As the demand for power continues to grow and natural resources become depleted, the world is turning to alternative energy for help. Various solar panels based on silicon, or thin film cadmium telluride and cadmium sulfide (CdTe/CdS) account for about 85% of all solar batteries being manufactured worldwide.

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, has discussed the amazing properties of perovskites and various possibilities they open up for humankind.

Mr. Zakhidov, could you say a few words about perovskites and their discovery? 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.

How many types of perovskites are there: dozens, hundreds or thousands? 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 synthesise new types of these organic-inorganic perovskites, trying to get rid of potentially hazardous lead, Pb.

Why did you start working with perovskites? 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%. 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.

 
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