Influence of Backbone Curvature on the Organic Electrochemical Transistor Performance of Glycolated Donor–Acceptor Conjugated Polymers

Abstract Two new glycolated semiconducting polymers PgBT(F)2gT and PgBT(F)2gTT of differing backbone curvatures were designed and synthesised for application as p‐type accumulation mode organic electrochemical transistor (OECT) materials. Both polymers demonstrated stable and reversible oxidation, accessible within the aqueous electrochemical window, to generate polaronic charge carriers. OECTs fabricated from PgBT(F)2gT featuring a curved backbone geometry attained a higher volumetric capacitance of 170 F cm−3. However, PgBT(F)2gTT with a linear backbone displayed overall superior OECT performance with a normalised peak transconductance of 3.00×104 mS cm−1, owing to its enhanced order, expediting the charge mobility to 0.931 cm2 V−1 s−1.

donor polymers. The transconductance is a key figure of merit relating to the product of the active material volumetric capacitance and charge mobility, [13,15,17] highlighting the delicate balance that must be achieved, between charge and ionic mobility, in the design of OECT active materials. Further material improvements therefore rely on better understanding the interplay between ion and charge conduction in OECT active materials. Here, for the first time, we investigate the influence of backbone curvature on the OECT performance of two p-type D-A glycolated polymers PgBT-(F)2gT and PgBT(F)2gTT, that perform contrastingly as excellent active materials.
The design of PgBT(F)2gT and PgBT(F)2gTT as p-type accumulation mode OECT active materials combines several design facets (Figure 1). Each repeating unit of both polymers feature three triethyleneglycol monomethylether sidechains, to strike a workable balance between ion and charge transport, as well as solution processability. Attachment of the glycol sidechains via aryl-ether linkages to either a thiophene or thieno [3,2-b]thiophene affords building blocks with an angular or linear template, resulting in curved or linear conjugated backbones, respectively. The mesomeric electron donating effect of the aryl ethers is paired against the presence of an electron deficient, fluorinated, glycolated benzothiadiazole (BT) acceptor, tuning the electrochemical oxidation potentials of both polymers to within the aqueous electrochemical window, as required for p-type OECT operation. [23] The BT acceptor also serves to bridge a procession of non-covalent S···O and S···F planarising interactions along the backbones of PgBT(F)2gT and PgBT(F)2gTT, to promote backbone planarity, stronger orbital hybridisation/ delocalisation and enhanced charge transport. [24] To investigate the role of non-covalent interactions along backbones of PgBT(F)2gT and PgBT(F)2gTT, DFT calculations of trimeric models at the B3LYP/6-31G(d,p) level were performed (Figures 1, S1-S8). [25] Potential energy scans were also performed on 3,4-dimethoxythiophene-thiophene as well as 3,6-dimethoxythienothiophene-thiophene components, revealing an optimal anti arrangement, with a small dihedral angle of 68 between the two heterocycles in both systems and an approximate 5 kJ mol À1 lower energy than the syn conformer ( Figures S9,S10). The preference for anti over syn is in agreement with crystal structures of related methoxy substituted oligothiophenes, [26,27] as well as earlier studies examining the role of non-covalent S···O interactions in bithiophenes. [28,29] The planarity and preferred conformation (thiophenes trans with respect to thiadiazole) of the dithie-noBT unit common to both PgBT(F)2gT and PgBT(F)2gTT has been established previously. [30,31] Starting from preferred conformations, energy minimisations of trimeric species revealed both backbones are almost fully coplanar. The positioning of the planarising interactions and sidechains on one side of the conjugated backbone of PgBT(F)2gT forces it to adopt a curved backbone geometry, whereas even distribution of sidechains and planarising interactions about both sides of the PgBT(F)2gTT backbone template its more linear geometry. This differing curvature is maintained regardless of the regiochemistry of the asymmetric BT within both backbones. In OFET applications, backbone curvature of conjugated polymers has been found to significantly influence their charge transport properties. [32,33] We observed good orbital mixing in PgBT(F)2gT and PgBT(F)2gTT as evidenced by their highly delocalised highest occupied molecular orbitals (HOMOs).
