# Polymerization-Charge Controlled Threshold Voltage Tunability in Organic Electrochemical Transistors

Getnet Kassahun, Noor Azmira Binti Ahmad, François Danzé, Gilles Pecastaings, Martial Leyney, Daniele Mantione, Gerardo Salinas, Alexander Kuhn, Damien Thuau, Mamatimin Abbas

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Getnet Kassahun, Noor Azmira Binti Ahmad, François Danzé, Gilles Pecastaings, Martial Leyney, et al.. Polymerization-Charge Controlled Threshold Voltage Tunability in Organic Electrochemical Transistors. Advanced Electronic Materials, 2025, pp.e00357. 10.1002/aelm.202500357. hal-05375980

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# Polymerization-Charge Controlled Threshold Voltage Tunability in Organic Electrochemical Transistors

Getnet Kassahun, Noor Azmira Binti Ahmad, François Danzé, Gilles Pecastaings, Martial Leyney, Daniele Mantione, Gerardo Salinas, Alexander Kuhn, Damien Thuau,* and Mamatimin Abbas*

Threshold voltage ($V_{th}$) is an important parameter in organic electrochemical transistors (OECTs), which are widely used in bio-sensing and neuromorphic applications. In bio-sensors, low $V_{th}$, consequently near zero gate voltage at transconductance maximum, is appreciated in device/cell interfaces, while in neuromorphic devices, the threshold voltage determines the switching behaviour and energy efficiency of artificial synapses and neurons. In this work, electropolymerization is used to fine-tune the $V_{th}$ of azidomethyl-PEDOT based OECTs, a clickable analogue of PEDOT. The tuning is achieved through the control of the deposited charge density in a chronoamperometry mode. Azidomethyl-PEDOT based OECTs yield similarly high transconductance as PEDOT based ones, while reaching maximum transconductance near zero gate voltage. Comprehensive comparisons with PEDOT based OECTs in terms of morphologies, ion transport and capacitance show that Azidomethyl-PEDOT forms a thinner and flatter morphology under the same electropolymerization conditions, a contributing factor for its low ion mobility, and in turn the strong dependence of switching speed on the film thickness, revealing the underlying mechanisms for $V_{th}$ tuneability. This approach provides a simple and effective strategy for $V_{th}$ modulation in OECTs in certain organic mixed ionic electronic conductors of particular interest.

## 1. Introduction

Organic Electrochemical Transistors (OECTs) have emerged as powerful bioelectronic devices thanks to their mixed ionic-electronic conduction, high transconductance and natural adaptability to biological environments.[1,2] OECT-based biosensors

typically employ two strategies: functionalizing the gate electrode or the channel/electrolyte interface.[3] Until now, by far the most widely used OECT channel material is the polyelectrolyte complex of poly(3,4-ethylenedioxythiophene) with polystyrene sulfonate (PEDOT: PSS).[4] However, PEDOT lacks native functional groups for bioconjugation, and the excess PSS in PEDOT:PSS can be acidic or cytotoxic, hindering biomolecule interfacing.[5] Consequently, engineering the channel layer by introducing chemical handles has become essential particularly in OECT biosensing applications: the goal is to tether analytes (aptamers, enzymes, antibodies, etc.) without degrading the film conductivity or device performance.[6-8] To create biorecognition sites on PEDOT channels, several strategies have been developed. One approach is to synthesize EDOT monomers bearing pendant groups (e.g., -COOH, $-SO_3H$, $-N_3$) and homopolymerize or copolymerize them with EDOT, so that the resulting PEDOT films bear reactive “handles”.[9,10] For example, carboxylated-EDOT has been copolymerized with EDOT to give PEDOT-acid films; these surface -COOH groups were then coupled to laminin-derived peptides, which significantly enhanced neuronal adhesion without sacrificing conductivity.

G. Kassahun, N. A. B. Ahmad, F. Danzé, M. Leyney, D. Thuau, M. Abbas
Université de Bordeaux, CNRS, Bordeaux INP, IMS
UMR 5218, Pessac F-33607, France
E-mail: damien.thuau@ims-bordeaux.fr;
mamatimin.abbas@ims-bordeaux.fr

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/aelm.202500357

© 2025 The Author(s). Advanced Electronic Materials published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

DOI: 10.1002/aelm.202500357

G. Pecastaings
Centre de Recherche Paul Pascal
Université de Bordeaux, CNRS
UMR 5031, 115 avenue du Dr Albert Schweitzer, Pessac F-33607, France

