
==== Front
Nat Commun
Nat Commun
Nature Communications
2041-1723
Nature Publishing Group UK London

51883
10.1038/s41467-024-51883-9
Article
Small signal analysis for the characterization of organic electrochemical transistors
http://orcid.org/0000-0002-9584-821X
Kim Youngseok ykim@chalmers.se

Kimpel Joost
Giovannitti Alexander
http://orcid.org/0000-0001-7859-7909
Müller Christian christian.muller@chalmers.se

https://ror.org/040wg7k59 grid.5371.0 0000 0001 0775 6028 Department of Chemistry and Chemical Engineering, Chalmers University of Technology, Göteborg, Sweden
1 9 2024
1 9 2024
2024
15 76067 5 2024
19 8 2024
© The Author(s) 2024
2024
https://creativecommons.org/licenses/by/4.0/ Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.
A method for the characterization of organic electrochemical transistors (OECTs) based on small signal analysis is presented that allows to determine the electronic mobility as a function of continuous gate potential using a standard two-channel AC potentiostat. Vector analysis in the frequency domain allows to exclude parasitic components in both ionic and electronic conduction regardless of film thickness, thus resulting in a standard deviation as low as 4%. Besides the electronic mobility, small signal analysis of OECTs also provides information about a wide range of other parameters including the conductance, transconductance, conductivity and volumetric capacitance through a single measurement. General applicability of small signal analysis is demonstrated by characterizing devices based on n-type, p-type, and ambipolar materials operating in accumulation or depletion modes. Accurate benchmarking of organic mixed ionic-electronic conductors through small signal analysis can be anticipated to guide both materials development and the design of bioelectronic devices.

Accurate determination of the performance of organic electrochemical transistors is challenging. The authors present a method for the device characterization based on small signal analysis, enabling the determination of the electronic mobility as a function of continuous gate potential.

Subject terms

Electronic devices
Electrical and electronic engineering
https://doi.org/10.13039/100010663 EC | EU Framework Programme for Research and Innovation H2020 | H2020 Priority Excellent Science | H2020 European Research Council (H2020 Excellent Science - European Research Council) 101043417 101116071 Giovannitti Alexander Müller Christian https://doi.org/10.13039/501100004063 Knut och Alice Wallenbergs Stiftelse (Knut and Alice Wallenberg Foundation) 2021.0295 2022.0034 Müller Christian https://doi.org/10.13039/100010661 EC | Horizon 2020 Framework Programme (EU Framework Programme for Research and Innovation H2020) 955837 Müller Christian issue-copyright-statement© Springer Nature Limited 2024
==== Body
pmcIntroduction

Organic electrochemical transistors (OECTs) receive considerable interest as sensor devices and as a basic building block of more advanced bioelectronic circuitry1–5. Moreover, OECTs are widely employed for the characterization of organic mixed ionic-electronic conductors (OMIECs), which constitute the channel material. Hence, accurate determination of the device performance6–8 as well as in-depth studies of the electrochemical properties of OMIEC materials9–12 require accurate characterization methods. The most important parameters are the volumetric capacitance C*, representing the change of the number of charge carriers stored per unit volume upon a small fluctuation in potential, and the mobility μ, i.e. the electric field-normalized velocity of electronic charge carriers13. The product μC* is often used to benchmark OMIEC materials, but techniques are lacking that allow to determine the two parameters and in particular μ independently, which leads to ambiguities when comparing materials14.

A widely used method for the determination of C* and μ involves two measurements: (1) OECT characterization and the analysis of transfer curves and (2) electrochemical impedance spectroscopy (EIS)13. OECT measurements allow to determine the volumetric transconductance gm* from transfer curves, i.e. the volumetric source-drain current IDS* is recorded as a function of gate potential VGS, according to:1 gm*=gm/wdLch=dIDS*dVGS=−μC*⋅VDS,linearregimeμC*⋅VGS−Vth,saturationregime

where gm is the transconductance, w, d and Lch are the width, thickness, and length of the channel, respectively, VDS is the drain potential and Vth is the threshold voltage. The product μC* can thus be obtained from transfer curves in either the linear (VDS>VGS−Vth for p-type and VDS<VGS−Vth for n-type OECTs) or saturation regime (VDS<VGS−Vth for p-type and VDS>VGS−Vth for n-type OECTs)9. Subsequently, μ is obtained by dividing μC* by corresponding C* values, which must be independently determined through EIS (see Table 1). The use of two distinct characterization methods for the determination of μC* and C* introduces uncertainty in the calculated mobility due to the propagation of errors. In addition, the OECT active layer, which swells during operation due to the uptake of hydrated ions (i.e., ions that are accompanied by water molecules15), is typically only tens of nanometers thin. As a result, a large relative error in d can arise due to an uneven film thickness across the channel, the adventitious uptake of water, as well as errors inherent to film thickness measurements with atomic force microscopy (AFM) or surface profilometry. An analysis of the recent literature indicates a coefficient of variation c^v=SD/μ¯ of up to 100% for mobility values obtained via the traditional transfer curve/EIS method (standard deviation SD and mean mobility μ¯; Fig. 1).Table 1 Input/output signals and extractable parameters G, σ, C*, gm, μC* and μ for conventional OECT characterization methods (transfer curve/EIS, constant gate current, sinusoidal gate potential) and small signal analysis

	Transfer curve	EIS	Constant gate current	Sinusoidal gate potential	Small signal analysis	
Input signal	VGS, VDS	Stepwise VDC

+ VAC

	Stepwise IGS, VDS	Stepwise VDC

+ VGS,AC,VDS

	Continuous VGS,DC

+ VGS,AC, VDS

	
Output signal	IGS, IDS	IAC	VGS, IDS	IGS,AC, IDS,AC	IGS,DC, IGS,AC,

IDS,DC, IDS,AC

	
G, σ, gm	✓	–	✓	✓	✓	
μC*	✓	–	✓	✓	✓	
C*	–	✓	✓	✓	✓	
μ	(✓)	–	✓	✓	✓	

Fig. 1 Correlation between the mean mobility μ̄ and the coefficient of variationc^v.

a Measurement scheme for the characterization of OECT devices. b Coefficient of variation c^v=SD/μ¯, where μ¯ and SD are the mean mobility and corresponding standard deviation, extracted from literature since 2014 (open symbols)6,9,10,31–46 and obtained in this work using the conventional transfer curve/EIS method (blue diamond) and small signal analysis (red circle); dashed lines represent c^v = 1, 5, 10 and 100%.

