Stacking Turns Organic Transistors Up
Organic electronics appear to be, as the name might imply, quite good at interacting with a biological body and brain. Now scientists have created record-breaking, high-performance organic electronic devices using a potentially cheap, easy, and scalable approach that adopts a vertical architecture instead of a flat one, according to a new study.
Modern electronics rely on transistors, which are essentially switches that flick on and off to encode data as ones and zeros. Most transistors are made of inorganic semiconductors, but organic electronics depend on organic compounds. Whereas organic field-effect transistors (OFETs) have ions that accumulate only on the surface of the organic material, organic electrochemical transistors (OECTs) rely on ions flowing in and out of organic semiconductors. This feature helps make OECTs efficient switches and powerful amplifiers.
“Our vertically stacked electrochemical transistor takes performance to a totally new level.”
—Tobin Marks, Northwestern University
Organic electronics have a number of advantages over their standard counterparts, such as flexibility, low weight, and easy, cheap fabrication. The way in which OECTs communicate—using ions, just as biology does—may also open up applications such as biomedical sensing, body-machine interfaces, and brain-imitating neuromorphic technology. In addition, previous research found that OECTs can possess exceptionally low driving voltages of less than 1 volt, low power consumption of less than 1 microwatt, and high transconductances—a measure of how well they can amplify signals—of more than 10 millisiemens.
However, previous research into OECTs was hindered by problems such as slow speeds and poor stability during operation. Until now, the best OECTs could achieve switching speeds of roughly 1 kilohertz and a stability on the order of 5,000 cycles of switching. In addition, manufacturing these devices often required complex, expensive fabrication techniques as well as channel lengths—the distance between the source and the drain electrodes—that were at least 10 micrometers long.
Now scientists have developed OECTs with switching speeds greater than 1 kHz, a stability across more than 50,000 cycles, channel lengths of less than 100 nanometers, as well as transconductances of 200 to 400 mS—figures that are the highest seen yet in OECTs. The key to this advance is a vertical architecture in which these devices are built like sandwiches, instead of the flat architecture seen with most previous OECTs and conventional transistors (in which they are laid out like street maps).
“Our vertically stacked electrochemical transistor takes performance to a totally new level,” says study cosenior author Tobin Marks, a materials chemist at Northwestern University in Evanston, Ill.
OECTs have three electrodes—a source and drain electrode connected by a thin film, or channel, of an organic semiconductor, plus a gate electrode connected to an electrolyte material that covers the channel. Applying a voltage to the gate electrode causes ions in the electrolyte to flow into the channel, altering the current passing between the source and drain electrodes.
In the new study, the researchers sandwiched the channel between two gold electrodes—the source on the bottom and the drain on top, with neither electrode completely covering the channel. The channel was made of a semiconducting ion-permeable compound mixed with another polymer that helped make the channel structurally robust and more stable during operation. The electrolyte lay on top of both the channel and the drain electrode.
The scientists noted they could fabricate these vertical OECTs in a simple and scalable way using standard manufacturing techniques. The vertical architecture also means these devices can be stacked on top of each other to achieve high circuit density, they say. The gate can also readily be modified—say, with biomolecules designed to latch onto specific molecules—to help serve as a sensor, says study coauthor Jonathan Rivnay, a materials scientist and biomedical engineer at Northwestern University.
In addition, the scientists could make the channel using either an n-type semiconductor, which carries negative charges in the form of electrons, or a p-type semiconductor, which carries positive charges in the form of holes. Previously, high-performance n-type OECTs, which are crucial for sensors and logic circuits, have proven difficult to build. In the new study, the research team’s vertical n-type OECTs outperformed any previous n– and p-type OECTs when used in complementary logic circuits that use both n– and p-type OECTs. (This work also marked the first vertically stacked complementary OECT logic circuits.)
The researchers are now exploring how to modify the materials and fabrication techniques used to make the vertical OECTs to further boost their speed and stability, Marks says.
The scientists detailed their findings in the 19 January issue of the journal Nature.