Defects responsible for photoluminescence in little-studied perovskite

A little-studied member of the perovskite family of materials could find use in a range of electronic devices, after researchers at King Abdullah University of Science and Technology (KAUST) in Saudi Arabia discovered the secret of its strong photoluminescence. The researchers report their findings in a paper in Chemistry of Materials.

Perovskites are a wide group of materials that are known to have remarkable optical and electronic properties. Perovskites with the general formula ABX₃, and particularly the perovskite methylammonium lead trihalide, have attracted almost all the research attention thanks to their great promise as low-cost, high-efficiency solar cell materials. But other members of the perovskite family and perovskite derivatives are also worthy research subjects, says Michele De Bastiani, a postdoctoral researcher in Osman Bakr’s group at KAUST.

De Bastiani and his colleagues have been testing Cs₄PbBr₆, a perovskite of the A₄BX₆ branch of the family. This material is noted for its strong photoluminescence – the ability to absorb light at one wavelength and re-emit it at another.

The material’s potential applications include color-converting coatings on LED light bulbs, lasers and photodetectors. But to be able to fine-tune the material’s optoelectric properties for each application, researchers need to solve the mystery of why the perovskite photoluminesces so strongly.

“We investigated the structural and optoelectronic properties of Cs₄PbBr₆ to understand the origin of its photoluminescence,” explains De Bastiani. Subjecting the material to a barrage of tests, the team discovered that heating a Cs₄PbBr₆ crystal to 180°C caused its photoluminescence to be irreversibly destroyed.

Photoluminescence is a two-step process: absorption of light generates a pair of quasi-particles called excitons within the perovskite, which must recombine to re-emit the light. Using temperature-dependent X-ray diffraction to track structural changes within the material as heat was applied, the team discovered that at 180°C, CsPbBr₃ nanocrystals form within the mineral.

The heat-induced structural rearrangements that create these nanocrystals also swallow natural defects in the original crystal where bromine atoms are missing, the researchers found. These bromine vacancies act as traps for passing excitons. Confined in these traps, the excitons are much more likely to recombine and emit light.

“Now that we have this fundamental understanding, our next step is to move on to potential applications,” De Bastiani says. “The unique photoluminescence manifested by Cs₄PbBr₆ makes these perovskites compelling materials for electroluminescence devices, lasers and light converters.”

Meanwhile, many other little-explored members of the perovskite family with interesting properties are waiting to be revealed, De Bastiani adds. “One example is CsPb₂Br₅, a single crystal we recently synthesized for the first time with unseen optoelectronic properties.”

This story is adapted from material from KAUST, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.