Semiconducting Small Molecules as Active Materials for p‐Type Accumulation Mode Organic Electrochemical Transistors

A series of semiconducting small molecules with bithiophene or bis‐3,4‐ethylenedioxythiophene cores are designed and synthesized. The molecules display stable reversible oxidation in solution and can be reversibly oxidized in the solid state with aqueous electrolyte when functionalized with polar triethylene glycol side chains. Evidence of promising ion injection properties observed with cyclic voltammetry is complemented by strong electrochromism probed by spectroelectrochemistry. Blending these molecules with high molecular weight polyethylene oxide (PEO) is found to improve both ion injection and thin film stability. The molecules and their corresponding PEO blends are investigated as active layers in organic electrochemical transistors (OECTs). For the most promising molecule:polymer blend (P4E4:PEO), p‐type accumulation mode OECTs with µA drain currents, μS peak transconductances, and a µC* figure‐of‐merit value of 0.81 F V−1 cm−1 s−1 are obtained.

, [23] is explored as a simple and easily modifiable starting point for the design of a proofof-concept small molecule mixed ionic-electronic conductor for OECT applications. We initially substituted the peripheral hexyl chains with triethylene glycol chains to allow for aqueous ion penetration in a thin film. Subsequently, we increased the electron-rich character of the molecule by introducing 3,4-ethylenedioxythiophene (EDOT) units and further extended A series of semiconducting small molecules with bithiophene or bis-3,4-ethylenedioxythiophene cores are designed and synthesized. The molecules display stable reversible oxidation in solution and can be reversibly oxidized in the solid state with aqueous electrolyte when functionalized with polar triethylene glycol side chains. Evidence of promising ion injection properties observed with cyclic voltammetry is complemented by strong electrochromism probed by spectroelectrochemistry. Blending these molecules with high molecular weight polyethylene oxide (PEO) is found to improve both ion injection and thin film stability. The molecules and their corresponding PEO blends are investigated as active layers in organic electrochemical transistors (OECTs). For the most promising molecule:polymer blend (P4E4:PEO), p-type accumulation mode OECTs with µA drain currents, µS peak transconductances, and a µC* figure-of-merit value of 0.81 F V −1 cm −1 s −1 are obtained.

Introduction
Organic electrochemical transistors (OECTs), as depicted schematically in Figure 1, have been widely explored for interfacing organic electronics with biologically and medically relevant systems. [1,2] The burgeoning field of organic bioelectronics has seen OECTs employed in a wide range of applications from neural interface the molecule length to improve π-stacking and intermolecular charge transport. Following this approach, an OECT with a small molecule active layer is demonstrated, while improved device performance is achieved when blending the small molecule with high molecular weight polyethylene oxide (PEO).

