Dual‐Supporter and Dual‐Salt Strategy for Solid Polymer Electrolyte with High Ionic Conductivity and Elastic Toughness

In recent years, ionic liquid (IL)‐based solid polymer electrolytes (SPEs) have attracted much attention as conducting or capacitive materials for stretchable electronics. To fabricate fast and mechanically robust electronic devices, the high ionic conductivity and high elastic toughness of the SPE are essential. However, it has been challenging to achieve both high ionic conductivity and high elastic toughness simultaneously because high ionic conductivity generally requires low crystallinity of the polymer chains. Herein, a facile strategy for fabricating highly conductive, mechanically robust, and thermally stable SPE is demonstrated. A glass fiber mesh and La0.57Li0.29TiO3 particles as dual‐supporters are introduced, and 1‐ethyl‐3‐methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][TFSI]) and bis(trifluoromethylsulfonyl)amine lithium salt (LiTFSI) (having the same anion) as the dual salt in the polymer electrolyte is introduced. Consequently, the SPE exhibits a superior ionic conductivity of 2.4 × 10−2 S cm−1 at room temperature and an outstanding elastic toughness of ≈170.3 kJ m−2. Finally, the dual‐salt/dual‐supporter SPE is successfully applied to high‐performance organic electrolyte‐gated transistors as gate dielectric materials and highly sensitive capacitive pressure sensors as force‐sensitive dielectric layers.


Introduction
With the development of the Internet of Things (IoT), multifarious sensors have been integrated into daily clothing and accessories, such as glasses, watches, and bands, to facilitate effective and convenient real-time information gathering. Electronic skin (e-skin), which has outstanding potential in health monitoring systems, will be one of the most ideal forms of wearable electronic devices in the future. [1][2][3][4][5][6][7][8][9][10] E-skin is an artificial multifunctional skin composed of sensors, transistors, batteries, etc. When eskin is attached to human skin or functions as the skin of robots, it undergoes various mechanical deformations, such as tension, compression, or twisting. Therefore, for e-skin to be stably attached to moving surfaces, the stretchability of e-skin is important. [11,12] In an effort to develop stretchable electronic devices, ionic liquid (IL)-based solid polymer electrolytes (SPEs) have been evaluated as conducting or capacitive materials for many stretchable electronic devices. Ionic conductivity and elastic toughness (the maximum energy that can be absorbed by the material up to the elastic limit) are two important figures of merit that represent the electrical and mechanical properties of SPE, respectively. The high ionic conductivity and high elastic toughness of SPE are required for fast and mechanically robust electronic devices; however, achieving both is a challenging task because, for ions to move fast in the polymer matrix, low crystallinity, and more mobile polymer chains are generally required. [13] In addition, an increase in the amount of ILs in the polymer network is a typical strategy to enhance the ionic conductivity, [14][15][16] but when the IL content in the polymer network is increased, the polymer electrolytes generally become more liquid-like. ILs may leak from the polymer network. Therefore, it is essential to explore a new strategy for achieving high ionic conductivity and elastic toughness in IL-based SPEs.
In this study, we demonstrate a highly conductive, mechanically robust, and thermally stable SPE using a dual-salt/dualsupporter strategy. For dual-salt, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][TFSI]) and bis(trifluoromethylsulfonyl)amine lithium salt (LiTFSI) was used to enhance the ionic conductivity, whereas a dual-supporter (La 0.57 Li 0.29 TiO 3 (LLTO) particles, glass fiber mesh) was introduced to improve both the mechanical properties and ionic conductivity. In other words, the LLTO particles, glass fiber mesh, [EMIM][TFSI], and LiTFSI were incorporated into a poly(ethylene glycol) diacrylate (PEGDA) network. The electrical, mechanical, and thermal properties were systematically investigated to determine the intrinsic properties of dualsalt/dual-supporter SPE compared to dual-salt/supporter-free dual-salt/single-supporter SPEs. Among the samples, the SPE fabricated using the dual-salt/dual-supporter strategy exhibited the highest ionic conductivity (≈10 −2 S cm −1 ) and elastic toughness (≈170.3 kJ m −2 ), large specific capacitance (≈30 μF at 1 kHz), and high thermal stability (thermal degradation temperature T d ≈300°C). Electrolytes have been successfully applied to electrolyte-gated transistors (EGTs) as gate dielectric materials and capacitive pressure sensors as force-sensitive dielectric layers.

