Organic wires pave way for bendy electronics

STANFORD (US)—Scientists are zeroing in on new low-cost way to produce flexible electronics, paving the way for advances in solar panels, memory devices, and digital paper. The key may be making transistors out of high-performance organic microwires, according to engineers at Stanford University and Samsung.

For years, scientists have been working to create flexible electronics based on inexpensive organic materials. These materials can be cheaper than silicon and metal materials (albeit slower in performance), and amenable to cheaper manufacturing processes such as roll-to-roll printing of photovoltaic cells. They also are more compatible with flexible substrates, such as plastics.

“In our process we can create organic semiconducting microwires with the most desirable electronic properties, flow a dispersed solution of them into a stencil, or mask, and then stamp them onto a pattern of electrodes,” says Zhenan Bao, a Stanford associate professor of chemical engineering and a senior author of the paper. “Because these wires can be precisely aligned with high density, the result is high-performance transistors.”

Although the research alone is not enough to enable economical mass production of low-cost, high-performance flexible electronics, it could make their eventual manufacturing more feasible, says Jong Min Kim, a Samsung Fellow and senior vice president and a coauthor of the paper.

“This technology can be applied to printable electronics such as low-cost and large-area display device components, radio frequency ID tags, sensors, memory devices, and many different types of energy devices,” Kim adds.

In electronics, transistors act as switches. The team reported measurements showing that in their “on” state—when they transmit current—the group’s dense microwire transistors operated about two-and-a-half times more quickly than the organic transistors most other research groups have announced to date. The transistors also transmit more current. In a flexible electronic display, faster operation results in blur-free motion, and higher current yields a brighter picture.

The performance improvements come from three factors, Bao says: the inherently fast conductivity of the single crystalline microwires, the new alignment method they developed, and the ability to pack a high density of wires onto the electrodes. Because almost all of the wires span the electrodes, a large number of them make the connection, ensuring that more current gets across.

The Stanford-Samsung team’s transistors are also among the best of a rare breed of organic “n-type” transistors, which transmit negative charges. They are just as necessary as more common “p-type” transistors for making integrated circuits, but have been harder to build.

In addition, the microwires are formulated to be “air stable,” which means their electrical properties are not spoiled by exposure to oxygen, as are many n-type organic transistors.

Because the process depends only on a stencil to align and concentrate the wires, the team was able to create patterns in which wires could be aligned in different directions in different places, a necessary capability for producing complex circuit designs. Also, Bao says, the team fabricated transistors over an area of several square centimeters, which suggests that patterning a large area could be feasible. a key goal for future work.

Funding sources include the U.S. National Science Foundation, Samsung, a Korea Research Foundation Fellowship, and a Sloan Research Fellowship.

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