Fully Printed and Encapsulated SWCNT-Based Thin Film Transistors via a Combination of R2R Gravure and Inkjet Printing

Fully printed thin film transistors (TFT) based on poly(9,9-di-n-dodecylfluorene) (PFDD) wrapped semiconducting single walled carbon nanotube (SWCNT) channels are fabricated by a practical route that combines roll-to-roll (R2R) gravure and ink jet printing. SWCNT network density is easily controlled via ink formulation (concentration and polymer:CNT ratio) and jetting conditions (droplet size, drop spacing, and number of printed layers). Optimum inkjet printing conditions are established on Si/SiO₂ in which an ink consisting of 6:1 PFDD:SWCNT ratio with 50 mg L^-1 SWCNT concentration printed at a drop spacing of 20 μm results in TFTs with mobilities of ∼25 cm² V^-1 s^-1 and on-/off-current ratios > 10⁵. These conditions yield excellent network uniformity and are used in a fully additive process to fabricate fully printed TFTs on PET substrates with mobility values > 5 cm² V^-1 s^-1 (R2R printed gate electrode and dielectric; inkjet printed channel and source/drain electrodes). An inkjet printed encapsulation layer completes the TFT process (fabricated in bottom gate, top contact TFT configuration) and provides mobilities > 1 cm² V^-1 s^-1 with good operational stability, based on the performance of an inverter circuit. An array of 20 TFTs shows that most have less than 10% variability in terms of threshold voltage, transconductance, on-current, and subthreshold swing.