Printing Innovations Improve Organic Electronics

A printing technique that focuses on the physics of the process rather than the chemical makeup of the semiconductor provides a tenfold improvement in organic electronics. The findings could yield flexible electronic displays, tiny sensors and lightweight, low-cost solar cells.

Organic electronics hold great potential for a variety of applications, but even the highest-quality films available today fall short in how well they conduct electrical current. Now researchers from SLAC National Accelerator Laboratory and Stanford University have devised a printing process called Fluence (fluid-enhanced crystal engineering) that for some materials results in thin films capable of conducting electricity 10 times more efficiently than those created using conventional methods.

“Even better, most of the concepts behind Fluence can scale up to meet industry requirements,” said Ying Diao, a SLAC/Stanford postdoctoral researcher and lead author of the study, which appeared in Nature Materials (doi: 10.1038/nmat3650).

An array of 1-mm-wide by 2-cm-long single-crystal organic semiconductors. The neatly aligned blue strips are what provide greater electric charge mobility. The Stanford logo, shown here, is the same size as a dime. Images courtesy of Y. Diao et al.

Diao engineered the process to produce strips of large, neatly aligned crystals that enable the flow of an electrical charge while preserving the benefits of the “strained lattice” structure and “solution shearing” printing technique developed previously in the lab of professor Zhenan Bao of the Stanford Institute for Materials and Energy Sciences, a joint SLAC-Stanford institute. She focused on controlling the flow of the liquid in which the organic material is dissolved.

“It's a vital piece of the puzzle,” she said. If the ink flow does not distribute evenly, as is often the case during fast printing, the semiconducting crystals will be riddled with defects. “But in this field there's been little research done on controlling fluid flow.”

The scanning electron micrograph shows the micropillars embedded in the shearing blade used in the SLAC printing process. The pillars are 35 × 42 µm and mix the organic semiconductor solution, ensuring that it’s evenly deposited.

Diao designed a printing blade with micropillars embedded in it that mix the ink so that a uniform film is created. She also engineered a way around another hurdle: the tendency of crystals to form randomly across the substrate. A series of chemical patterns on the substrate suppress the formation of disordered crystals that would otherwise grow out of alignment with the printing direction. The result is a film of large, well-aligned crystals.

X-ray studies of the organic semiconductors at the Stanford Synchrotron Radiation Lightsource enabled the investigators to inspect their progress and continue to make improvements, eventually showing neatly arranged crystals at least 10 times longer than crystals created with other solution-based techniques, and of much greater structural perfection.

A cross-polarized micrograph comparing a sample of an organic semiconducting film created without micropillars (top) and with micropillars (bottom) at scales of both 1 mm and 50 µm. Note the uniformity of the crystals in the bottom image compared with the top image.

The experiment was repeated using a second organic semiconductor material with a significantly different molecular structure, which also showed a notable improvement in the quality of the film — a sign that the techniques will work across a variety of materials, the researchers say.

Next, the investigators hope to pin down the underlying relationship between the material and the process that enabled their results. Such a discovery, they say, could provide an unprecedented degree of control over the electronic properties of printed films.

“That could lead to a revolutionary advance in organic electronics,” Bao said. “We've been making excellent progress, but I think we're only just scratching the surface.”

For more information, visit: www.slac.stanford.edu