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

Abstract

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 – , PF 6 – or BF 4 – ), 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.

Poly(ethylene glycol)-Based Poly(ionic liquid) Block Copolymers through 1,2,3-Triazole Click Reactions

“…28 Figure 3A shows that the EDL is beginning to form for poly(S)-poly(VB-Me-TGME-TFSI) as early as 1 × 10 6 Hz and has fully formed at 1 × 10 3 Hz, which is comparable to other high-performing PILs in our groups previous work. 10 Next, the poly(S)-poly(VB-Me-TGME-TFSI) had a maximum capacitance density of 2.5 μF/ cm 2 , as seen in Figure 3B. The measured capacitance of the poly(S)-poly(VB-Me-TGME-TFSI) is similar to other values of 1, 2, and 4 μF/cm 2 reported for PILs with a TFSI anion.…”

“…The observation of two glass transition temperatures is also a sign of block copolymer microphase separation. 10 The poly(S)-poly(VB-Me-TGME-TFSI) block copolymer was annealed and freeze fractured. At room temperature, the poly(VB-Me-TGME-TFSI) homopolymer was too soft and could not be properly handled and therefore was not further characterized.…”

“…These results are similar to previously reported PIL-containing block copolymers further demonstrating the ability of poly(S)-poly(VB-Me-TGME-TFSI) to selfassemble into ordered domains. 10,25,26 Metal insulator metal (MIM) capacitors (Figure 2B) were fabricated using poly(S)-poly(VB-Me-TGME-TFSI) and characterized using impedance spectroscopy under ambient conditions. The poly(S)-poly(VB-Me-TGME-TFSI) block copolymer was readily soluble in dichloromethane facilitating the fabrication of multiple functional MIM capacitors.…”

“…The measured capacitance of the poly(S)-poly(VB-Me-TGME-TFSI) is similar to other values of 1, 2, and 4 μF/cm 2 reported for PILs with a TFSI anion. 10,17,23,29 Our previous study characterized poly(VB-Me-TFSI) PILs with a T g of 8 °C, which demonstrated an ion conductivity of 3 × 10 −8 S/cm under ambient conditions. 17 As mentioned, the reduced T g leads to an increase in ion mobility, which correlates to faster EDL formation.…”

Insights into Ionic Liquids: From Z-Bonds to Quasi-Liquids

“… 10 − 15 More importantly, due to their rich interactions and microstructures (which researchers can modulate), ILs function as both solvents and catalysts, rendering them broadly useful in green chemistry and opening new directions in green chemical engineering. 16 − 21 …”