Ion Gel‐Gated Quasi‐Solid‐State Vertical Organic Electrochemical Transistor and Inverter

Parallel‐type organic electrochemical transistors (p‐OECTs) with aqueous electrolyte gate dielectrics have been widely studied for transducing biological signals into electrical signals. However, aqueous liquid electrolyte‐based p‐OECTs suffer from poor device stability, low transconductance (gm), and limited applications. In this study, a quasi‐solid‐state ion gel‐gated vertical‐type OECT (v‐OECT) and NOT logic gate are successfully demonstrated by combining both p‐type and n‐type v‐OECTs for the first time. Indacenodithiophene (IDT) polymers with alkyl (PIDTC16‐BT) and oligoethylene glycol (OEG) substituents (PIDTPEG‐BT) are studied as a channel material, and an ionic liquid in a crosslinked polymer matrix is adopted as a quasi‐solid electrolyte. Compared to aqueous devices, an enlarged electrochemical window and improved operational stability are observed. Notably, the v‐OECTs have a significantly larger channel area (50 × 50 µm2) and shorter channel length (≈30 nm), yielding a dramatically increased gm. As‐spun PIDTC16‐BT films exhibit a noticeably higher gm of 72.8 mS than that of previous p‐OECTs along with superior device stability, despite a low volumetric capacitance. In the case of v‐OECTs, face‐on intermolecular packing is required to increase the carrier transport in a vertical direction. Logic gates are successfully demonstrated using p‐ and n‐type v‐OECTs, suggesting the potential of OECT‐based next‐generation electronics.

Parallel-type organic electrochemical transistors (p-OECTs) with aqueous electrolyte gate dielectrics have been widely studied for transducing biological signals into electrical signals. However, aqueous liquid electrolyte-based p-OECTs suffer from poor device stability, low transconductance (g m ), and limited applications. In this study, a quasi-solid-state ion gel-gated vertical-type OECT (v-OECT) and NOT logic gate are successfully demonstrated by combining both p-type and n-type v-OECTs for the first time. Indacenodithiophene (IDT) polymers with alkyl (PIDTC16-BT) and oligoethylene glycol (OEG) substituents (PIDTPEG-BT) are studied as a channel material, and an ionic liquid in a crosslinked polymer matrix is adopted as a quasi-solid electrolyte. Compared to aqueous devices, an enlarged electrochemical window and improved operational stability are observed. Notably, the v-OECTs have a significantly larger channel area (50 × 50 µm 2 ) and shorter channel length (≈30 nm), yielding a dramatically increased g m . As-spun PIDTC16-BT films exhibit a noticeably higher g m of 72.8 mS than that of previous p-OECTs along with superior device stability, despite a low volumetric capacitance. In the case of v-OECTs, face-on intermolecular packing is required to increase the carrier transport in a vertical direction. Logic gates are successfully demonstrated using p-and n-type v-OECTs, suggesting the potential of OECT-based next-generation electronics.
sandwiched between source and drain electrodes and its thickness defines the channel length. While the channel length is typically in the range of tens to hundreds of µm in p-OECT, that of v-OECTs is decreased to a few tens of nanometers. [8] The enlarged channel area and decreased channel length in v-OECTs can further increase the transconductance compared to p-OECTs. Furthermore, the 3D geometry of the active layer in v-OECTs allows the reduced device footprints while maintaining high transconductance per given area. [2b,8c,9] Malliaras et al. also demonstrated v-OECTs with PEDOT:PSS channels to show a reduced footprint, which is particularly suited for applications where high density is needed (i.e., implantable devices). [9b] Blom et al. reported the submicron v-OECTs based on the electrodeposited poly (3,4-ethylenedioxythiophene):polys tyrene sulfonate (PEDOT:PSS) with a channel length down to 60 nm, showing ultrahigh g m up to 275 mS. [9d] Recently, Facchetti et al. fabricated v-OECTs to show footprint current densities exceeding 1 kA cm −2 , g m of 0.2-0.4 S, transient times less than 1 ms, and stable switching more than 50 000 cycles. They further fabricated the vertically stacked complementary logic circuits, demonstrating a great potential of v-OECTs. [9e] Although there have been a few demonstrations of v-OECTs previously, a relatively little attention has been paid to the vertical architecture of OECTs (compared to p-OECTs) and how different structures influence the device properties. These issues should be carefully considered when applying OECT devices; the study of new device architectures and approaches for replacing the aqueous gate dielectric are necessary to further extend real applications of OECTs.