An alternative method for the determination of μ is an analysis of the transient response in either the time or frequency domain. The transit time τe of electronic charge carriers through the channel is obtained by comparing the current values at the gate and drain electrodes, IGS and IDS, in case of a constant gate current16 or a sinusoidal gate potential signal17 according to:2 dIDSdt=−IGS/τe

Then, μ can be obtained regardless of the active-layer thickness according to:3 μ=Lch2τeVDS

While this approach facilitates a more accurate determination of μ than the method using transfer curves and EIS, repeated measurements with a stepwise gate current or gate potential would need to be carried out to obtain information of the change in μ as a function of gate potential. Further, the extracted μ tends to strongly deviate from the true value once the electrochemical redox peak dominates the output signal. It should be noted that IGS comprises both the non-Faradaic (capacitive) current as well as the Faradaic (resistive) current. Faradaic processes, which could be identified via the detection of hydrogen or the oxygen reduction response, would need to be ruled out for precise characterization (see Table 1 for summary of characterization methods)18,19.

Here, we introduce a method for the determination of μ that is based on vector analysis in the frequency domain and small signal analysis. For OECTs with a well-defined active layer, the mobility can be reliably extracted regardless of the channel thickness, yielding a very low c^v = 4% for 40 devices (Fig. 1). This is achieved by counting and comparing the number of charge carriers associated with the non-Faradaic part of IGS and the corresponding IDS. The here developed method uses a mixed VGS signal composed of a linear sweep to which a small sinusoidal signal is added. This approach allows to simultaneously characterize devices under steady-state and transient conditions, yielding device parameters as a function of continuous gate potential. Thus, essential parameters such as the conductance G, gm*, the conductivity σ, C* and μ can be extracted through one single measurement (Table 1), which can be readily carried out with a commercial two-channel AC potentiostat. Finally, versatility of the developed small-signal analysis method is demonstrated by characterizing OECTs based on a range of n-/p-type and ambipolar materials as well as accumulation/depletion mode materials.

Results and discussion

We prepared OECTs with a Lch = 20 μm and w = 100 μm with two contact regions with a length Lcontact = 100 μm that served as the source and drain electrode (Fig. 2a, see method section for fabrication procedure). A three-electrode configuration was used consisting of an Ag/AgCl reference and a Pt counter electrode. VGS comprised two waveforms, i.e. a pseudo steady-state triangular potential (VGS,DC, from +0.4 to −0.6 V, scan rate SR = 10 mV s−1) and a sinusoidal AC small potential (VGS,AC=Asin(2πfACt), amplitude A = 10 mV, frequency fAC = 10 Hz), while a constant VDS = 10 mV was applied. The conjugated polymer p(g3TT-T2) with triethylene glycol side chains was used as the active material (see Supplementary Fig. S1 for chemical structure), which features a high electronic mobility upon electrochemical oxidation (Supplementary Figs. S2 and S3) with good stability during cyclic operation20.Fig. 2 Small signal analysis for OECT device characterization.

a Device and measurement scheme for small signal analysis. A gate potential VGS consisting of a triangular potential VGS,DC and a small-amplitude sinusoidal potential VGS,AC is applied to the 100 mM NaCl aqueous electrolyte via a three-electrode configuration with a counter (CE) and a reference electrodes (RE), with a constant drain potential VDS. The lower inset depicts an optical microscopy image of the channel region of an OECT device (scale bar = 100 μm). b Time-varying input gate potential VGS (black) as well as output gate IGS (red) and drain current IDS (blue). c Phasor diagram of output currents relative to VGS, and (d) traces of VGS (black), IGS (red) and IDS (blue) at t = 95 s + △t, and centered at VGS = 0.55 V, IGS = 0 A, and IDS = 8 μA (y-axis scale bar = 10 mV, 100 nA, and 1 μA for VGS, IGS, and IDS). Since the holes are accumulated through the electrical double layer in case of p-type accumulation mode OECTs (see inset of c), the gate and drain currents show a time delay from the gate input bias of π/2 and π, respectively. e Amplitude of AC and DC components of IGS (upper panel) and IDS (lower panel) as a function of VGS,DC. f Steady-state current response IDS,DC (solid line) and IGS,DC (dashed line) as a function of the VGS,DC from small signal analysis, and transfer curve IDS vs. VGS,DC from conventional analysis (open circles). g Amplitude of IGS,AC (upper panel) and IDS,AC (lower panel) and its real and imaginary components as a function of VGS,DC, obtained from small signal analysis. The real (red) and imaginary component (blue) were extracted from the output current values (black) by vector analysis as shown in c.

OECT devices with a VDS = 10 mV were operating in the linear regime within the potential window of VGS ranging from −0.6 V to +0.4 V (Supplementary Fig. S3b). Thus, IGS can be described by the Bernards–Malliaras model16:4 IDS=μC*wdLVGS−VthVDS−12VDS2

We recorded IGS and IDS as a function of time t, which are the mixed current responses from the steady-state VGS,DC and sinusoidal VGS,AC input signals. As VGS,DC is scanned from +0.4 to −0.6 V, the amplitude of IGS increases and reaches a plateau, while IDS gradually increases. Considering the electrochemical double layer at the interface between the electrolyte and active layer21 and the p-type operation of the material20, a sinusoidal current response at the gate and drain electrode, IGS,AC and IDS,AC is expected with a phase shift of −90° and −180° relative to the VGS,AC signal, respectively. Hence, we can rule out any unintended redox response (e.g., oxygen reduction response22), which would contribute an additional real component to IGS and a corresponding imaginary component to IDS, resulting in an additional phase shift. We also separated the measured IGS and IDS values into the real (IGS,AC′ and IDS,AC′) and imaginary components (IGS,AC″ and IDS,AC″; Fig. 2c). The measured current traces at the gate and drain electrodes show an expected phase shift near −90° and −180° from the signal of the gate potential at t = 95 s when the active layer is heavily doped (Fig. 2d).

