Keywords: mélange de polymères semi-conducteurs, transistors électrochimiques organiques mixed conductors, semiconducting polymer blends, organic electrochemical transistors Days in water

Mélange de polymères conjugués pour conducteurs ionique et électronique organiques plus rapides

Le fonctionnement de divers dispositifs électrochimiques organiques relève de leur propriété intrinsèque de conductivité mixte par les polymères pi-conjugués. Un facteur clé, qui généralement limite les performances d'un dispositif, est le transport trop lent des ions à travers le polymère semiconducteur, nécessaire pour le dopage et dé-dopage en masse. De plus, de nombreux efforts ont été réalisés pour améliorer la conductivité ionique de différents polymères conjugués en travaillant chimiquement sur la polarité de la chaîne latérale sur ces matériaux. De tels groupements peuvent cependant impacter la structure de ces matériaux et rendre leurs propriétés électroniques sensibles aux conditions environnementales. Ce travail démontre la viabilité d'un mélange de polymères multicomposants comme alternative pour régler la conductivité mixte. Un copolymère à bloc hydrophile de poly(3-hexylthiophène) et poly(oxyde d'éthylène) (P3HT-b-PEO) est mélangé avec un copolymère aléatoire conducteur mixte de P3HT et poly(3-hydroxyhexylthiophène) (P3HTco-P3HHT). Les transistors électrochimiques du mélange montrent une performance de transduction similaire au matériau P3HT-co-P3HHT pur à l'état stable, cependant le temps de réponse est grandement réduit. On attribut cela à un dopage électrochimique plus rapide puisque la captation des ions s'améliore quand on augmente l'hydrophilie du matériau grâce à l'ajout du copolymère à bloc. En parallèle, la partie P3HT du copolymère à bloc semble agir comme un compatibilisant qui améliore la stabilité structurale du mélange, améliorant ainsi la stabilité opérationnelle du dispositif. Par conséquent, notre travail impliquant cette ingénierie des copolymères à blocs pour fabriquer des mélanges compatibles est une méthode prometteuse pour améliorer les performances et la stabilité des matériaux conducteurs mixtes.

Mots clés : conducteurs mixtes,

Les performances des récents dispositifs électrochimiques organiques reposent principalement sur leur propriété unique à être à la fois des conducteurs ioniques et des conducteurs électroniques via les liaisons pi-conjuguées. Les propriétés électrochimiques de ces matériaux permettent un dopage en masse ainsi qu'un dé-dopage en présence d'électrolytes qui permettent de changer efficacement leur conductance. Ceci peut être exploité, par exemple, dans des bio-capteurs où la transduction entre les flux biochimiques ioniques et électroniques est nécessaire. De plus, les interactions équilibrant les charges entre les ions injectés et les charges déjà présentes dans le polymère entraînent de larges capacitances volumétriques. Ces dernières peuvent contribuer d'avantage aux fortes magnitudes des signaux de transduction, lorsqu'elles sont exploitées dans un dispositif.

Cependant, ces phénomènes semblent reposer sur le mouvement des ions au niveau de l'interface polymère-analyte, ainsi que dans le polymère lui-même. La vitesse de réponse des dispositifs est ainsi limitée par la conductivité ionique du matériau actif. Par conséquent de nombreux travaux ont été menés pour élargir la gamme des matériaux disponibles, ayant des propriétés conductives mixtes, en améliorant l'efficacité du transport ionique dans ces matériaux.

Nous avons démontré que l'association d'un polymère conducteur mixte avec un composé hydrophile conducteur d'ions, est une stratégie simple et efficace pour améliorer le transport ionique dans ces matériaux. Cette méthode permet d'augmenter la fonctionnalité du matériau sans avoir recours à de modifications chimiques complexes de la structure du polymère semi-conducteur. En effet ces dernières requièrent souvent une substitution par des groupements complexes, encombrants ou polaires, qui peuvent avoir un effet néfaste sur l'ordre moléculaire du polymère et donc sur ses propriétés électriques.

Pour le conducteur mixte actif, nous avons sélectionné le copolymère poly (

Courants transitoires mesurés pour des OECTs faits de P3HT-co-P3HHT et leurs mélanges avec 25 wt% de P3HT-b-PEO avec des constantes de temps (τ) indiquées en rouge (au milieu). Influence du taux de balayage de la tension de grille (V s -1 ) sur mise à l'échelle de la transconductance (gm) avec la géométrie du dispositif et les paramètres de polarisation dans les OECT du matériau pur et de son mélange (en bas). μC* moyennes extraites de la régression linéaire des pentes représentées en vert.

Figure i.3. Enthalpies (ΔΗ) de mélange du P3HT (incluant le P3HHT) et du PEO dans

des solutions de diverses compositions extraites de leur thermogrammes (haut, gauche).

Les tirets indiquent les valeurs «idéales» pour des phases complètement séparées. Les enthalpies inférieures à leur valeur idéale indiquent un manque de cristallisation dû à la compatibilité entre les phases. Image de microscopie optique d'un mélange ne montrant aucune séparation de phase à grande échelle même lorsqu'elle est vue sous polarisation croisée pour observer la biréfringence (en haut à droite). Angles de contacts avec l'eau des mélanges (au milieu à gauche). Absorbance transitoire lors du dopage électrochimique des polymères avec des ajustements exponentiels indiqués en rouge

pour extraire les taux de dopage (k ; au milieu à droite). Normalisation de k à l'épaisseur du film (en bas à gauche). K normalisé par l'épaisseur en fonction de la composition du mélange (en bas à droite). Plus spécifiquement, les deux mélanges présentent une diminution de l'angle de contact à 70° avec l'eau et une amélioration similaire des taux de dopage électrochimique par rapport au matériau pur. Cela pourrait indiquer que l'effet du mélange sur les propriétés transitoires du conducteur mixte est dicté par la fraction de PEO. En outre, nous déterminons que seulement 20 wt% de teneur en PEO dans l'un ou l'autre système suffit à moduler efficacement les propriétés du matériau. En conséquence, les OECTs du mélange avec PEO présentent un comportement transitoire amélioré indiqué par des temps de réponse relativement faibles (τ = 70 ms). En même temps, la capacité volumétrique des mélanges avec PEO est relativement similaire à celle du matériau pur (190 ± 30 F cm -3 ). Par la suite, les OECTs comprenant le mélange avec le PEO présentent des caractéristiques à l'état d'équilibre similaires et un paramètre μC* (14 F cm -1 V -1 s -1 ) dans la plage de celle du P3HTco-P3HHT pur (10-55 F cm -1 V -1 s -1 ). Toutefois, le système comprenant 25 % en masse de l'homopolymère PEO présente une plus grande séparation de phases et moins de preuves

General Introduction

The rapid growth in the field of organic bioelectronics over the past two decades demonstrates the potential of π-conjugated polymers to be the primary active materials in future commercial medical technologies. Among the key features of these materials are their unique electrochemistry and mixed ionic and electronic conducting properties that leads to bulk doping and/or de-doping in the presence of electrolytes, which can be exploited in applications where transduction between biochemical ionic fluxes and electronics is required. For instance, the charge-balancing interaction of ionic species with polarons during electrochemical oxidation (or reduction) of the conjugated backbone not only modulates the conductivity of the material but also induces large volumetric capacitances, which contributes to high signal amplification when integrated in biosensors.

However, these phenomena seem to rely on the movement of ions across the polymeranalyte interface and within the polymer itself, thereby limiting the response speed of devices across various applications to the ionic conductivity of the active material. Thus, in the quest to expand the range of options for organic mixed conducting materials there has been great interest to enhance the ionic transport efficiency of various conjugated polymers.

One common strategy to develop new classes of polymeric mixed conductors involves chemically engineering the polarity of side chains functionalized on a conjugated backbone. Polythiophene derivatives with different substituents including acidic moieties, ionic species, and oligo(ethylene glycol) side chains have been explored. The resulting materials show mixed conductor properties such as enhanced ionic conductivity, bulk electrochemical doping, and high volumetric capacitances, and offer features such as depletion, accumulation, or ambipolar modes of device operation. Although this has led to the steady growth of the polymer mixed conductor library, the limitations of singlecomponent materials have also become more apparent. For example, most of conjugated polyelectrolytes are usually water-soluble and require cross-linkers or bulky counter-ions to be compatible with aqueous applications. In another case, although glycol modification of sidechains results in the highest augmentation of the capacitance in conjugated polymers, which leads to materials exhibiting the highest steady-state transduction performance in organic electrochemical transistors (OECTs), significant water uptake during operation can bring about irreversible morphological changes in the conjugated structure that heavily impact their electrical properties and device stability. This renders the optoelectronic properties of these materials sensitive to humidity. Furthermore, bulky and charged side groups can lead to torsional disorder in the backbone which has detrimental effects on charge-carrier mobility. Often the synthesis of a series of derivatives incorporating multiple monomers to rearrange side group and backbone unit distribution is required to further tune material performance. The mechanisms of OECT operation, the key features of mixed conducting polymers, and the recently reported polymers that have been designed and applied to OECTs are discussed further in Chapter 1 of the manuscript.

Polymer blending is another viable method that can incorporate various functionalities into a material without the need for complex chemical design on the active semiconducting polymer itself. We demonstrate here that we can greatly expedite the response speed of electrochemical devices operating in accumulation mode by blending a mixed conductor with an ion-conducting block copolymer, while using simple lowtemperature, solution-based techniques, and without the need for special post-processing treatments. As the active mixed conductor, we selected the random copolymer of poly(3hexyl thiophene) and poly [3-(6-hydroxy)hexyl thiophene] (P3HT-co-P3HHT, 50% comonomer composition) in which the hydroxyhexyl (-C6H12OH) side chain modification enhances this material's electrochemical response at low potentials in aqueous media, without drastically changing the molecular order and mechanical properties of solution-cast films even after operation. This was blended with a block copolymer of P3HT and poly(ethylene oxide) (P3HT-b-PEO) to enhance the hydrophilicity and ionic conductivity of the material. Furthermore, the P3HT block as a compatibilizer may limit large scale phase separation, which can help preserve the electronic properties of the semiconducting material and maintain its stability especially in the context of water-rich environments. The rationale for choosing these materials is detailed more in the conclusion of Chapter 1. In Chapter 2, we show that adding P3HT-b-PEO to P3HT-co-P3HHT does indeed permit faster OECT operation while apparently preserving its ion-electron transduction performance at steady state. This correlates with further electrochemical characterization of the blends, discussed in Chapter 3, which indicate that blending enhances the electrochemical doping kinetics of the material operating in aqueous electrolyte but without changing its volumetric capacitance. Finally, in Chapter 4, we show the potential of block copolymers as co-components in mixed conducting polymer blends to sustain long-term structural stability and device performance during operation in water-rich media.

CHAPTER 1:

Bibliographic research

Introduction

Mixed ionic-electronic conducting polymers have attracted much attention over recent years due to their increasing application in the field of bioelectronics. 1 These materials are ideal candidates for interfacing between biology and electronics for several reasons, one of which is that their chemistry consists mainly of carbon, hydrogen, nitrogen, and oxygen atoms, which matches that of biological molecules. 2 Their mechanical stiffness can also be tuned to be more compatible with that of biological tissue. Furthermore, they can be easily biofunctionalized using a variety of synthetic techniques. Meanwhile, owing to the unique electrochemical properties of conjugated polymers, they may conduct both ions and electrons, which offers a mechanism to efficiently transduce between ionic and electronic signals.

One particular device whose operation relies heavily on the intrinsic properties of mixed conducting polymers is the organic electrochemical transistor (OECT). 3 Owing to its device architecture, OECTs are amplifiers. However, with a mixed conducting polymer as the active channel material, OECTs can yield large output signals compared to that of other related organic thin film transistors. Additionally, multiple conductance states of the channel can be achieved depending on the applied input voltage. As a result, OECTs have gained interest for applications in biosensing and neuromorphic memory storage. OECTs employing different channel materials can also be used to benchmark mixed conducting polymers. Thus, OECTs are indispensable tools in the quest to expand the range of polymers that can be used for applications in bioelectronics.

Organic electrochemical transistors

Basic operating principles

The typical architecture of an OECT shown in Figure 1.1 consists of a gate electrode immersed in an electrolyte that is in contact with a conjugated polymer film, which is also in contact with a source and a drain electrode separated over a length L that makes up the channel across which holes and electrons flow during operation.

Accumulation-mode OECT Depletion-mode OECT

Bulk doping processes in the OECT channel, owing to the electrochemical properties and soft nature of the conjugated polymer as well as the presence of mobile ions in the electrolyte, is the defining characteristic of OECTs that distinguishes them from other closely related organic transistors such as organic field effect transistors (OFET) and even electrolyte gated OFETs (EGOFETs). In the latter two devices, application of gate bias induces field-effect doping of the semiconducting polymer, that is, the accumulation of charge at the polymer-dielectric (in OFETs) or polymer-electrolyte (in EGOFETs) interface, whose architectures and charging behavior are depicted in Figure 1.3. Consequently, the amount of charge and the drain currents induced by device operation is limited by the area of these interfaces. Contrastingly, bulk doping in OECTs results to greater charge build up in the channel and significantly higher drain currents. Indeed, OFETs show capacitances of roughly 10 -9 -10 -6 F cm -2 regardless of semiconductor thickness, 4,5 while an OECT comprising a 130-nm thick PEDOT:PSS channel, for example, can show up to 5 x 10 -4 F cm -2 . 6 Thus, in describing the OECT device physics, a bulk capacitance term is used.

Steady-state characteristics

The electronic circuit is described by Ohm's law and the ionic circuit is treated as an ideal polarizable electrode, exhibiting the same behavior as a charging capacitor.

Furthermore, the channel is considered to be a volumetric capacitor, rather than an interfacial one. This makes it possible to obtain the equation of the drain current at saturation (Id,sat) given in Equation 1.1. 6 𝐼 𝑑,𝑠𝑎𝑡 = 𝑊𝑑 2𝐿 𝜇𝐶 * (𝑉 𝑡 -𝑉 𝑔 ) 2

1.1

Where W, d, and L are the channel width, thickness, and length, respectively, μ is the channel charge carrier mobility (cm 2 V -1 s -1 ), C* is the volumetric capacitance (in F cm -3 ) and

Vt is the threshold voltage. Note that this equation is almost the same as that derived for MOSFETs 9 , except that the areal capacitance term (C') is replaced by the C* term for OECTs. The drain current at saturation is independent of the drain voltage, which simplifies subsequent derivation of performance parameters.

The transconductance (gm), which quantifies the transduction performance of the transistor, is experimentally obtained by measuring the change in drain currents over the change in gate voltage from differentiating the transfer curves (Id versus Vg) of OECTs. 3 We can also predict trends in gm by taking the first derivative of Equation 1.

1.3

Where ISS is the steady state drain current obtained at saturation, ΔISS is the total change in the drain current resulting from stepping Vg from zero to a certain bias, f is a geometrical factor defining the fraction of the ionic current measured at the drain, τe is the electron (or hole) transport time constant, and τi is the ionic transport time constant. Essentially there are three contributing terms to the total current transient: the drain current at saturation, the change in current due to conductance changes upon doping or de-doping the polymer, and a fraction of the gate current flowing into the channel, which can be understood to be split between the source and drain to account for the spatial non-uniformity of the doping or dedoping process across the channel. For simplicity, f can be made equal to 0.5 (equal division of the gate current at the source and drain) 11 , however variations of the model define more accurate values of f by assuming its dependence on both the gate and drain voltages 10,12 , for instance by extracting f from multi-parameter fits to experimental data or directly measuring the ratio of the gate and drain currents. Additionally, the exponential form in

𝑰(𝒕, 𝑰

𝒈 ) = 𝑰 𝟎 -𝑰 𝒈 (𝒇 + 𝒕 𝝉 𝒆 ) 1.4
Where I0 is the initial drain current prior to application of Ig. τe measures the time it takes for the charge carriers to drift across the source and drain under an applied potential, and is related to their mobility (μe) by τe = L 2 (μeVd) -1 . 13 Thus, plotting the slope of the transient drain current

( ∂I d ∂t )
versus Ig allows extraction of τe from the inverse of the slope, subsequently allowing calculation of μe for the active channel material. For example, the hole mobility for PEDOT:PSS extracted from OECT measurements, μOECT = 2.7 cm 2 V -1 s -1 , shows reasonable agreement with those obtained from Hall effect measurements, μH ≈ 1-3 cm 2 V - 1 s -1 , 14,15 which gives some validation of the OECT method to extract hole mobilities. This is especially useful as an alternative tool to characterize semiconducting polymers that show unintentional bulk doping behavior, which can interfere with the stability of measurements in typical OFET configuration for hole mobility determination. 16 1.2.3. Influence of channel geometry on OECT performance The same color coding and shape is used for each channel material across all panels.

Figures from ref. According to Equation 1.3, the response time of an OECT upon application of a step gate potential is the time constant, τi = RSC, of the ionic circuit across the electrolyte and channel (refer to Figure 1.4). Using impedance spectroscopy, the solution resistance, RS, and the channel capacitance, C, can be measured for devices with various channel dimensions. The C in OECTs of PEDOT:PSS has been shown to be proportional with WdL over a wide range of channel volumes, the slope of which can be used to extract the volumetric capacitance (C*), about 40 F cm -3 for PEDOT:PSS. 6 On the other hand, the solution resistance is inversely proportional with the area of the electrode in a simple electrochemical cell such as the one used to model the OECT ionic circuit. Indeed, RS in PEDOT:PSS electrodes have been shown to scale linearly with (WL) -1/2 . 18 Combining the scaling laws of CCH and RE yields τi = RECCH ∝ d(WL) 1/2 , which states that the response time of OECTs is proportional with channel size. This implies that thinner films may lead to faster devices but at the expense of the transduction performance. Therefore, the channel geometry should be tuned according to the requirements of intended applications.

The scaling laws derived from the basic model of OECTs discussed above provide practical implications for the design of devices and the active channel material, however certain conditions may cause real devices to deviate from these predictions. For instance, the capacitance only scales linearly with the channel volume if devices are operated such that enough time is given for the ions to penetrate completely throughout the bulk of the material. Above a characteristic frequency, fion ≈ μionVd -2 , that depends on the ion mobility (μion), devices will show sublinear dependence of the capacitance with channel volume. At even higher frequencies devices eventually perform similar to OFETs. Additional effects that may result from either the specific device architecture (ex. overlapping electrodes 19 ) or even functionalized devices (ex. blocking membranes 10 ), will also be a factor in the final response times and transconductances. The Bernard-Malliaras model can be adapted and extended further with parameters that account for such effects to improve the accuracy in predicting OECT measurements.

Key features of mixed ionic-electronic conducting polymers

According to our understanding of the basic operating principles and device physics of OECTs discussed in the previous section, we can identify some key features of mixed ionic-electronic conducting polymers. First, the conductance state of the material must be electrochemically controllable. This is possible with conjugated polymers due to their unique semiconducting properties arising from their electronic structure. As will be discussed, the incorporation of ions in the polymer supports charge carrier formation and is therefore necessary to modulate its electronic conductivity. Consequently, efficient transport of ions in the material is important for fast device operation. The presence of ions can also lead to electrostatic interactions that may contribute to the high volumetric capacitance of these materials, which is relevant for high-transconductance OECTs. High OECT performance also relies on high charge carrier mobility, which is dependent on complex charge transport mechanisms occurring in conjugated polymers due to their inherently disordered structure.

Semiconducting behavior of conjugated polymers

The unique semiconducting behavior of conjugated polymers arises due to the electronic structure of their backbone. 20,21 The carbon atoms of a conjugated polymer chain are sp 2 hybridized, each sharing three σ-bonds with neighboring atoms via the sp 2 hybridized orbitals and π-bonds with adjacent sp 2 hybridized carbons via the non-hybridized pz orbitals, depicted in increasing length (bottom). Adapted from ref. 22 .

The linear combination of the non-hybridized pz orbitals results in bonding (π) and antibonding (π*) states that form the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), respectively (Figure 1.6, bottom). The energy difference between the HOMO and the LUMO can be understood as the organic equivalent of the semiconductor band gap, Eg. In ethylene, the simplest alkene, the energy between the HOMO and LUMO is 7.6 eV, which is too large for it to be considered a semiconductor (assuming around Eg ≤ 4 eV for semiconductors). Increasing the carbon chain length to form conjugated oligomers and polymers increases the density of π and π* energy levels, which then approach continuous band-like energy distributions and lead to narrowing of Eg.

At the limit of infinitely long conjugated polymers, a minimum energy gap persists owing to dimerization of the carbon atoms in the form of alternating single and double bonds (i.e., alternating bond lengths) along the carbon chain. This occurrence can be understood as a Peierls distortion; 23,[START_REF] Castiglioni | Peierls distortion in trans polyacetylene: Evidence from infrared intensities[END_REF] dimerization changes the lattice periodicity and induces a band gap at which the energy of the HOMO is lowered and the energy of the LUMO is increased. As a result, the ground state electronic structure of conjugated polymers is semiconducting.

Thus, to induce electronic conductivity in conjugated polymers, mobile charge carriers must first be formed by doping.

Doping conjugated polymers

Like inorganic semiconductors, polymeric (organic) semiconductors may be p-("positive" charge carriers) or n-type ("negative" charge carriers) doped. However, unlike inorganic semiconductors in which their doping involves introducing impurity atoms in the crystal lattice to donate electrons or holes as free charge carriers, doping polymeric semiconductors typically involves redox reactions. [START_REF] Heeger | Semiconducting and Metallic Polymers: The Fourth Generation of Polymeric Materials (Nobel Lecture)[END_REF] Such reactions only involve electrons from the non-hybridized pz orbitals, while the sp 2 electrons maintain the structure of the polymer chain.

P-type doping of a conjugated polymer (P) involves its partial oxidation (P 0 + A → P + A -). This may be accomplished either chemically, in the presence of a strong electron acceptor, or electrochemically, by application of a positive potential against a counter electrode in the presence of electrolytes. Similarly, n-type doping of a conjugated polymer involves its partial reduction (P 0 + A → P -A + ) to form a negative charge on the backbone either by chemical reaction with an electron donor or by application of a negative potential in an electrochemical cell. Charge neutrality is maintained in the doped polymer by interactions with counterions (A -or A + ), which may originate from the dopant molecule or supporting electrolyte. Additionally, unlike chemical doping, electrochemical doping allows precise control of intermediate doping levels in conjugated polymers depending on the voltage applied. This particular feature is exploited in OECTs and other related devices.

Doping conjugated polymers, either by oxidation (loss of an electron) or reduction (gain of an electron), results in a radical ionic species called a "polaron". [START_REF] Bredas | The role of mobile organic radicals and ions (solitons, polarons and bipolarons) in the transport properties of doped conjugated polymers[END_REF][START_REF] Salaneck | Electronic structure of conjugated polymers: Consequences of electron-lattice coupling[END_REF] This may be positively or negatively charged with spin ½. The polaron is stabilized by delocalization over a few monomer units and by polarization of the surrounding environment. Several polarons can be formed in a long polymer chain. At high dopant concentrations, or in the case of confinement due to chain defects, polarons may combine by pairing of their radicals to form a spinless charged species with twice the charge. This species is called a "bipolaron" and may be positive or negative depending on the type of doping. Such polarons and bipolarons are the charge carriers in conducting polymers. A depiction of a positive polaron and bipolaron in a polythiophene is given in Figure 1.7.

Polaron and bipolaron formation affect the chemistry and geometry of the backbone.

For instance, in conjugated polymers with aromatic backbones such as the polythiophene illustrated in Figure 1.7, polaron formation induces a structural transition from a benzenoidtype to a planar quinoid-type (shaded in gray). [START_REF] Bredas | The role of mobile organic radicals and ions (solitons, polarons and bipolarons) in the transport properties of doped conjugated polymers[END_REF] This impacts the electronic structure of the polymer by the formation of localized energy states within the initial bandgap as shown in This is exploited in applications of electrochromic display devices, [START_REF] Howard | Cost-Effective, Flexible, and Colorful Dynamic Displays: Removing Underlying Conducting Layers from Polymer-Based Electrochromic Devices[END_REF] and can also be used to track doping of the polymer through practical methods such as spectroelectrochemistry (employed in this work, see section 3.3.2 Spectroelectrochemistry). Electrochemical doping of conjugated polymers to generate polarons necessitates counterions to maintain charge neutrality. This implies that the formation of electronic carriers is coupled with the transport of ions in these materials, which subsequently affects the performance and response speed of their electrochemical devices. For these reasons, the design of mixed conducting polymers as active materials for OECTs over recent years have involved modifying conjugated polymers to better accommodate ion transport, especially from an external electrolyte. As a starting point to understand how ionic conductivity can be enhanced in conjugated polymers, we first recall some basic notions about ion transport in polymer electrolytes.