D. Mantione
POLYMAT
University of the Basque Country UPV/EHU
Joxe Mari Korta Center, Donostia-San Sebastian 20018, Spain

D. Mantione
Ikerbasque
Basque Foundation for Science
Bilbao 48013, Spain

G. Salinas, A. Kuhn
Université de Bordeaux, CNRS, Bordeaux INP, ISM
UMR 5255, Pessac F-33607, France
Alternatively, biomolecule-friendly additives (for example polyethylene glycol,[11,12] gelatin,[13] chitin,[14] polypeptides[15] or biopolymer blends) have been mixed into PEDOT formulations to provide linking sites for biomolecules. Post-polymerization coupling strategies (e.g., carboxylated-EDOT plus EDC/NHS) and electrografting approaches provide complementary routes to install covalent handles on PEDOT films.[15–17] These functionalization schemes have enabled the stable anchoring of enzymes, antibodies, DNA strands and other bioreceptors to PEDOT channels, while largely preserving electrochemical properties of the polymer.[18]

Building on these ideas, recent work has introduced “clickable” OECT channels by incorporating azide or alkyne groups that can undergo Cu(I)- or strain-catalyzed cycloaddition. Fenoy et al. first demonstrated a PEDOT-based clickable OECT by synthesizing an azidomethyl-EDOT monomer (EDOT–N₃) and electrodepositing it onto an interdigitated electrode.[19] The resulting PEDOT–N₃ OECTs featured greatly reduced turn-on voltages compared to standard PEDOT devices, with the peak transconductance occurring near V<sub>G</sub>≈0 V. The azide-functional channel was then used to attach various alkyne-tagged probes. In all cases, the devices retained their electrical characteristics after functionalization, demonstrating that high-yield click chemistry can stably decorate OECT channels without compromising operation. Further interface engineering was reported by Fenoy et al. in 2023, in which attaching an azide-terminated HD22 aptamer via SPAAC (Strain-promoted azide-alkyne cycloaddition) produced thrombin-sensing OECTs with higher aptamer surface density and larger dynamic signal change than previous designs.[20] These studies illustrate that azide/alkyne OECTs offer a modular platform: click reactions can graft redox reporters or biorecognition layers with high yield and stability, and the devices maintain robust transconductance and low-threshold operation.

A near zero V<sub>G</sub> (g<sub>m</sub>, max) (gate voltage at the maximum transconductance) achieved through threshold voltage tuning is particularly desirable in bioelectronics, especially at device-cell interfaces, as cells/organs can be highly sensitive to applied electrical potentials.[21] Moreover, the threshold voltage of OECTs is a critical device parameter that must be taken into account when designing integrated circuits for various applications.[22] For example, in neuromorphic devices, the threshold voltage mainly determines the switching behavior and energy efficiency of artificial synapses and neurons, directly impacting the ability of the device to mimic biological neuronal responses.[23] Several strategies have been employed to tune the V<sub>th</sub> in OECTs, including modifying the channel length,[24] implementing dual-gate architectures,[25] incorporating chemically doped polymer gate electrodes,[26] or chemically adjusting the electrochemical potential at the gate.[27] While effective, these methods often involve complex fabrication processes and application limitations. In contrast, a recently reported approach using monomer blending offers a much simpler alternative.[28] Although this method enables V<sub>th</sub> tunability, the V<sub>G</sub> (g<sub>m</sub>, max) remains significantly offset from zero. While low threshold voltage and a near zero V<sub>G</sub> (g<sub>m</sub>, max) have been reported for PEDOT–N<sub>3</sub> based OECTs in the prior studies, the underlying mechanisms remain poorly understood. In this work, electropolymerization of EDOT–N<sub>3</sub> as a channel layer of OECTs is carried out and their performance is compared to that of EDOT-based devices. The devices were fabricated

under various electropolymerization conditions with a focus on the influence of the deposited charge. Our findings reveal that PEDOT-N₃ based OECTs exhibited tunable V<sub>th</sub> and V<sub>G</sub> (g<sub>m</sub>, max) depending on the amount of deposited charge, whereas PEDOT based OECTs display relatively constant V<sub>th</sub> and V<sub>G</sub> (g<sub>m</sub>, max) for the same conditions. The resulting channel layers were characterized in terms of the morphologies and the electrochemical properties through the measurement of ionic and electronic conductivities, as well as electrochemical impedance spectroscopy in order to understand the underlying mechanisms.

## 2. Results and Discussion

As shown in Figure 1a, a three-electrode potentiostatic cell allows depositing polymer from EDOT or EDOT-N₃ monomers onto Au source-drain electrodes, as described in our earlier studies.[29,30] The optical micrograph shows a continuous polymer film bridging the channel gap. The inset in Figure 1b compares single-cycle CVs (50 mV s⁻¹) for EDOT and EDOT-N₃ monomers. The EDOT oxidation peak is located near ≈1.4 V (vs. Ag<sup>+</sup>), whereas EDOT-N₃ requires ≈1.45 V. This anodic shift is expected as the azide pulls electron density, lowering the HOMO and rising the oxidation potential. In order to better control the polymer growth dynamics (thickness and conductivity), we used chronoamperometry to pass total charges ranging from 20 to 80 µC, enabling systematic investigation of charge-dependent film growth (Figure 1b). Films were grown potentiostatically at 1.4 V versus Ag<sup>+</sup>, a potential selected as the optimal bias to achieve maximal film conductivity for both monomers under identical conditions.

