Perovskite nanocrystals stabilised for LEDs

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. US and Taiwanese researchers have now created stable perovskite nanocrystals for more efficient and longer-lived LEDs, with their results published in the journal Nature Photonics.

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 previous attempts to create nanocrystal LEDs were thwarted by the nanocrystals degrading back to the unwanted bulk phase, losing their 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.

Researchers from the US Department of Energy’s Argonne National Laboratory, Brookhaven National Laboratory, Los Alamos National Laboratory and SLAC National Accelerator Laboratory, along with Academia Sinica in Taiwan, stabilised the nanocrystals by fabricating them within the matrix of a porous structure called a metal–organic framework (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.

The team’s 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-stabilised LEDs can be fabricated to create bright red, blue and green light, along with varying shades of each.

“Our photophysical studies show that this approach allows us to enhance the brightness and stability of the light-emitting nanocrystals substantially,” said Xuedan Ma of Argonne National Laboratory.

​“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,” added Hsinhan Tsai, a former postdoc fellow at Los Alamos.

Based on earth-abundant materials and fabricated at room temperature, the 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. 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.

“In this work, we demonstrated for the first time that perovskite nanocrystals stabilised in a MOF will create bright, stable LEDs in a range of colours,” said corresponding author Wanyi Nie, from the Center for Integrated Nanotechnologies at Los Alamos. “Metal-halide perovskite nanocrystals offer a continuously tuneable optical bandgap covering almost all the visible spectrum, meaning we can create different colours, improved colour purity and high photoluminescence quantum yield.”

Image caption: LEDs made from perovskite nanocrystals (green) embedded in a metal–organic framework can be created at low cost, use earth-abundant materials, and remain stable under typical working conditions.

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