One possible mechanism of ion transport in dry solid state polymer electrolytes involves ion motion coupled with the segmental motion of polymer chains. [START_REF] Aziz | A conceptual review on polymer electrolytes and ion transport models[END_REF][START_REF] Kuo | Solid polymer electrolytes V: Microstructure and ionic conductivity of epoxide-crosslinked polyether networks doped with LiClO4[END_REF] depicts how segmental motion can facilitate ion "hopping" between different sites in a single polymer chain (intrachain ion hopping) or between sites of adjacent chains (interchain ion hopping). This implies that an amorphous rubbery polymer matrix that has more segmental motion is most favorable to support ion mobility. Substitution of ion-coordinating moieties such as nitrogen or oxygen atoms on the polymer chain can further impact ion transport. [START_REF] Sai | Role of polar side chains in Li+ coordination and transport properties of polyoxetane-based polymer electrolytes[END_REF] Ultimately, however, the mobility of the ions is limited by the polymer mobility. To enhance ion diffusion in polymers, attempts have been made to decouple ion motion from polymer chain motion. [START_REF] Bresser | Decoupling segmental relaxation and ionic conductivity for lithium-ion polymer electrolytes[END_REF] One approach is to graft ion-coordinating moieties like polar or ion-terminated sidechains to distance the ions from the polymer backbone. For example, polymethyl methacrylates grafted with longer ethylene glycol sidechains show better ionic conductivities than their counterparts comprising shorter substituents. [START_REF] Bennington | Role of solvation site segmental dynamics on ion transport in ethylene-oxide based side-chain polymer electrolytes[END_REF] Another approach is to dilute the polymer chain density surrounding the ions by solvent-induced swelling of the polymer, resulting in a gel. [START_REF] Baskoro | Strategic Structural Design of a Gel Polymer Electrolyte toward a High Efficiency Lithium-Ion Battery[END_REF] This allows fast ion diffusion to occur through the mostly liquid phases.

For mixed conducting polymers that are exposed to aqueous electrolytes when operated in electrochemical devices, the swelling ability of the polymer seems to be one major factor of efficient ion injection. For instance, results from electrochemical strain microscopy (ESM) of semicrystalline P3HT operated with a DC offset bias and aqueous electrolyte (20 mM KCl), in combination with amplitude-modulation/frequency-modulation (AM-FM) viscoelastic mapping, provides some evidence of heterogeneous swelling of the polymer due to non-uniform ion penetration in the film favoring the softer amorphous phases over the stiffer ordered phases. [START_REF] Giridharagopal | Electrochemical strain microscopy probes morphologyinduced variations in ion uptake and performance in organic electrochemical CHAPTER 1: Bibliographic research | 67 transistors[END_REF] Crosslinking of PEDOT:PSS has also been shown to decrease the ion drift mobility extracted from electrochromic moving front experiments, which suggests that decreasing the swelling ability of the polymer results in slower ion uptake. [START_REF] Stavrinidou | Direct measurement of ion mobility in a conducting polymer[END_REF] Furthermore, substituting conjugated polymers with hydrophilic, ion-coordinating sidechains (e.g., oligo(ethylene glycol) sidechains) can induce passive (without bias) swelling and expansion of the crystal lattice of semicrystalline polymers upon exposure to aqueous electrolyte. [START_REF] Flagg | Polymer Crystallinity Controls Water Uptake in Glycol Side-Chain Polymer Organic Electrochemical Transistors[END_REF] As a result, ions can readily access polaron sites upon application of bias leading to much faster doping kinetics than their non-substituted counterparts. In fact, chemical modification of various p-type and n-type conjugated polymers with ethylene glycol sidechains has been a successful approach to yield mixed conducting polymers exhibiting high OECT performance (discussed more in section 1.4 Progress in mixed conducting polymer design). [START_REF] Giovannitti | Controlling the mode of operation of organic transistors through side-chain engineering[END_REF][START_REF] Giovannitti | N-type organic electrochemical transistors with stability in water[END_REF] One issue that arises due to hydration-induced swelling is excessive water uptake that can eventually degrade the mixed conducting properties and structural integrity of the film. For instance, it has been shown in electrochemical quartz crystal microbalance with dissipation (EQCM-D) measurements of P(g2T-TT), a polythiophene functionalized with ethylene glycol sidechains, that the injection of ions with large hydration spheres (high number of water molecules) can actually slow down the response speed of OECTs. [START_REF] Savva | Influence of Water on the Performance of Organic Electrochemical Transistors[END_REF] Also, QCM and spectroelectrochemical measurements of an n-type mixed conducting polymer functionalized with ethylene glycol sidechains show that excessive swelling in polymer electrodes leads to irreversible mass changes. [START_REF] Szumska | Reversible Electrochemical Charging of n-Type Conjugated Polymer Electrodes in Aqueous Electrolytes[END_REF] This results in irreproducible doping A simplified understanding on how C* arises in PEDOT:PSS involves an electronic double layer (EDL) model of the interaction of injected hydrated ions with the sulfonate ions in PSS. [START_REF] Proctor | Understanding volumetric capacitance in conducting polymers[END_REF] This layer forms a capacitor (C'DL) with a thickness (t) defined by the distance between the center of the injected ion and the polymer chain, depicted in the left panel of The performance of an OECT can be further enhanced by improving the charge carrier mobility of the mixed conducting polymer channel. However, designing polymers to improve their electronic mobilities remains a difficult task due to the complexity of charge carrier transport in these materials, which originates from their inherent disordered nature. [START_REF] Noriega | A general relationship between disorder, aggregation and charge transport in conjugated polymers[END_REF][START_REF] Fratini | Charge transport in high-mobility conjugated polymers and molecular semiconductors[END_REF] Since conjugated polymers are "soft" materials bonded by weak van der Waals interactions, they have numerous degrees of conformational freedom that can result in microstructures ranging from almost completely amorphous to semicrystalline.

Consequently, transport mechanisms involve multiple processes occurring at different length scales. The most efficient pathway for charge transport in polymers is by polaron delocalization along an extended conjugated segment in the backbone (intrachain transport). However, charges will eventually encounter obstacles from conformational defects, kinks and chain-ends that are associated with localized energy states and act as electronic traps. Thus, for charge transport to proceed over more practical dimensions, [START_REF] Noriega | A general relationship between disorder, aggregation and charge transport in conjugated polymers[END_REF][START_REF] Fratini | Charge transport in high-mobility conjugated polymers and molecular semiconductors[END_REF] Charge transport remains confined within the ordered regions of the material owing to the large energy barriers that must be overcome for charges to cross between ordered and amorphous phases. [START_REF] Noriega | A general relationship between disorder, aggregation and charge transport in conjugated polymers[END_REF] Also, polymer chains in the ordered regions have longer conjugation lengths and increased intermolecular coupling that can support charge transport. Meanwhile, high activation energies are needed for charge transport to occur in fully amorphous materials that do not show molecular aggregation behavior (right panel of Figure 1.13), making these materials inefficient electronic conductors. Increasing the molecular weight of polymers can increase the probability of tie chains and enhance charge mobility, but only to a certain extent. For P3HT, it has been estimated that only 10 -3 of all chains need to form tie chains to support macroscopic charge transport, above which mobility is independent of tie chain fraction. [START_REF] Gu | Assessing the Huang-Brown Description of Tie Chains for Charge Transport in Conjugated Polymers[END_REF] Also, charge mobility has been shown to be independent of molecular weight in flexible conjugated polymers at a molecular weight greater than the critical entanglement molecular weight (Mc). For instance, the field-effect mobility of P3HT saturates to about 0.02 cm 2 V -1 s -1 at a molecular weight greater than Mc ≈ 25-35 kg mol -1 . [START_REF] Koch | The impact of molecular weight on microstructure and charge transport in semicrystalline polymer semiconductors-poly(3-hexylthiophene), a model study[END_REF] In this case, charge mobility can be enhanced further under certain solidification conditions (e.g., high pressure or strain) that lead to chain-extended structures. In mixed conducting polymers operating in electrochemical devices, increasing the charge carrier concentration by doping impacts the charge transport behavior. A simplified explanation can be given according to charge transport models for disordered semiconductor materials such as conjugated polymers. [START_REF] Baranovskii | Theoretical description of charge transport in disordered organic semiconductors[END_REF] Charge hopping rates depend on the distances between localized carrier sites and their relative energies, which can be described by a Gaussian distribution of the electronic states (density of states, DOS) in energy space, depicted in Figure 1.14. At low charge carrier concentrations, the carriers only populate sparsely packed energy states, which leads to slow hopping. As the carrier concentration is increased by doping, carriers fill higher energy states that are more densely packed. Thus, hopping between sites becomes faster, resulting in higher charge-carrier mobilities. Models that incorporate carrier-concentration-dependent mobilities describe reasonably well the non-monotonic increase of the transconductance with applied gate voltage in OECTs. [START_REF] Friedlein | Influence of disorder on transfer characteristics of organic electrochemical transistors[END_REF][START_REF] Friedlein | Device physics of organic electrochemical transistors[END_REF]

Progress in mixed conducting polymer design

Several publications over the years have already demonstrated mixed ionicelectronic conducting polymers as promising high performance materials for bioelectronics. 2,[START_REF] Berggren | Ion Electron-Coupled Functionality in Materials and Devices Based on Conjugated Polymers[END_REF]54 In particular, PEDOT:PSS, a mixed conductor comprising a conjugated polymer (PEDOT) copolymerized with a polyelectrolyte (PSS), has been one of the most researched materials for such applications and for understanding OECT device physics due to its high conductivity (reaching up to 5000 S cm -1 with acid post-treatment), [START_REF] Kim | Highly Conductive PEDOT:PSS Nanofibrils Induced by Solution-Processed Crystallization[END_REF][START_REF] Shi | Effective Approaches to Improve the Electrical Conductivity of PEDOT:PSS: A Review[END_REF] biocompatibility, [START_REF] Stříteský | Printing inks of electroactive polymer PEDOT:PSS: The study of biocompatibility, stability, and electrical properties[END_REF] and commercial availability in aqueous solutions. 58 However, there are certain issues in using PEDOT:PSS. One is the acidity of the PSS chain, which can lead to degradation of devices, [START_REF] Cameron | The damaging effects of the acidity in PEDOT:PSS on semiconductor device performance and solutions based on non-acidic alternatives[END_REF] incompatible interfaces with biological species at physiological pH, [START_REF] Mantione | Poly(3,4-ethylenedioxythiophene):GlycosAminoGlycan Aqueous Dispersions: Toward Electrically Conductive Bioactive Materials for Neural Interfaces[END_REF] as well as corrosion of industrial equipment like common inkjet print heads. [START_REF] Nitta | Printing PEDOT from EDOT via plasma-assisted inkjet printing[END_REF] The latter would negate the advantages of an aqueous polymer dispersion for roll-to-roll mass production of devices. Additionally, since PEDOT:PSS is intrinsically doped, its OECTs operate only in depletion mode. This can be impractical for several applications and would also require constant voltage input to maintain devices in the "off" state. Usually up to 0.8 V must be applied, which can risk parasitic side reactions with ambient water and oxygen. [START_REF] De Leeuw | Stability of n-type doped conducting polymers and consequences for polymeric microelectronic devices[END_REF] Furthermore, OECTs of PEDOT:PSS exhibit relatively poor long-term operational stability without specific additives or post-treatment of the film. [START_REF] Kim | Influence of PEDOT:PSS crystallinity and composition on electrochemical transistor performance and long-term stability[END_REF] Thus, in the interest of developing tailored devices that can meet requirements such as different modes and polarity of operation, and good stability as bioelectronic implants, the demand for alternative materials persists. To this end, the past decade has seen much progress in the development of numerous mixed conducting polymers. Here, we discuss the different structural motifs of polymer families that have been studied in OECTs while highlighting some of the design rationale in engineering these materials. A summary of the OECT and mixed conductor performance indicators of some materials recently reported in literature and mentioned in this section are given in Table 1.1. Copolymer p(NDI-T2)-co-p(gNDI-T2) with 90% fraction of p(gNDI-T2); All devices measured in aqueous electrolyte except for marked b that were measured with a polymer-in-salt electrolyte; c Interdigitated microelectrodes were used as source and drain, and width is calculated from the product of the number of "fingers" and the width of their overlap; electrodes separated by micro-scale gaps as the channel material, and the device was gated by an organic electrolyte. [START_REF] White | Chemical Derivatization of an Array of Three Gold Microelectrodes with Polypyrrole: Fabrication of a Molecule-Based Transistor[END_REF] Although the device showed accumulation mode behavior reaching transconductances of 0.1 mS and on-off current ratios of 3000, it also displayed slow switching speeds of around 10 s. Furthermore, Ppy displays poor oxidative stability in its undoped form, leading to irreversible autooxidation in humid environments and a tendency to crosslink. PANI, on the other hand, shows similar conductivity to Ppy while also showing reversible oxidation in aqueous conditions, high environmental stability, and ability to limit electrode fouling, all of which are advantageous for bioelectronics. [START_REF] Dhand | Recent advances in polyaniline based biosensors[END_REF] As early as 1985 it was successfully used to construct OECTs with aqueous electrolyte, showing operational stability up to 10 hours. [START_REF] Paul | Resistance of polyaniline films as a function of electrochemical potential and the fabrication of polyaniline-based microelectronic devices[END_REF] While promising, PANI exhibits acid doping behavior such that its redox states are controlled by the surrounding pH. Maximum conductivity can be achieved in its emeraldine salt state, which occurs only at pH < 5.5, above which it transforms to its non-conductive emeraldine base state, limiting its applicability in biological applications. Despite their drawbacks, reports on both Ppy and PANI as mixed conductors for applications in bioelectronics are still ongoing. [START_REF] Li | Towards conductive hydrogels in e-skins: a review on rational design and recent developments[END_REF] Ppy blends with dopants have been incorporated into hydrogel networks, resulting in adhesive, stretchable, and conducting free standing films that can be applied as electronic skins and wound dressings. [START_REF] Han | Transparent, Adhesive, and Conductive Hydrogel for Soft Bioelectronics Based on Light-Transmitting Polydopamine-Doped Polypyrrole Nanofibrils[END_REF] The high transduction ability and environmental stability of PANI-based materials have been exploited to create sensors detecting peroxides, glucose, cholesterol, and various enzymes. 83 and moisture, which may be attributed to the presence of the sulfur atom which helps to stabilize positive charge on the polymer backbone. [START_REF] Arya | Stability of Conducting Polythiophene and Derivatives[END_REF] Functionalization of the thiophene monomer with alkyl side chains, such as methyl (P3MT) and hexyl (P3HT) groups, promotes the solubility of their polymers in various organic solvents, allowing extensive control over the semicrystalline morphology and subsequent optoelectronic properties of the resulting films using facile solution processing techniques. [START_REF] Hotta | Novel organosynthetic routes to polythiophene and its derivatives[END_REF] Furthermore, chemical synthesis of substituted polythiophenes can yield highly regioregular structures displaying head-to-tail configuration, leading to very well-oriented self-assembled solid state structures with narrowly distributed stacking distances in the crystalline domains of solution-cast films. [START_REF] Osaka | Advances in molecular design and synthesis of regioregular polythiophenes[END_REF] As a result, these films show high electrical conductivities upon doping with I2 vapor, reaching >100 S cm -1 in P3HT and up to 1000 S cm -1 in a dodecyl-substituted polythiophene (P3DDT). [START_REF] Mccullough | Self-Orienting Head-to-Tail Poly(3-alkylthiophenes): New Insights on Structure-Property Relationships in Conducting Polymers[END_REF] Substituting hydrophilic side chains consisting of short glycol (-(OCH2CH2)n) or hydroxyl end groups (-OH) onto thiophene-based polymers, shown in chains to yield materials with low electrochemical potentials and high volumetric capacitances while maintaining their charge transport properties. [START_REF] Giovannitti | Controlling the mode of operation of organic transistors through side-chain engineering[END_REF][START_REF] Nielsen | Molecular Design of Semiconducting Polymers for High-Performance Organic Electrochemical Transistors[END_REF] Noteworthy are polymers containing glycolated polythiophene units (gT), which show the highest overall mixed conductivity in terms of their μC* parameter (see polymers comprising g2T units in Table 1.1). Devices of these materials also show fast switching speeds in the submillisecond range, which can be attributed to ion transport facilitated by high degrees of hydration and swelling of the glycol side chains. One drawback is that excessive swelling can lead to irreversible volumetric expansion of the conjugated polymer, which may compromise the operational stability of devices (discussed in 1.

Ionic transport in mixed conducting polymers).

Furthermore, the highly hygroscopic nature of the glycol side chains can render the thermal and optoelectronic properties of these materials sensitive to humidity. [START_REF] Dyson | Managing Local Order in Conjugated Polymer Blends via Polarity Contrast[END_REF] Consequently, the synthesis of a series of polymers varying the ethylene glycol sidechain lengths and distribution is necessary to minimize water uptake in the material. [START_REF] Moser | Side Chain Redistribution as a Strategy to Boost Organic Electrochemical Transistor Performance and Stability[END_REF] This has led to optimization of the performance and stability of OECTs of p(g2T2-g4T2), the structure of which is shown in the conjugated backbone and the glycol side chains in the polymer P3MEEET, presented in Figure 1. 16 with the ethyl spacer highlighted in bold, has also been shown to maintain relatively low passive swelling (upon immersion in water/electrolyte) while enhancing active swelling (during electrochemical doping). [START_REF] Schmode | The Key Role of Side Chain Linkage in Structure Formation and Mixed Conduction of Ethylene Glycol Substituted Polythiophenes[END_REF] As a result, P3MEEET shows higher volumetric capacitance and higher μOECT than its counterpart with no ethyl spacer on the sidechain.

One drawback in employing an alkyl spacer is that it distances the electron-donating side group from the polythiophene backbone, which leads to higher electrochemical oxidation potentials and about 300 mV increase of the OECT threshold voltage. Alternatively, hydroxylated polythiophenes (P3HHT) show significantly less passive and active swelling relative to the glycolated polythiophene derivatives, however at the expense of switching speed, indicated by slow sweep rates required to measure OECT characteristics, and lower μC* performance. [START_REF] Nicolini | A Low-Swelling Polymeric Mixed Conductor Operating in Aqueous Electrolytes[END_REF] Nonetheless, OH-functionalization does not seem to change the shortand long-range ordering relative to non-functionalized P3HT, suggesting that the hole transport may be unaffected. [START_REF] Pacheco-Moreno | The Importance of Materials Design to Make Ions Flow: Toward Novel Materials Platforms for Bioelectronics Applications[END_REF] Thus, polymers based on hydroxylated thiophene building blocks can serve as reliable model systems to study structure/property interrelations in mixed conducting materials without factors such as excess swelling nor additional backbone disorder induced by the sidechains.

Conjugated polyelectrolytes

Polythiophenes substituted with ionic moieties as shown in Figure 1.17 have also gained attention since the presence of mobile counterions in the material leads to ionic conductivity, while anionic sidechains may induce self-doping. The pristine state of these polymers can exist either in the acidic form (R-A -H + ) or conjugate base form (R-A -) depending on the acid-base equilibria of the groups. Polythiophene functionalized with alkylsulfonate sidechains (PTHS) exists as a semiconducting polymer salt in its pristine state, leading to accumulation mode behavior in OECTs. [START_REF] Inal | A high transconductance accumulation mode electrochemical transistor[END_REF] The doping mechanism may involve stabilization of the positive polarons by counterions from the external electrolyte, or stabilization by the sulfonate anion, possibly dissociated from the internal counterion, in a self-doping manner. Sulfonate-terminated side chains have also been substituted onto PEDOT (PEDOT-S) to yield conducting acids that yield depletion type OECTs. 94 A problem with employing conjugated polyelectrolytes such as PEDOT-S and PTHS is that they tend to be water-soluble and therefore need to be crosslinked with additives, a common one being (3-glycidyloxypropyl) trimethoxysilane (GOPS), to improve their stability in aqueous media. Alternatively, the use of bulky intrinsic counterions may yield waterinsoluble salts. Protonated PEDOT-S could be converted to ammonium salt forms by ion exchange processes with either dioctylammonium chloride ((Oct)2NH2Cl) or nonylammonium chloride ((Nonyl)NH3Cl). [START_REF] Zeglio | Highly Stable Conjugated Polyelectrolytes for Water-Based Hybrid Mode Electrochemical Transistors[END_REF] This changes the solubility of the material from

PEDOTS PTHS P3CPT R = C 4 H 9 C 4 H 9 C 4 H 9 C 4 H 9 counterions P3KBT
aqueous-to organic-soluble and shifts its electrochemical potential. Consequently, OECTs of PEDOT-S[(Oct)2NH2] exhibit hybrid operation, being either accumulative or depletive depending on the gate bias polarity due to a threshold voltage > 0 V and saturated "on" state at more negative potentials. Also, greater steric hindrance in the counterion of PEDOT-S[(Oct)2NH2] offers better operational stability in its devices than those of PEDOT-S[H]. In another case, carboxylate-functionalized (-COO -K + ) polythiophene salts, poly(3-(potassium-4-butanoate)-thiophene-2,5-diyl) (P3KBT), have also been used to fabricate accumulation mode OECTs operating in 0.1 M NaCl aqueous electrolyte. [START_REF] Khau | Carboxylic Acid Functionalization Yields Solvent-Resistant Organic Electrochemical Transistors[END_REF] Although P3KBT is water-soluble, spray-cast films could be treated in an acidic organic solution of paratoluenesulfonic acid in methanol to yield solvent-resistant (water and several other organic solvents) carboxylic acid-functionalized polythiophene films, poly [3-(4-carboxypropyl)thiophene] (P3CPT).