Subsequently, Fourier transform analysis was used to separate the current responses at the gate and drain electrodes from the mixed gate input potential (Fig. 2e). The resulting steady-state gate and drain current response (IGS,DC and IDS,DC) as a function of VGS,DC are in very good agreement with the transfer curves recorded in the linear regime, which were obtained through the conventional characterization method (Fig. 2f; see Method section for details). Upon dedoping of the active material through application of a positive VGS,DC, the amplitude of IGS,AC′ (denoted as ΔIGS,AC′) approaches zero, while the amplitude of IGS,AC″ (ΔIGS,AC″) has a value of 4.5 nA. This offset is explained with the capacitance of the electrical double layer between the active layer and the underlying metal electrode. As the VGS,DC is changed to negative values, ΔIGS,AC″ gradually increases and reaches a plateau with 85 nA, reflecting the electrochemical capacitance C of the active material (vide infra). At VGS,DC = −0.15 V, a distinct peak in ΔIGS,AC′ is observed in both backward and forward scans (Fig. 2g). This peak is assigned to the contact resistance between the active layer and metal electrode (Supplementary Fig. S2d), which is also observed in corresponding EIS measurements (Supplementary Fig. S4; c.f. the resistive response showing a flat region near fAC 10 Hz in the Bode plot). Unlike the EIS measurement, the small signal analysis is conducted with one frequency value, so that the parasitic response is superimposed onto the current value.

We used the output current values obtained from the small signal analysis to extract material and device parameters as a function of VGS,DC. Initially, the conductance G of the active material was determined by using a steady-state input VDS,DC, which resulted in an output IDS,DC with G=IDS,DC/VDS,DC (Fig. 3a). The conductivity σ was obtained by normalizing G with wd/Lch (Fig. 3b). Concurrently, the capacitance C, i.e. the accumulation of additional charge carriers dq upon a change in electrochemical potential dVGS, was extracted according to the ionic non-Faradaic current value (IGS,AC″=dq/dt). Then, C* can be obtained according to (see Supplementary Information for derivation of Eq. (5)):5 C*=ΔIGS,AC″2πfACΔVGS,AC⋅vol

where, vol=wd⋅(Lch+2Lcontact) is the volume of the active layer (Fig. 3c). The change in C* as a function of VGS.DC indicates that the active layer material is gradually doped for VGS.DC < −0.1 V, which agrees with the onset potential extracted from the x-axis intercept of a linear fit near the maximum slope of the current transient of p(g3TT-T2) recorded during oxidation (Eonset = +0.1 V, vs. Ag/AgCl, see Supplementary Fig. S2a). We would like to point out that C* obtained from the small signal analysis tends to be underestimated compared with values obtained from EIS characterization (Supplementary Fig. S2d), which can be explained with limited ionic motion at fAC = 10 Hz. A decrease in fAC would enable a more accurate measurement of C* (see also Supplementary Fig. S5d).Fig. 3 Parameters extracted via small signal analysis.

Traces of (a) conductance G, (b) conductivity σ, (c) volumetric capacitance C*, (d) transconductance gm, (e) product of mobility and volumetric capacitance μC*, and (f) mobility μ as a function of continuous VGS,DC. The parameters obtained from the transient and steady-state response are depicted with solid lines and open circles, respectively. The required parameters for each calculation are denoted in each panel.

The transconductance gm was extracted from the steady-state (gm,DC=∂IDS,DC/∂VGS,DC) as well as transient response (gm,AC=∂IDS,AC′/∂VGS,AC). The benchmarking parameter μC* was obtained from both the DC and AC response via Eq. (1), denoted as μC*DC and μC*AC, respectively (Fig. 3d, e). Note that μC*AC is identical to the product of μAC (vide infra) and C*, which were extracted separately from the transient response. Since small signal analysis underestimates C*, values for μC*AC obtained from the AC response are about 7% smaller than μC*DC from the DC response. Before extracting the mobility, the transit time τe, the time taken for charge carriers to traverse the channel, was obtained according to Eq. (2), with IGS given by the non-Faradaic gate current, i.e. IGS=IGS,AC″⋅Ach/Aactive, and IDS given by the modulated drain current, i.e. IDS=IDS,AC′. The area correction factor Ach/Aactive, where Ach and Aactive are the area of the channel region and total active layer, respectively, is introduced to account for the additional charge carriers that are generated in the active layer in contact with the metal electrodes. Then, μAC can then be obtained according to Eq. (3). It is noteworthy that the mobility extracted through small-signal analysis does not depend on the thickness d of the active layer. For p(g3TT-T2), μAC gradually increased with increasing doping level for VGS,DC < −0.3 V (solid line, Fig. 3f), and a highest value of 1.95 cm2 V−1 s−1 was achieved at VGS,DC = −0.6 V. At VGS,DC < −0.6 V, μAC tends to decrease slightly (data not shown), which is expected due to formation of less-mobile bipolarons23,24 and/or an increased distance between polymer chains due to excessive swelling25. Also, the traces agree with those obtained from the steady-state response (μDC = μC*DC/C*). Moreover, μAC only changed by 1% when increasing fAC from 5 to 60 Hz even though C* decreased by 10% when increasing fAC from 5 to 60 Hz (Supplementary Fig. S5).

As with traditional small signal analysis, which is often used for the analysis of field-effect transistors26, a decrease in the magnitude of the offset potential between source and drain electrodes i.e., VDS, increases the accuracy of the extracted parameters. For the same reason, the slope of ΔIGS,AC and C* near the onset potential VGS,DC = −0.1 V became sharper as the amplitude of VDS was decreased from 100 to 1 mV (Supplementary Fig. S6). However, a reduction in VDS also reduces IDS leading to a lower signal-to-noise ratio at VDS < 5 mV. Further, a lower scan rate of VGS,DC is desirable not only for minimizing the undesired transient response from the VGS,DC signal, but also for reducing computational errors during Fourier transform analysis (Supplementary Fig. S7).

