Poly(nickel-ethylenetetrathiolate) and Its Analogs: Theoretical

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Poly(nickel-ethylenetetrathiolate) and Its Analogs: Theoretical Prediction of High-Performance Doping-Free Thermoelectric Polymers Wen Shi, Gang Wu, Kedar Hippalgaonkar, Jian-Sheng Wang, Jianwei Xu, and Shuo-Wang Yang J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b08270 • Publication Date (Web): 02 Oct 2018 Downloaded from http://pubs.acs.org on October 4, 2018

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Poly(nickel-ethylenetetrathiolate) and Its Analogs: Theoretical Prediction of High-Performance Doping-Free Thermoelectric Polymers Wen Shi,† Gang Wu,† Kedar Hippalgaonkar,‡,§ Jian-Sheng Wang,# Jianwei Xu,‡ and Shuo-Wang Yang*,† †

Institute of High Performance Computing, Agency for Science, Technology and Research, 1 Fusionopolis Way, #16-16 Connexis, Singapore 138632 ‡ Institute of Materials Research and Engineering, Agency for Science, Technology and Research, 2 Fusionopolis Way, #0803 Innovis, Singapore 138634 § School of Materials Science & Engineering, Nanyang Technological University, 50 Nanyang Ave, Singapore 639798 # Department of Physics, National University of Singapore, 21 Lower Kent Ridge Rd, Singapore 117551 Supporting Information ABSTRACT: It is generally deemed that doping is a must for polymeric materials to achieve their high thermoelectric performance. We herein present the first report that intrinsically metallic behaviors and high-performance thermoelectric power factors can coexist within doping-free linear-backbone conducting polymers, poly(nickel-ethylenetetrathiolate) and its analogs. Based on density functional calculations, we have corroborated that four crystalline π-d conjugated transition-metal coordination polymers, including poly(Ni-C2S4), poly(Ni-C2Se4), poly(Pd-C2S4) and poly(Pt-C2S4) exhibit intrinsically metallic behavior arising from the formation of dense inter-molecular interaction networks between sulfur/selenium atoms. They show moderate carrier concentrations (1019-1021 cm-3) and decent conductivities (103-104 S cm-1), among which, poly(Ni-C2S4), poly(Ni-C2Se4) and poly(PdC2S4) possess high power factors (~ 103 µW m-1 K-2).

The cutting-edge solid-state thermoelectric (TE) materials based on Seebeck effect and Peltier effect, have enormous potentials in the applications of waste-heat recovery and refrigeration. The efficiency of a TE material is determined by a dimensionless figure of merit, ‫ ܵ( = ܶݖ‬ଶ ߪܶ)/ߢ, where S, σ, κ and T are the Seebeck coefficient, electrical conductivity, thermal conductivity and the average absolute temperature of the hot and cold junctions, respectively. A good TE material must possess a high zT, which requires an excellent conductivity and a high Seebeck coefficient while a poor thermal conductivity.1 Promising polymeric TE materials have emerged in recent twenty years.2-5 They exhibit numerous advantages as compared with conventional inorganic TE materials, such as biocompatibility, flexibility, low lattice thermal conductivity, etc. Historically, doping is regarded to be indispensable to achieve acceptable carrier concentrations and thereby decent conductivities for polymeric TE applications.6 However, dopants inevitably alter the wellordered microscopic packing structures of conjugated polymers, influence their charge transport properties and consequently degrade the TE performance.7,8 On the other hand, owing to the poor chemical doping efficiency in semiconducting polymers, the realization of high carrier concentration is another challenging task.9 Very recently, intrinsically metallic behaviors defined as no bandgap with the Fermi energy lying inside the band and intrinsi-