Diketopyrrolopyrrole-based conjugated polymers

One recently emerging family of mixed conducting polymers employed in OECTs are diketopyrrolopyrrole-based conjugated polymers (PDPP). These polymers have been extensively studied as high charge carrier mobility materials in OFETs whose majority charge carrier can be varied from p- [START_REF] Li | A high mobility P-type DPP-thieno[3,2-b]thiophene copolymer for organic thin-film transistors[END_REF] to n-type [START_REF] Jeong | Acceptor-acceptor-type conjugated polymer for use in n-type organic thin-film transistors and thermoelectric devices[END_REF] and even ambipolar [START_REF] Sonar | A low-bandgap diketopyrrolopyrrole-benzothiadiazole-based copolymer for high-mobility ambipolar organic thin-film transistors[END_REF] semiconductor character upon copolymerizing with other aromatic monomers. They have also been used as acceptor units in low band gap donor-acceptor polymers for sensitizing layers in organic solar cells. [START_REF] Li | Diketopyrrolopyrrole Polymers for Organic Solar Cells[END_REF] Like polythiophenes, PDPPs can be functionalized with solubilizing sidechains, typically branching alkyl chains, to facilitate processability. The nature of the comonomers and flanking units can also influence the polymer's molecular geometry, optoelectronic properties, and subsequent device performance. [START_REF] Li | Diketopyrrolopyrrole Polymers for Organic Solar Cells[END_REF] For instance, strongly electron-donating thienyl groups (T) as flanking units in PDPPs (polymers consisting of -T-DPP-T-units) leads to lower bandgap materials, coplanar structures and efficient orbital overlap leading to high mobilities. 100 Figure 1.18 presents the chemical structures of thienyl-flanked glycolated PDPPs linked with various comonomers including EDOT, triethyleneglycol-subtituted thiophene (MEET), thienothiophene (TT) and methoxylated thiophene (MeOT2) units that have been investigated as p-type mixed conductors for accumulation mode OECTs. [START_REF] Moser | Polaron Delocalization in Donor-Acceptor Polymers and its Impact on Organic Electrochemical Transistor Performance[END_REF][START_REF] Krauss | Polydiketopyrrolopyrroles Carrying Ethylene Glycol Substituents as CHAPTER 1[END_REF][START_REF] Wang | Ethylenedioxythiophene incorporated diketopyrrolopyrrole conjugated polymers for high-performance organic electrochemical transistors[END_REF] Glycol sidechains have also been incorporated onto these DPP monomers to enhance ion uptake and capacitance when operating in aqueous electrolytes. EDOT and MEET comonomers both exhibit similar hole mobilities around 0.1 cm 2 V -1 s -1 , however MEET comonomers lead to much higher capacitances, which results in better overall μC* performance of 45 F cm -1 V -1 s -1 . [START_REF] Krauss | Polydiketopyrrolopyrroles Carrying Ethylene Glycol Substituents as CHAPTER 1[END_REF] On the other hand, PDPP polymerized with EDOT comonomers exhibit slightly better operational stability, perhaps due to the electrochemical stability of EDOT. [START_REF] Wang | Ethylenedioxythiophene incorporated diketopyrrolopyrrole conjugated polymers for high-performance organic electrochemical transistors[END_REF] PDPP copolymerized with TT (p(gDPP-TT)) and bithiophene (p(gDPP-T2)) comonomers stand out, showing impressively high μC* performances of 125 and 342 F cm -1 V -1 s -1 , respectively. [START_REF] Moser | Polaron Delocalization in Donor-Acceptor Polymers and its Impact on Organic Electrochemical Transistor Performance[END_REF] The difference in performance is due to their mobilities of 0.6 and 1.55 cm 2 V -1 s -1 for p(gDPP-TT) and p(gDPP-T2), respectively. On the other hand, PDPP copolymerized with MeOT2 units (p(gDPP-MeOT2)) show a lower hole mobility of 0.28 cm 2 gDPP-based copolymers V -1 s -1 and, consequently, relatively lower μC* performance of only 57 F cm -1 V -1 s -1 . The lower mobility of p(gDPP-MeOT2) has been attributed to polaron localization owing to the large differences in energy levels between DPP and MeOT2 units. Specifically, DFT simulations suggest that charges are localized on the MeOT2 units. On the other hand, p(gDPP-TT) and p(gDPP-T2), in which the energies between the units are more closely matched, exhibit more delocalized HOMO orbitals and greater polaron charge distribution over the different segments. In terms of operational stability, however, p(gDPP-MeOT2) outperforms p(gDPP-T2) and p(gDPP-TT). This has been attributed to its lower ionization potential that could better stabilize the hole polaron. These examples clearly highlight the tunability of the mixed conducting properties of PDPP-based copolymers and their devices by molecular design. 1.4.5. N-type polymers N-type conducting polymers whose majority charge carriers are electrons rather than holes can be very interesting in bio-transistors since their conductivity can be sensitive to cation fluxes that govern several physiological functions. Furthermore, these materials can be employed as electron acceptors for mediator-free sensing of enzymatic redox reactions 101 and anodes in enzymatic fuel cells. 102 Unfortunately, extensive practical application of n-type polymers is limited due to their scarcity and instability in air as a result of charge carrier trapping by water and oxygen. [START_REF] De Leeuw | Stability of n-type doped conducting polymers and consequences for polymeric microelectronic devices[END_REF]103 Despite such difficulties, a few n-type polymers showing mixed ionic and electronic conductivity, such as those given in Glycol sidechain-functionalized napthalene diimide-based polymers (PNDI) polymerized with bithiophene comonomers units (T2) in a donor-acceptor (D-A) type configuration (p(gNDI-T2)) show accumulation mode behavior in OECTs when operated in aqueous electrolytes. [START_REF] Giovannitti | The Role of the Side Chain on the Performance of N-type Conjugated Polymers in Aqueous Electrolytes[END_REF] It should be noted that since the polaronic and bipolaronic states in NDI-based polymers is sensitive to parasitic side reactions involving the reduction of oxygen, deoxygenation of the electrolyte is required. [START_REF] Szumska | Reversible Electrochemical Charging of n-Type Conjugated Polymer Electrodes in Aqueous Electrolytes[END_REF] Although substitution of PNDI with glycol sidechains improves its capacitance, it also degrades its electronic mobility significantly, relative to its alkylated PNDI counterpart. Furthermore, polymers with 100% fraction of glycolated NDI units exhibit excessive hydration-induced swelling that is detrimental to its properties. [START_REF] Szumska | Reversible Electrochemical Charging of n-Type Conjugated Polymer Electrodes in Aqueous Electrolytes[END_REF][START_REF] Giovannitti | The Role of the Side Chain on the Performance of N-type Conjugated Polymers in Aqueous Electrolytes[END_REF] Water uptake can be controlled by decreasing the fraction of glycolated units, which has led to the optimized copolymer of p(NDI-T2)-co-p(gNDI-T2) with 90% glycolated fraction ("P-90"). Subsequently, P-90 can be molecularly doped by admixing with tetra-n-butylammonium fluoride (TBAF) in solution to increase its mixed conductivity in terms of electron mobility and volumetric capacitance. 104 This also increases the operational stability of its devices with long-term exposure to water-rich media. Adding alkyl spacers between the NDI-backbone and glycol sidechain is another effective way to p(gNDI-T2) pgNgN BBL R = limit detrimental swelling of the polymer, while enhancing its μC* performance and improving its electrochemical stability at low potentials. 105 Alternatively, an n-type polymer based on condensed electron-deficient aryl lactam backbones has also been engineered with glycol sidechains (pgNgN) to yield high mixed conductivity. [START_REF] Chen | n-Type Rigid Semiconducting Polymers Bearing Oligo(Ethylene Glycol) Side Chains for High-Performance Organic Electrochemical Transistors[END_REF] These polymers exhibit highly rigid and planar conjugated backbone structures with no single linking bonds. The lack of donor-acceptor character was also intended in the synthesis of these polymers to yield highly delocalized LUMO frontier orbitals, resulting in electron mobilities in the 10 -3 cm 2 V -1 s -1 range. With a high volumetric capacitance of 240 F cm -3 , pgNgN exhibits a μC* approaching 0.05 F cm -1 V -1 s -1 . This is less than the performance of its counterpart with less glycol sidechain fraction (pgNaN), which shows higher electron mobilities resulting in a μC* of about 0.65 F cm -1 V -1 s -1 . This has been linked to the longer coherence length of the π-stacks estimated for pgNaN from GIWAXS measurements relative to that of the fully glycolated pgNgN, which hints at the negative effects of glycol side chains to the structural order of the films. The development of n-type mixed conducting polymers without necessitating any hydrophilic sidechains is also possible as demonstrated by reports of devices based poly(benzimid azobenzophenanthroline) (BBL) showing accumulation mode OECT behavior. [START_REF] Surgailis | Mixed Conduction in an N-Type Organic Semiconductor in the Absence of Hydrophilic Side-Chains[END_REF] This polymer shows good electrochemical stability and high electron mobility owing to its ladder-type structure leading to significant order along the backbone direction and tight π-stacking. On the downside, BBL-based OECTs exhibit slower switching times relative to P(gNDI-T2) based devices. Surprisingly, BBL also shows high volumetric capacitance, early reduction onsets hinting at low energy barriers to ion penetration, and high degrees of irreversible swelling despite the absence of any hydrophilic moiety. It has been suggested that this may be the result of electrolyte penetration into void spaces of the film structure. The high capacitance of about 700 F cm -3 combined with high electron mobility reaching 2 x 10 -3 cm 2 V 1 s -1 in BBL results in μC* performance up to 2 F cm -1 V -1 s - 1 , the highest reported so far for n-type mixed conducting polymers.

Multicomponent polymer systems

Arguably the most common multicomponent material employed in OECTs to date is PEDOT:PSS and its mixtures with other components as shown in Figure 1.20 and mentioned in the following text. PEDOT, a polymer based on a bicyclic ring thiophene monomer, was synthesized in order to prevent steric effects and support higher conductivities. 106 Meanwhile, the 3,4-dialkoxy substitutions can stabilize positive charges thereby enhancing the redox stability of the material. However, PEDOT alone is insoluble.

To improve material processability, EDOT is polymerized with polymeric salts in water to yield aqueous dispersions of oxidized PEDOT stabilized by a counter-polyanion. The most common one is polystyrene sulfonate (PSS). As a result of incorporating a polyelectrolyte, PEDOT:PSS also conducts ions with an ion mobility of about 10 -3 cm 2 V -1 s - form stable films, especially against operation in aqueous media. Increasing film rigidity by crosslinking can make it more difficult for ions to flow through the material. 107 Another issue with incorporating PSS is that it seems to decrease the conductivity of the material, which is attributed to excess PSS preventing dense packing and interconnectivity of the electronically conducting PEDOT:PSS-rich phases. To resolve this, conductivity enhancing additives such as surfactants (dodecylbenzenesulfonic acid, DBSA) or polar and highboiling point solvents (e.g., ethylene glycol or EG, DMSO) are usually mixed with the aqueous dispersions before casting films. [START_REF] Shi | Effective Approaches to Improve the Electrical Conductivity of PEDOT:PSS: A Review[END_REF] Films cast from a typical formulation of PEDOT:PSS (PH-1000 from Heraeus Clevios GmbH) including additives ethylene glycol (5 %v/v) and DBSA (0.2 %v/v) show hole mobilities (measured in an OECT) of about 2 cm 2 V - 1 s -1 , volumetric capacitances up to 39 F cm -3 , and a μC* performance of 47 F cm -1 V -1 s -1

as measured from OECTs. 6,14,[START_REF] Khodagholy | High transconductance organic electrochemical transistors[END_REF] Although such additives can tune the performance of commercially acquired dispersions, they can also make it more difficult to predict morphological changes. Alternatively, ionic liquids can also be added as "conductivity enhancers". Films cast from PEDOT:PSS dispersions mixed with 1-ethyl-3methylimidazolium tricyanomethanide ([EMIM][TCM]) exhibit higher μOECT (reaching 4 cm 2 V -1 s -1 ), C* (90 F cm -3 ), and, subsequently, μC* performance (335 F cm -1 V -1 s -1 ) than typical formulations with ethylene glycol. [START_REF] Wu | Ionic-Liquid Doping Enables High Transconductance, Fast Response Time, and High Ion Sensitivity in Organic Electrochemical Transistors[END_REF] It is suggested that the enhanced performance might be related to the microfibrillar morphology observed under bright-field TEM. Yet another highly successful approach to tune the mixed conducting properties of PEDOT:PSS is by sulfuric acid (H2SO4) post-treatment of the films (labelled as "Crys-P"). [START_REF] Kim | Highly Conductive PEDOT:PSS Nanofibrils Induced by Solution-Processed Crystallization[END_REF] This method can significantly increase the crystallinity of the PEDOT:PSS-rich phases (observed from GIWAXS) while removing excess PSS (evidenced by x-ray photoelectron spectroscopy, XPS) upon rinsing the films, both which might explain the high conductivities of resulting films (up to 5000 S cm -1 compared to 1 S cm -1 of pristine PEDOT:PSS films). Subsequently, Crys-P exhibits high μOECT (4 cm 2 V -1 s -1 ), but especially high C* (113 F cm -3 ) and, therefore, high μC* (490 F cm -1 V -1 s -1 ). Such enhancement of performance is again attributed to a fibrillar morphology in nano-length scales observed under combined electron microscopy techniques. In fact, for most of the methods mentioned above, it is believed that the mechanism of enhancing the conductivity involves inducing phase separation of PEDOT:PSS-rich and PSS-rich phases, which allows removal of excess PSS by rinsing.

Meanwhile, it has been suggested that methods that result in microfibrillar film morphologies can support high C* due to the high aspect ratio microstructure that leads to a large interfacial area between the electron-rich and ion-rich phases. [START_REF] Wu | Ionic-Liquid Doping Enables High Transconductance, Fast Response Time, and High Ion Sensitivity in Organic Electrochemical Transistors[END_REF] Multicomponent systems based on PEDOT have also been designed to facilitate biofunctionalization of the material towards building OECT biosensors. For instance, PEDOT:PSS dispersions mixed with polyvinyl alcohol (PVA) yields films with alcohol groups on the surface. 108 These react with chemical-vapor-deposited GOPS to yield silane groups than can later chemically link with various biomolecules. PEDOT-derivatives doped and stabilized by other electrolytes can also be biofunctionalized. One such derivative is the tosylate anion-doped PEDOT (PEDOT:TOS), polymerized by oxidation using Fe(III) ptoluenesulfonate (tosylate). PEDOT:TOS dispersions have been mixed with carboxylic acidfunctionalized polyethylene glycol (PEG(COOH)), a common biocompatible material often used in tissue engineering applications. 109 The alcohol groups allow further biofunctionalization of the deposited films. Similarly, PEDOT:TOS has also been mixed with polyallylamine hydrochloride (PAH, a polycation) comprising of NH2 moieties, which allow the electrostatic anchoring of enzymes such as acetylcholinesterase (AchE). 110 An important detail is that all of these approaches allow biofunctionalization after film fabrication and treatment, which typically involves solvents and temperatures too harsh for biomolecules. Furthermore, in all the blends mentioned here, blending does not drastically change the OECT performance relative to those of the neat PEDOT:PSS or PEDOT:TOS.

Incorporating additional components to PEDOT:PSS can also yield other interesting device functionalities. For instance, mixing PEDOT:PSS with polyethylenimine (PEI) results over multiple colors (red, green, and blue, in order, with increasing applied voltage). [START_REF] Zeglio | Conjugated Polyelectrolyte Blends for Electrochromic and Electrochemical Transistor Devices[END_REF] While PTEB-S is responsible for most of the electrochromic behavior, it is a poor conductor and has poor redox stability. Blending it with PEDOT-S increases the OECT transconductance and cycling stability, while also increasing the device response speed. The issue with these polyelectrolyte blends, however, is that they tend to be water soluble (even with the bulky hydrophobic counterion). Thus, a polymer-in-salt electrolyte (poly-2-hydroxyethyl methacrylate, PHEMA, network in BMIMBF4 ionic liquid) is used in the devices.

Instead of polyelectrolytes, non-ionic semiconducting polymers may also be blended with ion-conducting polymers to develop mixed conductors. To enhance the volumetric doping of P3HT in aqueous electrolyte, one attempt is to blend it with PEO. [START_REF] Frankenstein | Blends of polymer semiconductor and polymer electrolyte for mixed ionic and electronic conductivity[END_REF] Encouragingly, EQCM-D suggests that addition of PEO does enhance hydrated ion uptake in the material relative to the neat P3HT, which shows no signs of mass changes. Meanwhile, the cyclic voltammograms of the films in aqueous electrolyte do show oxidations onsets for the blends (0.55 to 0.65 V) within the investigated potential window (-0.6 to 0.8 V), unlike neat P3HT.

Consequently, the P3HT blend with PEO does exhibit higher μC* (0.6 F cm -1 V -1 s -1 ) than the neat P3HT (0.02 F cm -1 V -1 s -1 ), however the performance is very poor compared to current state-of-the-art p-type mixed conducting polymers (μC* reaching 500 F cm -1 V -1 s -1 ).

Thus, the opportunity currently exists to explore non-ionic mixed conducting polymer blends in OECTs and establish their design criteria.

Summary and research direction

Several key features of mixed conducting polymers can be identified from the performance of their electrochemical transistors. For instance, the mode and polarity of device operation, as well as the potential window of the device, will be determined by the redox properties of the active channel material. Also, since the electrochemical response of the polymer relies on counterions to maintain charge neutrality, electronic charge injection is coupled with the ionic transport properties of the material. Consequently, the response speed of devices may be limited by the mobility of ions. Furthermore, the OECT transconductance, gm, which measures the performance of the device according to the change in drain current with applied gate voltage, will depend on how well ions can penetrate the active material to support changes in the bulk conductance. One parameter that can reflect this is the volumetric capacitance, C*. Finally, the conductivity of the channel will also depend on the electronic mobility, μ, of the active material. Thus, the design of high-performance mixed conductors typically aims at increasing electronic mobility and ionic transport, while minimizing the redox potentials to allow operation at low bias. The latter is especially important for applications interfacing with water, which gives a narrow potential window beyond which parasitic side reactions may occur, and for applications with sensitive biological species.

Recent efforts to develop mixed conducting polymers usually involves engineering polar sidechains onto high-mobility conjugated polymer backbones, previously designed for other applications such as OFETs and organic solar cells, to increase their ionic conductivities. A commonly employed functional group substituted onto various p-and ntype conjugated polymers is ethylene glycol. Indeed, this approach has yielded some of the best performing mixed conducting materials in terms of the μC* parameter. One issue that may arise from this method is that it can lead to excessive swelling of the conducting materialwith water molecules, either from the external aqueous electrolyte or even from a humid environment. Excessive water uptake has been shown to be detrimental to the response time of devices and can also lead to irreversible mass changes in the active polymer. Consequently, this disturbs the structural order of the semiconductor and effectively degrades its electronic properties. Eventually, the structural integrity of the films may also be compromised and lead to detachment from underlying electrodes. As a result, such systems can have poor operational stability. Fortunately, despite initial impressions that hydration-induced swelling is necessary for more efficient ion uptake from an external electrolyte, results from low-swelling polymers such as P3HHT and the remarkable n-type BBL ladder polymer indicate that good mixed conducting performance can be achieved without excessive water uptake in the material. Meanwhile, most efforts to control water uptake involve the tedious synthesis of a series of polymers with various sidechain distributions and lengths.

Alternatively, we propose blending modification of mixed conducting polymers with hydrophilic, ion-conducting co-components as a simple strategy to enhance ionic transport in these materials. This method avoids the direct modification of the active semiconducting backbone to prevent inducing further conformational disorder that can decrease its electronic performance. Specifically, we blend P3HT-co-P3HHT (a random copolymer comprising 50% -3HT or -3HHT monomer composition) with P3HT-b-PEO (a block copolymer of P3HT and PEO) using a straightforward solution-based method (detailed in the appendix section A.1 Materials and film deposition) without involving any further chemical synthetic steps nor specific post-deposition film treatment (e.g., crosslinking or annealing). We choose P3HT-co-P3HHT as the active mixed conducting component according to previous work showing that copolymerization of hexylthiophene and hydroxyhexylthiophene with up to 50% comonomer composition does not change the shortand long-range order of resulting films. [START_REF] Pacheco-Moreno | The Importance of Materials Design to Make Ions Flow: Toward Novel Materials Platforms for Bioelectronics Applications[END_REF] This is an initial indicator that the material might maintain its electronic properties, linked to its solid state structure, even after functionalization. Furthermore, preliminary GIWAXS investigations given in Figure 1.21 reveal that the structure of P3HT-co-P3HHT is less affected upon exposure to humidity than the P3HHT homopolymer, which is a promising indication of low degrees of hydrationinduced swelling. Consequently, we expect high operational stability from the random copolymer. As the "additive", we choose the amphiphilic block copolymer of P3HT and PEO (P3HT-b-PEO). This has been previously reported to exhibit phase separation and simultaneous electronic and ionic conductivity. 113 Thus, the block copolymer potentially offers bicontinuous pathways for electronic and ionic conduction, which can be exploited in blends with the mixed conducting P3HT-co-P3HHT. The block copolymer might also maintain the structural stability of the blend since phase separation should be limited to small scales due to the chemical linkage between each block. Additionally, the P3HT moiety might also act as a compatibilizing unit that can further limit large scale phase separation in the blend. This might contribute to preserving the electronic properties of the material and would be beneficial for the long-term operational stability of the resulting devices.

Overall, our method combines both chemical and blending modification of the archetypal conjugated polymer P3HT with an ionically conducting PEO moiety to increase its ionic transport properties while potentially maintaining its electrical properties. Indeed, the results discussed in the following chapters indicate the viability and compatibility of this combined approach to develop high-performance mixed conducting materials. Similar techniques can be easily applied to other semiconducting polymers to expand their functionality and enhance their electrochemical performance.

Introduction

Much of the operating mechanisms of emerging organic electrochemical devices relies on the unique properties of mixed ionic and electronic conductors. One factor that can limit the performance of these devices is slow ionic transport processes occurring in the active material, typically a polymeric semiconductor. Thus, we propose blending modification of mixed conducting polymers with hydrophilic, ion-conducting co-components as a simple strategy to enhance ionic transport in these materials and subsequently improve the performance of their electrochemical devices. Specifically, we blend P3HT-co-P3HHT

(provided by Dr. Olivier Dautel, Institut Charles Gerhardt Montpellier) with P3HT-b-PEO (provided by Dr. Cyril Brochon, Laboratoire de Chimie des Polymères Organiques) using a straightforward, solution-based method without involving any further chemical synthetic steps nor specific post-deposition film treatment (e.g., cross-linking or annealing). To evaluate the impact of blending on the mixed conductivity of the material and the performance of its devices, we fabricate and characterize organic electrochemical transistors (OECTs). We also investigate the influence of varying operating speed and channel dimensions on the OECT characteristics. Results indicate that blending primarily affects the transient performance of the OECTs, which demonstrates the viability of our initial materials design strategy. Additionally, we find that the behavior of devices exhibiting slow response can rely heavily on the operating speed employed. This should therefore be considered when evaluating the performance of various mixed conductor channel materials for practical applications.

OECT characteristics at various operating rates

To begin investigating the mixed conductivity of P3HT-co-P3HHT (number-average molecular weight Mn = 30 kg mol -1 , dispersity Ɖ = 1.9) and its blends with P3HT-b-PEO (Mn, Additionally, the maximum |Id| (≈ 1 μA) measured in these conditions appears to be even less than the maximum gate current, |Ig| (≈ 10 μΑ), implying failed amplification. To remedy this, we operate the devices more slowly by decreasing the sweep rate to give enough time for the polymer to be doped with increasing bias. This results in steeper rise of Id at distinct Vg onsets, marking the threshold gate voltages, Vt, at which devices transition from the "off" (low Id, Vg < Vt) to "on" (high Id, Vg > Vt) state. The device reaches up to 60 μA maximum Id and "on" to "off" current ratios (ION/OFF) over two orders of magnitude for a Vg sweep rate of 0.01 V s -1 at 50 mV increments, from which we estimate a response time of at least 5 seconds. However, even at these slow conditions, we observe in the logarithmic plot of the transfer curve (left panel of Figure 2.3) that the current hysteresis persists, which implies that the performance can still be improved with longer device operation. These results provide initial indications of inefficient mixed conductivity in P3HT-co-P3HHT involving slow

I d (μA) g m (mS) V g (V) 0.0 0.2 0.4 P3HT-co-P3HHT 0.06 V s -1 V d = -0.6 V g m,max 0.1 -0.2 -0.5 -0.8 0 -40 -80 I d (μΑ) V g (V) g m (mS) +25 wt% P3HT-b-PEO 0.06 V s -1 V d = -0.6 V g m,max 0.0 0.2 0.4
We note that gm,max occurs at a Vg of -0.8 V in both devices, the highest magnitude of the biasing range chosen here. This is unlike devices of other mixed conductors like PEDOT:PSS 2 and p(g2T-TT) 3 that show non-monotonic growth of gm with Vg, which has been attributed to be the result of the intrinsic charge transport properties of disordered polymeric semiconductors. 4 In our case, the "real" peak gm may occur at higher Vg, however performing measurements at these conditions risks device degradation. Thus, for practical reasons, we consider the gm at the maximum Vg of our biasing range as the gm,max for these devices.

To quantify the current hysteresis observed in the transfer curves, we use the )

V g (V) P3HT-co-P3HHT 0.06 V s -1 V d = -0.6 V 0.1 -0.2 -0.5 -0.8 0 4 8 ΔV t |I d | 1/2 (μA 1/2 ) V g (V) +25 wt% P3HT-b-PEO 0.06 V s -1 V d = -0.6 V
devices of the blend operated at sweep rates up to 1 V s-1, low ΔVt is maintained at about 60 mV and only a small decrease of the gm,max from 0.4 to 0.25 mS is observed. Even after increasing the speed to an unreasonably fast rate of 10 V s-1 the increase of ΔVt and decrease of gm,max in devices of the blend are less than that observed for devices of the neat material when operated at a slower rate of 1 V s-1. These results imply that blending modification of P3HT-co-P3HHT by adding the P3HT-b-PEO component significantly enhances the electrochemical reactivity of the mixed conductor to application of bias without altering its bulk electronic properties. Therefore, blending improves the stability of the OECT performance to varying operating speeds over a wide range of sweep rates while maintaining the magnitude of its transduction performance.

Performance across different channel dimensions

We device geometry and to assess whether blending modification of the active channel material offers further control over the device performance independent of its size. We note that, for practical reasons, fabrication of the larger devices used in this work do not include deposition of an epoxy layer to insulate the source and drain contacts from the polymer film and electrolyte, resulting in a simpler architecture than that of the micro-scale devices. The OECT transfer curves of P3HT-co-P3HHT and its blends with P3HT-b-PEO at various Wd L -1 measured at a Vd of -0.6 V are presented in Figure 2.8.

Figure 2.8. OECT transfer curves measured for devices of neat P3HT-co-P3HHT (left) and its blend with 25 wt% P3HT-b-PEO (right) at various channel dimensions (units given

in cm for consistency with standard units of C*, F cm -3 ).

Both materials show higher "on" currents in their devices with larger Wd L -1 , as expected for OECTs and predicted in Equation 2.1. We also note that the large-scale devices fabricated without insulated source and drain electrodes (Wd L -1 > 5 x 10 -5 cm) show higher Id in the "off" state, which might be the result of some leak currents.

