S.
Thiemann
a,
S. J.
Sachnov
b,
M.
Gruber
a,
F.
Gannott
a,
S.
Spallek
c,
M.
Schweiger
a,
J.
Krückel
a,
J.
Kaschta
a,
E.
Spiecker
c,
P.
Wasserscheid
b and
J.
Zaumseil
*a
aInstitute of Polymer Materials, Friedrich-Alexander-Universität Erlangen-Nürnberg, Martensstraße 7, D-91058 Erlangen, Germany. E-mail: jana.zaumseil@fau.de
bDepartment of Chemical and Bioengineering, Friedrich-Alexander-Universität Erlangen-Nürnberg, Egerlandstraße 3, D-91058 Erlangen, Germany
cCenter for Nanoanalysis and Electron Microscopy, Friedrich-Alexander-Universität Erlangen-Nürnberg, Cauerstraße 6, 91058 Erlangen, Germany
First published on 21st February 2014
We introduce a new type of silane-based ionogels that are produced by gelation of the ionic liquid 3-methyl-1-(3-(triethoxysilyl)propyl)-imidazolium bis(trifluoromethylsulfonyl)imide ([(EtO)3SiPMIM][TFSI]) with tetramethylorthosilane and formic acid. In the obtained ionogels the cations are involved in the network formation while the anions can move freely. The ionogels show advantageous properties for application in flexible electronics, such as low modulus, solution processability and high specific capacitance. Spray-coated ionogels were used as high capacitance gate dielectrics for organic (poly[3-hexylthiophene], P3HT) electrolyte-gated transistors (EGTs) that operated at very low voltages (<2 V) with high on/off ratios in air over weeks. Devices fabricated on polymer foil remained functional during repeated bending cycles with strains up to 2.3%.
In an EGT the gate dielectric is replaced by an electrolyte (e.g., an ionogel). When a gate voltage is applied, the anions and cations move toward the gate electrode and the semiconductor, respectively. There they form nm-thick electric double layers (EDLs) at the interfaces with large effective capacitances in the μF cm−2 range, which enable the accumulation of large charge carrier densities at low voltages. In the case of polymer semiconductors the ions penetrate into the bulk so that opposite charges are accumulated to compensate.20,21 In both cases the carrier density and thus conductivity of the channel vary with the applied gate voltage as in a typical field-effect transistor. Given that ions within the ionogel retain their high mobility good switching speeds are attainable.12,14
Besides triblock-copolymer ionogels, gelation of ionic liquids is also achievable by mixing the ILs with other synthetic polymers (e.g., poly[vinylidenefluoride-co-hexafluoro-propylene]),22 biopolymers, such as cellulose,23 or silicon dioxide nanoparticles,24 to name a few. Furthermore, ionogels can be obtained by forming silica-like networks via a sol–gel process in the presence of ionic liquids. Néouze et al. investigated the properties of such ionogels based on silicon alkoxide precursors like tetramethylorthosilane (TMOS) and tetraethylorthosilane (TEOS), formic acid (FA) and different ionic liquids, e.g., 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([BMIM][TFSI]).25–29 The precursors crosslink via a condensation reaction and form an open silica-like network, through which the anions and cations can percolate. Depending on the amount and type of the IL, hard, transparent, and insoluble ionogels can be formed with high ionic conductivities and good thermal stability. Horowitz and Panzer optimized the TMOS:FA:IL ratio to obtain a compliant gel with high ionic conductivity and high capacitance, comparable to the pure IL.30 Importantly, in these ionogels the ionic liquid is not involved in the network formation itself but simply fills the voids and all ions remain free to move.
Here we introduce an ionogel, in which the ionic liquid itself takes part in the network formation. We use an ionic liquid with silicon alkoxide (triethoxysilane) functional groups attached to the imidazolium cation. This type of ionic liquid was previously used to immobilize palladium complexes on silica particles for heterogeneous catalysis.31 Without any additional ILs we produce ionogels by gelation with TMOS and FA that are viscous but mechanically stable. The new silane-based ionogels are soluble in acetone and ethylacetate thus enabling solution processing, e.g., printing and spray-coating, at low temperatures. We successfully apply these ionogels in low-voltage electrolyte-gated polymer transistors and simple circuits on rigid and flexible substrates.
