Sequential Cyanation of Polythiophenes: Tuning Charge Carrier Polarity in Organic Electrochemical Transistors

Polythiophenes, built on head‐to‐head (HH) linked 3,3′‐oligomer ethylene glycol‐2,2′‐bithiophene (gT2), have simple chemical structure, good backbone planarity, and relatively high‐lying the lowest unoccupied molecular orbital (LUMO) energy levels, hence showing excellent p‐type performance in organic electrochemical transistors (OECTs) with µC* of several hundred F cm−1 V−1 s−1. In this study, sequential cyano substitution is utilized to enable transformation of charge carrier polarity from p‐type to n‐type in OECTs, based on the parent polythiophene g4T2‐T2. With the increase of cyano group number, the polythiophenes exhibit gradually lowered LUMO levels from −2.55 to −3.90 eV. As a result, from g4T2‐T2 to CNg4T2‐CNT2, the p‐type performance dramatically diminishes accomplished by the enhancement of n‐type one when applied in OECTs. To the authors' delight, polymer CNg4T2‐CNT2 with the highest content of cyano groups exhibits a remarkable n‐type µC* of 27.01 F cm−1 V−1 s−1 and high gm,norm of 6.75 S cm−1 with negligible p‐type character. This study demonstrates that sequential cyano substitution provides a powerful approach for developing high‐performance n‐type polymers for OECT applications.

Polythiophenes and their derivatives are widely used in ptype organic thin-film transistors (OTFTs) and organic solar cells (OSCs). [26]With introduction of hydrophilic oligo(ethylene glycol) (OEG) side chains on polythiophene backbones, a series of high-performance p-type OMIECs have been obtained, such as g2T-T [27] and p(g2T2-g4T2) (Figure 1a). [28]Interestingly, the noncovalent intramolecular S•••O interactions can form in the headto-head (HH) linked bithiophene unit enabled by the oxygen atom of OEG side chains directly attached to the 3,3′ positions of bithiophene, [29] which result in planar backbones without extra ring-fusion synthetic steps.Such planar backbone will facilitate intramolecular charge delocalization and promote intermolecular charge hoping because of the improved intermolecular stacking and shortened cofacial distance.Thanks to these distinctive features, bithiophene-based polymer p(g2T2-g4T2) showed very planar backbone, low energetic disorder, and high charge carrier mobility, thus yielding a record high μC* of 522 F cm −1 V −1 s −1 in p-type OECTs. [28]Because of the electron-rich property of bithiophene moiety and the resultant high-lying lowest unoccupied molecular orbital (LUMO) energy level, almost all polythiophenes and their derivatives exhibited hole dominant transport character in OTFTs. [30]For the same reason, they also showed p-type performance in OECTs. [5,31]In fact, there are only a few examples of n-type polythiophenes for OTFT application, and no n-type OECT performance has been reported till now. [32]yano functionalization is an effective approach to enable ntype organic semiconductors because the very large dipole moment of cyano group can lead to the resulting semiconductors with high electron deficiency, [33] such as cyanated PDI [34] and dicyano benzothiadiazole (DCNBT)-based polymers. [35]For example, the DCNBT-based n-type polymers showed deep-positioned LUMOs and excellent device performance in both OSCs [36] and OTFTs. [37]In fact, introducing cyano groups into fused-ringsmall-molecular electron acceptors changed the field of OSCs, yielding unprecedented power conversion efficiencies. [35]However, the more bulky nature of cyano group (versus hydrogen and fluorine atoms) can cause steric hindrance, which in combination of its lack of solubilizing capability requires more judicious molecular design to minimize its unfavorable effects on the framework conformation, material solubility, film morphology, and hence device performance.