In this study, quasi-solid-state ion gel-gated v-OECTs based on indacenodithiophene (IDT) polymers with alkyl (PIDTC16-BT) and OEG substituents (PIDTPEG-BT) were demonstrated for the first time. The vertical architecture of v-OECTs was achieved by sandwiching the active channel layer between the source and drain electrodes. To overcome the drawbacks of the aqueous gate dielectric, an ionic liquid (IL) in a crosslinked polymer matrix was adopted as a quasi-solid-state electrolyte in our v-OECT system, which can enlarge the electrochemical window (compared to water) and improve the operational stability without water evaporation. [10] Compared to p-OECTs, v-OECTs yielded a significantly larger channel area (50 × 50 µm 2 ) and shorter channel length (≈30 nm), which dramatically increased the transconductance although alkyl-substituted PIDTC16-BT having a low volumetric capacitance applied to the channel layer. As-spun PIDTC16-BT films exhibited a noticeably higher g m of 72.8 mS compared to that of PIDTPEG-BT with OEG side chains. The g m value of 72.8 mS obtained in our v-OECT geometry is significantly higher than those of typical p-OECTs reported previously. Furthermore, a superior stability of PIDTC16-BT OECTs was measured during device operation, compared to the PIDTPEG-BT ones. The interchain orientation along the vertical direction was controlled by thermal annealing and its effect on the OECT performance was systematically investigated. Finally, the CMOS NOT gate was successfully demonstrated by incorporating both the p-type and n-type v-OECTs based on the IDT polymers. Our approach introduces a new direction for the future development of OECT-based next-generation flexible electronics. Figure 1a presents a schematic of the quasi-solid-state ion gel-gated v-OECT. First, a Cr/Au (3/17 nm) source electrode was patterned via thermal evaporation. The semiconducting polymer solution (5 mg mL −1 in chloroform) containing an ethane-1,2-diyl bis(4-azido-2,3,5,6-tetrafluorobenzoate) photocrosslinker (90:10 by volume) was spin-coated onto the substrate with a patterned source electrode. [11] The channel area was exposed by UV light through a shadow mask and the unexposed region was removed by the solvent. The polymer semiconductor channel was thermally annealed at different temperatures of 100 and 200 °C. Au (40 nm thick) was thermally evaporated through a shadow mask to form the drain electrode. The crossed area (50 × 50 µm 2 ) defined the channel area of v-OECTs. A solution containing 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (EMIM + TFSI − ) IL, poly(ethylene glycol) diacrylate (PEGDA) monomer, and 2-hydroxy-2-methylpropiophenone (HOMPP) photoinitiator (21:2:1 by weight) was flattened onto the substrate, and then the gelation process occurred through UV exposure. Finally, the Au gate electrode was thermally deposited with a mask onto the channel region to form the gate electrode. The schematic experimental process is summarized in Figure S1 (Supporting Information). An optical microscopic view of the fabricated device is shown in Figure 1b.

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The UV-Vis absorption spectra of PIDTC16-BT and PIDTPEG-BT are displayed in Figure S3a (Supporting Information). PIDTC16-BT and PIDTPEG-BT show the maximum absorptions at λ max = 664 and 644 nm in the solution (chloroform), respectively. In the film, both polymers showed slightly red-shifted absorption spectra compared to those in the solution with λ max = 674 and 662 nm for PIDTC16-BT and PIDTPEG-BT, respectively. The more red-shifted absorption of PIDTC16-BT in the solution and film may be related to its stronger interchain packing. The optical onset was measured at 720 nm for both polymers in the film, and the corresponding optical bandgap was calculated to be 1.72 eV. Cyclic voltammetry (CV) measurements were conducted relative to an internal standard of ferrocene/ferrocenium (Fc/Fc + ) using a three-electrode system with a platinum counter electrode, polymer film-coated platinum electrode as a working electrode, and Ag/AgNO 3 reference electrode in 0.1 m tetrabutylammonium tetrafluoroborate (Bu 4 NBF 4 ) acetonitrile solution ( Figure S3b, Supporting Information). The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels were determined to be -5.44/-3.72 eV and -5.09/-3.37 eV for PIDTC16-BT and PIDTPEG-BT, respectively. The higher HOMO of PIDTPEG-BT with electronreleasing OEG substituents can facilitate oxidation during electrochemical doping in OECT operations. The optical, electrochemical, and thermal properties of polymers are summarized in Table 1.