Fabrication Process of Solid Polymer Electrolytes
The detailed structure of the dual-salt/dual-supporter SPE and the fabrication procedure for the dual-salt/dual-supporter SPE is schematically illustrated in Figure 1a. With the glass fiber mesh as the starting framework, an electrolyte precursor composed of PEGDA, 2-Hydroxy-2-methylpropiophenone (HOMPP, UV cross-linking initiator), [EMIM][TFSI], LiTFSI, and LLTO particles was poured onto the glass fiber mesh and photocrosslinked to form cross-linked PEGDA under UV light. UV irradiation generates free radicals from HOMPP, which initiate the polymerization of acrylate end groups on PEGDA. [17] The crosslinked PEGDA was then annealed in air. The as-obtained sample consisted of a glass fiber mesh and LLTO particles embedded in the bulk of crosslinked-PEGDA, including [EMIM][TFSI] and LiTFSI. A dual-salt ([EMIM][TFSI], LiTFSI) was introduced to improve the ionic conductivity because plasticizing effect of the ionic liquid was expected. [18,19] The plasticizing effect of the dual salt was confirmed by measuring the glass transition temperature (T g ) of PEGDA before and after the addition of the dual salt ( Figure S1, Supporting Information). The T g of crosslinked PEGDA before the addition of the dual salt was ≈−40°C, but it decreased to below −85°C after the incorporation of the dual salt. Next, a dual-supporter (glass fiber mesh, LLTO particles) was used to enhance both the mechanical strength and ionic conductivity. Digital photographs of the samples (Figure 1b) show that dual-salt/supporter-free SPE is so weak that it cannot show selfstanding behavior, but it can be visually confirmed that SPEs can maintain their shape by introducing supporters. To ensure the difference according to the presence of supporters, the morphologies of the dual-salt/supporter-free, dual-salt/single-supporter, and dual-salt/dual-supporter SPEs were examined using scanning electron microscopy (SEM) images of Figure 1b. The dualsalt/supporter-free SPE exhibited only a polymer network without any supporter. After introducing a glass fiber mesh, a dual-salt/single-supporter SPE consisting of glass fibers hundreds of micrometers in length was embedded within the polymer network. With the addition of uniformly distributed LLTO particles, the dual-salt/dual-supporter SPE was composed of a polymer network with a glass fiber mesh and uniformly distributed LLTO particles.

Electrical and Mechanical Properties of Solid Polymer Electrolytes
When [EMIM] [TFSI] or LiTFSI was used independently, the ionic conductivity was low, at 10 −7 S cm −1 . However, a drastic increase occurred in the ionic conductivity when [EMIM][TFSI] and LiTFSI were used simultaneously. Because [EMIM][TFSI] and LiTFSI contain the same anion, the chances of cross-contact ion pair formation are minimized and the ionic conductivity is enhanced. In contrast, mixed-anion systems have a chance to form contact or cross-contact ion pairs, which do not participate in the conduction mechanism and decrease the ionic conductivity. [20] Therefore, we adopted a dual-salt strategy, and the ionic conductivity of the dual-salt SPEs was investigated at room temperature using electrochemical impedance spectroscopy (EIS) (Figure 2a). The Nyquist plots show the typical capacitance dispersion behavior of SPEs. [21,22] Since the x-intercept of Nyquist www.advancedsciencenews.com www.advelectronicmat.de plots is equal to the bulk electrolyte resistance, [23] the inset of Figure 2a indicates that the addition of supporters significantly decreases the bulk resistance of the electrolytes. Further insight is gained from the phase angle (tan ф = Z Im /Z Re ) versus frequency plot in Figure 2b. The phase angle of the dual-salt/dualsupporter SPE in the low-frequency region was close to −80°. A phase angle of −90°is indicative of a purely capacitive response. Therefore, dual-salt/dual-supporter SPE are suggested to operate mainly as capacitors for frequencies below 10 kHz. Above this frequency, the phase angle decayed, indicating a transition to a more resistive response rather than a capacitive response.
