Development of Synthetically Accessible Glycolated Polythiophenes for High‐Performance Organic Electrochemical Transistors

Four glycolated polythiophene‐based organic mixed ionic‐electronic conductors (OMIECs), PE2gTT, PE2gT, PT2gTT, and PT2gT are prepared by atom‐efficient direct arylation polymerization, avoiding the need for toxic organometallic precursors. PE2gT, PT2gTT, and PT2gT are operable in p‐type accumulation mode organic electrochemical transistors (OECTs), with PT2gT displaying the best device performance with a µC* product figure‐of‐merit of 290 F cm−1 V−1 s−1. A record volumetric capacitance among p‐type glycolated polythiophene OMIECs of 313 F cm−3 is observed for PE2gT, ascribed to the high proportionality of polar components in its materials design. The good OECT performance of PE2gT with µC* = 84.2 F cm−1 V−1 s−1, comparable with state‐of‐the‐art poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) devices, coupled with its synthetic accessibility and favorable accumulation mode operation makes PE2gT an ideal glycolated alternative to PEDOT:PSS in bioelectronics. PE2gT with the least negative threshold voltage also displays the best OECT operational cycling stability, linked to better resistance of its oxidized state against parasitic redox side reactions . Shelf life stability of OECTs stored (without bias) is observed to be better for materials with a more negative threshold voltage and higher average molecular weight (PT2gT), that are less susceptible to ambient auto‐oxidation and film delamination.


Introduction
Contrasting against the traditional solidstate electronics that operate predominantly within the realm of electrons, signal processing in biology employs the concurrent movement of both charges and ions within an aqueous electrolytic environment, giving rise to functional incompatibilities between the two. [1]In order to achieve electronics that interface more effectively with biology, recent research efforts have focussed on the development of bioelectronics, that exploit the mixed conductivity of organic mixed ionic-electronic conductors (OMIECs) to perform signal processing, with potential applications encompassing biosensing, integrated healthcare devices and neuromorphic computing. [2,3]6][7][8][9][10][11] Unlike organic field-effect transistors which are gated via a dielectric medium, the modulation of chargecarrying polarons (and bipolarons) throughout the OMIEC active channel of an OECT is achieved through its gate potential induced electrochemical oxidation and reduction, entailing the stoichiometric and volumetric penetration of counterions into the channel material. [12]ECT operation is often characterized according to Bernard and Malliaras' model, which applies orthogonal ionic and electronic circuits to describe the concurrent movement of both charges and ions within a device. [13]The OECT transconductance, a key figure-of-merit, is derived from Bernard and Malliaras' model to be proportional to the product of active layer volumetric capacitance and charge mobility (μC*).OECTs can be operated in either accumulation or depletion mode, depending on the electrochemical properties of the active OMIEC material. [14]Accumulation mode OECTs employ OMIECs that have negligible charge carriers in the absence of a gate bias and grow upon increased biasing.Depletion mode devices use materials with sufficient charge carriers to engender intrinsic conductivity, that are depleted through biasing.As OMIECs for depletion mode OECTs are inherently conductive under no gate bias, such devices exhibit higher power consumption, owing to the need for continuously applied gate voltage in order to access and maintain their OFF state. [15]18] Despite its depletion mode profile, poly(3,4ethylenedioxythiophene) (PEDOT, most commonly obtained as a water-dispersible two-component blend with anionic poly(styrenesulfonate), i.e., PEDOT:PSS) remains a well-known and popular selection for bioelectronics research in the anodic regime (i.e., p-type). [19][21][22][23] To combat electrochemical instability, PEDOT:PSS can be de-doped by reaction with excess alkylamines, to afford neutral PEDOT:PSS that can be operated as an accumulation mode OECT. [18]Biocomposite PEDOT blends with deoxyribonucleic acid (DNA) counteranions have also been developed, with the aim of creating a less acidic and more biocompatible