To investigate the electrochemical doping process for two polymer films, an ion gel/polymer film between two ITO electrodes was fabricated and the UV-Vis-NIR spectral changes were measured by changing the applied voltage ( Figure S4, Supporting Information). [14] As the applied voltage increased, the polymers underwent electrochemical oxidation (p-doping) with positive (bi)polaron generation, and negative TFSI − ions (from IL layer) migrated into the polymer films for charge compensation. For PIDTC16-BT films, the bandgap transition  at 650 nm starts to bleach at 1.2 V and new broad absorption peaks at >800 nm concomitantly emerge, indicating the generation of (bi)polarons along the conjugated polymer backbone. The bleaching of the bandgap transition occurred at a smaller voltage of 0.9 V for PIDTPEG-BT owing to its higher HOMO and facile TFSI − penetration into the polymer film with polar and hydrophilic OEG side chains, which sufficiently agrees with the results of previous studies. [15] When the voltage was adjusted back to 0 V, the original absorption of the undoped state was fully recovered for both polymers. The electrochemical stability of the PIDTC16-BT and PIDTPEG-BT films was examined by multicycle CV experiments in a 0.1 m EMIM + TFSI − solution in acetonitrile under a bias between 1 V and -1 V against the Ag/AgNO 3 electrode ( Figure S5a,b, Supporting Information). In contrast to PIDTC16-BT, PIDTPEG-BT demonstrated poor cycling stability with a significant loss in the measured current as the scan number increased. To evaluate the effect of polymer hydration on the electrochemical stability, multi-cycle CV measurements were further conducted in a 0.1 m Li + TFSI − aqueous solution ( Figure S5c,d, Supporting Information). A significant electrochemical instability of PIDTPEG-BT was also observed to show that the current completely diminished at the 20 th cycle. Previous studies reported that OEG-substituted conjugated polymers undergo irreversible oxidation/reduction, and hydrophilic OEG groups induce the uncontrolled hydration of polymers to cause over-swelling of the polymer layers, which disrupts the interchain packing and resulting electrical properties, even with a detachment of the active layer from the electrode. [5b,7b,16] In a contrary, the change in the current measured in acetonitrile was negligible for PIDTC16-BT; however, a slight current decay was observed in the aqueous media.
The electrical properties of PIDTC16-BT and PIDTPEG-BT were investigated by fabricating ion gel-gated quasi-solidstate v-OECTs with the device geometry shown in Figure 1a. Figure 2a shows the transfer curves and g m of the v-OECTs based on the as-spun PIDTC16-BT. The drain voltage (V D ) was fixed at -1 V. The devices turned on as the V G negatively increased, which is a direct signature of typical p-type operation. This p-type gate modulation was also confirmed by the output curve shown in Figure 2b. The higher hysteresis was observed for PIDTC16-BT compared to PIDTPEG-BT, suggesting much hindered ion penetration (or movement) into the channel of PIDTC16-BT with hydrophobic alkyl side chains. Although PIDTC16-BT has hydrophobic alkyl substituents, PIDTC16-BT v-OECTs exhibited a maximum g m (g m, max ) of 72.8 mS, which is more than five times higher than that of the 200 °C-annealed PIDTPEG-BT with OEG side chains (13.4 mS), as shown in Figure 2c,d. The average g m, max value with a standard deviation was obtained from 20 devices. Furthermore, we also prepared the ion-gel gated p-OECTs of both polymers and the detailed fabrication is described in the Supporting Information. It is noteworthy to emphasize that v-OECTs with a vertical geometry exhibited a three order of magnitude higher on-current, compared with that of typical p-OECTs ( Figure S6, Supporting Information). It is assumed that the IL electrolytes with a larger molecular size (compared to typical KCl or NaCl in an aqueous solution) may significantly disturb the intermolecular packing of both polymers during operation. This damage in the crystalline packing in the film can lead to a greater deterioration of the charge transport in the parallel device architecture with a significantly longer channel length (≈20 µm compared to ≈30 nm in v-OECT). As shown in Figure S7 (Supporting Information), the use of solid-state electrolytes slows down the transient time (see the response time in the range of ≈microsecond for aq. electrolyte-based OECTs): PIDTC16-BT based device presented τ on (10% to 90% rise time) = 2.73 s, τ off (90% to 10% fall time) = 0.65 s, whereas PIDTPEG-BT-based device presented τ on = 27 ms and τ off = 6.65 ms. The response time of PIDTC16-BT OECTs (with hydrophobic alkyl side chains) was ≈100 times slower compared to PIDTPEG-BT ones.
