UrFU Scientists Discover Atomic Mechanisms to Increase Efficiency of Solar Batteries

16:08, December 23, 2020

The research is innovative and ground-breaking for several scientific domains

A team of scientists from Ural Federal University, Skolkovo Institute of Science and Technology, Institute for Problems of Chemical Physics of Russian Academy of Sciences together with National Taiwan University have discovered how to increase the stability of solar cells. Scientists investigated structures in which halogens (iodine and bromine) served as anions, organic compounds (methylammonium and formamidinium, cesium) were one of the cations, and lead (Pb) was another cation.

An article describing the research was published in one of the most prestigious journals, Nano Energy. The following is compiled from an interview with Dr. Ivan Zhidkov.

Physicists have made breakthroughs in several directions at once. Investigating hybrid perovskites of lead halides, they found a mechanism to stop their degradation. Stabilization is extremely important as it allows perovskites to be used as absorbent materials in solar cells.

“It is found that stability of APbX3 perovskites with respect to light soaking and heat stress is connected with the formation of PbX2 degradation product, which can be monitored by high-energy resolved XPS Pb 4f, I 3d-spectra and valence band spectra. The obtained results are confirmed by DFT calculations of the formation energies of intrinsic PbX2-defects. Furthermore, our results show that the resistance of hybrid lead halide perovskites to stress factors (light or heat) is gradually increased while substituting the A-site cation going from CH3NH3+ to CH(NH2)2+ and then to Cs+,” explains Ivan Zhidkov, associate professor of the Department of Electrophysics of the Ural Federal University, a member of the research team and co-author of the article.

The most important product that is released during the degradation of perovskite and accompanies it is PbX2, where X is iodine or bromine. The process of release is as follows: the organic residue - a volatile compound methylammonium iodide, decomposes, comes out in the form of gases - iodomethane CH3I and ammonia NH3, and lead iodide remains in the perovskite structure. Perovskite degradation begins with the formation of a small portion of lead iodide defects. The higher the concentration of such defects, the stronger the decomposition of perovskite. Therefore, if we stop the "multiplication" of PbI2 defects and its subsequent decomposition to Pb0, the degradation of perovskite also stops, and it becomes stable. This is exactly what scientists from Russia and Taiwan did.

In addition, by creating solar cells based on hybrid lead halides, scientists have reduced the volume of the cell of its crystal lattice and increased its rigidity. This made it possible to inhibit the formation of defects and the degradation of perovskite. They also found that if an organic cation — from methylammonium or formamidinium — was replaced with cesium, then under certain conditions there were few defects.

“We have shown that in the case of long-term irradiation in the completely inorganic perovskite CsPbI3, instead of the expected 'classical' decomposition, in reality, a certain amount of PbI2 defects is actually first observed, which then stop forming due to the compaction of the crystal lattice and decompose into Pb0 and I2. Thus, during the evaporation of iodine, metallic lead is also formed, and the bulk of the perovskite remains in its original form - CsPbI3, its decomposition stops. In this case, the replacement of the organic cation with cesium provides a significant improvement in the stability of the perovskite. I would like to note that the formation of Pb0 under the action of photons on CsPbI3 was detected by us for the first time in the world, ”says Ivan Zhidkov.

The ultimate goal of scientists is to create highly efficient, easy-to-manufacture and inexpensive, durable solar cells, the energy of which will be several times and tens of times cheaper than the energy of traditional silicon batteries.

The study received two grants: from the Russian Foundation for Basic Research and the Sverdlovsk Region (project No. 20-42-660003 "Electronic structure of lead-free hybrid perovskite solar cells and their resistance to irradiation and temperature") and from a joint Russian-Taiwanese project of the Russian Foundation for Basic Research (No. 21-52-52002 "Investigation of the physicochemical properties of organic-inorganic perovskite interfaces and their influence on the possibilities of photoelectric applications").

About Solar Sells and Perovskites

Solar cells based on hybrid perovskites are a rapidly developing technology in the last decade. To date, they have achieved high efficiency in converting solar energy (up to 25.2%). For comparison - the efficiency of early commercial photovoltaic technologies based on silicon - 26.7%, gallium arsenide - 29.1%, cadmium telluride - 22.1%, and compounds of copper, indium, gallium and selenium - 23.4%.

Perovskites are a class of materials with a structure similar to that of calcium titanate, where one anion (negatively charged ion) and two cations (positively charged ions) are present. Anion, most often an oxidizing agent from the halogen series, replaces the oxygen position, one of the cations replaces the calcium position, and the other replaces titanium.