Electrochemical Characterization
The molecules were initially characterized by cyclic voltammetry (CV) in dichloromethane (DCM) solution with results summarized in Table 1 and Section S5.0 in the Supporting Information. Reversible oxidation events were observed for PTTP-C 6 for the formation of the radical cation and dication with half-wave potentials (E 1/2 ) of 0.51 and 1.01 V, respectively, versus ferrocene/ferrocenium (Fc/Fc + ), whereas PTTP exhibited slightly lower half-wave potentials of 0.39 and 0.75 V. The reduced half-wave potentials of PTTP are likely a result of the mesomeric effect of direct attachment of oxygen on the phenyl ring, increasing electron density on the conjugated backbone. For P2E2, clearly resolved first and second oxidation events with E 1/2 values of 0.04 and 0.50 V for cation and dication formation, respectively, are observed. Compared to PTTP, the electron-rich EDOT units lower the half-wave potentials of P2E2. Similarly, P4E4 has further reduced halfwave potentials with two less well-resolved oxidation events with E 1/2 values of 0.01 and 0.23 V. The lower half-wave potential of P4E4 compared to that of P2E2 for the second oxidation event to the dication is attributed to the increasing electron-rich core combined with a longer conjugation length, resulting in lower Coulombic repulsion of charges on the backbone.
Electrochemical conditions more relevant to OECT operation were probed by solid-state CV in aqueous electrolyte with resulting voltammograms depicted in Figure 2 and data summarized in Table 1. Unsurprisingly, the hydrophobic PTTP-C 6 could not be oxidized as the resistance to ion injection is too high to allow oxidation below the electrolysis potential of water. Introduction of the triethylene glycol chains in PTTP allows ion injection with the film showing a nonreversible oxidation event with a high onset of oxidation of 0.93 V versus Ag/Ag + . P2E2 exhibited a well-resolved reversible oxidation event with E 1/2 of 0.46 V and an onset potential of 0.34 V. Compared to P2E2, the longer and more electron-rich P4E4 showed a significantly lower onset of oxidation of 0.22 V with the first half-wave potential observed at 0.10 V as well as a second half-wave potential at 0.73 V for the oxidation event from radical cation to dication. Following a strong initial charge injection or memory peak previously noted for conducting polymers, [26] P2E2 and P4E4 films exhibited stable reversible redox behavior over many CV scans (Section S5.5, Supporting Information).
To improve charge injection into the film and overcome the low viscosity of small molecule solutions, films with 10 wt% high molecular weight PEO (M v 900 kDa) were also investigated. [27,28] The molecule:PEO blend solutions were drop cast onto the glassy carbon electrodes under the same conditions as for the neat molecule-based films. The P2E2:PEO blend ( Figure 2b) exhibited an E 1/2 value of 0.46 V and an onset potential of 0.34 V for the first oxidation event which is identical to what was observed for the neat molecule thin film. Slightly improved long-term electrochemical cycling stability was observed for the P2E2:PEO blend compared to the neat electroactive material (Section S5.6, Supporting Information). For the P4E4:PEO blend, we likewise observed nearly identical first and second half-wave potentials to what was observed for the neat P4E4 film. We do however note upon blending with PEO that the first onset of oxidation was lowered by ≈0.1 V concurrent with a significant increase in current density (Figure 2c), suggesting more effective ion injection in the blend film. Introduction of PEO had no effect on the aqueous switching of PTTP-C 6 and PTTP thin films (Sections S5.9 and S5.10, Supporting Information). From variable scan rate CV experiments, a linear relationship between peak current and the square-root of the scan rate was observed for both neat P2E2 and P4E4 films and their PEO blends (Section S5.11, Supporting Information). [29] This indicates a diffusion-controlled charge injection process in all four cases with extracted ionic diffusion coefficients plotted in Figure 2d. An increase of ionic diffusion coefficient across the series P2E2 < P2E2:PEO < P4E4 < P4E4:PEO suggests that P4E4:PEO has the most favorable ion injection under these conditions. The measured ionic diffusion coefficients are higher than those of partially glycolated polythiophenes and the value for the P4E4:PEO blend is only marginally lower than that of the high-performing OECT material p(g2T-TT). [30]

Optical Characterization
Optical characterization of solutions and films of the molecules was carried out by UV-vis-near-IR spectroscopy as depicted in Figure 3 and Sections S5. 1   Information. A gradual red-shift in absorption maximum (λ max ) was observed across the series going from PTTP (380 nm) to P2E2 (401 nm) and P4E4 (480 nm) with vibronic features observed for P2E2 and P4E4 owing to the planarizing intramolecular SO interactions. The thin film spectra exhibited the same trend in λ max as the solutions with P4E4 having a broadened and slightly red-shifted spectrum with λ max 485 nm and additional features around 462 and 532 nm. P2E2 thin films exhibited a significant red-shift over the solution phase by 51 nm with λ max 452 nm and significant vibronic shoulders at 284 and 420 nm. PTTP exhibited a slight blue-shift in thin film over the solution phase, suggesting poorer packing or H-aggregates in the solid state with λ max of 358 nm. Optical band gaps were calculated from the onset of absorption in the thin films and found to be 2.88, 2.66, and 2.13 eV for PTTP, P2E2, and P4E4, respectively. Thin film spectroelectrochemistry was employed to probe the formation and stability of oxidized species in aqueous electrolyte. For P2E2, we observed a gradual increase of the radical cation bands with absorption maxima at 630 and 1072 nm with concurrent bleaching of the π-π* transition at 401 nm for voltages between 0.1 and 0.5 V (Figure 3c). Subsequently, at increasing voltages, a band attributed to the dication P2E2 2 2+ with absorption maximum at 934 nm was observed. [31] The film could subsequently be reduced to the neutral state with around 50% loss of absorbance ascribed to degradation and partial film delamination. The P2E2:PEO film (Figure 3d) behaved similarly with the formation of the radical cation (λ max values at 628 and 1062 nm) and subsequently the dication (λ max 928 nm) combined with quenching of the π-π* transition. Compared to neat P2E2, the PEO-containing film degraded less during electrochemical cycling and could be reduced back to around 75% of initial absorbance. For P4E4, growth of two broad radical cation absorption features was observed with absorption maxima at 620 and 1018 nm for the neat film and at 600 and 1018 nm for the P4E4:PEO blend (Figure 3e,f). We note that the P4E4 radical cation appears to form at slightly lower potential in the blend (≈0.4 V) than in the neat film (≈0.5 V), most likely due to the faster ion diffusion in the blend (Figure 2d). The P4E4 dication afforded a peak with λ max at 810 and 804 nm, respectively, for the neat and PEO-containing film. Both the neat P4E4 and the blend film could be reduced back to neutral with the retention of 75% of the initial absorption.

