LEDs made from perovskite nanocrystals (green) embedded in a MOF can be created at low cost, using earth-abundant materials, and remain stable under typical working conditions. Image: Los Alamos National Laboratory.
LEDs made from perovskite nanocrystals (green) embedded in a MOF can be created at low cost, using earth-abundant materials, and remain stable under typical working conditions. Image: Los Alamos National Laboratory.

Light-emitting diodes (LEDs) are an unsung hero of the lighting industry. They run efficiently, give off little heat and last for a long time. Now scientists are looking at new materials to produce more efficient and longer-lived LEDs for use in consumer electronics, medicine and security.

In a paper in Nature Photonics, researchers from the US Department of Energy (DOE)'s Argonne National Laboratory, Brookhaven National Laboratory, Los Alamos National Laboratory and SLAC National Accelerator Laboratory report that they have prepared stable perovskite nanocrystals for such LEDs. Also contributing to the effort was Academia Sinica in Taiwan.

Perovskites are a class of material with a particular crystalline structure that gives them light-absorbing and light-emitting properties that are useful in a range of energy-efficient applications, including solar cells and various kinds of detectors.

Perovskite nanocrystals have been prime candidates as a new LED material but have proved unstable on testing. To address this issue, the researchers stabilized the nanocrystals in a porous structure called a metal-organic framework (MOF). Based on earth-abundant materials and fabricated at room temperature, these LEDs could one day lead to lower cost TVs and consumer electronics, as well as better gamma-ray imaging devices and even self-powered X-ray detectors with applications in medicine, security scanning and scientific research.

“We attacked the stability issue of perovskite materials by encapsulating them in MOF structures,” explained Xuedan Ma, a scientist in Argonne’s Center for Nanoscale Materials (CNM), a DOE Office of Science User Facility. “Our studies showed that this approach allows us to enhance the brightness and stability of the light-emitting nanocrystals substantially.”

“The intriguing concept of combining perovskite nanocrystal in MOF had been demonstrated in powder form, but this is the first time we successfully integrated it as the emission layer in an LED,” said Hsinhan Tsai, a former postdoc fellow at Los Alamos National Laboratory.

Previous attempts to create nanocrystal LEDs were thwarted by the nanocrystals' tendency to degrade back to the unwanted bulk phase, losing their nanocrystal advantages and undermining their potential as practical LEDs. Bulk materials consist of billions of atoms, but materials such as perovskites in the nano phase are made of groupings of just a few to a few thousand atoms, and thus behave differently.

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

The MOF-stabilized LEDs can be fabricated to create bright red, blue and green light, along with varying shades of each.

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

The research team used the Advanced Photon Source (APS), a DOE Office of Science User Facility at Argonne, to perform time-resolved X-ray absorption spectroscopy, a technique that allowed them to spot changes in the perovskite material over time. This meant the team could track electrical charges as they moved through the material and learn important information about what happens when light is emitted.

“We could only do this with the powerful single X-ray pulses and unique timing structure of the APS,” said Xiaoyi Zhang, group leader with Argonne’s X-ray Science Division. “We can follow where the charged particles were located inside the tiny perovskite crystals.”

In durability tests, the material performed well when exposed to ultraviolet radiation, heat and an electrical field, none of which caused it to degrade or lose its light-detecting and light-emitting efficiency, a key condition for practical applications such as TVs and radiation detectors.

This story is adapted from material from Argonne National Laboratory, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.