cally electrically conductive characteristics have been observed in some two-dimensionally (2D) layered metal-organic complex nanosheets, which warrants them as the potential doping-free TE materials. For instance, Cu-BHT (BHT = benzenehexathiol) exhibits intrinsically metallic behavior via ultraviolet photoemission valance band spectrum, and possesses ultrahigh room-temperature conductivity of 2500 S cm-1.10,11 In addition, crystalline Ni3(HIB)2 and Cu3(HIB)2 (HIB = hexaiminobenzene) show intrinsically metallic behavior via ultraviolet-photoelectron spectroscopy, and their room-temperature conductivity can approach up to 1000 S cm-1.12 However, to our best knowledge, intrinsically metallic behavior has never been reported in linear-backbone conducting polymers. To date, alkali metal doped π-d conjugated nickel coordination polymers are one of the best polymeric TE materials.6 For example, negatively charged poly(nickel-ethylenetetrathiolate) with potassium counter cations, i.e., poly[Kx(Ni-ett)] powder has a high experimental zT of 0.2 at 440 K,13 and its zT can even reach up to 0.32 in films at 400 K.14 We have elucidated theoretically that the isolated polymer chains for poly(Ni-C2S4), poly(nickel-ethylenetetraselenol) [poly(Ni-C2Se4)] and poly(palladium-ethylenetetrathiolate) [poly(Pd-C2S4)] possess narrow band gaps (< 1 eV) and large bandwidths (> 1 eV).15 The inter-molecular S-S/Se-Se interactions widely exist in organic materials, which strengthen the inter-molecular forces and thereby result in metallization.16 For these reasons, we hypothesize intermolecular S-S/Se-Se interaction networks could be formed in the crystalline poly(Ni-C2S4) and its analogs, which may narrow down the band gaps and even facilitate the intrinsically metallic behavior. To verify our hypotheses, we systematically investigate the stacking structures, electronic features and TE performance of crystalline poly(Ni-C2S4), poly(Ni-C2Se4), poly(Pd-C2S4), and poly(platinum-ethylenetetrathiolate) [poly(Pt-C2S4)] using firstprinciples calculations.

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C2S4) (3.194 Å) and poly(Ni-C2Se4) (3.325 Å)] (Table 1, Figure 1b, S2a, S2d and S2g). Among them, poly(Ni-C2Se4) possesses the shortest inter-chain distance (Table 1 and Figure S2c).

Table 1. The distance of two adjacent metal centers, d(M…M), the width of polymer backbone, Wb, the inter-chain distance, d(Inter-chain), and the parallel-chain distance, d(Parallel-chain) for crystalline poly(Ni-C2S4), poly(Ni-C2Se4), poly(Pd-C2S4) and poly(Pt-C2S4), respectively. d(M..M) (Å)

Wb (Å)

d(Inter-chain) (Å)

d(Parallel-chain) (Å)

poly(Ni-C2S4)

5.982

3.086

3.175

5.695

poly(Ni-C2Se4)

6.420

3.325

3.028

6.484

poly(Pd-C2S4)

6.209

3.194

3.345

6.152

poly(Pt-C2S4)

6.214

3.171

3.459

5.645

Figure 1. (a) Chemical structure of poly(Ni-C2S4). (b) Isolated polymer chain structure. (c) Top and (d) side views of crystalline structure for poly(Ni-C2S4), respectively. The second layer of polymer chains in the top view are shown in the line model. (e) Close inter-molecular contacts (black dashed lines) for S-S in crystalline poly(Ni-C2S4). The inter-molecular distances for S-S contacts are displayed in Å. The packing structures of abovementioned four crystalline polymers (Figure 1 and S2) are predicted through first-principles molecular dynamics (MD) simulations combined with simulated annealing, and the details are shown in Section 1 of Supporting Information (SI). Using extended X-ray absorption fine structure measurement, the bond lengths of Ni-S, C-S, and C-C in poly[Nax(Ni-C2S4)] are measured to be 2.169, 1.75 and 1.35 Å correspondingly; and the bond angles of S-Ni-S and Ni-S-C are 95° and 103°, respectively,17 which are well reproduced by our prediction at Perdew-Burke-Ernzerhof functional18 with dDsC dispersion correction19 (PBED) level (see Figure 1b, Figure S3a and Table S1). Besides, our simulated X-ray diffraction pattern for crystalline poly(Ni-C2S4) shows the peaks at 15.0° and 27.1° correspond to the parallel-chain distance (5.695 Å) and interchain distance (3.175 Å) (Figure S4a), which is consistent with the experimental observation for poly[Kx(Ni-C2S4)] films.14 These four polymers adopt a face-to-face packing with a shift of one five-membered ring distance along the backbone direction, so that the metal centers face the carbon-carbon bonds in the adjacent polymeric chain along the π-π stacking direction (top views of Figure 1c, S2b, S2e and S2h). This is because the metal centers are relatively positively charged, while the two carbon atoms are relatively negatively charged, which are confirmed via electron localization function maps (Figure 2a and S8) and Bader charge analysis (Figure 2b and S8). Meanwhile, the packing structures show slip-stacked conformation from the side views, and the polymer planes shift half inter-chain distance along the ππ stacking direction (Figure 1d, S2c, S2f and S2i). The distance of two adjacent polymeric backbone metal centers increases from poly(Ni-C2S4), poly(Pd-C2S4), poly(Pt-C2S4) to poly(Ni-C2Se4) (Table 1, Figure 1b, S2a, S2d and S2g) due to the increment of coordination bond lengths which are determined by the radii of metallic ions [Ni2+ (0.83 Å), Pd2+ (1.00 Å) and Pt2+ (0.94 Å)] and the van der Waals radii of coordination atoms [Se (1.90 Å) and S (1.80 Å)]. At the same time, the parallel-chain distances increases from poly(Ni-C2S4), poly(Pt-C2S4), poly(PdC2S4) to poly(Ni-C2Se4) (Table 1, Figure 1d, S2c, S2f and S2i), because the width of polymer backbones for poly(Ni-C2S4) (3.086 Å) is smaller than those of poly(Pt-C2S4) (3.171 Å), poly(Pd-