Based on the considerations of polymer backbone planarity and the frontier molecular orbital (FMO) levels, we herein report the strategy of sequential cyanation of polythiophene derivative g4T2-T2 (Figure 1), which is built based on a HH linkage containing gT2 bearing OEG side chain as the donor unit and bithiophene as the co-unit.Importantly, the presence of the S•••O conformation lock in g4T2-T2 greatly enhances its backbone planarity and film ordering, [38] thus yielding an excellent μC* of 133.62 F cm −1 V −1 s −1 in p-type OECTs.From g4T2-T2 to g4T2-CNT2 to CNg4T2-T2 and to CNg4T2-CNT2, the polymer LUMO level is gradually decreased from −2.55 to −3.90 eV with the increment of cyano groups.As the sequential cyano substitution, the p-type performance was greatly reduced and the ntype one was obviously increased in OECT devices.Moreover, CNg4T2-CNT2 with the highest content of cyano groups demonstrated pronounced n-type character with negligible p-type one in OECTs, achieving a remarkable μC* of 27.01 F cm −1 V −1 s −1 and a high g m,norm of 6.75 S cm −1 . [13]To the best of our knowledge, this is the first report of n-type OECT application for polythiophene derivatives.The results demonstrate that the introduction of intramolecular noncovalent interaction is an effective and elegant strategy to develop high-performance organic semiconductors, and sequential cyano substitution provides a powerful approach for developing high-performance n-type OMIECs.

Polymer Properties
Optical properties of the polythiophene derivatives were measured by UV-vis-NIR absorption in dilute chloroform solution and as thin film.The absorption spectra are shown in Figure 2 and the corresponding absorption parameters are summarized in Table 1.The polymer solutions showed a strong absorption in the range from 400 to 900 nm with the maximum absorption wavelength ( max sol ) of 572, 627, 555, and 624 nm for g4T2-T2, g4T2-CNT2, CNg4T2-T2, and CNg4T2-CNT2, respectively.From solution to film, all polymers exhibited redshifted absorption due to the more planar polymer backbone and stronger interchain interaction in solid state.Moreover, all polymer films featured a pronounced 0-0 vibronic transition, indicative of a good ordering of polymer chains.The optical bandgaps (E g opt s) of the polythiophene derivatives are determined based on their film absorption onsets, which are found to be 1.73, 1.43, 1.70, and 1.66 eV for g4T2-T2, g4T2-CNT2, CNg4T2-T2, and CNg4T2-CNT2, respectively.Please note that the absorption of g4T2-CNT2 with two cyano groups attached on T2 unit has an ≈70 nm bathochromic shift in both solution and film states and the E g opt of g4T2-CNT2 is significantly narrower compared with those of CNg4T2-T2 with two cyano groups attached on g4T2 unit.Such phenomenon is mainly attributed to the stronger electron-rich character of g4T2 in comparison to that of T2.When functionalized with cyano groups, CNT2 also shows higher electron deficiency than CNg4T2, thus leading to stronger intramolecular charge transfer character between CNT2 and g4T2 in polymer g4T2-CNT2 than that between T2 and CNg4T2 in CNg4T2-T2.
