# A Universal Method for Achieving Ultra-Low Contact Resistances in Organic Electrochemical Transistors

Luis-abraham Lozano-Hernández, Patrice Rannou, Yvan Bonnassieux,

Sébastien Sanaur

► To cite this version:

Luis-abraham Lozano-Hernández, Patrice Rannou, Yvan Bonnassieux, Sébastien Sanaur. A Universal Method for Achieving Ultra-Low Contact Resistances in Organic Electrochemical Transistors. Advanced Materials Interfaces, 2025, 12 (16), 10.1002/admi.202500208. hal-05383177

HAL Id: hal-05383177

https://hal.science/hal-05383177v1

Submitted on 3 Dec 2025

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
# A Universal Method for Achieving Ultra-Low Contact Resistances in Organic Electrochemical Transistors

Luis-Abraham Lozano-Hernández, Patrice Rannou, Yvan Bonnassieux, and Sébastien Sanaur*

Organic ElectroChemical Transistors (OECTs) are intensively studied for enabling their use in organic bioelectronics, neuromorphic systems, and biosensors. Beyond device geometry, reaching optimal operation of organic electronic circuits requires the optimization of the physico-chemical properties of the channel. Toward this end, the effects of a “bulk” doping of the channel material and its influence on the contact resistance ($R_C$) at the interface between a Polymeric Mixed Ionic-Electronic conductors (PMIECs) and the Source (S) and Drain (D) electrodes are presented. An easy-to-implement method to achieve ultra-low contact resistances in OECTs is introduced. By incorporation of LiTFSI, a 4x transconductance improvement is achieved, and a decrease of $R_C$ by a factor of ≈2 and ≈40 has been observed for p-type or n-type PMIECs, respectively. It reaches an unprecedented width-normalized contact resistance value as low as 1 Ohm.cm with the p(g2T-T) polymer. The formation of very localized domains in the polymeric matrix in the vicinity of the electrodes, as a result of the reduction of TFSI⁻ anions, which modulates the energy barrier at the S/D interface, is suggested here. Furthermore, both p(g2T-T) and p(gNDI-gT2) polymers exhibit low water uptake with minute amounts of LiTFSI. Worth noticing, doped p(g2T-T) preserves its volumetric capacitance and demonstrates an exceptional long-term stability. Finally, a universal strategy to fine-tune OECT performances, drawing prospects for implementing next-generation applications in organic bioelectronics and neuromorphics, is proposed.

L.-A. Lozano-Hernández, S. Sanaur

Mines Saint-Etienne

Department of Flexible Electronics

Centre Microélectronique de Provence

Gardanne F-13541, France

E-mail: sanaur@emse.fr

P. Rannou

Univ. Grenoble Alpes, Univ. Savoie Mont Blanc, Grenoble INP, LEPMI

CNRS

Grenoble 38000, France

Y. Bonnassieux

LPICM (UMR 7647 CNRS)

École Polytechnique - Institut Polytechnique de Paris

Route de Saclay, Palaiseau 91128, France

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/admi.202500208

© 2025 The Author(s). Advanced Materials Interfaces 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/admi.202500208

## 1. Introduction

Polymeric Mixed Ionic-Electronic Conductors (PMIECs) are channel materials of OECTs within which an ionic-to-electronic transduction occurs across their volumes. Due to the excellent mechanical and electronic properties of PMIECs, OECTs are flexible,[1] biocompatible,[2] present high transconductance and operation stability in an aqueous environment.[3-5] OECTs are three-terminal devices composed of a channel, consisting of a PMIEC thin-film, located between two electrodes (Source (S) and Drain (D)) and in direct contact with a liquid, a gel, or a solid electrolyte. The third electrode (Gate (G)) is in contact and/or immersed within the electrolyte (Figure 1a). When a gate voltage ($V_G$) is applied through the gate, the PMIEC layer is doped with ions penetrating from the electrolyte inside the channel. Consequently to the incorporation of these ions, an increase of opposite sign electronic ‘free’ charge carriers is occurring in the $\pi$-conjugated PMIEC backbone. The “injected” ions

are transduced into electronic charges throughout the channel and extracted at the drain,[6] sourcing the drain current ($I_D$) of the OECT (Figure 1a). The transconductance in saturation regime, $g_{m, sat}$, is one of the most important parameters of the transistor because it determines the gain of an amplifier made using a transistor. $g_{m, sat}$ is defined as:[7]

$$g_{m, sat} = \left. \frac{dI_D}{dV_G} \right|_{V_D>V_{Dsat}} = \frac{Wd}{L} \mu C^* (V_G - V_T) \quad (1)$$

where $V_G$ is the gate voltage, $V_T$ is the threshold voltage, $C^*$ is the volumetric capacitance, and $\mu$ is the charge carrier mobility. L, W, and d are the channel length, width, and thickness, respectively. The transconductance $g_{m, sat}$ drives indirectly the dynamic behavior of OECTs and its switching speed.[8] As Equation 1 indicates, $\mu C^*$ product is directly affecting $g_{m, sat}$ performances. These last years, the $\mu C^*$ product has been widely used as a Figure of Merit (FoM) for benchmarking PMIECs.[9]

Other than $g_{m, sat}$, others parameters also determine the final performance of OECTs. This is the case for contact
<figure><img src="image_2.png" /><figcaption>**Figure 1.** Schematic of an Organic ElectroChemical Transistor (OECT). The channel (PMIEC based) is patterned between two (S&D) electrodes with an electrolyte on the top. A third electrode (Ag/AgCl) is immersed in the electrolyte and used as the Gate electrode. Parylene C (PaC) is used as an isolating layer (a). Chemical structures of the p-type p(g2T-T) polymer, the n-type p(gNDI-gT2) polymer as channel materials and LiTFSI dopant (b). Bulk doping of the channel thin film is achieved via spin-coating (c), by mixing the polymer (dissolved in chloroform) with LiTFSI dissolved in acetone (a “bad solvent” for the polymer). Both solutions are combined in equal volume, with the LiTFSI concentration tailored according to the desired doping ratio “r”.</figcaption></figure>

resistance ($R_C$), a critical parameter in (organic) thin-film transistors ((O)TFTs). Surprisingly, $R_C$ is not widely reported[10] and/or controlled in OECTs. In the case of (O)TFTs (and also in OECTs), the $R_C$ physical mechanism is determined by a Mott–Schottky barrier at the Source (Drain, respectively) metal/channel interface[11,12] where high $R_C$ values penalize the performance of these electronic devices.[10,13] The Mott–Schottky barrier is reduced with the increase in the $V_C$. The $V_C$ application increases the number of free-charge carriers in the channel, leading to a HOMO/LUMO band bending and reduction of the energetic barrier between the electrodes and the channel. Because of its inert nature and biocompatibility, gold is usually used for the metal S&D electrodes of OECTs. Thus, it is important to notice that no oxidized layer is established at the surface of gold-based S&D electrodes of an OECT. It is also largely used as S&D electrodes because its Work Function (WF) matches the energetic level of the HOMO level of *p*-type semiconducting polymers and remains quite performant even with *n*-type semiconducting polymers.

Poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) is the most studied channel material for OECTs.[14] PEDOT: PSS is a commercially available and highly doped polymer capable of *moderate-to-high* OECT performance. Importantly, it can be used only in a depletion-mode OECT, which is a synonym of approving non-standard design rules in electronics and thus highly limiting the investigation of CMOS-like based OECTs circuits.[15] The development of new PMIECs is therefore essential. Accordingly, new polymers and a few small molecules have been designed and synthesized in the last years[16,17] to work as the active layer of accumulation-mode

OECTs.[8,18] One of the main characteristics of these PMIECs is the differences in their side-chains, for example, the replacement of alkyl pending groups by oligo(ethylene glycol) (OEG)-based side-chains in order to improve the hydrophilicity around their $\pi$-conjugated backbones. Macromolecular engineering allows for tuning the $\mu$C\* performance depending on the OEG side-chain length and position.[19] In this study, we investigate two *state-of-the-art* PMIECs, poly[3,3′-bis[2-[2-(2-methoxyethoxy)ethoxy]-2,2′:5,2′'-terthiophene-5,5′'-diyl] [p(g2T-T)] and the naphthalenediimide–bithiophene alternated copolymer [p(gNDI-gT2)], as *p*-type and *n*-type channel materials of OECTs, respectively (Figure 1b).[20]

The most straightforward strategies to increase and/or tune the electrical properties of organic semiconductors are to control their doping through oxidation or reduction for *p*-type and *n*-type semiconducting materials, respectively. This can be easily implemented by introducing “dopants” (oxidizing or reducing agents) to generate a host polymeric matrix-dopant mixture that can be processed into thin films to form a bulk-doped layer between S&D electrodes. In this way, it is possible to improve/modify the performance of organic semiconductors without the need to modify their original (macro)molecular structure.[21] So-called “bulk” and “contact” doping are two related concepts used in OFETs. The former aims to increase and tune the channel properties (current modulation) while the latter is set to overcome the high energy barrier at contacts (decreasing the contact resistance).[22] The advantage of contact doping, when compared to bulk/channel doping, is that the dopant is confined close to the contacts and, as a consequence, the channel remains unaffected. In OFETs, as in OECTs, the performance is strongly limited by
the contact resistance, reducing the device mobility, inducing a channel-length mobility dependence,[23] or affecting the switching speed of the devices.

Here, we investigate both *p*-type and *n*-type polymers by using LiTFSI as a bulk dopant to study its impact on the contact resistances and performance of OECTs (Figure 1b). There are only a few reports exploring the doping in the channel volume of OECTs. Moreover, the impact and mechanism of this type of doping remain unclear due to its scarce studies. Ohayon et al.[5] reported the use of organic salts as solution additives to improve the transduction and stability of *n*-type OECTs. Owing to its performance in boosting the ionic conductivity in OEG-containing salt-in-polymer solid polymer electrolytes,[24] the prototypical LiTFSI organic salt seemed worth testing with OEG-containing side-chains PMIECs. It results from our investigations that the ultra-low doping proposed here works as a universal method leading to an effective reduction in the values of contact resistance. Indeed, we propose that the use of LiTFSI as a bulk dopant at a sufficiently low quantity should avoid an important water uptake increasing within the channel during the OECT operation. We suggest the generation of thin domains of products reduced from TFSI⁻ at the electrode/channel material interface. Some of these products, such as LiF, are widely reported in the field of organic optoelectronics. They favorably reduce the interfacial energetic barrier by modifying the WF of the metal electrode or even acting as a tunneling layer because of its large bandgap.[25] Our work provides hints with regard to the use of organic salts such as LiTFSI, compatible with PMIECs featuring polar side-chains, as a universal method to induce a significant improvement in the contact resistances and performance of OECTs.

## 2. Results and Discussion

### 2.1. Contact Resistance Versus Doping

We first investigated the influence of doping *p*-type polymers on OECTs performance and the contact resistance. LiTFSI and p(g2T-T) were used as a bulk dopant and for the channel material of *p*-type OECTs, respectively. The chemical functionalization of the gold S/D electrodes by self-assembled monolayers (SAMs) is a well-known and established methodology to adjust the energetic levels at the electrode/semiconductor interface in OTFTs[26,27] and equally performs for OECTs.[28] Unambiguously efficient, it requires strictly controlled atmosphere conditions (e.g., glove box with controlled level of water and oxygen) to graft SAMs on the metal electrode surfaces. Beyond the lab-scale (e.g., one or a few device substrates at a time), it remains nevertheless difficult to control this critical chemisorption step, especially when considering surface homogeneity subjected to batch-to-batch variations. Furthermore, this strategy does not allow for doping in the volume of channel materials and potentially improving their performance, as is the case for OTFTs. In this work, we use doping in the PMIEC channel volume by adjusting the incorporation of a dopant (LiTFSI) fine-tuned content in order to have a control parameter that adjusts or even optimizes the *g*<sub>m, sat</sub> and *R*<sub>C</sub> values of *p*-type accumulation-mode OECTs. The “bulk” doping process is simply realized by mixing the two solutions (in equal volume ratio) of LiTFSI solubilized in acetone (a bad sol-

vent for the polymer) and a p(g2T-T) chloroform solution, respectively (Figure 1c). The molar ratio *r* = Li⁺/OEG is the adjustable parameter where Li⁺ represents the number of cations per number of Oligo(ethylene glycol) units in the side-chains of the *p*-type PMIEC (Figure 1b). The *R*<sub>C</sub> values are highly dependent on *V*<sub>G</sub>. At low *V*<sub>G</sub>, the variation and magnitude of the *R*<sub>C</sub> values are large. At high *V*<sub>G</sub>, the *R*<sub>C</sub> values are “lowered” or optimal. When *V*<sub>G</sub> is applied, ions are injected from the electrolyte into the channel, changing the doping state of the channel. This will shift the Fermi level and reduce the Mott-Schottky barrier, explaining the reduction of *R*<sub>C</sub>.[29–31] The analysis of *R*<sub>C</sub> values is thus performed at *V*<sub>G</sub> = −0.6 V.[32] p(g2T-T)-based OECTs work in accumulation mode, in which the current flowing between S and D electrodes (*I*<sub>D</sub>) is maximized at high negative *V*<sub>G</sub> magnitude values. At low magnitude *V*<sub>D</sub> values, |*V*<sub>D</sub>| < 0.06 V, a linear *I*<sub>D</sub> behavior is observed. The OECT is not in an operating range, which is limited by a Space Charge Limited Current (SCLC) regime and thus allows for extracting properly the contact resistances at the metal electrode/PMIEC interface. We use the Transfer Length Method (TLM) to extract the contact resistances at the S/D-channel interfaces.[10,28,29,33,34] By varying channel lengths from *L* = 2 to 100 µm, for two different fixed widths (*W* = 20 and 40 µm), a series of OECTs were measured at low *V*<sub>D</sub> (|*V*<sub>D</sub>| < 0.06 V). TLM is a simple analytical analysis to set up in order to extract directly some device physics parameters without modifying the pristine transistor’s properties. Along the channel length axis of the OECT, the total OECT resistance (*R*<sub>OECT</sub>) is expressed as:

$$R_{OECT} = \left( \frac{\partial V_D}{\partial I_D} \right)_{V_D \to 0} = R_S + R_{Ch} + R_D \\ = 2R_C + \frac{L}{Wd} \frac{1}{\mu_i C_i^* (V_G - V_T)} = 2R_C + Lr_{Ch} \quad (2)$$

