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    Stretchy, Flexible LEDs – Made With an Inkjet Printer

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    Junyi Zhao, a PhD candidate in Chuan Wang’s lab, used flexible PeLED to show off the spirit of a small school. Credits: Wustl / Wang Lab

    Sure, you can hinge the two screens together and call your phone “foldable,” but what if you could roll it up and put it in your wallet? Or do you stretch it around your wrist and wear it as a watch?

    The next step in digital displays being developed at the McKelby Institute of Technology at Washington University in St. Louis could make that happen.

    First, there were light emitting diodes, or LEDs. Next, organic LEDs, or OLEDs. Researchers in Chuan Wang’s lab, now an assistant professor at Preston M. Green’s Department of Electrical Systems Engineering, are using inkjet printers to develop new materials with the strengths of both technologies and new ways to manufacture them. bottom.

    The study was published in the journal this month Advanced material..

    Organic LEDs made of small organic molecule or polymer materials are cheap and flexible. “It can be bent and stretched, but it has relatively low performance and a short lifespan,” says Wang. “Inorganic LEDs such as microLEDs are high performance, very bright and very reliable, but inflexible and not very expensive.”

    “We made organic and inorganic compounds,” he said. “It has the best of both worlds.”

    Flexible PeLED in operation.Credit: Wanglab video courtesy

    They used a specific type of crystalline material called the organometallic halide perovskite, but with a new twist. The traditional method of creating a thin layer of liquid perovskite is to drop it onto a flat, rotating substrate, such as a spin art toy, in a process called spin coating. As the substrate rotates, the liquid spreads and eventually becomes covered with a thin layer.

    You can retrieve it from there and turn it into a perovskite LED or PeLED.

    However, like spin art, the process wastes a lot of material. When the board rotates at thousands of RPMs, some of the dripping perovskite scatters without sticking to the board.

    “I imagined that we could use an inkjet printer instead of spin coating because it comes in liquid form,” Wang said.

    Inkjet manufacturing allows perovskite to be deposited only where it is needed, saving material as well as printing letters and numbers on paper. There is no splattering and there is little waste. This process is also much faster, reducing manufacturing time from more than 5 hours to less than 25 minutes.

    Another advantage of using inkjet printing is that it has the potential to change the future of electronics. Perovskite can be printed on a variety of unconventional substrates, including those that do not contribute to rotational stability (such as rubber).

    “Imagine a device that initially fits the size of a cell phone, but can grow to the size of a tablet,” Wang said.

    However, printing a hard LED on the rubber to make the display flexible does not work. The LED itself needs to be flexible. Perovskite is not.

    Junyi Zhao, the lead author of a PhD candidate in Wang’s lab, was able to solve the problem by embedding inorganic perovskite crystals in an organic polymer matrix made of polymer binder. This makes the perovskite and the associated PeLED itself elastic and stretchable in nature.

    The best of both worlds.

    The process wasn’t exactly simple. It took a long number of days and nights in the lab to do it right. Wang and Zhao agreed that the biggest obstacle was to keep different layers of material from mixing.

    Since all parts of the PeLED (perovskite layer and two electrodes and buffer layer) are made of liquid, the main concern was to prevent all layers from mixing.

    The LED is constructed in a sandwich-like configuration with at least a light emitting layer, an anode layer, and a cathode layer. Additional layers, such as electron and hole transport layers, may also be used from time to time. Zhao needed to keep the perovskite layer safe so that it would not mix with other layers. Running a highlighter on freshly written ink can stain the perovskite layer.

    He needed to find suitable polymers that could be inserted between the perovskite and the other layers and protected them from them without significantly impairing the performance of PeLED.

    “We have found the right material and thickness to balance device performance and protection,” says Zhao. Then he printed the first elastic PeLED.

    The university’s technical management office is applying for patents on technology and manufacturing methods.

    These PeLEDs may be just the first step in the electronics revolution. The walls can provide lighting or even display the newspaper of the day. They can be used to create wearable devices such as pulse oximeters that measure blood oxygen, and even smart wearables.

    Most excitingly, the ability to print elastic, flexible PeLEDs cheaply and quickly can lead to new technologies that have yet to be dreamed of.

    Reference: “High-speed production of all-inkjet-printed organic metal halide perovskite light-emitting diodes on elastic substrates”, Junyi Zhao, Li-Wei Lo, Haochuan Wan, Pengsu Mao, Zhibin Yu, Chuan Wang, October 8, 2021 Day Advanced material..
    DOI: 10.1002 / adma.202102095

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