Simultaneous Enhancement in Electrical Conductivity and Thermopower of n‐Type NiETT/PVDF Composite Films by Annealing

Abstract

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...

Thermoelectric Materials for Textile Applications

“…Other developments in solution processible n-type OTEs include metal-coordination compounds, specifically metal-dithiolene coordination polymers, consisting of bridging ethenetetrathiolate ligands and nickel metal centers (nickel ethenetetrathiolate, NiETT) (poly[Na(NiETT)]) have resulted in higher performing n-type OTEs with electrical conductivity of 44 S/cm and power factor of 20 µW/mK 2 [ 170 , 171 ]. Elmoughni et al demonstrated a textile-integrated TE generator consisting of both p and n-type organic semiconductors: PEDOT:PSS and poly[Na(NiETT)] respectively, stencil printed onto a knitted polyester fabric, as shown in Figure 6 [ 135 ].…”

Progress in Nickel‐Coordinated Polymers as Intrinsically Conducting n‐Type Thermoelectric Materials

“…Na(NiETT) [50] Film with PVDF/DMSO 6 -28 0.5 Na(NiETT) [51] Film with PVDF/DMSO, annealed at 160 °C 48 -70 23.5…”

A Printable Paste Based on a Stable n-Type Poly[Ni-tto] Semiconducting Polymer

“…This allows us to perform a comparative characterization and apply the same processing and analysis methods to poly [Ni-tto] as are applied to poly[K x (Ni-ett)]. For example, since Wolfe and co-authors [17] explain the significant improvement in the thermoelectric characteristics of the polymer composite film after its annealing due to the removal of volatile impurities, we also carried out a heat treatment of the obtained polymer before the paste preparation and after the film deposition. Heat treatment of the polymer powder leads to an improvement in electrical conductivity ( [17].…”

“…For example, since Wolfe and co-authors [17] explain the significant improvement in the thermoelectric characteristics of the polymer composite film after its annealing due to the removal of volatile impurities, we also carried out a heat treatment of the obtained polymer before the paste preparation and after the film deposition. Heat treatment of the polymer powder leads to an improvement in electrical conductivity ( [17]. Since the value of the Seebeck coefficient of the poly[Ni-tto], both in the sample not subjected to heat treatment and after heat treatment, coincides with the value of the Seebeck coefficient of the post-annealing poly[K x (Ni-ett)] powder, this serves as additional evidence of the hypothesis that we proposed in the previous article [9]-that tetrathiooxalate is the true monomer for a polymer of this type and does not need any additional chemical converting.…”

“…In our case, it obviously completely destroys the poly[Ni-tto] structure with the formation of a stronger chelate compound, nickel diethyldithiocarbamate. The preparation of a paste based on poly[K x (Ni-ett)] powder, which does not require its complete dissolution, is described in several sources [17,[19][20][21][22]. It was previously shown that DMSO, used as a solvent, and PVDF, as a binder, provide sufficient dispersibility of the polymer and the resulting paste (or ink) is capable to form a film.…”