Control of the coffee ring effect during R2R gravure printing for minimizing threshold voltage variation in printed carbon nanotube-based thin film transistors

Devising a high-throughput additive manufacturing method (HTAM) could aid in fabricating flexible electronic devices with marginal greenhouse gas (GG) emissions and net zero by-products. A roll-to-roll (R2R) gravure printing method is considered a typical platform of HTAM to fabricate carbon nanotube-based flexible electronic devices. However, the large variation in threshold voltage (Vth) in the printed carbon nanotube devices restricts device yield, thereby limiting their practical applications. In this study, the authors would like to show a way of minimizing the variation of Vth in the R2R gravure printed carbon nanotube thin-film transistors (TFTs) by understanding how the morphology of the printed dielectric layer can be controlled during ink drying. Since the formation of an uneven morphology after drying with higher edge thickness, also known as the coffee ring effect (CRE), is one of the key factors causing the variation in Vth, regulating the morphology of the dielectric ink is vital in minimizing the Vth variation of the TFTs printed via R2R gravure. By optimizing the rheological characteristics of the dielectric ink, a homogeneous morphology was attained without CRE, consequently minimizing the Vth variation from 19.6% to 6.9%. Furthermore, the printed device stability was improved by removing the CRE and avoiding the localized bias stress during device operation. Based on this work, the yield, and the stability of R2R gravure printed devices could be improved, enabling the rapid commercialization of additively manufactured flexible devices without the emission of GG or any harmful by-products.