Eiffel Tower-shaped luminescent structures 3D-printed from supramolecular ink. Each 2-centimeter-tall device is fabricated from supramolecular ink that emits blue or green light when exposed to 254-nanometer ultraviolet light. (Image: Peidong Yang and Cheng Zhu/Berkeley Lab. Courtesy of Science)

A research team led by Lawrence Berkeley National Laboratory (Berkeley Lab) has developed “supramolecular ink,” a new technology for use in OLED (organic light-emitting diode) displays or other electronic devices. Made of inexpensive, Earth-abundant elements instead of costly scarce metals, supramolecular ink could enable more affordable and environmentally sustainable flat-panel screens and electronic devices.

“By replacing precious metals with Earth-abundant materials, our supramolecular ink technology could be a game changer for the OLED display industry,” said principal investigator Peidong Yang, a Faculty Senior Scientist in Berkeley Lab’s Materials Sciences Division and Professor of Chemistry and Materials Science and Engineering at UC Berkeley. “What’s even more exciting is that the technology could also extend its reach to organic printable films for the fabrication of wearable devices as well as luminescent art and sculpture,” he added.

If you have a relatively new smartphone or flat panel TV, there’s a good chance it features an OLED screen. OLEDs are rapidly expanding in the display market because they are lighter, thinner, use less energy, and have better picture quality than other flat-panel technologies. That’s because OLEDs contain tiny organic molecules that emit light directly, eliminating the need for the extra backlight layer that is found in a liquid crystal display (LCD). However, OLEDs can include rare, expensive metals like iridium.

But with the new material — which the Berkeley Lab team recently described in a new study published in the journal Science — electronics display manufacturers could potentially adopt a cheaper fabrication process that also requires far less energy than conventional methods.

The new material consists of powders containing hafnium (Hf) and zirconium (Zr) that can be mixed in solution at low temperatures — from room temperature up to around 176 °F (80 °C) — to form a semiconductor “ink.”

Tiny molecular “building block” structures within the ink self-assemble in solution — a process that the researchers call supramolecular assembly. “Our approach can be compared to building with LEGO blocks,” said Cheng Zhu, the co-first author on the paper and a Ph.D. candidate in materials science and engineering at UC Berkeley.

These supramolecular structures enable the material to achieve stable and high-purity synthesis at low temperatures, explained Zhu. He developed the material while working as a research affiliate in Berkeley Lab’s Materials Sciences Division and graduate student researcher in the Peidong Yang group at Berkeley Lab and UC Berkeley.

Spectroscopy experiments at UC Berkeley revealed that the supramolecular ink composites are highly efficient emitters of blue and green light — two signifiers of the material’s potential application as an energy-efficient OLED emitter in electronic displays and 3D printing.

Subsequent optical experiments revealed that the blue- and green-emitting supramolecular ink compounds exhibit what scientists call near-unity quantum efficiency. “This demonstrates their exceptional ability to convert nearly all absorbed light into visible light during the emission process,” Zhu explained.

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. In an exciting result, they found that the material is suitable for programmable electronic displays.

“The alphabet movie serves as a compelling example that illustrates the application of emissive thin films like supramolecular ink in the creation of fast-switching displays,” said Zhu.

Additional experiments at UC Berkeley showed that the supramolecular ink is also compatible with 3D printing technologies such as for the design of decorative OLED lighting.

Zhu added that manufacturers could also use the supramolecular ink to fabricate wearable devices or high-tech clothing that illuminates for safety in low-light conditions, or wearable devices that display information through the supramolecular light-emitting structures.

With their demonstrated stability and shelf life, the supramolecular ink compounds could also help in the commercial advancement of ionic halide perovskites, a thin-film solar material that the display industry has been eyeing for decades. With their low-temperature synthesis in solution, ionic halide perovskites could potentially enable cheaper manufacturing processes for the manufacturing of displays. But high-performance halide perovskites contain the element lead, which is concerning for the environment and public health. In contrast, the new supramolecular ink — which belongs to the ionic halide perovskite family — offers a lead-free formulation without compromising performance.

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