OMIEC. [24]The popularity of PEDOT can (at least in part) be attributed to its ease of synthesis and availability.Commercial PEDOT:PSS is made by the oxidative polymerization of 3,4-ethylenedioxythiophene (EDOT) in the presence of PSS, whilst electropolymerization of EDOT can also be performed to create OECT channels of tunable characteristics. [25,26][29][30][31] Singlecomponent OMIECs afford specific advantages including stronger ionic-electronic coupling and higher volumetric capacitances when compared to their multi-component brethren. [32]n order to engender ionic conductivity into a single-component organic semiconductor, polar sidechains are incorporated.35][36] However, much like PEDOT:PSS itself, these CPEs often exhibit aqueous solubility, necessitating the application of performancediminishing crosslinkers in their OECT channels. [6,33,37]As such, research attention shifted towards developing polythiophenes with oligomeric ethylene glycol sidechains, that enable ion conduction without conferring aqueous solubility. [38]Seeded from the seminal studies by Giovannitti et al. and Nielsen et al. that describe P(g2T-TT) and P(g2T-T) respectively, [39,40] there is now an expanding library of high-performance p-type glycolated polythiophene based OMIECs.Materials design studies have explored modifications to both sidechain and backbone configuration, for example in altering sidechain distribution, [41] adopting propylene and butylene glycol substitutes, [42] as well as incorporating the cyclopentadithiophene unit, [43] to attain OECT μC* product performances of 322, 342 and 370 F cm −1 V −1 s −1 respectively.However, most of these glycolated polythiophenes require multistep syntheses employing Stille polymerization, which is undesirable from both an upscaling and toxicity point of view.
Drawing inspiration from the high OECT performances attainable with glycolated polythiophenes, as well as the prevalence of PEDOT in bioelectronics, we designed a set of glycolated polythiophene-based OMIECs with systematic change in their materials design by the co-polymerization of either thiophene (T) or the more electron rich EDOT (E), with doubly glycolated comonomers based on thiophene (2gT) and thieno [3,2-b]thiophene (2gTT).The incorporation of two triethylene glycol sidechains in 2gT and 2gTT aims to facilitate the volumetric penetration of counterions into films of resulting co-polymers, without conferring undesirable aqueous solubility and causing undue compromise to electronic charge conduction pathways.Designed variability of the proportion of polar glycol sidechains (and ethylenedioxy bridges) in each polymeric repeat unit was expected to lead to diverse volumetric capacitances and swelling characteristics that can also impact OECT performance.
The resulting PE2gTT, PE2gT, PT2gTT, and PT2gT materials were synthesized according to a streamlined scheme employing atom-efficient direct arylation polymerizations that also circumvents the need for toxic organometallic precursors.Electrochemistry revealed onsets of oxidation at −0.5, −0.7, −0.2, and 0.3 V versus Ag/AgCl for PE2gTT, PE2gT, PT2gTT, and PT2gT respectively.PT2gT displayed the overall champion accumulation mode OECT performance with a μC* product figure-of-merit of 290 F cm −1 V −1 s −1 , ascribed to its high degree of polymerization that enable its superior charge mobility at 1.00 cm 2 V −1 s −1 .PE2gT exhibited a record volumetric capacitance among p-type glycolated polythiophene OMIECs of 313 F cm −3 , attributed to the high proportion of polar glycol and ethylenedioxy components in its materials design.The good OECT performance of PE2gT, with a μC* figure-of-merit of 84.2 F cm −1 V −1 s −1 that is comparable with state-of-the-art PEDOT:PSS devices, coupled with its synthetic accessibility and favorable intrinsic accumulation mode operation makes PE2gT a suitable glycolated alternative to PE-DOT:PSS in bioelectronics.PE2gT with the least negative threshold voltage also displayed the best OECT cycling performance, owing to better resistance of its oxidized state against redox side reactions with products of the oxygen reduction reaction (ORR), whilst shelf life stability was found to be best for PT2gT with the most negative threshold voltage and highest average molecular weight, due to greater material resistance against ambient autooxidation and film delamination.
