A Highly Conductive n-Type Conjugated Polymer Synthesized in Water

Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is a benchmark hole-transporting (p-type) polymer that finds applications in diverse electronic devices. Most of its success is due to its facile synthesis in water, exceptional processability from aqueous solutions, and outstanding electrical performance in ambient. Applications in fields like (opto-)electronics, bioelectronics, and energy harvesting/storage devices often necessitate the complementary use of both p-type and n-type (electron-transporting) materials. However, the availability of n-type materials amenable to water-based polymerization and processing remains limited. Herein, we present a novel synthesis method enabling direct polymerization in water, yielding a highly conductive, water-processable n-type conjugated polymer, namely, poly[(2,2′-(2,5-dihydroxy-1,4-phenylene)diacetic acid)-stat-3,7-dihydrobenzo[1,2-b:4,5-b′]difuran-2,6-dione] (PDADF), with remarkable electrical conductivity as high as 66 S cm–1, ranking among the highest for n-type polymers processed using green solvents. The new n-type polymer PDADF also exhibits outstanding stability, maintaining 90% of its initial conductivity after 146 days of storage in air. Our synthetic approach, along with the novel polymer it yields, promises significant advancements for the sustainable development of organic electronic materials and devices.


Synthesis
All chemical reagents were purchased from Sigma-Aldrich and used as received unless otherwise indicated.Duroquinone (TMQ) was purchased from TCI EUROPE N.V. PEDOT:PSS (Clevios PH1000) was purchased from Heraeus Holding GmbH.

Absorption spectra
All the Fourier-transformed infrared (FTIR) spectra samples were prepared by evaporating solvent to form a solid sample on a 70 °C hotplate and measured with PerkinElmer Spectron 3 in ATR mode.The ultraviolet-visible-near-infrared (UV-vis-NIR) absorption spectroscopy measurements were performed using a Perkin Elmer Lambda.

XPS and NEXAFS spectroscopy
The samples were deposited in ambient on Au substrates by drop-casting, followed by washing with acetone and water, and then quickly transferred into the load lock chamber of the ultrahigh vacuum (UHV) system for the following steps.X-ray photoemission spectroscopy (XPS) was performed with a Scienta-200 hemispherical analyzer using a monochromatized Al Kα source with a photon energy of 1486.6 eV.All photoelectron spectroscopy measurements were carried out with a base pressure lower than 1 ×10 -9 mbar.Near edge x-ray absorption fine structure (NEXAFS) spectroscopy measurement was performed at AU-Matline in ASTRID2 synchrotron source, Aarhus University, Denmark, in partial electron yield mode recording C, O Auger electrons using hemispherical analyzer operated in angular mode with a pass energy of 100 eV.

Grazing-incidence wide-angle X-ray scattering and AFM characterization
GIWAXS experiments were performed following the previous procedure. 4The samples were measured at Beamline 9A at the Pohang Accelerator Laboratory in South Korea.The X-ray energy was 11.08 eV, and the incidence angle was 0.12°.Samples were measured in vacuum, and the total exposure time was 10 s.The scattered X-rays were recorded by a charge-coupled device detector located 220.1727 mm from the sample.AFM images were recorded with an Icon XR from Bruker, using a silicon nitride cantilever with a spring constant of 40 N m -1 .

Thermoelectric generators
The TEGs had one p/n-leg pair module with the in-plane geometry design on the 100 µm-thick polyethylene naphthalate (PEN) substrate.PEDOT:PSS (PH1000) treated with ethylene glycol (EG 5 vol%) was used for the p-leg.The width of the p/n legs was set to 2 mm/10 mm due to the different conductivity of p/n materials; the leg length and thickness were both 2.5 mm and 1.5 µm, respectively.First, Cr/Au (5 nm/ 50 nm) were deposited onto the PEN substrate by evaporation through a shadow mask.Then, PDADF (50 wt% TW80) was cast on PEN in air, followed by submersion in acetone to remove impurities, and dried with a nitrogen gun.Next, PEDOT:PSS with EG was printed by drop casting.All the processes and measurements were done in air without encapsulation.

Electrical characterization
Electrical conductivity was measured by a four-point probe technique.The conductivity was calculated through the following equation: σ = IL/VWd, where W is the sample width, d is the thickness of the film, and L is the length between two electrodes, I is the applied current and V is the measured voltage.Here, W = 7 mm, L = 3 mm, and d is dependent on different samples.The Seebeck coefficients were measured inside the glovebox by a pair of Peltier elements to provide a temperature difference.All these measurements used a Keithley 4200-SCS semiconductor characterization system.