Where *R*<sub>S</sub> is the source resistance, *R*<sub>Ch</sub> is the channel resistance, and *R*<sub>D</sub> is the drain resistance. We consider here, as a first approximation, that S and D are symmetric, thus leading to *R*<sub>S</sub> = *R*<sub>D</sub>. If the width-normalized total resistance (*R*<sub>OECT</sub>·*W*) is plotted as a function of the channel length (*L*), a linear fit gives the width-normalized contact resistance (*R*<sub>C</sub>·*W*); with 2*R*<sub>C</sub> = *R*<sub>S</sub> + *R*<sub>D</sub> at the intercept with the y-axis, while the slope represents the channel resistance per channel-length unit (*r*<sub>Ch</sub>). *μ*<sub>i</sub> and *C*<sub>i</sub><sup>*</sup> are the intrinsic charge carrier mobility and the intrinsic volumetric capacitance, respectively, which do not included and consider effects at the contacts (i.e., channel/electrode interface). On the contrary, the μC<sup>*</sup> product depicted in Equation 1 incorporates *R*<sub>C</sub> effects.

Under “high” LiTFSI molar doping ratio (i.e. *r* = 0.20), *R*<sub>C</sub> values exhibit significant variations primarily due to the substantial disruption of the thin film morphology linked to the addition of LiTFSI. This disruption is clearly demonstrated by the spreading of the measured *R*<sub>C</sub> values as depicted in Figure 2a. On the other hand, at ultra-low doping values (*r* = 0.005 or 0.002), *R*<sub>C</sub> shows notably lower values and variations (Figure 2a,b). Finally, “medium” doping values (*r* = 0.01, 0.05, and 0.10) do not show a clear tendency in the *R*<sub>C</sub> values, which could be associated with significant effects in both the polymer layer and the contacts. Therefore, it is of utmost importance to improve *R*<sub>C</sub>
<figure><img src="image_3.png" /><figcaption>**Figure 2.** Width-normalized contact resistance $R_C \cdot W$ as a function of LiTFSI doping ($r = Li^+/OEG$) ratio for $p(g2T-T)$ OECTs (20µm channel width). The red star value depicts the result without bulk doping. 8 devices at each channel length (L) were fabricated to generate these data (a). TLM method for width-normalized OECT resistance ($R_{OECT} \cdot W$) as a function of the channel length L for several $r = Li^+/OEG$ ratios in $p(g2T-T)$ OECTs (b).</figcaption></figure>

since it is the main limiting factor of OECT devices (see Figure S2, Supporting Information). Note here that these $R_C$ values are comparable and even lower than values reported for PEDOT:PSS-based OECTs ($R_C = 1.4 \ \Omega$ cm). For PEDOT:PSS-based OECTs, the HOMO energetic level is aligned with the WF of the Au electrode ($WF_{Au} = 5.1$ eV)[35] explaining the low values. In the case of $p(g2T-T)$, the HOMO energy level is reported as 4.38 eV through cyclic voltammetry measurements[36] or 4.55 eV through XPS characterizations.[37] This is far to match the energetic levels alignment with gold S&D.

As can be seen in Figure 2a, at very low $Li^+/OEG$ ratios (i.e., ultra-low doping), the $R_C \cdot W$ values are always improved with respect to the reference, bad solvent, undoped situation. By relating the number of LiTFSI molecules compared to $g2T-T$ monomer (1 molecule vs 83, respectively), it is difficult to link the enhanced performance of our OECT devices at ultra-low doping conditions to a “doping effect” linked to the $\pi$-conjugated backbone of $p(g2T-T)$.[38] By comparison, OTFTs show a volumetric “doping” effect for ratios up to 0.4% weight, leading to reduced width-normalized contact resistance down to a few $k\Omega$ cm.[39] More interestingly, such contact resistances in OTFTs are positively affected by surface engineering doping, where extra free-charges are permanently accumulated at the interface between electrodes/active layer.[40–42] Contrary to OFETs, ions diffusion is occurring and constitutes the working principle of an OECT.[43] The injected ions are free to move under the $V_D$ bias voltage, until they are stopped from accumulating at the interface between the metallic S&D electrodes and the PMIEC.

On the other hand, following Equation 2, we can extract the channel resistances ($r_{Ch}$) from Figure 2b. $r_{Ch}$ does not show a clear trend as a function of the doping ratio (see Figure S1, Supporting Information). Moreover, further experimental investigations were performed by 4-point measurements to extract $r_{Ch}$, not giving a clear trend of the dispersion of $r_{Ch}$ values as a function of the doping ratio either. Contact resistances are therefore the major bottleneck of the OECT, with values of a

higher order of magnitude (Figure S2, Supporting Information), confirming that contact resistances have a direct impact on the performance of OECTs.[15,16,24] Extracted from Figures S3–S5 (Supporting Information), the $\mu C^*$ product for $p(g2T-T)$-based $p$-type OECTs under a $V_G$ set to -0.6 V are gathered in the Table 1.

The lower $\mu C^*$ values for the “high” doping ratio in $p(g2T-T)$ suggest that the water uptake into the channel is slightly higher than in the undoped case. The volumetric capacitance, $C^*$, will increase with the water uptake while the charge carrier mobility will be altered by the size reduction of $\pi-\pi$ stacked crystalline domains, which is thus the dominant parameter for $p(g2T-T)$ in the $\mu C^*$ product.[20] On the other hand, at ultra-low doping, it makes sense to consider that a change in the $\pi-\pi$ stack domains and morphology is negligible. Only a few localized changes in the channel would take place as the ratio of doping is very low. However, we observe a significant decrease in the $R_C$ values at these very low doping ratios. The width-normalized contact resistance is significantly reduced from $2.31 \pm 0.14 \ \Omega$ cm to less than half ($1.06 \pm 0.30 \ \Omega$ cm). On the other hand, the performance of the OECT is largely improved, manifesting in a 4 fold increase of its Drain current (Figure 3a). $r_{Ch}$ values extracted from Figure 2b bring us to calculate an accumulated sheet carrier concentration as high as $n_{2D} = 1.2 \ 10^{18} \ cm^{-2}$.[8] This constitutes a very high accumulation of charge carrier density for any thin-film-based transistors reported so far. Nonetheless, such ultra-low LiTFSI