V TH is a threshold voltage and A/L are the channel area/length. C* was measured by electrochemical impedance spectroscopy (EIS) of the capacitors with a metal-insulator-semiconductor (MIS) geometry ( Figure S8, Supporting Information). As shown in Figure 2f, the as-spun PIDTPEG-BT and PIDTC16-BT films exhibited the C* values of 20.6 and 0.34 F cm −3 , respectively, indicating that the OEG side chains in PIDTPEG-BT facilitate ion penetration into the semiconductor layer. Based on the C* values measured by EIS, µ h was calculated according to the aforementioned equation (Figure 2g). The higher g m value of v-OECTs based on as-spun PIDTC16-BT is ascribed to its higher µ h value (8.2 × 10 −2 cm 2 V −1 s −1 ) with a face-on dominant interchain orientation (Figures S9-S11, Supporting Information) despite the smaller C*. The g m value of 72.8 mS obtained in our v-OECT geometry is significantly higher than those of typical p-OECTs reported previously. [17] Compared to typical p-OECTs, the enlarged channel area with a short channel length in v-OECTs further improved the transconductance. The vertical device architecture has a great advantage for realizing high transconductance OECTs.
The opposite trends in g m, max with thermal annealing can be interpreted in terms of both µ h and C*. First, we performed the 2D grazing incidence wide-angle X-ray scattering (2D GIWAXS) measurements for two polymer films before (pristine films without crosslinker) and after photo-crosslinking (with crosslinker). As shown in Figure S9 and Table S1 (Supporting Information), 2D GIWAXS images, corresponding line-cut profiles (along the out-of-plane (OOP) and in-plane (IP) directions), and packing parameters show negligible changes before and after crosslinking, indicating little effects of photo-crosslinking on the interchain packing structure. After thermal annealing, the face-on dominant packing was maintained and the interchain packing was further enhanced for both polymers; the crystal coherence length (CCL) of a (010) π-π stacking peak in OOP direction increased from 16.47 Å (as-spun) to 26.20 Å (annealed at 200 °C) for PIDTC16-BT, and from 14.32 Å (as-spun) to 27.16 Å (annealed at 200 °C) for www.advelectronicmat.de

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PIDTPEG-BT, respectively (Table S2, Supporting Information). Therefore, the enhanced packing structure of both PIDTC16-BT and PIDTPEG-BT impeded the counterion penetration, which resulted in a decreased C*. In addition, the edge-on characteristic was more developed in PIDTC16-BT compared to that of PIDTPEG-BT after thermal treatments. As shown in Figures S10 and S11 and Tables S2 and S3 (Supporting Information), a strong (100) lamellar scattering at q z = 0.32 Å −1 in the OOP direction was measured for PIDTC16-BT by thermal annealing at 200 °C. Owing to enhanced edge-on ordering, the decrease in µ h along with a smaller C* at higher annealing temperatures resulted in a reduced g m, max of PIDTC16-BT. Contrary to PIDTC16-BT, PIDTPEG-BT films demonstrated an enhanced packing of the face-on orientation in the 200°C-annealed film. The enhanced vertical charge transport (increased µ h ) in thermally annealed PIDTPEG-BT films contributed to the increase in g m, max despite thermal annealing decreasing C*. The overall trends are summarized in Figure S12 (Supporting Information).
The operational stability of both v-OECTs was then compared, as shown in Figure 2h,i. The square wave input pulse V G = +2 to -4 V at V D = -0.1 V was applied to the devices for 1500 s. The PIDTC16-BT v-OECTs exhibited superior opera-tional stability compared to PIDTPEG-BT v-OECTs, which is consistent with the CV reversibility measurements. The hydrophilic OEG side chains in PIDTPEG-BT may induce an overswelling of the polymer layers with the penetration of ions, which disrupts the interchain packing and resulting electrical properties, demonstrating the same trend as the previous results of p-OECTs with an aqueous electrolyte solution. [5f ] Note, PIDTC16-BT with normal alkyl side chains performed better than the OEG-substituted PIDTPEG-BT in our ion gel-gated v-OECTs with higher g m and µ h values along with a higher operational stability, although it demonstrated a smaller C* value compared to that of PIDTPEG-BT. Furthermore, the face-on orientation of the semiconductor needs to be carefully considered in the material design and device fabrication of v-OECTs. The enlarged area and decreased length of the channel in v-OECTs can increase the transconductance despite the small C*, contrary to p-OECTs.