Subsequently, the LUMO and highest occupied molecular orbital (HOMO) levels of the polythiophene derivatives were estimated by using cyclic voltammetry (CV) with 0.1 m  tetrabutylammonium hexafluorophosphate electrolyte.As shown in Figure 2c, from top to bottom, the reduction peaks become more obvious accompanied by gradually weakened oxidation peaks, indicating that the sequential cyanation resulted in enhanced n-type with clearly diminished p-type characteristics.The onset oxidation potentials of g4T2-T2, g4T2-CNT2, CNg4T2-T2, and CNg4T2-CNT2 are −0.06,0.34, 0.72, and 1.26 eV relative to the ferrocene/ferrocenium (Fc/Fc + ) redox couple, corresponding to the HOMO levels of −4.28, −4.68, −5.06, and −5.60 eV, respectively.Based on the film onset reduction potentials, the LUMO levels of g4T2-CNT2, CNg4T2-T2, and CNg4T2-CNT2 were determined to be −3.54,−3.57, and −3.90 eV, respectively.The LUMO level of g4T2-T2 was estimated by the equation of E LUMO = E HOMO + E opt g due to the absence of reduction peak, which was found be to −2.55 eV.From these results, the LUMO/HOMO levels were gradually lowered as the num-ber of cyano groups increased in repeating units (Figure 2d).Among them, the lowest-lying LUMO level of CNg4T2-CNT2 (−3.90 eV) will be beneficial to facilitating electron generation and enhancing operational stability in aqueous solution.According to the density functional theory (DFT) calculation, the calculated LUMO/HOMO levels are −2.24/−4.32,−3.07/−4.92,−3.08/−5.32,and −3.39/−5.88eV for the trimers of g4T2-T2, g4T2-CNT2, CNg4T2-T2, and CNg4T2-CNT2 repeating units, respectively.This trend is in good agreement with the CV results.Furthermore, g4T2-T2 displayed a high degree of backbone planarity (Figure S6, Supporting Information), which is beneficial to microstructural ordering of polymer chains and charge transport.To our delight, with the incorporation of cyano groups, g4T2-CNT2, CNg4T2-T2, and CNg4T2-CNT2 exhibit a comparatively planar backbone with dihedral angles ≤11°between two neighboring thiophenes.Especially for CNg4T2-CNT2,  Transconductance values measured at V g of −0.5 (g4T2-T2), −0.9 V (g4T2-CNT2), −0.9 V (CNg4T2-T2), and 1.0 V (CNg4T2-CNT2); the values are normalized by active layer  each thiophene is functionalized with one cyano group, which also exhibits high backbone planarity, resulting from the intramolecular S•••O conformation lock and small steric hindrance associated with cyano group. [28]The results from DFT calculations demonstrate that the synergistical combination of the intramolecular S•••O conformation lock and cyano functionalization is a very powerful strategy for developing polymer semiconductors with deep-lying LUMO level and highly coplanar backbone.
The electrochemical properties of polythiophene derivatives in aqueous electrolyte were investigated using CV and UV-vis-NIR spectroelectrochemistry.As shown in FigureS7, Supporting Information, g4T2-T2 and g4T2-CNT2 display pronounced and reversible oxidative event with a good cycling stability when biased from −0.5 to 0.5 V for g4T2-T2 and from −0.2 to 0.8 V for g4T2-CNT2 against the Ag/AgCl reference.However, due to the higher threshold voltage (V th ) of CNg4T2-T2 (vide infra), the bias was further increased to 0.9 V, the current density gradually decreased with increasing cycle numbers, indicative of its lower stability.Furthermore, CNg4T2-CNT2 showed highly reversible reduction and excellent operational stability over 20 cycles when biased from 0 to −1.0 V. Hence, polymers g4T2-T2, g4T2-CNT2, and CNg4T2-CNT2 with the polar side chains featured stable and reversible electrochemical properties switching in both aqueous and organic media, indicating their promising operational stability when applied in OECTs.The onsets of oxidation in aqueous media are −0.2,0, and 0.52 V for g4T2-T2, g4T2-CNT2, and CNg4T2-T2, respectively, showing a same trend as that found in tetrabutylammonium hexafluorophosphate electrolyte.A relatively large difference in oxidation potential was noticed from organic to aqueous media, ascribed to variations in the charge density and size of the counter-anion and deviations in the solute-solvent interactions between the OMIECs and the electrolyte. [40]V-vis-NIR spectroelectrochemistry (Figure 2e-h; FigureS8, Supporting Information) was used to gain insights into the ion doping process of these polythiophene derivatives.As shown in Figure 2e,f, both g4T2-T2 and g4T2-CNT2 were doped under ambient condition, thus