Highly conductive carbon-based aqueous inks toward electroluminescent devices, printed capacitive sensors and flexible wearable electronics

Abstract

Carbon-based conductive inks are one of the most important materials in the field of printing electronics. However, most carbon-based conductive inks with small electrical resistance are expensive, such as graphene. It limits the commercial use of carbon inks in the fields of flexible electronics and printed electronics. Here, we propose a low-cost and environmentally friendly formula based on dihydroxyphenyl-functionalized multi-walled carbon nanotubes (MWNT-f-OH)/carbon black/graphite as conductive fillers and waterborne acrylic resins as binders for preparing highly conductive carbon-based aqueous inks (HCCA-inks). Our study showed that when the mass fraction of carbon black, graphite and MWNT-f-OH was 3.0%, 10.2% and 4.1%, respectively, on a thickness of 40 μm, optimal conductivity (sheet resistance up to 29 Ω sq^-1) was achieved, and the printed HCCA-inks on a paper could withstand extremely high folding cycles (>2000 cycles) while the resistance value of the flexible circuit only increased by 11%. The carbon-based aqueous inks showed high electrical conductivity and excellent mechanical stability, which makes it possible for them to be used as flexible wearable electronics, electroluminescent (EL) devices and printed capacitive sensors.

Fig. 1. (a) Relationship between grinding time and sheet resistance of each group of conductive inks. (b) Comparison of conductivity of inks prepared by different conductive fillers. (c and d) Schematic diagram of CNTs network and graphite, CB filled CNTs network. (e) SEM image of the pure MWNT-f-OH, (f) SEM image of the CB, graphite and MWNT-f-OH are mixed conductive ink filled with each other. (g) SEM images of the cross-section of the ink layer. (h) TEM images of the few-layer graphene in ink.

Fig. 2. Raman spectrum of the graphite before grinding and few-layer graphene obtained from the ink after grinding.

Fig. 3. (a) Bending test of the paper-base flexible circuit and the relationship between resistance and bending angle of the paper-based flexible circuit. (b) The structure diagram of EL device. (c–f) EL devices prepared based on the HCCA-inks as a back electrode. (g) The power and performance parameters test of the EL devices.

Fig. 4. (a–c) The construction and working principle of a simple proximity sensor. (d–f) Three kinds of sensors with different structures. (g) The printing area of the three kinds of sensors. (h) The maximum detection distance of the three kinds of sensors. (i) The detection coefficient α of the three kinds of sensors. (j) The maximum induction height of different printing size sensors with the same detection coefficient α.