Inkjet printing of graphene for flexible electronics


how-nanotechnology-could-change-solar-panels-photovoltaic_66790_600x450(Nanowerk Spotlight) Graphene has a unique combination  of properties that is ideal for next-generation electronics, including  mechanical flexibility, high electrical conductivity, and chemical stability.  Combine this with inkjet printing, already extensively demonstrated with  conductive metal nanoparticle ink (see for instance: “Low-cost  nanotechnology substitute for gold and silver in printable electronics”),  and you get an inexpensive and scalable path for exploiting these properties in  real-world technologies.

Although liquid-phase graphene dispersions have been  demonstrated (see: “Inkjet-printed  graphene opens the door to foldable electronics”), researchers are still  struggling with sophisticated inkjet printing technologies that allow efficient  and reliable mass production of high-quality graphene patterns for practical  applications. There are several challenges that need to be overcome:  

  • a  good ink should possess proper fluidic properties, in particular the right  viscosity and surface tension;
  • the  graphene concentration in these solvents is often quite low so that several tens  of print passes are required to obtain functional films, reducing efficiency of  the technique;
  • graphene  flakes easily aggregate in inks or during solvent evaporation, which decreases  the ink stability and/ or degrades the film/device performance;
  • ideal  solvents for graphene dispersions are toxic so that their corresponding inks  cannot be used in an open environment;
  • most  studies published thus far on inkjet printing of graphene are actually based on  graphene oxide inks, not graphene inks.

Recent work by researchers at the KTH Royal Institute of  Technology in Sweden has addressed these issues and proposes an approach to  overcome these problems. Reporting their findings in a recent issue of  Advanced Materials (“Efficient Inkjet Printing of Graphene”), a team  led by Max Lemme and Mikael Östling, professors at the School of  Information and Communication Technology at KTH, demonstrates a mature but  simple technology for inkjet printing of high-quality few-layer graphene.

Inkjet printed graphene patterns

Inkjet printed graphene patterns. a–c) Optical images of as-printed  patterns on glass slides: a) droplet matrix, b) lines, and c) a film corner.  (Reprinted with permission from Wiley-VCH Verlag)  

 

The approach is based on the team’s previously published  distillation-assisted solvent exchange technique to prepare high-concentration  graphene dispersions (“A simple route towards high-concentration  surfactant-free graphene dispersions”). They first  exfoliate graphene from  graphite flakes in dimethylformamide (DMF), and then DMF is exchanged by  terpineol through distillation by virtue of the large difference between their  boiling points.

Therefore, graphene can be significantly concentrated if  terpineol is of much lower volume than DMF. More importantly, the solvent is  changed from low-viscosity and toxic DMF to high-viscosity (about 40 cP at 20°  C) and environmentally-friendly terpineol.   They write, though, that the disadvantages of the technique in  the previous work – a short stable period (the dispersion can only be stable for  about 10 hours) and severe flake aggregation during solvent evaporation –  prevent the dispersions from being practical inks. “In this work, we have  improved the ink formulation mainly through polymer stabilization.

Before  distillation, a small amount of polymer (ethyl cellulose) is added into the  harvested graphene/DMF dispersion to protect the graphene flakes from  agglomeration. After printing, the stabilizing poly mers can be effectively  removed through a simple annealing process.”   The resulting graphene dispersion had a stable period of at  least several weeks.

The researchers point out that the inks provide  well-directed and constant jetting out of all nozzles at an even velocity, which  is comparable to the performance of commercially available inks.   To investigate the quality of the printed graphene, the team  fabricated large-area centimeter-scale graphene thin films with between 1 and 6  printing layers on glass slides.

 

“Printed transparent conductive films attain a sheet resistance  around 200 /sq at a transmittance of about 90%,” they summarize their results.  “Printed narrow-line resistors exhibit a resistance range from a few kΩ to  several MΩ. Printed few-layer graphene thin film transistors can be modulated by  the electric field effect.

Printed micro-supercapacitors achieve a high specific  capacitance of 0.59 mF cm-2 and a rapid  frequency response time around 13 ms.”   The team concludes that the present technology provides an  efficient and low-cost method to fabricate a variety of graphene electronic  devices with good performance and is a promising alternative for future  commercial applications in printed and flexible electronics.

 

By Michael Berger. Copyright © Nanowerk
Read more: http://www.nanowerk.com/spotlight/spotid=31868.php#ixzz2d5jygcw5

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