To compare the accuracy of different OECT characterization methods, we measured 40 devices with each method and compared the extracted μC* and μ values (Fig. 4). The evolution of μC*AC with VGS,DC, obtained from small signal analysis, matches that of μC*lin and μC*sat obtained through the conventional characterization methods that utilize the transfer curves in the linear or saturation regime, respectively (Fig. 4a). In case of the analysis of transfer curves in the saturation regime, the potential at maximum μC*sat tends to be shifted from VGS,DC < −0.6 V to −0.55 V because the operation regime is at the boundary between the saturation and linear regimes (see Supplementary Fig. S3b). The small signal analysis at fAC = 10 Hz yields lower μC* values because C* is underestimated but that does not affect the extracted value of μ, which is determined independently (see Figs. 3c, 4b and Supplementary Fig. S2d). With regard to the extraction of μC* values, all three methods require knowledge of the thickness (d = 36.1 ± 1.7 nm from AFM), which results in a similar SD ≈ 7% for μC*AC, μC*lin and μC*sat. However, the mobility obtained through the small signal analysis has a lower SD = 4% than values from the conventional methods with SD ≈ 9% (Fig. 4c, d) because only the latter two require knowledge of d as well as C*.Fig. 4 Statistical analysis of the extracted [μC *] and μ values.

a, c μC* and μ as a function of VGS,DC, measured by small signal analysis (black solid line), and conventional methods from the transfer curves in the linear (red triangles) and saturation region (blue squares), respectively. b, d Maximum [μC*] and μ values from characterization of 40 OECT devices (open circles), as well as corresponding mean values and standard deviations (gray boxes).

Similar to other characterization methods (see Table 1), the film quality of the active layer, e.g. its roughness and thickness uniformity, is important for an accurate determination of μ with small signal analysis. Furthermore, it is necessary to consider additional capacitive or resistive components that contribute to ionic conduction such as (1) a parasitic capacitance at the gate electrode (especially in case of a two-electrode configuration with a small gate electrode consisting of a noble metal or OMIEC such as PEDOT:PSS)27 and/or at the metal source/drain electrode (e.g., electrical double layer capacitance), or (2) a parasitic resistance at the lead wires and interface between the active layer and source/drain metal electrode28,29. Those parasitic components can lead to an incorrect gate potential at the channel, an overestimated C*, and an incorrect drain potential through the channel, respectively.

To establish to which extent small signal analysis can be applied to a wide range of materials, we characterized OECT devices based on various OMIECs including p(g42T-T) (p-type, accumulation mode), PEDOT:PSS (p-type, depletion mode), and p(gNDI-gT2) (n-type/p-type, accumulation mode) (see Supplementary Note 1 and Supplementary Figs. S8–13 for detailed results of various OMIEC material based OECTs).

The μAC and C* values, which were obtained by the small signal analysis, are summarized in a μ – C* plot with four quadrants according to the type of electronic (y-axis) and ionic charge carriers (x-axis) that are associated with the different operation regimes of the various investigated devices (Fig. 5; see Supplementary Note 1 and Supplementary Figs. S8–13 for detailed results of p(g42T-T), PEDOT:PSS and p(gNDI-gT2) based OECTs). Evidently, the suggested method is valid not only for the evaluation of OECTs based on high-mobility polymers such as p(g3TT-T2) that show hole-anion conduction (p-type, accumulation mode), but also for various devices exhibiting different combinations of hole-cation (p-type, depletion mode), and electron-cation conduction (n-type, accumulation mode). In addition to providing accurate values of the electronic mobility as a function of continuous gate potential, small signal analysis allows to monitor the ionic and electronic behavior simultaneously. As a result, it would allow not only to characterize ionic, electronic and mixed conduction through one single measurement, but also to gain additional information about device degradation during cyclic operation, e.g. by monitoring the charge-carrier density or contact resistance.Fig. 5 μ – C * map with its quadrants representing different combinations of the type of majority ionic/electronic charge carriers within the active layer material.

Orange and blue regions depict accumulation and depletion mode operation, respectively. Can* and Ccat*: volumetric capacitance enabled by the ingression anions and cations, respectively. μe and μh: hole and electron mobility, respectively.

We have introduced an accurate method for determination of the electronic mobility of OMIECs, which is based on small signal analysis of OECTs. Gate and drain currents, which were acquired from a commercial two-channel AC potentiostat, were analyzed through vector analysis in the frequency domain, so that parasitic components from the materials and devices could be separated and excluded when calculating various device parameters. The number of charge carriers through the gate and drain electrodes were recorded simultaneously. As a result, the precise determination of the electronic mobility with a low standard deviation of 4% was achieved within a designated gate potential window for 40 OECT devices, which did not require knowledge of the active layer thickness. Moreover, various material and device parameters such as the conductance, transconductance, conductivity and volumetric capacitance could be obtained as a function of the continuous gate potential through one single OECT device measurement. Lastly, the small signal analysis of OECTs could be used for characterizing a range of devices based on both n- and p-type as well as ambipolar materials operating in accumulation or depletion mode, showing that the here introduced method is of general applicability. Small signal analysis allows to accurately benchmark the electronic charge-carrier mobility of OMIECs and likely also other types of materials such as MXenes and semiconducting single-walled carbon nanotubes. It can be anticipated that small signal analysis will aid the identification of best-performing materials that are required for the development of viable bioelectronic devices.

Methods

Chemicals and materials

P(g3TT-T2) (number-average molecular weight Mn,SEC = 29 kg mol−1, Mn,NMR = 39 kg mol−1, polydispersity index PDI = 2.2) and p(g42T-T) (Mn,SEC = 24 kg mol−1, PDI = 3.3) were prepared as previously described20,30. p(gNDI-gT2) (Mn,SEC = 30 kg mol−1, PDI = 2.5) and PEDOT:PSS dispersion (PH-1000) were purchased from 1-Material Inc and Heraeus, respectively. Ethylene glycol (extra pure grade), chloroform (analytical reagent grade) and sodium chloride (analytical reagent grade) were purchased from Thermo Fisher Scientific.

