Look Out, Silicon—These Printable Electronics are Fully Recyclable

What you’ll learn:

How carbon and cellulose-based materials have been adopted for inks used to print transistors.
The role of each ink formulation in the multi-layered inkjet printing process.
How these transistors can be recycled, and their constituent components reused.
How the performance of these devices compares to other printed active devices.

There’s been a lot of interest, some progress, and even modest success in recycling electronics to recover many of the constituent materials, including precious metals, but there’s still the “waste” of the ICs themselves with which to contend. Now, a team based at Duke University has developed and tested printable electronics using a menu of inks and an inkjet printer.

Their devices show complete recyclability of all materials in the printed, all-carbon electronics comprised of paper substrates, semiconducting carbon nanotubes, conducting graphene, and insulating crystalline nanocellulose. Not only did the devices also exhibit stable performance over six months in ambient conditions followed by controllable decomposition, but the graphene and carbon-nanotube inks were even recaptured for recycling (>95% recapture efficiency) and reprinting of new transistors.

Led by Aaron Franklin, Addy Professor of Electrical and Computer Engineering, the group demonstrated a completely recyclable, fully functional transistor made out of three carbon-based inks that they printed onto paper and other flexible, environmentally friendly surfaces. Carbon nanotubes and graphene inks are used for the semiconductors and conductors, respectively. Franklin noted that these materials aren’t new to the world of printed electronics, but the path to recyclability was opened with the development of a wood-derived insulating dielectric ink called nanocellulose.

“Nanocellulose is biodegradable and has been used in applications like packaging for years,” said Franklin. “And while people have long known about its potential applications as an insulator in electronics, nobody has figured out how to use it in a printable ink before. That’s one of the keys to making these fully recyclable devices functional.”

For insulators, the researchers developed a method for suspending crystals of nanocellulose that were extracted from wood fibers, which, with the addition of a common salt, produced an ink that functioned as an insulator in their printed transistors. Using the three inks in a direct-write aerosol jet printer at room temperature (20°C), the team shows that their all-carbon transistors perform well enough for use in a wide variety of applications.

Utilizing their crystalline nanocellulose (CNC) dielectric ink, all-carbon thin-film transistors (TFTs) were printed on a paper substrate with graphene source/drain/gate electrodes and a carbon-nanotube (CNT) channel (Fig. 1). There were four distinct set of inks, with one each for graphene, semiconducting carbon nanotube, crystalline nanocellulose, and silver nanowire printing. They also printed a capacitor as well as a lactate sensor; the latter is useful in medical applications.

1. CNC-based all-carbon TFT printing and testing. (a) All-carbon electronic components. (b) Printing fabrication schematic. (c) Optical image of all-carbon TFT used for (d) the surface map of transistor Ion/Ioff as a function of gating location. (e) Subthreshold curves as a function of CNT print passes. Data represent average (line) ± standard deviation (shaded region) of 12 devices without added salt at a Vds of -0.5 V. (f) Frequency-dependent capacitance vs. ink salt concentrations with values at 1000 Hz inset. Subthreshold curves measured with different delay times for devices with (g) no added salt and (h) addition of 0.15-mM (millimolar) NaCl to CNC ink; inset is photo of an array of all-carbon transistors. All devices in this report have a channel length of 250 µm and a channel width of 200 µm.

They evaluated both short- and longer-term performance in a special fixture (Fig. 2).

2. This fixture was used to test the transistor, which is comprised of fully recyclable, printed electronics. The recycling process recovers nearly 100% of the materials used and the materials lose very little of their performance capabilities.

Prof. Franklin added that previous reports claim “all-carbon” electronics actually incorporate inorganic components, focusing only on a carbonaceous channel and electrodes. However, the printed CNC gate dielectric in their TFTs yields a truly all-carbon device that they dubbed “all-carbon recyclable electronics” (ACRE). They also compared performance of their printed device to others reported by other researchers (Fig. 3).

3. Benchmarking printed all-carbon TFT performance against other printed transistors. (a) Width-normalized on-current plotted against source-drain voltage for related (thin-film transistors) TFTs. (b) Width-normalized on-current plotted against gate voltage. Colors represent transistor types (both semiconducting material and gating scheme); carbon-nanotube TFTs (CNT-TFTs) use a semiconducting carbon-nanotube channel, organic TFTs (OTFTs) use an organic semiconductor channel and IGZO is indium gallium zinc oxide. Ion gel-based transistors use an ionic liquid as the dielectric. Shapes represent the processing technique, where circles demarcate fully printed transistors and triangles represent transistors that utilize clean-room techniques in at least some of the processing. Finally, datapoints with a border are transistors on a paper substrate.

What about the recover/reuse considerations? By submerging their devices in a series of baths, gently vibrating them with sound waves, and centrifuging the resulting solution, the carbon nanotubes and graphene are sequentially recovered with an average yield of nearly 100%. Both materials may then be reused in the same printing process while losing very little of their performance viability, and the nanocellulose, being made from wood, can simply be recycled along with the paper on which it was printed (Fig. 4).

4. ACRE-TFTs with controlled recycling demonstrated. (a) Schematic of ACRE system demonstrating printing, use, recycling, and then reprinting, reuse, etc. Top right inset shows array of ACRE-TFTs with excess graphene (left) and CNTs (right) printed to the side. (b) Stable TFT characteristics over six months storage in air. Transistor fabricated without salt and run with 500-ms delay time. (c) UV-Vis-NIR spectra of new and recycled CNT inks. SEM images of printed (d) new and (e) recycled CNT inks. (f) Subthreshold curves of transistors from new and recycled inks. Data represent average ± standard deviation of 4 devices with a salt concentration of 0.15 millimolar (mM) in the CNC and a delay time of 10 ms.

Franklin explained that by demonstrating a fully recyclable, multifunctional, printed transistor first, he hopes to make a first step toward the technology being commercially pursued for simple devices. For example, the technology could find a role in a large building needing thousands of simple environmental sensors to monitor its energy use, or in customized biosensing patches for tracking medical conditions.

Their published article in Nature Electronics “Printable and recyclable carbon electronics using crystalline nanocellulose dielectrics” is highly readable, discussing how they devised the idea and developed the specific inks, including the rationale, modifications, and testing of each (it’s behind a paywall but an open version is posted here). In addition, there’s a short video of the printing here. Furthermore, a “Supplementary Information” posting shows that the team has truly done its homework, as it presents a detailed table listing 28 other somewhat-similar projects with their reference citations, and the key technical attributes and specifications of each, to demonstrate why their approach is a distinct improvement. Perhaps, some day, “printed circuits” will truly be made that way?