<table><caption>**Table 1.** Performance characteristics of undoped and LiTFSI-doped $p(g2T-T)$ OECTs at $V_G = -0.6$ V.</caption><thead><tr><th>Polymer</th><th>r</th><th>μC* [F cm<sup>-1</sup> V s]</th></tr></thead><tbody><tr><td rowspan="4">p(g2T-T)</td><td>0</td><td>103 ± 11</td></tr><tr><td>0.002</td><td>140 ± 17</td></tr><tr><td>0.005</td><td>27 ± 4</td></tr><tr><td>0.200</td><td>99 ± 11</td></tr></tbody></table>
<figure><img src="image_4.png" /><figcaption>**Figure 3.** Output characteristics for doped ($r$ = 0.005, red dotted line) and “the bad solvent” or undoped ($r$ = 0, black continuous line) $p(g2T-T)$-based OECTs with overlapping channel/source-drain length (overlap = overlap channel/source-overlap channel/drain, 25–25 µm for example) (a). Transconductance of OECTs with same channel length (L) but different overlap for ultra-low doping ($r$ = 0.005) and “bad solvent” (undoped, $r$ = 0) states (b).</figcaption></figure>

bulk doping is *simple-to-implement* in order to induce a signifi-
cant improvement without the need to use i) low-energy metal
integration, tunneling contacts, semi-metal contacts, etc.,[45] and
ii) related vacuum micro(nano)-technologies to set up such
contacts.

Figure 3b shows the transconductance dependency on V<sub>G</sub>, for undoped vs. doped (with r = 0.005) states. We observe that the larger the channel overlap on top of Source-Drain electrodes, the higher the relative transconductance enhancement. When the OECT is slightly doped with LiTFSI, one can note that the maximal V<sub>G</sub> for the maximum transconductance is shifted by ≈0.1–0.2 V toward lower voltages. In the fabrication process we set up in this study, an observation by optical microscopy clearly indicates that the PMIEC layer covers the (ca. 100 nm-thick) step formed by the gold S/D electrodes. The improvement tendency with an increased area of contact, channel/source-drain, is an additional indicator of the enhancement of the channel/source- drain interface, i.e., contact resistance dependency. However, the overlap is showing to be a very important and frequently under- rated characteristic to optimize for enabling high-performance OECTs. This kind of doping shows that it is possible to signifi- cantly increase the transconductance performance of the OECTs by increasing the channel/source-drain overlap. Strikingly, such a doping effect on the contact resistances in OECTs is universal since we report the same favorable decreasing R<sub>C</sub> trend when us- ing an *n*-type polymeric semiconductor, here p(gNDI-gT2). In- deed, Figure S6 (Supporting Information) shows R<sub>C</sub>.W values as a function of channel length (L) as low as 100–300 Ω cm in the case of *ultralow-to-low* doping (r = 0.002; 0.005; 0.010). These values compared favorably with respect to bad solvent (un- doped) conditions, where the R<sub>C</sub>.W value is at least 40 times higher than in the case of low doping conditions. Such a re- sult is particularly interesting from the perspective of designing CMOS-like OECTs circuits. Indeed, *n*-type PMIECs present to- day lower µC\* product than *p*-type PMIECs, but this holds true without considering the contact resistance contribution. From this analysis, we recommend taking into consideration the intrin- sic product (µC)<sub>i</sub>\* as a FoM in PMIECs development. A higher R<sub>C</sub>.W amplitude value (in Ω cm) reported here for *n*-type is ex- plained by an energy mismatch between the gold work func-

tion and the LUMO level of p(gNDI-gT2), which is ≈0.4 eV
higher when compared to the case of p(g2T-T) case. The LUMO
level of p(gNDI-gT2) is 4.12 eV.[18] The energy difference ΔE =
E<sub>p(gT-2T)</sub><sup>LUMO</sup> - E<sub>p(gNDI-gT2)</sub><sup>LUMO</sup> is ≈0.3 eV. A tunneling current being ex-
ponentially dependent on the energetic ΔE barrier amplitude,
it is not surprising to measure a contact resistance ratio equal
to 4640/2 = 2320, from *p*-type to *n*-type PMIEC cases. Interest-
ingly, doping the *n*-type PMIEC reduces this ratio to ≈100 if we
consider a *p*-type PMIEC without doping. Such a strategy could
contribute to reducing the performance gap between *p*- and *n*-
type PMIECs, in a very simple materials science engineering
way.

## 2.2. Effect of LiTFSI on the Swelling Behavior and the Volumetric Capacitance of thin Films

The enhanced initial drain current with very low LiTFSI doping (0.33 vs 0.1 mA for pristine and 0.25 mA for mixed solvent p(g2T-T)) strongly suggests that a small amount of LiTFSI can significantly improve the initial device performance. The preserved stability over 12 h is equally important. Even with the added salt, it demonstrates that there is no rapid degradation, which is often a concern in OECTs. This could imply an inflexion in the doping regime where it is possible to get the electronic benefits of LiTFSI without introducing instability or morphological disruption (Figure 4).

On the other hand, when one looks at the swelling of thin films (Figure 5b–d), we can see that the doped p(g2T-T) is showing a significantly lower and faster swelling compared to the pristine p(g2T-T) and the “bad solvent” p(g2T-T). Nevertheless, the volumetric capacitance for doped p(g2T-T) and pristine p(g2T-T) are roughly similar (Figure 5a), leaving then as plausible explanations that thin films are presenting subtle differences in the phase separation between ionic and electronic domains and/or slightly different crystallinity values. In the case of *n*-type p(gNDI-gT2), we observed (Figure S7b–d, Supporting Information) a reduction in the swelling of the film when the mixture of solvents was used and the same values when the polymer was doped with a minute amount of LiTFSI (r = 0.005).
<figure><img src="image_5.png" /><figcaption>**Figure 4.** Long-term stability measurements for pristine p(g2T-T), “bad solvent” (chloroform + acetone) p(g2T-T) and doped (r = 0.005) p(g2T-T)-based OECTs. The Drain current is normalized to the peak value of the initial pulse. Gate voltage pulses of 5 s were applied with 5 s intervals between successive pulses.</figcaption></figure>

## 2.3. LiTFSI Influence at the Source-Drain Interface

Savva et al. suggested that minimizing the channel swelling in the doping process could be an essential parameter for highly efficient and fast ion-to-electron transduction devices.[46] Ohayon et al. used organic salts to improve n-type OECTs, with the best result showing a reduced water uptake.[5] In our study, we demonstrate that the water uptake is reduced by doping the channel with an ultralow quantity of LiTFSI when used for the p(g2T-T) channel material but not for p(gNDI-gT2). The only plausible explanation left, supported by the significant improvement in R<sub>C</sub>, is that there is a phenomenon enhancing the performance of both p- and n-type OECTs occurring directly at the channel/S-D interface.