Nonetheless, ION/OFF of at least two orders of magnitude are maintained. Furthermore, we consistently observe less hysteresis in the OECT transfer curves of the blends than in those of the neat material. Thus, while tuning the channel geometry effectively modulates the current magnitudes, it appears that only by modifying the active channel material itself can Although we do see modulation of the gm with channel dimensions, we continue to observe its maximum value only at the highest Vg. This is unlike the reported behavior of PEDOT:PSS OECTs, where the Vg at which devices exhibit maximum transconductance, Vg(gm,max), can be shifted to lower potentials by as much as -0.3 V through decreasing channel thickness, from 640 to 25 nm, or by decreasing the width to length ratio, WL -1 , from 10 to 0.1. 6 In our devices, the thicknesses only range from 80 to 160 nm, and the WL -1 from -0.1 -0.3 -0.5 -0.7 -0.9 0 4 8 12 16

V g (V)

P3HT-co-P3HHT 0.06 V s -1 g m,fwd

(mS) V s -1 0.06 0.13 0.25 0.50 This is in accordance with the transconductance equation for OECTs described in literature and given in Equation 2.2.

μC* = 35 P3HT-co-P3HHT g m (S) |V t -V g | W d L -1 (V cm)
𝑔 𝑚 = 𝜇𝐶 * (| 𝑉 𝑡 -𝑉 𝑔 |)(𝑊𝑑/𝐿) 2.2
Note that this is derived from solving the first derivative of Id,sat in Equation 2.1 with respect to Vg. 5 It is therefore interesting to extract the μC* product, often used as the materials-system figure-of-merit for OECTs independent of device geometry and bias, from the slope of gm plotted against |Vt-Vg|WdL -1 . 7 However, due to the lagging current response in devices of the neat P3HT-co-P3HHT, we obtain largely dispersed values of gm across sweep rates (Figure 2.10, left). As a result, the extracted slopes significantly deviate depending on the operating speed used, from 10 to 55 F cm -1 V -1 s -1 , averaging at about 35 F cm -1 V -1 s -1 . We note that this matches the performance of the P3HHT homopolymer measured in previous work using similar OECT architecture, 8 which implies that only a fraction of polar sidechain substitution is necessary to maximize the mixed conducting performance of the polythiophene operating in aqueous electrolyte. However, although these results imply that the neat P3HT-co-P3HHT can potentially exhibit higher gain with slower device operation, these conditions are not necessarily practical for certain target applications requiring sharp off-to-on current response. This also highlights the importance of measurement speed when evaluating mixed conductor performance. Contrastingly, significantly less disperse gm is obtained from devices of the blend (Figure 2.10, right) due to better stability of its performance over a wider range of sweep rates. Consequently, an average slope of 28 F cm -1 V -1 s -1 is calculated, well within the range of that of the neat material.

Influence of channel dimensions on current hysteresis

The Bernard's model depicts the OECT transient behavior as a charging capacitor with an RC time constant, τ, proportional to d(WL) 1/2 , 9,10 suggesting that smaller channel width and thickness should improve the device response speed. However, upon closer inspection of the ΔVt, which we assume is proportional to the device response time, measured in several devices of P3HT-co-P3HHT and its blend with P3HT-b-PEO, we find it difficult to observe any significant scaling when plotted over each channel dimension in This shows that tuning the channel geometry within the range investigated here is not a highly effective method to minimize the Id hysteresis observed in devices of the neat material. On the other hand, we could decrease the sweep rates from 0.5 to 0.06 V s -1 to reduce the Id lag in all devices of P3HT-co-P3HHT and lower ΔVt from 2 V to about 0.2 V, but this value is still quite large considering that OECTs typically operate at Vg < 1 V. Fortunately, despite the limited effects of tuning the device parameters, we can further minimize ΔVt to values even less than 0.1 V simply by adding P3HT-b-PEO to the channel.

Since we observe no significant influence of the channel dimensions on ΔVt, we quantify the general hysteresis measured in devices of each material by calculating the average value over channel volume. Figure 2.12 plots ΔVt over channel volume, the data yielding average ΔVt ≈ 0.3 -1.5 V for devices of the neat P3HT-co-P3HHT and average ΔVt ≈ 0.04 -0.08 V for that of its blend with P3HT-b-PEO across Vg sweep rates of 0.06 -0.5 V s -1 .

Figure 2.12. ΔVt plotted against the OECT channel volume for various devices of neat

P3HT-co-P3HHT (left) and its blend with 25 wt% P3HT-b-PEO (right).

Performance across blend composition

We further investigate the extent at which blending effectively tunes the performance of the OECTs by varying the "additive" content.

Behavior of the neat material and the 10 wt% blend significantly changes at higher sweep rates (bold lines). All devices compared hold similar channel dimensions (W = 2500 μm, L = 50 μm, d = 120±20 nm).

At a slow sweep rate of 0.06 V s -1 , we see that increasing the P3HT-b-PEO content from 0 to 45 wt% leads to progressively lower Id,max and, subsequently, gm,max, while also reducing the ΔVt. Meanwhile, Vt,fwd does not seem to change significantly, verifying that blending does not affect the energy needed for hole and/or ion injection even at higher compositions. Upon increasing the sweep rate to 0.50 V s -1 , both the neat material and its blend at 10 wt% P3HT-b-PEO show larger hysteresis in the transfer curves, yielding lower gm,fwd and larger ΔVt, while the behavior of the blends at higher block copolymer content do not change significantly, maintaining Id and gm magnitudes comparable to that of the neat material. This confirms that the electronic properties of the neat material are not lost even after blending. Similar trends are observed upon extracting the μC* product and average

ΔVt for several devices of different channel dimensions and blend composition, shown in ΔVt) (bottom) extracted for various blend compositions at indicated scan rates. Average μC* extracted for neat P3HT-co-P3HHT is labelled (dashed green line) for reference.

|V t -V g | W d L -1 (V cm) +45 wt% P3HT-b-PEO V s -1 0.06 0.50
This summary offers a sample guideline to how tuning the blend composition can effectively tailor the overall performance of the OECT to meet conditions of specific applications. In cases where slightly enhanced response speed is desired but maximizing the current magnitude of devices is still prioritized, as little as 10 wt% P3HT-b-PEO can be added to reduce the Id lag while maintaining the average μC* parameter of the neat material, 35 F cm -1 V -1 s -1 . Otherwise, blends of 25 wt% P3HT-b-PEO can be prepared to minimize the response time as low as possible while still showing high μC* of 28 F cm -1 V -1 s -1 . To economize the quantity of active mixed conductor used, we can choose to employ blends with P3HT-b-PEO content up to 40 wt% and still achieve better transduction performance than the neat material at faster operating speeds. Ave.

ΔV t (V)

P3HT-b-PEO content (wt%)

Transient response

We've shown so far that blending P3HT-co-P3HHT with P3HT-b-PEO can effectively reduce the drain current lag observed in the transfer curves of their OECTs while fairly sustaining their transconductances, and thus assumed that these devices should also show faster response times with high on-to-off current ratios. The results presented in this section confirm that we do indeed obtain shorter time constants in the transient drain currents of these devices upon application of a gate potential. Our micro-scale OECTs (W = 100 μm and L = 10 μm) were chosen for these measurements to conform with the prediction that τ ~ d(WL) 1/2 , while the presence of the insulating layer might help minimize additional resistive elements that could affect the transient behavior. We limit our investigation to the performances of the neat material and its blend with 25 wt% P3HT-b-PEO, which appears to be the optimum composition to maximize μC* and minimize ΔVt.

2.3

Where ISS refers to steady state drain current measured at saturation during a Vg step, ΔISS is the total change in current between that at zero Vg and that at the step potential, τe is the electronic time constant, and f is a geometric factor that accounts for the complex, nonuniform distribution of Id at the source and drain electrodes depending on Vg and Vd. In both devices the values of ISS are relatively equal at about 10 μA given similar biasing conditions and channel dimensions, confirming once again that blending does not change the conductivity of the material. Meanwhile, we determine τ ≈ 8 s for devices of the neat material and τ ≈ 80 ms for that of its blend, which approximately translates to cut-off frequencies, fc = (2πτ) -1 , around 0.02 Hz and 2 Hz, respectively. We note that, even after blending, the fc we predict is low compared to that measured for the homopolymer P3HHT, with fc = 75 Hz, 8 and even lower relative to state-of-the-art OECTs of accumulation-mode p(g2T-TT) (fc ≈ 1

V d = -0.4 V V g (V)
Time (ms) 0.0 -0.6 kHz) 3 and depletion-mode PEDOT:PSS (fc reaching as high as 10 kHz). 12 We could possibly increase fc further by modifying device parameters such as the applied drain bias 13 or changing the transistor architecture itself, 12,14,15 however this is beyond the scope of this work. What our results do show is the decrease of the response time by two orders of magnitude upon blending, which indicates that adding P3HT-b-PEO to P3HT-co-P3HHT enhances its ionic mobility. proof-of-concept on how a simple blending technique can drastically enhance the response speed of initially "slow" polymer mixed conductors, rendering them more suitable for applications requiring fast operation, for example, in biosensing 16 or neuromorphic memory devices. 17

Response to various

Summary

We characterize OECTs of P3HT-co-P3HHT and its blends with P3HT-b-PEO to implies that the slow device response is intrinsic to the channel material. Consequently, the μC* parameter, used as the material-specific figure-of-merit to describe the performance of the OECT channel material and is obtained from characterizing several devices of different channel sizes, varies greatly within the range of sweep rates employed here. The values of μC* extracted for P3HT-co-P3HHT range from 10 to 55 F cm -1 V -1 s -1 (measured at sweep rates of 0.5 to 0.06 V s -1 ), averaging at about 35 F cm -1 V -1 s -1 . These results suggest that the charge injection kinetics involved in electrochemically doping P3HT-co-P3HHT during device operation is slow. This also highlights the importance to consider temporal resolution when evaluating the performance of particularly "slow" mixed conductors. On the other hand, upon blending P3HT-co-P3HHT with 25 wt% P3HT-b-PEO, the devices exhibit faster response speeds indicated by the minimized Id hysteresis in their transfer curves and relatively short response time (τ = 80 ms). It follows that the gm values of the devices of the blend are stable over a wide range of Vg sweep rates, even up to 10 V s -1 . Upon investigating blends of various compositions, we found that only 25 wt% P3HT-b-PEO is needed to effectively minimize the hysteresis in the transfer curves while maintain a high μC* of 28 F cm -1 V -1 s -1 . However, even up to 40 wt% P3HT-b-PEO could be added to the material and still achieve a μC* of 14 F cm -1 V -1 s -1 , still greater than that of the neat P3HT-co-P3HHT when operated at a fast sweep rate of 0.5 V s -1 . Overall, the results of this chapter show that blending P3HT-co-P3HHT with P3HT-b-PEO can effectively enhance the response speed of its OECTs without greatly affecting its steady state performance.

CHAPTER 3:

Modulation of electrochemical doping kinetics via blending

Introduction

Blending-modification of P3HT-co-P3HHT with P3HT-b-PEO can significantly enhance the response speed of its OECTs without compromising its steady state characteristics and μC* parameter. We suspect that the improved transient performance of the OECT is due to the increased hydrophilicity of the channel material upon adding PEO, which would support more efficient interdiffusion of ions between the polymer film and the external aqueous electrolyte. We also suspect that the P3HT-block chemically linked to the PEO will limit phase separation to microscales and lead to macroscopically uniform distribution of the components. This might help maintain the bulk electrical properties of the material, which would explain the relatively sustained transconductances of the OECTs even after blending. To explore these notions further, we investigate how blending, using the same methodology as that employed in OECT fabrication (detailed in appendix section A.1 Materials and film deposition), affects the physical and electrochemical properties of the material. Results provide some evidence of compatibility in the blends with the block copolymer despite phase separation. Also, it appears that our blending strategy primarily influences ionic transport in the mixed conductor leading to modulation of its electrochemical doping kinetics. On the other hand, blending does not seem to change the steady state electrochemical properties of the material relative to that of the neat P3HT-co-P3HHT. These findings agree with the trends observed in the performance of the OECTs.

Physical properties of P3HT-co-P3HHT blended with P3HT-b-PEO

Solid-state structure

We begin by examining how incorporation of P3HT-b-PEO influences the solid-state structure of P3HT-co-P3HHT since this can have consequences on its electronic properties.

Inspection of the UV-Vis absorbance spectra of the films in Figure 3.1 gives insight into the impact of blending on the local molecular order of the conjugated polythiophene segments.

We observe distinct features attributed to the 0-0 and 0-1 vibronic transitions (A0-0 and A0-1) associated with exciton coupling between locally ordered π-stacks (interchain coupling) and between conjugated segments along the polymer backbone (intrachain coupling), which indicate short-range order in conjugated polymers in the form of molecular aggregates. [1][2][3] Blending with different P3HT-b-PEO compositions (0-45 wt%) results in identical absorbance maxima at λ ≈ 560 nm and only slight differences in the 0-1/0-0 vibronic peak ratios (A0-0/A0-1). This suggests that the aggregation behavior and related optoelectronic properties of P3HT-co-P3HHT do not change with addition of P3HT-b-PEO. This may be attributed to phase-separation of the components, which minimizes interaction of the PEO segments with the polythiophene backbone, rendering the optoelectronic properties of the latter virtually unaffected.

ΔH1 a (J g -1 )
Tm2,end (°C)

ΔH2 b (J g -1 )
Tc1,on (°C)

ΔH1 a (J g -1 )
Tc2,on (°C)

ΔH2 b (J g -1 )
Tm1,end (°C)

ΔH1 a (J g -1 )
Tm2,end (°C) As-cast P3HT-co-P3HHT exhibits a broad melting peak that ends at Tm2,end = 259 °C, which is close to that of solution-cast P3HT homopolymer of similar molecular weight (≈ 250 °C), 5 while as-cast P3HT-b-PEO shows a single sharp peak and Tm1,end = 71 °C attributed to PEO melting. The absence of a melting peak corresponding to the P3HT moiety in the block copolymer might be due to its low molecular weight (Mn, P3HT = 5 kg mol -1 ) relative )

ΔH2 b (J g -1 ) 0 1.6 -- -- 259
to the PEO block (Mn, PEO = 20 kg mol -1 ) and the strong tendency of PEO of this molecular weight to crystallize. After melting at 300 °C, neat P3HT-co-P3HHT exhibits a melting temperature (Tm,end = 233 °C) significantly lower than that of the as-cast material measured in the first scan. This is likely due to an increased entanglement density where melt processing leads to a smaller lamellae thickness, which is directly related to the melting temperature. 6 Indeed, the molecular weight of P3HT-co-P3HHT (Mn,P3HT-co-P3HHT = 30 kg mol -

1

) used here is around the critical molecular weight of entanglement for P3HT (Mc,P3HT = 25-35 kg mol -1 ). 5 Similarly, the molecular weight of the PEO moiety is well above the reported molecular weight between entanglements (Me,PEO ≈ 1.6-2.5 kg mol -1 ), 7 thus, its melting during the second heat trace occurs at a temperature less than that from the first heat (i.e., a solution-processed sample). This supports the view that the polymers are thermally stable within the temperature range employed here.

The thermograms of the as-cast blends (1 st heat) display distinct melting peaks attributed to the polythiophene and PEO moieties, which indicates phase separation and aligns with results from GIWAXS analysis. However, closer inspection of the thermal transitions of each moiety also provides evidence of compatibility of the blend. Increasing the P3HT-b-PEO content in the blend clearly leads to lowering of the melting enthalpy of the P3HT-rich phase (refer to Table 3.1; note that the values are estimated by normalizing the integrated endothermic peak area to the combined weight fraction of the P3HT-co-P3HHT component and the P3HT-block, assuming the latter might also contribute to the P3HT-rich phase). Likewise, the melting enthalpy attributed to PEO decreases with increasing P3HT-co-P3HHT content. Thus, the crystallization of both phases is, to an extent, suppressed in the blends, which implies sufficient interaction between the components. This is highlighted in the left panel of

Surface polarity

Owing to a certain degree of microphase separation, the short-and long-range order of P3HT-co-P3HHT is not drastically changed upon blending, as we've observed from the UV-Vis absorption and GIWAXS, respectively. On the other hand, addition of the block copolymer does have a strong impact on the water contact angles of the films, plotted in Figure 3.6. Angles were measured immediately after the water droplet was in contact with the surface and 30 seconds after contact to inspect for the possibility of droplet uptake in the film, of which none was largely apparent. Results show that increasing the block copolymer content decreases the water contact angle of the film surface. As little as 25 wt% block copolymer content can be added to efficiently reduce the contact angle from about 90° down to 70°, with no significant further decrease observed at higher compositions. This demonstrates efficient enhancement of the surface hydrophilicity, which may be pertinent to applications interfacing with water-rich media. We also assume that this reflects bulk material hydrophilicity, which may be more relevant for mixed conductors operating by bulk electrochemical mechanisms, since the blends appear to be compatible and may therefore exhibit relatively uniform macroscopic properties.

Electrochemical properties

We suspect that the increased hydrophilicity of the material upon blending explains the enhanced transient behavior observed in the OECTs, since this could facilitate interdiffusion of ions from aqueous electrolytes through the material and support bulk electrochemical operation. In order to assess this further, we tested the electrochemical behavior of P3HT-co-P3HT and its blends with P3HT-b-PEO. The polymer electrodes were fabricated by depositing films onto cleaned ITO substrates, following the same blending and deposition protocols as that used to make the OECTs. The details of the instrumentation and methodology of the various experiments are outlined under the electrochemical characterization section of the appendix (A.4 Electrochemical characterization).

Cyclic Voltammetry

We start by comparing the cyclic voltammograms of the blends measured in different media. However, upon closer examination of the anodic (charging) currents plotted in Figure 3.8, we notice subtle differences between the neat material and its blends when measured in organic or aqueous electrolyte, which could hint towards enhanced ion uptake efficiency in the blends. For instance, the oxidation onset potentials identified for the blends are generally less than that of the neat material, by about 100 to 200 mV depending on the composition and electrolyte, as outlined in Table 3.2. Also, operating the neat P3HT-co-P3HHT in aqueous electrolyte appears to result in higher onsets (Eox,on(aq) = 0.22 V) and peak anodic potentials (Epa(aq) = 0.75 V) relative to those measured with organic electrolyte (Eox,on(ACN) = 0.14 V, Epa(aq) = 0.70 V). In contrast, the onsets and peak potentials extracted for the blends measured in the different media are practically in agreement (<50 mV difference). This might suggest that blending effectively lowers the energy barriers associated with electrochemical oxidation of the semiconducting P3HT-co-P3HHT in aqueous electrolyte, potentially due to the PEO moiety facilitating hydrated ion uptake and transport throughout the material.

Table 3.2. Oxidation onsets (Eox,on) and anodic peak potentials (Epa) extracted from

voltammograms measured against Ag/AgCl as reference at 2 mV s -1 . We determine from the voltammograms measured in organic media that hydroxyhexyl substitution of P3HT lowers its oxidation onset by about 200 mV, which we attribute to the improvement of ion uptake efficiency facilitated by the polar sidechains.

0.1 M TBAPF6 (ACN) 0.1 M KCl (aq) Polymer Eox,on (V) Epa (V) Eox,on (V) Epa (V) P3HT-co-P3HHT 0
Conversely, blending P3HT with P3HT-b-PEO increases the oxidation onset slightly by about 60 mV. When operating in aqueous electrolyte, no significant oxidation behavior is observed for P3HT within the potential window investigated here, likely the result of the hydrophobic material being impenetrable to the hydrated ions. Blending this with the block copolymer does seem to enhance its electrochemical activity in aqueous media, indicated by a steep increase of the anodic current. However, the potential range at which this occurs is too high for practical applications and risks side reactions with water. This highlights the importance of engineering substituents on the active semiconductor in combination with blending modification when developing new polymeric mixed conducting materials.

Spectroelectrochemistry

We obtain more evidence supporting that blending P3HT-co-P3HHT with P3HT-b-PEO enhances the efficiency of ion uptake in the material leading to faster electrochemical device operation by probing the electrochemical doping kinetics of the films via spectroelectrochemistry. The experimental design is illustrated in Figure 3.10 and involves measuring the evolution of the absorbance spectra of the film over time during application of a potential step in the presence of an electrolyte. Oxidation (doping) of the p-type semiconducting polymer changes the structure of the backbone from benzenoid-to quinoidlike as positively charged polarons and bipolarons are created. This is accompanied by the formation of corresponding electronic states within the energy gap of the initially neutral material, thereby allowing lower energy transitions. Practically, this can be observed as bleaching of the film involving a decrease of absorbance corresponding to the initial π-π* transition and an increase of absorbance at higher wavelengths. 8 Thus, this phenomenon

Electrochemical bleaching

The evolution of the absorbance spectra in P3HT-co-P3HHT and its blends with P3HT-b-PEO of different compositions (0-45 wt%) during electrochemical oxidation in aqueous electrolyte (0.1 M KCl; the same as that employed in the OECTs) is presented in Figure 3.11. The left panel shows that, at a potential of 0 V (t = 0 s), the neutral materials exhibit similar maxima at λ = 560 nm. Application of a potential step from 0 to 0.75 V, chosen as the doping bias based on the anodic peak potentials observed from cyclic voltammetry, leads to a decrease of the absorbance intensity at 560 nm followed by an increase of a broad absorbance centered at around 800 nm. This behavior matches that of P3HT reported in literature, in which the increase of absorbance at around 800 nm with bias up to 0.9 V vs Ag/AgCl in organic electrolyte (0.1 M TBAPF6 in acetonitrile) corresponds to subgap electronic transitions (≈ 1.7 eV) associated with polaronic states. 8 Mapping the change of the normalized absorbance over time during application of the doping bias given in the We also observe in Figure 3.12, which plots the normalized peak absorbance (560 nm) over time under constant doping bias, that all the materials investigated here display similar relative decrease of the absorbance intensity at steady state (norm. abs. (t = ss) ≈ 0.5 at 0.75 V). This suggests that incorporation of the PEO moiety does not seem to largely affect the steady state spectroelectrochemical response of the polythiophene-rich phases, which we can attribute again to phase separation in the blend. Additionally, recovery of the initial absorption upon application of zero to negative bias indicates that the films can be easily de-doped, which assures that the electrochemical behavior is highly reversible within the potential range investigated here.

Electrochemical doping rates

The influence of blending on the absorbance transients of P3HT-co-P3HHT is emphasized in Figure 3.13, which focuses on the normalized peak absorbance rescaled from 0-1 (norm. rel. abs.) measured within the first few seconds of applying bias. Blends with higher block copolymer content exhibit significantly faster absorbance transients (top

Monophasic exponential kinetic model

Assuming that the change in absorbance is directly proportional to the decrease of neutral polythiophene segments to form polaronic species and follows first order kinetics, we estimate the electrochemical doping rates from the rate constants (k) extracted from exponential decay fits to the transients. Figure 3.14 shows that a single-phase exponential decay function fits adequately with the experimental data until about 90% of the total change, beyond which the data deviates from the model as it approaches steady state. Plots of k1 and k2 obtained for each blend composition in Figure 3.17 shows the same trend as that observed in the rate constants extracted from monophasic exponential fits. Meanwhile, although logarithmic plots show that both rate constants increase with higher block copolymer content, the increase in k1 is much more significant than k2. Concurrent doping mechanisms with different kinetics in polymeric mixed conductors may be the result of a distribution of charge injection energy barriers in the material due to its complex solid-state structure. For instance, we could speculate that the faster processes indicated by k1 can be attributed to more facile ion injection in the amorphous phases of the material while the slower processes represented by k2 might reflect ion injection into the densely packed ordered aggregates. This is inspired from literature investigating the structure of P3HT in electrochemical devices gated with an ionic liquid, 9 which suggests that at low voltages and high frequencies, ions only enter the amorphous fraction of the polymer, while at high voltages and low frequencies, ions may penetrate the crystallites. We could then further surmise that the stronger influence of blending on k1 reflects that the hydrophilic, ionically conducting PEO segments are well integrated with the active semiconducting P3HT-co-P3HHT segments in the amorphous phases. Meanwhile, ion uptake in the ordered P3HT-co-P3HHT domains indicated by k2 is less affected by blending, perhaps due to the large extent of micro-scale phase separation leading to greater purity in the ordered phases, as we deduced from investigating the physical properties of the blend.

Impedance behavior

Examination of the results from spectroelectrochemistry reveals that the incorporation of P3HT-b-PEO in blends with P3HT-co-P3HHT strongly influences its electrochemical transient behavior, while the steady state response appears to be unaffected. We further investigate the doped state of the materials using electrochemical impedance spectroscopy (EIS), which also provides insight into the bulk capacitance of the films.