To elucidate the structural characteristics and to confirm the incorporation of azide functionalities within the electropolymerized azidomethyl-EDOT OECT channel, we conducted Fourier-transform infrared attenuated total reflectance (FTIR-ATR) spectroscopy on the EDOT and EDOT-N₃ monomers (Figure S5, Supporting Information), as well as the resulting electropolymerized films (Figure 1d). Within the 800–1500 cm⁻¹ range, stretching modes of the alkylenedioxy group (1019, 1080, and 1181 cm⁻¹), and C = C stretching vibrations (1380 and 1480 cm⁻¹) were observed.[31] Critically, a distinct vibration band at 2100 cm⁻¹, characteristic of the azido group,[32] was evident in the spectra of both the EDOT-N₃ monomer and the electropolymerized azidomethyl-PEDOT film, consistent with prior reports.[19,33] These spectroscopic findings unequivocally establish that the electropolymerized EDOT-N₃ monomer is indeed azidomethyl-PEDOT (PEDOT-N₃).

Electropolymerization conditions govern the nucleation and growth of PEDOT-type films, resulting in strikingly different morphologies for PEDOT versus its azide-derivatized analogue PEDOT-N₃. Under identical charge-controlled deposition, PEDOT builds up thick, porous networks, whereas PEDOT-N₃ remains comparatively thin and smooth. When EDOT is oxidized in a TBAPF₆/acetonitrile medium, PEDOT films grow vertically via instantaneous nucleation into cauliflower-like clusters several micrometres in height and exhibit high root-mean-square (RMS) roughness (hundreds of nm).[34,35] Quantitative thickness measurements (AFM profile) are provided in the Supporting Information; for equal deposition charge PEDOT films are significantly thicker than PEDOT-N₃ films (Figure S10, Supporting Information). AFM measurements by Scavetta et al.[36] show
<figure><img src="image_2.png" /><figcaption>**Figure 1.** Synthesis and characterization of PEDOT and PEDOT-N₃ organic electrochemical transistors (OECTs). a) Schematic of the three-electrode electropolymerization setup used for OECT fabrication (WE: working electrode; RE: reference electrode; and CE: counter electrode) and optical micrograph of the resulting polymer channel for PEDOT-N₃. b) Current (dot) evolution depending on the injected amount of charge (solid line) as a function of time in chronoamperometry for EDOT-N₃. Inset: Cyclic voltammograms (CVs) during electropolymerization of PEDOT (blue) and PEDOT-N₃ (red) OECTs in 10 mM EDOT/EDOT-N₃ monomer solutions with 0.1 M TBAPF₆ electrolyte (scan rate: 50 mV s⁻¹, 1 cycle). c) Microscope images of a PEDOT-N₃ channel formed with different deposited charge (from 20 to 80 µC. d) Fourier transform infrared (FTIR) spectra of PEDOT and PEDOT-N₃ polymer films. 5 × 5 µm AFM height image of e) PEDOT and f) PEDOT-N₃.</figcaption></figure>

that, for equal 10 CV cycles, PEDOT:PSS is ≈2.5× thicker than PEDOT-N₃:PSS (≈540 ± 30 nm versus 210 ± 10 nm); our AFM measurement (Figure S10, Supporting Information) confirms this similar proportional thickness difference.

Atomic force microscopy (AFM) (Figure 1e,f) confirms this disparity: PEDOT coatings reveal granular, high-surface-area textures (RMS roughness >100 nm) characteristic of 2D/3D growth, while PEDOT-N₃ films display smooth topographies with RMS ≈50 nm, indicating a layer-by-layer growth regime. Mechanistically, conventional EDOT undergoes rapid oxidation and immediate oligomer precipitation (“instantaneous nucleation”), producing fibrous, porous networks.[37] Although the PEDOT-N₃ nucleation mode has not yet been investigated in the literature, we hypothesize, by analogy to other substituted thiophenes, that the bulky azide substituent on EDOT-N₃ rises the critical chain

length for precipitation, delaying nucleation and favoring a more uniform 2D growth mode. Such substitution-controlled nucleation has been observed in other alkyl- versus aryl-thiophene systems, where longer side chains suppress early-stage island formation, yielding smoother morphologies.[38] Indeed, for the same amount of deposited charge, much faster growth of PEDOT comparing to PEDOT-N₃ is evident from the time dependent charge injection curves (Figure S4, Supporting Information).