As “additional” water is drifted into the channel, the Li⁺ and TFSI⁻ ions will be able to coordinate to these molecules. Since the charge carrier mobility will remain similar to the undoped case, the amount of additional water molecules expected should not be high (high μ and medium C* are expected[46]). Under these internal conditions, the water in the channel compared to the amount of LiTFSI molecules will lead to a high and localized concentration of LiTFSI, especially when using an ultra-low doped LiTFSI-based channel OECT. The behavior of LiTFSI-based electrolytes at high and low concentrations has shown that their interaction with gold-based electrodes is completely dependent on the LiTFSI concentration.[47] In a diluted LiTFSI electrolyte (Figure 6a), the Li⁺ cations are fully solvated by H₂O (4 molecules) in the primary solvation sheath. On the other hand,
<figure><img src="image_6.png" /><figcaption>**Figure 5.** a) Dependence of the film capacitance on geometry, where the slope of the linear fit corresponds to the volumetric capacitance $C^*$ for pristine p(g2T-T), bad solvent p(g2T-T), and doped p(g2T-T). Mass change ($\Delta m$), measured by quartz crystal microbalance (QCM), for b) pristine p(g2T-T), c) 'bad solvent' processed p(g2T-T), and d) doped p(g2T-T) films in the presence of deionized water and 0.1 M NaCl.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>**Figure 6.** Interaction between $Li^+$ and $TFSI^-$ at low concentration a) and high concentration b) in water.[48]</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>**Figure 7.** Zoom-in on the channel for LiTFSI ultralow doping (a) and high doping (b) at $V_G = 0$ (left) and $V_G < 0$ (right). At ultralow doping a), the water penetration is limited when $V_G < 0$ and the $Li^+$ cations are attracted to the D (negative) electrode, drifting with them TFSI$^−$ anions. On the other hand, when under high doping b), the water uptake increases and the $Li^+$ ions can be, or not, attracted to the drain but will not drift TFSI$^−$ anions with them as there are numerous water molecules around to bond with. (In each representation, the interface of the PMIEC channel with the electrolyte is represented at the top of the schematic; the S electrode on the left and the D electrode on the right of each schematic, respectively).</figcaption></figure>

the TFSI$^−$ anions cannot directly be in contact with $Li^+$, staying relatively free (also known as solvent-separated ion pairs). In this scenario of a low LiTFSI content, there are also significant free $H_2O$ molecules. As shown in Figure 6b, it is reported that at a high LiTFSI concentration, the amount of $H_2O$ molecules decreases, and an average of 2 TFSI$^−$ anions will be observed in each primary solvation shell of $Li^+$ (known as contact ion pairs).[47]

In the OECT case, the $Li^+$ cation, characterized by its very small size and highly localized charge, tendentiously highly attracted to the electrode to counteract the negative charge on the drain. The above is also true for the high concentration of “salt-in-water,” which is linked to the complexed form $Li(TFSI)_2$.[49,50] In general, it is expected to be difficult for TFSI$^−$ anions to reach the electrode because the cations are strongly tightened and block their reduction, except if the cation is $Li^+$, in which case TFSI$^−$ is attracted and drifted by $Li^+$ to the electrode.

For an ultralow doped channel (Figure 7a), we expect to generate a negligible mechanical and morphological change in the channel since the amount of LiTFSI cannot induce any drastic changes in the amorphous and crystalline domains. For ultralow LiTFSI doping, at $V_G < 0$ and $V_D < 0$, $Li^+$ cations will be easily attracted toward the negative (D) electrode, dragging TFSI$^−$ anions with them.

As can be seen in Figure 7, at $V_G < 0$, water molecules absorbed by the channel accumulate between the OEG chains and in the amorphous domains, creating free spaces for ion movement.[46] $Li^+$ cations movement, along with TFSI$^−$ anions, is only possible with limited water penetration, inducing a highly concentrated LiTFSI “electrolyte” in some regions of the channel. Some water molecules will be drifted by the electrolyte (NaCl in our case) when ions are pushed inside the channel by the applied $V_G < 0$. We expect that if a microregion has a high LiTFSI concentration (i.e., limited water able to coordinate with both $Li^+$ cations and TFSI$^−$ anions), TFSI$^−$ anions will coordinate with $Li^+$ cations, as depicted in the case of Figure 6b. In lithium-ion batteries, a key-enabling electrochemical interfacial layer called solid electrolyte interphase (SEI) forms on the electrode surface because of the reduction of electrolyte products.[51] Tułodziecki et al. reported the formation of a SEI through the reduction of TFSI$^−$ anions that is catalyzed by the simple presence of $Li^+$ cations.[49] As we mentioned above, $Li^+$ cations will be easily attracted to the D electrode with coordinated TFSI$^−$ anions, attaching this last one to the gold surface by Au–O bonds and promoting its reduction, which will lead to the formation of new products such as LiF, $CF_3$, etc.[47,52] As a $Li^+$ cation will coordinate to two TFSI$^−$ anion subunits, $[Li(TFSI)_2]^{−}$, the adsorption of coordinated $Li^+$ cations will
<figure><img src="image_9.png" /><figcaption>**Figure 8.** TFSI⁻ anion reduction at the D electrode, forming micro/nano-domains incorporating new products such as LiF, $CF_3$.</figcaption></figure>

induce an increase in TFSI⁻ anions at the gold surface, making even easier its reduction.[49] In the case of our OECTs, instead of having a unique, uniform and continuous “layer” of $[Li(TFSI)_2]⁻$ or LiTFSI (and the products of their reduction), one can expect to have small domains at the polymer/gold interface (Figure 8) since polymers cannot completely cover the entire deposition surface (gold electrodes S and D electrodes).

We proposed that these products act as enhancers of the gold electrode by reducing the WF of the electrode and/or facilitating the injection of electrons from the D electrode by a tunneling mechanism.[25] We suggest that this phenomenon only occurs at ultra-low doping, where the uptake of free water molecules is quite limited and present in and around amorphous domains more than in crystalline ones.[20] Such a phenomenon does not thus hinder the movement of LiTFSI molecules toward the gold electrodes of the OECT under bias voltage. To the contrary (Figure 7b), when a high LiTFSI doping is used, morphological and mechanical changes are expected to occur. The number and size of crystalline domains are modified with the numerous LiTFSI molecules through a plasticizing effect in a similar way to the situation encountered for salt-in-polymer PEO-based electrolytes. These several LiTFSI molecules will induce new “free” spaces within the amorphous domains, facilitating and increasing water penetration. Under these circumstances, a significant swelling effect may occur due to the significant “defects” introduced along the polymer, leading to a low local concentration of LiTFSI with $Li⁺$ cations and TFSI⁻ anions unable to coordinate with each other (Figure 7b).

In such a process, we believe that the increase or reduction of WF at the electrode could depend on the dopant chemical structure, i.e., the anion used as the reduced products formed at the electrode interface being inherently dependent on the organic salt used. Using the $Li⁺$ cation to attract and reduce one (or more) anions could simply prove to be a new way to fine-tune the contact resistance in OECTs. Illustratively, one alternative to LiTFSI among the handful of Lithium salts used in the arena of Lithium-ion batteries is Lithium bis(fluorosulfonyl)imide (LiFSI), which, when reduced, is reported to be extremely effective in generating ionically conductive LiF as the main reduction product.[53] This is calling for sampling the existing library of Lithium salts (developed within the context of electrochemical energy storage devices) with the promise to improve the performances of both

$p$- and $n$-type OECTs with our universal minute amount bulk doping recipe, applying to past, present, and future polymer and small molecule mixed ionic-electronic conductors.