Organic electrochemical transistor (OECT) device fabrication

Source and drain metal electrodes were defined via a conventional lift-off process using a Karl Suss MA6 contact aligner and a Kurt J Lesker PVD e-beam evaporator on cleaned Marienfeld soda lime glass slides, resulting in channels with a length Lch = 20 μm. Two parylene films were sequentially deposited with a thickness of 1 and 2 μm with an anti-adhesive soap layer between them. Two parylene films were patterned via a conventional dry-etching process using a Karl Suss MA6 contact aligner and reactive ion etcher (O2, 300 W). Then, a solution of the active layer material (7–8 g L−1 in chloroform for p(g3TT-T2), p(g42T-T) and p(gNDI-gT2), and as received PEDOT:PSS solution with 5% ethylene glycol) was spin-coated onto the patterned substrate, followed by peeling away of the second parylene film to pattern the active layer. For PEDOT:PSS thin films, the devices were treated with oxygen plasma before spin-coating for better wetting.

Electrochemical characterization

Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were conducted using 100 mM NaCl aqueous electrolyte and a three-electrode configuration (Ag/AgCl reference electrode (3 M KCl), and Pt wire as the counter electrode) using an electrochemical workstation (Biologic, SP-300). Before and during characterization, the electrolyte was purged with nitrogen gas.

OECT device characterization

All OECT characterization was conducted with a nitrogen-purged 100 mM NaCl aqueous electrolyte using a three-electrode configuration with an Ag/AgCl reference electrode and Pt counter electrode for applying the gate potential. All analysis was conducted using MATLAB and Origin software (see Data Availability for MATLAB source code).

Steady-state transfer/output type characterization was conducted with two Matlab-controlled source-measure units (Keithley 2400). For the gate electrode the built-in ‘four-wire mode’ function of the source-measure unit was used. The Ag/AgCl reference electrode and Pt counter electrode, which were immersed in the electrolyte, were electrically connected to the HP and HC ports of the source-measure unit, respectively, and the other LP and LC were connected to the source electrode (HP: high potential, HC: high current, LP, low potential, LC: low current). For the drain-source potential, drain and source electrodes were connected to the high and low ports of the other source-measure unit using a conventional ‘two-wire mode’. The applied potential values for transfer and output characteristic curves are described in the manuscript.

Small signal analysis was conducted with a two-channel electrochemical workstation (Biologic, SP-300). Both channels for the gate and drain potential were set to the ‘CE to ground’ electrode connection mode available through the EC-Lab software, to assign the voltage based on the potential of the source electrode. The former and latter electrode were operated with a three-electrode and two-electrode configuration, respectively. For application of the gate potential, the Ag/AgCl electrode and Pt electrode were connected to the S1 and P1 ports respectively, and the source electrode of the device was connected to the S2, S3 and GND ports (S1: high potential, P1: high current, S2 and S3: low potential, GND: ground). For application of the drain potential, the drain electrode was connected to the P1 and S1 ports, and the source electrode was connected to the S2, S3 and GND electrode ports of the instrument. The gate and drain potentials were assigned with the ‘AC-Voltammetry’ and ‘Constant-Voltage’ functions available through the EC-Lab software, respectively (AC-Voltammetry; triangular potential with superimposed small sinusoidal waveform, Constant-Voltage; constant voltage bias, Waveform parameters are described in the manuscript). The specific potential values for each type of device are described in the manuscript. For a synchronized measurement on both channels, the built-in ‘synchronize’ function was used.

Supplementary information

Supplementary Information

Peer Review File

Supplementary information

The online version contains supplementary material available at 10.1038/s41467-024-51883-9.

Acknowledgements

We gratefully acknowledge financial support from the European Union’s Horizon 2020 research and innovation programme through the Marie Skłodowska-Curie grant agreement no. 955837 (HORATES; C.M.), the Knut and Alice Wallenberg Foundation (grant agreement nos. 2021.0295 and 2022.0034; C.M.) and the European Research Council (ERC) under grant agreement nos. 101043417 (C.M.) and 101116071 (A.G.). Myfab is acknowledged for support and access to the nanofabrication laboratory at Chalmers.

Author contributions

Y.K. and C.M. conceived the study and wrote the manuscript. Y.K. performed device preparation, measurement and analysis. J.K. synthesized polymers. J.K. and A.G. contributed to analyzing results. C.M. supervised the study.

Peer review

Peer review information

Nature Communications thanks the anonymous reviewers for their contribution to the peer review of this work. A peer review file is available.

Funding

Open access funding provided by Chalmers University of Technology.

Data availability

The data used in this study are available in the Zenodo database under accession code 11093663.

Code availability

The Matlab code for carrying out small signal analysis of OECTs is available in the Zenodo database under accession code 11093663.

Competing interests

The authors declare no competing interests.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
==== Refs
References

