Russia discusses the amazing properties of these materials and various possibilities they open up for humankind.
Question: Mr. Zakhidov, could you say a few words about perovskites and their discovery?
UT Dallas Nano-Tech Institute.
Anvar Zakhidov: Classic perovskite crystals with the same type of crystal structure as calcium titanate (CaTiO3), known as the perovskite ABX3 structure, were first discovered in 1839 in the Ural region. The mineral was named after Count Leo Perovsky, a Russian collector of minerals.
Scientists later discovered other types of perovskites, including barium titanate, lithium niobate and lanthanides. All perovskites have a similar structure with the ABX3 pattern. This complicated structure features crystal atoms that are intricately bound by ion bonds, but each of the three elements can be replaced. If we take a new hybrid perovskite on which we are currently working, then one of its elements should be organic molecular ion, and this element should consist of carbon and hydrogen. For example, A denotes the so-called methylamine (CH3NH3), B denotes lead (Pb) and X denotes iodine (I). It is not easy to make a three-element combination because their sizes should be of a certain value, fitting the so called tolerance factor.
Question: How many types of perovskites are there: dozens, hundreds or thousands?
Anvar Zakhidov: There are very many types of perovskites, and their total number probably exceeds several hundred. One can single out superconducting, ferroelectric and non-linear/optical perovskites. I have a large chunk of perovskite crystal called lithium niobate, beautiful as a diamond, at my laboratory. However we are now particularly interested in representatives of a new special class, that is, organic-inorganic perovskites which are also called organometal halide perovskites (OHPs), with iodine, bromide and chlorine acting as halides. Scientists continue to synthesize new types of these organic-inorganic perovskites, trying to get rid of potentially hazardous lead, Pb.
Question: Why did you start working with perovskites?
Anvar Zakhidov: I have been dealing with photovoltaics most of my life, and I have been studying current-generation processes inside various materials under the influence of light. At the beginning of my research, I worked with a team of physics theorists at the Institute of Spectroscopy at the Russian Academy of Sciences in Troitsk. Members of that team dealt with excitons originating inside molecular semiconductors and organic crystals during light absorption. I have studied virtually all light-absorbing materials, including molecular crystals, silicon, gallium arsenide, conducting polymers and the so-called charge transfer complexes. Of course, I started working actively with perovskites when they appeared because perovskites have very exciting physical properties, not found in other classes of materials, and particularly interesting excitons with substantial binding energy and strong interaction with light.
I learned about the existence of perovskites by sheer coincidence. In 2013, world-famous scientist Michael Grätzel, an expert on photo-chemistry, delivered a report about perovskites at the Solar Energy for World Peace International scientific congress in Istanbul. Grätzel announced that members of his research team had managed to boost the efficiency of perovskite-based photovoltaic elements to 15.5 percent. At the same time, surprisingly, he was unable to explain how exactly the perovskite crystals functioned inside photo cells. My colleagues and I started actively discussing the unusual hybrid material’s possible principle of operation. I took up perovskite research right after returning to my laboratory in Dallas, simply because of this puzzle: they were too good to be a truth, and it was not clear why.
Question: Silicon-based solar batteries provide 20-25 percent efficiency. Gallium arsenide batteries boast 30 percent efficiency, and you have mentioned 15 percent efficiency with regard to perovskites. Why are scientists focusing on perovskites if their efficiency is so unimpressive? What makes them so unique?
Anvar Zakhidov: In 2013, Science magazine listed perovskite among the top 10 breakthroughs of the year. This was no coincidence because perovskite is a highly promising material for the solar power industry. Its efficiency levels have already reached 22.1 percent and continue to increase, while it is very easy to obtain thin films of these OHPs, just from solutions in conventional organic solvents. These perovskites in general are full of riddles and mysteries. They have such a unique crystal structure, making it possible to obtain variety of unconventional physical properties: I already mentioned the best high-temperature superconductors (in Ba-based inorganic perovskites).
Their structure is so diverse that it offers many other useful properties, including ferroelectricity and non-linear optical activity. If we compare our particular OHP hybrid perovskites with inorganic superconductors, we can see that this material functions in a rather unusual manner, as far as photovoltaics are concerned. Charge carriers live too long and travel too far, after they are photo-generated by light, and it is still not clear, why they can be so easily separated. Maybe because of internal tiny electric fields, created by organic electric dipoles of CH3NH3 methylamine. Or because of ions of iodine, moving in a certain way. All these questions are still awaiting some answers, which creates an exciting challenge for researchers.
cells (source NUST MISiS).