For the transfer and the transient characteristics, the V_D was fixed at -0.1 V. By fixing the drain bias at -0.1 V, we remove V_D as an additional variable when comparing PEDOT and PEDOT-N_3 devices, ensuring that any observed differences in transconductance or transient kinetics arise solely from intrinsic material properties (mobility, volumetric capacitance) or geometry rather than bias-dependent channel modulation. **Figure 2a,b** show that
<figure><img src="image_3.png" /><figcaption>**Figure 2.** a) Transfer characteristics (solid line) and corresponding transconductance ($g_m$, dot) for a representative OECT as a function of electropolymerization charge ($V_D = -0.1$ V, 0.1 M NaCl aqueous electrolyte): a) PEDOT and b) PEDOT-N$_3$. Performance metrics of PEDOT and PEDOT-N$_3$ based OECTs as a function of electropolymerization charge: c) Threshold voltage ($V_{th}$) and d) Gate voltage ($V_G$) at transconductance maximum ($g_m$, max).</figcaption></figure>

PEDOT-N$_3$ based OECTs turn off at a gate voltage closer to 0 V than PEDOT based OECTs at low electropolymerization charge, and $V_{th}$ of PEDOT-N$_3$ based OECTs shifts upward (more positive) as the electropolymerization charge (and thus film thickness) is increased (Figure 2c). In contrast, $V_{th}$ of PEDOT based OECTs is relatively invariant to electropolymerization charge variations (modest thickness changes). In concurrence, $V_G$ ($g_m$, max) of PEDOT based OECTs remains nearly constant across deposition charges, whereas that of PEDOT-N$_3$ based OECTs moves toward zero as the films get thinner (Figure 2d). The obtained AFM height profiles (Figure S10, Supporting Information) demonstrates that PEDOT film thickness scales with electropolymerization charge; nonetheless, the threshold voltage and peak transconductance remain largely invariant. We attribute this to PEDOT's large intrinsic hole density and high µC* (which saturate the device response once a modest thickness is reached). We note as a hypothesis that non-ideal current extraction (contact resistance/current crowding), morphology-dependent mobility saturation, and partial ionic access to the deepest regions of very thick films can all reduce the apparent linear dependence of $I_D$, max on thickness. The transient gating behavior in Figure S7c,d (Supporting Information) shows that PEDOT-N$_3$ based OECTs switch more slowly (longer $t_{ON}$ and $t_{OFF}$) than PEDOT based OECTs, especially for higher deposition charges (thicker films). In OECTs, switching is almost always governed by ion motion.[39] Thicker channels contain more ionic capacitance and therefore switch more slowly (response time $\propto$ capacitance), which turns out to be the case for relatively thick PEDOT based OECTs. However, longer switching time of thinner PEDOT-N$_3$ (comparing to

PEDOT) based OECTs alludes a rather different ion transport in these two systems, which can also explain a strong thickness dependent switching time, consequently the threshold voltage variations in PEDOT-N$_3$ based OECTs. Therefore, we further investigated the ion/electron transport in these two channel materials.

In organic mixed ionic-electronic conductors (OMIECs) systems, electrochemical (de)doping dramatically changes both the electronic and ionic conductivities by orders of magnitude, often accompanied by swelling of the polymer matrix.[40–42] Thus, quantitatively tracking both conductivities in real time has been challenging. Zayat et al. recently demonstrated an in situ method using electrochemical impedance spectroscopy (EIS) on interdigitated electrodes to measure both ionic ($\sigma_i$) and electronic ($\sigma_e$) conductivity as a function of applied potential.[41] Their approach uses two electrode geometries on a single interdigitated array, allowing independent extraction of $\sigma_i$ and $\sigma_e$ under identical doping conditions. This combined measurement is advantageous because it directly reveals the interplay between ion and electron transport during doping, whereas conventional techniques typically measure only one channel or rely on ex-situ methods. For PEDOT (Figure 3a) and PEDOT-N$_3$ (Figure 3c) in TBAPF$_6$/ACN, characteristic redox peaks appear, and selected potentials (marked by circles on the CVs) were chosen for conductivity measurements. At each potential, the polymer film was held potentiostatically to reach steady state, then impedance spectra were acquired in two configurations: i) Three-electrode mode, with both interdigitated electrodes shorted as the working electrode, platinum as counter electrode and a silver wire as pseudo reference electrode, to extract the ionic conductivity by fitting the
<figure><img src="image_4.png" /><figcaption>**Figure 3.** Electrochemical and charge-transport properties of PEDOT and PEDOT-N₃ films under varying doping conditions. a) CV of a PEDOT film in monomer-free electrolyte (0.1 M TBAPF₆/acetonitrile) at a scan rate of 50 mV s⁻¹. b) Dual-axis plot of electronic (σₑ, triangle) and ionic (σᵢ, square) conductivity for PEDOT as a function of electrochemical doping potential. c) CV of a PEDOT-N₃ film under identical conditions to a. d) Dual-axis plot of electronic (σₑ, triangle) and ionic (σᵢ, square) conductivity for PEDOT-N₃ as a function of electrochemical doping potential.</figcaption></figure>

high-frequency impedance; ii) Two-electrode mode, measuring between the separated digits to extract the electronic conductivity (σₑ). This protocol follows Zayat et al.'s method, allowing σᵢ and σₑ to be obtained at each doping potential.[⁴¹] (Further experimental details and analysis models are provided in Supporting Information.)

