« Previously: Perovskite creates exciting research challenge  

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 answered questions about perovskites and various possibilities they open up for humankind.

Silicon-based solar batteries provide 20-25% efficiency. Gallium arsenide batteries boast 30% efficiency, and you have mentioned 15% efficiency with regard to perovskites. Why are scientists focusing on perovskites if their efficiency is so unimpressive? What makes them so unique? 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% 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.

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 6W perovskite solar battery measuring 15cm x 20 cm costs about ₹203.98 ($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-sensitised 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-colour 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-colour perovskite solar cells.

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? 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-colour 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 5-10nm. 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.

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? 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.

Julia Shabunina is Editor at the NUST MISiS Press Office

First published by EE Times Europe

 
« Previously: Perovskite creates exciting research challenge