Organic light-emitting diodes (OLEDs) are behind many high-quality flat screens, smartphones and smartwatches today. Now researchers have developed a new material with which such OLEDs can be produced more cost-effectively - through 3D printing. Unlike previous OLEDs, the “supramolecular ink” is not based on rare metals, but rather more common elements, as the chemists report in “Science”. The ink is also stable, flexible and extremely energy efficient. The material can therefore be used to print devices that are cheaper and use less energy than before.
Newer smartphones and flat-screen televisions are increasingly equipped with an OLED screen. These displays are lighter and thinner, use less energy and offer better picture quality than flat screens based on other technologies. This is made possible by the OLEDs (organic light-emitting diode) they contain – tiny organic molecules that emit light when excited. This means that no additional layer of material is required in the screens for backlighting, as is found in a liquid crystal display (LCD). The big disadvantage of OLEDs, however, is that they usually contain rare and therefore expensive metals such as iridium and their production is very energy-intensive. This makes their use unsustainable and expensive.
So-called ionic halide perovskites, which are made up of octahedron-shaped molecular clusters, are a possible alternative. The display industry has been working on the development of such metal and mineral-containing OLED materials for thin luminous layers for decades. However, so far they contain the element lead, which is harmful to the environment and health. In addition, these materials are not yet durable enough because they clump together over time, and they cannot display colors with short wavelengths such as green and blue well.
What is the new OLED ink made of?
A research team led by Cheng Zhu from the University of California (UC) in Berkeley has now developed a new, lead-free OLED material from the halide perovskite class. The powdered base materials contain either hafnium (Hf) or zirconium (Zr). Both metals are very stable and are more common on Earth, although not in huge quantities. Even at room temperature the powders ((18C6@K)2HfBr6 or (18C6@K)2ZrCl4Br2) can be dissolved in an organic solvent, as Zhu and his colleagues report. Tiny molecular building blocks within the ink organize themselves into stable, octahedron-shaped supramolecular structures that capture light.
Thanks to this structure, the resulting liquid “ink” functions like a semiconductor diode. When the “supramolecular ink” is excited with UV light, it even emits green and blue light with high efficiency, as spectroscopy images showed. The researchers speak of a “quantum efficiency close to one”. That means the ink has the “extraordinary ability to convert almost all of the absorbed light into visible light during the emission process,” Zhu explains. At the same time, the ink is flexible enough to be used in a 3D printer, as shown by small printed works of art made from the glowing material.
Thanks to these properties, the material can be used as a new, more cost-effective OLED class for high-quality devices that use less energy than previous devices in both production and use, the researchers conclude. “By replacing precious metals with materials more common on Earth, our supramolecular ink technology could transform the OLED display industry,” says senior author Peidong Yang of UC Berkeley.
Prototype demonstrates areas of application
To demonstrate the use of the new ink in OLED screens, the scientists made a prototype that contained a thin layer of the new material. In initial tests, the device achieved the desired properties in terms of colors, contrast and brightness of the screen, as Zhu and his colleagues report. They conclude that their novel ink can be used in a wide variety of electronic devices - from flat screen televisions and smartphones to wearables such as smartwatches or fitness trackers to high-tech clothing that glows in the dark. “The technology could also be used for organic printable films to make wearable devices as well as luminous art and sculptures,” Yang adds.
In further tests, he and his team now want to research whether and how well the new OLED material lights up even after being stimulated with electricity. “This step is important to understand the full potential of our material for producing efficient light-emitting devices,” says Zhu.
Source: Cheng Zhu (University of California Berkeley) et al., Science, doi: 10.1126/science.adi4196