Physical Properties
Using the B3LYP density functional theory (DFT) method with 6-311++g (d,p) basis set using the self-consistent reaction field model, we calculated the frontier orbital energies as well as optimized geometries. The DFT data are in good agreement with the experimentally observed narrowing of the optical band gap and decrease in the ionization potential when going from PTTP to P2E2 and to P4E4 (Section S8.0, Supporting Information). The oxygen atoms on the EDOT ethylene bridge have a well-known dipolar interaction with sulfur atoms of neighboring thiophene rings. [32] The DFT calculations corroborate these planarizing interactions with P2E2 and P4E4 having highly coplanar energy-minimized conformations.
A crystal structure for P2E2 was obtained from single crystals grown via antisolvent precipitation (CCDC deposition Number 1944170), (Section S9.1, Supporting Information). In the crystal, P2E2 exhibited a completely planar backbone with a SCCS dihedral angle of 0° as a result of the SO planarizing interaction in good agreement with the DFT data. The torsion angle was 1.68° for the SCCC dihedral (phenyl-EDOT linkage), which is significantly more coplanar than predicted by DFT, most likely due to intermolecular interactions not accounted for in the gas phase DFT. Despite this high degree of backbone co-planarity, the crystal packing revealed a preference for P2E2 to pack in a staggered arrangement as seen in Figure 4. Molecules are slipped in both the x and y directions with respect to the previous molecule enforcing an edge to face structure pre-venting effective π-overlap (Figure 4a). The layers of molecules are packed at an angle of 65° with respect to the plane of the π-conjugated backbone which favors packing of peripheral side chains further discouraging π-overlap.
In order to assess the microstructure of neat and PEOblended films of P2E2 and P4E4, we carried out grazing incident wide angle X-ray diffraction scattering (GIWAXS) (Figure 4c-f). The 2D GIWAXS patterns for P2E2 neat and blended films reveal many discrete spots consistent with an oriented polycrystalline film (Section S9.2, Supporting Information). Qualitatively, PEO-containing P2E2 films took on a marginally more isotropic ordering indicated by the subtle smearing out of the scattering peaks. In contrast, the neat film of P4E4 scatters much more similarly to a semicrystalline thin film. P4E4 displays a strong lamellar like out-of-plane scattering peak at low q z (≈0.3 Å −1 ) and a broad π-stack like peak at higher q z (≈1.55 Å −1 ). The calculated lamellar-like d-spacing of 21 Å is much shorter than the conjugated core of the molecule (≈30 Å), much less than the entire molecule including peripheral triethylene glycol chains. If this is in fact a lamellar-like scattering, this implies a significant tilt to the stacking of the conjugated molecular core, which is supported by the out-of-plane π-stacking which shows a maximum ≈±27° with respect to the q z direction. Also observed were two distinct in-plane scattering peaks. A strong peak with weak second order at ≈0.45 and 0.91 Å −1 , respectively, matches the interlayer d-spacing (≈14 Å) of EDOT-based thin films. A second strong in-plane peak at ≈0.8 Å −1 is potentially due to interchain spacing of the end-capped triethylene glycol chains. Overall, these data are interpreted as indicating a tilted end-on orientation of P4E4 molecules. When blended with PEO, the out-of-plane lamellar-like scatter shifts to lower q z (≈0.26 Å −1 ), effectively an ≈4.5 Å lamellar expansion to ≈25.5 Å. Further, a new out-of-plane peak appeared at similar q to the inplane peak is attributed to the interlayer spacing. This implies the presence of a new edge-on scattering population. While the in-plane peak positions remain the same in the PEO blended films, the relative peak intensities of the two major peaks were inverted.
Atomic force microscopy (Section S10.0, Supporting Information) was carried out on thin films of the neat molecules and their PEO blends. For PTTP, P2E2, and P4E4, continuous films were obtained with the smoother films obtained for P2E2 with negligible difference between neat and blended films and mean surface roughness (R a ) values of 2.21 and 2.14 nm, respectively. P4E4 exhibited a rougher film with smaller crystallites than P2E2. The P4E4:PEO blend was also slightly rougher than the P2E2 film. The increased roughness is likely as a result of more rapid nucleation due to the lower solubility of P4E4, with R a values of 6.31 nm and 8.36 nm for neat and blended films, respectively.

