Additive-Induced Morphology Change of Polymer Film Enables Enhanced Charge Mobility and Faster Organic Electrochemical Transistor Switching

Abstract

Conjugated polymers (CPs) play an important role in organic electrochemical transistors (OECTs) for bioelectronics and related applications, where they serve as channel materials. Currently, most successful polymers for CPs are re-engineered from traditional CPs by replacing hydrophobic alkyl side chains with hydrophilic ethylene glycol or ionic groups. Frustratingly, the enhanced ion transport often compromises the charge mobility of the original CP. In this work, we present an additive-mediated method to construct a modified poly­(3-hexylthiophene) (P3HT) film to enable efficient ion migration. The additive is designed with a cleavable diazo group that releases nitrogen and 2-methoxyethanol, a volatile compound, to alter the P3HT film morphology. OECTs based on the film exhibit improved response times. Interestingly, the process also enhances the crystallinity of P3HT, leading to higher hole mobility compared with pristine P3HT. This study proposes an in situ strategy to achieve the functionality of the OMIEC via morphological regulation, offering a promising route to simultaneously enhance both ion accessibility and charge mobility.

Introduction

Organic mixed ionic–electronic conductors (OMIECs) have emerged as a highly promising class of materials integrated into organic electrochemical transistors (OECTs) for next-generation bioelectronics, − neuromorphic computing, , electrophysiology, , and soft logic circuits , due to their high transconductance, low operation voltage, and compatibility with aqueous environments. − Compared to the conventional organic field-effect transistors (OFETs), OECTs distinguish themselves by modulating channel conductivity of OMIECs through gate-bias-driven ion migration into the film, coupled with an electrochemical doping/dedoping process. This unique feature makes OECTs well-suited for bridging biological and electronic interfaces. , Given their broad application potential, the design and synthesis of polymer-based OMIECs are of great importance. However, a longstanding challenge in the field is the inherently limited ionic conductivity of conventional conjugated polymers (CPs), which are typically engineered for electronic performance alone and thus are hydrophobic and ion-impermeable. ,

To address this challenge, extensive efforts have been devoted to synthesizing new OMIEC polymers incorporating ion-conducting (hydrophilic) side chains or backbone modifications. − While these approaches have successfully improved ionic transport, the syntheses often lack broad modularity. The necessity to design and synthesize entirely new polymers for each specific application presents significant barriers to scalability, reproducibility, and high-throughput material screening. In contrast to the well-studied traditional CPs used in OFETs, charge transport in OECTs occurs within a dynamic 3D network that is intricately coupled with ion migration. , Consequently, the structure–property relationships critical to OMIEC performance in OECTs remain insufficiently and poorly understood, thereby hindering the development of high-performance OMIEC materials.

A promising alternative would be to directly endow ion migration to the existing well-established CPs without the need for an extensive molecular redesign. This can be achieved by using several different methods. For instance, the introduction of an extra ionic exchange gel interfacial layer between the electrolyte and the channel has been shown to enhance the faster transient response and increase higher transconductance of hydrophobic poly­[2,5-bis­(3-tetradecylthiophen- 2-yl) thieno­[3,2-b]­thiophene] (PBTTT) and poly­[(2,6-(4,8-bis­(5-(2-ethylhexyl)­thiophen-2-yl)-benzo­[1,2-b:4,5-b′]­dithiophene))-alt-(5,5-(1′,3′-di-2-thienyl-5′,7′-bis­(2-ethylhexyl)­benzo­[1′,2′-c:4′,5′-c′]­dithiophene-4,8-dione))] (PBDB-T). Similarly, replacing aqueous electrolytes with polymeric ionic liquids has demonstrated to improve the compatibility with the hydrophobic poly­(3-hexylthiophene) (P3HT). More recently, blends of hydrophobic CPs and hydrophilic photo-cross-linker were constructed to achieve separated ionic–electronic conduction within the film, leading to much higher transconductance and lower threshold voltage. In addition, the morphology regulation approaches have also been proven to improve the OECT performance of hydrophobic CPs. These include physically blending regioregular and regiorandom P3HT, constructing porous P3HT films through the breath figure growth method, and patterning striped microstructures in P3HT through photolithography. However, a common limitation of these approaches is the trade-off between ion migration and charge mobility. Enhancing ion transport often comes at the expense of reduced electronic mobility in CPs.