Derived electronic (σₑ) and ionic (σᵢ) conductivities as a function of applied potential for a PEDOT film are shown in Figure 3c. We note that σᵢ is reported as an effective, potential-dependent ionic conductivity measured under the stated electrochemical conditions and electrode geometry; σᵢ therefore represents the net contribution of the mobile ionic species (anion and/or cation) present under that bias rather than a species-resolved mobility. As the potential becomes more anodic (increasing polymer oxidation/doping), σₑ increases, which is expected, as oxidative doping generates more mobile hole carriers in the conjugated PEDOT backbone, leading to electronic conduction enhancement. The measured σₑ of PEDOT reaches 3.35 S cm⁻¹ at high doping, consistent with the literature. (For comparison, the four-point probe conductivity of PEDOT:PSS is typically a few S cm⁻¹.[⁴³]) The ionic conductivity (σᵢ) also increases with doping, but remains much smaller (on the order of 10⁻²–10⁻¹ S cm⁻¹) at high doping potential. In PEDOT, electrochemical oxidation injects positively charged holes stabilized by counter-anions (PF₆⁻) and a more highly doped polymer contains more mobile ions, enhancing σᵢ. However, σᵢ is intrinsically limited by slow ion diffusion through the polymer. Indeed, in OMIECs, it is common for σₑ to exceed σᵢ by several orders of magnitude.[⁴¹] The overall trend in Figure 3b – a monotonic rise and eventual saturation of σₑ

and σᵢ with doping matches expectations from literature on p-type OMIECs. However, the PEDOT-N₃ film (Figure 3d) shows a markedly different behavior. As with PEDOT, σₑ generally increases with anodic potential (more hole doping), but the curve is non-monotonic. At the most cathodic bias tested (−0.8 V), both σᵢ and σₑ are relatively high. Moving to −0.4 V, both drop to lower values, then at more positive potentials (greater oxidation) they rise again and saturate. The initial high conductivities at −0.8 V suggest that in the relatively undoped state, the PEDOT-N₃ film already supports notable ion and hole transport, probably due to residual trapped charges after deposition. The drop in both σᵢ and σₑ around −0.4 V implies a temporary reduction in transport efficiency. One hypothesis is that partial doping triggers a conformational change or reorganization of the PEDOT-N₃ matrix. The bulky azide side chains could lead to microstructural rearrangements when the polymer gains or loses charges. For instance, formation of new polarons/bipolarons might locally stiffen or collapse parts of the film, reducing pathways for ion diffusion and electronic hopping at that intermediate potential.[⁴⁴,⁴⁵] Once the potential is increased further, as expected, oxidative doping generates more mobile hole carriers in the conjugated backbone, causing the eventual rise in σᵢ and σₑ. In contrast, the PEDOT film (Figure 3b) did not exhibit such a dip, indicating that its microscopic morphology remains more stable throughout the doping range. This contrast suggests that the azide functionalization introduces additional steric or morphological complexity, which transiently impedes transport at intermediate doping. Nevertheless, the ionic conductivity σᵢ of PEDOT-N₃ at high doping is ≈10⁻³–10⁻² S cm⁻¹ roughly an order of magnitude
<figure><img src="image_5.png" /><figcaption>**Figure 4.** Electrochemical impedance spectroscopy (EIS) measurements: a) Capacitance (C) versus frequency (f) for a PEDOT based OECT; b) Phase angle ($\theta$) versus frequency (f) for the same device; c) Capacitance (C) versus frequency (f) for PEDOT-N$_3$ OECT; d) Phase angle ($\theta$) versus frequency (f) for the same device.</figcaption></figure>

lower than that of PEDOT, while the $\sigma_e$ is comparable. The more porous structure of the PEDOT film, as evidenced by AFM images, is clearly preferable for ion transport. This explains the similar switch on of thin PEDOT-N$_3$ based OECTs (80 µC) as that of much thicker PEDOT based OECTs (80 µC) as well as the observed stronger thickness dependent $V_{th}$ in PEDOT-N$_3$ based OECTs.

Due to this low ionic conductivity, ions migrate sluggishly in PEDOT-N$_3$ and the device response is dominated by this ionic transport time. As a result, $t_{ON}$ increases with film thickness (deposition charge) for PEDOT-N$_3$ in Figure S7c (Supporting Information), whereas PEDOT channels (with better ionic pathways) charge and discharge more quickly, confirming the difference in ionic conductivity. The higher $t_{OFF}$ at low charges for PEDOT-N$_3$ (Figure S7d, Supporting Information) likely reflects residual trapped ions and a poorer removal of charge from a thinner, more disordered film. In short, the reduced ionic mobility in PEDOT-N$_3$ dominates the switching dynamics, as expected for ion-limited OECT operation.[39]