1. Bonafè F AC amplification gain in organic electrochemical transistors for impedance-based single cell sensors Nat. Commun. 2022 13 5423 10.1038/s41467-022-33094-2 36109508
Bonafè, F. et al. AC amplification gain in organic electrochemical transistors for impedance-based single cell sensors. Nat. Commun. 13, 5423 (2022).36109508 10.1038/s41467-022-33094-2
2. Andersson Ersman P All-printed large-scale integrated circuits based on organic electrochemical transistors Nat. Commun. 2019 10 5053 10.1038/s41467-019-13079-4 31699999
Andersson Ersman, P. et al. All-printed large-scale integrated circuits based on organic electrochemical transistors. Nat. Commun. 10, 5053 (2019).31699999 10.1038/s41467-019-13079-4
3. Jo YJ Kwon KY Khan ZU Crispin X Kim T Gelatin Hydrogel-Based Organic Electrochemical Transistors and Their Integrated Logic Circuits ACS Appl. Mater. Interfaces 2018 10 39083 39090 10.1021/acsami.8b11362 30360103
Jo, Y. J., Kwon, K. Y., Khan, Z. U., Crispin, X. & Kim, T. Gelatin Hydrogel-Based Organic Electrochemical Transistors and Their Integrated Logic Circuits. ACS Appl. Mater. Interfaces 10, 39083–39090 (2018).30360103 10.1021/acsami.8b11362
4. Guo K Rapid single-molecule detection of COVID-19 and MERS antigens via nanobody-functionalized organic electrochemical transistors Nat. Biomed. Eng. 2021 5 666 677 10.1038/s41551-021-00734-9 34031558
Guo, K. et al. Rapid single-molecule detection of COVID-19 and MERS antigens via nanobody-functionalized organic electrochemical transistors. Nat. Biomed. Eng. 5, 666–677 (2021).34031558 10.1038/s41551-021-00734-9
5. Hamedi M Forchheimer R Inganäs O Towards woven logic from organic electronic fibres Nat. Mater. 2007 6 357 362 10.1038/nmat1884 17406663
Hamedi, M., Forchheimer, R. & Inganäs, O. Towards woven logic from organic electronic fibres. Nat. Mater. 6, 357–362 (2007).17406663 10.1038/nmat1884
6. Jo I-Y High-Performance Organic Electrochemical Transistors Achieved by Optimizing Structural and Energetic Ordering of Diketopyrrolopyrrole-Based Polymers Adv. Mater. 2024 36 2307402 10.1002/adma.202307402
Jo, I.-Y. et al. High-Performance Organic Electrochemical Transistors Achieved by Optimizing Structural and Energetic Ordering of Diketopyrrolopyrrole-Based Polymers. Adv. Mater. 36, 2307402 (2024).10.1002/adma.202307402
7. Rivnay J High-performance transistors for bioelectronics through tuning of channel thickness Sci. Adv. 2015 1 e1400251 10.1126/sciadv.1400251 26601178
Rivnay, J. et al. High-performance transistors for bioelectronics through tuning of channel thickness. Sci. Adv. 1, e1400251 (2015).26601178 10.1126/sciadv.1400251
8. Kim Y Strain-Engineering Induced Anisotropic Crystallite Orientation and Maximized Carrier Mobility for High-Performance Microfiber-Based Organic Bioelectronic Devices Adv. Mater. 2021 33 2007550 10.1002/adma.202007550
Kim, Y. et al. Strain-Engineering Induced Anisotropic Crystallite Orientation and Maximized Carrier Mobility for High-Performance Microfiber-Based Organic Bioelectronic Devices. Adv. Mater. 33, 2007550 (2021).10.1002/adma.202007550
9. Hidalgo Castillo TC Simultaneous Performance and Stability Improvement of a p-Type Organic Electrochemical Transistor through Additives Chem. Mater. 2022 34 6723 6733 10.1021/acs.chemmater.2c00632
Hidalgo Castillo, T. C. et al. Simultaneous Performance and Stability Improvement of a p-Type Organic Electrochemical Transistor through Additives. Chem. Mater. 34, 6723–6733 (2022).10.1021/acs.chemmater.2c00632
10. Rivnay J Structural control of mixed ionic and electronic transport in conducting polymers Nat. Commun. 2016 7 11287 10.1038/ncomms11287 27090156
Rivnay, J. et al. Structural control of mixed ionic and electronic transport in conducting polymers. Nat. Commun. 7, 11287 (2016).27090156 10.1038/ncomms11287
11. Keene ST Rao A Malliaras GG The relationship between ionic-electronic coupling and transport in organic mixed conductors Sci. Adv. 2023 9 eadi3536 10.1126/sciadv.adi3536 37647402
Keene, S. T., Rao, A. & Malliaras, G. G. The relationship between ionic-electronic coupling and transport in organic mixed conductors. Sci. Adv. 9, eadi3536 (2023).37647402 10.1126/sciadv.adi3536
12. Giovannitti A The Role of the Side Chain on the Performance of N-type Conjugated Polymers in Aqueous Electrolytes Chem. Mater. 2018 30 2945 2953 10.1021/acs.chemmater.8b00321 29780208
Giovannitti, A. et al. The Role of the Side Chain on the Performance of N-type Conjugated Polymers in Aqueous Electrolytes. Chem. Mater. 30, 2945–2953 (2018).29780208 10.1021/acs.chemmater.8b00321
13. Inal S Malliaras GG Rivnay J Benchmarking organic mixed conductors for transistors Nat. Commun. 2017 8 1767 10.1038/s41467-017-01812-w 29176599
Inal, S., Malliaras, G. G. & Rivnay, J. Benchmarking organic mixed conductors for transistors. Nat. Commun. 8, 1767 (2017).29176599 10.1038/s41467-017-01812-w
14. Shahi M The organic electrochemical transistor conundrum when reporting a mixed ionic–electronic transport figure of merit Nat. Mater. 2024 23 2 8 10.1038/s41563-023-01672-4 37880535
Shahi, M. et al. The organic electrochemical transistor conundrum when reporting a mixed ionic–electronic transport figure of merit. Nat. Mater. 23, 2–8 (2024).37880535 10.1038/s41563-023-01672-4
15. Cendra C Role of the Anion on the Transport and Structure of Organic Mixed Conductors Adv. Funct. Mater. 2019 29 1807034 10.1002/adfm.201807034
Cendra, C. et al. Role of the Anion on the Transport and Structure of Organic Mixed Conductors. Adv. Funct. Mater. 29, 1807034 (2019).10.1002/adfm.201807034
