Pen lithography for flexible microsupercapacitors with layer-by-layer assembled graphene flake/PEDOT nanocomposite electrodes

An energy device using an all solid-state microsupercapacitor (MSC) has to play the roles of both a current collector and an electrode material, as well as demonstrating properties of high charge storage, conductivity, and flexibility. Despite the complexity and processing costs, microfabrication techniques are being employed in fabricating a great variety of MSC devices. In this work, simpler and cheaper concepts are proposed to fabricate flexible MSCs based on graphene flakes and polyethylenedioxythiophene (PEDOT) with a layer-by-layer assembly method using pen lithography, without the need for complex processing, or a cleanroom environment. In order to fabricate interdigitated finger patterned electrodes for the MSC, we report the preparation of highly conductive graphene flakes and PEDOT on a polyethylene terephthalate (PET) film formed by inducing the polymerization of 3,4-ethylenedioxythiophene (EDOT) monomers. The sheet resistance of 15 Ω sq.^-1 measured for 3-layer graphene/PEDOT is much lower than that displayed by the graphene flake/EDOT (14.8 ± 0.6 kΩ sq.^-1). For a flexible MSC application, the MSC exhibits a maximum energy density of 1.5 mW h cm^-3, a power density of 141 W cm^-3, and a volumetric capacitance of 7.7 F cm^-3 (at a current density of 0.02 A cm^-3), which are higher than those values obtained for other solid state MSCs. The graphene/PEDOT MSC also shows good long-term cycling stability, with a capacitance retention rate of 81% after a large cycling number of 2500 times. The simplicity and wide scope of this innovative strategy can open up new avenues for easy and scalable fabrication of a wide variety of devices.