The structural identities of PE2gTT, PE2gT, PT2gTT, and PT2gT were confirmed through a combination of hot 1 H NMR at 65 °C as well as the fragmental analysis of their matrixassisted laser desorption/ionization (MALDI) mass spectrometry traces (Figures S1-S8, Supporting Information).Interpretation of PE2gTT, PE2gT, PT2gTT, and PT2gT IR spectra were complicated by the overpowering presence of glycol sidechain signals (Figure S9, Supporting Information).However, for PT2gTT and PT2gT a signal at 3040 cm −1 was seen and assigned to C-H stretching on the 3-and 4-positions of the unsubstituted thiophene present in the repeating unit of both materials.No signal near 3040 cm −1 was seen for PE2gTT and PE2gT featuring 3,4-disubstituted EDOT in place of unsubstituted thiophene.Gel permeation chromatography (GPC) was applied to probe the macromolecular sizes of all four polymers (Table 1;   PE2gTT and PE2gT respectively.Integration analysis of their hot 1 H NMRs indicated average molecular weights of 3.0 and 3.6 KDa for PE2gTT and PE2gT respectively, confirming their low degree of polymerization, equivalent to an average chain length of five to seven repeating units.The lower degree of PE2gTT and PE2gT polymerization may be explained by their tendency to auto-oxidize upon heating (see below paragraph discussing UV/Vis traces), which potentially compromises the Pd(0) catalytic cycle. [50]No thermal processes were recorded between 40 -300°C in the differential scanning calorimetry traces of PE2gTT, PE2gT, PT2gTT, and PT2gT (Figures S14-S17, Supporting Information).

Optical and Electronic Properties
Density functional theory (DFT) computations of PE2gTT, PE2gT, PT2gTT, and PT2gT trimeric models were conducted at the B3LYP/6-31G(d,p) level of theory to probe the energetics and physical conformation of their materials design.The DFT energetics of all four polymers are summarized in Table 2.It is evident polymers with a higher ratio of aryl ether substitutions per thiophene unit are calculated to have shallower HOMO levels (PT2gTT < PT2gT < PE2gTT < PE2gT), owing to the mesomeric electron-donating effect of aryl ether linkages into the backbone.All four polymers are predicted to feature highly planar backbones that are required for good orbital delocalization and charge transport, achieved through the incorporation of favorable S…O non-covalent interactions (Figures S18-S33, Supporting Infromation). [51]he solution state UV/Vis of PE2gTT, PE2gT, PT2gTT, and PT2gT in chloroform (Figures S34-S38, Supporting Infromation) all showed evidence of polymer aggregation, with their S0-S1 transitions at  max = 552, 583, 529, and 528 nm respectively exhibiting red-shifted shoulders, that dissipate upon warming for PT2gTT and PT2gT.Low energy polaron bands at 780 and 760 nm were identified in the UV/Vis traces of PE2gTT and PE2gT respectively, that intensified upon heating in ambient atmosphere.This suggests PE2gTT and PE2gT can both be oxidized (i.e., doped) in the as-synthesized "resting" state.Polaron bands were not observed for PT2gTT and PT2gT, including at elevated temperatures.The optical bandgaps of PE2gTT, PE2gT, PT2gTT, and PT2gT were estimated from their onsets of S0-S1 absorption in the solution state, to be 1.9, 1.8, 2.0, and 2.0 eV respectively.UV/Vis of PE2gTT, PE2gT, PT2gTT, and PT2gT thin films showed broadened transitions when compared against the solution state, which is explained by packing effects in the solid state (Figure S39, Supporting Information).
