high conductivity and electron‐transfer validation …...eral solution-processed n-doped organic...

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High Conductivity and Electron-Transfer Validation in an n-Type Fluoride-Anion-Doped Polymer for Thermoelectrics in Air

Xingang Zhao, Deepa Madan,* Yan Cheng, Jiawang Zhou, Hui Li, Susanna M. Thon, Arthur E. Bragg, Mallory E. DeCoster, Patrick E. Hopkins, and Howard E. Katz*

DOI: 10.1002/adma.201606928

While stable p-type conductive polymers, especially those based on thiophene subu-nits, have long been available,[5] solution-processable conductive n-type polymers remain rare, and few are stable in air.[6] Deficiencies of reported solution-process-able conductive n-type polymers are their low electrical conductivity σ and the air instability, severely limiting their extensive application. Chemical doping, inducing more free carriers (holes for p-doping or electrons for n-doping) into the host materials via redox reactions, is the most widely used strategy for increasing σ of conjugated polymers.[6] Recently, sev-eral solution-processed n-doped organic conjugated polymers were reported, most of which show typical conductivi-ties of <10−2 S cm−1. For example, the classic bithiophene donor-naphtha-lenediimide acceptor n-type poly mer P(NDI2OD-T2) (lowest unoccupied molecular orbital (LUMO) ≈ −4.0 eV) was doped with dimers of rhodocene, dimeth-ylaminophenyl dimethylbenzimidazo-line (N-DMBI), tetrakis(dimethylamino)

ethylene (TDAE), and with even more strongly n-doping N-DMBI dimer derivatives, which can effectively and quan-titatively dope P(NDI2OD-T2) by electron transfer, with elec-tron conductivity of 10−4–10−3 S cm−1 in spin-coated films under vacuum or N2.[7] Recently, a series of halo-substituted

Air-stable and soluble tetrabutylammonium fluoride (TBAF) is demonstrated as an efficient n-type dopant for the conjugated polymer ClBDPPV. Electron transfer from F− anions to the π-electron-deficient ClBDPPV through anion–π electronic interactions is strongly corroborated by the combined results of electron spin resonance, UV–vis–NIR, and ultraviolet photoelectron spec-troscopy. Doping of ClBDPPV with 25 mol% TBAF boosts electrical con-ductivity to up to 0.62 S cm−1, among the highest conductivities that have been reported for solution-processed n-type conjugated polymers, with a thermoelectric power factor of 0.63 µW m−1 K−2 in air. Importantly, the See-beck coefficient agrees with recently published correlations to conductivity. Moreover, the F−-doped ClBDPPV shows significant air stability, maintaining the conductivity of over 0.1 S cm−1 in a thick film after exposure to air for one week, to the best of our knowledge the first report of an air-stable solution-processable n-doped conductive polymer with this level of conductivity. The result shows that using solution-processable small-anion salts such as TBAF as an n-dopant of organic conjugated polymers possessing lower LUMO (lowest unoccupied molecular orbital), less than −4.2 eV) can open new opportunities toward high-performance air-stable solution-processable n-type thermoelectric (TE) conjugated polymers.

Thermoelectrics

Dr. X. Zhao, Prof. D. Madan, Dr. H. Li, Prof. H. E. KatzDepartment of Materials Science and EngineeringJohns Hopkins University3400 North Charles Street, Baltimore, MD 21218, USAE-mail: [email protected]; [email protected]. D. MadanDepartment of Mechanical EngineeringUniversity of Maryland Baltimore County1000 Hilltop Circle, Baltimore, MD 21250, USAY. Cheng, Prof. S. M. ThonDepartment of Electrical and Computer EngineeringJohns Hopkins University3400 North Charles Street, Baltimore, MD 21218, USA

Dr. J. Zhou, Prof. A. E. Bragg, Prof. H. E. KatzDepartment of ChemistryJohns Hopkins University3400 North Charles Street, Baltimore, MD 21218, USAM. E. DeCoster, Prof. P. E. HopkinsDepartment of Mechanical and Aerospace EngineeringUniversity of Virginia122 Engineer’s Way, Charlottesville, VA 22904, USA

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adma.201606928.