requiring the infliction of a negative bias for de-doping first, corresponding to −0.5 and −0.2 V, respectively.For CNg4T2-T2 and g4T2-CNT2 with different cyano group positions, g4T2-CNT2 with the stronger intramolecular charge transfer character led to its higher-lying HOMO level compared with CNg4T2-T2, which resulted in a more facile p-doping for g4T2-CNT2 (Figure 2f-g).The bias (−0.1 V) was still applied to CNg4T2-T2 in order to ensure the complete de-doping.27a] Notably, the decrement of optical absorption of CNg4T2-T2 manifests the gradual dissolution/degradation of film in the aqueous environment when biased from −0.1 to 0.5 V (Figures S8g,S9a, Supporting Information).With the further increment of bias, CNg4T2-T2 showed a strong residual absorption of ICT at a high bias of 0.9 V with concurrent appearance of weak polaronic absorption.At the highest applied potential of 0.5 V (oxidation) for g4T2-T2, 0.8 V (oxidation) for g4T2-CNT2, and −1.0 V (reduction) for CNg4T2-CNT2, the neutral absorption was mostly bleached, indicative of a high degree of doping level.In comparation to g4T2-T2 and g4T2-CNT2, the low p-doping efficiency of CNg4T2-T2 is attributed to its lower-lying HOMO level revealed by the CV results.To explore the reversibility of the doping progress of the polymers, the applied bias was reversed to −0.5 V for g4T2-T2, −0.2 V for g4T2-CNT2, −0.1 V for CNg4T2-T2, and 0 V for CNg4T2-CNT2.29a] However, as shown in Figure 2g and Figure S9b, Supporting Information, the optical absorption of the de-doped CNg4T2-T2 cannot be fully restored, due to the degradation of this polymer film induced by the applied high bias, which is in accord with the CV results in aqueous electrolyte.

Device Performance of Organic Electrochemical Transistors
In order to evaluate the effect of sequential cyanation, OECT devices were fabricated to characterize the performance of the polymers.The details of device fabrication are included in supporting information.It should be noted that patterned parylene layers were used to designate the channel area since it can not only minimize the overlap between source/drain electrodes and active layers for fast device response, but also provide insulation between the source/drain electrodes and aqueous electrolyte in OECTs for suppressed leakage currents. [13]Then, the OECTs were tested in 0.1 m NaCl aqueous solution under ambient air with Ag/AgCl as the gate electrode.
As shown in Figure 3, the sequential cyano substitution resulted in remarkable performance difference in OECTs.g4T2-T2 displayed p-type characteristic and no n-type behavior when the gate voltage was applied from −0.5 to 1.5 V (Figure 3a).With the introduction of cyano groups, g4T2-CNT2, CNg4T2-T2, and CNg4T2-CNT2 showed a certain degree of ambipolar characteristics (Figure 3b-d).g4T2-CNT2 and CNg4T2-T2 are dominated by p-type behaviors, whereas CNg4T2-CNT2 with the highest content of cyano groups exhibits n-type dominant performance with negligible p-type one.Among the p-type OMIECs, g4T2-T2 displayed a lowest V th of −0.21 V, small off-current (I off ) of ≈10 −7 A, and high on-current/off-current (I on /I off ) ratio of 10 4 .V th increased significantly for the polymers functionalized with cyano groups, V th was extrapolated to be −0.74V for g4T2-CNT2, and this further increased to −0.83 V for CNg4T2-T2, in agreement with the low-lying HOMO levels.Both g4T2-CNT2 and CNg4T2-T2 exhibited a low I off of 10 −8 A, but I on /I off ratio was only 10 2 owing to their small I on .With the attachment of the highest number of cyano groups, CNg4T2-CNT2 displayed typical n-type character with a V th of 0.75 V, low I off of ≈10 −8 A, and high I on /I off ratio up to 10 4 .With the drain voltage (V ds ) at −0.4 V, the g4T2-T2-based devices showed a maximum drain current (I ds,max ) of −1.25 mA at the gate voltage (V g ) of −0.5 V, which is far higher than the g4T2-CNT2-(I ds,max = −1.54× 10 −3 mA; V g = −0.8V) and CNg4T2-T2-based OECTs (I ds,max = −2.25 × 10 −3 mA; V g = −0.9V).Moreover, the maximum transconductance (g m,max ) of 6.43 mS was obtained for g4T2-T2, much higher than g4T2-CNT2 (0.044 mS) and CNg4T2-T2 (0.050 mS), and the g m,max similarly reached at a lower bias of −0.5 V for g4T2-T2 compared with that for g4T2-CNT2 and CNg4T2-T2 (both at −0.9 V).Due to the lowest-lying FMO levels, the CNg4T2-CNT2-based devices exhibited an improved I ds,max of 4.65 × 10 −2 mA (V g = 1.0 V) at the V ds of 0.4 V, and a sizable g m,max of 0.5 mS.
