PORTLAND, Ore. Samsung and its university research partners claim to have overcome two of the last hurdles to large-scale printable electronics on plastic substrates: an n-type organic semiconductor material and a patterning method for separately fabricating nano- and micro-wire transistor channels.
Solutions to both problems, using self-aligning organic transistor channels with performance that rivals amorphous silicon, were demonstrated by Samsung Advanced Institute of Technology (Gyunggi-do, South Korea), Stanford University (Palo Alto, Calif.) and Sungkyunkwan University (Suwon, South Korea). The researchers claimed their technique is amenable to printable plastic electronics applications such as large-scale displays, sensor arrays, smart merchandise tags, flexible solar panels and digital paper.
"We studied both single wires and many wires between source and drain electrodes," said team leader and Stanford professor Zhenan Bao. "Single wires were studied to measure their charge carrier mobility, which is the largest of any reported n-type organic transistors."
|Organic transistor channels 1 micron wide and 100 microns in length were self-aligned over a prefabricated source and drain electrodes before being transfered to plastic substrates.|
The biggest drawback to widespread adoption of printable electronics has been a lack of matching n- and p-type transistors for power-saving complementary circuits--the organic equivalent of CMOS. P-type transistors are comparatively easy to fabricate with organic "inks" for printable electronics, but n-type transistors have not offered the same performance as p-type transistors, making CMOS-like circuits impractical.
According to the researchers, the solution was using an organic perylene-diimide derivative to fabricate pure single crystalline transistor channels with comparable performance to amorphous silicon. The trick was to precipitate wires in solution about 1 micron in width and 100 microns long. They were then aligned in parallel over prefabricated electrodes serving as the source and drain for transistors.
"The length of the wires was 100 microns or more, but their diameter ranged from just 10 nanometers to a few microns," said Bao. "Our best performance came from microwires about one micron in diameter and 100 microns long."
The hardest part of fabricating thin-film transistors from channels precipitated in solution is properly aligning them over the electrodes. The team's method combined fluid dynamics with liquid filtration techniques that deposited thin layers of semiconducting perylene-diimide wires over the correct electrode arrays. Then, the patterned transistors were transfered onto a flexible polymer substrate. The result was a circuit said to be as useful as amorphous silicon circuits but fabricated at room temperature and pressure.
"The most important thing is that we have shown a new fluidic method of flowing these wires from liquid suspension onto electrodes prefabricated on a substrate to make high performance, air-stable n-channel transistors," said Bao.
A reusable mask was made with openings wherever microwires were to be patterned above a device electrode. Suction ensured that the fluid containing the semiconducting microwires flowed into openings on the mask and over the correct electrodes, thereby self-aligning wires between source and drain. Once the mask was removed, the finished transistor circuits were transferred onto a polymer substrate.
"Our method is called filtration and transfer," said Bao. "When we filter the liquid, the suction force caused by filtration aligns the microwires as they flow through the mask."
Next, the researchers want to scale up to larger areas, then demonstrate how to allign complex circuits with different types of both organic and inorganic materials.
Funding for the research was provided by the National Science Foundation, Samsung, the Korea Research Foundation Fellowship and by a Sloan Research Fellowship.