Multiple organic electronics applications require complemen-tary pairs of n- and p-type conductive polymers, including the charge injection/extraction layers of organic light-emitting diodes and hybrid solar cells,[1] complementary inverter cir-cuits[2] and sensors,[3] and organic thermoelectric modules.[4]

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benzodifurandione-phenylenevinylene (BDPPV) derivatives polymers were developed by Pei and co-workers. These are the most electron deficient conjugated polymers, possessing low-lying LUMO levels of around −4.20 eV.[8] Strikingly, doped ClBDPPV and FBDPPV with N-DMBI reached electrical con-ductivities of up to >4 S cm−1 and even to 14 S cm−1 for FBDPPV in N2, the highest electrical conductivity for solution-processed n-type conjugated polymers in N2. Compared to P(NDI2OD-T2), a more efficient doping process using the same n-dopant con-tributes to the lower LUMO and low crystallinity of BDPPV polymers. More recently, by inducing longer polaron delocaliza-tion length by minimizing the donor–acceptor character of the backbone, the amorphous acetylene-linked acceptor–acceptor P(PDI2OD-A) can achieve high conductivities of 0.45 S cm−1 by solution n-doping using (2-cyclohexyl-DMBI)2 while D–A type P(NDI2OD-T2) showed three orders of magnitude lower con-ductivity than that of P(PDI2OD-A) using the same n-dopant.[9] Although having the same LUMO (approximately −4.0 eV) with D–A type P(NDI2OD-T2), conductivities of up to 2.4 S cm−1 are obtained for the linear—“torsion-free”—ladder-type conducting polymers poly(benzobisimidazobenzophenanthroline (BBL) after exposure to strong reducing TDAE vapor for 3 h, which are three orders of magnitude higher than those of distorted donor–acceptor polymer P(NDI2OD-T2).[7d] The results indi-cate that larger polaron and spin delocalization in BBL result in an easier intramolecular transfer, and thus a higher polaron mobility along the backbone of the ladder-type polymer. How-ever, the weakness of BBL is that films have to be fabricated from hazardous and proton-conductive methanesulfonic acid due to its poor solubility in common organic solvents.[10] It is worth noting that all of the films mentioned above are spin-coated to be <100 nm thick, and were only described as working in N2 or vacuum. To the best of our knowledge, there are no reports of air-stable n-doped conductive conjugated polymers.

Compared to the great p-doping progress, such as achieving 1000 S cm−1 conductivities in poly(ethylenedioxythiophene),[11] the low ionization energies of highly reducing n-dopants and radical anions, inducing vulnerability to reaction with O2, make solution n-doping a greater challenge than p-doping.[6b,12] Cleav-able dimeric organometallic complexes and hydride donors like N-DMBI as solution-processed precursors have been the most widely used n-dopants, convertable into neutral radicals with higher highest occupied molecular orbitals (HOMOs) facili-tating n-doping through thermal- or photoactivation.[7a–c,8,9,13] However, the synthesis of the cleavable dimeric organometallic complexes and N-DMBI derivatives requires multiple steps. Moreover, post-activation of doped films can be inefficient. To overcome these issues, development of low-cost, stable, solu-tion-processable, and effective n-type dopants for thermoelec-tric (TE) conjugated polymer is highly desirable.

Recently, commercial organic tetrabutylammonium salts (X− = F−, OH−, AcO−, Br−, I−) or tetrabutyl phosphonium bro-mide have been reported as effective n-dopants for fullerene derivatives and a π-electron deficient molecules to obtain signif-icantly increased conductivities.[14] Surprisingly, an unprec-edented anion → π-electron-poor unit electron transfer event arises from strong electronic interactions between lone-pair electrons of anions (such as F−, OH−, AcO−, Br−, and I−) and π*-orbitals of the π-electron deficient units, which results in

effective n-doping.[14a,15] During the preparation of this manu-script, a report appeared demonstrating that the use of a very small amount of tetrabutylammonium fluoride (TBAF) to fill trap sites in the top-gate bottom-contact (TGBC) organic field effect transistors (OFETs) based on n-type P(NDI2OD-T2) results in enhanced electron mobility and air stability of TGBT OFET devices.[16]

Over the past decade, organic conjugated polymers for TE applications have been considered for lightweight and portable power generators because of their advantages of typically low thermal conductivities, tunable electronic structures (through precise molecular chemistry and doping), mechanical flexi-bility, and ease of scalable processing.[4,6,17] TE performance is typically benchmarked by the dimensionless figure of merit, ZT = σS2T/λ, composed of the electrical conductivity, σ, See-beck coefficient (the thermopower), S (also commonly denoted by α), the thermal conductivity, κ, and the absolute tempera-ture, T. Given that the thermal conductivities (0.1–1 W m−1 K−1) of organic materials are low, fairly constant, and sometimes dif-ficult to measure, the power factor σS2 (numerator of Z) is often reported instead of ZT in characterizing organic TE materials.[6]