The aqueous thin film electrochemistry of all four polymers was recorded in 0.1 m KCl/H 2 O (Figure 1).Cyclic voltammetry (CV) and square wave voltammetry (SQW) experiments revealed PE2gTT, PE2gT, PT2gTT, and PT2gT onsets of oxidation at −0.5, −0.7, −0.2 and 0.3 V versus Ag/AgCl respectively.The HOMOs of PE2gTT, PE2gT, PT2gTT, and PT2gT were calculated to lie at −4.1, −3.9, −4.4, and −4.9 eV respectively, by assuming the Ag/AgCl process occurs at −4.6 eV, which for PE2gTT, PE2gT, and PT2gTT are in good agreement with DFT calculations.The slight difference between onset of PT2gT electrochemical oxidation and its HOMO level from DFT may be explained by the swelling behavior of its films (see Discussion on Device Performance and Thin Film Microstructure and Swelling sections); electrochemical quartz crystal microbalance (EQCM, Figure 5a) measurements on PT2gT films revealed minimal electrolyte swelling at potentials more cathodic than ≈0.5 V versus Ag/AgCl, impacting counterion diffusion within PT2gT films, reasoning the observed discrepancy between its oxidation onset and calculated HOMO. [52]Despite this, the volumetric penetration of counterions into thin films of PE2gTT, PE2gT, PT2gTT, and PT2gT upon bulk electrochemical oxidation was confirmed through scan rate dependence CV data, that revealed the diffusion-limited nature of their electrochemical oxidation in accordance with the Randles-Sevcik equation. [53]CV cycling for 12 scans were applied to confirm the excellent electrochemical stability and reversibility of all four polymers (Figures S40-S43, Supporting Information).To complement aqueous electrolyte results, the electrochemistry of all four polymers was also recorded in 0.1 M [n-Bu 4 N]PF 6 /MeCN, revealing similar oxidatively active behaviors (Figures S44-S57, Supporting Information).
In order to identify the nature of their electrochemically oxidized states, thin films of all four polymers were also probed by UV/Vis spectroelectrochemistry (SEC) in 0.1 M KCl/H 2 O.For PT2gTT thin films (Figure 2), upon application of a potential of −0.2 V versus Ag/AgCl, which was incremented anodically to 0.5 V, a gradual quenching of the polymer ground state transitions was observed concurrent with the appearance of a polaron band at 850 nm.As the applied potential was further incremented above 0.5 V, to 0.8 V, a reduction in polaron and (remaining) ground state transition intensities were observed, which is consistent with bipolaron formation. [54]Reversing the applied potential incrementally back to −0.2 V resulted in the initial re-intensification of polaron state transitions, followed by the restoration of ground state spectra, indicating full electrochemical reversibility of polaronic and bipolaronic states.Thus, the UV/Vis SEC of PT2gTT indicates the reversible accession of polaron and bipolaron charge carriers on its backbone at anodic potentials above −0.2 and 0.5 V respectively.Similarly, reversible generation of both polarons and bipolarons on PE2gTT, PE2gT, and PT2gT backbones were also observed in their UV/Vis SEC (Figures S58-S69, Supporting Information), at onset potentials which are summarized in Table 3.

OECT Performance
Fabrication of OECTs with all four polymers PE2gTT, PE2gT, PT2gTT, and PT2gT was attempted, however ambient operability in 0.1 m KCl/H 2 O was observed for PE2gT, PT2gTT, and PT2gT only.Channel delamination of PE2gTT OECTs occurred upon contact with aqueous electrolyte, irrespective of posttreatment (annealing) procedures, preventing its stable device operation, which may relate to its low molecular weight.For PE2gT, PT2gTT, and PT2gT p-type accumulation mode OECTs, their performance was interpreted using the transconductance expression (Equation 1, Table 4, Figure 3; Figures S70-S74, Supporting Information; V G = gate voltage, I D = drain current). [12] G is interpreted relative to the Ag/AgCl reference potential, where negative V G corresponds to oxidative biasing of channel materials.