In the absence of inductive effects, the complex impedance (Z) of a simple AC circuit defined in Equation 3.1 arises from the combination of an ohmic resistance (R) equivalent to the real part of the impedance (Zre) and a capacitive reactance (Xc = -(ωC) -1 ), constituting the imaginary part of the impedance (jZim). 10,11 𝑍 = 𝑍 𝑟𝑒 + 𝑗𝑍 𝑖𝑚

3.1

Where

𝑍 𝑟𝑒 = 𝑅 𝑍 𝑖𝑚 = 𝑋 𝐶 = -(𝜔𝐶) -1
Conventionally, the reactance of a capacitive element (XC) is negative to reflect the phase lag of the AC voltage with respect to the AC current in a charging capacitor. 12 It follows that the phase angle (θ) between Zre and Z vectors is also negative, but for the purpose of clarity we temporarily omit the sign. For an ideal capacitor, θ = 90°. However, typically θ < 90° in real capacitors due to parasitic resistances. In the case where effects from diffusion processes (e.g., diffusion of reactive species, 13 adsorption of ions on porous electrodes 14 ) dominate, the phase shift may approach 45° and the impedance response can be modelled by a Warburg element. 15 Dominance of resistive effects is indicated by the imaginary part of the complex impedance (θ and Zim) approaching zero. 11 The impedance response of P3HT-co-P3HHT and its blends with P3HT-b-PEO was measured over a frequency range of 10 -1 to 10 4 Hz while applying a DC offset bias at 0.75

V to probe the doped state of the materials. We note that the doping bias was applied prior to beginning the impedance measurement over a sufficient time period to reach equilibrium capacitive effects dominating the impedance response at longer time periods. We note that the blends with higher block copolymer content display onsets of the phase angle peaks at slightly higher frequencies, which could suggest that their capacitive behavior manifests at shorter time periods relative to the neat P3HT-co-P3HHT. This might correlate with the trends observed in the OECT transient current data and absorption transients measured during electrochemical doping.

(
Major differences in the impedance response of the neat material and the blends arise at the mid-frequency range between 1 and 10 3 Hz. Neat P3HT-co-P3HHT exhibits a plateau of |Z| and Zim at about 1 and 3 kΩ, respectively, and a local maximum phase angle of -30°, which we attribute to diffusion effects dominating the impedance response. We speculate that the diffusion-limited mid-frequency impedance response results from hampered penetration of hydrated ions into the relatively hydrophobic film. This could also explain the depressed semi-circle feature apparent in the Nyquist plots of the neat material (Figure 3.18, bottom-right). Blending appears to supress this behavior, indicated by the drop of the impedance plateaus to lower magnitudes and the phase angle approaching zero at higher block copolymer compositions. This trend might correlate with enhanced ion transfer efficiency across the polymer-electrolyte interface in the more hydrophilic material.

Equivalent circuit model-fitting

To facilitate interpretation of the impedance response and extraction of physical parameters such as the capacitance of the polymer films, we fit the experimental data with an equivalent circuit model. The spectra could be fit reasonably well with a second order Voigt-type equivalent circuit typically used in simplified models of Li-ion battery and supercapacitor cells. 16,17 The model is shown in Figure 3. 19 and consists of a capacitor (C) describing the capacitance of the film in parallel with a resistor (RP), in series with a constant phase element (CPEi) in parallel with a resistor (Ri), and in series with a solution resistance (RS). A CPE was included in place of an ideal capacitor element to better describe the midfrequency impedance behavior and depressed semi-circle feature in the Nyquist plots of neat P3HT-co-P3HHT. The total impedance of the equivalent circuit described above is given in Equation Note that for the CPE, when n = 1, it is equivalent to a capacitor element (Q = C), and when n = 0.5, it is equivalent to a Warburg element that models diffusion processes. 18 Results from extracting the various circuit parameters given in Table 3. 4 show that the values of n for the neat P3HT-co-P3HHT range from about 0.55 to 0.65, which could indicate a diffusion-dominated response. Furthermore, we observe that increasing the P3HT-b-PEO content yields lower Ri and significantly higher Q parameter, implying reduced impedance contribution by the CPE. This describes well the decrease of the mid-frequency impedances and local phase angle maxima, as well as suppression of the semi-circle feature in the Nyquist plots upon blending. Consequently, the equivalent circuit approaches that of a simple Randle's cell (Rs(C||RP)), frequently used in literature to model polymer mixed conductors and deduce the volumetric capacitance (C*), which describes the strength of ionic-electronic coupling in the material and can be related to the charge carrier density. 19- 22 Assuming that C* originates from the combination of double-layer capacitances formed between injected ions and charge carriers in the bulk of mixed conductors and is therefore dependent on the bulk charge carrier concentration, 21 it appears that blending does not change the polaron density of the electrochemically doped material. One explanation for this might be that a significant fraction of the neat material itself does not participate in polaron formation. Hence, the bulk electronic properties of the material may remain insensitive to the incorporation of an insulating moiety, so long as it maintains the structural order of the active semiconducting component necessary for charge carrier formation and transport. Indeed, it has been shown in literature that as little as 3 wt% P3HT in blends with different commodity plastics like polystyrene and polyethylene can yield FET mobilities approaching that of the neat P3HT with appropriate control of the blend crystallization. 23 This might also explain why the OECTs of the blends exhibit performance, indicated by maximum Id, gm, and μC*, similar to that of devices of the neat P3HT-co-P3HHT, as discussed in the previous chapter.

Summary

Initial characterizations of the solution-processed blends of P3HT-co-P3HHT and P3HT-b-PEO indicate some intermixing or compatibility in the blend despite evidence of phase-separation. Also, the short-and long-range molecular order of P3HT-co-P3HHT in the resulting film is not largely affected upon blending, while no large-scale phase separation is observed via optical microscopy. On the other hand, blending notably decreases the contact angles of water on the films, which could indicate increased hydrophilicity of the surface and, therefore, better compatibility of the polymer interface with water-rich media. As a result, the blends seem to exhibit enhanced ion uptake efficiency when operated in aqueous electrolyte, leading to drastically faster electrochemical doping processes observed via spectroelectrochemistry. The thickness-normalized spectroelectrochemical rate constant (kd -1 ) of the neat P3HT-co-P3HHT (≈ 2.4 ± 0.3 s -1 μm - 1 ), estimated from exponential fits to the absorption transients of the films of different thicknesses and assumed to be proportional to the electrochemical doping rate, could be enhanced at least a full order of magnitude by blending with only 25 wt% P3HT-b-PEO (≈ 35 ± 8 s -1 μm -1 ), while less dramatic increase is observed with higher block copolymer content (≈ 48 ± 10 s -1 μm -1 ). Conversely, the volumetric capacitance of doped P3HT-co-P3HHT (190 ± 30 F cm -3 ), extracted from equivalent circuit model fits to the impedance spectra of the doped films, does not seem to change upon blending. This might indicate that the density of charge carriers, which we assume interacts with injected ions to contribute to double-layer capacitances in the bulk, is relatively similar across the different

Introduction

We have shown so far that blends of P3HT-co-P3HHT and P3HT-b-PEO exhibit enhanced electrochemical doping kinetics resulting in faster OECT performance.

Meanwhile, the transduction performance of the devices related to the electronic conductivity and charge storage capacity of the films do not seem to change upon blending, which we suspect is facilitated by phase separation in the blend despite some intermixing of the components. We also believe that the P3HT-block chemically linked to the PEO compatibilizes the solution-cast blends, which should help maintain their structural stability and, subsequently, their long-term device performance. To test this, we examine the performance of their OECTs during prolonged immersion in water and probe the stability of the blends in aqueous media through quartz crystal microbalance measurements (QCM).

Additionally, we investigate whether we can improve the mixed conducting properties and transistor performance of P3HT-co-P3HHT by blending with a PEO homopolymer instead, which might intercept the need for additional synthetic steps to make the block copolymer.

However, results show relatively poorer stability of the blends with PEO, implicating the advantages of employing block copolymers.

Operational stability of blends with P3HT-b-PEO

To investigate the operational stability of the devices we track the steady-state and transient performance of OECTs of P3HT-co-P3HHT and its blends with P3HT-b-PEO over time with prolonged immersion of the transistors in water. To facilitate comparison of the different materials, we test their devices constituting similar channel dimensions (W = 100 μm, L = 10 μm, d = 100±10 nm). The initial performance is measured in aqueous electrolyte, followed by rinsing and continuous storage submerged in water before subsequent characterizations. The devices are measured every other day to every week for over one month. To minimize the possibility of electrically deteriorating the polymers, the potentials applied are limited to a Vd of -0.4 V and a Vg range of 0.2 to -0.6 V. Sufficiently slow sweep rates are employed when measuring the transfer characteristics of the neat material in order to operate in the saturation regime using lower potentials (according to observations made in section 2.2 OECT characteristics at various operating rates, Figure 2.2) and yield detectable "on" currents. Finally, the transient behavior of the devices is measured upon application of a Vg step from 0 V ("off") to -0.6 V ("on") and held over a period long enough to observe steady state conditions.

Transfer characteristics

Investigation of the transfer characteristics reveals that the OECT of the neat material shows good operational stability while that of the blend shows gradual degradation of its performance with continuous long-term exposure to aqueous media. does not display significant deterioration of its "on" currents and transconductances during one month of continuous immersion in water, which suggests that the structural and mixed conducting properties of the P3HT-co-P3HHT film is sturdy in ambient, aqueous environment. Meanwhile, the blend exhibits only small decrease of the "on" currents and slightly larger current hysteresis after one week. However, after two weeks being stored in water, significant decrease of the transconductance is eventually observed. These findings could indicate either degradation of the electronic conductivity in the blend or simply slower device response relative to the operating speed (50 mV s -1 ). (Ion/off ≈ 10 3 ) even after one month of repeated measurement and immersion in water. On the other hand, the values of τ indicated by the shoulders on the drain current transients are observed to decrease for both material systems. The neat material, which already shows initially long response times lasting up to 10 s, exhibits slightly further increase of τ over time. However, the device of the blend exhibits more significant change of τ from an initial (μS)

V g (V)
value of 80 ms to about 5 s after a month, two orders of magnitude higher, which explains the conspicuous decline of the drain currents and transconductances observed uniquely in the transfer curves of the blend. We attribute this to less efficient charge injection resulting from prolonged exposure to water that may have been preceded by the disintegration of the blend induced by the water-soluble, low molecular weight PEO component. After each measurement in wet conditions, the sensors were lightly rinsed with water and allowed to dry under nitrogen flow, then re-immersed in the water bath for subsequent measurements. At least three dry-wet cycles (x = 3) were recorded for each material system. Further details on the methodology may be found in the appendix.

Results from QCM measurements on P3HT-co-P3HHT plotted in this is equivalent to an areal density of 11 μg cm -2 , which, assuming uniform thickness over the sensor area and a volumetric density of 1 g cm -3 , translates to a film thickness of about 110 nm matching results from profilometry measurements on previous films spun-cast using the same parameters. which might result from swelling of the film. Similar behavior has been observed for films of p(g2T-TT) and crosslinked PEDOT:PSS that swell by up to 15% and 85%, respectively, in wet conditions. [START_REF] Wang | Ethylenedioxythiophene incorporated diketopyrrolopyrrole conjugated polymers for high-performance organic electrochemical transistors[END_REF] Furthermore, the frequency measured during the second and third immersion of the blend-coated sensor at Δfwet,2 = Δfwet,3 ≈ -1.1 kHz is 0.1 kHz higher than that of the first immersion, Δfwet,1, translating to a decrease in areal mass by about 2 μg cm - 2 (≈ -10%). This suggests that blends with P3HT-b-PEO are susceptible to both swelling and loss of material when exposed to water-rich environments, which might explain how the initially fast response speed of their OECTs eventually slows down over time.

Blends of P3HT-co-P3HHT with PEO

To investigate whether the instability observed in the transient performance of the blends is driven by the solubility of the PEO moiety, we examine the properties of P3HTco-P3HHT blended with a PEO homopolymer of similar molecular weight as that in the block copolymer (Mn,PEO ≈ 20 kg mol -1 ). First, we determine whether we can also achieve compatible blends processed from solution without chemically linking the PEO with a P3HTblock, using the same film-deposition protocols as that of blends with the block copolymer.

We then investigate the electrochemical properties of these materials to forecast the performance of their devices.

Physical properties

Inspection of the thermal properties extracted from the thermograms plotted in

ΔH1 a (J g -1 )
Tm2,end (°C)

ΔH2 b (J g -1 )
Tc1,on (°C)

ΔH1 a (J g -1 )
Tc2,on (°C)

ΔH2 b (J g -1 )
Tm1,end (°C)

ΔH1 a (J g -1 )
Tm2,end (°C) On the other hand, the cooling traces show significantly suppressed crystallization of PEO and P3HT-co-P3HHT in the blend leading to lower enthalpies and larger supercooling (ΔΤ = Tm,end (2 nd heat) -Tc) compared to the neat materials. This might indicate that the melt-process blend with PEO is compatible, which is also reflected in the right panel of Figure 4.6. This suggests sufficient interaction between PEO and P3HT-co-P3HHT, which appears to be diluted when processed from solution. Meanwhile, we do not observe large differences in the local molecular ordering of P3HT-co-P3HHT upon blending with PEO. Specifically, the UV-Vis absorption spectra of the blend given in Figure 4.8 is almost identical with that of the neat P3HT-co-P3HHT and shows only slightly higher A0-0/A0-1 intensity ratio. The small influence of blending on the optical properties could reflect minimal interaction of the components due to their microphase separation as implied by the thermal properties of the solution-cast blend. While blending with PEO does not strongly influence the short-range order of P3HTco-P3HHT when compared to the neat material, it does seem to increase the hydrophilicity of the films, much like the effect of blending with the block copolymer. Intriguingly, the angle decreases and approaches 70° in both blend systems despite there being even less PEO fraction in blends with the block copolymer (e.g., 20 wt% PEO in a blend with 25 wt% P3HT-b-PEO according to its molecular weight fraction). This suggests that the hydrophobic P3HT-block linked to PEO does not limit its influence on the hydrophilicity of the blends and that the trend saturates at low PEO compositions. Indeed, we find from plotting the compiled data from measuring both blend systems in Figure 4.10 that no further significant decrease of the contact angle can be achieved at compositions above 20 wt% PEO. This leads us to predict that we can enhance the electrochemical properties of the material operating in aqueous conditions, linked to its hydrophilicity, by blending with PEO at relatively low compositions.

ΔH2 b (J g -1 ) 0 1.6 -- -- 259

+25 wt% PEO +25 wt% PEO

Electrochemical properties

The strong similarities between the physical properties of the blends comprising either P3HT-b-PEO or the PEO homopolymer allows us to predict that adding PEO should also enhance the transient electrochemical properties of the material. We begin by verifying that blending with PEO does not drastically change its charging behavior relative to that of the neat material or its blend with the block copolymer through comparing their cyclic Thus, we use similar potentials to operate and compare electrodes of the different materials. Closer inspection of the relative peak absorption changes, given in the left panel of It follows that the regression slopes of the k-d plots corresponding to the thicknessnormalized rate constants (kd -1 ) are virtually the same for blends comprising either 25 wt% PEO (44±2 s -1 μm -1 ) or 25 wt% P3HT-b-PEO (35±8 s -1 μm -1 ), despite the latter containing less PEO fraction (20 wt% PEO), which suggests that the effect of PEO on the transient electrochemical properties of the material saturates at low compositions. To investigate this further, we compile the kd -1 values obtained for the different blend systems and plot them according to PEO content in Figure 4.14. Significant enhancement of the rate constants in the blends is displayed only up to kd -1 ≈ 40 s -1 μm -1 at a composition of 20 wt% PEO, above which no further increase is achieved. This is reminiscent of the trend observed in the water contact angles of the materials, also plotted in describes the mixed conducting performance of the active channel material independent of the channel dimensions and biasing. A value of about 14 F cm -1 V -1 s -1 is obtained for the blend with 25 wt% PEO, which is within the range of values obtained for the neat P3HT-co-P3HHT and its blends with P3HT-b-PEO (10 to 55 F cm -1 V -1 s -1 ). Thus, we establish that blending the mixed conducting P3HT-co-P3HHT with the electronically insulating PEO homopolymer is also a feasible strategy to enhance its transient electrochemical behavior and, subsequently, the response speed of its devices without losing its electronic properties.

Stability of blends with PEO

Having determined that blending the mixed conductor with PEO can efficiently enhance its transient electrochemical properties and transistor performance, we proceed to examine their long-term operational stability in aqueous context with reference to the instability observed in the blends of the block copolymer. We predict that, without the P3HTsegment chemically linked to the PEO to limit its solubility in water, the structural stability of the blends and their subsequent long-term device performance will be relatively poor. (μS)

V g (V)
the time constant (τ), indicated by the shoulder of the transient current plots, shifts drastically by two orders of magnitude after only a week. to sustain a τ almost one order lower than the neat material even after one month in water.

This might reflect less fraction of the ion-conducting phases lost to the aqueous environment, which would suggest a more structurally stable blend and aligns with the deductions from QCM analysis. We suppose that this behavior is supported by better compatibility of the blend with the block copolymer. The hydrophobic P3HT-block chemically linked to the PEO might also contribute to limiting material dissolution. Notwithstanding that the stability of the transient performance can still be improved further, the results presented here allude to potential advantages in choosing amphiphilic block copolymers as cocomponents in designing compatibilized mixed conducting polymer blends. weeks, indicated by an increase of its time constant from τ = 80 ms to about 4 s. Preliminary QCM analysis suggests that the blend may lose around 10% of the material during prolonged exposure to aqueous media, which we attribute to the water-soluble low molecular weight PEO moiety. To test this, we investigate blends of P3HT-co-P3HHT and a PEO homopolymer of the same molecular weight as the PEO moiety in the block copolymer. We find that blending with PEO affects the material properties much like blending with the block copolymer. Specifically, both blend systems show a decrease of the water contact angle to 70° and similar enhancement of the electrochemical doping rates relative to the neat material. This might indicate that the effect of blending on the mixed conductor transient properties is dictated by the PEO moiety. Furthermore, we determine that only up to 20 wt% PEO content in either blend system is needed to effectively modulate the material properties. As a result, OECTs of the blend with PEO show enhanced transient behavior indicated by relatively low response times (τ = 70 ms). Meanwhile, the volumetric capacitance of the blends with PEO is essentially equivalent to that of the neat material (190±30 F cm -3 ). Subsequently, OECTs comprising the PEO blend show similar steady state characteristics and μC* parameter (14 F cm -1 V -1 s -1 ) within range of that of the neat P3HT-co-P3HHT (10-55 F cm -1 V -1 s -1 ). Although these results imply that blending with the PEO homopolymer may be a viable method to enhance the properties of the mixed conductor, preliminary QCM analysis suggests that the blends with PEO can exhibit loss of about 20 wt% of material during prolonged exposure to aqueous media, likely due to the water-solubility of the PEO. As a result, its devices exhibit poor long-term operational stability, showing more rapid degradation of its transient performance and to a greater extent, from τ = 70 ms to about 30 s, than that observed in the blend with the block copolymer within a week of continuous storage in water. The superior long-term stability of the blends with the block copolymer might be due to its compatibility and due to the the hydrophobic P3HT-block linked to the PEO, which helps limit its solubility and could therefore support the mechanical stability of the blend. These findings imply opportunities parameters and device geometry, even after incorporating an electronically insulating component in the channel. We also find that the μC* performance measured for the neat P3HT-co-P3HHT at practical device operating speeds (≈ 13 to 23 F cm -1 V -1 s -1 at sweep rates of 0.5 and 0.25 V s -1 ) is sustained after blending (≈ 14-35 F cm -1 V -1 s -1 for blends comprising between 10 and 40 wt% P3HT-b-PEO). These results imply that the electronic conductivity of P3HT-co-P3HHT is virtually unaffected by blending with the PEO-based block copolymer investigated here. This further suggests that a percolation pathway is maintained despite some intermixing of the components as evidenced by the vitrification effect observed in DSC (lowering of melting enthalpies).

Furthermore, we conclude that employing block copolymer "additives" possessing moieties compatible with the mixed conducting component supports the stability of the blend. Specifically, the P3HT-block chemically linked to the PEO compatibilizes its blend with P3HT-co-P3HHT and therefore supports its structural stability and long-term device performance. This is in strong contrast to blends with the PEO homopolymer. Analysis of the thermal properties of both blend systems (blends with the P3HT-b-PEO block copolymer and blends with the PEO homopolymer) provides evidence that the blends with the block copolymer exhibit more compatibility than the blend with the PEO homopolymer.

Furthermore, preliminary QCM results suggest that films of the blends with the block copolymer "lose" less material upon immersion in water than blends with the PEO homopolymer. This might imply that the hydrophobic P3HT-block helps to limit dissolution These results suggest further work focusing on block copolymer engineering to improve the stability and performance of mixed conducting polymer blends. For instance, molecular weights of either moiety in P3HT-b-PEO may be increased to offer more robust films when continuously operated in water, but without drastically affecting the mixed conductivity of the material. Additionally, semicrystalline polymer mixtures and block copolymers can follow different solidification pathways that may be manipulated via temperature and solution-processing techniques. These may be exploited to control the solid-state structure of resulting films and investigate their consequences on their mixed ionic and electronic conducting properties to establish widely applicable structureprocessing--property relationships.

For the general development of new polymeric mixed conductors, polymer blending may provide a way to distinctly manipulate ionic and electronic transport in organic systems, notwithstanding the coupled nature of these processes, by addressing each polymer component and controlling phase separation in the blend (degree of intermixing; lengthscale of phase-separation). This promises to open up new avenues for semiconducting polymers and polymer electrolytes that have been originally designed for field effect transistors and batteries. Overall, the blending approach presented in this thesis can be employed in designing materials for applications such as electrochemical biosensing that require fast switching device operation, high signal amplification, and long-term stability.

1 , ƉP3HT = 1 . 2 ,

 112 3hexylthiophène) et poly(3-(6-hydroxyhexylthiophène) (P3HT-co-P3HHT; masse molaire moyenne en nombre Mn = 30 kg/mol, dispersité Ɖ = 1.9, Figure i.1, gauche). En effet, des travaux précédents ont montré que ce matériau permet une réponse électrochimique rapide, sans gonflement excessif qui pourrait détériorer ses propriétés sur le long terme, lorsqu'il est utilisé en électrolyte aqueux. Le P3HT-co-P3HHT a été mélangé avec un copolymère à bloc de P3HT et de poly(oxyde d'éthylène) (P3HT-b-PEO, Mn, P3HT = 5 kg mol - Mn, PEO = 20 kg mol -1 , ƉPEO ≤ 1.2, Figure i.1, droite), pour améliorer le caractère hydrophile et la conductivité ionique du matériau. De plus, le bloc P3HT pourrait agir comme compatibilisant pour limiter la séparation de phase à grande échelle, contribuant ainsi la stabilité structurale du mélange, particulièrement lorsqu'il est utilisé dans des milieux riches en eau.

Figure i. 1 .Figure i. 2 .