Here, we report an additive-mediated approach to improve the response time in an OECT, without the need to synthesize a new polymer. The morphology of the P3HT film is altered by using a degradable and evaporable small-molecule additive (IML), which interacts with P3HT during film formation before being cleared during thermal annealing (TA) (Figure ). Surprisingly, the morphology change enabled by the IML does not disrupt the P3HT crystallinity; instead, it enhances its order, which results in higher hole mobility compared to pristine P3HT in OFETs. Benefiting from the enhanced surface roughness, films processed from P3HT:IML blends (referred to as IML-processed P3HT or P3HT:IML in figure legends for brevity) exhibit greater ion accessibility. These results demonstrate a new strategy for creating OMIECs directly from traditional hydrophobic CPs, offering promising potential for OECT fabrication.

1.

Illustration of the molecule-mediated formation of OMIECs through morphology regulation.

Experimental Section

IML Synthesis

The scheme shown in Figure S4 was followed.

Step 1: A mixture of 2-methoxyethanol (7.4 g, 97 mmol) and triethylamine (13.1 g, 130 mmol) was prepared using dichloromethane (200 mL), to which phenylacetyl chloride (10.1 g, 65 mmol) was added dropwise over a period of 30 min. After completion, the reaction mixture was cooled to room temperature and added to ice water. It was then extracted three times by using dichloromethane and washed with saturated aqueous NaCl. The organic residue was dried using anhydrous MgSO₄ and filtered. The organic solvent was removed by rotary evaporation. The crude product was further purified by chromatography on silica gel, yielding a colorless liquid as the product (11.1 g, yield 88%).

¹H NMR (400 MHz, chloroform-d) δ 7.37–7.22 (m, 5H), 4.27–4.23 (m, 2H), 3.66 (s, 2H), 3.60–3.56 (m, 2H), 3.36 (s, 3H). ¹³C NMR (101 MHz, CDCl₃) δ: 171.63, 133.93, 129.31, 128.57, 127.11, 70.40, 63.92, 59.00, 41.15.

Step 2: To a round-bottom flask with the compound obtained in the first step (6.0 g, 30.9 mmol) were added DBU (7.7 g, 46.4 mmol) and p-acetamidobenzene-sulfonyl azide (p-ABSA) (8.9 g, 37.1 mmol) in MeCN (150 mL). The mixture was stirred overnight at room temperature. After completion indicated by TLC, the resulting mixture was poured into water, extracted with dichloromethane three times, and then washed with saturated aqueous NaCl. The organic residue was dried with anhydrous MgSO₄ and filtered. The organic solvent was removed by rotary evaporation. Then, the crude product was purified by chromatography on silica gel, and an orange liquid of IML was obtained (5.8 g, yield 85%).

¹H NMR (400 MHz, chloroform-d) δ 7.51–7.46 (m, 2H), 7.42–7.35 (m, 2H), 7.18 (ddt, J = 7.7, 7.1, 1.2 Hz, 1H), 4.46–4.37 (m, 2H), 3.71–3.63 (m, 2H), 3.41 (s, 3H). ¹³C NMR (101 MHz, CDCl₃) δ: 165.11, 128.95, 125.86, 125.48, 124.01, 70.55, 63.91, 59.05.

OFET Fabrication and Measurements

Top-Gate–Bottom-Contact (TGBC) Devices

TGBC devices were fabricated on an alkaline-free glass substrate, where Cr/Au source and drain electrodes were patterned via photolithography. The glass substrates were sonicated with isopropanol for 10 min, rinsed in boiled isopropanol for 10 min, and then subjected to UV-ozone treatment for 30 min. The cleaned substrates were treated with 1-octanethiol (OT) to modify the Au source and drain electrodes. Polymer thin films were spin-coated from chlorobenzene solutions (5 g L^–1) for 30 s in a glovebox and were then thermally annealed at 150 °C for 30 min. The PMMA dielectric layer (C _i = 3.6 nF cm^–2) with ca. 550 nm thickness was spin-coated on top of the polymer layer and then was dried at 80 °C for 3 h. Finally, the Ag layer (100 nm) was evaporated on top of the gate electrode through a shadow mask.