Overall, PT2gT OECTs exhibited the best performance, achieving an excellent μC* figure-of-merit benchmark of 290 F cm −1 V −1 s −1 , which can be attributed to its large volumetric capacitance of 290 F cm −3 , that is balanced against a good charge mobility calculated at 1.00 cm 2 V −1 s −1 .An even higher volumetric capacitance of 313 F cm −3 was attained in PE2gT OECTs, that is a record among p-type glycolated OMIECs (Table S1, Supporting Information), however, its overall modest μC* figure-of-merit of 84.2 F cm −1 V −1 s −1 corresponds with a lower calculated charge mobility at 0.270 cm 2 V −1 s −1 , offsetting any performance advantages afforded by its exceptional volumetric capacitance.The observed V Th of PE2gT accumulation mode OECTs (−0.106V) is at a noticeably more oxidative potential compared to its electrochemical onset of oxidation measured under inert atmosphere, indicating that generation of stable electronic charge carriers (holes) in PE2gT thin films to a sufficient concentration to facilitate electronic conductivity can only be achieved at anodic potentials above 0.1 V versus Ag/AgCl.PT2gTT exhibited the poorest (accumulation mode) OECT performance among the trio of polymers tested, with a μC* figure-of-merit of 61.2 F cm −1 V −1 s −1 , owing to a relatively lowercharge mobility calculated at 0.277 cm 2 V −1 s −1 and volumetric capacitance of 221 F cm −3 .All OECTs displayed commendable ON/OFF ratios at factors of 10 4 .
2][43][55][56][57] PE2gT, PT2gTT, and PT2gT all achieve volumetric capaci- tances that are among the highest reported for p-type glycolated OMIECs (Table S1, Supporting Information), with further augmentation possible through electrolyte and morphology optimization, [7,55,[58][59][60] which sets the precedent for their potential application in energy storage, in addition to their use in bioelectronics. [54]he shelf lives of PE2gT, PT2gTT, and PT2gT OECTs were investigated by comparing peak source-drain currents attained with freshly fabricated versus two-day-aged devices.The ratio of peak currents attained with new versus aged devices were calculated to represent active material shelf life stability and are plot against the corresponding threshold voltages of new devices in Figure 3e, to illustrate the correlation between glycolated polythiophenes of a more negative OECT threshold voltage displaying improved shelf lives, owing to better material resistance against ambient auto-oxidation. [16,61]OECT shelf life stabilities also reflect the average molecular weights of each active material; PT2gT with the highest M n had the best shelf life stability, whereas PE2gT with poorer shelf life stability had the lowest M n .Shelf life stability of PE2gT OECTs was observed to be compromised by channel delamination (Figure S75, Supporting Information), which contextualized with the OECT inoperability of PE2gTT, leads to the proposition that OECT shelf life stability is optimal for high molecular weight OMIECs with the best film-forming properties and strongest resistance against ambient auto-oxidation. [62]dditionally, operational device stabilities of PE2gT, PT2gTT, and PT2gT were investigated by OECT cycling measurements, by recording the linear and saturation transfer characteristics every 6 seconds for over 300 cycles (Figure S76, Supporting  EIS and μ calculated from peak transconductance; f) Volumetric capacitance, measured by EIS (electrochemical impedance spectroscopy, using a conventional 3-electrode system); g) Charge mobility calculated by dividing volumetric capacitance (C*) from the figure-of-merit (μC*) determined using peak transconductance.
Information), with switching of gate voltage to −0.7 V for PE2gT as well as −0.8 V for PT2gTT and PT2gTT, to replicate aggressive cycling conditions that reach beyond the peak transconductance states of each material, thereby rigorously probing operational cycling behaviors. [16,63]Of the three polymers, PE2gT OECTs demonstrated the best operational cycling stability, retaining 34% of its initial source-drain current on its 333rd cycle (2000 s).On the other hand, PT2gTT and PT2gT retained 0.12% and 0.05% of their respective initial source-drain current on the 333rd cycle.