 12 Figure i.1. Structures chimiques du P3HT-co-P3HHT (à gauche) et P3HT-b-PEO (à droite).

  t -V g | W d L -1 (V cm) que le mélange affecte tout d'abord les performances transitoires des OECT. Plus particulièrement, le P3HT-co-P3HHT pur a un long temps de réponse OECT (τ = 8 s) qui est extrait depuis le courant transitoire au niveau du drain après application d'une différence de potentiel. Il en résulte que les performances en transduction des OECTs (indiquées par la transconductance gm et extraite de mesures de Id après un balayage en tension de la grille (Vg)), sont particulièrement sensibles à la vitesse de balayage des paramètres étudiés ici. En effet, ces résultats de transduction peuvent diminuer de deux ordres de grandeur lorsque la vitesse de balayage Vg passe de 0.1 à 1 V s -1 . La détermination de μC*, figure de mérite des OECTs, permet de différencier et décrire les performances des matériaux utilisés comme canal des transistors. Cette figure de mérite peut être obtenue en caractérisant le gm de différents dispositifs en faisant varier la taille des canaux, dans la gamme de Vg utilisée ici. Le paramètre μC* varie ainsi grandement suivant la vitesse de balayage utilisée (Figure i.2, en bas à gauche). Les valeurs de μC* extraites pour P3HT-co-P3HHT varient de 10 à 55 F cm -1 V -1 s -1 , avec une moyenne à 35 F cm -1 V -1 s -1 . Ces résultats semblent indiquer que la vitesse d'injection des charges impliquées dans le dopage électrochimique de P3HT-co-P3HHT, lors de l'utilisation du dispositif, est faible. Ces résultats soulignent aussi l'importance de la résolution temporelle lorsque des conducteurs mixtes « lents » sont étudiés. D'autre part, mélanger du P3HT-co-P3HHT avec 25 wt% de P3HT-b-PEO permet d'obtenir une vitesse de réponse plus courte (temps de réponse plus court, τ = 80 ms, Figure i.2, au milieu à droite). Les valeurs de transconductance gm de ces dispositifs sont stables sur une large gamme de tension de grille Vg, même jusqu'à 10 V s -1 , et un μC* de 28 F cm -1 V -1 s -1 avec des variations minimales est mesuré (Figure i.2, en bas à droite). Nous remarquons que cette valeur n'est pas très différente du matériau pur, impliquant que le mélange n'a pas d'effet marqué sur les performances en masse du matériau dopé. De plus, le Id à saturation des courants transitoires (Figure i.2, au milieu) pour les deux polymères sont virtuellement équivalents (≈ 10 -5 A) (en considérant les mêmes paramètres et dimensions des canaux). Ces résultats montrent que mélanger P3HT-co-P3HHT avec du P3HT-b-PEO permet d'améliorer efficacement la vitesse de réponse des OECTs, sans trop affecter négativement sa performance à l'état d'équilibre. Nous supposons que cette amélioration des performances à l'état d'équilibre des OECT est dû à une augmentation de l'hydrophilie du matériau de canal lors de l'ajout de PEO, ce qui pourrait entrainer une interdiffusion plus efficace des ions entre le film polymère et l'électrolyte aqueux externe. Nous supposons également que le bloc P3HT, lié chimiquement au PEO, contribue à stabiliser le mélange et limite la séparation de phase à une échelle microscopique, entraînant une distribution macroscopique uniforme des composants. Ceci pourrait aider au maintien des propriétés électriques en masse du matériau, expliquant ainsi que les propriétés à l'état d'équilibre des OECTs restent relativement constantes, même après mélange. Pour étudier d'avantage ces notions, nous avons examiné en quoi le mélange (réalisé de la même façon que lors de la fabrication des OECTs) affecte les propriétés physiques et électrochimiques du matériau. Les premières caractérisations de mélange de P3HT-co-P3HHT/P3HT-b-PEO, préparé en solution, indiquent que les composants ne sont pas dans la même phase, mais il semblerait qu'ils aient un certain degré de compatibilité, lié à leur comportement en température (Figure i.3, en haut à gauche). De par cette séparation de phase, l'ordre à courte et longue distances obtenu dans les films de P3HT-co-P3HHT, n'est pas particulièrement affecté après mélange (d'après mesures d'absorbance et de GIWAXS, respectivement). Aucune séparation de phase à grande échelle n'a pu être observée par microscopie optique, à cause de certaines compatibilités dans le mélange (Figure i.3, en haut à droite).

  part, le mélange diminue nettement l'angle de contact de l'eau sur les films (Figure i.3, à gauche), ce qui pourrait indiquer une augmentation de l'hydrophilic de la surface, et donc une meilleure compatibilité entre l'interface polymère et un milieu riche en eau. Par conséquent, les mélanges semblent augmenter l'efficacité de fixation des ions lorsqu'ils sont utilisés dans des électrolytes aqueux, ce qui entraîne une augmentation drastique de la vitesse des procédés de dopage, observés par spectro-électrochimie (Figure i.3, au milieu à droite). La constante de vitesse spectro-électrochimique kd -1 rapportée à l'épaisseur du film peut être estimée par une courbe de tendance exponentielle des absorptions transitoires des films de différentes épaisseurs, en considérant qu'elle est proportionnelle à la vitesse de dopage électrochimique. Cette constante kd -1 , d'environ 2.4 ± 0.3 s -1 μm -1 pour le P3HT-co-P3HHT pur, pourrait être augmentée d'au moins un ordre de grandeur en le mélangeant avec seulement 25 wt% de P3HT-b-PEO (kd -1 ≈ 35 ± 8 s -1 μm -1 ), alors qu'une augmentation plus faible est observée avec une quantité de copolymère à blocs plus importante (kd -1 ≈ 48 ± 10 s -1 μm -1 ) (Figure i.3, en bas). Inversement, la capacitance volumétrique du P3HT-co-P3HHT (190 ± 30 F cm -3 ), extraite d'un modèle de circuit équivalent qui s'adapte au spectre d'impédance des films dopés, ne semble pas changer lors du mélange. Cela pourrait indiquer que la densité des porteurs de charges, dont nous supposons qu'ils interagissent avec les ions injectés pour contribuer aux capacités à double couche en masse, est relativement similaire dans les différentes compositions de mélange. Cela implique qu'une grande partie du P3HT-co-P3HHT pur ne participe pas lui-même à la formation et au transport des porteurs de charges. Nous soupçonnons alors que les propriétés électriques du matériau en masse peuvent être relativement insensibles au mélange avec un composant électroniquement isolant, tant qu'il maintient l'ordre structurel nécessaire du composant semi-conducteur actif dans les films résultants. Nous pensons que cela est le cas dans nos mélanges car ils présentent une séparation de phase sur des échelles de longueur très fines. Ceci expliquerait donc les performances électriques C* et OECT soutenues des mélanges par rapport à celles du matériau pur. Outre l'induction d'une séparation de phase à des échelles de longueurs microscopiques, nous soupçonnons également que l'effet de compatibilité du bloc P3HT, chimiquement lié au PEO lorsqu'il est mélangé avec P3HT-co-P3HHT, devrait aider à maintenir la stabilité structurale et, par la suite, les performances à long terme du dispositif. Pour tester cela, nous examinons les performances de leurs OECTs lors d'un stockage prolongé dans l'eau. Le dispositif de P3HT-co-P3HHT pur présente un fonctionnement stable indiqué par des courants transitoires relativement reproductibles même après un mois de mesures répétées et d'immersion continue dans l'eau (Figure i.4, en haut à gauche). Lors du mélange avec P3HT-b-PEO, bien que le transistor montre des performances transitoires initialement améliorées, sa vitesse de réponse ralentit finalement après deux semaines, indiquée par une augmentation de sa constante de temps de 80 ms à environ 4 s (Figure i.4, en haut à droite). L'analyse via microbalance de Quartz (QCM) préliminaire suggère que le mélange semble perdre environ 10 % de sa masse lors d'une exposition prolongée à des milieux aqueux, ce que nous attribuons à la fraction PEO soluble dans l'eau ainsi qu'à sa faible masse moléculaire. Pour explorer un peu plus cela, nous étudions des mélanges de P3HT-co-P3HHT et d'un homopolymère PEO du même masse moléculaire que la fraction PEO dans le copolymère à blocs (Mn,PEO = 20 kg mol -1 ). Nous constatons que le mélange avec le PEO affecte les propriétés du matériau tout comme le mélange avec le copolymère à blocs, ce qui semble-t-il, pourrait permettre d'éviter les étapes de synthèse supplémentaires nécessaire à la fabrication d'un copolymère à blocs.

  de compatibilité, notamment indiquée par ses propriétés thermiques, ce qui pourrait conduire à une stabilité structurale moindre. En effet, une analyse QCM préliminaire suggère que les mélanges avec PEO peuvent présenter une perte d'environ 20 % en poids de matière lors d'une exposition prolongée à des milieux aqueux, plus élevée que celle observée dans les mélanges avec le copolymère à blocs. En conséquence, ces dispositifs présentent une mauvaise stabilité opérationnelle à long terme (Figure i.4, en bas à gauche), montrant ainsi une dégradation plus rapide et plus importante de ses performances transitoires, de τ = 70 ms à environ 30 s, que celle observée dans le mélange avec le copolymère à blocs dans la semaine de stockage continu dans l'eau (Figure i.4, en bas à droite). La stabilité supérieure à long terme des mélanges avec le copolymère à blocs pourrait être due à sa compatibilité et également au bloc P3HT hydrophobe lié au PEO, qui aide à limiter sa solubilité et soutient donc la stabilité structurale du mélange. Ces résultats ouvrent des opportunités pour de futures recherches sur la conception et l'ingénierie de copolymères à blocs en tant que co-composants prometteurs pour améliorer les performances et la stabilité des mélanges de polymères conducteurs mixtes.

Figure i. 4 .

 4 Figure i.4. Evolution de l'Id transitoire du système P3HT-co-P3HHT (en haut à gauche) et ses mélanges avec le P3HT-b-PEO (en haut à droite) et le PEO (en bas à gauche) lors d'une immersion continue dans l'eau en fonction du temps. Evolution de la constante de temps (τ, en bas à droite) extraite à partir des courants transitoires mesurés lors de cemême stockage dans l'eau.

2 , 3

 23 Another practical use for OECTs is that their characterization can offer plenty of insight into the properties of the active mixed conducting polymer. This requires some understanding of the basic operating principles and device physics of OECTs. Determining the key performance indicators of OECTs allows the extraction of materials-specific parameters such as the charge carrier mobility and volumetric capacitance. Comparison of the performance of

Figure 1 . 1 . 23 1. 2 ,

 11232 Figure 1.1. Schematic diagram of a typical OECT. Image from ref.3.

Figure 1 . 2 .

 12 Figure 1.2. Graphical representations elucidating the processes involved in the polymeric channel material for accumulation and depletion mode OECTs (top), as well as the general profiles of the drain current that can be expected during application of a gate potential sweep from each device mode (bottom). Image from ref 3 .

Figure 1 . 3 .CHAPTER 1 : 25 1. 2 . 2 .Figure 1 . 4 . 8

 1312522148 Figure 1.3. Differences between the architectures and charging behaviors of OFETs (left), EGOFETs (middle), and OECTs (right) upon application of gate potential. Here, OSC stands for "organic semiconductor". Image from ref.7.

Figure 1 . 4 .

 14 Figure 1.4. Electric circuit models used to describe the ionic and electronic parts of OECTs in the Bernards-Malliaras model. Image from ref.3.

Equation 1 . 3

 13 implies that the time constant extracted from Id transients is determined by the ionic time constant τi, suggesting that the overall response speed in an OECT is limited by the rate of ion injection into the channel.Another operating condition that provides useful information about the device physics and channel material properties is the measurement of the transient drain current upon application of a constant gate current. This fixes the kinetics of the ionic circuit and focuses on the electronic transport behavior in the device. The transient behavior at constant Ig derived from the Bernards-Malliaras model is given in Equation 1.4.

Equations 1 .1 and 1 . 2

 112 imply that the magnitude of the OECT Id and gm scale with the channel thickness (d), unlike that of FETs, and reiterates the bulk doping phenomenon unique to OECT behavior. Furthermore, Equation 1.2 allows the extraction of the μC* parameter from the values of gm measured for several devices composed of different channel dimensions. The left-most panel of Figure 1.5 shows the gm scaling of devices over different channel geometry and bias, WdL -1 |Vt-Vg|, for several types of OECT channel materials reported in literature. The slope of each plot gives the μC* product (marked [μC*]OECT in Figure 1.5). The values almost directly agree with the products of individually extracted μ, from electron transit times in OECTs, and C*, from equivalent circuit modelling of the impedance, in a 1:1 fashion, as shown in the middle panel of Figure 1.5. Since the terms μ and C* capture the electronic transport properties and ionic charging behavior of the channel material, respectively, and are independent of the device dimensions and operating conditions, the μC* product serves as the material-system figure-of-merit to benchmark OECT channel materials. 17 Plots of μOECT versus C* such as that given in the right-most panel of Figure 1.5 provide a quick summary of the relative performance of different OECT materials, the optimum region being on the northeast part of the graph.

Figure 1 . 5 .

 15 Figure 1.5. Scaling of gm with OECT channel dimensions and bias, WdL -1 |Vt-Vg| (left), whose slope, [μC*]OECT, is plotted against the product of individually obtained values of μ and C* (middle) showing almost 1:1 agreement (dashed line), and a μOECT-C* map of different OECT channel materials as labelled (right). Dotted lines indicate constant μC*.

17

 17 Figures from ref.17.

  Figures from ref.17.

Figure 1 . 6 (

 16 top).

Figure 1 . 6 .

 16 Figure 1.6. Illustration depicting the general electronic structure of double-bonded carbon atoms (top) and energy diagram of the frontier orbitals of conjugated carbon chains with

Figure 1 . 8 .

 18 Figure 1.8. Positive polarons and bipolarons provide holes in localized electronic states

Figure 1 . 7 .

 17 Figure 1.7. The structures of a neutral polythiophene and its polarons and bipolarons upon p-type doping. Charge neutrality is maintained by interaction with counterions (encircled by a red dashed line).

Figure 1 . 8 . 3 .

 183 Figure 1.8. Simplified energy diagram depicting the localized energy levels within the band gap of an initially neutral polymer induced by polaron and bipolaron formation.

Figure 1 . 9

 19

Figure 1 . 9 .CHAPTER 1 :

 191 Figure 1.9. Illustration of ion transport in polymer matrices involving intra-and interchain ion hopping assisted by segmental motions of polymer chains. Ion-coordinating moieties along the chains are represented by open circles. Image from ref.32.

  behavior and deterioration of OECT performance, as well as disintegration of the film during continuous cycling operation, illustrated in Figure 1.10.

Figure 1 . 10 .

 110 Figure 1.10. Illustration of the charging mechanism of ethylene glycol-substituted n-type mixed conducting polymer electrodes operating in aqueous electrolyte. For polymers with mixed hydrophilic and hydrophobic sidechains, water uptake is limited ("controlled") leading to reversible swelling and charging behavior (top). For polymers that are

Figure 1 . 11 .

 111 Figure 1.11. Additionally, there exists several charged "sites" separated by a distance α at

Figure 1 .

 1 Figure 1.11 by depicting the charged polymer segments as parallel plates separated by a distance α throughout the film volume (with width W, length L, and thickness, d). Thus, C*

Figure 1 . 11 . 44 1. 3 . 5 .

 1114435 Figure 1.11. Illustration of double layers formed between injected cations and sulfonate anions in PEDOT:PSS upon hole extraction (left). Simplified illustration of double layercapacitances in PEDOT:PSS as a block of parallel plate capacitors (right).[START_REF] Proctor | Understanding volumetric capacitance in conducting polymers[END_REF]

CHAPTER 1 :Figure 1 . 12 .

 1112 Figure 1.12. Charge transport in semicrystalline polymers (PBTTT exhibiting edge-on packing given for example) occurs along the polymer backbone (intrachain transport) andalong the π-stack and lamella-stack directions (interchain transport). Image from ref.[START_REF] Noriega | A general relationship between disorder, aggregation and charge transport in conjugated polymers[END_REF] .

Figure 1 . 13 .CHAPTER 1 :

 1131 Figure 1.13. Schematics depicting the microstructures of semicrystalline (left) and completely amorphous (right) flexible chain polymers. Note coexistence of ordered (darker shade) and amorphous regions. Long polymer chains connecting ordered regions are highlighted in red.

Figure 1 . 14 .

 114 Figure 1.14. Map of the distribution of localized sites, as indicated, over space and energy according to a Gaussian distribution of the density of states (DOS) of a disordered semiconductor (left panel, from ref. 52 ). Disorder model fit (solid blue line) to the experimental transconductance (grey scatter plot) of a P(g2T-TT)-based OECT (right panel, from ref. 50 ). Predicted drain current (solid red line) from disorder model and experimental drain current (black scatter plot) also shown.

1. 4 . 1 .

 41 Early mixed conducting polymers: Ppy and PANI Polypyrrole (Ppy) and polyaniline (PANI), shown in Figure 1.15, are among the earliest reported materials that have been used in OECTs. In the first OECT invented by Wrighton and colleagues in 1984, Ppy was anodically electropolymerized across gold

Figure 1 . 15 .CHAPTER 1 :

 1151 Figure 1.15. Chemical structures of early OECT channel materials.

Figure 1 .

 1 16, leads to bulk electrochemical doping in accumulation-mode devices operating at low potentials (< 1 V) in aqueous electrolyte. Since these semiconducting polymers are solution-processable in organic solvents only, their films do not require special treatments like cross-linking to operate in aqueous media. Polythiophenes and their derivatives consisting of fused aromatic rings such as poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]-thiophenes (PBTTT) and polybenzo[1,2-b:4,5-b']dithiophenes (PBDT) have been functionalized with glycol

Figure 1 . 16 .

 116 Figure 1.16. Various polythiophene derivatives that have been used as the active channel material in OECTs gated with aqueous electrolyte.

Figure 1 . 16 .

 116 Incorporating alkyl spacers to increase the distance between

Figure 1 . 17 .

 117 Figure 1.17. Some conjugated polyelectrolytes and their counterions used as the mixed conducting channel in OECTs.

Figure 1 . 18 .

 118 Figure 1.18. Polymers of thienyl-flanked glycolated DPP copolymerized with different aromatic monomers X have recently been tested in OECTs.

CHAPTER 1 :

 1 Bibliographic research | 53

Figure 1 . 19 ,

 119 have been reported.

Figure 1 . 19 .

 119 Figure 1.19. Reported n-type mixed conducting polymers.

CHAPTER 1 :

 1 Bibliographic research | 59 in an electrochemical doping mechanism that can be exploited in non-volatile memory devices.111 Specifically, protonation of PEI during device operation induces its electrostatic interaction with PSS, effectively stabilizing the de-doped state of PEDOT. This allows multiple non-volatile conductance states without relying on slow doping or ion uptake kinetics. Another example is the addition of molecular de-dopants in the form of aliphatic amines to PEDOT:PSS dispersions to yield accumulation mode OECTs.112 For instance, Nmethyl-2,2′-diaminodiethylamine (DEMTA) has been mixed into PEDOT:PSS. Heating the films induces electron transfer from the amine to the PEDOT and effectively de-dopes the polymer. As a result, OECTs may operate in accumulation mode (gm,max = 74 S cm -1 , ION/OFF = 10 3 ) with high cycling stability, while the threshold voltages can be tuned further by careful design of the molecular additive. Several examples of multicomponent mixed conductors can be named based on PEDOT:PSS alone, however other polymers have also been modified by blending. Blends of the conjugated polyelectrolytes PTHS, PEDOT-S[H], and PEDOT-S[(Oct2)NH2] (bulky counterion) allow manipulation of their OECT threshold voltages at values between those of the neat materials. 73 It is suggested that this control is driven by the difference in the solubility of PEDOT-S[H] (soluble in water) and PEDOT-S[(Oct2)NH2] (soluble in methanol). Thus, blending PTHS with PEDOT-S[H] in water shifts the Vt from -0.2 V to 0 V, while blending PTHS with PEDOT-S[(Oct2)NH2] in methanol shifts the Vt to -0.4 V. Meanwhile, although the OECT transconductances and ION/OFF of the different blends are quite similar to that of PTHS, the blends do exhibit faster response speeds than the neat PTHS. In another case, PEDOT-S has also been blended with poly(2-(3-thienyl)ethoxy-4butylsulfonate) (PTEB-S) to yield a single OECT that can exhibit electrochromic behavior

Figure 1 . 21 .

 121 Figure 1.21. In-situ GIWAXS analysis of humidity-induced swelling in P3HHT and the random copolymer P3HT-co-P3HHT. Shift of peak positions in P3HHT to lower Qz values with increasing relative humidity (RH%) indicates swelling of the material (top-left). Up to 2-3% increase in the lamellar stacking distance is observed in P3HHT at 100 %RH (topright). No apparent change can be observed for P3HT-co-P3HHT (bottom). Note that the swelling is reversible upon changing the environment from 100 to 0 %RH.

P3HT = 5

 5 kg mol -1 , ƉP3HT = 1.2, Mn, PEO = 20 kg mol -1 , ƉPEO ≤ 1.2), we evaluate the performance of their OECTs built on the general architecture illustrated in Figure2.1.

Figure 2 . 1 .Figure A. 2 )

 212 Figure 2.1. Schematic diagram of the OECTs used in this work. An Ag/AgCl pellet electrode was used as the gate (G) while patterned gold electrodes were used as the source (S) and drain (D), which were partially insulated from the electrolyte (0.1 M KCl aqueous solution) by a negative photoresist (SU-8).

Figure 2 . 2 .

 22 Figure 2.2. Output characteristics of an OECT with P3HT-co-P3HHT as the active channel material measured at a Vd sweep rate of 0.5 (left) and 0.25 V s -1 (right) to show shift of onset of Id saturation.

  ions, which are necessary to counterbalance the formation of charge bearing polarons and induce electronic conductivity across the device channel.

Figure 2 . 3 .ForFigure 2 . 4 )

 2324 Figure 2.3. Transfer characteristics of an OECT with P3HT-co-P3HHT as channel material plotted linearly (left) and logarithmically (right), showing large Id hysteresis and variation of ION/OFF across various Vg sweep rates (V s -1 ). Linearly plotted Ig is shown in dashed green lines.

Figure 2 . 4 .CHAPTER 2 :. 1 Figure 2 . 5

 242125 Figure 2.4. OECT output (top) and transfer characteristics (bottom) plotted linearly (left) and logarithmically (right) for a blend of P3HT-co-P3HHT with 25 wt% P3HT-b-PEO as active channel material measured at various Vg sweep rates (V s -1 ), showing minimal hysteresis in the Id loops and relatively similar ION/OFF.

Figure 2 . 5 .

 25 Figure 2.5. First derivatives (pink line-and-symbol plots, right axis) of OECT transfer curves (black lines, left axis) yielding the transconductance plots (gm) for neat P3HT-co-P3HHT (right) and its blend with 25 wt% P3HT-b-PEO (left), showing almost similar gm,max.

Equation 2 . 1 . 5 𝐼

 215 difference between the threshold voltages in the forward and reverse scan, ΔVt. The values of Vt are estimated by extrapolating the intercept of the line in plots of |Id| over Vg. This is in accordance with the definition of the OECT Id in the saturation regime, Id,sat, given in Where μ and C * is the hole mobility and volumetric capacitance of the channel material, respectively. As demonstrated in Figure 2.6, larger values of ΔVt are measured for the device of P3HT-co-P3HHT, reflecting the lag of Id due to slower rate of conductance change in the material relative to the Vg sweep rate. It follows that devices of the blend exhibiting less Id lag show less ΔVt.

Figure 2 . 6 .

 26 Figure 2.6. Extraction of the threshold voltage differences between the forward and reverse scans (ΔVt) of transfer curves of neat P3HT-co-P3HHT (left) and its blend with 25 wt% P3HT-b-PEO (right), showing significantly less ΔVt in the latter given similar biasing conditions.

Figure 2 . 7 .

 27 Figure 2.7. ΔVt and gm,max plotted against Vg sweep rate for devices of P3HT-co-P3HHT and its blend with 25 wt% P3HT-b-PEO, showing more stable performance of the blend over a wider range of sweep rates.

  continue our investigation of OECTs based on P3HT-co-P3HHT and its blends with P3HT-b-PEO by testing devices with larger channel dimensions (W = 2500 μm, L = 50, 110, 180, and 360 μm) to elucidate the factors of OECT behavior strongly influenced by

  of the devices be significantly manipulated. It follows then that higher values of gm are measured in larger devices of both materials, while devices of the blends all show less ΔVt than those of the neat P3HT-co-P3HHT, plotted in Figure2.9.

Figure 2 . 9 .

 29 Figure 2.9. Plots of gm (left) and |Id| 1/2 to extrapolate threshold voltages (right) for OECTs of neat P3HT-co-P3HHT (top) and its blend with 25 wt% P3HT-b-PEO (bottom) at various channel dimensions.

Wd L - 1 Figure 2 .

 12 Figure 2.10 shows that the values of gm measured from several devices of the neat

Figure 2 . 10 .

 210 Figure 2.10. Scaling of gm with |Vt -Vg|WdL -1 for OECTs of neat P3HT-co-P3HHT (left) and its blend with 25 wt% P3HT-b-PEO (right) at various Vg sweep rates. Data for micro-OECTs are plotted in red gradient while the rest correspond to large-scale OECTs. μC* extracted from model fits (dashed green line) is shown.

Figure 2

 2 Figure 2.11.

Figure 2 . 11 .

 211 Figure 2.11. ΔVt measured for various devices of neat P3HT-co-P3HHT plotted against the OECT channel length (top), thickness (middle), and width (bottom), showing no significant variation across each dimension at a given Vg sweep rate.

  -b-PEO L = 50 μm, d = 100 nm Channel width, W (μm)

Figure 2 .Figure 2 . 13 .

 2213 Figure 2.13. Transfer curves (top), plots of gm (middle), and |Id| 1/2 to extrapolate Vt (bottom) for OECTs of P3HT-co-P3HHT and its blends with P3HT-b-PEO at various indicated compositions, operated at sweep rates of 0.06 V s -1 (left) and 0.50 V s -1 (right).

Figure 2 .

 2 Figure 2.14 and Figure 2.15.

Figure 2 . 14 .

