Researchers from Florida State University and Washington University in St. Louis have developed a new material for displays and a novel way to fabricate it—using an inkjet printer. The team used organometal halide perovskites — with a novel twist.
The traditional way to create a thin layer of perovskites, which is in liquid form, is to drip it onto a flat, spinning substrate, in a process known as spin coating. As the substrate spins, the liquid spreads out, eventually covering it in a thin layer. From there, it can be recovered and made into perovskite LEDs, or PeLEDs. A lot of material, however, is wasted in that process—as the substrate spins at several thousand RPM, some of the dripping perovskite splatters and flies away, not sticking to the substrate. The researchers substituted this process with one based on an inkjet printer.
Inkjet fabrication saves materials, as the perovskite can be deposited only where it's needed. The process is much faster as well, cutting fabrication time from more than five hours to less than 25 minutes. Another benefit of using the inkjet printing method: perovskite can be printed onto a variety of unconventional substrates, including those that wouldn't lend themselves to stability while spinning—materials such as rubber.
For a display to be flexible, however, printing stiff LEDs on rubber won't do the trick. The LEDs themselves need to be flexible. Perovskite is not.
First author Junyi Zhao, was able to solve the problem by embedding the inorganic perovskite crystals into an organic, polymer matrix made of polymer binders. This made the perovskite and, by association, the PeLEDs, themselves, elastic and stretchable in nature.
The process wasn't exactly straightforward. It took a long time before getting it right. Zhao and Chuan Wang, assistant professor in the Preston M. Green Department of Electrical & Systems Engineering, agreed that the biggest roadblock was making sure the different layers of material didn't mix.
Because all parts of the PeLED were made from liquid—the perovskite layer as well as the two electrodes and a buffer layer—a major concern was keeping all of the layers from mixing.
LEDs are constructed in a sandwich-like configuration, with at least an emissive layer, an anode layer and a cathode layer. Additional layers such as electron and hole transporting layers may sometimes also be used. Zhao had to keep the perovskite layer safe from mixing with any of the others, the way running a highlighter over freshly written ink might smear it.
He needed to find a suitable polymer, one that could be inserted between the perovskite and the other layers, protecting it from them while not interfering too much with the PeLED's performance.
"We found the best material and best thickness to balance performance and protection of the device," Zhao said. After that, he went on to print the first stretchy PeLEDs.
The team envisions these PeLEDs as possibly being just the first step in an electronics revolution: Walls could provide lighting or even display the day's newspaper. They can be used to make wearable devices, even smart wearables, like a pulse oximeter to measure blood oxygen.