Figure 2. (a) Electron localization function maps and (b) Bader charge analysis for isolated poly(Ni-C2S4) chain. (c) Band structures and density of states (DOS) for crystalline poly(NiC2S4). The projected DOS of C-p orbitals, S-p orbitals and Ni-d orbitals are displayed. The horizontal red dashed lines are Fermi energy levels. These four polymers all show intrinsically metallic band structures with zero bandgap (Figure 2c and S6), which is the first observation of metallic behavior in doping-free linear-backbone conducting polymers. Our previously theoretical work has verified that 2D multilayered nickel bis(dithiolene) sheets show intrinsically metallic behavior due to the interlayer interaction between the sulfur atoms in adjacent layers.20 Interestingly, the inter-molecular S-S/Se-Se distances in these polymers are in the range of 3.0-4.0 Å; and the shortest ones are along the π-π stacking direction, namely, 3.3, 3.0, 3.3 and 3.5 Å for crystalline poly(Ni-C2S4), poly(Ni-C2Se4), poly(Pd-C2S4) and poly(Pt-C2S4), respectively (Figure 1e and S7). Such distances are shorter than or comparable with the sum of van der Waals radii of S (3.60 Å) or Se (3.80 Å) atoms. Accordingly, we can confirm nonbonding SS/Se-Se attractive interaction networks form, which greatly strengthen the inter-molecular forces and stabilize the polymeric stacking structures. So, the inter-molecular S-S/Se-Se attractive interaction networks are the origin of the metallization of these four polymers. This finding indicates the intrinsically metallic characteristic can be realized not only in 2D layered transitionmetal coordination nanosheets but also in linear-backbone π-d conjugated transition-metal coordination polymers. Moreover, the p orbitals of coordination atoms (S and Se), d orbitals of metallic centers (Ni, Pd and Pt) and p orbitals of carbon atoms all contribute to the states near Fermi energy (Figure 2c and S6), indicating the high degree of π-d conjugation along the polymeric backbones.