 214 Figure 2.14. Scaling of gm with |Vt -Vg|WdL -1 for OECTs of blends of P3HT-co-P3HHT and P3HT-b-PEO at various indicated compositions and Vg sweep rates. Average μC* extracted from model fits (dashed green line) is labelled.

Figure 2 . 15 .Figure 2 . 16 ,

 215216 Figure 2.15. ΔVt plotted against the OECT channel volume for various devices of blends of P3HT-co-P3HHT and P3HT-b-PEO at the indicated compositions.

Figure 2 . 16 .

 216 Figure 2.16. Summary of µC* (top) and average gate threshold voltage difference (ave.

2. 5 . 1 .

 51 Response to a Vg pulseThe transient drain currents in response to a Vg step are measured in devices of the neat P3HT-co-P3HHT and its blend with P3HT-b-PEO, plotted in Figure Figure2.17. Vd is limited to -0.4 V and Vg is limited to -0.6 V (-1 V total potential) to prevent rapid degradation of the devices during longer periods of biasing.

Figure 2 . 17 .

 217 Figure 2.17. Id over time upon applying a Vg pulse measured for OECTs of neat P3HT-co-P3HHT (left) and its blend with 25 wt% P3HT-b-PEO (right) of similar channel dimensions (W = 100 μm, L = 10 μm, d = 100±10 nm). Time constants extracted from exponential fits (red solid line) are labeled.

  Vg waveforms We further demonstrate the enhanced transient performance in OECTs of the blend by measuring their Id response to square, saw, and asymmetric triangle Vg inputs. Each measurement plotted in Figure 2.18 clearly demonstrates superior signal detection in OECTs of the blend over those of the neat material. Owing to its faster response speed, the drain currents in devices of the blend can almost replicate the different waveforms with significantly less distortion than that of the neat material and at higher Id magnitudes. The Id transient in response to a symmetric triangle wave (top-right in Figure 2.18) shows the best quality of reproduced signal owing to the gradual rise and fall of the input Vg, in contrast with the sharp steps of the square wave input (top-left in Figure 2.18). We also observe that application of a sawtooth-type asymmetric triangle wave (bottom-left in Figure 2.18), where Vg is ramped up and then stepped back down to the initial value, yields sharper Id peaks than those resulting from application of an inverse sawtooth triangle wave (bottomright in Figure 2.18). This highlights faster discharging of the OECTs, which further implies easily reversible doping behavior in the channel material. Overall, these results provide a

Figure 2 . 18 .

 218 Figure 2.18. Normalized Id over time upon applying various Vg waveforms (square, saw, and asymmetric triangle waves) measured for OECTs of neat P3HT-co-P3HHT and its blend with 25 wt% P3HT-b-PEO at similar channel dimensions (W = 100 μm, L = 10 μm, d = 100±10 nm).

  evaluate the impact of blending modification of the mixed conductor on the performance of its electrochemical devices. Devices of neat P3HT-co-P3HHT exhibit slow response speeds indicated by the large Id hysteresis in their transfer curves (Id vs Vg) and long response time (τ = 8 s) extracted from the Id transients (Id vs t) upon application of a step potential. As a result, the transduction performance of the OECTs of P3HT-co-P3HHT, indicated by gm, is highly sensitive to operating speed within the range of parameters investigated here, decreasing by over two orders of magnitude when the Vg sweep rate is increased from 0.1 to 1 V s -1 . Similar behavior is also observed in devices of various channel dimensions, which -

2

 2

Figure 3 . 1 .

 31 Figure 3.1. UV-Vis absorption spectra of P3HT-co-P3HHT and its blends with P3HT-b-PEO. No significant change in the A0-0/A0-1 ratio implies that blending does not strongly influence the short-range aggregation behavior of P3HT-co-P3HHT.

Figure 3 . 2 .A. 3 . 2 Figure 3 . 3

 323233 Figure 3.2. GIWAXS patterns of P3HT-co-P3HHT, P3HT-b-PEO, and their blends, as labelled. Distinct diffraction features corresponding to the ordered crystallites of each component can be observed in the blends, implying that blending does not disrupt the long-range ordering of each component.

Figure 3 . 3 .

 33 Figure 3.3. Thermograms of solution-cast P3HT-co-P3HHT and its blends with P3HT-b-PEO from the first heat (left-most), cool (middle), and second heat (right-most). The presence of melting peaks corresponding to each component in the first heating scan of the blends suggests a phase separated structure.

Figure 3 . 4 , 6 Figure 3 . 4 . 3 . 4 .

 3463434 Figure 3.4. Enthalpies of the P3HT (black, left axis) and PEO (pink, right axis) moieties in blends of P3HT-co-P3HHT and P3HT-b-PEO cast from solution (left) and cooled from the melt (right), extracted from thermograms. The dashed lines indicate "ideal" values for completely phase-separated components. Data at 0 wt% P3HT extracted from measurements of a PEO homopolymer (open symbol) with the same molecular weight as the PEO moiety in the block copolymer (Mn,PEO = 20 kg mol -1 ). Enthalpies below the ideal line indicates suppressed crystallization of the component in the blend relative to neat films.

Figure 3 . 5 .

 35 Figure 3.5. Optical microscopy images of P3HT-co-P3HHT and its blends with P3HT-b-PEO in transmittance mode (left) and corresponding cross-polarized mode (right), showing no distinct large-scale domains in the blends. Scale bar measures 200 μm.

Figure 3 . 6 .

 36 Figure 3.6. Water contact angles of P3HT-co-P3HHT and its blends with P3HT-b-PEO, suggesting increased hydrophilicity with higher block copolymer content.

Figure 3 . 7

 37 plots the voltammograms of P3HT-co-P3HHT and its blends with P3HTb-PEO of various compositions (0-45 wt%) measured in organic (left) and aqueous electrolyte (right). Slow scan rates were used after observing that it was more difficult to identify peak anodic currents with faster cycling (see appendix section A.4.1 Cyclic voltammetry, Figure A.5). Quick inspection of the graphs gives the impression that blending does not drastically change the oxidation behavior of the material for a given electrolyte, electrode area, and scan rate. This implies that similar potentials can be applied to probe the electrochemical response of the different blends and obtain comparable behavior.

Figure 3 . 7 .

 37 Figure 3.7. Cyclic voltammograms of P3HT-co-P3HHT and its blends with P3HT-b-PEO of different compositions as indicated (wt%) measured at 2 mV s -1 against Ag/AgCl in organic (left) and aqueous (right) electrolyte. Ferrocene in organic electrolyte is also shown for reference (E1/2 = 0.48 V).

Figure 3 . 8 .

 38 Figure 3.8. Anodic currents extracted from cyclic voltammetry sweeps of P3HT-co-P3HHT and its blends with P3HT-b-PEO measured in organic (transparent lines) and aqueous (opaque lines) electrolyte. Solid green lines indicate oxidation onset and peak anodic potential of neat P3HT-co-P3HHT measured in 0.1 M TBAPF6 for reference. Peak anodic currents indicated by red ticks.

2 )

 2 These findings lead us to wonder whether blending modification with an ionconducting component can enhance the oxidation behavior of hydrophobic conjugated -in aqueous media, without engineering substituents on the active semiconducting component. To test this, we characterized blends of P3HT-b-PEO with a non-hydroxyhexyl substituted P3HT of molecular weight similar to that of the P3HT-co-P3HHT (Mn = 30 kg mol -1 ). Figure3.9 plots the voltammograms measured for P3HT-co-P3HHT, P3HT, and blends of P3HT with 25 wt% P3HT-b-PEO in organic (left) and aqueous (right) electrolyte. The values of the oxidation onsets and peak anodic currents are tabulated in Table3.3.

Figure 3 . 9 .Table 3 . 3 .

 3933 Figure 3.9. Cyclic voltammograms of P3HT, its blend with 25 wt% P3HT-b-PEO, and P3HT-co-P3HHT measured against Ag/AgCl as reference at 10 mV s -1 in organic (left) and aqueous (right) electrolyte. Ferrocene in organic electrolyte is also shown for reference (E1/2 = 0.4 V).

CHAPTER 3 :

 3 Modulation of electrochemical doping kinetics | 123provides a simple way to directly probe the electrochemical doping behavior of the polymer semiconductor via its optical properties.

Figure 3 . 10 .

 310 Figure 3.10. Schematic diagram of experimental setup used for spectroelectrochemical measurements.

Figure 3 .

 3 11 shows that the blends approach steady state conditions much faster than the neat material.

Figure 3 . 11 .

 311 Figure 3.11. Normalized UV-Vis absorbance spectra of P3HT-co-P3HHT and its blends with P3HT-b-PEO measured upon application of a step potential (left). Color map showing evolution of spectra over time (right).

CHAPTER 3 :

 3 Modulation of electrochemical doping kinetics | 125

Figure 3 . 12 .

 312 Figure 3.12. Normalized peak absorbance of P3HT-co-P3HHT and its blends with P3HTb-PEO plotted over time at various indicated biasing conditions. Data are for films of similar thickness (d = 110 ± 10 nm) to facilitate comparisons.

Figure 3 . 13 ).

 313 This trend correlates with the greater hydrophilicity of the films, which might contribute to decreasing the energy barriers associated with the injection of hydrated ions into the bulk necessary to electrochemically dope the polymer. Faster absorbance change is also measured in thinner films of P3HT-co-P3HHT (bottom-left panel of Figure3.13), which suggests that doping occurs throughout the bulk of the film even in the relatively more hydrophobic material. However, the blends generally show faster absorbance transients in thicker films (210 nm), approaching steady state within a second of applying bias (bottom-right panel of Figure3.13), than the thinnest films of the neat material (110 nm), only approaching steady state after 3 seconds. This behavior demonstrates the efficacy of blending to tune the electrochemical doping kinetics of the material.

Figure 3 . 13 .CHAPTER 3 :

 3133 Figure 3.13. Normalized peak absorbance transients rescaled 0-1 (norm. rel. abs) ofP3HT-co-P3HHT and its blends with P3HT-b-PEO (film thickness d = 110 ± 10 nm) upon application of bias (top). Normalized peak absorbance transients of P3HT-co-P3HHT

Figure 3 . 14 .

 314 Figure 3.14. First-order kinetic model fits (solid red line) to the initial 90% of normalized absorbance transients of P3HT-co-P3HHT and its blends with P3HT-b-PEO to extract electrochemical doping rates (k).

Figure 3 . 15 . 3 . 1 )Figure 3 . 16 .

 31531316 Figure 3.15. Summary of electrochemical doping rates (k) extracted for P3HT-co-P3HHT and its blends with P3HT-b-PEO at various film thicknesses (left). Thickness-normalized rates (kd -1 ) plotted against blend composition (right).

Figure 3 . 17 .

 317 Figure 3.17. Summary of decay rates extracted for P3HT-co-P3HHT and its blends with P3HT-b-PEO of similar film thickness (110±10 nm) using biphasic exponential functions, plotted linearly (left) and logarithmically (right).

  about 30-60 seconds depending on the blend composition and film thickness, based on observations during spectroelectrochemical measurements). The frequency-dependence of the impedance magnitude (|Z| vs f), imaginary part of the impedance (-Zim vs f), and phase angle (θ vs f), as well as the Nyquist plots (Zim vs Zre) measured for the different blends are shown in Figure 3.18.

Figure 3 . 18 .

 318 Figure 3.18. Impedance magnitude (|Z|, top-left), imaginary impedance (-Zim, top-right) and phase angle (bottom-left) plotted against frequency, and Nyquist plots (Zre vs -Zim, bottom-right) of P3HT-co-P3HHT and its blends with P3HT-b-PEO. Scatter plots refer to experimental data while solid lines correspond to equivalent circuit model fits.

Figure 3 . 19 .

 319 Figure 3.19. Equivalent circuit model fitted to experimental impedance data in Figure 3.17

(

  solid green lines) consisting of a resistor (RS) in series with a constant phase element (CPEi) and resistor (Ri) in parallel, also in series with a capacitor (C) and resistor (RP) in parallel.

Figure 3 .

 3 20 show relatively small variation in the values between the neat P3HT-co-P3HHT and its blends with P3HT-b-PEO under similar doping conditions (DC offset at 0.75 V).

Figure 3 . 20 .

 320 Figure 3.20. Summary of volumetric capacitances calculated for P3HT-co-P3HHT and its blends with P3HT-b-PEO, showing relatively similar values.

Figure 4 . 1

 41 plots the transfer characteristics and transconductances measured for devices of P3HT-co-P3HHT and its blends comprising 25 wt% P3HT-b-PEO. The OECT of the neat material

Figure 4 . 1 .

 41 Figure 4.1. OECT transfer characteristics of P3HT-co-P3HHT and its blends with P3HT-b-PEO before and after 1 week of repeated measurements and storage in water (left). Color maps showing evolution of gm in devices constantly stored in water over time (right).

Figure 4 . 2 . 4 . 1 . 1 ∆𝑓

 42411 Figure 4.2. Evolution of OECT Id transients from devices of P3HT-co-P3HHT and its blend with P3HT-b-PEO stored in water over time.

Figure 4 . 3 suggest

 43 that the film is structurally stable when immersed in water. A quartz sensor coated with the neat material exhibits a frequency shift of Δfdry ≈ -0.61 kHz relative to the initial frequency of the uncoated sensor in nitrogen gas environment. According to Equation 4.1,

Figure 4 . 3 .

 43 Figure 4.3. Shifts of the fundamental frequency (overtone order n = 1) of a QCM sensor before ("blank") and after coating with P3HT-co-P3HHT, measured under N2 gas atmosphere (Δfdry) and while immersed in water (Δfwet,x) for x consecutive dry-wet cycles (dark to light pink solid lines indicate x = 1, 2, and 3, in that order). The frequency change upon immersing the blank quartz crystal sensor in water is also shown (Δfwet,0).

Figure 4 . 4 .

 44 Figure 4.4. Shifts in the frequency (overtone order n = 1) of QCM sensors before ("blank") and after coating with the 25 wt% P3HT-b-PEO blend under N2 gas (Δfdry), and in water (Δfwet,x) for x consecutive dry-wet cycles (dark to light pink solid lines indicate x = 1, 2, and 3, in that order).

Figure 4 . 5

 45 Figure 4.5 and tabulated in Table 4.1 provides evidence of a phase-separated blend. The

Figure 4 . 5 .

 45 Figure 4.5. Thermograms of P3HT-co-P3HHT and its blend with PEO during the 1st heat, cool, and 2nd heat scans.

Figure 4 . 6 . 1 )CHAPTER 4 :

 4614 Figure 4.6. Enthalpies of the P3HT (black, left axis) and PEO (pink, right axis) moieties in blends of P3HT-co-P3HHT and PEO cast from solution (left) and cooled from the melt (right), extracted from thermograms. The dashed lines indicate "ideal" values for completely phase-separated components.

Figure 4 . 7 .

 47 Figure 4.7. Optical microscopy images of PEO and its blend with P3HT-co-P3HHT in transmittance mode (left) and corresponding cross-polarized mode (right), showing no large-scale phase separation. Scale bar measures 200 μm.

Figure 4 . 8 .

 48 Figure 4.8. UV-Vis absorption spectra of P3HT-co-P3HHT and its blend with PEO.

Figure 4 . 9

 49 plots the water contact angles measured for the neat P3HT-co-P3HHT and its blends with 25 wt% P3HT-b-PEO and 25 wt% PEO.

Figure 4 . 9 .

 49 Figure 4.9. Water contact angles of P3HT-co-P3HHT and its blends with P3HT-b-PEO and PEO.

CHAPTER 4 :

 4 Stability of mixed conducting blends | 157

Figure 4 . 10 .

 410 Figure 4.10. Compiled water contact angles measured from blends with P3HT-b-PEO and PEO, showing plateau above 20 wt% PEO.

  aqueous electrolyte (0.1 M KCl), given in Figure4.11. The PEO blends do exhibit oxidation onsets and peak anodic potentials close to that of the blends with the block copolymer and the neat P3HT-co-P3HHT, as indicated in Table4.2.

Figure 4 . 11 .

 411 Figure 4.11. Cyclic voltammograms of films of P3HT-co-P3HHT and its blend with PEO in aqueous electrolyte measured at a scan rate of 2 mV s-1. Oxidation onsets and peaks are indicated (dashed red lines and ticks). Blend with P3HT-b-PEO is also shown for comparison.

CHAPTER 4 :

 4 Stability of mixed conducting blends | 159 accelerate doping of the material with aqueous electrolyte. The absorption spectra change depicted in Figure 4.12 reflects oxidation of the thiophene aggregates and formation of positive polarons (p-type doping) upon application of bias (0.75 V). The peak absorption (λ = 560 nm) decreases to half the initial value (norm. abs. ≈ 0.5) at steady state within 500 ms, faster than what was observed for the neat P3HT-co-P3HHT (see section 3

Figure 4 . 12 .

 412 Figure 4.12. Evolution of absorption spectra over time upon application of a step bias (0.75 V) to a blend of P3HT-co-P3HHT and PEO cast on an ITO electrode in aqueous electrolyte.

Figure 4 .

 4 Figure 4.13 for different film thicknesses (d), allows us to extract the electrochemical doping

Figure 4 . 13 .

 413 Figure 4.13. Normalized absorption change (rescaled 0 to 1) at initial peak wavelength (λ = 560 nm) in blends with PEO of various thicknesses (left). Doping rates (k) plotted against thickness for P3HT-co-P3HHT and its blends with P3HT-b-PEO and PEO (right).

Figure 4 .Figure 4 . 14 .Figure 4 . 15 .

 4414415 Figure 4.14. Thickness-normalized rate constants (kd -1 ) plotted against PEO content using compiled data from blends with either PEO or P3HT-b-PEO, showing a plateau of the rate increase above 20 wt% PEO. Data from water contact angle measurements also shown (pink transparent plot, right axis) for comparison.

3 . 3 . 3 . 1

 3331 Equivalent circuit model-fitting) is employed to fit the experimental data (solid lines in Figure 4.15) and extract the parameters tabulated in Table 4.3. For the blend with PEO, we determine a C* value of about 200±30 F cm -3 , which is essentially the same as that obtained for the blends with the block copolymer and the neat P3HT-co-P3HHT, as shown in Figure 4.16.

Figure 4 . 16 . 3 )Figure 4 . 17 .

 4163417 Figure 4.16. Summary of volumetric capacitances, C*, calculated from values extracted from equivalent circuit model fits to impedance data of P3HT-co-P3HHT and its blends with P3HT-b-PEO and PEO.

Figure 4 . 18 .Figure 2 . 17 ).

 418217 Figure 4.18. OECT transient performance upon application of a Vg pulse for a blend of P3HT-co-P3HHT and PEO. The time constant extracted from an exponentiall fit (solid red line) is shown.

Figure 4 . 19 .

 419 Figure 4.19. Scaling of gm with |Vt -Vg|WdL-1 for OECTs of blends of P3HT-co-P3HHT and PEO at the indicated Vg sweep rate. μC* extracted from model fit (dashed green line) is shown.

4. 4 . 1 .

 41 Preliminary QCM analysis QCM measurements of the blends with PEO plotted in Figure4.20 do seem to indicate that the film partially delaminates when immersed in water. The quartz sensor exhibits a frequency shift of Δfdry ≈ -1 kHz relative to the initial frequency of the uncoated sensor in the dry state. Using Equation 4.1, this is equivalent to an areal density of about 18 μg cm -1 and translates to a film thickness of approximately 180 nm, which is near the values of the thickness measured using profilometry on previous films cast using similar protocols. During the first immersion in water the frequency of the coated quartz decreases further to about -1.8 kHz, yielding a difference of Δfwet,1 ≈ -0.9 Hz between the wet coated sensor and wet uncoated sensor (Δf0,wet ≈ -0.87 Hz). The change from Δfdry to Δfwet,1 by about +0.1 Hz might suggest that during the first immersion in water the film already starts to lose around 2 μg cm -1 worth of material (≈ -10%). In subsequent immersions the frequency shifts even more to Δfwet,2 = Δfwet,3 ≈ -0.8 Hz, equivalent to a total change of +0.2 Hz relative to the dry coated sensor and indicating a cumulative decrease of areal mass by approximately 4 μg cm -1 (≈ -20%). Conversely, we recall that the blends with P3HT-b-PEO exhibit just up to 10% mass loss, which we observe only after the first immersion (see section 4.2.2 Stability in water: preliminary QCM investigation, Figure4.4). We gather from these preliminary investigations that the blends with PEO potentially disintegrates faster and to a greater extent than the blends with P3HT-b-PEO when interfaced with waterrich media.

Figure 4 . 20 .

 420 Figure 4.20. Shifts of the fundamental frequency (overtone n = 1) of a QCM sensor before ("blank") and after coating with P3HT-co-P3HHT blended with PEO under N2 gas atmosphere (Δfdry) and while immersed in water (Δfwet,x) for x consecutive dry-wet cycles (dark to light pink solid lines indicate x = 1, 2, and 3, in that order). The frequency change upon immersing the blank quartz crystal sensor in water is also shown (Δf0,wet).

  Df 0,wet Df dry left panel of Figure 4.21 decline rapidly, reaching insignificant values after just two days. As expected, the performance loss in devices of the blends with PEO occurs at a faster rate than what was observed for blends with the block copolymer, the latter exhibiting steady transfer characteristics up to two weeks before degradation (refer to Figure 4.1).

Figure 4 . 21 .

 421 Figure 4.21. OECT transfer characteristics of P3HT-co-P3HHT and its blends with PEO before and after 1 week of repeated measurements and storage in water (left). Color maps showing evolution of gm in devices stored in water over time (right).

4. 4 . 3 .

 43 OECT transient characteristicsExamining the evolution of the OECT transient currents during continuous immersion in water over time, depicted in Figure4.22, verifies that the observed loss in performance originates solely from the retardation of the device response and not from diminished electronic properties. Specifically, no significant change in the drain current at steady state (ISS) upon application of a Vg step from 0 to -0.6 V is detected after 1 month, which implies that the conductivity of the doped channel at equilibrium is preserved. This is in accordance with the electronic stability observed in devices of the neat P3HT-co-P3HHT presented here (see Figure4.1 and Figure4.2) and in previous work.6 On the other hand,

Figure 4 . 22 . 35 CHAPTER 4 :

 422354 Figure 4.22. Evolution of OECT Id transients from devices of P3HT-co-P3HHT blended with PEO stored in water over time.

Figure 4 . 23 .

 423 Figure 4.23. Evolution of steady state Id (ISS) and time constants (τ) extracted from exponential model fits to transient data measured for OECTs of P3HT-co-P3HHT and its blends with P3HT-b-PEO and PEO stored in water over time.

  or delamination induced by the water-soluble PEO-rich phases. Consequently, upon evaluating the long-term operation stability of OECTs that have been continuously immersed water, devices of the blends with the block copolymer sustain faster transient performance than those of the neat P3HT-co-P3HHT and its blend with the PEO homopolymer. While OECTs comprising P3HT-b-PEO do show around 50-fold slower response time after one month stored in water, devices of the blend with the PEO General conclusions and perspectives | 179 homopolymer degrade more rapidly (500-fold slower) and approaches that of the devices made of the neat P3HT-co-P3HHT.

  through a steel mask onto cleaned glass slides. The chromium layer was integrated to allow better adhesion of gold onto the glass surface. Each substrate consisted of electrodes worth three devices. For practical reasons, the large devices did not consist of a patterned insulator between electrodes. Instead, the devices were parted by simply scratching or wiping off the semiconducting polymer around the active channel area and electrode interconnects. A PDMS well was positioned over the channels to hold an aqueous electrolyte of 0.1 M of KCl (Sigma-Aldrich) dissolved in deionized water (18.2 MΩ, dispensed from Purelab Flex, ELGA LabWater). The circuit was completed by submerging an Ag/AgCl cylindrical pellet electrode (Warner Instruments, height = 2 mm, diameter = 2 mm) in the electrolyte as the gate electrode. We note that measurements of the transconductances and μC* parameter of devices of P3HT-co-P3HHT in Figure A.3 (Vd = -0.6 V, Vg sweep rate = 0.04 V s -1 , |Vt -Vg| = 0.2 V) show that no significant difference in performance was observed depending on how the large devices were separated, indicating a relatively robust architecture.

Figure A. 3 .

 3 Figure A.3. Scaling of transconductances (gm, left) lead to values of μC* (right) measured for various large devices of P3HT-co-P3HHT. No significant difference is observed whether the devices are separated either by scratching the polymer film or completely wiping the contact electrodes, as shown in the inset images.