Fig. 1 Molecular structures of the ionic liquid [(EtO)3SiPMIM][TFSI] and network precursors with optical image of the resulting ionogel. |
The morphology of the ionogels (spray-coated onto TEM-grids) was studied by high resolution transmission electron microscopy (HRTEM, Philips CM300). The film thickness of the spray-coated ionogel was measured by laser profilometry with a UBM microfocus system. Raman spectra were acquired using a Renishaw inVia Reflex Confocal Raman Spectrometer with an excitation laser wavelength of 785 nm. Precursor solutions were kept in a sealed glass cuvette and spray-coated ionogels and pure ionic liquid were deposited onto aluminum foil for background-free measurements.
Differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA) were performed with a Q 2000 differential scanning calorimeter and a Q5000 IR thermo-gravimetric analyzer (TA Instruments), respectively.
Oscillatory shear measurements were performed on a Gemini rotational rheometer (Malvern) in the plate–plate geometry with a diameter of 25 mm and a gap of 1.8 mm. The dynamic shear moduli and the viscosity were recorded as a function of the angular frequency in the linear viscoelastic regime. The temperature was 25 °C and the stress amplitude was 200 Pa in the small amplitude oscillatory shear (SAOS) mode.
Smooth, homogeneous films of the ionogel with approximately 100 μm (±20 μm) thickness were obtained by spraying the ionogel–acetone solution with an air-brush gun twice for roughly 15 s at a distance of 10 to 15 cm onto the substrates at 60 °C in ambient air. A thin polydimethylsiloxane film (PDMS, Corning Sylgard 184, thickness 450 μm) with rectangular openings acted as a simple shadow mask. Samples were stored in a vacuum at 40 °C for 12 hours to remove residual solvent and air-bubbles from the spray-coated ionogel. Finally, a gold gate electrode was evaporated onto the ionogel by electron-beam evaporation (50 nm, 0.05 nm s−1).
Current–voltage characteristics of ionogel-gated P3HT transistors and inverters were measured with an Agilent 4155C parameter analyzer in a dry nitrogen glovebox, and in ambient air where noted. Resistor-loaded inverters were realized with an external resistor (2 MΩ).
In order to verify the crosslinking and formation of a network, we recorded Raman spectra of the pure IL [(EtO)3SiPMIM][TFSI], the precursor solution (IL:TMOS:FA) and the spray-coated ionogel. In Fig. 2a the spectral regions between 100–2000 cm−1 and 2800–3200 cm−1 are shown, and Fig. 2b–d show the most important peaks in detail. Five peaks at 739, 1020, 1132, 1238, and 1877 cm−1 are dominant in all three samples. These arise from the characteristic vibrations of the [TFSI] anion and imidazolium cation and do not change after gelation.34,35 More informative are those peaks that increase or decrease after gelation. A small peak at 490 cm−1 emerges after gelation. This Raman mode is representative of Si–O–Si bonds corroborating at least partial formation of a silica-like network. A peak at 665 cm−1 corresponding to a Si–O–Cp.h. (p.h. – partially hydrated) stretching mode and two peaks at 1448 and 1476 cm−1 corresponding to asymmetric and symmetric bending modes of –CH3, respectively,34 decrease substantially. From these spectra and in accordance with Martinelli and Nordstierna34 we can summarize the sol–gel reaction as follows: the formic acid hydrolyzes the alkoxy silane groups of the IL and the TMOS. Ethylformate and methylformate are most likely formed and evaporate immediately at the reaction temperature of 150 °C as indicated by the reduction of the Si–O–Cp.h. peaks and the mass loss found in the TGA. The silanol groups then undergo a condensation reaction creating a Si–O–Si network (indicated by the peak at 490 cm−1) containing the cations of the ionic liquid and silica originating from the TMOS. Although the Raman signals of the Si–O–Cp.h. and –CH3 groups are strongly reduced in the final ionogel they have not completely vanished indicating free network ends and remaining free [(EtO)3SiPMIM] cations. We conclude that the ionogel is partially crosslinked by Si–O–Si bonds, with the cations of the IL involved in the network. The softness (see rheology data below) of the obtained ionogel suggests that this is not a fully formed, three-dimensional network as those demonstrated by Néouze et al.25–27 but rather a highly branched polyelectrolyte or ionomer with fixed cations and mobile anions. The lack of complete crosslinking and the presence of positive charges within the network would also explain the solubility of the ionogel in polar solvents such as acetone and ethylacetate after sonication. Polyelectrolytes based on, for example, poly(styrene sulfonic acid) and others have been previously used for EGTs.36–38
