Scientists visualize record exciton diffusion length

(Nanowerk News) Solar cells, LEDs, and a huge number of other modern electronics rely on optoelectronics. These are components that give off, detect, or control light. Within these devices, the movement of excitons (pairs of negative electrons and positive holes) determines how well the device performs.
A type of nanoscale crystals called perovskite—smaller than a grain of dust—shows promise for optoelectronic devices. Researchers have now created a new perovskite nanocrystal system and taken direct visualizations of the movement of an exciton from crystal to crystal over very long distances (ACS Nano, "Long-Range Exciton Diffusion in Two-Dimensional Assemblies of Cesium Lead Bromide Perovskite Nanocrystals").
Visualizing Exciton Diffusion Length
By coating a substrate with chemicals that perovskite nanocrystals attach to, researchers formed a layer of tightly packed nanocrystals. This system had a record exciton diffusion length, measured by directly visualizing diffusion with a custom microscope. (Image: Berkeley Lab)
Until now, scientists had not seen excitons travel more than about 30 nanometers in conventional optoelectronic systems. In addition, scientists had no clear picture of how the excitons move. For the first time, researchers have built a microscope that can take direct visualizations of exciton movement. They observed movement at a record distance of 200 nanometers, an order of magnitude longer than previous achievements. This work paves the way for commercial application in optoelectronic devices.
A team of researchers at the Molecular Foundry, a Department of Energy Office of Science user facility, created the nanocrystal system by coating a silicon surface with a polymer containing chemical groups to which the perovskite nanocrystals would attach. This formed a single layer of tightly packed perovskite nanocrystals. This system provided the researchers with a useful case study for looking at how excitons move, or diffuse, in depth.
In the past, researchers measured exciton movement by adding defects, imperfections in a crystal that trap excitons. Researchers could then indirectly measure the movement of excitons by comparing samples with different amounts of defects. Now, the research team can actually visualize the exciton movement by directly imaging it with a custom-built microscope. They were able to measure record values of exciton diffusion in perovskite nanocrystals that match the absorption depth of the same material, indicating that the material could be of great use in photovoltaic solar power devices and other optoelectronic applications.
Source: U.S. Department of Energy, Office of Science
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