SEOUL, November 06 (AJP) - South Korean chemists have found a way to build more efficient perovskite light-emitting diodes without a separate chemical treatment step that the technology has long needed, Kookmin University said Friday.
The new light-emitting diodes (LEDs) turned about 11 of every 100 electrons sent through them into light that escaped the device. That external quantum efficiency of 11.4 percent is higher than 2.7 percent for devices built from conventionally made crystals. Peak brightness rose more than fivefold, from 59 to 314 candelas per square meter.
Perovskite nanocrystals are tiny crystals that glow brightly in pure colors and can be handled in liquid form, much like ink. These traits make them a leading candidate material for next-generation display LEDs.
The crystals have a built-in drawback. Standard synthesis leaves their surfaces covered in organic molecules with long carbon chains, called ligands, which hinder electric charge from moving from one crystal to the next.
To build a high-performance device, researchers have generally had to strip the long ligands away after synthesis and swap in shorter ones. The process is known as post-synthetic ligand exchange.
The Kookmin team, working with researchers from Sungkyunkwan University and Hanyang University, controlled the crystals' surface chemistry during synthesis instead. The team used a heat-up method built on a type of chemical reaction known as SN2, in which one molecule displaces part of another.
The method produced cesium lead bromide (CsPbBr3) nanocrystals carrying far less oleate, the long-chain ligand that normally coats them. Bromide ions took over much of the job of stabilizing the surface.
Through experiments and theoretical calculations, the researchers showed that pairing bromide with oleylamine, another surface molecule, formed more stable bonds than the conventional pairing of oleylamine with oleate. As a result, the crystals suppressed surface defects and kept strong optical properties even with fewer organic molecules on them.
Films made from the new crystals showed less variation in energy levels and more orderly crystal packing than films of conventionally made crystals. Charge also moved between crystals far more easily.
The team placed the crystals directly into the LEDs' light-emitting layer, skipping ligand exchange entirely. The range of wavelengths in the emitted light narrowed from 21 nanometers to 17 nanometers, a sign of purer color.
Kookmin said that controlling ligand density and surface stabilization at the synthesis stage simplifies device fabrication and could widen the approach's use to other devices that convert between light and electricity.
The results come from laboratory devices. Their peak brightness of 314 candelas per square meter falls below what many smartphone screens reach. The university did not report how long the devices keep working, a basic test for any display material.
Lee Yu-bin, a Kookmin chemistry master's student who has completed coursework, served as co-first author with Kookmin researcher Kim Ji-geon and Seok Gyeong-eun, a researcher at Sungkyunkwan University (SKKU). Lee worked on analyzing the crystals' surface properties and applying them to devices, the university said.
Kookmin chemistry professor Kim Young-hoon, SKKU professor Lee Bo-ram and Hanyang University professor Lee Tae-kyung served as co-corresponding authors.
The study is set to be published in Advanced Functional Materials, a materials science journal with an impact factor of 19.9 that ranks in the top 5 percent of its field by Journal Citation Reports.
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