16. Bernards DA Malliaras GG Steady-State and Transient Behavior of Organic Electrochemical Transistors Adv. Funct. Mater. 2007 17 3538 3544 10.1002/adfm.200601239
Bernards, D. A. & Malliaras, G. G. Steady-State and Transient Behavior of Organic Electrochemical Transistors. Adv. Funct. Mater. 17, 3538–3544 (2007).10.1002/adfm.200601239
17. Rivnay J Organic electrochemical transistors for cell-based impedance sensing Appl. Phys. Lett. 2015 106 043301 10.1063/1.4906872
Rivnay, J. et al. Organic electrochemical transistors for cell-based impedance sensing. Appl. Phys. Lett. 106, 043301 (2015).10.1063/1.4906872
18. Volkov AV Understanding the Capacitance of PEDOT:PSS Adv. Funct. Mater. 2017 27 1700329 10.1002/adfm.201700329
Volkov, A. V. et al. Understanding the Capacitance of PEDOT:PSS. Adv. Funct. Mater. 27, 1700329 (2017).10.1002/adfm.201700329
19. Österholm AM Ponder JF Jr. De Keersmaecker M Shen DE Reynolds JR Disentangling Redox Properties and Capacitance in Solution-Processed Conjugated Polymers Chem. Mater. 2019 31 2971 2982 10.1021/acs.chemmater.9b00528
Österholm, A. M., Ponder, J. F. Jr., De Keersmaecker, M., Shen, D. E. & Reynolds, J. R. Disentangling Redox Properties and Capacitance in Solution-Processed Conjugated Polymers. Chem. Mater. 31, 2971–2982 (2019).10.1021/acs.chemmater.9b00528
20. Kimpel J High-Mobility Organic Mixed Conductors with a Low Synthetic Complexity Index via Direct Arylation Polymerization Chem. Sci. 2024 15 7679 7688 10.1039/D4SC01430H 38784738
Kimpel, J. et al. High-Mobility Organic Mixed Conductors with a Low Synthetic Complexity Index via Direct Arylation Polymerization. Chem. Sci. 15, 7679–7688 (2024).38784738 10.1039/D4SC01430H
21. Proctor CM Rivnay J Malliaras GG Understanding volumetric capacitance in conducting polymers J. Polym. Sci. Part B Polym. Phys. 2016 54 1433 1436 10.1002/polb.24038
Proctor, C. M., Rivnay, J. & Malliaras, G. G. Understanding volumetric capacitance in conducting polymers. J. Polym. Sci. Part B Polym. Phys. 54, 1433–1436 (2016).10.1002/polb.24038
22. Zhang S Toward Stable p-Type Thiophene-Based Organic Electrochemical Transistors Adv. Funct. Mater. 2023 33 2302249 10.1002/adfm.202302249
Zhang, S. et al. Toward Stable p-Type Thiophene-Based Organic Electrochemical Transistors. Adv. Funct. Mater. 33, 2302249 (2023).10.1002/adfm.202302249
23. Enokida I Furukawa Y Doping-level dependent mobilities of positive polarons and bipolarons in poly(2,5-bis(3-hexadecylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT-C16) based on an ionic-liquid-gated transistor configuration Org. Electron. 2019 68 28 34 10.1016/j.orgel.2019.01.045
Enokida, I. & Furukawa, Y. Doping-level dependent mobilities of positive polarons and bipolarons in poly(2,5-bis(3-hexadecylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT-C16) based on an ionic-liquid-gated transistor configuration. Org. Electron. 68, 28–34 (2019).10.1016/j.orgel.2019.01.045
24. Voss MG Driving Force and Optical Signatures of Bipolaron Formation in Chemically Doped Conjugated Polymers Adv. Mater. 2021 33 2000228 10.1002/adma.202000228
Voss, M. G. et al. Driving Force and Optical Signatures of Bipolaron Formation in Chemically Doped Conjugated Polymers. Adv. Mater. 33, 2000228 (2021).10.1002/adma.202000228
25. Paulsen BD Frisbie CD Dependence of Conductivity on Charge Density and Electrochemical Potential in Polymer Semiconductors Gated with Ionic Liquids J. Phys. Chem. C. 2012 116 3132 3141 10.1021/jp2093934
Paulsen, B. D. & Frisbie, C. D. Dependence of Conductivity on Charge Density and Electrochemical Potential in Polymer Semiconductors Gated with Ionic Liquids. J. Phys. Chem. C. 116, 3132–3141 (2012).10.1021/jp2093934
26. Taur, Y. & Ning, T. H. Fundamentals of Modern VLSI Devices. 3rd Ed. (Cambridge University Press, 2021).
27. Tarabella G Effect of the gate electrode on the response of organic electrochemical transistors Appl. Phys. Lett. 2010 97 123304 10.1063/1.3491216
Tarabella, G. et al. Effect of the gate electrode on the response of organic electrochemical transistors. Appl. Phys. Lett. 97, 123304 (2010).10.1063/1.3491216
28. Donahue MJ High-Performance Vertical Organic Electrochemical Transistors Adv. Mater. 2018 30 1705031 10.1002/adma.201705031
Donahue, M. J. et al. High-Performance Vertical Organic Electrochemical Transistors. Adv. Mater. 30, 1705031 (2018).10.1002/adma.201705031
29. Kaphle V Liu S Al-Shadeedi A Keum C-M Lüssem B Contact Resistance Effects in Highly Doped Organic Electrochemical Transistors Adv. Mater. 2016 28 8766 8770 10.1002/adma.201602125 27511804
Kaphle, V., Liu, S., Al-Shadeedi, A., Keum, C.-M. & Lüssem, B. Contact Resistance Effects in Highly Doped Organic Electrochemical Transistors. Adv. Mater. 28, 8766–8770 (2016).27511804 10.1002/adma.201602125
30. Kroon R Polar Side Chains Enhance Processability, Electrical Conductivity, and Thermal Stability of a Molecularly p-Doped Polythiophene Adv. Mater. 2017 29 1700930 10.1002/adma.201700930
Kroon, R. et al. Polar Side Chains Enhance Processability, Electrical Conductivity, and Thermal Stability of a Molecularly p-Doped Polythiophene. Adv. Mater. 29, 1700930 (2017).10.1002/adma.201700930
31. Inal S A High Transconductance Accumulation Mode Electrochemical Transistor Adv. Mater. 2014 26 7450 7455 10.1002/adma.201403150 25312252
Inal, S. et al. A High Transconductance Accumulation Mode Electrochemical Transistor. Adv. Mater. 26, 7450–7455 (2014).25312252 10.1002/adma.201403150
32. Giovannitti A Controlling the mode of operation of organic transistors through side-chain engineering Proc. Natl Acad. Sci. 2016 113 12017 12022 10.1073/pnas.1608780113 27790983
