Poly(ionic liquid) Gating Materials for High-Performance Organic Thin-Film Transistors: The Role of Block Copolymer Self-Assembly at the Semiconductor Interface.

The widespread realization of wearable electronics requires printable active materials capable of operating at low voltages. Polymerized ionic liquid (PIL) block copolymers exhibit a thickness-independent double-layer capacitance that makes them a promising gating medium for the development of organic thin-film transistors (OTFTs) with low operating voltages and high switching speed. PIL block copolymer structure and self-assembly can influence ion conductivity and the resulting OTFT performance. In an OTFT, self-assembly of the PIL gate on the semiconducting polymer may differ from bulk self-assembly, which would directly influence electrical double-layer formation. To this end, we used poly{[N,N'-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5'-(2,2'-bithiophene)} (P(NDI2OD-T2)) as a model semiconductor for our OTFTs, on which our PILs exhibited self-assembly. In this study, we explore this critical interface by grazing-incidence small-angle X-ray scattering (GISAXS) and atomic force microscopy (AFM) of P(NDI2OD-T2) and a series of 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-). We investigate the thin-film self-assembly of block copolymers as a function of device performance. Overall, a mixed orientation at the interface leads to improved device performance, while predominantly hexagonal packing leads to nonfunctional devices, regardless of the anion present. These PIL gated OTFTs were characterized with a threshold voltage below 1 V, making understanding of their structure-property relationships crucial to enabling the further development of high-performance gating materials.

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 g of 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 DL is not strictly proportional to OTFT output currents.

Controlled Synthesis of Poly(pentafluorostyrene-ran-methyl methacrylate) Copolymers by Nitroxide Mediated Polymerization and Their Use as Dielectric Layers in Organic Thin-film Transistors.

A library of statistically random pentafluorostyrene (PFS) and methyl methacrylate (MMA) copolymers with narrow molecular weight distributions was produced, using nitroxide mediated polymerization (NMP) to study the effect of polymer composition on the performance of bottom-gate top-contact organic thin-film transistors, when utilized as the dielectric medium. Contact angle measurements confirmed the ability to tune the surface properties of copolymer thin films through variation of its PFS/MMA composition, while impedance spectroscopy determined the effect of this variation on dielectric properties. Bottom-gate, top-contact copper phthalocyanine (CuPc) based organic thin-film transistors were fabricated using the random copolymers as a dielectric layer. We found that increasing the PFS content led to increased field-effect mobility, until a point after which the CuPc no longer adhered to the polymer dielectric.