Organic electrochemical transistors (OECTs) and inverters
OECTs were fabricated following a procedure reported previously. 4The devices have a width and length of 100 μm and 12 μm, respectively.The inverters were assembled by connecting a 4 kΩ resistor in series with a PDADF-based OECT.The OECTs and inverters were characterized using a Keithley 4200A-SCS, utilizing 0.1 M NaCl electrolyte and Ag/AgCl pellet gate electrodes.

General materials characterization
The Nuclear magnetic resonance (NMR) data were collected from a 500 MHz Bruker system.The thermogravimetric analysis (TGA) was performed by TA Q500.The samples were heated from 50 °C to 850 °C with a heating rate of 10 °C min −1 under a nitrogen atmosphere.The differential scanning calorimetry (DSC) was performed by TA DSC250 under nitrogen flow at heating/cooling rates of 10/10 °C min −1 .The zeta potential of the solutions was characterized by dynamic light scattering at Zetasizer Nano ZS90 (laser wavelength = 632.8nm) at room temperature.All solutions are diluted to about 0.1 mg ml -1 (without filtering) and sonicated for 5 min before testing.Cyclic voltammetry (CV) was performed using a Potentiostat BioLogic SP-200.CV measurements were performed using 0.2 M tetrabutylammonium bis-trifluoromethane-sulfonimidate (TBA-TFSI) in propylene carbonate.After the films had dried, some small white crystals could be observed in the film.These spectra are recorded with ATR-FTIR.Hence, differences in signal strength can be attributed to variations in contact between the polymer powder and the diamond crystal, likely resulting from differences in coverage or applied pressure.FTIR spectra of HPDA derivatives relative to the OH vibration region.(c) PDADF in equilibrium with its quinone structure.We observe that HPDA shows appreciable O-H vibrations spanning 3300-3500 cm -1 .However, when HPDA is mixed with PBFDO, we note that the O-H vibration is lost or obscured in the gently sloped baseline (a).Similarly, we find in PDADF that there is no clear O-H vibration present in the FTIR spectrum.We attribute this obscuring of the O-H vibration to broadening due to (disordered) H-bonding of the copolymer and/or to the formation of a quinone structure.In both cases, the O-H vibration is expected to broaden to an extent where it is difficult to discern.We find a similar trend for HPDA, HPDA-Na, and HPDA-2Na, whereby further Na substitution causes an increased shift and broadening of the O-H vibration (b).This could be similarly ascribed to increased H-bonding of the molecules HPDA-Na and HPDA-2Na.The formation of a PDADF quinone structure (c) could not be ruled out, which might be another explanation for the obscuring/broadening O-H vibration.PBFDO shows smooth and uniform film morphology, with a roughness of around 2 nm and negligible particles on its surface compared to PDADF.On the contrary, PDADF exhibits a much rougher surface (∼70 nm) with aggregates.It is important to note that mixing PDADF with TW80 lowers its film roughness by more than a third (42.6 nm) and slightly increases film uniformity.The GIWAXS data reveal that PBFDO and PDADF films have similar edge-on orientation with respect to the substrate, with PDADF showing a weaker π-π stacking (010) signal compared to PBFDO (see Fig. S15a).PDADF exhibits a lamellar packing at qz = 0.657 Å⁻¹ (d-spacing = 9.53 Å), increasing to a d-spacing of 13.46 Å upon the addition of TW80 (Fig. S15b).Additionally, both PDADF and PDADF:TW80 (50 wt%) show reduced π-π stacking crystallinity with a decreased coherence length and increased paracrystalline disorder, both in-plane and out-of-plane, compared to PBFDO (see Table S2).Table S2.Summary of the calculated π-π stacking and lamellar distances and FWHM of (010) and (100) peaks from GIWAXS 1D data.(100)

Supplementary Figures and Tables
PBFDO by TMQ  FTIR spectra of polymers obtained from the polymerization scheme in (a).(d) Offset FTIR spectra of PDADF from 2 eq NaOH and HPDA derivatives.The highest electrical conductivity is reached for 1 eq NaOH.Similarities in the FTIR spectra for the samples up to 1.5 eq NaOH indicate that the polymers have a comparable defective structure.We tentatively attribute the lower electrical conductivity at NaOH < 1 eq to a lower degree of polymerization originating from the lower solubility of TMQ-PA.The use of higher equivalents of NaOH (i.e., 2 eq) yields a polymer with a higher content of opened lactone moieties, which is likely responsible for the lower electrical conductivity.Moreover, 2 eq leads to a more pronounced peak around 1692 cm −1 and 1553 cm −1 , corresponding to COOH and asymmetric COO − vibrations, respectively.These features are qualitatively similar to those observed in the FTIR spectra of fully ring-opened HPDA derivatives HPDA-Na and HPDA-2Na, suggesting a higher content of opened lactone moieties when 2 eq of NaOH are used.