Giovannitti, A. et al. Controlling the mode of operation of organic transistors through side-chain engineering. Proc. Natl Acad. Sci. 113, 12017–12022 (2016).27790983 10.1073/pnas.1608780113
33. Giovannitti A Energetic Control of Redox-Active Polymers toward Safe Organic Bioelectronic Materials Adv. Mater. 2020 32 1908047 10.1002/adma.201908047
Giovannitti, A. et al. Energetic Control of Redox-Active Polymers toward Safe Organic Bioelectronic Materials. Adv. Mater. 32, 1908047 (2020).10.1002/adma.201908047
34. Yang W High-Performance n-Type Polymeric Mixed Ionic-Electronic Conductors: The Impacts of Halogen Functionalization Adv. Mater. 2024 36 2305416 10.1002/adma.202305416
Yang, W. et al. High-Performance n-Type Polymeric Mixed Ionic-Electronic Conductors: The Impacts of Halogen Functionalization. Adv. Mater. 36, 2305416 (2024).10.1002/adma.202305416
35. Ding B Influence of Backbone Curvature on the Organic Electrochemical Transistor Performance of Glycolated Donor–Acceptor Conjugated Polymers Angew. Chem. Int. Ed. 2021 60 19679 19684 10.1002/anie.202106084
Ding, B. et al. Influence of Backbone Curvature on the Organic Electrochemical Transistor Performance of Glycolated Donor–Acceptor Conjugated Polymers. Angew. Chem. Int. Ed. 60, 19679–19684 (2021).10.1002/anie.202106084
36. Mone M Mechanically Adaptive Mixed Ionic-Electronic Conductors Based on a Polar Polythiophene Reinforced with Cellulose Nanofibrils ACS Appl. Mater. Interfaces 2023 15 28300 28309 10.1021/acsami.3c03962 37262133
Mone, M. et al. Mechanically Adaptive Mixed Ionic-Electronic Conductors Based on a Polar Polythiophene Reinforced with Cellulose Nanofibrils. ACS Appl. Mater. Interfaces 15, 28300–28309 (2023).37262133 10.1021/acsami.3c03962
37. Friedlein JT Donahue MJ Shaheen SE Malliaras GG McLeod RR Microsecond Response in Organic Electrochemical Transistors: Exceeding the Ionic Speed Limit Adv. Mater. 2016 28 8398 8404 10.1002/adma.201602684 27457055
Friedlein, J. T., Donahue, M. J., Shaheen, S. E., Malliaras, G. G. & McLeod, R. R. Microsecond Response in Organic Electrochemical Transistors: Exceeding the Ionic Speed Limit. Adv. Mater. 28, 8398–8404 (2016).27457055 10.1002/adma.201602684
38. Nielsen CB Molecular Design of Semiconducting Polymers for High-Performance Organic Electrochemical Transistors J. Am. Chem. Soc. 2016 138 10252 10259 10.1021/jacs.6b05280 27444189
Nielsen, C. B. et al. Molecular Design of Semiconducting Polymers for High-Performance Organic Electrochemical Transistors. J. Am. Chem. Soc. 138, 10252–10259 (2016).27444189 10.1021/jacs.6b05280
39. Giovannitti A N-type organic electrochemical transistors with stability in water Nat. Commun. 2016 7 13066 10.1038/ncomms13066 27713414
Giovannitti, A. et al. N-type organic electrochemical transistors with stability in water. Nat. Commun. 7, 13066 (2016).27713414 10.1038/ncomms13066
40. Inal S Organic electrochemical transistors based on PEDOT with different anionic polyelectrolyte dopants J. Polym. Sci. Part B Polym. Phys. 2016 54 147 151 10.1002/polb.23938
Inal, S. et al. Organic electrochemical transistors based on PEDOT with different anionic polyelectrolyte dopants. J. Polym. Sci. Part B Polym. Phys. 54, 147–151 (2016).10.1002/polb.23938
41. Moser M Polaron Delocalization in Donor–Acceptor Polymers and its Impact on Organic Electrochemical Transistor Performance Angew. Chem. Int. Ed. 2021 60 7777 7785 10.1002/anie.202014078
Moser, M. et al. Polaron Delocalization in Donor–Acceptor Polymers and its Impact on Organic Electrochemical Transistor Performance. Angew. Chem. Int. Ed. 60, 7777–7785 (2021).10.1002/anie.202014078
42. Harman DG Poly(3,4-ethylenedioxythiophene):dextran sulfate (PEDOT:DS) – A highly processable conductive organic biopolymer Acta Biomater. 2015 14 33 42 10.1016/j.actbio.2014.11.049 25484333
Harman, D. G. et al. Poly(3,4-ethylenedioxythiophene):dextran sulfate (PEDOT:DS) – A highly processable conductive organic biopolymer. Acta Biomater. 14, 33–42 (2015).25484333 10.1016/j.actbio.2014.11.049
43. Moser M Side Chain Redistribution as a Strategy to Boost Organic Electrochemical Transistor Performance and Stability Adv. Mater. 2020 32 2002748 10.1002/adma.202002748
Moser, M. et al. Side Chain Redistribution as a Strategy to Boost Organic Electrochemical Transistor Performance and Stability. Adv. Mater. 32, 2002748 (2020).10.1002/adma.202002748
44. Osazuwa PO Surface Functionalization with (3-Glycidyloxypropyl)trimethoxysilane (GOPS) as an Alternative to Blending for Enhancing the Aqueous Stability and Electronic Performance of PEDOT:PSS Thin Films ACS Appl. Mater. Interfaces 2023 15 54711 54720 10.1021/acsami.3c09452 37962428
Osazuwa, P. O. et al. Surface Functionalization with (3-Glycidyloxypropyl)trimethoxysilane (GOPS) as an Alternative to Blending for Enhancing the Aqueous Stability and Electronic Performance of PEDOT:PSS Thin Films. ACS Appl. Mater. Interfaces 15, 54711–54720 (2023).37962428 10.1021/acsami.3c09452
45. Zokaei S Toughening of a Soft Polar Polythiophene through Copolymerization with Hard Urethane Segments Adv. Sci. 2021 8 2002778 10.1002/advs.202002778
Zokaei, S. et al. Toughening of a Soft Polar Polythiophene through Copolymerization with Hard Urethane Segments. Adv. Sci. 8, 2002778 (2021).10.1002/advs.202002778
46. Winther-Jensen B West K Vapor-Phase Polymerization of 3,4-Ethylenedioxythiophene:  A Route to Highly Conducting Polymer Surface Layers Macromolecules 2004 37 4538 4543 10.1021/ma049864l
Winther-Jensen, B. & West, K. Vapor-Phase Polymerization of 3,4-Ethylenedioxythiophene:  A Route to Highly Conducting Polymer Surface Layers. Macromolecules 37, 4538–4543 (2004).10.1021/ma049864l
