Nickel ethenetetrathiolate (NiETT) polymers are promising n-type thermoelectric (TE) materials, but their insolubility requires the use of an inert polymer matrix to form films, which is detrimental to the TE performance. In this work, the use of thermal annealing as a post-treatment process simultaneously enhances the electrical conductivity from 6 ± 2 to 23 ± 3 S cm −1 and thermopower from −28 ± 3 to −74 ± 4 µV K −1 for NiETT/PVDF composite films. Spectroscopic characterization reveals that the underlying mechanism involves removal of residual solvent and volatile impurities (carbonyl sulfide and water) in the NiETT polymer backbone. Additionally, microscopic characterization reveals morphological changes caused by a densification of the film that improves chain packing. These effects result in a 25 × improvement in power factor from 0.5 to 12.5 µW m −1 K −2 for NiETT/PVDF films and provide insight into the composition of these coordination polymers that maintain their stability under ambient conditions.The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adfm.201803275.techniques. There has been considerable effort recently in the area of organic TEs; however, much of this has focused on derivatives of poly(3,4-ethylenedioxythiophene) [1][2][3][4] that have high electrical conductivities, σ, but often suffer from a low Seebeck coefficient or thermopower, S. In contrast, organic n-type materials tend to have high S and low σ values, [5][6][7] necessitating the use of doping, which often drastically reduces the thermopower and can render the materials air sensitive. These results illustrate the inherent inverse correlation between these TE properties, and these trade-offs have been well studied in inorganic TE materials [8] within the framework of band-like transport. [9] Polymers, however, are unique in that the material performance is strongly determined by the processing conditions that impact film morphology, [10,11] and transport is better described by thermally activated hopping conduction. [3,12,13] Metal-coordination compounds, specifically metal-dithiolene coordination polymers, consisting of bridging ethenetetrathiolate ligands and nickel metal centers (nickel ethenetetrathiolate, NiETT), are one of the best-performing n-type organic TE materials. Pressed pellets of poly[K(NiETT)] have demonstrated high TE properties with S = −120 µV K −1 and σ = 44 S cm −1 at room temperature, but the insoluble nature of the material limits its applicability. [14] One route to make NiETTs solution processable is by blending it with an inert matrix ( e.g., poly(vinylidene fluoride), PVDF) and casting from low vapor pressure solvents (e.g.. dimethyl sulfoxide, DMSO) albeit at the sacrifice of electrical conductivity. [15] Other strategies include fabricating composites with carbon nanotubes that yield high power factors, but the resulting material is often p-type. [16,17] This indicates the need for studying NiETTs further, and developing an n-type material that...
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