Bottom-Gate–Bottom-Contact (BGBC) Devices

BGBC devices were fabricated on a heavily doped p⁺-Si­(100) wafer with 300 nm thickness thermally grown SiO₂ (C _i = 10.0 nF cm^–2) where Au electrodes were patterned. The Si/SiO₂ substrates were sonicated with water for 3 min thrice and acetone and isopropanol for 10 min, respectively, rinsed in boiled isopropanol for 10 min, and then subjected to UV-ozone treatment for 30 min. The cleaned substrates were treated by octadecyltrichlorosilane (ODTS) as surfactant to modify the SiO₂ surface and then by 1-octanethiol (OT) to modify the Au source and drain electrodes. For the ODTS treatment, the substrates were soaked in a solution of ODTS in toluene (3 mM) and heated at 60 °C for 1 h on the hot plate. For the OT treatment, the substrates were soaked in a solution of OT in toluene (10 mM) for 10 min at room temperature. The substrates were then rinsed with water and boiled isopropanol. Polymer thin films were spin-coated using the same condition as the TGBC devices.

Measurement and Characterization

Current–voltage characteristics were measured at room temperature under vacuum conditions with a KEYSIGHT B2902A and semiconductor parameter analysis software (SYSTEMHOUSE SUNRISE). Threshold voltages were estimated from the transfer plots by extrapolating the square root of the drain current to the horizontal axis. Hole field-effect mobility (μ) was extracted from the square root of the drain current in the saturation regime by using the following equation:

$$\mu =\frac{2L}{W{C}_i}{\left(\frac{d\sqrt{|I_d|}}{d{V}_g}\right)}^2$$ 1

where L (25 μm) and W (10000 μm) are channel length and width, respectively, and C _i is capacitance of the gate insulator. The average hole mobilities and threshold voltages were obtained for more than eight devices.

OECT Device Fabrication and Measurements

Fabrication Process

Gold electrodes were patterned by using a photolithography process, as described below: First, silicon wafers (p-type, boron-doped, 300 nm SiO₂, Sokatek) were cleaned by sonication in acetone and isopropyl alcohol, then dried with a nitrogen flow and heated on a hot plate at 120 °C for 10 min. A negative photoresist (NR9-3000PY, Futurrex) was spin-coated onto the cleaned substrates and exposed to UV light through a photomask (MA6 mask aligner, Suss Microtech). Development was performed in 2.38% TMAH. A 10 nm chromium layer followed by a 100 nm gold layer were deposited by electron beam evaporation (MEB550S, Plassys). Resist lift-off was achieved by soaking the substrates in acetone with gentle agitation. The OECT devices were then cleaned by sonication in acetone and isopropyl alcohol and dried with a nitrogen flow. Inside a nitrogen-filled glovebox, polymer films were formed by spin-coating at 1000 rpm for 60 s. The polymer was carefully removed around the channel area by using a cotton swab dipped in chloroform. Finally, an insulating layer (nail polish, Sally Hansen) was applied to cover the electrodes. Devices with a channel length of 10 μm and widths ranging from 100 to 4000 μm were characterized in air at room temperature using a homemade setup. The setup included a 3D-printed substrate holder with a PDMS electrolyte reservoir, a Ag/AgCl pellet electrode (Warner Instruments) as the gate, and two Keithley 2401 source meter units.