The ionic conductivity of the dual-salt/supporter-free SPE was 9.0 × 10 −3 S cm −1 , which was enhanced to 1.8 × 10 −2 S cm −1 upon the addition of the glass fiber mesh and was further increased to 2.4 × 10 −2 S cm −1 by the incorporation of uniformly dispersed LLTO particles. We also confirmed that the ionic conductivity of both dual-salt/supporter-free SPE and dual-salt/dualsupporter SPE increased as the amount of added LLTO particles increased ( Figure S2, Supporting Information), demonstrating the synergistic effect of LLTO particles on the ionic conductivity. The ionic conductivity of the dual-salt/dual-supporter SPE was comparable to that of liquid electrolytes at room temperature. The high ionic conductivity of the dual-salt/dual-supporter SPE can be attributed to the following reasons. First, the addition of glass fiber promotes the dissociation of lithium salts by forming weak bonding between oxygen in Si-O groups of the glass fiber and Li cations. [24,25] Second, acidic groups present on the surface of LLTO particles may show a strong affinity for TFSI anions, [26] which separate cations and anions. Finally, the surface of the LLTO particles with Li vacancies provides a fast pathway for Li-ion diffusion to distances without interruption. [27] In addition to ionic conductivity, the dual-salt/dual-supporter SPE exhibited excellent mechanical properties. Figure 2c shows the stress-strain curves of the three types of electrolytes. A sample without a supporter (dual-salt/supporter-free SPE) was insufficient to show self-standing behavior and could not be tested ( Figure 1b). After the introduction of glass fiber mesh (dualsalt/single-supporter SPE), the electrolyte had an elastic modulus of 3.34 ± 1.17 MPa and the yield strain of 1.4% (corresponding elastic toughness is 46.8 kJ m −2 ). The strengthening mechanism of the electrolyte due to the glass fiber mesh is similar to that of concrete-rebar structure. Here, glass fiber mesh acts as a rebar and polymer act as a concrete in concrete-rebar structure. The successful reinforcement implies strong bonding between PEGDA and glass fiber, which prevents slip or separation of the two materials under mechanical deformation. More interestingly, with the addition of uniformly distributed LLTO particles (dualsalt/dual-supporter SPE), the elastic modulus of the electrolyte reached 7.74 ± 1.09 MPa and the yield strain was also enhanced to 2.2%; the corresponding elastic toughness was 170.3 kJ m −2 , which is 3.6 times higher than that of dual-salt/single-supporter SPE. In addition, the elongation at break also increased from 7.8% to 15.6% after the addition of LLTO particles. We believe that the LLTO particles interact with the polymer chains of PEGDA via ion-dipole interactions between the defect sites of the LLTO surface and PEGDA. The ion-dipole interaction physically, not chemically, crosslinks some polymer chains, strengthening the SPE without sacrificing the yield strain. [28] Figure 2d shows the elastic modulus versus the ionic conductivity of the previously reported [EMIM] [TFSI]-based SPEs with different host polymer networks. [14,29] The ionic conductivity of the dual-salt/dual-supporter SPE is the highest among those of mechanically robust SPEs (with an elastic modulus exceeding 1 MPa) based on various network structures using [EMIM] [TFSI]. In summary, we achieved the desired mechanical properties of SPEs for various stretchable electronic devices while maintaining high electrical properties.
Understanding the enhanced thermal stability of dualsalt/dual-supporter SPEs is difficult due to the complexity of thermal degradation in polymers. However, the formation of chemical bonds between PEGDA and glass fibers and between PEGDA and LLTO, which contribute to enhanced mechanical stability as discussed, could also impact the thermal stability of the SPEs. In addition, no endothermic peak for any of the SPEs appears in the differential scanning calorimetry (DSC) analysis up to 200°C ( Figure S3, Supporting Information). This indicates that the crystallinity of all the samples was very low. This indicates that the addition of glass fiber or LLTO particles barely induced densification of the polymer chains, which may significantly degrade the ionic transport inside SPEs.
The capacitance was measured as a function of frequency to confirm the electrochemical properties ( Figure S4, Supporting Information). At a low frequency of 100 Hz, all samples displayed a high capacitance of more than 5 μF cm −2 , originating from the formation of an electric double layer (EDL) at the interface of the electrolyte and the electrode. The capacitance of the dualsalt/dual-supporter sample was the highest at all frequencies. As the AC signal frequency increased, the capacitances decreased for all the samples, indicating that the ion mobility of the electrolytes limits the polarization response time. [30] Therefore, the high capacitance of the dual-salt/dual-supporter SPE-EGT can be attributed to its higher ionic conductivity.

