Uniformly connected conductive networks on cellulose nanofiber paper for transparent paper electronics

We demonstrate the fabrication of highly transparent conductive networks on a cellulose nanofiber paper, called cellulose nanopaper. Uniform coating of the conductive nanomaterials, such as silver nanowires (AgNWs) and carbon nanotubes, is achieved by simple filtration of their aqueous dispersions through the cellulose nanopaper, which acts as both filter and transparent flexible substrate. The as-prepared AgNW networks on the nanopaper offer sheet resistance of 12 Ω sq.−1 with optical transparency of 88%, which is up to 75 times lower than the sheet resistance on a polyethylene terephthalate film prepared by conventional coating processes. These results indicate that the ‘filtration coating’ provides uniformly connected conductive networks because of drainage in the perpendicular direction through paper-specific nanopores, whereas conventional coating processes inevitably cause self-aggregation and uneven distribution of the conductive nanomaterials because of the hard-to-control drying process, as indicated by the well-known coffee-ring effect. Furthermore, the conductive networks are embedded in the surface layer of the nanopaper, showing strong adhesion to the nanopaper substrate and providing foldability with negligible changes in electrical conductivity. This filtration process is thus expected to offer an effective coating approach for various conductive materials, and the resulting transparent conductive nanopaper is a promising material for future paper electronics. Paper has been used throughout history for numerous purposes and continues to be extremely useful. The cellulose-based material has even been investigated as a support for flexible electronics. Traditional paper, however, does not provide the transparency that is increasingly sought after in this field. Now, Hirotaka Koga, Masaya Nogi and co-workers from Osaka University, Japan, have coated a type of transparent paper that they previously developed with conductive materials. Their paper is based on cellulose nanofibers rather than the microfibers from which traditional paper is composed. The team used silver nanowires or carbon nanotubes as the conductive material—both show promise in transparent flexible electronics that employ plastic substrates. By depositing the silver or carbon on the ‘cellulose nanopaper’ through a simple filtration process, the researchers obtained uniform networks, thus avoiding the uneven distribution that typically occurs with plastic-based substrates. The resulting flexible materials showed good conductivity and optical transparency. Highly transparent and strongly adhesive conductive networks embedded in the surface of cellulose nanofiber paper are prepared by a simple filtration coating process. As-prepared transparent conductive paper shows sheet resistance of 12 Ω sq.−1 with specular transmittance of 88%, which is up to 75 times lower than the sheet resistance on a polyethylene terephthalate film prepared by conventional coating processes. In addition, the transparent conductive paper is folded with negligible changes in electrical conductivity, opening new doors for future paper electronics.