Transistor Characterization
P2E2 and P4E4, which both exhibited good electrochromic responses, were tested as the active material with and without PEO in organic electrochemical transistors according to previously reported procedures. [18,25] An aqueous NaCl electrolyte was dropped onto the device, a Ag/AgCl gate electrode was submerged in the electrolyte and the device was cycled three times before measurement to ensure stable recordings. Representative output and transfer curves can be seen in Figure 5 and in Section S10.0 in the Supporting Information. Initially, using neat P2E2 as the active layer, we observed no current across a number of devices and different spin-coating conditions despite promising electrochromism in aqueous electrolyte. We attribute the poor performance to the staggered crystal packing in the solid state where molecules exhibit minimal π-stacking and therefore negligible electronic charge transport. When blending P2E2 with 10 wt% high molecular weight PEO, a device which exhibited a threshold voltage V t of −0.28 V and a peak drain current of 6 nA was obtained (Figure 5a,b); the transconductance (g m ) could not be reliably extracted.
For OECTs fabricated with neat P4E4 as the active material, we observed a turn on for the devices and a moderate nanoamp drain current over three devices with an on/off ratio of 10 1 (Section S11.1, Supporting Information). The devices showed some hysteresis which could be mitigated by cycling of the devices and a slower sweep rate. We extracted a peak transconductance, g m , of 0.14 μS at a gate voltage of −0.65 V averaged across three devices, representing to the best of our knowledge the first p-type small molecule-based OECT. The most promising results were obtained from a blend of P4E4 and 10 wt% PEO (Figure 5c,d). The devices were measured to have an average peak transconductance of 0.65 μS at a gate voltage of −0.47 V and on/off ratios around 10 2 for an average device thickness of 31 nm. The μC* product, considered as a good material figure-of-merit for organic mixed ionic-electronic conductors, was found to be 0.81 F V −1 cm −1 s −1 in the saturation regime for P4E4:PEO while the inferior performance of the neat P4E4-based device prevented a reliable extraction of the μC* product in that case. [18]  the CV and spectroelectrochemistry data, the OECTs with the PEO blend turned on at lower bias (V t of −0.15 V) than the neat P4E4-based devices (V t of −0.32 V) while considerably lower hysteresis was also observed for the PEO-containing active layer. The relatively low currents observed for both neat P4E4 and its PEO blend made it difficult to extract the OECT charge carrier mobility with confidence and decoupling of the μC* product is consequently the subject of further investigations.

Conclusion
Given their often-excellent electronic charge transport properties, with mobilities exceeding those of semiconducting polymers, combined ease of synthesis and modular construction, small molecules have been widely used as active materials in organic electronics. We have shown here that mixed ionic-electronic conduction can also be facilitated in small molecules through molecular design. This has resulted in the first p-type small molecule OECT material, namely, P4E4, to complement the rapidly expanding library of p-and n-type mixed conduction materials. [33,34] A μC* product, widely adopted as one of the main figures-of-merit for OECTs, of 0.81 F V −1 cm −1 s −1 was achieved for P4E4 with 10 wt% high molecular weight PEO. Although lower than what has been reported for the best performing p-type OECT materials, this value is comparable to the μC* product for p(gNDI-g2T), a glycolated naphthalene diimide-based n-type polymer, and aforementioned partially glycolated polythiophene-based p-type materials. [18,30,35] From a molecular design point of view, in order to explore the concept of mixed ionic-electronic conduction in small molecules, we have synthesized a series of small molecules based on thiophene or more electron-rich EDOT cores with glycolated phenyl flanking groups. For the small molecules comprising electron-rich EDOT cores and peripheral oligoether side chains, we have shown very efficient charge injection from aqueous environment with stable electrochemical cycling in the solid state and low onset potentials well below the potential for water electrolysis. By thin film spectroelectrochemistry, facile formation of radical cation and dication species was evidenced by strong electrochromic responses at low potential with an aqueous supporting electrolyte in both neat molecule and PEO blends. Despite charge injection being observed for P2E2 films, no working OECTs were obtained, likely as a result of the polycrystalline nature of the film with inefficient π-stacking. Conversely, for neat P4E4 films, we measured ion injection as well as moderate OECT transconductances and on/off ratios. Upon blending with high molecular weight PEO, we obtained a functioning device for P2E2 with moderate nanoamp drain current, while much better performance was achieved with P4E4 affording an OECT threshold voltage of −0.15 V and a peak transconductance value of 0.65 μS at a gate voltage of −0.47 V.  We attribute the more efficient charge transport in P4E4 compared to P2E2 to its semicrystalline structure, which should be more tolerant to ion intercalation necessary for OECT operation.
With further optimization, and better understanding of important parameters such as charge carrier mobility and ionic mobility, we envision small molecules being good candidates for new mixed ionic-electronic conduction materials and an ideal platform for elucidating in much greater detail the intricate structure-property relations associated with mixed conduction.
CCDC 1944170 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

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