Cellulose Nanocrystal Aqueous Inks Evaluated for Printed Electronics and Application to Thin-film Transistors
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With increasing demand for smart features on consumer items, it is imperative that a new class of environmentally sustainable processing methods and materials are developed in order to enable smart functionality on mass produced goods. The challenge faced is to find a suitable material system and process that is bio-sourced, degradable, and compatible with novel substrates for electronics, such as flexible paper and plastic, which are often sensitive to processing conditions. We have demonstrated significant first steps towards this goal through development of conductive, aqueous-based inks of cellulose nanocrystal (CNC) for low-waste additive deposition via inkjet printing. Aqueous CNC dispersions were tuned for printability through addition of ethanol and ethylene glycol before successful patterning unto amorphous silicon (a-Si:H) thin-film transistors (TFTs) as proof-of-concept source and drain contacts. Bio-mass-sourced CNC nanoparticles functionalized with conductive polypyrrole polymer (CNC-PVP-PPY) through oxidative polymerization in water were found to withstand temperatures of 150°C and displayed conductivity as high as 77.8 S/m, making this ink a viable candidate to replace conventional polymeric conductors such as PEDOT, which are expensive to produce and require organic solvents and strict processing conditions. Two key observations were made: TFTs fabricated with CNC-PVP-PPY successfully showed switching behaviour with typical mobilities on the order of 0.2 cm^2/Vs, on-off ratio of 10^6, threshold voltage of 7.6V, and subthreshold swing of 264 mV/dec, however, the larger contact resistance for CNC-PVP-PPY had an impact on the extracted parameters when compared to sputtered metal and printed annealed-silver nanoparticle contact devices. In this work we report the first instance of conductive CNC nanoparticles successfully print-processed from aqueous inks and integrated into a-Si:H TFTs as proof of concept for the use of CNC inks in electronic devices.