We investigated the rheological properties of the obtained ionogel by means of oscillatory shear measurements. Fig. 3a shows the storage modulus (G′) and the loss modulus (G′′) as well as the absolute value of the complex viscosity (|η*|) as a function of the angular frequency (ω). Over the investigated frequency range G′′ is proportional to ω and exceeds G′. This behaviour is typical of a liquid. G′ exhibits a plateau between 0.1 and 1 rad s−1 indicating a second relaxation process. According to Capek39 ionic polymers frequently show microphase separation leading to ion-rich domains due to strong ion–dipole attractions. Due to the formation of a second phase and surface energy differences between the phases an additional relaxation process occurs at low frequencies or elevated temperatures. At frequencies below and above the plateau G′ scales roughly with ω2 indicating typical terminal rheological behaviour of a viscoelastic liquid.40 The rheological data show that for the low TMOS concentrations used here the produced ionogel behaves more like a highly viscous liquid than a gel. This indicates again that crosslinking is incomplete and branched ionomers or polyelectrolytes are formed instead of a complete network.
HRTEM images (see Fig. 3b) as well as electron diffraction patterns (not shown) indicate an amorphous morphology. There is no indication for the presence of any larger silica structures.
For the application of ionogels as high capacitance electrolytes their electrochemical properties are crucial. The electro-chemical window of the pure [(EtO)3SiPMIM][TFSI] extends from −1.5 V to 2.0 V. The ionogel exhibits the same overall electrochemical window of 3.5 V but slightly shifted (see Fig. 4a). Electrochemical impedance spectroscopy (EIS) was carried out to obtain phase angle versus frequency (f) plots for the ionogel and the pure IL. In typical electrolyte systems small phase angles at high frequencies are indicative of resistor-like behaviour and phase angles close to −90° are representative of capacitive behaviour.41 The pure IL shows an almost constant phase angle (max. −83°) at frequencies up to 100 Hz, which drops to 0° at 100 kHz. In contrast to that, the ionogel shows a maximum phase angle of −66° and starts to drop at frequencies larger than 10 Hz. At much higher frequencies (>104 Hz) the phase angle rises again up to 54°, possibly due to dipolar relaxation of the material, as previously shown by Larsson et al. for polyelectrolytes.37 The frequency-dependent capacitances of the ionogel and IL (see Fig. 4c) were calculated according to Dasgupta et al.42 Note that, a serial model of a resistor and a constant phase element was used to extract the capacitances. The IL and the ionogel exhibit similar specific capacitances of 10.4 μF cm−2 at the lowest frequency of 0.1 Hz. The decrease of capacitance of the ionogel around 100 Hz indicates a relatively low ionic mobility within the ionogel, which is in agreement with the measured ionic conductivity of 14.5 μS cm−1 compared to the pure IL (900 μS cm−1). The resulting RC-time constant of 740 μs for an ionogel thickness of 100 μm would result in a relatively low switching speed for transistors gated with these ionogels. Clearly, the involvement of the cation in the network and its interaction with the anions slow down the ion movement and thus the formation of electric double-layers. Thinner ionogel films, e.g., produced by printing, would improve the switching speed as previously shown for other ionogels.41
Despite the partial crosslinking the obtained ionogels are soluble in common organic solvents like acetone and thus solution processable, e.g., by spray-coating at low temperatures, which enables the use of low-cost, flexible polymer substrates. Also, these new ionogels allow for the evaporation of thin metal films on top as gate electrodes (see the inset in Fig. 5b). This is unusual because most metals penetrate common ionogels used for EGTs during evaporation and do not form continuous or conducting films.