To demonstrate the logic gate operation based on the v-OECT devices, an n-type IDT-based semiconducting polymer, DCNBT-IDT with octyl side chains, was synthesized by following a previously reported procedure (Figure 3a). [18] DCNBT-IDT performed well as an n-type electron acceptor in all-polymer solar www.advelectronicmat.de cells with face-on orientation. Figure 3b presents the transfer curve of the OECTs based on DCNBT-IDT without thermal annealing at V D = +0.1 V. The value of I D increased as V G positively increased, directly indicating an n-type operation. The DCNBT-IDT v-OECT exhibited a maximum on-current level of ≈10 −5 A and g m of ≈7 µS. The on/off current ratio was approximately 10 3 . Figure 3c is the output curve of the n-type DCNBT-IDT v-OECT, which demonstrates a successful gate modulation, although its transistor characteristics are significantly poorer than the p-type PIDTC16-BT. Figure 3d presents the schematic circuit diagram of the CMOS NOT gate. The p-type v-OECT was connected to the supply voltage (V DD ), and the n-type v-OECT was connected to the ground. Figure

Conclusion
In summary, we reported ion gel-gated quasi-solid-state v-OECTs based on IDT-based p-type semiconductors. Without any posttreatments, PIDTC16-BT with normal alkyl side chains demonstrated a noticeably higher g m of 72.8 mS compared to that of PIDTPEG-BT. The higher g m of PIDTC16-BT was mainly ascribed to its higher µ h , despite the smaller C* compared to that of PIDTPEG-BT. Furthermore, the superior operational stability of PIDTC16-BT v-OECTs, compared to PIDTPEG-BT, was measured. In contrast to typical p-OECTs, face-on intermolecular packing is required to facilitate carrier transport vertically. Considering the vertical architecture, the channel area is significantly larger and the channel length is shorter (within a range of a few tens of nanometer) compared to those of p-OECTs, which can significantly increase the transconductance despite the poor electrochemical doping by quasi-solid-state ion gelgated dielectrics. The quasi-solid vertical device architecture proposed in this study may guide the future development of OECT-based next-generation electronics.
v-OECT Device Fabrication: Silicon wafer with 300 nm thick SiO 2 was cleaned by sonicating in acetone, 2-propanol, and deionized water for 10 min each. The source electrode (Cr/Au with 3 nm/17 nm thickness) was thermally deposited on a clean SiO 2 /Si ++ (100 nm thick thermallygrown SiO 2 ) substrate at a rate of 0.1 Å s −1 for Cr and 0.3 Å s −1 for Au. The semiconductor (PIDTC16-BT, PIDTPEG-BT, DCNBT-IDT) solution in chloroform (5, 5, and 10 mg mL −1 , respectively) was mixed with a photocrosslinker (ethane-1,2-diyl bis(4-azido-2,3,5,6tetrafluorobenzoate)) solution (5, 5, and 10 mg mL −1 in chloroform, respectively) with a 90:10 (by volume) ratio, which was spin-coated on the substrate with source patterns at 2000 rpm for 60 s. Thickness of semiconducting channel was measured using the surface profiler (DektakXT) and spectroscopic ellipsometer (Alpha SE). The thickness of PIDTC16-BT and PIDTPEG-BT was measured to be approximately 32 and 34 nm, respectively. The semiconducting film was selectively exposed by UV light (254 nm and 6 W cm −2 ) through a metal shadow mask for 25 s. Sequentially, an unexposed part was removed with chloroform to form the channel area, and the sample was dried for 12 h in an Ar-filled glovebox. After the drying process, a 40 nm thick Au drain electrode was thermally deposited at 0.8 Å s −1 . The thermal annealing process was performed after depositing a drain electrode. The ion gel solution was prepared by mixing PEGDA, HOMPP, and EMIM + TFSI − with a 2:1:21 (by weight) ratio, and flat-casted and then the gelation process occurred through UV exposure. Lastly, a 40 nm thick Au gate electrode was thermally deposited on the top of the channel at a rate of 0.8 Å s −1 .
v-OECT Device Characterization: Thickness of semiconducting channel was measured using the surface profiler (DektakXT) and spectroscopic ellipsometer (Alpha SE). The thicknesses of PIDTC16-BT and PIDTPEG-BT were measured to be approximately 32 nm and 34 nm, respectively. Electrical properties of the OECTs were measured with a probe station (MS Tech. Co.) using a Keithley 4200 semiconductor characterization system. The V G voltage sweep rate for transfer curve characterization was 380 mV s −1 .

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