The OECT operational stabilities of PE2gT, PT2gTT, and PT2gT thus follow the reverse correlation to that for shelf life stability, where materials of a higher threshold voltage/more cathodic oxidation onset show better operational cycling stabilities.This is explained by better energetic resistance of the oxidized state of glycolated polythiophenes with more cathodic oxidation onsets, against parasitic redox side reactions with products of the ORR, notably hydroxide, occuring at the OECT drain in the OFF state, as characterised previously. [63]

Thin Film Microstructure and Swelling
To investigatethe effects of their thin film microstructure on OECT performance, the grazing-incidence wide-angle X-ray scattering (GIWAXS) profiles of all four polymers were collected (Figure 4).PT2gTT thin films exhibited bimodal crystal orientation, with "lamellar" peaks (100) (at q = 3.3 nm −1 , d-spacing = 1.9 nm) and - stacking peaks (at q = 17.3 nm −1 , d-spacing = 0.36 nm) showing up along both q r and q z directions.PE2gT thin films displayed marginal preference to interact face-on with the substrate, as characterized by orientation of the - reflection  peak along the q z direction, at d-spacing = 0.36 nm.Thin films of PT2gT and OECT inoperable PE2gTT were found to be amorphous.
Electrolyte swelling of the active layer material in an OECT upon electrochemical biasing has a profound effect on device performance. [27,57,64]The swelling characteristics under electrochemical bias of PE2gTT, PE2gT, PT2gTT, and PT2gT films in 0.1 M KCl/H 2 O were probed using an EQCM, for five CV cycles of applied potential between −0.8 and 0.8 V versus Ag/AgCl, at the scan rate of 100 mV/s (Figure 5a).Upon initial incrementation of the applied potential for PE2gTT, PE2gT, PT2gTT, and PT2gT films past their onset of oxidation up to 0.8 V, significant mass uptakes of varying degrees were observed for all four materials, assigned to the ingestion of anions and water by films for charge balancing electrochemical oxidation (hole injection).
Upon further cycling from 0.8 back down to −0.8 V, a reversal in mass uptake was seen for all polymers, correlated to anion/water ejection commensurate with electrochemical de-doping (reduction).The baseline swelling of reduced films after the first cycle of applied potential were all higher (incrementing with each subsequent cycle) than that recorded for the pristine state at 0% mass uptake, owing to electrolyte retention within films. [65]eak mass uptakes at 0.8 V, representing absolute film swelling, [52] were seen to increase from PT2gTT < PT2gT < PE2gTT < PE2gT with average values of 40%, 55%, 56%, and 140% respectively.This observed trend in increasing EQCM peak mass uptakes reflects the proportionality of polar components in each materials design.Among the four polymers reported herein, PE2gT has the largest proportion of polar components drawing electrolyte into its films, with two triethylene glycol sidechains plus an ethylenedioxy bridge for each bisthiophene repeat unit, reasoning its highest peak EQCM mass uptake of 140%.The materials design of PE2gTT and PT2gT both feature a lower proportion of polar components compared to PE2gT, owing to the elongated thiophene-thienothiophene backbone repeat unit in PE2gTT, as well as the lack of an ethylenedioxy bridge in the repeat unit of PT2gT (c.f.PE2gT), explaining their lower peak EQCM mass uptakes.PT2gTT has the lowest proportion of two polar triethylene glycol sidechains within each thiophene-thienothiophene repeat unit, correlating with its peak EQCM mass uptake of 40% that is lowest among the four polymers tested.Differences in mass uptake between peak at 0.8 V and baseline swelling at −0.8 V (after the first cycle), representing active (bias-induced) film swelling only, [52] were calculated to be 40%, 75%, 30% and 30% for PE2gTT, PE2gT, PT2gTT, and PT2gT respectively.

Discussion on Device Performance
Calculated OECT mobilities of PE2gT, PT2gTT, and PT2gT were found to correlate best with their degrees of polymerization.PT2gT, the polymer with the highest average molecular weight at M n = 50 KDa, was calculated to exhibit the highest charge mobility in OECTs among all polymers tested.This is explained by the presence of effective electronic charge conduction pathways along long polymer strands within its thin films, [62,66] which act as tie-chains between crystalline domains and are less disrupted by thin film swelling upon electrochemical oxidation. [57]n the other hand, PE2gT with the largest EQCM peak electrolyte uptake at 140% upon electrochemical oxidation to 0.8 V versus Ag/AgCl and low average molecular weight of M n = 3.6 KDa, was calculated to have the lowest OECT charge mobility, owing to disruption of confined charge conduction pathways by thin film electrolyte swelling. [66]PT2gTT with median average molecular weight of M n = 11 KDa was determined to display an intermediary OECT charge mobility.
