Questioning the quantum behavior of perovskites

Scientists investigating the behavior of individual particles within a perovskite have made a discovery they claim challenges the widespread understanding of the materials, and which could have implications for the design of perovskite solar cells and light emitting devices.

The discovery, reported in the paper Emergence of multiple fluoropheres in individual cesium lead bromide nanocrystals, published in Nature Communications, concerns light emission properties and the source of photoluminescence in the perovskite.

University of Texas researchers said received wisdom was that light emission in perovskites followed a similar model to other semiconductor materials, with light emitted by mobile excitons within the material. Under that model, shrinking the material would restrict the movement of excitons, resulting in changes to the wavelength of light being absorbed and emitted.

However, the scientists used single particle spectroscopy to test the theory and found it was not proven. “We observed that perovskite light is remarkably consistent,” said Anton Malko, associate professor of physics at the School of Natural Sciences and Mathematics at the institution. “Despite examining a wide range of sizes, from 9 to 30mµ, the emission wavelength – the color of the light – was unchanged in the cesium-based perovskite samples. The emitted light was a specific green no matter the size of the material observed.”

Quantum confinement contradicted

Observing the material’s light emissions on the single nanoparticle level, the group found the source of the light came from strongly localized sources within the material tied to bromide atom vacancy sites in the cesium-lead bromide material the group worked with in the study.

“These findings contradict the quantum confinement model, which would dictate that the source of luminescence in these perovskites is from excitons delocalized over nanoparticles,” added Malko. “Perovskites of any size will demonstrate this behavior.”

The group said its discovery represents a major step forward in understanding the photoluminescence (PL) properties of perovskites and will inform further studies into solar cell and LED applications for the materials.

“This systematic study of light emission statistics provides vital insights into the nature of defects formation and the decisive role they play in the origin of PL in cesium-lead-halide perovskite nanocrystals,” read the paper’s conclusion.