The normalized transconductance (g m,norm ) was estimated to be 38.75, 0.89, and 0.77 S cm −1 for g4T2-T2, g4T2-CNT2, and CNg4T2-T2 in p-type OECTs, respectively, based on the formula: g m,norm = g m /(Wd/L), where W is the channel width (200 μm for g4T2-T2, 100 μm for g4T2-CNT2, CNg4T2-T2, and CNg4T2-CNT2), d is the active layer thickness (87.1 nm for g4T2-T2, 56.5 nm for g4T2-CNT2, 55.6 nm for CNg4T2-T2, and 73.3 nm for CNg4T2-CNT2), and L is the channel length (10 μm).Hence, it can be concluded that the introduction of cyano group on the polymers is not conducive to p-type OECT performance.In contrast, further incorporation of cyano groups leads to CNg4T2-CNT2 with remarkable n-type performance in OECTs having a g m,norm of 6.75 S cm −1 , which is attributed to its deep-lying LUMO level and strong ion uptake ability as confirmed by the CV and electrochemical impedance spectroscopy (EIS) results (vide infra).These results clearly illustrate the great power of sequential cyano substitution for developing high-performance n-type OECT polymers (Table 2).
In order to determine the long-term operational stability, these OECT devices were biased under suitably pulsed circulating gate voltage.As shown in Figure 4a, g4T2-T2 devices exhibited a current retention of ≈64% for 2400 s.The operational stability of g4T2-CNT2 devices deteriorated more rapidly, with the oncurrent retained at ≈30% of the initial I ds upon cycling for 1200 s (Figure 4b).It was found that CNg4T2-T2 showed an obviously decreased current after 170 s when switched from 0 to −0.9 V (Figure S12, Supporting Information), showing a poor stability in aqueous medium as confirmed by the CV and spectroelectrochemistry results.These significant differences could be related to the HOMO level variation of three polymers and to the irreversible changes of the film morphology upon doping.CNg4T2-CNT2 was biased from 0 to 0.8 V, showing a remarkable stability as visible from the constant peak current obtained for each cycle and current retention >80% after 2400 s measurement in air (Figure 4c).In continuous-mode bias voltage test (Figure S13, Supporting Information), the g4T2-T2-, g4T2-CNT2-, and CNg4T2-CNT2-based devices were biased with V g /V d = −0.4/−0.4,−0.8/−0.6, and 0.8/0.6V, respectively, the I ds was found to be nearly unchanged after 800 s, while the current of the CNg4T2-T2-based OECTs dropped rapidly after 130 s with V g /V d = −0.9/−0.6V. Furthermore, to access the transient response of these devices, the response time was determined by an exponential fitting the curves of I ds (Figure 4; Figure S14, Supporting Information).The on/off time constant ( on / off ) was extracted to be 971/2.0,471/2.7,981/4.0,and 102/4.2ms for g4T2-T2, g4T2-CNT2, CNg4T2-T2, and CNg4T2-CNT2, respectively.Among them, CNg4T2-CNT2 displayed relatively fast switching speed, reflecting the facile ion injection and the charge extraction at the source/drain contacts.
