Scientists use inkjet printers to make flexible LEDs with strong flexibility

At present, some mobile phone manufacturers have introduced folding screen phones, but what if you can roll up the phone screen and put it in your wallet? Or wear it on your wrist as a watch? The next step in digital displays being developed by the McKelvey School of Engineering at Washington University in St. Louis may make it a reality. Researchers in Wang Chuan’s laboratory, associate professor of Preston M. Green’s Department of System Engineering and Electrical Engineering, have developed a new material that has the advantages of both LED and OLED technologies, and has a new manufacturing method -Use an inkjet printer.

The research was published this month in the journal Advanced Materials.

OLEDs made of organic small molecules or polymer materials are cheap and flexible. Wang Chuan said: “You can bend or stretch them-but they have relatively low performance and short lifespan. Inorganic LEDs, such as microLEDs, are high-performance, super bright, very reliable, but not flexible, and very expensive.”

“What we do is an organic-inorganic compound,” he said. “It has the characteristics of these two worlds.” They used a special type of crystalline material called an organometal halide perovskite-although there was a new twist. The traditional method of creating a thin layer of perovskite is to drop it on a flat, rotating substrate, like a rotating art toy. This process is called spin coating. As the substrate rotates, the liquid spreads out, covering it with a thin layer. There, it can be recycled and made into perovskite LEDs, or PeLEDs.

However, just like rotating art, a lot of material is wasted in this process-when the substrate rotates at a speed of several thousand revolutions, some of the dripping perovskite splashes and flies away without sticking to the substrate.

Wang Chuan said: “Because it comes in liquid form, we imagine that we can use inkjet printers instead of spin coating.”

Inkjet manufacturing saves materials, because peroxides can be deposited only where needed, just like printing letters and numbers on a piece of paper with accuracy; there is no splashing, and waste is reduced. This process is also much faster, and the production time has been shortened from more than 5 hours to less than 25 minutes.

Another benefit of using inkjet printing methods has the potential to reshape the future of Electronic products: Perovskite can be printed on a variety of unconventional substrates, including those that are unstable when rotating-such as rubber. Wang Chuan said: “Imagine a device that is only the size of a mobile phone at the beginning, but it can be stretched to the size of a tablet.

However, in order to make the Display flexible, printing hard LEDs on rubber does not work. The LED itself needs to be flexible. Perovskite is not.

Researchers were able to solve this problem by embedding inorganic perovskite crystals in an organic polymer matrix made of a polymer binder. This makes peroxide and PeLED inherently elastic and stretchable. This process is not entirely simple. The researchers agree that the big obstacle is to ensure that the materials of the different layers do not mix.

Because all parts of the PeLED are made of liquid-the perovskite layer and the two electrodes and a buffer layer-a major problem is to keep all the layers unmixed.

The structure of the LED is similar to a sandwich, with at least one emitting layer, one anode layer and one cathode layer. Sometimes other layers may be used. Researchers must keep the perovskite layer from mixing with any other layers, just like applying a highlighter to newly written ink. They need to find a suitable polymer, a polymer that can be inserted between the peroxide and other layers, while protecting it without excessively disturbing the performance of the PeLED.

“We found the material and thickness to balance the performance and protection of the device,” the researcher said. After that, he went on to print the first flexible PeLEDs.

The university’s technology management office is applying for a patent for the technology and manufacturing method. These PeLEDs may be just the first step in the electronic revolution. They can be used to make wearable devices, even smart wearable devices, such as pulse oximeters that measure blood oxygen. Researchers believe that what is exciting is that being able to print flexible PeLEDs cheaply and quickly may bring new technologies that have not yet been dreamed of.

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