Regiorandom PgBT(F)2gT and PgBT(F)2gTT were synthesised by direct arylation polymerisation of dibrominated monomer 1 with glycolated thienothiophene 2 and thiophene 3 (Figure 1), bypassing the necessity for toxic organometallic reagents. [34] Direct arylation polymerisations were performed at 80 8C to suppress crosslinking. [35] Both polymers were isolated in high yield (> 80 %) following precipitation and sequential solvent washing to remove impurities and low weight material. As with many reported glycolated conjugated polymers, the analysis of molecular weight was complicated by the tendency of both polymers to aggregate in solution. [36,37] Examining different concentrations of PgBT-(F)2gT by GPC revealed an estimated M n = 10 kDa and = 1.7 against polystyrene standards, whereas the main peak of PgBT(F)2gTT exhibited unrealistically high values irrespective of concentration with a small peak also apparent at M n = 3.8 kDa and = 1.2 (Figures S11-S13). The structures of both polymers were confirmed by 1   According to both solid state cyclic voltammetry (CV) and square-wave voltammetry (SQW) electrochemical data in 0.1 M KCl/H 2 O (Figure 2), the onset of PgBT(F)2gT thin film oxidation occurs at 0.4 V vs. Ag/AgCl (HOMO = À4.8 eV [38] ), which lies well within the aqueous electrochemical window but sufficiently anodic to avoid material ambient auto-oxidation. Cycling CV experiments suggested excellent electrochemical stability and reversibility of PgBT(F)2gT oxidation (Figure 2 e). Scan rate dependence CV data were collected to demonstrate the volumetric penetration of counterions into PgBT(F)2gT thin films upon bulk electrochemical oxidation, which revealed the diffusion limited nature of thin film oxidation in accordance with the Randles-Sevcik equation. [39] Similarly, the onset of PgBT(F)2gTT thin film oxidation was observed at a felicitously accessible potential of 0.3 V during CV and SQW experiments (HOMO = À4.7 eV [38] ), with volumetric counterion diffusion upon oxidation, as well as its electrochemical stability and reversibility, confirmed by scan rate dependence and cycling CV studies, respectively. Thin film electrochemistry data for PgBT(F)2gT and PgBT(F)2gTT were also recorded in 0.1 M [n-Bu 4 N]PF 6 /MeCN to compliment aqueous electrolyte results ( Figures S37-S44), revealing similar behaviours.
Solid state thin film UV/Vis spectroelectrochemistry (SEC) in 0.1 M KCl/H 2 O was applied to identify the electrochemically oxidised state of PgBT(F)2gT (Figure 2 f). Upon applying an oxidative potential of 0.4 V vs. Ag/AgCl, which was incremented up to 0.8 V, gradual quenching of the PgBT(F)2gT ground state transitions was observed, concurrent with the appearance and intensification of a broad polaron band at 850 nm. Subsequently returning the applied potential to 0 V resulted in a restoration of the PgBT(F)2gT ground state UV/Vis spectrum, evidencing electrochemical reversibility ( Figure S46). Similarly, in the reversible UV/Vis SEC of PgBT(F)2gTT thin films (Figure S47), a broad polaron band (and a bleaching of ground state transitions) transpired at an oxidative potential of 0.3 V, peaking in intensity at an apogean anodic potential of 0.8 V. Thus, UV/Vis SEC confirms reversible generation of mobile charge carrying hole polarons on PgBT(F)2gT and PgBT(F)2gTT by electrochemical oxidation. The p-type accumulation mode OECT performance of devices fabricated using PgBT(F)2gT and PgBT(F)2gTT were interpreted using the transconductance expression [Eq. (1), Table 1, and Figures 3, S48-S51; I D represents source-drain current and V G represents gate voltage]. [6] OECTs fabricated using PgBT(F)2gT exhibited a higher volumetric capacitance of 170 F cm À3 at V G = À0.8 V, where channel mobility was calculated at 0.060 cm 2 V À1 s À1 , corresponding to a (channel dimension) normalised peak transconductance of 2.18 10 3 mS cm À1 . A higher normalised peak transconductance of 3.00 10 4 mS cm À1 at V G = À0.8 V was recorded for OECTs employing PgBT(F)2gTT, which was ascribed to its superior channel mobility of 0.931 cm 2 V À1 s À1 , despite an inferior volumetric capacitance of 111 F cm À3 . Note that both materials show volume-dependent capacitance increase ( Figures S52,S53), within a thickness range up to 200 nm (typical for devices). The OECT ON/OFF ratio of PgBT(F)2gTT was much higher at 10 5 than PgBT(F)2gT at 10 3 , and is comparable with state-of-the-art devices. [22] Furthermore, there is no significant difference in operational cutoff frequency between PgBT(F)2gT and PgBT(F)2gTT (Figure S54).  