Figure S17
. Different types of surfactants were added to the PDADF water dispersion to improve the electrical conductivity of the drop-cast films.(a) The use of anionic surfactants, both small molecules and polymer-based, was observed to diminish the electrical conductivity of the PDADF films.(b) The use of cationic surfactants like polyquaternium with varied functional groups has no discernible impact on the electrical conductivity.We attributed this to strong aggregation between PDADF and the cationic surfactants, causing non-uniform films and hindering any enhancement in electrical properties.(c) Nonionic surfactants such as linear PEI induced strong aggregation of PDADF, leading to a much-reduced electrical conductivity.Similarly, polyethylene glycol (PEG)based surfactants demonstrated poor conductivity.In contrast, the nonionic surfactant Tween 80 (TW80) displayed improved conductivity.We propose that the optimal combination of hydrophilic glycol chains and hydrophobic groups in TW80 results in the formation of well-defined micelles in conjunction with PDADF, contributing to enhanced conductivity.Table S3.Comparison of in-plane geometry polymer thermoelectric generator: thermoelectric leg materials, thermos module units (one unit = a pair of p and n legs), working temperature difference (ΔT), power output (Poutput), and power output per p-n pair.
Figure S1.Typical synthesis methods to prepare p-type and n-type CPs (a) PEDOT synthesis from water in the presence of PSS to form PEDOT:PSS water ink.(b) Stille polymerization of n-type CPs, Pd catalysts, and problematic solvents such as THF and toluene are typically used.(c) Suzuki polymerization of n-type CPs.(d) C-H activation of n-type CPs.(e) Aldol condensation of n-type CPs.

13 Figure S2 .
Figure S2.(a)Synthesis of PBFDO using TMQ as the catalyst, following Tang's protocol.3(b)Attempting to replace DMSO with water in the aforementioned synthesis resulted in the absence of polymerization when TMQ was utilized as the catalyst.(c) PBFDO synthesized from DMSO using TMQ-PA, highlighting the catalytic capability of TMQ-PA for both polymerization and insitu doping.(d) No polymerization was observed in water, employing TMQ-PA as the catalyst.(e) PDADF was successfully polymerized in water with TMQ-PANa as the catalyst.(f) Use of Tween 80 (TW80) surfactant to stabilize the water-based PDADF ink (50 wt% TW80).

Figure S3 .
Figure S3.Identification of PBFDO synthesized from TMQ and TMQ-PA by offset FTIR spectra of PBFDO synthesized from TMQ and TMQ-PA, indicating that PBFDO was formed regardless of catalyst.

Figure S6 .
Figure S6.(a) HBFDO is unstable in a basic environment, and a new compound, HPDA-Na, is formed.(b) Adding 1 eq NaOH induces the formation of HPDA-Na.(c) Adding 2 eq NaOH induces the formation of HPDA-2Na.(d) No PDADF was observed when HPDA was used as the monomer instead of HBFDO under the standard reaction conditions for synthesizing PDADF.

Figure S7. 1 H
Figure S7.1 H NMR spectra of HPDA, HPDA-Na, and HPDA-2Na.1 H NMR spectra indicate that the formation of sodium salt causes a slight shift of the methylene group.

Figure S8 .
Figure S8.(a) Comparison of PBFDO and PDADF's FTIR spectra in the OH vibration region.(b)FTIR spectra of HPDA derivatives relative to the OH vibration region.(c) PDADF in equilibrium with its quinone structure.We observe that HPDA shows appreciable O-H vibrations spanning 3300-3500 cm -1 .However, when HPDA is mixed with PBFDO, we note that the O-H vibration is lost or obscured in the gently sloped baseline (a).Similarly, we find in PDADF that there is no clear O-H vibration present in the FTIR spectrum.We attribute this obscuring of the O-H vibration to broadening due to (disordered) H-bonding of the copolymer and/or to the formation of a quinone structure.In both cases, the O-H vibration is expected to broaden to an extent where it is difficult to discern.We find a similar trend for HPDA, HPDA-Na, and HPDA-2Na, whereby further Na substitution causes an increased shift and broadening of the O-H vibration (b).This could be similarly ascribed to increased H-bonding of the molecules HPDA-Na and HPDA-2Na.The formation of a PDADF quinone structure (c) could not be ruled out, which might be another explanation for the obscuring/broadening O-H vibration.