In OECT devices, these morphological contrasts, being a determining factor for ionic transport, translate directly into threshold-voltage and transconductance tuning. Another factor to be considered is the ionic capacitance. The lower-volume PEDOT-N$_3$ channel can be modulated at low gate biases yielding small $V_{th}$ because the gate efficiently depletes its smaller ionic reservoir. By contrast, thick PEDOT channels maintain a large, fixed ionic background that pins $V_{th}$ in depletion mode and renders it insensitive to modest thickness variations. Volumetric ca-

pacitance (C*) of the OMIEC channel is a fundamental figure-of-merit for OECT performance.[46] In an OECT, ion penetration into the bulk of the polymer enables very high capacitance per unit volume, which in turn amplifies the drain current response (high transconductance). For example, PEDOT: PSS films typically exhibit C* ≈ 40 F cm⁻³,[47] enabling large $g_m$ values. Clickable' PEDOT-N$_3$ derivatives have been reported to attain much higher volumetric capacitance (≈226 ± 92 F cm⁻³ at 0.7 V).[19] Fenoy et al. attribute this increase to bias-driven electrochemical oxidation of the polymer (formation of polarons/bipolarons); the resulting positive charges are compensated by uptake of counter-anions from the electrolyte, which increases the film's ionic charge storage.[19] In general, higher volumetric C* increases the charge storage capacity and lowers the voltage required for a given hole density, improving OECT gain, but also causing slow response (more ionic charge must move).[47] Thus, understanding how C varies with applied potential is crucial. Examining the potential- and frequency-dependent capacitance via electrochemical impedance spectroscopy (EIS) therefore reveals the coupling of ions and electronic carriers in the polymer channel, and has direct implications for switching speed and signal amplification in OECT devices.[47] In the EIS setup, a small AC voltage (50 mV) was applied over frequencies from 10⁻¹ to 10⁵ Hz on top of each DC bias. The resulting C and phase angles as a function of frequency at each bias constitute Figure 4a–d.

We determined capacitance by two complementary approaches. First, a direct formula-based method was applied to the
EIS data. Here the film is treated as (approximately) a capacitor at each frequency and bias. This is valid when the phase angle indicates predominantly capacitive response. Second, an equivalent-circuit fitting approach was used: the impedance spectra were fitted to a circuit model to extract the polymer capacitance.[39] In our data, the two extraction methods yielded essentially identical capacitance values across the bias range, consistent with the expectation that an OMIEC with no interfacial blocking should give equivalent C by either approach.[39]

Figure 4a–d compare the frequency-resolved capacitance and phase for PEDOT and PEDOT-N₃ based OECTs at various DC biases. At low frequencies (below ≈10 Hz), the PEDOT based OECT exhibits a large capacitance that moderately increases with more positive gate bias (up to ≈10.6 µF at 0.7 V) (Figure 4a). All curves tend to collapse to a common value at high frequency (above ≈40 Hz), as the ionic motion is too slow to respond in that regime. This is the expected behavior for a well-doped PEDOT film: increasing bias boosts hole density and ionic charge storage, and at low frequency the ions fully equilibrate in the bulk.[46] The corresponding phase response (Figure 4b) initially exhibits resistive behavior at high frequencies (phase <10°), progressively shifting toward a capacitive regime (phase ≈45–90°) as the frequency decreases, a hallmark of mixed ionic–electronic systems governed by distributed resistor-capacitor dynamics. Notably, the curve at –0.8 V deviates strongly: its phase remains low even at low frequency, indicating that the undoped PEDOT film is nearly resistive with very small capacitance. This is consistent with the notion that insufficient doping yields minimal polarons and thus low ionic capacitance.

The PEDOT-N₃ based OECT (Figure 4c,d) also shows rising capacitance for more positive bias, with the maximum (≈12.45 µF) at 0.7 V. However, in contrast to PEDOT based OECT, the capacitance curves do not reach a low-frequency plateau within the measured range as we observed for the PEDOT based OECTs. Even at the lowest frequency (0.1 Hz) the capacitance continues to increase with decreasing frequency for most of the applied DC bias values. Additionally, the phase plot (Figure 4d) is anomalous: the PEDOT-N₃ based OECT behaves capacitively (phase ≈70–90°) at both very low and very high frequencies, dipping toward resistive behavior only at intermediate frequencies (≈10²⁻¹⁰³ Hz). In other words, two capacitive processes seem to contribute – one dominating the slow (low-frequency) regime and another unexpectedly active at the fastest measurement frequencies. This contrasts with the PEDOT based OECT (no capacitive response at high frequency) and suggests the presence of a second, faster capacitor in parallel with the bulk where PEDOT-N₃ modification was found to introduce an additional capacitive element.[20] Figure S9a (Supporting Information) illustrates the DC-bias dependence of the low-frequency capacitance (at 0.1 Hz) for both materials, compiled from Figure 4a,c. PEDOT shows the expected behavior: C increases with bias and begins to saturate at 0.7 V, reflecting that the film approaches full doping. PEDOT-N₃ by contrast exhibits a nearly continuous rise in C across the full bias range, with no sign of saturation at 0.7 V. This indicates that, within the bias range studied, the PEDOT-N₃ film is not reaching a fixed maximum charge storage; it continues to accept additional ionic charge as the voltage is increased. Figure S9b (Supporting Information) displays the capacitance and phase angle as a function of frequency of PEDOT-N₃ film in a planar interdig-

itated electrode (IDE) configuration. In the IDE geometry (a flat Pt comb with PEDOT-N₃), the PEDOT-N₃ capacitance and phase show a more conventional behavior: the phase remains low at high frequency and only rises at low frequency. This implies the extra high-frequency capacitance seen in Figure 4d is specific to the OECT geometry. The two configurations share the same polymer but differ in electric pathway and film morphology. The fact that the unusual phase response is absent in the IDE suggests that series resistance or contact effects in the OECT device accentuate the apparent high-frequency capacitance, probably due to the Au and PEDOT-N₃ interface in OECT configuration.

