Researchers have developed a process to make light-emitting diodes by spraying a substrate with quantum dots (ACS Photonics 2017, DOI: 10.1021/acsphotonics.7b00216). The quantum-dot LEDs (QLEDs) are 100 times as bright and efficient as similar devices, the researchers say. The process could someday be used to mass-produce inexpensive, vibrant, and flexible displays.
Quantum dots are semiconductor nanocrystals that emit a single wavelength when stimulated by electricity or light. This pure color can be tuned by changing the size or composition of the nanocrystals. In some electronic displays on the market today, quantum dots convert blue light from a conventional LED into red and green light, needed for a full-color display. But engineers want to make true QLED displays in which quantum dots at each pixel are electrically triggered to emit each of those three colors. Such displays should be rich, bright, and energy efficient. The question is how to make QLEDs on large areas.
A QLED contains quantum dots sandwiched between a layer that transports negatively charged electrons and one that transports positively charged holes. To make these various layers, researchers coat glass or plastic substrates with layers of charge-ferrying organic polymers and quantum dots using vacuum deposition or spin-coating. But the vacuum method is expensive and complicated, and it is difficult to make high-quality films over large areas with spin-coating. “These are not suitable for industry-scale production,” says Wenfa Xie, a professor of electronic science & engineering at Jilin University.
Spraying is a much simpler and cheaper way to coat large areas, including flexible plastic sheets. So Xie, Hanzhuang Zhang, and their colleagues created prototype 5-mm2 QLEDs by spraying a series of three nanoparticle solutions onto a glass substrate, depositing one layer at a time. For this, they used an ultrasonic spray machine, which vibrates to break down a liquid into tiny droplets and releases them from a nozzle as an ultrafine spray.
The quantum dots were made of cadmium selenide-cadmium sulfide cores and zinc sulfide shells, which glow green when excited with electricity. Xie and his colleagues chose inorganic metal oxides as charge-transport layers instead of organic polymers because organic materials don’t last long under air and humidity and require expensive encapsulation. The team used zinc oxide nanoparticles for the electron-transporting layer and nickel oxide nanoparticles for the hole-transporting layer. To boost efficiency, Xie and his colleagues sprayed a layer of aluminum oxide between the nickel oxide and quantum dot layers, which helps to quash charge-trapping defect sites in the nickel oxide.
The team produced green QLEDs with an efficiency of 20.5 candelas per ampere and a brightness of more than 20,000 cd/m2. These numbers are two orders of magnitude greater than previously reported QLEDs made with all inorganic materials, Xie says. The process could easily be used to make QLEDs of other colors.
But the devices still need to catch up in brightness and lifetime with organic polymer-based QLEDs, which can be twice as efficient and last 10 times as long. Xie says that his team is now trying to improve both traits by engineering better quantum-dot and hole-transport layers.
Paul Holloway, a materials science & engineering professor at the University of Florida, says the spray process is unique for making QLEDs. Manufacturers would still need to figure out how to pattern the QLEDs to make pixels for displays, he adds, but there are many ways to do that.