Evaluation of Additively Printed Dielectrics for Fully Printed Carbon Nanotube Thin-Film Transistors

Carbon-nanotube thin-film transistors (CNT-TFTs) have the potential for a wide variety of applications, such as chemical sensors and digital logic circuits, due to their high field-effect mobility and compatibility with versatile fabrication approaches. Aerosol jet printing is an additive, direct-write approach for integrating CNT-TFTs onto various substrates. Among all the layers making up a CNT-TFT, the gate dielectric is the most difficult to additively print owing to challenges in thickness scaling, pinhole formation, and reliance on high sintering temperatures. The most commonly used printed dielectrics include electrolytes and ion gels due to their high specific capacitance and compatibility with solution-phase dispersion into inks. Recently, crystalline nanocellulose (CNC) has been utilized as an electrolyte gate dielectric to realize a fully recyclable, printed CNT-TFT. In this work, we present a detailed evaluation and benchmarking of three additively printed gate dielectrics for fully printed, top-gated CNT-TFTs that operate at low voltages: an electrolyte consisting of elastomer PVDF-HFP; CNC with a salt concentration of 0.2%; and an ion gel consisting of triblock polymer PS-PMMA-PS, ionic liquid EMIM-TFSI, and ethyl acetate in a 1:9:90 w/w ratio. Because electrolyte dielectrics are performance-limited by ionic diffusion rates, the impact of voltage sweep rate on switching characteristics, such as threshold voltage, subthreshold slope, and hysteresis, is examined. Further, electrical bias stress testing is completed over 12 hours to determine the drift characteristics of these dielectrics. Additionally, these TFTs, printed on Kapton, a highly flexible substrate, undergo mechanical stress testing to evaluate the effect of bending on transistor performance. This extensive evaluation of three leading options for additively printed dielectrics will aid in moving the field of printed electronics closer to application-ready devices with stable, low-voltage operation CNT-TFTs.