### 3. Conclusion

In summary, we have demonstrated that electropolymerization can be used as an effective tool to control the V<sub>th</sub> of certain polymer systems in OECTs, such as clickable Azidomethyl-PEDOT. We found that when creating channel materials for OECTs using chronoamperometry, the total electropolymerization charge passed is a critical parameter that dictates much more than just the film thickness. It fundamentally influences the film morphology, percolation, ionic transport efficiency, and ultimately, the overall iono-electronic performance of OECTs. AFM investigations indicate that under identical charge-controlled deposition, PEDOT builds up thick, porous networks, whereas PEDOT-N₃ remains comparatively thin and smooth, which can be due to the bulky azide substituent on EDOT-N₃ that rises the critical chain length for precipitation, delaying nucleation and favoring a more uniform 2D growth mode. Such a different morphology causes rather different ion transport in these two systems. Our EIS studies show that the ion mobility of PEDOT-N₃ is about an order of magnitude lower than that of PEDOT. Ion mobility is ultimately responsible for the switching speed of OECTs, reflected by the V<sub>th</sub>. Thinner channel layers coupled with low ion mobility renders a more tunable V<sub>th</sub>. Indeed, the PEDOT-N₃ based OECT exhibited tunable V<sub>th</sub>, whereas PEDOT based OECTs display relatively constant V<sub>th</sub> across the same conditions. Consequently, we have achieved almost zero V<sub>G</sub> (g<sub>m</sub>, max) for PEDOT-N₃ based OECTs. Despite being relatively thin, PEDOT-N₃ channel layers offer quite high volumetric capacitance, especially in a high injection regime, as evidenced by EIS measurements, attesting similarly high transconductance achieved in both polymer channel layers. Our electropolymerization strategy involves navigating distinct growth regimes, where optimal performance is achieved by carefully balancing film connectivity, homogeneity, and ion transport efficiency. This complex interplay underscores the necessity for precise control over synthesis parameters to achieve desired performance targets. Understanding these growth dynamics is crucial for moving beyond empirical optimization to a more rational design of electropolymerization protocols for a variety of functional monomers with tailored properties such as tuned V<sub>th</sub>.

### 4. Experimental Section

**Materials and Chemicals:** Unless otherwise stated, all solvents and reagents were used as received. HPLC-grade acetonitrile, sodium chloride (NaCl), tetrabutylammonium hexafluorophosphate (TBAPF₆), and
3,4-Ethylenedioxythiophene (EDOT) was purchased from Sigma-Aldrich. Positive (AZ 1512 HS) and negative (SU-8 2002) photoresists, their respective developers, and the iodine-based gold etchant (TechniEtch AC12) along with 4-inch borosilicate glass wafers were obtained from Micro-Chemicals GmbH. The azide-functionalized EDOT-N₃ monomer was synthesized according to previously reported protocols (Details of materials synthesis procedures and characterization for EDOT-N₃ monomer are provided in the Supporting Information). Electrochemical impedance spectroscopy, cyclic voltammetry, and steady-state and transient transfer measurements were performed in Millipore water (18.2 MΩ cm) at 24 ± 2 °C.

**OECT Device Fabrication:** Device fabrication for organic electrochemical transistors was carried out using standard photolithography processes. The overall workflow comprised five sequential steps: 1) substrate cleaning; 2) metal deposition; 3) photoresist patterning of source/drain electrodes; 4) SU-8 passivation and channel definition; and 5) substrate dicing and cell assembly. 4-inch borosilicate glass wafers (700 µm thick; Micro-Chemicals GmbH) were first cleaned by sequential 10 min sonication in acetone, isopropanol and deionized water, then dried with air flow. A 5 nm titanium adhesion layer and 100 nm gold electrode film were deposited by electron-beam physical-vapor deposition, after which positive photoresist (AZ 1512 HS; MicroChemicals GmbH) was spin-coated at 4000 rpm for 60 s to yield ≈1.2 µm thickness. Following a 90 s soft bake at 110 °C, the resist was exposed (i-line, 40 mJ cm²) using EVG6200 mask aligner, post baked at 110 °C, developed in AZ 726 MIF for 40 s, rinsed, hard-baked at 115 °C for 50 s, and used as an etch mask for gold (TechniEtch AC12; iodine-based etchant) and titanium (CF₄ plasma, RIE (FLRIE300-C) at 150 W, 60 sccm for 120 s), before stripping in acetone. To define the channel and passivation, SU-8 2002 (MicroChemicals GmbH) was spin-coated at 3000 rpm for 60 s (≈1.4 µm), prebaked at 65 °C (1 min) then 95 °C (2 min), exposed at 80 mJ cm², post-exposure-baked (65 °C, 1 min; 95 °C, 2 min) and developed for 10 min in SU-8 developer. The resulting resist pattern encapsulated the source-drain leads and opened a 150 × 50 µm well and a 100 µm × 10 µm channel region. Finally, wafers were diced using a CO₂ laser system (Trotec Speedy 400fle) and a glass ring was affixed to each chip with biocompatible silicone (Elastosil E43, Wacker) to form an electrochemical measurement cell.

