A new material shows promise for the next generation of organic electronics

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For decades, field-effect transistors powered by silicon-based semiconductors have propelled the electronics revolution. But in recent years, manufacturers have come up against strict physical limits to further reduce the size and gain efficiency of silicon chips. That’s why scientists and engineers are looking for alternatives to conventional metal-oxide-semiconductor (CMOS) transistors.

“Organic semiconductors offer several distinct advantages over conventional silicon-based semiconductor devices: they are made from abundantly available elements, such as carbon, hydrogen, and nitrogen; they offer mechanical flexibility and low manufacturing cost; and they can be fabricated easily on a large scale,” notes Yon Visell, an engineering professor at UC Santa Barbara, who is part of a group of researchers working with the new materials . “Perhaps most importantly, the polymers themselves can be fabricated using a wide variety of chemical methods to endow the resulting semiconductor devices with attractive optical and electrical properties. These properties can be designed, tuned or selected in many more ways than inorganics (eg, silicon-based transistors).

The design flexibility described by Visell is exemplified by the reconfigurability of the devices reported by the UCSB researchers and others in the review. Advanced materials.

Reconfigurable logic circuits are of particular interest as candidates for post-CMOS electronics, as they allow simplification of circuit design while increasing power efficiency. A recently developed class of carbon-based (as opposed to, for example, silicon or gallium nitride-based) transistors, called organic electrochemical transistors (OECT), have proven to be well suited for reconfigurable electronics.

In the recent article, chemistry professor Thuc-Quyen Nguyen, who directs the UCSB Center for Polymers and Organic Solids, and co-authors including Visell, describe a breakthrough material – a flexible, semi-conductive carbon-based polymer – that can offer unique advantages over inorganic semiconductors currently found in conventional silicon transistors.

“Reconfigurable organic logic devices are promising candidates for the next generations of efficient computing systems and adaptive electronics,” the researchers write. “Ideally, such devices would be simple in structure and design, [as well as] energy-efficient and compatible with high-throughput microfabrication techniques.”

Conjugate for conductivity

A conjugated polyelectrolyte, or CPE-K, consists of a central conjugated backbone, with alternating single and double bonds, and multiple charged side chains with attached ions. “Having conjugated bonds throughout the polymer makes it conductive, because the delocalized electrons have high mobility along the length of the polymer,” says lead author Tung Nguyen-Dang, a postdoctoral researcher in Nguyen’s lab. , co-directed by Visell. “You are marrying two classic materials, the polymer and the semiconductor, in this molecular design.”

Artificial intelligence (AI) has played a role in the development of the hardware. “You can go through trial and error to make a material,” says Nguyen. “You can create a whole bunch of them and hope for the best, and maybe one in twenty works or has interesting properties; however, we worked with a professor from California State in Northridge, Gang Lu, who used AI to select building blocks and do calculations to get a rough idea of ​​how to proceed, given the energy level and properties we’re aiming for.”

Understanding reconfigurability

One of the main advantages of CPE-K is that it allows reconfigurable (“dual-mode”) logic gates, which means that they can be switched on the fly to operate in either depletion mode or depletion mode. accumulation, simply by adjusting the voltage at the gate. In depletion mode, the current through the active material between drain and source is initially high, before any gate voltage is applied (i.e. ON state). When the gate voltage is applied, the current drops and the transistor goes to OFF state. The accumulation mode is the opposite – with no gate voltage, the transistor is in the OFF position, and applying a gate voltage produces a higher current, turning the device to the ON state.

“Conventional electronic logic gates, which are the building blocks of all digital circuitry found in computers or smart phones, are hardware that only does the job it is designed to do,” says Nguyen. “For example, an AND gate has two inputs and one output, and if the inputs applied to it are all 1, then the output will be 1. Similarly, a NOR gate also has two inputs and one output, but if all inputs applied to it are 1, then the output will be 0. Electronic gates are implemented using transistors, and reconfiguring them (such as changing from an AND gate to a NOR gate) requires invasive modification, such as dismantling, which is usually too complicated to be practical.

“Reconfigurable gates, like the one we show, can behave like both types of logic gates, switching from AND to NOR and vice versa by changing only the gate voltage,” she continues. “Currently in electronics, functionality is defined by structure, but in our device, you can change the behavior and make it something else just by changing the voltage applied to it. If we extend this invention of a single gate to much more complex circuits made up of many such reconfigurable gates, we can imagine powerful hardware that can be programmed with far more functionality than conventional gates with the same number of transistors.”

Another advantage of CPE-K-based OECTs is that they can operate at very low voltages, making them suitable for use in personal electronics. This, combined with its flexibility and biocompatibility, makes the material a likely candidate for implanted biosensors, wearable devices, and neuromorphic computing systems in which OECTs could serve as artificial synapses or non-volatile memories.

“Our colleague is making devices that can monitor the drop in brain glucose levels that occurs just before a seizure,” Nguyen says of a collaborator at the University of Cambridge in England. “And after detection, another device – a microfluidic device – will deliver a drug locally to stop the process before it happens.”

Devices made from CPE-K exhibit simultaneous doping and dedoping depending on the type of ions, according to Nguyen. “You make the device and put it in a liquid electrolyte – sodium chloride [i.e., table salt] dissolved in water,” she says. “You can then migrate the sodium into the CPE-K active layer by applying a positive voltage to the gate. Alternatively, you can change the polarity of the gate voltage and drive the chloride to migrate to the active layer. Each scenario produces a different type of ion injection, and these different ions allow us to change the modes of operation of the device.”

Auto-doping also simplifies the manufacturing process by removing the extra step of adding dopants. “Often when you add a dopant, it’s not evenly distributed throughout the volume of the material,” says Nguyen. “Organic doping materials tend to clump together instead of dispersing. But because our material doesn’t need this step, you don’t have the problem of uneven dopant distribution. You also avoid the whole process optimizing the dopant and determining the right mix and proportions, all of which add steps and complicate processing.”

The team also developed a physical model for the device that explains its operating mechanism and correctly predicts its behavior in both modes of operation, demonstrating that the device does what it appears to do.

Visell concludes: “This remarkable new transistor technology is a perfect example of the amazing electronics and computing capabilities that are made possible through convergent research in chemistry, physics, materials and electrical engineering.

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