1 A. 4 . 1

 141 t -V g | Wd L -1 (V cm -1 )P3HT-co-P3HHT V g sweep rate: 0.04 V s -Cyclic voltammetry Cyclic voltammograms of polymer-coated ITO electrodes were measured at room temperature (22 °C) against a Pt/Ti wire as counter electrode (ET078, eDAQ) and Ag/AgCl reference electrode (MF-2056, BASi) in 0.1 M TBAPF6 in acetonitrile (Sigma-Aldrich) as organic electrolyte and 0.1 M KCl (Sigma-Aldrich) as aqueous electrolyte (active area = 0.18 cm 2 ). Voltammograms were measured over four cycles and showed no significant changes for all materials investigated. Results from the fourth cycle was reported here for each material.Anodic peak currents in the voltammograms of P3HT-co-P3HHT and its blends were more easily identifiable within the limits of the potential window that could be employed with an aqueous electrolyte (-0.4 to 0.8 V) at slower cycling speeds (see Figure A.5). Hence, only results from slow scan rates (2 mV s -1 ) were discussed in main text. Nonetheless, the voltammograms measured at faster scan rates (30 mV s -1 ) seem to indicate lower oxidation onset potentials in the blend (≈ 0.1 V) than in the neat P3HT-co-P3HHT (≈ 0.3 V), similar to what was observed in the results from slower measurements and discussed in the main text.

Figure A. 5 .

 5 Figure A.5. Cyclic voltammograms of P3HT-co-P3HHT measured against Ag/AgCl as reference at various indicated scan rates in 0.1 M KCl aqueous electrolyte. Identifiable peak anodic current is indicated (solid red tick).

  .2.1. Basic operating principles ............................................................................... 1.2.2. Physical model of OECTs ............................................................................... 1.2.2.1. Steady-state characteristics .....................................................................

1.1. Introduction ........................................................................................................... 1.2. Organic electrochemical transistors....................................................................... 11.2.2.2. Transient characteristics .......................................................................... 1.2.3. Influence of channel geometry on OECT performance ................................... 1.3. Key features of mixed ionic-electronic conducting polymers .................................. 1.3.1. Semiconducting behavior of conjugated polymers .......................................... 1.3.2. Doping conjugated polymers .......................................................................... 1.3.3. Ionic transport in mixed conducting polymers ................................................. 1.3.4. Volumetric capacitance in mixed conducting polymers ................................... 1.3.5. Electronic charge transport in conjugated polymers ....................................... 1.4. Progress in mixed conducting polymer design ...................................................... 1.4.1. Early mixed conducting polymers: Ppy and PANI ........................................... 1.4.2. Polythiophene derivatives ............................................................................... 1.4.3. Conjugated polyelectrolytes ........................................................................... 1.4.4. Diketopyrrolopyrrole-based conjugated polymers ........................................... 1.4.5. N-type polymers ............................................................................................. 1.4.6. Multicomponent polymer systems .................................................................. 1.5. Summary and research direction ........................................................................... 1.6. References ............................................................................................................

.3.3.1 Equivalent circuit model-fitting), among

	1.3.4. Volumetric capacitance in mixed conducting polymers
	Another consequence of incorporating ions in the bulk of conjugated polymer films
	is that they may interact electrostatically with oppositely charged species present in the
	polymer material, which may either be the counterions in conducting polymer electrolytes
	(e.g., sulfonate anion in PEDOT:PSS upon hole extraction) or the charge carriers formed
	upon doping conjugated polymers. This interaction is identified as the origin of the large
	volumetric capacitances in mixed conducting polymers. The concept was initially introduced
	to explain the charging behavior of depletion-type PEDOT:PSS electrodes that exhibit ideal
	volume-dependent capacitance. 6,43 Since then, C* has been adapted as a practical indicator
	of ion-electron coupling in both depletion-and accumulation-type mixed conducting
	polymers, and can be extracted from equivalent circuit model fits to the experimental
	impedance spectra of films of various thicknesses (employed in this work, see section
	3
	). For polymers that are
	completely functionalized with hydrophilic sidechains, excessive water uptake
	("uncontrolled") can lead to irreversible mass changes and material disintegration.
	Schematic from ref. 42 .

other methods.

Table 1 . 1 .

 11 OECT and mixed conductor performance indicators of some recent materials.

	Polymer ref	W	L	d	Vt gm,max Ion/off τon	τoff	μOECT	C*	μC*
		μm	μm nm	V	mS		ms	ms cm 2 V -1 s -1 F cm -3 F cm -1 V -1 s -1
	P-type materials operating in accumulation mode
	p(g2T2-g4T2) 64	100	10	50	0.0	6.5	10 5			1.7	190	5x10 2
	p(gDPP-T2) 65	100	10	90	-0.5 6.3	10 5			1.6	200	3x10 2
	p(g2T-TT) 39	100	10	40	-0.2 5.4	10 5	0.4 0.04	9.4x10 -1	240	3x10 2
	p(g2T-T) 16	100	10	100 0.0	7.9	10 5	1.4		3x10 -1	200	2x10 2
	p(g3T2) 64	100	10	80	0.0	8.9	10 5			9x10 -1	160	2x10 2
	p(gDPP-TT) 65	100	10	120 -0.5 3.1	10 5			5.7x10 -1	180	1x10 2
	p(gDPP-MeOT2) 65	100	10	80	-0.3 1.4	10 4			2.8x10 -1	170	6x10 1
	PDPP[T]2{TEG}3-MEET 66			60	0.4	1.9	10 3	1.9		1.3x10 -1	340	5x10 1
	P3HHT 67	100	10	140 -0.6	15	10 5	2			4x10 1
	p(gDTDPP-bis-EDOT) 68	60	30	290 -0.1 0.4	10 2	22	16	8.7x10 -2	190	2x10 1
	PDPP[T]2{TEG}-EDOT 66			40	0.4	1.4	10 3	1.4		8.4x10 -2	170	1x10 1
	P3MEEET 69	100	10		-0.6					3.5x10 -2	240	1x10 1
	ProDOT(OE)-co-DMP 70	50	50	280 -0.5 0.6	10 5			6.3x10 -2	110	1x10 1
	P3MEEMT 69	100	10		-0.7					7x10 -2	160	10
	PTHS + EG (5 vol%) 71	250	5	60	-0.8 2.0	10 2	0.4		1.3x10 -3	120	6
	p(gBDT-g2T) 16	100	10	110 -0.3 0.5	10 3			1x10 -2	80	4
	p(aDTDPP-bis-EDOT) 68	60	30	190 -0.5 0.1	1	2700 200	9.5x10 -2	20	2
	p(gPyDPP-MeOT2) 72	100	10	120 -0.5 0.2	10 5	0.8	0.5	3x10 -2	60	2
	p(gDTDPP-aBT) 68	60	30	120 -0.6 0.1	10 2 2800 800	4.4x10 -2	20	1
	P3MEET 69	100	10		-0.2					5x10 -3	80	4x10 -2
	P-type materials operating in depletion mode
	PEDOT-S 73	1125 75	20	0.2	16	10 3	360 300		2x10 2
	N-type materials operating in accumulation mode
	BBL 74	100	10	80	0.2 0.07 10 4	5		2.1x10 -3	730	2
	pgNaN 75	100	10 1500 0.3	0.2	10 4	100		6.5x10 -3	100	7x10 -1
	p(gNDI-gT2) 76	100	10	200 0.4 0.02 10 3	5	5	3x10 -4	400	2x10 -1
	p(gNDI-T2) "P-90" a 77	100	10	50		0.001 10 2	40		2.4x10 -4	200	5x10 -2
	p(gNDI-T2) 42	100	10		0.3 0.001					4x10 -2
	pgNgN 75	100	10	500 0.2 0.003 10 3			1.9x10 -4	240	4x10 -2
	p(gNDI-g3T2) 42	100	10		0.3 0.001					1x10 -2
	Multicomponent materials operating in accumulation mode
	PTHS:PEDOT-S[(Oct2)NH2] b 73 39000 c 20	112 -0.4	15	10 4	80	10
	PTHS:PEDOT-S[H] b 73	39000 c 20	102	0	14	10 4	90	20
	PEDOT-S:PTEB-S (3:2) b 78	39000 c 20	55	0	12	10 2	192	40
	P3HT:PEO 79	1000 30	120 -0.3 0.08 10 1				6x10 -1
	Multicomponent materials operating in depletion mode
	Crys-P 55	80	20	200 < 0	19				4	100	5x10 2
	PEDOT:PSS / [EMIM][TCM] 80	1000 100 200	0	14	10 3	4		4	100	3x10 2
	PEDOT:PSS + EG, DBSA 6,14,81	10	10	400	0	4		0.1		2	40	5x10 1
	a

  3.1. Introduction ......................................................................................................... 3.2. Physical properties of P3HT-co-P3HHT blended with P3HT-b-PEO .................... 3.2.1. Solid-state structure ..................................................................................... 3.2.2. Thermal properties .......................................................................................

3.2.3. Surface polarity ............................................................................................ 3.3. Electrochemical properties .................................................................................. 3.3.1. Cyclic Voltammetry ....................................................................................... 3.3.2. Spectroelectrochemistry ............................................................................... 3.3.2.1. Electrochemical bleaching ..................................................................... 3.3.2.2. Electrochemical doping rates ................................................................. 3.3.2.3. Monophasic exponential kinetic model .................................................. 3.3.2.4. Biphasic exponential kinetic model ........................................................ 3.3.3. Impedance behavior ..................................................................................... 3.3.3.1. Equivalent circuit model-fitting ............................................................... 3.3.3.2. Volumetric capacitance, C* .................................................................... 3.4. Summary ............................................................................................................ 3.5. References ..........................................................................................................

Table 3 .

 3

			1 st Heat	Cool	2 nd Heat
	P3HT-b-PEO	Mass	Tm1,end
	content (wt%)	(mg)	(°C)

1. Transition temperatures and enthalpies extracted from thermograms of blends of P3HT-co-P3HHT and P3HT-b-PEO.

Table 3 .

 3 Using the extracted values of C for films with different thicknesses, we could calculate the average C* of our materials. The results summarized in

	4. Parameters extracted from equivalent circuit model fits to experimental
	impedance data of P3HT-co-P3HHT and its blends with P3HT-b-PEO of various film
	thicknesses.
	P3HT-b-PEO content (wt%)	Thickness (nm)	RS (Ω)	Q (µS s n )	n	Ri (Ω)	RP (kΩ)	C (µF)	C* (F cm -3 )
	0	110 ± 10 217	28.8	0.643 1170	14	293	190 ± 30
		150 ± 10 181	51	0.569 803	7	542
		210 ± 10 179	19	0.657 1270	4	720
	10	110 ± 10 232	138	0.512 302	9.9	450	212 ± 6
		140 ± 10 206	129	0.509 318	7.5	550
		200 ± 10 213	122	0.505 339	5.5	750
	25	90 ± 10	198	5500	0.35	90	10.9	346	200 ± 20
		110 ± 10 205	6000	0.40	90	9.8	422
		190 ± 10 203	5100	0.50	40	4.3	630
	45	90 ± 10	224	6300	0.38	70	15.5	256	170 ± 20
		110 ± 10 232	6700	0.42	140	11.8	324
		140 ± 10 231	5000	0.52	130	8.9	490

Table 4 . 1 .

 41 Transition temperatures and enthalpies from thermograms of P3HT-co-P3HHTand its blends with PEO.

			1 st Heat	Cool	2 nd Heat
	PEO content	Mass	Tm1,end
	(wt%)	(mg)	(°C)

  Normalized to weight fraction of PEO moiety; b Normalized to weight fraction of P3HT and P3HHT moieties;

						40	--	--	195	-17	--	--	233	26
	25	1.4	70	159	254	40	20	-77	178	-6	68	91	220	10
	100	1.3	71	171	--	--	46	-157 --	--	71	162	--	--

a

Table 4 .2. Oxidation

 4

	Polymer	Eox,on (V)	Epa (V)
	P3HT-co-P3HHT	0.16	0.69
	+ 25 wt% P3HT-b-PEO	0.12	0.66
	+ 25 wt% PEO	0.14	0.66

onsets and peaks extracted from voltammograms of polymers measured against Ag/AgCl as reference at 2 mV s -1 in 0.1 M KCl.

Further examination of the oxidation behavior using spectroelectrochemistry to probe the electrochemical doping kinetics reveals that blending with PEO does indeed

Table 4 .3. Parameters

 4

	PEO content (wt%)	Thickness (nm)	RS (Ω)	Q (µS s n )	n	Ri (Ω)	RP (kΩ)	C (µF)	C* (F cm -3 )
	25	110 ± 10 234	5700	0.34	140	10	340	190 ± 30
		120 ± 10 230	3300	0.30	170	11	400
		160 ± 10 260	4000	0.30	220	7.5	640
	Fitting the impedance data with an appropriate equivalent circuit model allows us to

extracted from equivalent circuit model fits to EIS spectra measured for blends with PEO.

extract the capacitance of the doped material of known thickness, from which the volumetric capacitance (C*) may be calculated. A modified Randle's circuit (RS(CPEi||Ri)(C||RP), see

P3HT-co-P3HHT

Available at: https://www.thinksrs.com/downloads/pdfs/manuals/QCM200m.pdf.

blend compositions. This further implies that a large fraction of the neat P3HT-co-P3HHT itself does not participate in polaron formation and transport. We suspect then that the electrical properties of the bulk material may be relatively insensitive to blending with an electronically insulating component, as long as it maintains the necessary structural order of the active semiconducting component in the resulting films. We believe this is achieved in the compatibilized blends and, therefore, explains the sustained C* and the similar electrical performance of their OECTs relative to that of the neat material.

where Cf is the mass sensitivity factor specific to the quartz sensor (56.6 Hz μg -1 cm 2 for a 5 MHz crystal driven at its fundamental frequency). Thus, an increase of the resonance frequency typically indicates a decrease of mass at the surface of the sensor.

We note that quantification of mass changes using Equation 4.1 is only possible in the case of homogeneous rigid thin film coatings such as metals or dry polymers. However, when analyzing soft viscoelastic materials like polymer films in contact with a liquid, the viscoelastic properties of the material and the surrounding liquid must be considered for accurate calculation of mass changes since these can effectively dampen the crystal oscillation. 2,3 It is therefore useful to measure both Δf and the dissipation factor (ΔD) describing the oscillation damping, which can be related by viscoelastic models to the deposited material's thickness, density, viscosity, and shear modulus, as well as the density and viscosity of the surrounding media. 4 Furthermore, for more accurate fits of the data to such models, measurements are typically done at multiple higher-order overtones of the fundamental frequency (n = 3, 5, 7, 9, 11, and 13). 5 However, due to the limitations of the QCM equipment used in this work (see appendix section A.5 Quartz crystal microbalance for details on instrumentation and methodology), we could not access ΔD nor Δf at higherorder overtones n > 1. Therefore, the preliminary results presented here provide only a crude first impression of the behavior of the films when immersed in water.

The resonance frequency of the blank quartz sensors were first tested in "dry" conditions (f0 ≈ 5 MHz for AT-cut and polished quartz crystals with chromium/gold electrodes) under nitrogen flow (0.5 L min -1 ) in a sealed chamber and in "wet" conditions (Δf0,wet = f -f0) by immersing the sensor holder in a water bath, allowing 60-90 minutes in each state for the signal to stabilize. The sensors were then coated with the polymer films by spin-casting their solutions following the same protocols used for OECT fabrication.

Once dry, the frequencies of the coated sensors were measured under dry (Δfdry = f -f0) and wet conditions over several cycles (Δfwet,x = f -Δf0,wet, where x = cycle ordinal number).

OECT performance

Characterization of OECTs fabricated with the blends confirms that adding the PEO component, even without a chemically attached P3HT-block, to the channel material enhances the device response speed while maintaining its steady state performance. Specifically, inspection of the OECT output curves of the blend with PEO in the top panel of Figure 4.17 shows that the onset of saturation of the drain current with applied drain voltage occurs at Vd as low as -0.3 V for a sweep rate of 0.5 V s -1 . Additionally, the transfer curves measured for the blends with PEO in the bottom panel of Figure 4.17 show minimal hysteresis even at Vg sweep rates higher than 1 V s -1 and relatively reproducible maximum Id across a wide range of operating speeds. This is unlike the OECTs of the neat P3HT-co-P3HHT, which only show Id saturation at Vd more negative than -0.7 V for a Vd sweep rate of 0.5 V s -1 , large hysteresis in their transfer curves, and significant decrease of their Id,max with faster Vg sweep rates from 0.01 to 0.5 V s -1 (see results in section 2. for future research on block copolymer design and engineering as promising cocomponents to improve the performance and stability of mixed conducting conjugated polymer blends.

General conclusions and perspectives

Our work demonstrates that blending of a mixed conductor with a "compatibilizing" block copolymer is a viable strategy to enhance the transient electrochemical performance of an active material without the need for its direct chemical modification. Specifically, we find that adding hydrophilic, ion-conducting PEO-based additives (P3HT-b-PEO block copolymer) to the mixed conducting P3HT-co-P3HHT supports faster electrochemical doping of the material in aqueous media. This consequently improves the response speed of its transistors (from τ = 8 s in neat P3HT-co-P3HHT to τ = 80 ms in the blends) and appears to be more effective than tuning the geometry of the active channel. We attribute this to enhanced efficiency of ion uptake in the blend due to a more compatible interface with the aqueous electrolyte. Water contact angle measurements do suggest increased hydrophilicity of the material with increasing block copolymer content. This might also explain the reduced mid-frequency impedances observed upon blending if we interpret that the diffusion-dominated impedance response in the neat P3HT-co-P3HHT electrodes arises due to limited transport of hydrated ions across the relatively hydrophobic polymer surface. By comparing the trends observed from water contact angle measurements and the electrochemical doping rates extracted from spectroelectrochemistry, we find that only up to 25 wt% of the block copolymer is necessary to achieve efficient modulation of the transient properties by blending.

Results also suggest that our blending approach does not strongly affect the bulk electronic performance of the material. For instance, although counterions penetrate the material faster and, thus, are more readily available in the blends, the volumetric capacitance, which can be related to the polaron density of the electrochemically doped polymer, does not change (C* ≈ 190 F cm -3 ). Likewise, the threshold voltages (Vt ≈ 0.45-0.55 V), the transconductances (gm,max ≈ 0.4 mS), and the steady state currents (ISS ≈ 10 -5

A at Vg = -0.6 V and Vd = -0.4 V) of the OECTs are preserved, given the same biasing APPENDIX:

Methodology and supplementary information

A.1. Materials and film deposition

The random copolymer P3HT-co-P3HHT (50% -3HT random comonomer composition; Mn = 30 kg mol -1 , dispersity Ɖ = 1.9) was synthesized by Dr. Olivier Dautel at the Institut Charles Gerhardt Montpellier following previously described protocols. 1 Block copolymer P3HT-b-PEO (Mn, P3HT = 5 kg mol -1 , ƉP3HT = 1.2, Mn, PEO = 20 kg mol -1 , ƉPEO ≤ 1. The films were subsequently dried at 55 °C for 3-8 min and then stored in ambient conditions before characterization soon afterward.

A.2. Organic electrochemical transistors (OECT)

A.

Figure A.2. Optical microscopy images of the patterned source and drain electrodes of micro-scale OECTs with measurements of their widths and lengths indicated.

Larger devices were fabricated by the author. The source and drain electrodes in devices with large channels (W ≈ 2500 µm, L ≈ 50, 110, 190, and 380 µm) were patterned by vapor-deposition (ME400B, Plassys) of chromium (10 nm) followed by gold (100 nm)

A.2.3 OECT measurement parameters

The output curves were obtained by measuring Id over Vd (0 to -0.8 V at 50 mV increments) at constant Vg (0.2 to -0.8 V at 50 mV increments) using various Vd sweep rates (0.25 and 0.50 V s -1 , as indicated in the main text). The transfer curves were obtained by measuring Id over Vg (0.2 to -0.8 V at 50 mV increments) at constant Vd (-0.6 V unless otherwise stated in the main text) using various Vg sweep rates (0.01 to 10 V s -1 , as indicated in the main text). The values of gm were extracted from differentiating the transfer curves, while Vt and ΔVt were extracted from plots of |Id| 1/2 vs Vg, explained further in the main text.

The μC* parameter was extracted from plotting the gm extracted at |Vt -Vg| ≈ 0.2 V from devices of different channel dimensions. The transient currents were measured during application of a Vg step from 0 to -0.6 V or application of various Vg waveforms (detailed in the main text) at a constant Vd of -0.4 V.

A.2.4 OECT operational stability in water

To test their long-term operational stability when interfacing with water-rich media, the OECTs were first characterized upon fabrication then stored in deionized water. The same devices were measured periodically to track the evolution of their performance while continuously stored in water.

A.3. Characterization of physical properties

A.3.1 Grazing incidence wide angle x-ray scattering (GIWAXS)

The polymer samples were spin-cast onto cleaned silicon wafer chips following the same film deposition protocols outlined above. GIWAXS characterization was performed at NSLS-II (CMS 11-BM Beamline). Chips were cut into 5 x 5 mm² and positioned in a vacuum chamber. Automated sample alignment procedure was performed prior to irradiating the samples for data collection. X-rays with 13.5 keV beam energy (λ = 0.918402 Å) were used to probe the structure of the thin films and scattering patterns were collected onto a 2D Pilatus CCD detector (pixel size 0.172 x 0.172 mm²), positioned at a distance of roughly 26 cm from the sample. A silver behenate standard was used to calibrate sample-to-detector distance (258.1 mm) and beam center x-and y-positions using 9 ring-shaped reflections.

NIKA version 1.75 package 4 within IGORPro v.6.37, along with some custom macros developed at NIST, was used to perform calibration, GI correction and general image treatment.

A.3.2 Differential scanning calorimetry

The polymer samples (1-2 mg) were drop-cast from solution (80 µL of 15 mg mL

A.4. Electrochemical characterization

Cyclic voltammetry, spectroelectrochemistry and electrochemical impedance spectroscopy were performed on polymer-coated ITO electrodes using a VersaSTAT3 potentiostat (Princeton Applied Research) and accompanying VersaStudio software.

A.4.2 Spectroelectrochemistry

The polymers were characterized by spectroelectrochemistry to investigate their electrochemical doping behavior, tracking changes in the absorbance due to polaron formation upon electrochemical oxidation of the polymer backbone, as previously reported. 1 The experimental set-up is illustrated in

A.4.3 Electrochemical impedance spectroscopy

The impedance spectra of polymer-coated ITO electrodes of different thicknesses were measured against a Pt/Ti wire as counter electrode (ET078, eDAQ) and an Ag/AgCl wire as pseudo-reference electrode (ET073, eDAQ) in 0.1 M KCl electrolyte (active area = 0.18 cm 2 ). An AC signal with 10 mV amplitude was applied to the polymer over the frequency range of 0.1-10000 Hz, while simultaneously applying DC bias to characterize the behavior of the doped films. A chronoamperometric experiment was performed immediately prior to the impedance measurements by applying a DC potential to the polymer until currents stabilized (after about 10-30 s) to ensure that the polymer was in the targeted doped state. The raw data were fitted with the equivalent circuit described in the main text using an EIS spectrum analyser software available online (http://www.abc.chemistry.bsu.by/vi/analyser/). 6

A.5. Quartz crystal microbalance (QCM)

Measurements were performed using a QCM crystal oscillator (QCM25, Stanford Research Systems) connected to a digital controller (QCM200, Stanford Research Systems) equipped with a built-in frequency counter. Polished, AT-cut quartz crystals with chromium/gold keyhole style electrodes and resonance frequency of 5 MHz (O100RX1, Stanford Research Systems) were used as the QCM sensors, which could be fixed to a crystal holder with the active face exposed. The sample holder comprised of electrode contact springs connected to the oscillator via a BNC connector. For measurements in "dry" conditions, a home-built gas chamber was fixed around the crystal holder with inlet and outlet tubes for nitrogen gas flow (0.5 L min -1 ) regulated by a multichannel gas controller (647C, MKS Instruments). For measurements in "wet" conditions, the gas chamber was detached from the sample holder, which was then immersed in a water (milliQ) bath at 25 °C without removing or replacing the crystal sensor. Each instance that the surrounding media was changed, a varactor equipped in the QCM instrument was adjusted to nullify shunt capacitances for more accurate resonant frequency measurement as specified by the manufacturer. 7 Measurements were performed in each media for at least 60 to 90 minutes continuously to reach steady state conditions. The blank crystal sensors were first measured in dry (f0) and wet (f0,wet) conditions for reference. The pre-characterized sensors were then coated with the polymer films to be analyzed following the same deposition protocols described above. The coated crystals were first measured in dry conditions to characterize the as-cast films (Δfdry) then measured in wet conditions (Δfwet,x), after which the samples were removed from the water bath without detaching from the sample holder and dried by carefully purging the exposed sensor face and the sample holder with compressed air. These dry-wet measurement cycles were performed three times (X = 3) for each sample to detect any progressive effects of water immersion as discussed in the main text.