In order to calculate the field-effect mobilities the specific capacitance Ci of the ionogel was required. The formation of the EDL and therefore also the effective capacitance of the ionogel depend on the applied gate voltage and the nature of the interface and penetration of ions into the polymer. Hence, the capacitance was measured in the actual device instead of using the quasi-static values from EIS obtained at zero bias and with two gold electrodes. Displacement current measurements were carried out to extract the gate voltage dependent capacitance according to Xie and Frisbie.44 The source and drain electrodes were grounded and the displacement current (IDispl) (i.e. the gate current) was measured for different gate voltage sweep rates (dVG/dt) (see Fig. S2, ESI†). The extracted capacitance values increased from 0.6 μF cm−2 at 0.6 V to 3.2 μF cm−2 at −0.5 V. The capacitance values obtained by EIS (see above) and displacement current measurements differ considerably, probably due to the fact that P3HT is permeable for the [TFSI] anions leading to electrochemical doping instead of the formation of electric double layers on the metal electrodes used for EIS.20 For the mobility calculations above the capacitances extracted from displacement current measurements were used.
In order to test the applicability of these ionogel-gated transistors in simple circuits we fabricated a resistor loaded inverter consisting of a P3HT EGT and an external resistor (2 MΩ). The circuit diagram and the obtained transfer curve are shown in Fig. 5c. The hysteresis is probably a result of the limited ionic conductivity and thickness of the ionogel. The operating frequency was quite low with 0.02 Hz but the gain of 10 was reasonable. At higher operating frequencies the hysteresis increased and the gain was reduced. The switching time of the inverter would decrease for thinner ionogels and shorter channel lengths.
We tested the device performance of the P3HT EGTs not only in nitrogen atmosphere but also in air. For application in flexible electronics the long-term stability in both atmospheres is essential. The stability of the ionogel-gated P3HT EGTs was first tested in dry nitrogen by repeated transfer measurements every 5 min for about 13 hours. The on/off current ratio (∼4 × 104) did not change over time but the corresponding transfer curves showed a negative threshold shift of about 0.25 V (see Fig. S3, ESI†).
For measurements in air we expected a strong decrease of the on/off ratio and a threshold voltage shift to positive voltages due to the increased doping of P3HT by oxygen, which is usually observed for P3HT transistors in air and light.43,45 However, the ionogel and the gold top gate are likely to act as gas diffusion barriers and allow for greater air-stability. The ionogel-gated P3HT transistors showed remarkably stable on/off ratios and reversible threshold shifts. Fig. 6a shows the initial transfer characteristics measured in ambient air and after 12 days of storage in air. Additionally, for the first two days the device was stressed by performing transfer measurements every 5 min for 48 hours. The overall current–voltage characteristics did not change much, but the threshold shifted to more positive values and the on/off-ratio decreased slightly indicating p-doping. After storing the device for 10 days in ambient air the threshold voltage shifted back to its initial value. Further bias stress for two days again led to a positive threshold voltage shift, which likewise was reversible by storage in air for several days (see Fig. 6b).
Footnote |
† Electronic supplementary information (ESI) available: Differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA) of the silane precursor, displacement current characteristics of P3HT-EGTs and calculated capacitance vs. gate voltage, bias stress stability of P3HT-EGTs in nitrogen. See DOI: 10.1039/c3tc32465f |
This journal is © The Royal Society of Chemistry 2014 |