A new, 3D-printable material that is a highly efficient emitter could lead to cheaper, more sustainable manufacturing processes for OLED devices. The material, called supramolecular ink, demonstrated the ability to convert nearly all absorbed light into visible light during the emission process. Although OLEDs are lighter, thinner, and more energy-efficient than other flat-panel technologies and provide a higher-quality image, they often contain rare, expensive metals such as iridium. Supramolecular ink, which is made of inexpensive, Earth-abundant elements instead of costly, scarce metals, could enable more affordable, environmentally sustainable OLED displays and electronic devices. These 2-cm-high 3D-printed objects were fabricated from supramolecular ink that emits blue or white light. Courtesy of Jenny Nuss/Berkeley Lab and Science. “By replacing precious metals with Earth-abundant materials, our supramolecular ink technology could be a game changer for the OLED display industry,” said Peidong Yang, a scientist in Lawrence Berkeley National Laboratory's (Berkeley Lab's) Materials Sciences Division and professor at the University of California, Berkeley (UC Berkeley). Developed by researchers at Berkeley Lab, supramolecular ink consists of powders containing hafnium and zirconium, which can be mixed in solution at room temperature or up to around 176 °F, to form a semiconductor ink consisting of blue- and green-emitting compounds. The ink contains tiny, molecular structures that act as building blocks that self-assemble in solution through a process the team calls supramolecular assembly. “Our approach can be compared to building with LEGO blocks,” said co-first author Cheng Zhu, a PhD candidate at UC Berkeley. The supramolecular structures enable the ink to achieve stable, high-purity synthesis at low temperatures. A single crystal x-ray diffraction image of blue-emitting supramolecular ink (18C6@K)2HfBr6) reveals the atomic structure of a 1- to 2-nm unit cell. The tiny, molecular “building block” structures within the ink self-assemble in solution, enabling the material to achieve stable and high-purity synthesis at low temperatures. Courtesy of Peidong Yang and Cheng Zhu/Berkeley Lab and Science. In spectroscopy experiments performed at UC Berkeley, the supramolecular ink composites were shown to be highly efficient emitters of blue and green light, exhibiting near-unity quantum efficiency. A hafnium powder composite exhibited about 96% photoluminescence quantum yield for blue emission, and a zirconium analog demonstrated a photoluminescence quantum yield of about 83% for green emission. The researchers added a polymer to create solution-processable inks for printing luminescent thin films and structures. The highly emissive powders maintained a high photoluminescence quantum yield in solution-processable semiconductor inks under ambient conditions. To demonstrate the material’s color tunability and luminescence as an OLED emitter, the researchers fabricated a thin-film display prototype from the composite ink and demonstrated that the material was suitable for programmable electronic displays. Through additional experiments conducted at UC Berkeley, the researchers demonstrated the use of supramolecular inks to achieve emissive 3D-printed architectures with high spatial resolution. The ink’s compatibility with 3D-printing technologies could also make it useful for designing decorative OLED lighting. In addition to its potential role as an energy-efficient OLED emitter for electronic displays and 3D printing, the supramolecular ink could be used to fabricate high-tech clothing that illuminates individuals in low-light conditions, and wearable devices that display information through the supramolecular light-emitting structures. “The technology could also extend its reach to organic printable films for the fabrication of wearable devices as well as luminescent art and sculpture,” Yang said. Eiffel Tower-shaped luminescent structures, 3D-printed from supramolecular ink. Each 2-cm-high device is fabricated from supramolecular ink that emits blue or green light when exposed to 254-nm ultraviolet light. Courtesy of Peidong Yang and Cheng Zhu/Berkeley Lab and Science. The supramolecular ink also could help speed the commercialization of ionic halide perovskites for the display industry. Ionic halide perovskites are a thin-film solar material that can synthesize in solution at low temperatures. Ionic halide perovskites could lower the cost of display manufacturing processes, but high-performance halide perovskites contain lead. Supramolecular ink, which belongs to the ionic halide perovskite family, is based on a high-performance, lead-free formulation that is safe for the environment and public health. Supramolecular ink compounds are stable and have a long shelf life. Now that the team has established the supramolecular ink’s potential in OLED thin films and 3D-printable electronics, it has begun to explore the material’s electroluminescent potential. “This involves a focused and specialized investigation into how well our materials can emit light using electrical excitation,” Zhu said. “This step is essential to understanding our material’s full potential for creating efficient light-emitting devices.” The research was published in Science (www.doi.org/10.1126/science.adi4196).