During cyclic switching of OECT devices, both materials show good operational stability in V G = À0.6 V up to 100 cycles, but exhibit slight degradation under an excessive gating voltage of À0.8 V ( Figure S55). The reduced stability during cycling up to V G = À0.8 V maybe attributed to the negative effects of repeated volume expansions on the hopping dominated interchain transport occurring in both polymers. [40] In terms of the redox resilience of both polymers under ambient conditions, OFF-current rising of PgBT(F)2gT was lower than PgBT(F)2gTT due to its lower HOMO level (Figure 3 e). [20] The structural features of PgBT(F)2gT and PgBT(F)2gTT thin films were investigated to understand the influence of materials design on their differing OECT performances. Grazing-incidence wide-angle X-ray scattering (GIWAXS, Figure 4) revealed varying structural order in both materials. PgBT(F)2gTT thin films exhibited an approximate 1:1 mixture of face-on and edge-on crystallite orientations, as deduced from "lamellar" peaks (100) (at q = 3.49 nm À1 , dspacing = 1.8 nm) and a p-p stacking peak (at q = 17.3 nm À1 , d-spacing = 0.36 nm) showing up along both q r and q z directions. On the other hand, PgBT(F)2gT thin films exhibited preferred face-on orientation and an increased dspacing of 2.16 nm, with the (100) and p-p stacking peaks oriented along the q r and q z directions, respectively. These structural differences can be correlated to backbone geometries; linear PgBT(F)2gTT has greater macromolecular symmetry, facilitating bimodal crystallinity, whereas curved PgBT(F)2gT is of lower backbone symmetry, which limits the directions of stackable crystalline propagation. [41] Atomicforce microscopy (AFM, Figures S56-S59) images supported these findings, with PgBT(F)2gTT exhibiting a microfibrillar structure comprising of fibrils 10-20 nm wide and 30-50 nm long. AFM images of PgBT-(F)2gT appeared relatively featureless, in agreement with its less pronounced crystallinity.
The higher OECT channel mobility of PgBT-(F)2gTT is thus explained by its bimodal crystallinity, enabling effective sourcedrain charge transport. The limited geometric capacity for PgBT(F)2gT to stack only in the face-on orientation hinders charge transport in the source-drain direction of the OECT channel. However, ease of counterion diffusion along pervasive amorphous channels in PgBT(F)2gT enables its larger volumetric capacitance. [42] Fundamental characterisation and device performance of PgBT(F)2gT and PgBT(F)2gTT conclusively demonstrate the success of their design as state-of-the-art OECT active materials. PgBT-(F)2gT and PgBT(F)2gTT exhibit highly reversible and stable electrochemical oxidation enabling their application as p-type accumulation mode OECT materials, as shown by their CV, SQW and SEC behaviour. OECTs constructed with PgBT(F)2gT performed with a superior volumetric capacitance of up to 170 F cm À3 , ascribed to its low symmetry, curved backbone design templating comparatively amorphous (iondiffusive) films. In contrast, OECTs employing PgBT(F)2gTT attained a higher charge mobility of 0.931 cm 2 V À1 s À1 , owing to its linear backbone design facilitating bimodal crystallinity. Overall, OECTs featuring PgBT(F)2gTT displayed the best  performance, attaining a normalised peak transconductance of 3.00 10 4 mS cm À1 . To fine-tune OECT performance, future work will focus on further backbone functionalisation of both polymers by the nucleophilic aromatic subsitution of the fluorine atoms on BT. [43]