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 make more efficient and longer-lived LEDs with applications in consumer electronics, medicine and security.
Researchers from the U.S. Department of Energy’s (DOE) 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.
“Our studies showed that this approach allows us to enhance the brightness and stability of the light-emitting nanocrystals substantially.” — Xuedan Ma, scientist in Argonne’s Center for Nanoscale Materials
Perovskites are a class of material that share a particular crystalline structure giving 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. The research team stabilized the nanocrystals in a porous structure called a metal-organic framework, or MOF for short. Based on earth-abundant materials and fabricated at room temperature, these LEDs could one day enable 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,” said Xuedan Ma, 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.”
Hsinhan Tsai, a former J. R. Oppenheimer postdoc fellow at Los Alamos, added that, “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.”
Previous attempts to create nanocrystal LEDs were thwarted by the nanocrystals degrading back to the unwanted bulk phase, losing their nanocrystal advantages and undermining their potential as practical LEDs. Bulk materials consist of billions of atoms. 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 in the framework as the metal precursor and halide salts as the organic material. The solution of halide salts 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 separated, so they don’t interact and degrade. This method is based on a solution coating approach, far less expensive than the vacuum processing used to create the inorganic LEDs in wide 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, 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 the changes in the perovskite material over time. 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 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 under ultraviolet radiation, in heat and in an electrical field without degrading and losing its light-detecting and light-emitting efficiency, a key condition for practical applications such as TVs and radiation detectors.
This research appeared in Nature Photonics in a paper entitled, “Bright and stable light emitting diodes made with perovskite nanocrystals stabilized in metal-organic frameworks.” Argonne researchers contributing to this work include Xuedan Ma, Gary Wiederrecht and Xiewen Wen from the CNM, and Xiaoyi Zhang and Cunming Liu from the 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.
The DOE Office of Basic Energy Sciences funded the Argonne portion of this work. Part of this research used sector 8-ID-E and 11-ID-D of the Advanced Photon Source.
This press release was adapted from one prepared at Los Alamos National Laboratory.
About Argonne’s Center for Nanoscale Materials
The Center for Nanoscale Materials is one of the five DOE Nanoscale Science Research Centers, premier national user facilities for interdisciplinary research at the nanoscale supported by the DOE Office of Science. Together the NSRCs comprise a suite of complementary facilities that provide researchers with state-of-the-art capabilities to fabricate, process, characterize and model nanoscale materials, and constitute the largest infrastructure investment of the National Nanotechnology Initiative. The NSRCs are located at DOE’s Argonne, Brookhaven, Lawrence Berkeley, Oak Ridge, Sandia and Los Alamos National Laboratories. For more information about the DOE NSRCs, please visit https://science.osti.gov/User-Facilities/User-Facilities-at-a-Glance.
About the Advanced Photon Source
The U. S. Department of Energy Office of Science’s Advanced Photon Source (APS) at Argonne National Laboratory is one of the world’s most productive X-ray light source facilities. The APS provides high-brightness X-ray beams to a diverse community of researchers in materials science, chemistry, condensed matter physics, the life and environmental sciences, and applied research. These X-rays are ideally suited for explorations of materials and biological structures; elemental distribution; chemical, magnetic, electronic states; and a wide range of technologically important engineering systems from batteries to fuel injector sprays, all of which are the foundations of our nation’s economic, technological, and physical well-being. Each year, more than 5,000 researchers use the APS to produce over 2,000 publications detailing impactful discoveries, and solve more vital biological protein structures than users of any other X-ray light source research facility. APS scientists and engineers innovate technology that is at the heart of advancing accelerator and light-source operations. This includes the insertion devices that produce extreme-brightness X-rays prized by researchers, lenses that focus the X-rays down to a few nanometers, instrumentation that maximizes the way the X-rays interact with samples being studied, and software that gathers and manages the massive quantity of data resulting from discovery research at the APS.
This research used resources of the Advanced Photon Source, a U.S. DOE Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357.
Argonne National Laboratory seeks solutions to pressing national problems in science and technology. The nation’s first national laboratory, Argonne conducts leading-edge basic and applied scientific research in virtually every scientific discipline. Argonne researchers work closely with researchers from hundreds of companies, universities, and federal, state and municipal agencies to help them solve their specific problems, advance America’s scientific leadership and prepare the nation for a better future. With employees from more than 60 nations, Argonne is managed by UChicago Argonne, LLC for the U.S. Department of Energy’s Office of Science.
The U.S. Department of Energy’s Office of Science is the single largest supporter of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time. For more information, visit https://energy.gov/science.