High-Resolution Electrochemical Transistors Defined by Mold-Guided Drying of PEDOT:PSS Liquid Suspension

Ion-sensitive transistors with nanoscale or microscale dimensions are promising for high-resolution electrophysiological recording and sensing. Technology that can pattern polymer functional materials directly from a solution can effectively avoid any chemical damage induced by conventional lithography techniques. The application of a mold-guided drying technique to pattern PEDOT:PSS-based transistors with high resolution directly from the water-based suspension is presented in this paper. Gold electrodes with short channels were first defined by creating high-resolution polymer lines with mold-guided drying followed by pattern transfer through a lift-off process. Then, PEDOT:PSS lines were subsequently created through an identical mold-guided drying process on the predefined electrodes. Small-scale transistor devices with both shortened channel length and width exhibited a good high-frequency response because of the weak capacitive effect. This is particularly advantageous for electrochemical transistors since the use of conventional fabrication techniques is extremely challenging in this case. In addition, modified polymer chain alignment of the assembled PEDOT:PSS lines during the drying process was observed by optical and electrical measurement. The mold-guided drying technique has been proven to be a promising method to fabricate small-scale devices, especially for biological applications.


INTRODUCTION
Recent developments in solution-processable devices/systems have revived interest in micrometer-and nanometer-scale drying processes. Considerable efforts have been made to generate high-resolution patterns or structures using various methods including (a) controlling solvent evaporation, such as inkjet printing, 1,2 (b) line-wise deposition by pulling a sharp blade, 3−5 and (c) repeated pinning/depinning of the contact line between the drying solution and substrate. 6−8 Edge deposition induced by template confinement has also been used to form highresolution structures including the progressive shrinkage of capillary bridge and groove pinning. 9−11 Direct patterning of materials from the solution has several advantages: high resolution, inexpensive cost, and applicability to a range of situations where conventional lithography technology may not be suitable due to the induced material degradation by UV irradiation and chemical reaction during the development process.
Fine patterning of ion-sensitive materials to form small-sized devices that permit high-frequency detection and high-density device packing is undoubtedly attractive. For instance, a small ion-sensitive transistor array can be used for high-resolution biorecording where the ion concentration fluctuates within the subcellular domain. 12,13 Ion-based synaptic transistors with a proper dopant in the active layer could generate reprogrammable and multiple states with stable conduction, which are promising for next-generation neuromorphic computing, and the miniaturization of such synaptic transistors is significant for large-scale integration. 14 Poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) is a well-known conducting polymer, which is a blend of cationic polythiophene derivatives doped with a polyanion. 15−17 The polymer conductance is extremely sensitive to ion doping from the environment by modifying the redox conditions of the PEDOT polymer. 18−20 Various techniques have been used to pattern PEDOT:PSS materials, which include mask etching, 11,21,22 soft lithography, 23 nanoimprinting, 24 pulsed laser ablation, 25 direct UV patterning, 26 selective polymerization, 27,28 and electrospinning. 29 Most of the approaches are mainly limited by insufficient spatial resolution. Although electrospinning, nanoimprinting, and mask etching techniques can be used to produce submicron PEDOT:PSS, they demand further pattern transfer by plasma etching or face challenges to form a well-defined pattern. To this end, the development of techniques that enable direct patterning of PEDOT:PSS with high resolution is valuable.
In this paper, a novel patterning process based on the moldguided drying technique 11 was used in this paper to pattern PEDOT:PSS directly from its water suspension and to fabricate PEDOT:PSS-based organic electrochemical transistors (OECTs). In the process, gold electrodes with narrow channels were first defined by creating polymer lines with a mold-guided drying technique and transferring the pattern through a lift-off process. Then, the mold-guided drying process was repeated to form PEDOT:PSS lines over the electrodes through proper solution formulation. The OECT with both shortened channel length and width demonstrated good high-frequency and multifrequency responses. In addition, the alignment of PEDOT:PSS chains during formation of the assembled lines was observed, and it was confirmed by electrical and optical measurements.