Measurement and Characterization

Output curves were obtained by sweeping the drain voltage V _D from 0 V to −0.7 V (0.05 V steps with 10 s between each step) while keeping the gate voltage V _G fixed (from −0.1 V to −0.7 V with 0.1 V steps). Transfer curves were obtained by sweeping the gate voltage V _G from 0 V to −0.7 V (0.025 V steps with 10 s between each step) while keeping the drain voltage V _D constant at −0.7 V. The response times of the OECTs was extracted from time-resolved drain current measurements following a step change in gate voltage. A constant drain bias of −0.7 V was applied while the gate voltage was stepped from 0 V to −0.6 V, and the transient drain current was recorded. The steady-state drain current I _D,ss was defined as the average current value after the transient had fully stabilized. t ₉₀ was then determined as the time required for I _D to reach 90% of the steady-state drain current.

Results and Discussion

The IML was synthesized via a straightforward two-step procedure. Phenylacetyl chloride was reacted with 2-methoxyethanol to yield the intermediate ester, which was then reacted with excess p-acetamidobenzene-sulfonyl azide to form the cleavable and evaporable diazo compound (IML) in a yield of 85%. IML was further purified by column chromatography and obtained as an orange liquid. The synthetic route (Figure S4) is shown in the Supporting Information (SI), and detailed synthesis procedures are described in the Experimental Section. Thermogravimetric analysis (TGA) (Figure S5) shows that the decomposition of the IML begins at approximately 100 °C, indicating its thermal cleavability. Differential scanning calorimetry (DSC) analysis (Figure S6) further reveals that nitrogen release from the IML initiates at around 120 °C and is nearly complete at 150 °C.

The molecular design of IML is based on two key considerations: (1) a diazo compound can release nitrogen gas upon heating, which facilitates the formation of a rougher structure within the host CP thin film; (2) 2-methoxyethanol, with its low boiling point, serves as a cleavable protecting group that can readily evaporate, further contributing to enhanced surface roughness in the film. To fabricate the film, P3HT and IML were physically blended with a weight ratio of 3:2, and the resulting solution was spin-coated to form the film. The resulting film was thermally annealed (TA) at 150 °C to ensure complete decomposition of the IML and evaporation of its byproducts. To confirm the decomposition of the IML, the nuclear magnetic resonance (NMR) spectra of neat P3HT, a mixture of P3HT and IML, and the TA IML-processed P3HT film were compared (Figure S7). The disappearance of IML-associated NMR peaks in the annealed blend indicates near-complete decomposition of IML. Importantly, the peaks corresponding to P3HT remained unchanged, suggesting that the polymer is preserved during the process. Additionally, X-ray photoelectron spectroscopy (XPS) was used to characterize the thin film composition to further verify the removal of the IML (Figure S8). The XPS spectrum of the annealed IML-processed P3HT film showed no significant differences from that of pristine P3HT, providing further evidence for the effective elimination of IML after thermal annealing.

The decomposition of the IML was expected to promote the formation of a rougher film. To examine the resulting surface morphology, atomic force microscopy (AFM) was performed (Figure a–d). Mediated by the cleavage and evaporation of the small molecule, the film of IML-processed P3HT showed a dramatic change in morphology compared to that of pristine P3HT, revealing a significantly rougher surface. The root-mean-square roughness (R _q) of IML-processed P3HT is 3.42 nm, which is over 3.5 times that of pristine P3HT. In contrast to the smooth topography of pristine P3HT, the IML-processed P3HT film showed pronounced height variations across the surface (Figure e,f). To explore the generality of this morphological modulation strategy, two other classical CPs (N2200 and DPPTT) were also processed by the same method using IML as a morphology regulator. As shown in Figure S9, both IML-processed N2200 and DPPTT films displayed rougher surfaces compared to their pristine counterparts. The roughness values of IML-processed N2200 and DPPTT are approximately 3 and 2 times higher, respectively, than their unmodified pristine counterparts. Furthermore, scanning electron microscopy (SEM) was also used to investigate their morphological change, as in Figure S10. In the IML-processed P3HT film, the rougher surface can be clearly observed, consistent with the AFM findings. Collectively, these results demonstrate that the incorporation of IML offers an effective and general strategy for inducing surface roughness in CP films.

2.

(a–d) AFM height images of P3HT and IML-processed P3HT (labeled P3HT:IML) films. (e) Height distribution and (f) surface depth histograms of P3HT and IML-processed P3HT films.