We believe these results highlight the importance of considering the optimization of contact resistances in OECTs, especially when designing circuits. In such a case, a study of the additional incorporation of ionic species, set in motion under bias voltage, is mandatory. Indeed, it is customary in electronics to consider (rightly) that ionic species are the slowest “charges” to move and can harm the switching speed performance of these devices. However, such an effect is mitigated if particular OECT architectures are implemented, since OECTs operating in the MHz regime have already been reported in the literature.[54,55] This then means that the speed of movement of the ions can be optimized, without hindering, in practice, the implementation of the OECT in the considered applications. On the other hand, in the case of analog electronic circuits, such as biosensing applications, this effect does not seem to be detrimental, since the switching speed of the devices is not a limiting parameter. Finally, a compromise is still to be considered and studied before designing the use of OECTs in circuits, for applications in the fields of neuromorphics and bioelectronics.

### 3. Conclusion

Our study shows that OECTs can be highly limited by the contact resistances, $R_C$. Indeed, we demonstrated that the incorporation of LiTFSI molecules within a $p(g2T-T)$ polymer solution, thereafter spin-coated as an OECT's channel layer, leads to variations of its contact resistances according to the doping ratio. Particularly, “ultralow” doping is enough to decrease by a factor of 2 (i.e., 50%) the contact resistances in a $p$-type $p(g2T-T)$ semiconducting polymer. Moreover, we have shown that the transconductance value can be improved up to 4 times and the peak $V_G$ reduced, independently of the channel resistance variation. Remarkably, long-term operational stability is also achieved. Additionally, the inclusion of a very low amount of LiTFSI contributes to reducing water uptake while maintaining the volumetric capacitance characteristic of pristine $p(g2T-T)$. The decrease of the contact resistances is even more pronounced in the case of an $n$-type $p(gNDI-gT2)$ semiconducting polymer, which depicts a larger energy difference between the LUMO energy of the $p(gNDI-gT2)$ polymer
and the gold work function. We thus propose a universal mechanism that takes place in polar side-chain-containing polymeric mixed ionic-electronic conductors when incorporating Li-based organic salts in the channel layer. Our experimental work investigating the low concentration LiTFSI incorporation in the PMIEC channel is i) a simple-to-implement technique, and ii) easy and reliable. In the working principle we propose, the degree of LiTFSI concentration introduced in the PMIEC is key to reaching optimal OECT performances. At low content, the cations and anions of the Li-based organic salt are 'free' to drift under bias voltage within hydrophilic PMIECs equipped with polar side-chains. This leads to the formation of a layer equivalent to a beneficial "electrical double layer" which is triggered by the redox reaction in the PMIECs. This inevitably takes place near the metal electrodes where the ionic species stop their movements and therefore accumulate. Such a mechanism is an 'intrinsic' property of the electrochemical working principle of an OECT as a device which is driven under a voltage bias. The method we introduce in this work is then universal and is valid for any PMIEC used today and/or for new ones following the same design rules. The increase or reduction of WF at the electrode could be dependent on the organic salt-based dopant used, as the products at the metallic electrodes are coming from the reduction of their anions. Using the $Li^+$ cation to attract and reduce one (or more) anions could be a new way to improve contact resistance in both p- and n-type OECTs, opening doors to higher-performance organic circuitry.

## 4. Experimental Section

**OECTs Fabrication:** A 3 µm-thick PaC layer was deposited on a 4-inch CZ-Si dummy wafer in an SCS coater. Ti Prime was deposited on the wafer by spin-coating (@2000 rpm for 30 s) and annealed at 100 °C for 2 min. The wafer was spin-coated with AZ5214 photoresist (@4000 rpm for 40 s) followed by a soft bake treatment (@100 °C for 45 s). The photoresist was exposed for two cycles of 4 s, each at 9.5 mW cm⁻² (@365 nm), through a chromium photomask with a Süss MicroTec MJB4 mask-aligner. A post-exposure bake was performed@120 °C for 45 s, followed by flood exposure (@300 mJ cm⁻² for 35 s). Finally, the photoresist was developed in MF-26A developer for 30 s. An 8 nm thick Ti layer was evaporated, followed by a 100 nm thick Au layer (vacuum thermal evaporation@10⁻⁶ Torr vacuum pressure) onto the wafer. To define the gold electrodes, lift-off was performed by immersing the wafer in acetone. A first layer of 3 µm-thick PaC was deposited, followed by the spin-coating of 2% soap solution (anti-adherent layer). A second PaC layer (4 µm) was deposited atop. Photolithography and O₂ Reactive-Ion Etching (RIE) controlled by an Endpoint detector were used to open access to source and drain gold electrodes. Residual photoresist was removed by rinsing the wafer with acetone prior to channel deposition. 2 × 18 OECTs channel transistors were fabricated per one 4 inch-wafer. Two wafers were used to produce the reported data to serve the analyses developed within this research paper.

Polymers p(g2T-T) and p(gNDI-gT2) ($M_w$≈40 kDa eq. PS, PDI≈2 and $M_w$≈30 kDa eq. PS, PDI≈2.5, respectively) were purchased from 1-material Co. Ltd. Solution of p(g2T-T) was prepared inside a glove box at a concentration of 5 mg mL⁻¹ in chloroform (purchased from Merck, an-hydrous, ≥99%, contains 0.5–1.0% ethanol as stabilizer) and stirred 20–24 h before use. Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI, purchased from Merck) was dissolved in acetone (Merck) and mixed with p(g2T-T), at the specific ratios to achieve the next LiTFSI/OEG molar ratios: r = [Li⁺]/[OEG] = 0.002, 0.005, 0.010, 0.050, 0.100, and 0.200. OECT devices based on p(gNDI-gT2) were fabricated following the same procedure as p(g2T-T). The channel deposition was made by spin-coating. Two layers were deposited (700 rpm; 700 rpm s⁻¹) to ensure full coverage of the source and drain electrodes. The PaC layer was peeled-off, and then

an annealing treatment at 80°C for 15 min was applied. All the channel deposition and OECT characterizations were performed under a nitrogen atmosphere, in a glove box featuring below 0.1 ppm O₂ and 1 ppm H₂O levels. NaCl 0.1 M was used as electrolyte.

$R_C$ and $r_{Ch}$ were obtained by using the Transfer Length Method (TLM). The µC* was obtained from the $g_m$, sat of different OECTs plotted as a function of their biasing conditions and the geometry of each channel. As a result, a linear fitting was performed, and from the slope, the µC* product was obtained.[7] OECTs were fabricated and characterized under a nitrogen atmosphere using 0.1 M NaCl as the electrolyte. Stability measurements were carried out under ambient conditions by applying gate voltage pulses of 5 s duration with 5 s intervals between pulses ($V_G$ = -0.5 V, $V_D$ = -0.5 V).

