Ionic Liquid Containing Block Copolymer Dielectrics : Designing for High-Frequency Capacitance, Low-Voltage Operation, and Fast Switching Speeds

Polymerized ionic liquids (PILs) are a potential solution to the large-scale production of low-power consuming organic thin-film transistors (OTFTs). When used as the device gating medium in OTFTs, PILs experience a double-layer capacitance that enables thickness independent, low-voltage operation. PIL microstructure, polymer composition, and choice of anion have all been reported to have an effect on device performance, but a better structure property relationship is still required. A library of 27 well-defined, poly(styrene)-b-poly(1-(4-vinylbenzyl)-3-butylimidazolium-random-poly(ethylene glycol) methyl ether methacrylate) (poly(S)-b-poly(VBBI⁺[X]-r-PEGMA)) block copolymers, with varying PEGMA/VBBI⁺ratios and three different mobile anions (where X = TFSI^-, PF₆^-or BF₄^-), were synthesized, characterized and integrated into OTFTs. The fraction of VBBI⁺in the poly(VBBI⁺[X]-r-PEGMA) block ranged from to 100 mol % and led to glass transition temperatures (T_g) between -7 and 55 °C for that block. When VBBI⁺composition was equal or above 50 mol %, the block copolymer self-assembled into well-ordered domains with sizes between 22 and 52 nm, depending on the composition and choice of anion. The block copolymers double-layer capacitance (C_DL) and ionic conductivity (σ) were found to correlate to the polymer self-assembly and the T_gof the poly(VBBI⁺[X]-r-PEGMA) block. Finally, the block copolymers were integrated into OTFTs as the gating medium that led to n-type devices with threshold voltages of 0.5-1.5 V while maintaining good electron mobilities. We also found that the greater the σ of the PIL, the greater the OTFT operating frequency could reach. However, we also found that C_DLis not strictly proportional to OTFT output currents.