As shown in the UV–vis absorption spectra (Figure a) and summarized in Table , pristine P3HT exhibits a maximum absorption peak at 556 nm, while the IML-processed P3HT film shows a slight red-shift to 559 nm. In addition, the intensity of the 0–0 vibronic peak with respect to the 0–1 peak in the IML-processed P3HT film is significantly enhanced (Table ). This red-shift, along with the pronounced enhancement of the 0–0 peak, suggests increased intrachain π-electron delocalization. These changes are likely induced by the interaction between IML and P3HT during film formation. We speculate that the physical blending, followed by thermal cleavage and evaporation of IML, drives the P3HT chains into a more planar and ordered conformation, enhancing intrachain coupling and molecular packing. We also note that the peak absorbances are nearly identical for both films, implying that a similar amount of P3HT is present in both films. Given the measured thickness increase (∼30 nm for pristine P3HT to ∼35 nm for IML-processed P3HT), the similarity in absorbance suggests a reduction in the effective film density for the IML-processed film.

3.

(a) Unnormalized UV–vis spectra of P3HT and IML-processed P3HT films. (b) Cyclic voltammograms of P3HT and IML-processed P3HT films tested in 0.1 M aqueous KPF₆ (first cycle). (c, d) Spectroelectrochemical analysis results of P3HT and IML-processed P3HT films tested in 0.1 M aqueous KPF₆. (e, f) Absorbance and the change of absorbance of π–π* transition and polaron peak obtained from P3HT and IML-processed P3HT polymer films with different offset voltages: for P3HT, a π–π* maximum peak at 545 nm, a polaron maximum peak at 820 nm, for IML-processed P3HT,a π–π* maximum peak at 560 nm, a polaron at maximum peak at 820 nm.

1. Photophysical and Energy Level Properties.

Film	λmax,film [nm]	λonset,film [nm]	E g,opt [eV]	I 0–0/I 0–1	E ox,aq [V]
P3HT	556	648	1.91	0.72	0.34
P3HT:IML	559	649	1.91	0.82	0.36

^a E _g = 1240/λ_onset,film.

^bMeasured employing a 0.1 M aq. KPF₆ solution.

Cyclic voltammetry (CV) was performed on the polymer films in an aqueous solution of 0.1 M KPF₆ (Figure b). Both pristine P3HT and IML-processed P3HT films exhibit anodic and cathodic waves in their CV curves, indicating that both materials can undergo reversible oxidation and reduction in aqueous electrolytes. The onset oxidation potentials of the two polymer films are similar to each other (0.34 V for P3HT and 0.36 V vs Ag/AgCl for IML-processed P3HT). To quantify the charge storage properties, the volumetric capacitance (C*) was calculated using the measured film thicknesses (30 nm for pristine vs 35 nm for IML-processed) as described in the SI. The resultant values are comparable with C* being 32.8 ± 2.5 F cm^–3 for pristine P3HT and 31.4 ± 0.9 F cm^–3 for IML-processed P3HT (Figure S2). Obtaining C* values using EIS proved to be challenging due to difficulties in fitting the data to standard equivalent circuit models. The similarity in C* values suggests that the intrinsic charge storage capacity is largely unchanged by the processing. However, the IML-processed film exhibits a larger integrated area under the CV curve. Given the similar optical absorbance despite increased film thickness, the higher total charge suggests greater ion accessibility.

Spectroelectrochemical measurements were conducted in a 0.1 M KPF₆ aqueous solution to investigate the oxidation characteristics of the two polymer films. As shown in Figure c,d, both P3HT and IML-processed P3HT films exhibit clear electrochromic responses as the applied potential is swept from 0 to 0.9 V versus Ag/AgCl. Upon application of an oxidative potential starting at 0.4 V and increasing to 0.9 V (Figure e), both films show a gradual decrease in the intensity of their ground-state absorption bands, accompanied by the emergence of a broad polaron band centered around 820 nm, which is indicative of progressive oxidation and polaron formation. It is noteworthy that, even in the presence of the electrolyte, the IML-processed P3HT film displays a red-shifted absorption peak at 560 nm, compared to 545 nm for pristine P3HT, again suggesting greater intramolecular π-electron delocalization. As illustrated in Figure f, the change in absorbance upon oxidation of both films shares a similar trend but is larger for IML-processed P3HT than for pristine P3HT, indicating a higher degree of electrochemical activity. These results suggest that IML-mediated P3HT films exhibit improved electrochemical doping efficiency and more effective modulation of ionic and electronic transport, making them promising candidates for OECT applications.

