Quinoid‐Resonant Conducting Polymers Achieve High Electrical Conductivity over 4000 S cm−1 for Thermoelectrics

Abstract New conducting polymers polythieno[3,4‐b]thiophene‐Tosylate (PTbT‐Tos) are prepared by solution casting polymerization. Through tuning the alkyl group of TbT, the electrical conductivity can be effectively enhanced from 0.0001 to 450 S cm−1. Interestingly, the electrical conductivity of PTbT‐C1‐Tos increases significantly from 450 S cm−1 at room temperature to 4444 S cm−1 at 370 K, which is disparate from polyethylenedioxythiophene‐Tos exhibiting metallic conducting behavior. Quasi‐reversible phase transformation with temperature from 3D crystallites to lamellar‐stacking coincides with the increasing electrical conductivity of PTbT‐C1‐Tos with heating. Methyl‐substituted PTbT‐Tos with the best electrical property is further utilized for thermoelectrics and a power factor as high as 263 µW m−1 K−1 is obtained. It is believed that PTbT‐Tos will be a promising family of conducting polymers for solution‐processed organic electronics.


General tests and experimental details
Materials. All the reactions dealing with air-or moisture-sensitive compounds were carried out in a positive atmosphere of nitrogen. Unless stated otherwise, starting materials were obtained from Adamas, Aldrich and J&K and were used without any further purification.
Anhydrous THF were distilled over Na/benzophenone prior to use. The monomer thieno [3,4b] thiophene (TbT) with alkyl group ranging from C 8 H 17 to CH 3 and H were prepared according to the published procedures. [1] Measurements and General Methods. Hydrogen nuclear magnetic resonance ( 1 H NMR) and carbon nuclear magnetic resonance ( 13 C NMR) spectra were measured on BRUKER DMX 300 and BRUKER DMX 400 spectrometers. Chemical shifts for hydrogens are reported in parts per million (ppm,  scale) downfield from tetramethylsilane and are referenced to the residual protons in the NMR solvent and ( 13 C NMR spectra were recorded at 100 MHz. Chemical shifts for carbons are reported in parts per million (ppm,  scale) downfield from tetramethylsilane and are referenced to the carbon resonance of the solvent.
HR-ESI and HR-MALDI-TOF measurements were performed on an Applied Biosystems 4700 Proteomics Analyzer. The Ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS) measurements were carried out in a Kratos ULTRA AXIS DLD ultrahigh vacuum photoelectron spectroscopy connected to a custom-made high vacuum thermal evaporation system. The base pressure of the analysis chamber and the evaporation chamber were better than 5×10 −10 and 5×10 −9 Torr, respectively. The PTbT-Tos polymer films were obtained by spin-coating the solutions containing TbT monomers and Baytron C on 1 cm x 1 cm bare silicon wafer. After heating at 90 o C in air for 2 hours, the films were rinsed with ethanol three times and then transferred to hot plate at 90 o C heating for half an hour.
The PTbT-Tos films were transferred to the analysis chamber without breaking the vacuum for UPS and XPS measurements. An unfiltered He-discharge lamp (21.22 eV) and a monochromatic Al Kα X-ray (1486.6 eV) excitation sources were respectively equipped for UPS and XPS analysis. The energy resolution for UPS was 100 meV as estimated from the Fermi edge of an Ar + sputtered clean Au film. The samples were negatively biased at 9.0 V with respect to the electron analyzer for obtaining the secondary electron cutoff (SECO) spectra. The Fermi edge was calibrated from a UPS spectrum of the cleaned Au substrate.
UV-VIS-NIR spectra of the polymer thin films were recorded on a JASCO V-570 spectrometer. The temperature dependent X-ray diffraction (XRD) measurements were measured using a Bruker D8 Advance diffractometer. The 2θ scanning range was set from 5° to 30° with step size of 0.02° and integration time of 2s. A Cu rotating anode was used as the X-ray source with the generator voltage set as 40 kV to maximize the incident light flux. A LYNXEYE XE energy-dispersive 1D detector with an array of 192 point detectors was employed to deliver superior signal/noise ratio. Synchrotron-based grazing-incidence X-ray diffraction (GIXD) were measured at the SAXS/WAXS beamline at the Australian Synchrotron. [2] 15 keV photons were used with scattering patterns recorded on a Dectris Pilatus 1M detector. Images shown were acquired at an incident angle close the critical angle.
Such images were chosen from a series of images taken with incident X-ray angle varying from 0.02 to 0.15 in steps of 0.01 with the chosen image showing the highest scattering intesnity. The X-ray exposure time was 3 s such that no film damage was identified. The sample-to-detector distance as calibrated using a silver behenate sample. The results were analyzed by an altered version of the NIKA [3] 2D based in IgorPro. The Seebeck coefficient and electrical conductivity were measured with the same device. [4]          As shown in Table S1, the optimized power factor of PTbT-C1-Tos at room temperature is around two times higher than that of PEDOT-Tos under the same conditions prepared in this work. The electrical conductivity of PEDOT-Tos (600 S cm −1 ) is as high as the reported value, but the Seebeck coefficient is much lower (Energy Environ. Sci. 2013, 6, 788.). The reason is unclear and might be attributed to the polymer preparation and measurement.

Preparation and Characterization
Moreover, the power factor of PTbT-C1-Tos increased from 13.0 μW m -1 K -2 at room temperature to 263 μW m -1 K -2 at 370 K significantly, which is much higher than that of