It has already been proved that perovskite solar cells can surpass currently available silicon-based solar batteries in terms of their efficiency, and that they can probably catch up with gallium arsenide quite soon. Scientists continue to increase their efficiency, to develop ever new methods for obtaining materials. They continue to improve the properties of thin films, but no one can explain their impressive performance for the time being.
There is no need to use rare-earth or other precious metals for making perovskite-based solar batteries. Perovskite panels are made using a low-temperature process and inexpensive ordinary-salt solutions known for their low production costs. A six-watt perovskite solar battery measuring 15 by 20 centimeters costs about $3.
It is also easy to work with perovskites. ABX3 elements are linked by ionic bonds, and the crystals of organo-metal halide perovskites are easily dissolved in conventional organic solvents. The solvent disappears after being deposited on the basic layer, but the crystals remain. It is possible to coat any surface with extremely thin layers of this material, and it is also possible to manufacture thin flexible elements. In Italy now operates a pilot plant to manufacture the so-called Grätzel solar elements, also called dye-sensitized solar cells (DSSCs). Perovskites have been used as dyes in DSSC for the first time, and they can modify the operation of traditional electrolyte based DSSC, making them all solid state (without need for liquid electrolytes). Architects order huge multi-color DSSC panels for decorating various buildings. But these DSSC panels can be now upgraded: their liquid electrolytes can be replaced with solid-state materials, resulting in semi-transparent multi-color perovskite solar cells.
Question: Quite recently, Nature Photonics carried an article by Princeton University scientists claiming that, apart from solar batteries, perovskites can be used in laser systems, display screens, monitors, etc. What are their possible applications?
Anvar Zakhidov: Yes, that’s right, perovskites are not only used in photovoltaics. Members of my research team study the use of perovskites in laser systems and luminescent glowing screens. Samsung manufactures for their cell phones bright multi-color screens based on organic light emitting devices: OLEDs. It took several decades to develop extremely thin layers of these organic materials, and now OHP perovskites have every chance of surpassing this technology. Perovskite-based LEDs will be brighter, more efficient and, maybe, even more stable than OLEDs. However Perovskite LED stability is the main problem today, and the same can be said about OHP Perovskite Solar cells. Scientists are already making perovskites LEDs with a brightness of 0.5 million candelas per square meter (the Latin word “candela” means “candle”), while organic light-emitting diodes (OLEDs) generate about 20,000-50,000 candelas in terms of brightness levels.
Members of our team are also trying to obtain quantum dots from perovskites using the laser spray-coating method. These tiny cube-shaped or ball-shaped quantum dots measure between two and 5-10 nano-meters. Unlike ordinary 3D perovskites, these perovskite-based quantum dots will not disintegrate quickly, but they will remain stable because ball-shaped or cubic shrouds will be able to retain stable perovskite crystals.
Perovskites can also be used in photo-detectors. It turns out that perovskite-based detectors are highly sensitive. We have recently published an article on this issue. We can take perovskite, interspersed with an array of nanoimprinted thin perovskite strips, and obtain very sensitive photo-detectors. They can operate in the optical and infrared spectrum bands. Quite possibly, the practical applications of hybrid OHP perovskites will continue to expand.
Question: In January 2017, Nature published an article by scientists from the Okinawa Institute of Science and Technology who claim that it is impossible to use perovskites for making solar batteries because perovskites disintegrate due to iodine emissions (Editor’s Note: Iodine is a component of modern types of OHP perovskites). Are the Japanese experts right?
Anvar Zakhidov: Of course, perovskites have some drawbacks. First, they are really unstable in ambient atmosphere, and they disintegrate quickly in humid air. Second, their efficiency in photovoltaic devices remains still quite low. But I believe it will become possible to resolve these issues in the near future.
Numerous research projects are implemented in this area worldwide. When I spoke with Michael Grätzel during the congress in Istanbul, Turkey, in 2013, 30 to 50 articles about OHP perovskites were published annually. And their number has soared several hundred times over the past three years. Nowadays, several thousand articles about organo-metal halide perovskites are being published annually. Every more or less influential research team is dealing with perovskites. Scientists from the United States, the United Kingdom, Japan, China, Korea, Russia, Germany and many other countries are studying this issue very actively. Given this tremendous interest, we should see specific results quite soon. I think much more stable perovskites will be obtained quite soon. Moreover, we need to create an effective hybrid perovskite with non-toxic elements, eliminating Pb. At this time, basic organo-metal halide perovskites contain lead. It is possible to replace lead with other metals, including tin or silver. Who and how will provide the best and fastest solution? That is the main question nowadays.
About the author:
Julia Shabunina is Editor at the NUST MISiS Press Office – https://en.misis.ru/
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