Volumetric capacitances of polymers studied were found to correlate best with the proportion of polar components in each materials design, which correlates with peak EQCM mass uptakes linked to electrolyte swelling.[69] The removal of an ethylenedioxy bridge in the design of PT2gT (c.f.PE2gT) per bisthiophene repeating unit lowers the proportionality of polar components, correlating well with its comparatively decreased volumetric capacitance.PT2gTT has the lowest volumetric capacitance among the three OECT operational polymers reported herein, as it has the smallest glycol content per elongated thienothiophene-thiophene repeating unit.Despite an electrochemically recorded onset of oxidation at −0.7 V versus Ag/AgCl, generation of sufficient, stable electronic charge carriers (holes) in PE2gT thin films to facilitate electronic conductivity, thereby turning its OECT(s) ON, is only achieved at a threshold voltage of −0.106 V, making PE2gT OECTs accumulation mode devices.This discrepancy between its oxidation onset and threshold voltage can be explained by the lack of chemically anchored counteranionic components in PE2gT, compromising the persistence of holes in its unbiased state and at cathodic (reductive) potentials.This counteranion phenomenon has been observed previously by Keene et al. for PEDOT:PSS, which feature the equivalent conjugated backbone as PE2gT, where treatment of PEDOT:PSS with various alkylamine based de-dopants that react with acidic PSS to form 1:1 cation-anion pair complexes results in a shift in OECT threshold voltages to accumulation mode operation, despite the de-doped PEDOT:PSS materials retaining onsets of oxidation at ca. −0.7 V versus Ag/AgCl. [18]The favourable accumulation mode OECT performance of PE2gT without the need for chemical de-doping, its straightforward two step synthesis with a good overall 34% yield, [46] as well as stable OECT performance with a μC* product figure-of-merit of 84.2 F cm −1 V −1 s −1 comparable to state-of-the-art PEDOT:PSS OECTs, [32,70] demonstrates potential for broader applicability of PE2gT in bioelectronics, as an alternative to PEDOT:PSS.
The minimal electrolyte swelling change of PT2gT films at potentials below (more cathodic than) ≈0.5 V versus Ag/AgCl (Figure 5a) is hypothesized to restrict counterion diffusion within its films, explaining the anomalous discrepancy between its solid state electrochemical onset of oxidation and DFT computed HOMO energy level. [52]The anodic onset of PT2gT film oxidation/lower OECT threshold voltage, alongside its high molecular weight, does endow good shelf life stability, owing to better resistance against ambient auto-oxidation and channel delamination. [16,61,62]In contrast, the higher cycling stability of PE2gT may be attributed to its higher OECT threshold voltage/more cathodic oxidation onset, which improves resilience of its oxidized state to redox side reactions with products of ORR. [63]he inoperability of PE2gTT as an OECT material may be explained by its low molecular weight at M n = 3.0 KDa and high surface roughness, which compromises thin film integrity upon swelling, causing delamination.