To further investigate the impact of IML processing on P3HT morphology, grazing-incidence wide-angle X-ray scattering (GIWAXS) was performed to analyze the molecular packing and structural order (Figure ). Consistent with previously reported results, , the P3HT film predominantly adopted an “edge-on” orientation relative to the substrate. This was also seen in the IML-processed P3HT films. In the out-of-plane (OOP) direction, both films show a series of peaks corresponding to (100), (200), and (300) at Q _z = 0.39, 0.77, and 1.14 Å^–1, respectively (Table S3). The (100) peak corresponds to a lamellar spacing of 16.1 Å, which aligns with literature values and remains unchanged after being treated with the IML. As shown in Table S3, the relatively more pronounced higher-order (200) and (300) peaks in the IML-processed P3HT film further confirm that IML treatment can enhance the crystallinity and order of P3HT, resulting in a higher degree of long-range order in the lamellar stacking direction. In the in-plane (IP) direction, both films display similar (010) peaks at ∼1.65 Å^–1, corresponding to a π–π stacking distance of approximately 3.80 Å. As with the OOP direction, the (010) peak in the IML-processed P3HT film is similar to that of pristine P3HT, but the ratio of π-stacking intensity to halo intensity is greater for IML-processed P3HT than that of P3HT, indicating improved π–π stacking and greater molecular order in the IP direction. These findings suggest that IML induces densification of the polymer domains, leading to enhanced crystallinity, improved long-range lamellar order, and more coherent π–π stacking in both OOP and IP directions.

4.

(a, b) 2D GIWAXS spectra of P3HT and IML-processed P3HT films. (c, d) Linecuts of the 2D GIWAXS spectra in the in-plane (Q _xy ) and out-of-plane (Q _z ) directions.

To evaluate the effect of IML mediation on the charge carrier mobility, both top-gate–bottom-contact (TGBC) and bottom-gate–bottom-contact (BGBC) organic field-effect transistors (OFETs) were fabricated to understand their differences (Figure and Figure S11), following the established protocols. , The key operational characteristics of these devices are summarized in Table S2. Both devices operate as p-channel accumulation-mode transistors. As seen in Figure b, the slope for the IML-processed P3HT TGBC-structured device is significantly steeper than that for pristine P3HT. The average hole mobility (μ _hole) increases from 6.65 ± 1.6 × 10^–2 cm² V^–1 s^–1 for P3HT to 9.77 ± 1.5 × 10^–2 cm² V^–1 s^–1 for IML-processed P3HT. The lower threshold voltage can also be obtained in the IML-processed P3HT. The output characteristic curves (Figure c,d) further highlight the improved performance of the IML-processed P3HT. At the gate voltage of −80 V and drain voltage of −60 V, the IML-processed P3HT device achieves a maximum drain current of over 0.4 mA, whereas the pristine P3HT device reaches only ∼0.31 mA. This increase in drain current output also indicates more efficient charge transport in the IML-processed film. In addition, in BGBC-structured OFETs, the IML-processed P3HT-based device also shows higher mobility than that of P3HT (Table S2 and Figure S11). These enhancements in OFET performance correlate well with the increased crystallinity and improved molecular ordering observed in UV–vis absorption and GIWAXS measurements. The IML-mediated morphology modulation promotes tighter molecular packing and more coherent π–π stacking, facilitating more efficient intermolecular charge transport pathways.

5.

(a) Device architecture of TGBC-type OFETs. (b). Transfer characteristics of P3HT and IML-processed P3HT. (c, d) Output characteristic curves of P3HT and IML-processed P3HT under different drain voltages.