Hydrophilicity is an important index for ion injection in OECTs.As shown in Figure S15, Supporting Information, the introduction of cyano groups has no apparent effect on the hydrophilicity based on the results of water contact angle measurements.The slow transient response times of these polymers are likely due to the increased energy barrier of ion injection [27b] and slow ion penetration partly resulting from the low density of hydrophilic oligo(ethylene glycol) side chains.For example, in comparison to g4T2-T2, p(g4T2-T) showed a faster response time ( on = 0.59 ms) due to the larger density of oligo(ethylene glycol) side chains. [41]There are some strategies to alleviate this drawback.27a,43] Moreover, increasing the side chain length and density of hydrophilic oligo(ethylene glycol) would facilitate faster switching. [41]

Polymer Film Morphology
To illustrate the correlation between the chemical structure and device performance, the polymer film morphology was characterized by atomic force microscopy (AFM, Figure S16, Supporting Information) and 2D grazing incidence wide-angle X-ray scattering (2D-GIWAXS, Figure 5).AFM height images of these polythiophene derivatives showed smooth surface, with a root-meansquare (RMS) roughness of 1.24, 1.37, 1.8, and 1.2 nm, respectively.Based on the 2D-GIWAXS images displayed in Figure 5, and the corresponding packing parameters are listed in Table S2, Supporting Information.The g4T2-T2, g4T2-CNT2, CNg4T2-T2, and CNg4T2-CNT2 films showed a similar lamellar (100) scattering along both out-of-plane (OOP) and in-plane (IP) directions, and the corresponding lamellar distance was found to be 18.47, 17.94, 18.47, and 18.47 Å, respectively.All of polythiophene films share similar lamellar packing distances due to the same chain on the thiophene unit.Moreover, g4T2-T2, g4T2-CNT2, CNg4T2-T2, and CNg4T2-CNT2 displayed IP (010) - scattering peak at 1.75, 1.75, 1.75, and 1.70 Å −1 , which corresponds to a - stacking distance of 3.59, 3.59, 3.59, and 3.69 Å, respectively.In the case of CNg4T2-CNT2, the peak intensity of (010) in the IP direction and (300) in the OOP direction is lower than both of g4T2-T2, g4T2-CNT2, and CNg4T2-T2.The films of g4T2-T2, g4T2-CNT2, and CNg4T2-T2 showed a tighter - packing compared with CNg4T2-CNT2, which is attributed to the more planar backbone as confirmed by the DFT calculations.It should be noted that the incorporation of cyano group does not significantly affect the molecular packing, even with the highest content of cyano groups for polymer CNg4T2-CNT2, due to the intramolecular S•••O noncovalent interaction.The (010) peak in the IP direction together with the pronounced lamellar diffractions up to (300) in the OOP direction indicates a predominant edge-on orientation for the polymers, which is beneficial to the horizontal charge carrier transport in OECTs. [16]he crystal coherence lengths (CCLs) are crucial parameters to appraise the crystalline order range of the polymers using the Scherrer equation. [11]The CCL OOP (100) values of g4T2-T2, g4T2-CNT2, CNg4T2-T2, and CNg4T2-CNT2 were determined to be 199.6,160.8, 183.9, and 171.7 Å, respectively.In contrast with the cyano-functionalized polythiophene (g4T2-CNT2, CNg4T2-T2, and CNg4T2-CNT2), g4T2-T2 exhibited the largest CCL OOP (100) and shorter - packing distance, indicating that the highest crystallinity of g4T2-T2, which is favorable for the hole transport in OECTs.This result is in good accord with its best p-type performance.Going from g4T2-T2 to g4T2-CNT2 and to CNg4T2-T2, the CCL OOP (100) is decreased with the incorporation of cyano group, implying a consecutively reduced crystallinity, coupled with the consistently lowered HOMO level, results in the significantly diminished p-type performance.Interestingly, the introduction of four cyano groups in each repeating unit of polymer CNg4T2-CNT2 can not only deepen the FMO level to achieve n-type transport, but also not obviously affect the - stacking and film crystallinity, thus resulting in a remarkable ntype performance in OECTs.