Recently, great breakthroughs have been seen in p-type conjugated polymers with σ up to 1000 S cm−1 and reported ZT up to 0.42.[18] As for n-type materials, the polymers of Pei and co-workers show highest reported power factors of up to 28 µW m−1 K−2, projecting ZT between 0.01 and 0.1, the highest reported power factors for all-organic n-type polymers.[8] These were on very thin spincoated films maintained in inert atmos-phere. The BBL polymer mentioned above had a power factor of 0.43 µW m−1 K−2.[7d] With very thin films, it is very difficult to maintain the distinct temperature difference needed for TE applications. There are no reports using organic salts to dope n-type polymers through anion–π interactions to obtain the n-type conductive conjugated polymers for organic TEs.

Herein, we demonstrate that air-stable, solution-processed, efficient, and low-cost TBAF can be employed as effective and promising n-dopant for ClBDPPV to boost σ up to 0.62 S cm−1, which was maintained above 0.1 S cm−1 for a week or more in air, the first report of this level of air stability in a solution-pro-cessed n-type polymer. We also measure S of −99.2 ± 9.2 µV K−1, in accordance with increasingly accepted empirical models,[19] and consequently a power factor of 0.63 µW m−1 K−2 in air. Electron transfer from anions F− to the π-electron deficient poly mer ClBDPPV is supported by the combination of the results of electron spin resonance (ESR), UV–vis–NIR absorb-ance, and ultraviolet photoelectron spectroscopy (UPS). The results indicate that using soluble small-anion salts such as TBAF as n-dopants of organic conjugated polymers with lower LUMOs (less than −4.2 eV) can provide a promising strategy toward high-performance air-stable solution-processable n-type conjugated polymers.

Figure 1 shows the polymer structure and proposed reac-tion scheme for generation of ClBDPPV radical anion via reac-tion of TBAF and ClBDPPV. The monomer ClBDOPV and polymer ClBDPPV were synthesized according to a modified procedure in the reference and the details can be found in the Supporting Information.[8] Unlike the previous report using microwave reaction for the polymerization, ClBDPPV was syn-thesized with oil bath heating at 120 °C for two days. Polymer

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molecular weight was measured via gel permeation chroma-tography versus polystyrene standards using tetrahydrofuran (THF) as eluent. Number-average molecular weight (Mn) and polydispersity index (Mw/Mn) are 34 000 and 3.06, respectively, comparable to those of the reference. The HOMO and LUMO level of ClBDPPV based on the cyclic voltammetry measure-ment (Figure S3, Supporting Information) are −5.9 and −4.3 eV, respectively, also consistent with the reference.[8] All of the results indicate that ClBDPPV is successfully synthesized using oil bath heating. The calculated dihedral angles between the ClBDOPV and ethylene planes are small (θ1 and θ2, ≈10.06°) (Figure 2), indicating that ClBDPPV possesses an almost planar conjugated backbone. Moreover, computational results suggest that the HOMO and LUMO of ClBDPPV are well delocalized

over the polymer backbone, which indicate longer polaron delo-calization length and an easier intramolecular transfer, thus favoring a higher polaron mobility along the chain for almost planar ClBDPPV polymer.[7d,9] Considering that lower LUMO (−4.28 eV), high electron mobility, and longer polaron delocali-zation length would promote efficient n-doping and higher σ, ClBDPPV was chosen to evaluate the efficiency of the n-dopant TBAF.

The polymer was doped by mixing TBAF in THF and ClBDPPV in chlorobenzene (molar doping ratios mentioned below are dopant molecules: repeat units of the ClBDPPV). UV–vis–NIR spectroscopy is often utilized to evaluate the formation of charge transfer complexes in doped polymers. UV–vis–NIR absorption spectra of ClBDPPV polymer with or

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Figure 1. The polymer structure and proposed reaction scheme for generation of ClBDPPV radical anion via reaction of TBAF and ClBDPPV. Other F− addition sites and radical/anion resonance structures are possible.