Organic Electrolyte-Gated Transistors
SPEs were employed as gate dielectric materials in organic EGTs to verify the versatility and ionic functions of the electrolytes. The enhancement of the elastic modulus and yield strain, that is, elastic toughness, of the dual-salt/dualsupporter SPE provides a solvent-free fabrication of gate electrolytes for transistors. Solvent-free fabrication of the gate electrolyte is important because the solvent can contaminate the transistor channel during the solution process of the gate electrolyte. [16] Our fabrication methodology is shown in Figure 3a. i) The source and drain (S/D) electrodes, poly (3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), were fabricated directly on a poly(ethylene terephthalate) (PET) substrate using a commercial desktop inkjet printer. ii) Using a dispenser, a bank (PEGDA incorporating HOMPP) was printed along the rectangular outline of the active area of the transistor. The printed bank was irradiated with UV light to crosslink it. iii) The poly(3-hexylthiophene-2,5-diyl) (P3HT) solution was dispensed into the bank using a dispenser and the solvent was evaporated and dried. iv) Subsequently, a dual-salt/dual-supporter SPE cut into the appropriate shape was placed onto P3HT using a tweezer. During the cutting process, www.advancedsciencenews.com www.advelectronicmat.de a large stress or strain is inevitably applied to the SPEs. However, dual-salt/dual-supporter SPE can sustain a large mechanical stress and be free from plastic deformation owing to the enhanced elastic toughness of the electrolyte. Finally, UV light irradiation was performed to form a crosslinked PEGDA polymer matrix, and then the crosslinked PEGDA was annealed in air. v) The Ag layer was then coated on top of the SPE as a gate electrode using brush painting. Figure 3b shows an array of fabricated EGTs on flexible substrates.
In addition to the processing aspect, the use of dual-salt/dualsupporter SPE significantly enhanced the performance of EGTs in terms of switching properties and hysteresis in the transfer characteristics. The transfer characteristics of electrolyte-gated transistors with SPEs as gate dielectric materials are shown in Figure 3c. The drain current (I D ) versus gate voltage (V G ) curve was measured by sweeping V G from 0.5 to −2 V and back to 0.5 V at a constant drain voltage (V D ) of −2 V. The transfer characteristics of EGT with dual-salt/support-free SPE can be characterized by low current levels and large hysteresis. However, the transfer characteristics of the EGT with the dual-salt/dual-supporter SPE exhibited high current levels and negligible hysteresis. The subthreshold swing was reduced to 0.87 V dec −1 compared to that of EGT with dual-salt/support-free SPE (2.01 V dec −1 ). The current levels of EGT with dual-salt/single-supporter SPE were in between those of EGTs with dual-salt/support-free SPE and dualsalt/dual-supporter SPE. Hysteresis was also observed for EGT with dual-salt/single-supporter SPE, but the shift in the threshold voltage was small compared to the shift observed in EGT with dual-salt/support-free SPE.
When the gate voltage is stepped from V 0 to V 0 + ΔV G , the channel current is given by the following equation [31] where is the mobility, L is the channel length, C G is the volumetric capacitance of the SPE, V th is the threshold voltage, and is the RC delay time, which is a characteristic of the SPE. is given by = C 0 l/ where C 0 is the specific capacitance, l is the thickness, and is the ionic conductivity of the SPE. The first term on the right side of Equation (1) is the steady-state current when the gate voltage is V 0 and the second term represents the change in I D with time toward a new steady-state current when the gate voltage is V 0 + ΔV G ( . According to Equation (1), the difference in the current level depending on the type of SPEs can be attributed to the difference in the dynamic capacitances. A large dynamic capacitance of the SPE implies a large C G , which leads to a large I D . This is consistent with our observation that the current level is highest for the EGT with dualsalt/dual-supporter SPE. In addition, the current can only rapidly reach the steady-state current; thus, a large saturation current occurs when is small. Therefore, the high ionic conductivity of the dual-salt/dual-supporter SPE further enhances the current level.
