New low cost and energy efficient light source

Light Emitting Diodes made from perovskite (green) nanocrystals embedded in a metallo-organic structure can be created inexpensively, use materials abundant in soil, and remain stable under typical working conditions. Credit: Image from Los Alamos National Laboratory

A breakthrough in nanocrystallization introduces a low-cost, energy-efficient light source for consumer electronics, detectors and medical imaging.

Light Emitting Diodes (LEDs) are an unsung hero in the lighting industry. They work efficiently, give off little heat and last a long time. Today, scientists are studying new materials to make more efficient and durable LEDs with applications in consumer electronics, medicine, and security.

Researchers from the US Department of Energy (DOE) Argonne National Laboratory, Brookhaven National Laboratory, Los Alamos National Laboratory and SLAC National Acceleration Laboratory report that they prepared perovskite nanocrystals stable for such LEDs. Taiwan’s Academia Sinica also contributed to this effort.

“Our studies have shown that this approach allows us to significantly improve the brightness and stability of light-emitting nanocrystals.” – Xuedan Ma, scientist at the Center for Nanometric Materials in Argonne

Perovskites are a class of materials that share a particular crystal structure, giving them light absorption and emission properties that are useful in a range of energy efficient applications, including solar cells and various types of detectors.

Perovskite nanocrystals were prime candidates as a new LED material, but were found to be unstable in testing. The research team stabilized the nanocrystals in a porous structure called a metal-organic framework, or MOF for short. Based on materials abundant on earth and manufactured at room temperature, these LEDs could one day enable television sets and consumer electronics at a lower cost, as well as better gamma-ray imaging devices and even auto x-ray detectors. – powered with applications in medicine, security scanning and scientific research.

“We have attacked the stability problem of perovskite materials by encapsulating them in MOF structures,” said Xuedan Ma, scientist at the Center for Nanoscale Materials (CNM) in Argonne, a user facility at the DOE Office of Science. “Our studies have shown that this approach allows us to significantly improve the brightness and stability of electroluminescent nanocrystals. “

Hsinhan Tsai, former JR Oppenheimer postdoctoral fellow at Los Alamos, added that “the intriguing concept of combining the perovskite nanocrystal into the MOF has been demonstrated in powder form, but this is the first time we have successfully integrated it. as an emission layer in an LED. “

Previous attempts to create nanocrystalline LEDs have been thwarted by the degradation of nanocrystals to the unwanted mass phase, losing their nanocrystals advantages and undermining their potential as practical LEDs. Bulk materials are made up of billions of atoms. Materials such as nano-phase perovskites are made up of groups of a few to a few thousand atoms, and therefore behave differently.

In their new approach, the research team stabilized the nanocrystals by fabricating them in the matrix of an MOF, like tennis balls caught in a chain-link fence. They used lead knots in the framework as a metal precursor and halide salts as organic material. The halide salt solution contains methylammonium bromide, which reacts with lead in the framework to assemble nanocrystals around the lead core trapped in the matrix. The matrix keeps the nanocrystals separate, so they don’t interact and degrade. This method is based on a solution coating approach, which is much less expensive than the vacuum processing used to create the inorganic LEDs widely used today.

MOF stabilized LEDs can be fabricated to create bright red, blue and green light, as well as varying shades of each.

“In this work, we demonstrated for the first time that perovskite nanocrystals stabilized in an MOF will create bright and stable LEDs in a range of colors,” said Wanyi Nie, scientist at the Center for Integrated Nanotechnologies at Los Alamos National Laboratory. . ? “We can create different colors, improve color purity, and increase the quantum yield of photoluminescence, which is a measure of a material’s ability to produce light.”

The research team used the Advanced Photon Source (APS), a user facility at the DOE Office of Science in Argonne, to perform time-resolved x-ray absorption spectroscopy, a technique that allowed them to spot changes in the perovskite material over time. The researchers were able to track electrical charges as they moved through the material and learned important information about what happens when light is emitted.

“We could only do this with the unique powerful x-ray pulses and unique timing structure of APS,” said Xiaoyi Zhang, group leader at Argonne’s x-ray science division. “We can track the location of charged particles inside tiny perovskite crystals. “

In durability testing, the material performed well under ultraviolet radiation, heat and electric field without degrading or losing its light detection and emission efficiency, a key condition for practical applications such as televisions and radiation detectors.

Reference: “Bright and stable light-emitting diodes made with perovskite nanocrystals stabilized in metallo-organic frames” by Hsinhan Tsai, Shreetu Shrestha, Rafael A. Vilá, Wenxiao Huang, Cunming Liu, Cheng-Hung Hou, Hsin-Hsiang Huang, Xiewen Wen, Mingxing Li, Gary Wiederrecht, Yi Cui, Mircea Cotlet, Xiaoyi Zhang, Xuedan Ma and Wanyi Nie, September 6, 2021, Photonics of nature.
DOI: 10.1038 / s41566-021-00857-0

Argonne researchers contributing to this work include Xuedan Ma, Gary Wiederrecht and Xiewen Wen of the CNM, and Xiaoyi Zhang and Cunming Liu from APS. Researchers from other institutions include Hsinhan Tsai, Shreetu Shrestha, Rafael A. Vilá, Wenxiao Huang, Cheng-Hung Hou, Hsin-Hsiang Huang, Mingxing Li, Yi Cui, Mircea Cotlet, and Wanyi Nie.