Moreover, an OECT complementary inverter based on parent polythiophene g4T2-T2 as p-type channel material and cyanated CNg4T2-CNT2 as n-type counterpart was performed.The schematic diagram of the complementary was shown in Figure 6a,b.transfer characteristics (VTCs) of the corresponding inverter were measured by different V DD (Figure 6c).As shown in Figure 6d, the voltage gains (∂ OUT /∂ IN ) were extracted from the VTCs (Figure 6c).As V DD increases, the gains are gradually improved, and reached a peak value of 45 at V DD = 0.8 V with a low power consumption of ≈11 μW (Figure 6e).However, the peak gain at V DD = 0.8 V is at V IN = 0.62 V rather than V IN = V DD /2 = 0.4 V, indicating a non-ideal inverter, due to the mismatched p-and n-type performance.Thus, high-performance n-type OMIECs is important for developing complementary inverters.

Conclusions
A series of tetraethylene glycol-substituted polythiophene derivatives modulated with the distinct number of cyano groups were designed and synthesized.Benefiting from the head-to-head linked 3,3′-tetraethylene glycol-2,2′-bithiophene with a highly planar backbone conformation enabled by the intramolecular noncovalent S•••O interaction, four polythiophene derivatives showed high degree of backbone planarity and highly crystalline film morphology.As a result, polymer g4T2-T2 obtained an excellent μC* of 133.62 F cm −1 V −1 s −1 and high g m,norm of 38.75 S cm −1 in p-type OECTs by compensating the small C* with high hole mobility.The μC* is much higher than those of g4T2-CNT2 and CNg4T2-T2.With the sequential cyano substitution, the polymer FMO levels were gradually decreased from g4T2-T2 to CNg4T2-CNT2.As a consequence, CNg4T2-CNT2 with the highest content of cyano groups yielded a lowest-lying LUMO level of −3.90 eV, which is much lower than the non-cyanated parent polymer g4T2-T2, and the polymer exhibited an n-type transport character with a high electron mobility of 0.09 cm 2 V −1 s −1 .Hence, a remarkable μC* of 27.01 F cm −1 V −1 s −1 and high g m,norm of 6.75 S cm −1 was attained for CNg4T2-CNT2 in n-type OECTs.Such performance is originated from the integrated strategy of incorporating the intramolecular noncovalent S•••O interaction and cyano functionalization.The results demonstrate the effectiveness of sequential cyano substitution of the polythiophenes in enabling transformation of charge carrier polarity from p-type to n-type in OECTs.In addition, considering that there is still a large gap compared with the p-type OMIECs, further improvement of electron mobility is expected to advance the development of high-performance n-type OECTs through more precise structural innovation and optimization, and the structure-property-performance correlations from this work provide important insights into the material design for ntype polymers.

Figure 1 .
Figure 1.a) Chemical structures of the representative p-type OECT polythiophenes reported in the literature.b) With sequential cyano substitution, the polythiophene derivatives reported in this work can achieve a planar backbone conformation and low-lying LUMO level simultaneously, thus realizing charge carrier polarity transformation from p-type to n-type when applied in OECTs.
dimensions; b) Average film thickness determined by profilometer; c) The C* values extracted from EIS data; d) Calculated from the product μC* and EIS-measured C*; e) The values calculated using the equation: μC* = g m L/Wd(V th −V g ).

Figure 6 .
Figure 6.Schematic diagram of (a) complementary logic circuits and (b) OECT-based inverter.(c) Voltage transfer characteristics and (d) corresponding voltage gains of the inverters at different supply voltages.(e) Power consumption of the inverter at different supply voltages.

Table 1 .
Molecular weights and physicochemical properties of the polymers.

Table 2 .
OECT performance parameters of the polythiophene derivatives.