RESULTS AND DISCUSSION
2.1. Principle of PEDOT:PSS Patterning. The formation process used for PEDOT:PSS patterning is schematically shown in Figure 1. A formulated PEDOT:PSS water suspension was introduced between a substrate and a polydimethylsiloxane (PDMS) mold. As the solvent evaporated, capillary bridges of the solution formed, and the liquid/substrate contact lines were pinned at the mold grooves. Thin PEDOT:PSS lines were subsequently formed next to the groove-pinned contact lines. To be more specific, a controlled volume of the formulated solution (∼3 μL, for PDMS mold with a 1 cm 2 area of line pattern) was drop-casted onto the surface of the structured PDMS mold. Then, a substrate was gently brought into contact with the solution-wetted mold and left dried at room temperature for 4 h under a small applied pressure (about 5 MPa) (Figure 1a,b). Finally, the PDMS mold was removed, leaving the patterned PEDOT:PSS on the substrate (Figure 1c). The groove depth of the PDMS mold was 1.5 μm, and the line width/separations varied from 40 to 120 μm.
The PEDOT:PSS lines were formed by a single patterning process (Figure 2a), and a sequential double patterning process can be used to generate grid structures ( Figure 2b). The existing structure on the substrate does not affect the subsequent patterning because the capillary bridges are pinned at the mold grooves. We also observed that the PEDOT:PSS solution formulation was critical for the successful patterning, which was discussed in Section 4. Apart from the modification of the surface property of the substrate and surface tension of the solution, both ethylene glycol and triton can affect the polymer chain alignment and interaction between the PEDOT and PSS molecules, which might contribute to the patterning process and the resulted structures. 30 The typical line width was measured to be 700 nm, and the width of PEDOT:PSS wires can be controlled by adjusting the solution concentration. It was also found that the profile of the patterned PEDOT:PSS wire was approximately a triangle.
2.2. PEDOT:PSS Wire Transistors. The PEDOT:PSSbased OECT was fabricated by first preparing electrodes with small gaps using a similar process. Silicon with a 300 nm-thick SiO 2 layer or glass was used as the substrate. A 250 nm-thick polydimethylglutarimide (PMGI) layer was spin coated on the substrate and baked for 30 min at 200°C. A 10 nm-thick Ge layer was then thermally evaporated on top as an etching mask (in CF 4 ) for subsequent PMGI etching (in oxygen).Poly-4-  ACS Applied Electronic Materials pubs.acs.org/acsaelm Article vinylphenol (PVP) lines were formed on the Ge layer using the method described in Figure 3a. CF 4 plasma was used to etch through the Ge layer with the PVP line as the etching mask, and this was followed by oxygen plasma to etch through the PMGI using the Ge layer as the etching mask ( Figure 3b). Then, Au/Cr (30 nm/10 nm) films were deposited by thermal evaporation, and the lift-off process in a Microposit Remover (1165) was applied to complete the electrode fabrication ( Figure 3c). 31,32 Finally, PEDOT:PSS lines were created on top of the fabricated Au electrodes (Figure 3d) and annealed at 140°C for 1 h in air. Figure 4a,b shows the device image and testing circuit used for device characterization. The silver conducting wire and 0.1 M NaCl aqueous solution were used as the gate electrode and electrolyte, respectively. A plastic ring (6 mm in diameter) was placed over the sample to contain the electrolyte. The transistors were evaluated under a small gate and drain voltages (<1 V). The drain current decreased as the gate voltage increased, i.e., the device worked with a depletion model because of the partial balance of the negatively charged PSS − by Na + , which de-dopes the PEDOT (Figure 4c,d). The OECT with a small size is crucial for high-resolution sensing, like bio-recording, where the ion concentration varies within the subcellular domain. Due to the short device channel, the current density was one order of magnitude higher than that with the PEDOT:PSS wires defined by mask etching. 11 The PEDOT:PSS-based OECT with small size is also promising in applications like a synaptic device, which is an essential component in artificial neuromorphic network (ANN). Figure 4e shows the schematic illustration of signal transformation between neurons through synapses. The OECT can realize a similar function when the gate voltage and the drain current are analogically treated as pre-synaptic and post-synaptic signals. 14 2.3. Frequency Response of the PEDOT:PSS Wire Transistor. The PEDOT:PSS wire transistor with a narrow channel length and channel width has been successfully fabricated, and its switching properties have been demonstrated. The contact area between the PEDOT:PSS wire and the electrolyte is considerably reduced compared to the spin-coated PEDOT:PSS ( Figure 5). As a result, the PEDOT:PSS wire transistor has a smaller capacitance between the electrolyte and the active material. This is particularly important for highfrequency operation since the RC time constant (τ = R × C) is significantly reduced. In order to investigate the high-frequency response of the PEDOT:PSS wire transistor, experiments were conducted with various gate frequencies. The frequency response was measured with an oscilloscope on the voltage drop across a resistor connected in series in the drain current loop. Figure 6 shows the frequency response of different input signals, and the device exhibited a good frequency response. Multiple-input response was also measured, which widely exists in the signal processing in the neuron system ( Figure 4e) and can be potentially applied for logical calculation. 33,34 Multiple inputs were realized by using multiple gates, and each gate was powered by a signal generator. Figure 6a shows the single-input response at 1 kHz, while Figure 6b−e shows the multiple-input responses of the transistor and the simulated results. The resulting output spectra are well described by a summation of multiple input signals and can be conveniently decomposed at high accuracy, which makes the response to very small variations between inputs possible. For instance, a tiny phase change (less than 0.01π) of the inputs can be read out in the output spectra (Figure 6e). The over 1 kHz bandwidth of our OECT is sufficient to record the biosignal in medical application, like brain mapping. 19 Device performance can be further improved in future research by adding an insulating layer between the liquid and electrodes, which is expected to reduce the parasitic effects.
2.4. Molecular Alignment of PEDOT:PSS Wires during Formation. The device fabrication and characterization have been presented in the previous sections. Moreover, the molecular alignment of PEDOT:PSS during the guided drying process is expected due to the hydrophobic behavior of PDMS. This was confirmed during the experiment 11 by optical and electrical measurements. Polarized ultraviolet−visible absorption spectroscopy was first used to indicate the polymer alignment, and a maximum absorption is expected when the transition dipole moments align with the polarization direction of light since the transition dipole moment of the conjugated polymers are oriented in the direction along the polymer where I is the electrical current, l is the conductive length of the PEDOT:PSS, A is the cross-section area, and V is the voltage  ACS Applied Electronic Materials pubs.acs.org/acsaelm Article applied. As I × l/A = σ × V, the conductivity (σ) can be read out from the slope of the (I × l/A) ≈ V plot, as shown in Figure 8. Moreover, the ratio of conductivity between PEDOT:PSS wires and the thin film (σ line /σ film ) was extracted to be 2.05, which indicates that the PEDOT:PSS chains were preferentially aligned in the wire direction. The preferential alignment of the PEDOT chain can be explained by the deposition and collapsing of gel-like particles

