Missing Atoms Produce Luminescence in Perovskite Crystal

Perovskites are known to have remarkable optical and electronic properties. Perovskites with the general formula ABX₃, and particularly the perovskite methylammonium lead trihalide, show great promise as low-cost, high-efficiency solar cell materials. And now, 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 & Technology (KAUST) discovered the secret of its strong photoluminescence.

A Cs₄PbBr₆ crystal perovskite with a strong green fluorescence. Scientists believe the fluorescence is possibly emitted because of missing atoms. Courtesy of KAUST.

KAUST researchers have been testing Cs₄PbBr₆, a perovskite of the A₄BX₆ branch of the family. Cs₄PbBr₆ is noted for its strong photoluminescence that could potentially aid in color-converting coatings on LED light bulbs, lasers and photodetectors.

Researchers are trying to understand why the perovskite photoluminesces so strongly. To do so, they subjected the material to a barrage of tests. Through these tests, they discovered that when a Cs₄PbBr₆ crystal was heated to 180 °C, its photoluminescence was irreversibly destroyed.

Using temperature-dependent x-ray diffraction to track structural changes to the material as heat was applied, the team also discovered that at 180 °C, CsPbBr₃ nanocrystals formed within the mineral. The heat-induced structural rearrangements that create these nanocrystals also swallow natural defects in the original crystal where bromine atoms were missing. 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," said KAUST researcher Michele De Bastiani. "The unique photoluminescence manifested by Cs₄PbBr₆ makes these perovskites compelling materials for electroluminescence devices, lasers and light converters."

The researchers are looking into other members of the perovskite family to include CsPb₂Br₅, which has already shown unseen optoelectronic properties.

The KAUST research has been published in the journal Chemistry of Materials (doi: 10.1021/acs.chemmater.7b02415).