Electrochemical Impedance Spectroscopy (EIS) was conducted using a three-electrode configuration on OECT devices with varying geometries. 0.1 M NaCl, Platinum, and Ag/AgCl served as electrolyte, and the counter and reference electrodes, respectively. A VMP-3e potentiostat was employed to record data over a frequency range from 1 MHz to 1 Hz using a 50 mV AC modulation. Capacitance values were extracted by fitting the impedance spectra with a simplified Randles equivalent circuit model $R(R|C)$. The volumetric capacitance was determined by plotting the extracted capacitance values as a function of device geometry.

Quartz Crystal Microbalance (QCM) measurements were performed at 25 °C using a Qsense Analyzer. DI water and 0.1 M NaCl were injected at 50 µL.min⁻¹. The polymers were spin-coated onto gold QSensors. Data analysis was carried out using the third overtone, selected for its signal quality. Mass changes were calculated based on the Saurbrey model.

OECT samples for EIS and long-term stability measurements, as well as those used for QCM analysis, were fabricated and characterized under ambient atmospheric conditions.

## Supporting Information

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

## Acknowledgements

The authors gratefully acknowledge the funding from the Agence Nationale de la Recherche (ANR) (under the Grant ANR-20-CE06-022), supporting the research project MASTERMIND. This work was in part supported within the framework of the Centre of Excellence of Multifunctional Architectured Materials “CEMAM” (under the ANR-10-LABX-44-01) Grant. P.R. acknowledges support from the Centre National de la Recherche Scientifique (CNRS) at the UMR5279-LEPMI laboratory. L.L-H and S.S gratefully acknowledge the funding from the European Innovation Council (under the grant number #101129638), supporting the ICONIC research project.

## 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

contact resistances, electrochemical doping, organic electrochemical transistors, organic mixed ionic-electronic conductors, solvation
Received: March 5, 2025
Revised: June 20, 2025
Published online:

[1] D. Khodagholy, J. Rivnay, M. Sessolo, M. Gurfinkel, P. Leleux, L. H. Jimison, E. Stavrinidou, T. Herve, S. Sanaur, R. M. Owens, G. G. Malliaras, Nat. Commun. **2013**, *4*, 2133.

[2] D. Khodagholy, T. Doublet, P. Quilichini, M. Gurfinkel, P. Leleux, A. Ghestem, E. Ismailova, T. Hervé, S. Sanaur, C. Bernard, G. G. Malliaras, Nat. Commun. **2013**, *4*, 1575.

[3] M. Moser, J. F. Ponder, A. Wadsworth, A. Giovannitti, I. McCulloch, Adv. Funct. Mater. **2019**, *29*, 1807033.

[4] A. Marks, S. Griggs, N. Gasparini, M. Moser, Adv. Mater. Interfaces **2022**, *9*, 2102039.

[5] D. Ohayon, L. Q. Flagg, A. Giugni, S. Wustoni, R. Li, T. C. Hidalgo Castillo, A. H. Erwas, R. Sheelamanthula, I. McCulloch, L. J. Richter, S. Inal, ACS Mater. **2023**, *3*, 242.

[6] J. Rivnay, S. Inal, B. A. Collins, M. Sessolo, E. Stavrinidou, X. Strakosas, C. Tassone, D. M. Delongchamp, G. G. Malliaras, Nat. Commun. **2016**, *7*, 11287.

[7] D. Ohayon, V. Druet, S. Inal, Chem. Soc. Rev. **2023**, *52*, 1001.

[8] S. M. Sze, K. K. Ng, Physics of Semiconductor Devices, Wiley, Hoboken, NJ, USA **2006**.

[9] A. Marks, X. Chen, R. Wu, R. B. Rashid, W. Jin, B. D. Paulsen, M. Moser, X. Ji, S. Griggs, D. Meli, X. Wu, H. Bristow, J. Strzalka, N. Gasparini, G. Costantini, S. Fabiano, J. Rivnay, I. McCulloch, J. Am. Chem. Soc. **2022**, *144*, 4642.

[10] V. Kaphle, S. Liu, A. Al-Shadeedi, C. M. Keum, B. Lüssem, Adv. Mater. **2016**, *28*, 8766.

[11] D. Cahen, A. Kahn, Adv. Mater. **2003**, *15*, 271.

[12] S. Jung, C. H. Kim, Y. Bonnassieux, G. Horowitz, J. Phys. D Appl. Phys. **2015**, *48*, 395103.

[13] A. F. Paterson, T. D. Anthopoulos, Nat. Commun. **2018**, *9*, 5264.

[14] D. Khodagholy, M. Gurfinkel, E. Stavrinidou, P. Leleux, T. Herve, S. Sanaur, G. G. Malliaras, Appl. Phys. Lett. **2014**, *99*, 163304.

[15] S. T. Keene, T. P. A. Van Der Pol, D. Zakhidov, C. H. L. Weijtens, R. A. Janssen, A. Salleo, Y. Van De Burgt, S. T. Keene, D. Zakhidov, A. Salleo, T. P. A. Van Der Pol, C. H. L. Weijtens, R. A. J. Janssen, Adv. Mater. **2020**, *32*, 2000270.

[16] H. Kim, Y. Won, H. W. Song, Y. Kwon, M. Jun, J. H. Oh, Adv. Sci. **2024**, *11*, 2306191.

[17] S. Inal, G. G. Malliaras, J. Rivnay, Nat. Commun. **2017**, *8*, 1767.

[18] A. Giovannitti, C. B. Nielsen, D. T. Sbircea, S. Inal, M. Donahue, M. R. Niazi, D. A. Hanifi, A. Amassian, G. G. Malliaras, J. Rivnay, I. McCulloch, Nat. Commun. **2016**, *7*, 1.

[19] M. Moser, L. R. Savagian, A. Savva, M. Matta, J. F. Ponder, T. C. Hidalgo, D. Ohayon, R. Hallani, M. Reisjalali, A. Troisi, A. Wadsworth, J. R. Reynolds, S. Inal, I. McCulloch, Chem. Mater. **2020**, *32*, 6618.

[20] Y. Dai, S. Dai, N. Li, Y. Li, M. Moser, J. Strzalka, A. Prominski, Y. Liu, Q. Zhang, S. Li, H. Hu, W. Liu, S. Chatterji, P. Cheng, B. Tian, I. McCulloch, J. Xu, S. Wang, Adv. Mater. **2022**, *34*, 2201178.

[21] N. A. Kukhta, A. Marks, C. K. Luscombe, Chem. Rev. **2022**, *122*, 4325.

[22] C. H. Kim, Micromachines (Basel) **2021**, *12*, 742.