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Figure 3. (a) Intrinsic carrier concentration, N for poly(Ni-C2S4), poly(Ni-C2Se4), poly(Pd-C2S4) and poly(Pt-C2S4) at room temperature, respectively. (b) Carrier concentration, N vs. integral DOS for poly(Ni-C2S4), poly(Ni-C2Se4), poly(Pd-C2S4) and poly(Pt-C2S4), respectively. The room-temperature carrier concentration of poly(Ni-C2Se4) (2.56×1019 cm-3) is much lower than those of rest polymers [poly(Pd-C2S4) (7.34×1020 cm-3), poly(Pt-C2S4) (1.16×1021 cm-3) and poly(Ni-C2S4) (3.01×1021 cm-3)] (Figure 3a). This can be understood from their DOS near Fermi energy, where the intrinsic carrier concentration increases linearly along the increment of integral DOS in the order of poly(Ni-C2Se4), poly(Pd-C2S4), poly(Pt-C2S4) to poly(Ni-C2S4) (Figure 3b). Through field-effectmodulated doping, the carrier concentration for conducting polymers, indacenodithiophene-co-benzothiadiazole films can be realized around 4.0×1019 cm-3;21 and the carrier concentration for bismuth interfacial doped thiophene-diketopyrrolopyrrole-based quinoidal films can reach to 1.6×1019 cm-3.22 Compared with these two experimental examples, the same carrier concentration leavl can be easily achieved for these four doping-free polymers. We herein used Boltzmann transport theory23 and deformation potential theory24 to model the TE transport properties, and the details are shown in Section 1 of SI. Figure 4a shows the intrinsically excellent conductivities of these four polymers. Poly(NiC2S4) exhibit a high conductivity of 3.2×104 S cm-1 owning to its intrinsically high carrier concentration of 3.01×1021 cm-3 and moderate mobility of 66.65 cm2 V-1 s-1 (Figure 4b). Its conductivity is of the same order of magnitude as that for the high-quality polyacetylene films after doping with iodine (2×104 S cm-1).25 Recently, a high conductivity of 2.5×103 S cm-1 was detected for copper benzenehexathiolate coordination polymer films.11 Our predicted conductivities for poly(Ni-C2Se4) (4.1×103 S cm-1) and poly(Pt-C2S4) (1.4×103 S cm-1) are of the same order of magnitude. In addition, both poly(Ni-C2Se4) (-74 µV K-1) and poly(PdC2S4) (-106 µV K-1) show decent negative Seebeck coefficients, while poly(Ni-C2S4) (20 µV K-1) and poly(Pt-C2S4) (4 µV K-1) show small positive Seebeck coefficients, because Fermi energy levels for poly(Ni-C2S4) and poly(Pt-C2S4) are mainly located in the valence bands (Figure 2 and S6c). The Seebeck coefficients for poly(Ni-C2Se4) (−74 µV K-1) and poly(Pd-C2S4) (−106 µV K1 ) are comparable with that of poly[Kx(Ni-ett)] films (−125 µV K1 ) (Figure 4a).14 The trend of absolute Seebeck coefficients, |S| can be understood from Mott’s formula.26 As Figure S11 shows, |S| increases along the product of the differential DOS at Fermi energy and the reciprocal intrinsic carrier concentration. The doping-free poly(Ni-C2S4), poly(Ni-C2Se4) and poly(Pd-C2S4) with intrinsically metallic behavior possess not only ultrahigh conductivities (> 103 S cm-1), but also decent Seebeck coefficients, which makes them competitive to the currently state-of-the-art doped organic TE materials, such as n-type poly[Kx(Ni-C2S4)],14 p-type poly(styrenesulphonate) doped poly(3,4-ethylenedioxythiophene) (PEDOT)3 etc. (Figure 4a).

Figure 4. (a) Absolute Seebeck coefficients, |S| and conductivities, σ for poly(Ni-C2S4), poly(Ni-C2Se4), poly(PdC2S4) and poly(Pt-C2S4), respectively (red dots). Some experimental values are displayed (blue X-type). The materials for 1-8 are n-type 4-(1,3-dimethyl-2,3-dihydro-1Hbenzoimidazol-2-yl)phenyl)dimethyl amine doped dihydropyrrolo[3,4-c]pyrrole-1,4-diylidenebis(thieno[3,2b]thiophene),27 n-type bismuth doped thiophenediketopyrrolopyrrole-based quinoidal,22 p-type PEDOT:Tos,2 ntype poly[Kx(Ni-C2S4)] power,13 n-type poly[Kx(Ni-C2S4)] films,14 p-type PEDOT:Tos,4 p-type PEDOT:PSS,3 and p-type bis(trifluoromethylsulfonyl)imide (BTFMSI) doped PEDOT,5 respectively. (b) Power factors, S2σ and mobilities, µ for poly(NiC2S4), poly(Ni-C2Se4), poly(Pd-C2S4) and poly(Pt-C2S4), respectively. Poly(Ni-C2Se4) (2234 µW m-1 K-2), poly(Ni-C2S4) (1240 µW m-1 K-2) and poly(Pd-C2S4) (1034 µW m-1 K-2) have decent power factors. (Figure 4b and Table S5). The n-type power factor of hybrids of carbon nanotubes and PEDOT treated by tetrakis(dimethylamino)ethylene was measured to be 1050 µW m1 K-2.28 Compared with this report, our calculated n-type power factor for poly(Pd-C2S4) (1034 µW m-1 K-2) is comparable. Besides, an ultrahigh p-type power factor of 1270 µW m-1 K-2 has been achieved in tosylate doped PEDOT films,4 which is quite close to our predicted p-type power factor for poly(Ni-C2S4) (1240 µW m-1 K-2). In addition, these four polymers show decent mobilities (Figure 4b). Poly(Pd-C2S4) (7.764 cm2 V-1 s-1) and poly(Pt-C2S4) (7.367 cm2 V-1 s-1) exhibit nearly the same mobilities (Figure 4b and Table S3). Poly(Ni-C2Se4) has a high mobility of 989.6 cm2 V-1 s1 mainly due to its dispersed bands near Fermi energy. The intrachain hole mobility of isolated ladder-type poly(p-phenylenes) chains was detected to be around 600 cm2 V-1 s-1 through timeresolved microwave conductivity measurement,29 the same order of magnitude of poly(Ni-C2Se4). Meanwhile, poly(Ni-C2S4) show a good mobility of 66.65 cm2 V-1 s-1. Recently, a high field-effect electron/hole mobility of 116/99 cm2 V-1 s-1 were reported for CuBHT films composed 2D nanosheets.10 Compared with this experimental result, the mobility of poly(Ni-C2S4) is of the same order of magnitude. In conclusion, we have demonstrated for the first time the intrinsically metallic behavior can be realized in doping-free linear-backbone poly(Ni-C2S4), poly(Ni-C2Se4), poly(Pd-C2S4),