without TBAF doping in chlorobenzene solution and solid film are shown in Figure 3a. ClBDPPV exhibits similar absorption profiles as the previous reference showed,[4] but with different absorption intensity in solid films because the thick films were made with drop casting replacing the thin film spincoating reported. After doping with 25 mol% TBAF, in addition to the absorption bands at wavelengths <1000 nm belonging to characteristic of the neutral polymer without extra electrons, absorption bands arose including 1000–1600 nm and extending from 1600 nm to >2000 nm in the doped ClBDPPV solution and films, which is reasonably associated with polaronic tran-sitions (λpolaron),[20] in agreement with that of ClBDPPV doped with N-DMBI.[8] In addition, after adding 25 mol % TBAF, the dark films slightly turn light and transparent (inserted in Figure 3b), further indicating that a number of neutral units in the films accepted electrons, giving rise to polarons. The emergence of new polaron bands in the doped polymer in solu-tion and films clearly indicated that electron transfer between F− ions and ClBDPPV occurred. Based on the experiments and quantum-chemical modeling, the proximity of the F anion to π-electron deficient groups in solution and in solid can consid-erably affect the doping efficiency.[14a] As we see in Figure 3a, higher absorption intensity ratios (λpolaron/λmax) were observed in doped polymer film than in the doped polymer in solution, suggesting the presence of more polarons (or free electrons) in the doped film than in the solution. The higher doping effi-ciency in the solid arises from much closer contact between F− anion and polymer main chain in the solid than that in solu-tion, thus making the electron transfer from anion to poly mer more favorable. Moreover, higher polaron concentration in

the annealed film indicates that annealing of the film can further slightly enhance the doping efficiency relative to the film without annealing, suggesting that the doping reaction between anion and polymer can be further kinetically acti-vated by heating. Also, considering the electrophilicity of the ClBDPPV and nucleophilicity of F− (enhanced because of the nonpolar medium) as well as the large mismatch in standard reduction potentials between ordinary F− and ClBDPPV, a more likely alternative to direct electronic transfer is an initial chemical reaction between the nucleophile F− and ClBDOPV subunit in the polymer ClBDPPV to form an adduct, followed by electron transfer to a second ClBDOPV subunit on another polymer chain, as shown in Figure 1, which is consistent with prior reaction reported between nucleophiles F−/OH− and elec-trophile C60.[20b]

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Figure 2. Optimized geometries and corresponding frontier molec-ular orbitals obtained from DFT calculations with ClBDPPV at B3LYP/ 6-31G(d,p) level. The torsion parameters are labeled as θ1 (10.06°) and θ2 (10.06°). Gray = carbon; blue = nitrogen; red = oxygen; green = chlorine; white = hydrogen.

Figure 3. a) UV–vis–NIR absorption spectra of ClBDPPV under pristine and doping conditions (25 mol% of TBAF) in solution and in films with and without annealing (normalized at λmax of neutral molecules). b) UPS spectra of the undoped and 25 mol% TBAF-doped ClBDPPV polymer film. Photographic images (inserted) of pure and 25 mol% TBAF-doped ClBDPPV films on glass substrates.



To better understand the energetics of the doping process and to confirm the n-doping by the shift of the Fermi level (EF) toward the LUMO, the energy-level alignment of the doped and undoped ClBDPPV films was probed by UPS (Figure 3b). Compared to the pristine polymer, a distinct shifting of HOMO edge and the vacuum level toward higher binding energy in ClBDPPV films doped with 25 mol% of TBAF was observed, consistent with EF moving upward toward the LUMO level in the bandgap.[21] This is another indication that effective n-doping occurred in ClBDPPV, and is consistent with emer-gence of new polaron bands in the UV–vis–NIR spectra. As shown from ESR spectroscopy (Figure 4), the paramagnetic signal for anion radicals of 25 mol% TBAF-doped ClBDPPV polymer in solution and in film could also be observed. Stronger ESR intensity appearing in doped ClBDPPV film indicates more anion radicals induced by TBAF in the film relative to that in solution, also consistent with appearance of more polarons in the doped film in UV–vis–NIR spectros-copy (Figure 3a). X-ray photoemission spectra is also utilized to investigate the chemical compositions on the surface of the polymer films with and without TBAF. As seen in Figure S5 in the Supporting Information, the F(1s) peak at 689.2 eV was observed in the doped ClBDPPV, whereas there is no F(1s) peak observed in the pristine polymer, suggesting the presence of TBAF on the surface. Atomic force microscopy (AFM) was employed to investigate further the effect of dopant amount on surface morphologies of pure ClBDPPV and ClBDPPV doped with 25% TBAF (Figure S6, Supporting Information). AFM of doped films (Figure S6c, Supporting Information) suggest the formation of aggregates on the top surface which are not observed on the surface of pure ClBDPPV films (Figure S6a, Supporting Information).

