Printed Electronics from Solution-Processed 2D Materials

Abstract

The fervour created in the wake of the graphene gold-rush has fuelled investigative momentum over the last decade. The maturity of liquid-phase processing has meant that solution-processed layered crystals can be now be consolidated with additive manufacturing and devices based on printed nanosheet networks are now regularly reported. There are many challenges associated with printed architecture and this work aims to address the some of the issues faced by the current generation of nanosheet-network devices. As contemporary printed devices move from lateral to vertical geometry, it was first necessary to develop a methodology for printing continuous, pinhole-free films to prevent unwanted interlayer contact. This was undertaken through a capacitor study where the devices were printed through sequential deposition of conductive graphene and dielectric hexagonal boron nitride (h-BN) nanosheet networks. Such heterostructures will only function as capacitors once the dielectric layer is continuous and, through trial and error, the dielectric layers were found to be pinhole-free at thicknesses above 1.65 μm. Impedance spectroscopy showed that such heterostructures act as series combinations of a capacitor and a resistor, with the expected dimensional scaling of the capacitance. The areal capacitance ranged from 0.24 to 1.1 nF cm-2, with an average series resistance of 120 kΩ. The fitting of the Bode plots also provided an average RC time constant of 0.74 μs for these devices. This development thus paves the way toward more complex multilayer devices. While a transistor based on a printed graphene network was demonstrated in 2012, there has been little progress on transistors based on printed semiconducting networks. This is likely due to the electrostatic gate field being screened by the network structure. A liquid electrolyte was thus used to electrolytically gate the entire volume of a printed network and the efficacy of this technique was demonstrated for printed films of MoS2, WS2, WSe2, and MoSe2. These networks showed mobilities of > 0.1 cm2 V-1 s-1, on:off ratios of up to 600, and transconductances exceeding 5 milliSiemens. The transconductance was also found to scale with the thickness of the network, a feature atypical of traditional transistors. The large device capacitance, while hindering switching speeds, allows these devices to drive high currents at relatively low drive voltages, in contrast with other devices of comparable mobility. An all-printed, vertically stacked transistor was then demonstrated with graphene source, drain, and gate electrodes, a WSe2 channel, and a BN separator -all formed from nanosheet networks. The BN network contains the electrolyte within its porous interior, which facilitates electrolytic gating in a solid-like structure. This proof-of-concept transistor performs reasonably well, with an on:off ratio of ~25 and a transconductance of 22 μS. With the aim of increasing the carrier mobility in nanosheet-network devices, MoS2-graphene composites were investigated as active regions in printed photodetectors. Combining liquid exfoliation and inkjet printing, all-printed photodetectors were fabricated with graphene electrodes and MoS2-graphene composite channels with various graphene mass fractions (0 ≤ Mf,G ≤ 16 wt%). The increase in dark conductivity of the channel with Mf,G was consistent with percolation theory for composites below the percolation threshold. While the photoconductivity increased with graphene content, it did so more slowly than the dark conductivity, such that the fractional photoconductivity decayed rapidly with increasing Mf,G. This indicates that both mobility and dark carrier density increase with graphene content according to percolation-like scaling laws, while photo-induced carrier density is essentially independent of graphene loading. These data show that analysis of the photoconductivity in such composites is a useful way to differentiate the effects of filler content on mobility and carrier density, parameters which are usually aggregated in the conductivity.