[23] J. Zeng, D. He, J. Qiao, Y. Li, L. Sun, W. Li, J. Xie, S. Gao, L. Pan, P. Wang, Y. Xu, Y. Li, H. Qiu, Y. Shi, J. Bin Xu, W. Ji, X. Wang, Nat. Commun. **2023**, *14*, 324.

[24] Z. Xue, D. He, X. Xie, J. Mater. Chem. A **2015**, *3*, 19218.

[25] A. Turak, Electron. Mater. **2021**, *2*, 198.

[26] P. Marmont, N. Battaglini, P. Lang, G. Horowitz, J. Hwang, A. Kahn, C. Amato, P. Calas, Org. Electron. **2008**, *9*, 419.

[27] D. Boudinet, M. Benwadih, Y. Qi, S. Altazin, J. M. Verilhac, M. Kroger, C. Serbutoviez, R. Gwoziecki, R. Coppard, G. Le Blevennec, A. Kahn, G. Horowitz, Org. Electron. **2010**, *11*, 227.

[28] A. F. Paterson, H. Faber, A. Savva, G. Nikiforidis, M. Gedda, T. C. Hidalgo, X. Chen, I. McCulloch, T. D. Anthopoulos, S. Inal, Adv. Mater. **2019**, *31*, 1902291.

[29] M. Barret, S. Sanaur, P. Collot, Org. Electron. **2008**, *9*, 1093.

[30] Y. Xu, H. Sun, A. Liu, H. H. Zhu, W. Li, Y. F. Lin, Y. Y. Noh, Adv. Mater. **2018**, *30*, 1801830.

[31] J. Liu, X. Liu, Z. Chen, L. Miao, X. Liu, B. Li, L. Tang, K. Chen, Y. Liu, J. Li, Z. Wei, X. Duan, Nano Res. **2019**, *12*, 463.

[32] E. G. Bittle, H. W. Ro, C. R. Snyder, S. Engmann, R. J. Kline, X. Zhang, O. D. Jurchescu, D. M. DeLongchamp, D. J. Gundlach, J. Polym. Sci. Polym. Phys. **2017**, *55*, 1063.

[33] K. Terada, H. Muta, Jpn J. Appl. Phys. **1979**, *18*, 953.

[34] C. Liu, Y. Xu, Y. Y. Noh, Mater. Today **2015**, *18*, 79.

[35] K. Hong, S. H. Kim, C. Yang, T. K. An, H. Cha, C. Park, C. E. Park, Org. Electron. **2011**, *12*, 516.

[36] C. B. Nielsen, A. Giovannitti, D. T. Sbircea, E. Bandiello, M. R. Niazi, D. A. Hanifi, M. Sessolo, A. Amassian, G. G. Malliaras, J. Rivnay, I. McCulloch, J. Am. Chem. Soc. **2016**, *138*, 10252.

[37] R. K. Hallani, B. D. Paulsen, A. J. Petty, R. Sheelamanthula, M. Moser, K. J. Thorley, W. Sohn, R. B. Rashid, A. Savva, S. Moro, J. P. Parker, O. Drury, M. Alsufyani, M. Neophytou, J. Kosco, S. Inal, G. Costantini, J. Rivnay, I. McCulloch, J. Am. Chem. Soc. **2021**, *143*, 11007.

[38] Y. Yamashita, J. Tsurumi, M. Ohno, R. Fujimoto, S. Kumagai, T. Kurosawa, T. Okamoto, J. Takeya, S. Watanabe, Nature **2019**, *572*, 634.

[39] Y. Hu, Z. D. Rengert, C. McDowell, M. J. Ford, M. Wang, A. Karki, A. T. Lill, G. C. Bazan, T. Q. Nguyen, ACS Nano **2018**, *12*, 3938.

[40] C. G. Tang, M. C. Y. Ang, K. K. Choo, V. Keerthi, J. K. Tan, M. N. Syafiqah, T. Kugler, J. H. Burroughes, R. Q. Png, L. L. Chua, P. K. H. Ho, Nature **2016**, *539*, 536.

[41] B. Lüssem, C. M. Keum, D. Kasemann, B. Naab, Z. Bao, K. Leo, Chem. Rev. **2016**, *116*, 13714.

[42] A. A. Günther, M. Sawatzki, P. Formánek, D. Kasemann, K. Leo, Adv. Funct. Mater. **2016**, *26*, 768.

[43] Y. Kim, K. Broch, W. Lee, H. Ahn, J. Lee, D. Yoo, J. Kim, S. Chung, H. Sirringhaus, K. Kang, T. Lee, Adv. Funct. Mater. **2020**, *30*, 2000058.

[44] G. Sych, P. Rannou, M. Jullien-Palletier, S. Sadki, Y. Bonnassieux, S. Sanaur, Adv. Electron. Mater. **2023**, *9*, 2201067.

[45] W. Li, X. Gong, Z. Yu, L. Ma, W. Sun, S. Gao, Ç. Köroğlu, W. Wang, L. Liu, T. Li, H. Ning, D. Fan, Y. Xu, X. Tu, T. Xu, L. Sun, W. Wang, J. Lu, Z. Ni, J. Li, X. Duan, P. Wang, Y. Nie, H. Qiu, Y. Shi, E. Pop, J. Wang, X. Wang, Nature **2023**, *613*, 274.

[46] A. Savva, C. Cendra, A. Giugni, B. Torre, J. Surgailis, D. Ohayon, A. Giovannitti, I. McCulloch, E. Di Fabrizio, A. Salleo, J. Rivnay, S. Inal, Chem. Mater. **2019**, *31*, 927.

[47] L. Suo, O. Borodin, T. Gao, M. Olguin, J. Ho, X. Fan, C. Luo, C. Wang, K. Xu, Science **2015**, *350*, 938.

[48] X. Li, X. Wang, L. Ma, W. Huang, Adv. Energy Mater. **2022**, *12*, 2202068.

[49] M. Tułodziecki, J. M. Tarascon, P. L. Taberna, C. Guéry, Electrochem. Commun. **2017**, *77*, 128.

[50] A. Lahiri, T. Carstens, R. Atkin, N. Borisenko, F. Endres, J. Phys. Chem. C **2015**, *119*, 16734.

[51] A. Wang, S. Kadam, H. Li, S. Shi, Y. Qi, npj Comput. Mater. **2018**, *4*, 15.

[52] L. Wang, K. Uosaki, H. Noguchi, J. Phys. Chem. C **2020**, *124*, 12381.

[53] L. E. Camacho-Forero, T. W. Smith, P. B. Balbuena, J. Phys. Chem. C **2017**, *121*, 182.

[54] A. Melianas, T. J. Quill, G. LeCroy, Y. Tuchman, H. V. Loo, S. T. Keene, A. Giovannitti, H. R. Lee, I. P. Maria, I. McCulloch, A. Salleo, Sci. Adv. **2020**, *6*, abb2958.

[55] C. Cea, Z. Zhao, D. J. Wisniewski, G. D. Spyropoulos, A. Polyravas, J. N. Gelinas, D. Khodagholy, Nat. Mater. **2023**, *22*, 1227.