Based on the above characterizations, the IML-mediated films possess two key features relevant for the OMIEC performance: enhanced molecular packing (Figure ), leading to enhanced hole mobility (Figure ), and a rougher surface (Figure ). These findings suggest that the IML-processed P3HT film may be better suited in an OECT. To evaluate the combined effect of these two features, OECTs were fabricated using both P3HT and IML-processed P3HT films as active channels (Figure a). Under applied voltages (gate voltage (V _G) = −0.6 V, drain voltage (V _D) = −0.7 V), the IML-processed P3HT device achieved a higher and more stable “on” current of around −2.0 mA, in contrast to −1.7 mA for the pristine P3HT device (Figure b). Furthermore, the initial current peak for the IML-processed P3HT device is also sharper and higher than that of pristine P3HT, indicative of faster electrochemical doping. Further analysis indicates that the response time (t ₉₀) of the OECT devices is improved for the IML-processed films (Figure S3), decreasing from 17.9 s for pristine P3HT to 7.2 s. This improvement is maintained under repeated device operation after the initial break-in period, with IML-processed films having a t ₉₀ of 2.6 s compared to 6.8 s for P3HT after 48 cycles. As observed in Figure c, the output characteristics show that the IML-processed P3HT device achieves a higher saturation drain current (I _D) than the pristine P3HT. The transfer curves (Figure d) further support this trend; although both devices exhibit a similar threshold voltage (V _th) near −0.4 V, the IML-processed P3HT channel delivers a smoother and higher I _D response across the same range of gate voltages, with a slight shift in V _th.

6.

(a) Device architecture of the OECTs. (b). Transient response curves, (c) output characteristics, and (d) transfer characteristics of P3HT and IML-processed P3HT based OECTs using aq. 0.1 M KPF₆ as the electrolyte.

The OECT characteristics were extracted to calculate the product of mobility and volumetric capacitance (μC*) (Figure S1 and Table S1). The P3HT and IML-processed films exhibited comparable μC* values (130 ± 14 vs 134 ± 14 F cm^–1 V^–1 s^–1). We acknowledge that decomposing these values into separate μ and C* values involves geometric assumptions that may not be appropriate when comparing smooth vs rough films. Additionally, we acknowledge the C* values were not obtained using EIS due to the aforementioned issues. However, using the effective C* value from CV and the average profilometry thickness yields a calculated OECT mobility of 4.0 ± 0.5 cm² V^–1 s^–1 for P3HT, while the IML-processed P3HT shows a slightly higher mobility of 4.3 ± 0.5 cm² V^–1 s^–1, in line with the OFET mobility enhancement that was observed. Collectively, the results show that the IML-mediated processing introduces structural changes to the P3HT matrix, contributing to the distinct electronic and kinetic behavior of the resulting OECTs.

Conclusion

In summary, we demonstrated a simple, modular, and scalable strategy to achieve the OMIEC behavior directly from commercially available CPs, eliminating the need for additional synthetic procedures. Mediated by the IML, the morphology of P3HT can be altered from a smooth surface to a rougher surface, facilitating greater ion accessibility to the film. GIWAXS analysis revealed that the IML-processed P3HT film can achieve enhanced crystallinity and improved packing compared to the pristine P3HT. As a result, OFETs based on IML-processed P3HT achieve an enhanced hole mobility of 9.77 × 10^–2 cm² V^–1 s^–1, compared to that of the pristine P3HT. Benefiting from the rougher film, the IML-processed P3HT based OECTs exhibit a faster response time. This work demonstrates a straightforward method to modulate the properties of conjugated polymer films through morphological engineering. By adaptation of the microstructure of existing CPs, this additive-mediated approach offers a versatile tool for tuning device characteristics relevant to bioelectronics and neuromorphic applications.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsapm.5c04176.

• Details of the materials, instruments, experimental methods, characterization data, and figures (NMR, XPS, TGA, DSC, SEM) and the summary of OFET device performance and corresponding characteristics (PDF)

The authors declare no competing financial interest.