The hysteresis In the transfer characteristics of EGTs strongly depends on their operation mode. For EGTs with an ionpermeable channel layer, such as P3HT, EGT can operate in two different modes: electrochemical doping mode and field-effect transistor (FET) mode. [32] When EGT operates in the electrochemical doping mode, ions penetrate the semiconducting layer during current-voltage measurements, causing electrochemical doping of the channel semiconductor. Because both inward and outward diffusion of ions inside the channel layer are slow processes, such ion penetration causes hysteresis in the transfer characteristics. However, no ion penetration into the channel occurs in the FET mode of operation. Consequently, this operation mode showed negligible hysteresis in the transfer characteristics. In our case, SPEs with poor ionic conductivity resulted in a large hysteresis in the transfer characteristics, that is, the electrochemical doping mode. If the ionic conductivity of the SPE is low, a large fraction of anions that are adsorbed at the electrolyte/P3HT interface when V G <V th will remain near the electrolyte/P3HT interface even after the direction of the electric field inside the SPE is reversed owing to the limited ion mobility (Figure 3d). Therefore, the probability of anions being diffused into bulk P3HT increases as the ionic conductivity of the SPE decreases. The negligible hysteresis in the transfer characteristics of EGTs with dual-salt/dualsupporter SPE can be attributed to the high ionic conductivity of dual-salt/dual-supporter SPE, which leads to the operation of EGT in the field-effect transistor mode.
In addition to the effects of ionic penetration on the hysteresis, the RC-delay time ( in Equation (1)) may be attributed to the hysteresis behavior of EGTs. The low ionic conductivity of SPE will result in large , so the steady-state current of EGTs cannot be achieved during the measurement. The measurement of the-steady-state current during voltage sweep will cause the hysteresis. On the other hand, the high ionic conductivity of dual-salt/dual-supporter SPE will significantly reduce in Equation (1). Therefore, the use of dual-salt/dual-supporter SPE enables the measurement of the steady-state current during voltage sweep, suppressing the hysteresis behavior.

Capacitive Pressure Sensors
To demonstrate the excellent applicability of the dual-salt/dualsupporter SPE, we fabricated a capacitive pressure sensor that transduces an applied force into an electrical signal using a dual-salt/dual-supporter SPE (Figure 4a,b). By placing the dualsalt/dual-supporter SPE on top of the two electrodes, the SPE acted as the force-sensitive dielectric layer and was tested under various weight loads when an applied bias of 1 V was applied at an AC bias frequency of 100 Hz. When a higher pressure was applied to the dual-salt/dual-supporter-based pressure sensor, a larger number of free ions located in the amorphous region of the polymer matrix was squeezed out to the interface between the electrolyte and the electrode, which induced EDL formation through elastic deformation of the electrolyte (Figure 4a). [33] The time-dependent response and recovery signals of the dualsalt/dual-supporter SPE-based pressure sensor were tested under stepwise pressure conditions of 0.7, 1, 1.3, and 1.5 kPa. As a result, noticeable capacitance changes of 23, 83, 212, and 490 nF in response to dynamic pressure are shown in Figure 4c (see also Figure S8, Supporting Information). The loadingunloading test in Figure 4d displays high reliability under repeated mechanical stimuli. The measured capacitance change under dynamic pressure was normalized to evaluate the sensitivity of the electrolyte-based pressure sensor (Figure 4e). The normalized capacitance change (ΔC/C 0 ) of the electrolyte-based pressure sensor can reach up to 2.7 at 1.5 kPa. Importantly, the dual-salt/dual-supporter-based pressure sensor exhibits a sensitivity of up to 10.8 kPa −1 in the range of 1.3-1.5 kPa, which is superior to the recently reported capacitive pressure sensitivity values. [20,[34][35][36] As mentioned above, glass fibers with high dielectric constants and LLTO particles assist the dissociation of salts, increasing free ions in dual-salt/dual-supporter SPE. Therefore, the number of free ions squeezed out to the interface between the electrolyte and the electrode was significantly enhanced, leading to high sensitivity. The very high sensitivity, noticeable capacitance change, and simple fabrication of these pressure sensors based on SPE make them suitable candidates for wearable e-skin applications.

Conclusion
In this study, we successfully demonstrated a highly conductive, mechanically robust, and thermally stable dual-salt/dualsupporter SPE. Adopting [EMIM][TFSI] and LiTFSI as dual salts is beneficial for improving the ionic conductivity of the SPE, which is comparable to that of liquid electrolytes. More importantly, the incorporation of glass fiber mesh and LLTO particles as dual-supporters endowed the SPEs with outstanding electrical properties, good mechanical strength and toughness, and excellent thermal stability. Dual-salt/dual-supporter SPE can be incorporated easily and solvent-freely as a high-capacitance gate dielectric layer in EGTs. In addition, the use of a dualsalt/dual-supporter SPE improves the switching performance and eliminates the hysteresis in the transfer characteristics of the EGT. Finally, the capacitive pressure sensor using this SPE as a force-sensitive dielectric layer exhibited noticeable capacitance changes, high-pressure sensitivity, and high reliability under mechanical stimuli.
