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New insights into perovskite crystal behaviour for improved efficiencies

Perovskite crystals are a material consisting of calcium titanate which are very suitable for the development of engineered materials, as a great variety of different cations can be embedded in their structure. They play an important role in the development of new materials for efficiently harvesting solar energy. Perovskites are not as expensive as the materials currently used for solar panels, and so far their conversion efficiencies have yielded promising results. Therefore, it is of great importance to gain greater insights into the mechanisms at work at an atomic scale in order to improve efficiencies of perovskite materials.

Now (2021), scientists at Argonne National Laboratory have studied how perovskite materials function with the help of X-ray scattering and neutron scattering. This allowed the scientists to analyse the behaviour of the material at the atomic scale. The research revealed that it may possibly be the liquid-like back-and-forth motion of atoms in perovskites which was responsible for the production of electric currents.

The scientists analysed basic perovskites consisting of caesium, lead and bromine (CsPbBr3) and analysed the positions of the crystal atoms at different temperatures. X-ray scattering revealed that lead and bromine atoms formed rigid molecule-like structures which started to oscillate like a liquid. The scientists assumed that this movement was caused by free electrons when they traversed the material as the molecule structures possibly followed this movement. This assumption was supported by the experimental results and might play a decisive role in the design of new perovskite materials for solar cells. The scientists also discovered that the molecules oscillated in two-dimensional planes which also prevented recombination of electrons and enhanced the efficiency of the material.

For further investigation of the atom movement the scientists employed neutron scattering. Neutron scattering confirmed the two-dimensional movement of the molecules and also showed that almost no energy was needed for them to perform this movement, which would explain why the electrons were able to deform the lattice.

Scientists have been working relentlessly to improve efficiencies of perovskite solar cells. In 2020, a material for flexible perovskite solar cells was designed which could use electrons generated by sunlight more effectively and obtained a new record efficiency of 20.7%. The material consisted of a porous planar electron transport layer able to improve the interaction between the electrodes and the perovskite layer. The structure had a porous layer made of 20nm-sized Zn2SnO4 nanoparticles and a planar layer made of 2nm-sized SnO2 nanoparticles, which created an energy band structure for extracting electrons that were excited by light hitting the perovskite active layer and reducing the recombination of electron–hole pairs.

Also in 2020, an opto-electronic simulation framework intended for the computational analysis and optimisation of perovskite-silicon tandem solar cells was designed. This framework comprised a combination of a multiscale optical model for the simultaneous analysis of interference in thin coatings and scattering at textured interfaces in combination with a mixed electronic-ionic drift-diffusion transport model, taking into account the specifics of the materials used in the tandem architecture.

Perovskites hold several advantages over conventional material used for solar installations: they can increase the efficiency and lower the cost of solar energy as they employ low potential material and reduce processing costs. Perovskites can also react to various different wavelengths of light, which lets them convert more of the sunlight into electricity. Moreover, they are flexible, can be customised, and are lightweight, which opens up more possibilities for applications in solar cells. Gaining new insights into the atomic-scale mechanisms of this material can help tailor the perovskites to the needs of individual installations.

This study is an important milestone in researching solar technologies so that one day full advantage can be taken of the largely untapped sustainable energy from the sun, which would be beneficial for the environment as well as the economy.