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and poly(Pt-C2S4). The reason behind is the formation of dense inter-molecular S-S/Se-Se attractive interaction networks, which strengthen the inter-molecular forces. Moreover, we found these four polymers possess moderate carrier concentrations and decent conductivities. Poly(Ni-C2S4), poly(Ni-C2Se4) and poly(Pd-C2S4) exhibite intrinsically prominent power factors (~ 103 µW m-1 K-2), which are comparable with those of the currently state-of-the-art doped TE polymers. We anticipate this work would spark new routes to design intrinsically electrically conducting polymers for TE applications.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Supporting computational details, discussions, figures, tables, and supporting references (PDF)

AUTHOR INFORMATION Corresponding Author E-mail: [email protected]

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT

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(16) Mailman, A.; Leitch, A. A.; Yong, W.; Steven, E.; Winter, S. M.; Claridge, R. C. M.; Assoud, A.; Tse, J. S.; Desgreniers, S.; Secco, R. A.; Oakley, R. T. J. Am. Chem. Soc. 2017, 139, 2180-2183. (17) Vogt, T.; Faulmann, C.; Soules, R.; Lecante, P.; Mosset, A.; Castan, P.; Cassoux, P.; Galy, J. J. Am. Chem. Soc. 1988, 110, 1833-1840. (18) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1996, 77, 3865-3868. (19) Steinmann, S. N.; Corminboeuf, C. J. Chem. Phys. 2011, 134, 044117. (20) Li, S.; Lü, T.-Y.; Zheng, J.-C.; Yang, S.-W.; Wang, J.-S.; Wu, G. 2D Materials 2018, 5, 035027. (21) Venkateshvaran, D.; Nikolka, M.; Sadhanala, A.; Lemaur, V.; Zelazny, M.; Kepa, M.; Hurhangee, M.; Kronemeijer, A. J.; Pecunia, V.; Nasrallah, I.; Romanov, I.; Broch, K.; McCulloch, I.; Emin, D.; Olivier, Y.; Cornil, J.; Beljonne, D.; Sirringhaus, H. Nature 2014, 515, 384-388. (22) Huang, D.; Wang, C.; Zou, Y.; Shen, X.; Zang, Y.; Shen, H.; Gao, X.; Yi, Y.; Xu, W.; Di, C.-a.; Zhu, D. Angew. Chem. Int. Ed. 2016, 128, 10830-10833. (23) Ziman, J. M. Principles of the Theory of Solid; Cambridge University Press: Cambridge, 1971. (24) Bardeen, J.; Shockley, W. Phys. Rev. 1950, 80, 72-80. (25) Basescu, N.; Liu, Z. X.; Moses, D.; Heeger, A. J.; Naarmann, H.; Theophilou, N. Nature 1987, 327, 403. (26) Cutler, M.; Mott, N. F. Phys. Rev. 1969, 181, 1336-1340. (27) Huang, D.; Yao, H.; Cui, Y.; Zou, Y.; Zhang, F.; Wang, C.; Shen, H.; Jin, W.; Zhu, J.; Diao, Y.; Xu, W.; Di, C.-a.; Zhu, D. J. Am. Chem. Soc. 2017, 139, 13013-13023. (28) Wang, H.; Hsu, J.-H.; Yi, S.-I.; Kim, S. L.; Choi, K.; Yang, G.; Yu, C. Adv. Mater. 2015, 27, 6855-6861. (29) Prins, P.; Grozema, F. C.; Schins, J. M.; Patil, S.; Scherf, U.; Siebbeles, L. D. A. Phys. Rev. Lett. 2006, 96, 146601.

This work was supported by the Agency for Science, Technology and Research (A*STAR) of Singapore (1527200024 and 1527200019). Computational resources are provided by the National Supercomputing Centre Singapore (NSCC) and A*STAR Computational Resource Centre (A*CRC).

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Poly(nickel-ethylenetetrathiolate) and Its Analogs: Theoretical Prediction of High-Performance Doping-Free Thermoelectric Polymers

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