Preparation [TFSI]/LiTFSI) were added, and magnetic stirring was conducted overnight. When all the materials were uniformly distributed, this homogeneous precursor was poured onto the glass fiber mesh for swelling, and MeOH was allowed to evaporate under ambient conditions. Then, photocrosslinking under UV light (365 nm, 7000 μW cm −2 ) was performed to form crosslinked PEGDA. The cross-linked PEGDA was then annealed at 50°C for 2.5 h to evaporate the residual MeOH solvent.
Characterization of SPEs: The morphologies of the SPEs were analyzed by field-emission scanning electron microscopy (SEM, JEOL JSM-7401F). All samples for SEM were coated with a very thin platinum layer using sputter coating. The SPEs were sandwiched between two stainless steel electrodes for ionic conductivity measurements, which were tested by electrochemical impedance spectroscopy (EIS) using a BioLogic VSP-100 instrument in the frequency range of 1 Hz to 1 MHz with an AC amplitude of 10 mV at room temperature. The ionic conductivity was calculated using the equation = L/R b S, where is the ionic conductivity (S cm −1 ), R b is the bulk resistance (Ω), and L and S are the thickness (cm) and area (cm 2 ) of the samples, respectively. The mechanical properties of the SPEs were measured using an Instron 34SC-05 universal testing machine at a constant elongation rate of 3 mm min −1 . The thermal properties of the SPEs were characterized using differential scanning calorimetry (DSC, Discovery TA SDT 650) and thermogravimetric analysis (TGA, TA Instruments Q50). DSC curves were recorded in the second heating scans in the temperature range of 30-200°C, and TGA curves were collected from 30 to 250°C under a N 2 atmosphere. A temperature sweep rate of 10°C min −1 was used for both the DSC and TGA measurements. DSC (DSC Q1000) analysis for the temperature range between −85-30°C was performed in Korea Research Institute of Chemical Technology (KRICT). A temperature sweep rate of 10°C min −1 was used and a flow of N 2 gas at a rate of 50 mL min −1 was introduced for purging. Capacitance-frequency measurements of the MIM devices were performed using a Wayne Kerr 4100 LCR meter.
Dual-Salt/Dual-Supporter SPE EGT Fabrication and Characterization: S/D electrode ink was fabricated by diluting PEDOT:PSS with 60 vol% deionized water (DI water) and 10 vol% ethylene glycol (EG). Inkjet printing of the S/D electrode ink was performed on a PET film using a commercial desktop inkjet printer (HP Deskjet 1010). Using a dispenser, PEGDA incorporating HOMPP (volume ratio 2:1) was printed along the rectangular outlines of the active area of the transistor to fabricate a bank. The printed bank was irradiated with ultraviolet (UV) light (365 nm, 7000 μW cm −2 ) for 1 min. A solution of P3HT dissolved in chlorobenzene (1 mg mL −1 ) was printed using a dispenser inside the bank. The chlorobenzene solvent was evaporated at 50°C in air and dried under vacuum. Subsequently, a dual-salt/dual-supporter SPE cut into the appropriate shape was placed onto P3HT using a tweezer. UV light (365 nm, 7000 μW cm −2 ) irradiation was performed for 20 min. The crosslinked PEGDA was annealed for 2.5 h under air to evaporate the residual MeOH solvent. To form the gate electrode of the SPE-EGT device, an Ag layer was coated on top of the SPE by simple brush painting. The transistor performance was measured using a probe station (Janis st-500) connected to a semiconductor parameter analyzer (Keithley 4200-SCS).
Measurement of Mechanical Stimulus Response and Sensitivity of the Dual-Salt/Dual-Supporter SPE Pressure Sensor: All capacitances were measured using a Wayne Kerr 4100 LCR meter under dynamic pressure. The corresponding pressure was calculated by dividing the load by the pressing dimensions of the SPE. In addition, the sensitivity of the capacitive pressure sensors is defined as S = (ΔC/C 0 )/ P, where ΔC ( = C−C 0 ) is the relative change in capacitance, C and C 0 are the capacitances of the sensor with and without applied pressure, respectively, and P is the applied pressure. The capacitance change was measured at different pressure levels to evaluate the pressure sensitivity of the devices.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.