Figure S10 .
Figure S10.X-ray absorption spectroscopy (XAS) was employed to probe the transition from C and O core levels to the unoccupied states of PDADF and PBFDO.The similar C and O K-edges of PDADF compared with those of PBFDO reveal that they have very closed electronic properties.The slight differences in C K-edge spectra (b) on the first shoulder suggest changes in molecular structure of PDADF compared to PBFDO.

Figure S12 .
Figure S12.Batch-to-batch electrical conductivity variations based on 24 samples made from 6 batches of PDADF.

Figure S13 .
Figure S13.(Top panels) AFM height images of PBFDO synthesized using TMQ and TMQ-PA; (Bottom panels) AFM height images of PDADF and PDADF (50 wt% TW80) synthesized in water.PBFDO shows smooth and uniform film morphology, with a roughness of around 2 nm and negligible particles on its surface compared to PDADF.On the contrary, PDADF exhibits a much rougher surface (∼70 nm) with aggregates.It is important to note that mixing PDADF with TW80 lowers its film roughness by more than a third (42.6 nm) and slightly increases film uniformity.

Figure S16 .
Figure S16.Polymerization with different equivalents of NaOH.(a) Polymerization scheme.(b) Electrical conductivity as a function of NaOH equivalents used during polymerization.(c) OffsetFTIR spectra of polymers obtained from the polymerization scheme in (a).(d) Offset FTIR spectra of PDADF from 2 eq NaOH and HPDA derivatives.The highest electrical conductivity is reached for 1 eq NaOH.Similarities in the FTIR spectra for the samples up to 1.5 eq NaOH indicate that the polymers have a comparable defective structure.We tentatively attribute the lower electrical conductivity at NaOH < 1 eq to a lower degree of polymerization originating from the lower solubility of TMQ-PA.The use of higher equivalents of NaOH (i.e., 2 eq) yields a polymer with a higher content of opened lactone moieties, which is likely responsible for the lower electrical conductivity.Moreover, 2 eq leads to a more pronounced peak around 1692 cm −1 and 1553 cm −1 , corresponding to COOH and asymmetric COO − vibrations, respectively.These features are qualitatively similar to those observed in the FTIR spectra of fully ring-opened HPDA derivatives HPDA-Na and HPDA-2Na, suggesting a higher content of opened lactone moieties when 2 eq of NaOH are used.

Figure S20 .
Figure S20.Differential scanning calorimetry (DSC) curves of polymers under nitrogen flow at a heating/cooling rate of 10/10 °C min −1 .All curves exhibit no obvious exothermic or endothermic features in the range of 25 ~ 250 °C.

Figure S22 .
Figure S22.OECTs and OECT-based inverters performance.(a) Output and (b) transfer curves of PDADF-based OECTs.(c) Stability of PDADF-based OECTs under 0-0.4V gate voltage pulses.(d) Transient response of PDADF-based OECT.(e) Typical voltage transfer characteristics and (f) voltage gains of the PDADF-based inverters.The PDADF films were cast and patterned as the ntype channel active material (width of 100 μm and length of 12 μm).An Ag/AgCl pellet was used as the gate electrode together with a 0.1 M NaCl aqueous electrolyte.The measured output and transfer characteristics are consistent with those expected for a partially doped conductive polymer.The max transconductance (gm) reaches 2.26 mS at a gate voltage (VG) of -0.1 V and source-drain voltage (VD) of 0.6 V.No obvious degradation of the drain current (ID) was observed when pulsing the VG between 0 V and 0.4 V for 15 min.Additionally, the transient response shows τON = 5.28 ms.To further demonstrate PDADF-OECT applicability, we fabricated an inverter by connecting a 4 kΩ resistor in series with the PDADF-OECT.The resulting inverter operates at supply voltages (VDD) of 0.6 V with a voltage gain over 2.2 V V −1 .

Figure S23 .
Figure S23.Internal resistance of one p-n pair TEG combination of PDADF (50 wt% TW80) and PEDOT:PSS.The result shows the stable internal resistance of TEG during the measurement with a mean value of 78 ohms.

Table S1 .
Summary of representative n-type conjugated polymers polymerized or processed from water/alcohol-based solvents.