Conclusion
Motivated by a desire to create synthetically accessible p-type OMIECs for bioelectronics, we developed four glycolated polythiophenes, PE2gTT, PE2gT, PT2gTT, and PT2gT.All polymers were synthesized by atom-efficient direct arylation polymerization, circumventing the need for toxic organometallic precursors.Aqueous electrochemistry revealed PE2gTT, PE2gT, PT2gTT, and PT2gT onsets of oxidation at −0.5, −0.7, −0.2, and 0.3 V versus Ag/AgCl respectively.Overall, PT2gT displayed the champion accumulation mode OECT performance with an attained μC* product figure-of-merit of 290 F cm −1 V −1 s −1 , linked to its superior OECT charge mobility calculated at 1.00 cm 2 V −1 s −1 which is enabled by its high degree of polymerization (M n = 50 KDa).A record volumetric capacitance among p-type glycolated polythiophene OMIECs of 313 F cm −3 was observed for PE2gT, which is explained by the high proportion of polar gly-col and ethylenedioxy components in its materials design, that increases electrolyte swelling of its films upon electrochemical oxidation.The good OECT performance of PE2gT attaining a μC* product figure-of-merit of 84.2 F cm −1 V −1 s −1 that is comparable with state-of-the-art PEDOT:PSS devices, coupled with its favorable intrinsic accumulation mode operation and straightforward two-step synthesis with a sound overall yield of 34%, makes PE2gT a prime alternative to PEDOT:PSS in bioelectronics.Additionally, PE2gT OECTs with the least negative threshold voltage displayed the best operational cycling stability, owing to better resistance of its oxidized state against parasitic redox side reactions with products of the ORR, whilst PT2gT OECTs with the most negative threshold voltage and highest average molecular weight displayed better shelf life (unbiased) stability, attributable to better material resistance against ambient auto-oxidation and film delamination.Volumetric capacitances of PE2gT, PT2gTT, and PT2gT are all among the highest reported for glycolated polythiophenes, setting the precedent for their potential use in energy storage.
, Supporting Information), revealing a M n /Ð of 11 KDa/2.0 and 50 KDa/3.8for PT2gTT and PT2gT respectively, against polystyrene standards.Analysis of PE2gTT and PE2gT molecular weights by GPC was complicated by their tendency to aggregate in solution (owing to backbone auto-oxidation in air), with examination of UV detector responses suggesting an unrealistically broad M n /Ð of 3.1 KDa/31 and 4.1 KDa/34 for

Figure 2 .
Figure 2. UV/Vis SEC of PT2gTT thin films in 0.1 M KCl/H 2 O showing spectral changes upon applying an increasingly anodic potential from a) −0.2 to 0.5 V versus Ag/AgCl and from b) 0.5 to 0.8 V, as well as upon returning the applied potential from c) 0.8 to 0.5 V and from d) 0.5 to −0.2 V. Arrows indicate spectral progression.Incrementation of potentials was conducted at 0.1 V intervals.

Figure 3 .
Figure 3. OECT performance of PE2gT (violet), PT2gTT (red) and PT2gT (orange) showing a) device architecture; b) channel thickness normalized transconductance (top, transconductance used to calculate μC*) and transfer curves at V D = −0.7 V (bottom); c) OECT output curves at stepped V G in 0.1 V intervals within indicated ranges (arrows indicate data at more negative/channel oxidative V G ); d) plot of volumetric capacitances against OECT mobilities; and e) OECT shelf life stabilities of each material (quantified by the loss of peak source-drain currents between new versus aged devices), plot against corresponding threshold voltages of new devices.

Figure 4 .
Figure 4. GIWAXS of a) PE2gTT, b) PE2gT, c) PT2gTT,and d) PT2gT dry films; with corresponding isotropic (black) and linecut integrations shown below, in the q z (dark cyan, out of plane) and q r (light green, in plane) directions.

Figure 5 .
Figure 5. Thin film swelling and roughness data of PE2gTT (blue), PE2gT (violet), PT2gTT (red) and PT2gT (orange) showing a) fractional mass changes calculated from EQCM measurements in 0.1 M KCl/H 2 O, with five cycles of potential applied shown at top (black), as well as b) RMS mean surface roughness from AFM images of pristine thin films.
DFT [eV] E g,DFT [eV] Onset of Oxidation [V vs Ag/AgCl] HOMO [eV] a) Onset of Absorption [nm] E g,opt [eV] LUMO [eV] b) a) Calculated from the onset of electrochemical oxidation, by assuming the Ag/AgCl process occurs at −4.6 eV; b) Estimated by adding optical bandgap to corresponding HOMO energy level; c) Discrepancy with DFT computed HOMO caused by constriction of counterion diffusion.

Table 3 .
Onset potentials for accessing polaron and bipolarons states on