Charge transport mechanisms in inkjet-printed thin-film transistors based on two-dimensional materials

Printed electronics has emerged as a pathway for large scale, flexible, and wearable devices enabled by graphene and two-dimensional materials. However, the complexity of the ink formulations, and the polycrystalline nature of the resulting thin films, have made it difficult to examine charge transport in such devices. Here we identify and describe the charge transport mechanisms of surfactant- and solvent-free inkjet-printed thin-film devices of representative few-layer graphene (semi-metal), molybdenum disulphide (MoS2, semiconductor) and titanium carbide MXene (Ti3C2, metal) by investigating the temperature, gate and magnetic field dependencies of their electrical conductivity. We find that charge transport in printed few-layer MXene and MoS2 devices is dominated by the intrinsic transport mechanism of the constituent flakes: MXene exhibits a weakly-localized 2D metallic behaviour at any temperature, whereas MoS2 behaves as an insulator with a crossover from 3D-Mott variable-range hopping to nearest-neighbour hopping around 200 K. Charge transport in printed few-layer graphene devices is dominated by the transport mechanism between different flakes, which exhibit 3D-Mott variable range hopping conduction at any temperature. These findings pave the way for a reliable design of printed electronics with two-dimensional materials.