Full and In Situ Printing of Nanogenerators that Are Based on an Inherently Viscous Piezoelectric Polymer: An Effort to Minimize "Coffee Ring Effect" and Nonprinting Operations

Among various printingtechnologies for fabrication of electronicdevices, microplotter printing and inkjet printing provide the bestprinting resolution. For inkjet printing, which is compatible withonly low-viscosity inks, the "coffee ring effect" andtime-/labor-consuming nonprinting operations are among the most thatimpose challenges to its applications for rapid and large-scale fabricationof electronic devices. When printing a flexible electronic devicethat tends to bend spontaneously, relocating an unfinished/undriedworkpiece for post-printing treatments usually causes ink diffusion,resulting in deteriorated printing resolution, nonuniformity, anddefects. Taking the printing of a poly(vinylidene fluoride)- (PVDF-)based flexible nanogenerator as an example, this work utilized a high-viscositycompatible microplotter printer and a concentrated/viscous PVDF ink(which minimized the "coffee ring effect)" and an insitu fabrication process (which not only maximally facilitated andminimized the nonprinting operations but also avoided the deteriorationof the printing resolution and the nonuniformity/defects). A morphologicalcomparison investigation showed that a PVDF patch printed (for onlytwo passes with a concentrated 7 wt % PVDF ink) with a microplotterwas morphologically homogeneous with no observable defects, whilea PVDF patch printed (for 80 passes with a diluted 0.5 wt % PVDF ink)with a typical inkjet printer exhibited conspicuous nonuniformityand defects like grooves, pits, and cracks. Performance characterizationof such a nanogenerator showed that it generated negative-positivetwin pulses of short-circuit current during its cyclic bending andunbending, with a short-circuit current density magnitude up to & SIM;0.4 & mu;A/cm(2). A flexible carbon nanotube-based chemiresistivegas sensor was also fully printed in situ, in order to demonstratethat the in situ printing method utilized in this work is also compatiblewith fine features and low-viscosity inks.