To evaluate the TE properties, the electrical conductivity and Seebeck coefficient of the doped ClBDPPV as a function of the dopant molar concentration were investigated (Figure 5). Solu-tions of mixtures of the ClBDPPV and TBAF in varied molar fraction were drop-cast on glass substrates with prepatterned

gold contacts. To obtain good packing of the polymer chain, these mixtures were covered with a Petri dish overnight and the solvent was evaporated very slowly. When dry, the films were annealed at 130 °C for 12 h. All of these processes were carried out in the glovebox. A four-probe method was employed for the measurement of σ, and S was obtained by directly measuring

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Figure 4. ESR spectra of ClBDPPV under pristine and doping conditions (25 mol% of TBAF) in solution and film.

Figure 5. Thermoelectric properties of doped ClBDPPV at different TBAF doping concentrations. a–c) Electrical conductivities and Seebeck coef-ficients (a), power factor (b), and air stability (c) of 25 mol% TBAF-doped ClBDPPV films.



the thermovoltages induced by imposing a temperature gra-dient across the doped polymer. σ as low as 1.7 × 10−6 S cm−1 is obtained in pristine ClBDPPV film. Introduction of a small molar ratio (≤25 mol %) of TBAF in ClBDPPV results in dra-matically progressive increasing of σ up to 0.62 S cm−1 in air, among the highest conductivities reported to date for solu-tion-processed n-doped polymers and the highest reported in air, accompanied with a decreasing of S from −1250 to −99.2 ± 9.2 µV K−1 in 25 mol % TBAF-doped ClBDPPV film, boosting the power factor of up to 0.63 µW m−1 K−2 in air. Notably, the negative sign of S confirmed electrons being the majority carrier in the doped ClBDPPV.[8] The evolution of the electrical conductivities with the dopant concentration is oppo-site to the change of S, consistent with their negative correla-tion with carrier concentrations. Moreover, the considerable increase in σ is in consistent with the appearance of polaron absorption bands in the UV–vis–NIR spectra, anion radical peak of ESR and an upward movement of EF to LUMO level, strongly corroborating the successful n-doping of ClBDPPV with TBAF. When increasing the molar ratio of TBAF to higher than 25 mol %, a rapid decrease of σ was observed, possibly stemming from the disruption of the π–π stacking of doped ClBDPPV induced by more dopants, which is detrimental for electron transport in the film.[22] Meanwhile, S also continues to decline, consistent with variation of S in n-type polymers with excess dopants.[7b,8,22] We also evaluated the electron mobilities using TGBC field-effect transistors based on ClBDPPV to fit with the recently published empirical model by Snyder, which demonstrates the relationship between the Seebeck coefficient and transport edge conductivity. Representative transfer and output characteristics of the devices are shown in Figure S7 in the Supporting Information. An electron mobility of up to 0.104 cm2 V−1 s−1 was achieved for ClBDPPV in TGBC OFETs using poly(methyl methacrylate) (PMMA) (880 nm) as dielec-tric for OFET devices in the ambient atmosphere, comparable to a prior report.[8] Very importantly, S for the two well-behaved compositions (10 and 25 mol% doping) agree with recently published correlations to conductivity in both empirical models[19,23] reported by Chabinyc and co-workers (Figure S8, red spots, Supporting Information) and Snyder (Figure S9, red spots, Supporting Information). Agreement is poor after the emergence of the disruption of the π–π stacking of doped ClBDPPV, namely, when the concentration of TBAF employed to dope ClBDPPV is greater than 25 mol%. The combination of good fitting to both empirical models and lack of a significant but transient (in 100 s) increase in S (Figure S10, Supporting Information), that would have arisen from a large ionic See-beck contribution, suggest that ionic contributions to σ and S can be reasonably ruled out.[24] Besides, the less basic anion salt tetrabutylphosphonium bromide (TBPB) was investigated as n-type dopant for ClBDPPV. σ of 0.013 S cm−1 was obtained in the 25 mol% TBPB-doped ClBDPPV films, 40 times lower than that of the 25 mol% TBAF-doped ClBDPPV films. This result agrees well with the more basic anion having stronger electronic interactions between lone-pair electrons of anions and π*-orbitals of the π-electron deficient units and conse-quently more efficient n-doping.[14a,15a]