**Electropolymerization of EDOT and EDOT-N₃ Films:** Potentiostatic electropolymerization of 3,4-ethylenedioxythiophene (EDOT) and its azide-functionalized derivative (EDOT-N₃) was performed at ambient temperature on cleaned OECT substrates using a PalmSens4 bipotentiostat (PSTrace v5.9) in a three-electrode configuration with the source-drain terminals serving as the working electrode, a platinum wire as counter electrode and an Ag pseudo-reference electrode at ambient temperature. Monomer solutions (10 mM EDOT or EDOT-N₃) were prepared in acetonitrile containing 0.1 M tetrabutylammonium hexafluorophosphate and deoxygenated by nitrogen bubbling. Substrates underwent a 10 min oxygen plasma treatment immediately prior to deposition. Electropolymerization was carried out potentiostatically at 1.4 V versus Ag⁺ using chronoamperometry to pass total charges ranging from 20 to 80 µC, enabling systematic investigation of charge-dependent film growth. Resulting polymer films were rinsed sequentially in acetonitrile and deionized water, then stored under ambient conditions until characterization.

**Electrochemical Measurements:** EIS and CV experiments were performed with a PalmSens4 bipotentiostat (PSTrace v5.9) in a standard three-electrode configuration. The channel layer was used as the working electrode by shorting the source and drain terminals of the OECT together. A platinum wire served as the counter electrode, and an Ag wire as pseudo reference electrode. For the aqueous measurements, an Ag/AgCl reference electrode and a platinum wire as counter electrode were employed. 0.1 M NaCl was used as electrolyte. Potentiostatic EIS measurements were performed by applying a small sinusoidal voltage (50 mV amplitude) over the frequency range 10⁻¹–10⁵ Hz, superimposed on the desired DC offset (gate) voltage ranging from –0.8 to 0.7 V.

**Electrical Characterization:** The transfer characteristics of the OECTs were recorded using a Keithley 4200 semiconductor analyzer in 0.1 M NaCl electrolyte while a Ag/AgCl pellet was employed as gate electrode. To accurately determine the threshold voltage (Vth) for each OECTs, we plotted

the drain current (I_D) versus gate voltage (V_G) transfer characteristics at a constant drain voltage (V_D = –0.1 V). We then linearly extrapolated the steepest slope of the I_D curve in its quasi-linear regime (typically observed between 75% and 25% of maximum I_D) to the V_G axis. Such empirically derived Vth signifies the transition from the non-degenerate to the degenerate doping regime of the OECT channel, where volumetric ion penetration leads to a relatively uniform charge distribution along the channel, enabling a predictable current response.

**ATR-FTIR Measurements:** ATR-FTIR spectra were recorded using the PRO ONE attenuated total reflectance (ATR) attachment on a FTIR-4600 spectrometer (JASCO, UK). The samples on the gold electrode were placed in contact with the diamond ATR element for analysis in the range of 450–4000 cm⁻¹ at room temperature and at laboratory humidity conditions. The Spectra manager software program (JASCO, UK) was used for data acquisition and processing.

**AFM Measurements:** AFM images were acquired with a Bruker Bioscope Resolve microscope controlled by a Nanoscope V software. The height images of the films in air were obtained in Peak Force Tapping mode with ScanAsyst-Air probes (Bruker). In this mode, the probe and the sample are intermittently brought into contact for a short period and can be viewed as a succession of approach-retract curves performed at high frequency (2 kHz) and limited by a maximum set force which can be finetuned.

## Supporting Information

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

## Acknowledgements

This work was financially supported by French National Research Agency (ANR) grants CHIRASENSEO (ANR-23-CE42-0004) and APTAWINS (ANR-24-CE93-0008-01). D. M. thanks financial support of MCIN/AEI/10.13039/501100011033 (RYC2021-031668-I and PID2023-152287OA-100) and European Union NextGeneration EU/PRTR.

## Conflict of Interest

The authors declare no conflict of interest.

## Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.

## Keywords

electropolymerization, organic electrochemical transistor, threshold voltage

Received: May 29, 2025

Revised: October 1, 2025

Published online:

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