CONCLUSIONS
We have applied the mold-guided drying process to pattern PEDOT:PSS with high resolution directly from its water-based suspension, and a small-sized OECT was successfully fabricated. The transistor characteristics of the OECT with small-scale channel width and channel length were demonstrated, and its high-frequency and multiple-input response were also investigated. In addition, PEDOT:PSS chain alignment of the assembled lines was observed and verified by optical and electrical measurements. Such ion-sensitive transistors with small dimensions are particularly promising for high-resolution electrophysiological recording and sensing.

EXPERIMENTAL METHOD
4.1. Template Preparation and Line Forming. The PDMS template with proper line patterns was made with a commercial silicone elastomer (Sylgard 184, Dow Corning). The silicone elastomer, consisted of a two-part liquid component kit (a 10:1 mix ratio), was poured onto a photoresist master predefined by optical lithography. The thickness of the photoresist carried on a Si wafer was 1.5 μm, which defines the groove depth of the PDMS template. After curing of the silicone elastomer along with the photoresist master at 70°C for 1 h, the PDMS elastomer with a duplicated pattern was peeled off from the master. PEDOT:PSS and PVP patterning was performed with a selfdeveloped stainless-steel clamping platform, which enabled the proper alignment of the substrates and PDMS template, and adjustment of the applied pressure was done.

Device Fabrication and Measurements.
To fabricate the patterned PEDOT:PSS wire transistors, the PEDOT:PSS solution was formulated with a PEDOT:PSS (Clevios PH-1000) water-based suspension (from Heraeus) by adding 20% ethylene glycol and 0.92% Triton X-100 (Sigma-Aldrich) (both in volume percentage) to the PEDOT:PSS suspension in order to improve material conductivity and reduce the surface tension. It was observed that the PEDOT:PSS solution formulation was critical for the PEDOT:PSS patterning, which is the reason why direct patterning of PEDOT:PSS could not be previously achieved by mold-guided drying. The pattern quality is particularly sensitive to the amount of Triton X-100 added to the solution. It was found that a high-quality pattern is obtained when 0.92 mg/mL Triton X-100 was contained in the formulated solution. Insufficient addition of Triton causes line noncontinuity induced by the high surface tension of the liquid. In contrast, a higher concentration of Triton often induces residual material between the patterned lines. The PEDOT:PSS wire transistors were tested with an Agilent 4156A precision semiconductor parameter analyzer (Yokogawa-Hewlett-Packard Ltd., Tokyo, Japan).

Analysis of Molecule Alignment and Electric
Conductivity. To analyze electric conductivity, a 90 nm-thick PEDOT:PSS film was spin-coated onto a SiO 2 (300 nm)/Si substrate and annealed at 100°C for 10 min. PEDOT:PSS wires were fabricated using the method developed and annealed at the same condition. The Jasco V-670 absorption spectrometer with an attached linear polarizer was used to measure the absorption of the patterned PEDOT:PSS wires. The cross-section area of the PEDOT:PSS wires was measured by a profilometer.     J.L. and X.C. contributed equally to this work, carrying out the experiments and analyzing the data. S.L. designed and carried out the experiments. E.K.W.T. and P.K.S. helped to optimize the fabrication process. S.L., J.L., X.C., and D.C. prepared the manuscript. D.C. supervised and managed the project.

Notes
The authors declare no competing financial interest. The data that support the findings of this study are available from the corresponding author upon request.

■ ACKNOWLEDGMENTS
We would like to thank the Bio-lab of CPDS, University of Cambridge, and the financial support of UK Engineering and Physical Sciences Research Council (EPSRC) through the EPSRC Centre for Doctoral Training in Integrated Photonic and Electronic Systems (EP/L015455/1). S.L. thanks the financial support from Guangdong Basic and Applied Basic Research Foundation (grant no. 2019A1515011673). X.C. thanks the China Scholarship Council for PhD studentship funding. All data are available in the main text and the Cambridge University's repository.