Air instability of n-type polymers is mostly attributed to the susceptibility of transported electrons to H2O or O2 in air,

severely suppressing electron transporting processes.[10] Here, the air-stability of TBAF-doped ClBDPPV was investigated by measuring σ as a function of time (Figure 5c). The doped ClBDPPV shows good stability with a slow decreasing of σ. Sur-prisingly, after storing the devices under ambient conditions for one week, σ over 0.1 S cm−1 can be still obtained in ClBDPPV thick films doped by 25 mol% TBAF. The intensity of the ESR peak (Figure S11, Supporting Information) and the polaron peak of UV–vis–NIR (Figure 3a) of ClBDPPV doped with 25 mol% TBAF after exposure to air for one week decreased slightly compared to the fresh ClBDPPV doped film, suggesting that considerable amount of radical anion still present in the doped ClBDPPV film stored in the air for one week. This is con-sistent with the observed air stability of ClBDPPV films doped with 25 mol% TBAF. To the best of our knowledge, this is the first solution-processable n-doped conductive polymer reported to show any degree of stability in air. This stability is induced by the combination of lower LUMOs (−4.28 eV) of the ClBDPPV, the compensatory effect of extra electrons of doped ClBDPPV on the O2 and H2O electron traps, and self-encapsulation in the µm level thick film against penetration of H2O and O2. Self-encapsulation seems to promote the air stability; much thinner films (<100 nm) lost their conductivity within an hour.

Finally, the thermal conductivity of the doped CIBDPPV films with 25% TBAF was measured with time-domain ther-moreflectance. The thermal conductivity of the TBAF-doped film was 0.34 ± 0.11 W m−1 K−1. Details of the measurement, analysis and associated uncertainties are discussed in the Sup-porting Information. We note that the thermal conductivity values are in line with typical thermal conductivities of electri-cally conducting polymers.[25–27]

In conclusion, we have reported that the simple, air-stable, and solution-processable TBAF can function as a strong and efficient n-type dopant for ClBDPPV by solution processing. The effective and successful n-doping, arising from electron transfer from anions F− to the electron deficient polymer ClBDPPV via anion-π electronic interactions, is strongly sup-ported by the appearance of polaron absorption bands in the UV–vis–NIR spectra, an anion radical peak of ESR, and an obvious shift of EF to the LUMO level. σ up to 0.62 S cm−1 and a power factor of 0.63 µW m−1 K−2 were observed in ClBDPPV thick film doped with 25 mol%, among the highest σ reported for solution-processed n-type conjugated polymers. Surpri-singly, after one week storage at ambient conditions, the doped µm-level ClBDPPV films can still retain σ of over 0.1 S cm−1, the first report of air stability in a solution-processable n-doped polymer with this level of conductivity. The result reveals that a new and effective n-doping strategy employing simple air-stable, low-cost, solution-processable anion salts such as TBAF via electron transfer from anions F− to the π-electron deficient units induced by anion–π electronic interactions can drive the development of high-performance air-stable solution-process-able n-type conductive and thermoelectric conjugated polymers.

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

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AcknowledgementsThe authors thank Cambridge Display Technologies and the Johns Hopkins Applied Physics Laboratory for support of the initial synthesis and thermoelectric demonstration. The spectroscopy, modeling, electrochemistry, conductivity optimization, and transistor study were supported by the Department of Energy, Office of Basic Energy Sciences, Materials Chemistry Program, Grant Number DE-FG02-07ER46465. The thermal-transport measurements were supported by the Army Research Office, Grant Number W911NF-16-1-0320. The authors acknowledge graduate student Michael Barclay and Professor D. Howard Fairbrother for acquiring the UPS spectra. The authors thank Professors Michael Chabynic and G. Jeffrey Snyder for helpful discussions of Seebeck coefficient/electrical conductivity correlations, and Prof. Yonghao Zheng for helpful discussions about the EPR spectroscopy.

Conflict of InterestThe authors declare no conflict of interest.

Keywordschemical doping, fluoride salts, n-type conjugated polymers, organic semiconductors, thermoelectrics

Received: December 22, 2016Revised: June 11, 2017

Published online: July 14, 2017

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