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May 2019

Steering Emitted Light with Flat Perovskite Crystals

By Brooke Kuei

Light-emitting diodes (LEDs), devices that convert electrical current into emitted light, are used in lighting, displays, and sensors, and offer the advantages of lower energy consumption, longer lifetime, and smaller size. An emerging material for LEDs is perovskites, a class of nanocrystals that can emit light across the entire visible spectrum, including the deep blue range, an area that some LED technologies struggle with.

The perfect LED would emit all of its light out of its surface, but in reality, the light is emitted in many different directions and some is trapped inside the device. In a study recently published in Nano Letters, a team of researchers led by Molecular Foundry scientists have created thin platelets of cesium lead halide perovskites in which the asymmetric shape of the nanocrystals causes the direction of emitted light to be nearly perpendicular to the surface.

“We found that by making tiny platelets, we are able to ‘squeeze the light’ and steer the emitted light outwards,” said Yi Liu, facility director of the Organic and Macromolecular Synthesis Facility at the Foundry and principal investigator of the study.

This study was spurred by the observation that perovskite cubes, which are symmetric and should emit light equally in all directions, actually emit light in a specific way – parallel to the surface they are on, which is less useful for LEDs. “When cubes are placed onto a surface, you create an asymmetric system,” explained Matthew Jurow, lead author of the study. “But the cubes direct the light the wrong way. So we moved from cubes to flat platelets and found that we were able to push the light in the right direction.”

However, changing the shape of the perovskite crystals into thin, flat platelets only partially controlled the emitted light. To increase light emission outwards even more, the researchers added a layer of aluminum oxide on top of the perovskites. Since aluminum oxide creates a similar electrical environment to the underlying surface on which the perovskites are lying, the additional layer can counter the interactions from the surface and push the emitted light in a direction more perpendicular to the surface.

Now that the researchers have created a system that enhances emitted light in the outward direction, Liu says that the next step will be to make actual LED devices out of their perovskite platelets. The researchers are also designing electrodes that would be integrated with perovskite nanocrystals to apply an externally controlled electric field as another means of controlling the direction of emitted light.

Read the paper here.