Innovation in printed, organic, flexible electronics advances new electronic devices
Printed, carbon-based, flexible electronics were first suggested 20 years ago as a possible way to enable the mass production of cheap, flexible circuits for diverse applications ranging from foldable displays to healthcare wearables. Most electronics are currently made from silicon on rigid small chip substrates, but flexible electronics can accommodate larger areas. Constraints inherent in their properties have meant that printed, organic, flexible electronics have been limited to low-speed applications, such as simple temperature or humidity devices. This has precluded some applications, notably for wireless communications. The HEROIC project took advantage of semiconducting polymers to develop faster printed transistors. The team achieved a record operational frequency of 160 MHz, in the very high frequency range using an organic transistor made with a printed polymer. “Wireless communications using printed polymer electronics is no longer simply a distant, exotic possibility,” says Mario Caironi, project coordinator from the Italian Institute of Technology, the project host. “Before HEROIC, no one considered this seriously; now roadmaps have been suggested.”
Beyond mobility
One of HEROIC’s key successes was first addressing the low mobility issue. Mobility defines how fast an electric charge can flow when subjected to an electric field. Silicon electronics, for example, benefit from 100 or 1 000 times higher mobility than that achieved by the best semiconducting polymers. “The motivation for HEROIC came from simple, yet surprising calculations I made showing that printable polymers were already capable of speeds of 100 MHz to 1 GHz. But that still left us with other issues to resolve,” adds Caironi. The team had to rebuild the architecture of transistors, the fundamental building blocks of any circuit, including their physical interfaces. They also reduced the dimensions of printed electrodes to enable short gaps between electrodes, reducing the time it takes a charge to cross. A narrow electrode is key, as it is also responsible for reducing one of the main factors limiting transistor speed, known as parasitic capacitance, which disrupts transistor operation at high frequency. They also reworked methods to print semiconductor films with a microstructure for high mobility. “State-of-the-art printing tools only offered resolutions tens of times below what was required. We coupled ultra-fast lasers capable of pulsing at hundreds of femtoseconds, which offered micron and submicron resolution, with common printing techniques,” Caironi explains. The team also managed to ensure good charge injection in the transistor channel, while maintaining compatibility with high throughput manufacturing methods.
A wide-ranging enabling technology
HEROIC could revolutionise many applications and processes. It may, for example, be used for biomedical wearable sensors. By avoiding the integration of silicon chips, it would simplify designs and work flows. Cheap and widespread access to such tools could offer transformative opportunities for less developed countries. The team is already exploiting HEROIC’s results to develop a pre-industrial roll-to-roll tool, combining flexography and https://en.wikipedia.org/wiki/Femtosecond (femtosecond)-laser ablation, for high-resolution patterning of conductors on cheap plastic substrates. This is part of the Italian iLabel (website in Italian) project to produce high-performance smart labels for retail goods. Additionally, some of the project’s results have fed into a new ERC project, ELFO, which aims to make edible printed circuits by using food and derivatives with electronic properties. More immediately, the team is working on suitable flexible substrates for high-frequency devices which avoid the problem of heat dissipation on insulating plastics. Operation voltages will have to be decreased to reduce power consumption of the fastest devices envisaged.
Keywords
HEROIC, voltage, frequency, substrate, circuit, electronics, polymer, wearables, silicon, wireless communication, print
