Molecular Design of Semiconducting Polymers for High-Performance Organic Electrochemical Transistors

The organic electrochemical transistor (OECT), capable of transducing small ionic fluxes into electronic signals in an aqueous environment, is an ideal device to utilize in bioelectronic applications. Currently, most OECTs are fabricated with commercially available conducting poly(3,4-ethylenedioxythiophene) (PEDOT)-based suspensions and are therefore operated in depletion mode. Here, we present a series of semiconducting polymers designed to elucidate important structure–property guidelines required for accumulation mode OECT operation. We discuss key aspects relating to OECT performance such as ion and hole transport, electrochromic properties, operational voltage, and stability. The demonstration of our molecular design strategy is the fabrication of accumulation mode OECTs that clearly outperform state-of-the-art PEDOT-based devices, and show stability under aqueous operation without the need for formulation additives and cross-linkers.


Methods
Column chromatography was carried out with silica gel for flash chromatography from VWR Scientific. Microwave experiments were performed in a Biotage Initiator V 2.3. 1 H-and 13 C-NMR spectra were recorded on a Bruker AV-400 spectrometer at 298 K. UV-Vis absorption spectra were recorded on a UV-1601 Shimadzu UV-Vis spectrometer. Number-average (M n ) and weight-average (M w ) molecular weights were determined with an Agilent Technologies 1200 series GPC in either N,N-dimethylformamide at 40 °C or in 1,2,4-trichlorobenzene at 130 C, using two PL mixed B columns in series, and calibrated against narrow weight-average dispersity (Dw < 1.10) polystyrene standards. Film thicknesses were measured with a Dektak profilometer.
X-ray diffraction (XRD) measurements were carried out with a Panalytical Model X'Pert PRO MRD diffractometer equipped with a nickel-filtered Cu K 1 beam and a X'Celerator detector, using a current of I = 40 mA and an accelerating voltage of U = 40 kV.
Cyclic voltammograms were obtained using an Autolab PGSTAT101 potentiostat with a standard three-electrode setup with an ITO coated glass slide as the working electrode, a platinum mesh counter electrode and a Ag/Ag + reference electrode calibrated against Fc/Fc + . The measurements were carried out with 0.1 M tetrabutylammonium hexafluorophosphate as the supporting electrolyte at a scan rate of 100 mV/s. Ionization potentials were obtained using the equation: IP = (E ox -E Fc + 4.8V).
Spectroelectrochemistry was carried out using an Ocean Optics Spectrometer and an Autolab potentiostat with a standard three-electrode setup with a platinum wire working electrode and a Ag/Ag + reference electrode in 0.1 M NaCl in deionized water. Thin films were prepared by spin coating on ITO coated glass substrates.
The OECT fabrication process, similar to that reported previously, 1 included the deposition and patterning of metal, parylene, and the conducting polymers. We used photolithography to define the channels of the transistors with Au source and drain electrodes and interconnects on glass substrates.
These interconnects were insulated from the aqueous electrolyte by a vapour-polymerized parylene-C layer. The active channel and the insulating parylene layer were simultaneously patterned by using a second sacrificial parylene layer. After the polymer film was spin cast, the device was baked at 110 C for 60 min with subsequent rinsing in DI water. The thickness of the films at each channel was measured using a Dektak profilometer.

S3
Device Characterization: Devices were operated in the common source configuration. The electrolyte was a 0.1 M NaCl water solution dropped on top of the transistors. The gate electrode was a Ag/AgCl pellet which was immersed in the electrolyte. The IV-characteristics of the OECTs were measured using a National Instruments PXIe-1062Q system. An NI-PXI-4071 digital multimeter was used to measure drain current, and a NI-PXI 6289 measured drain and gate voltage. All the measurements were triggered through the built-in PXI architecture. The recorded signals were saved and analyzed using customized LabVIEW software.
Field-effect transistor devices: substrates were sonicated in Acetone, IPA, and ethanol for 15 minutes.
UV ozone cleaning 15 minutes before OTS treatment. After PFBT treatment baked at 150 C.
Polymers were deposited by spin coating at 2500 rpm. Thermal annealing for 30 min at 100 C in air.
Various device architectures were tested: BCBG (short channels), BCBG (long channels), TCBG and TCTG (CYTOP dielectric); best results reported herein were achieved with BCBG and short channels. Output curves could not be obtained for gBDT-T due to device instabilities.

3-(2-(2-(2-Methoxyethoxy)ethoxy)ethoxy)thiophene.
According to a modified procedure by Marks, 2 a dry 250 mL round bottom flask was charged with copper iodide (2.80 g, 14.70 mmol, 0.20 equiv), potassium tert-butoxide (12.40 g, 110.71 mmol, 1.5 equiv). The flask was purged with Argon after which triethylene glycol monomethyl ether (16.10 mL, 147.20 mmol, 2.00 equiv) was added and left to stir for 1 h at room temperature. 3-Bromothiophene (6.90 mL, 73.60 mmol, 1.00 equiv) was added and the reaction was heated to 100 o C for 24h. After cooling to room temperature, the mixture was put through a short silica plug eluting with DCM:MeOH 95:5 to remove inorganics. The crude material was concentrated in vacuo and purified via column chromatography, eluting with 3:2 hexanes:ethyl acetate (v/v). The desired product was isolated as a pale yellow oil (11.41 g, 63%). 1   hexanes (9.75 mmol) was then added dropwise over 10 min, and the reaction was stirred for two h at 0 C. This solution was then added to a dry 250 mL two-neck round bottom flask with condenser and S5 magnetic stir bar that had been charged with 3.46 g iron(iii) acetylacetonate (9.80 mmol) and 70 mL of dry THF. The reaction mixture was then heated to reflux and stirred for two h. After cooling to room temperature, the mixture was put through a short silica plug eluting with DCM:MeOH 95:5 to remove inorganics. The crude material was concentrated in vacuo and purified via column chromatography, eluting with 97:3 dichloromethane:methanol (v/v) giving of a yellow tinted oil that solidified at reduced temperature. The material was recrystallized from diethyl to afford white crystals 2,2'-bithiophene (0.80 g, 1.63 mmol, 1.00 equiv) in dry degassed chloroform (30 mL) was cooled to -30  C. N-bromosuccinimide (0.58 g, 3.27 mmol, 2.0 equiv) was added in small portions over 10 minutes at -30 o C. After the addition, the reaction was stirred at -30  C for 2 h. The reaction was then warmed to room temperature and was quenched by the addition of 30 mL saturated aqueous sodium sulphite. The reaction was extracted three times with dichloromethane (30 mL), the combined organic were washed with brine (50 mL), dried over magnesium sulphate, filtered, concentrated in vacuo to give a green oil. The oil was put through a short silica plug eluting with 95:5 dichloromethane:methanol (v/v), concentrated in vacuo to give a pale yellow oil which solidified upon cooling. The residue was recrystallized two times from diethyl ether, to give yellow microcrystals (0.81 g, 77%). 1  b']dithiophene 4 (0.14 mmol, 1.00 eq.) and 57.21 mg 2,5-bis(trimethylstannyl)thiophene (0.14 mmol, 1.00 eq.) were dissolved in 1.00 mL of degassed anhydrous chlorobenzene. 2.56 g Pd 2 (dba) 3                g2T-T