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

Radical and diradical states of bis(molybdenocene dithiolene) complexes

The synthesis and characterization of two bis(dithiolene) proligands involving heteroatomic linkers such as 1,4-dithiine and dihydro-1,4-disiline between the two protected dithiolene moieties are described. Two bimetallic complexes involving these heteroatomic bridges between two redox active bis(cyclopentadienyl)molybdenum dithiolene moieties have been synthesized and characterized by electrochemistry, spectroelectrochemistry, and their properties rationalized with (TD-)DFT. Cyclic voltammetry experiments show sequential oxidation of the two redox centers with ΔE values between successive one-electron transfers varying according to the nature of the bridge. Depending on the nature of the heteroatomic bridge, the bis-oxidized complexes exhibit either a diradical character with both radicals essentially localized on the metallacycles, or a closed-shell dicationic state.

Controlling the thermoelectric properties of organo-metallic coordination polymers through backbone geometry

Poly(nickel-benzene-1,2,4,5-tetrakis(thiolate)) (Ni-btt), an organometallic coordination polymer (OMCP) characterized by the coordination between benzene-1,2,4,5-tetrakis(thiolate) (btt) and Ni2+ ions, has been recognized as a promising p-type thermoelectric material. In this study, we employed a constitutional isomer based on benzene-1,2,3,4-tetrakis(thiolate) (ibtt) to generate the corresponding isomeric polymer, poly(nickel-benzene-1,2,3,4-tetrakis(thiolate)) (Ni-ibtt). Comparative analysis of Ni-ibtt and Ni-btt reveals several common infrared (IR) and Raman features attributed to their similar square-planar nickel-sulfur (Ni-S) coordination. Nevertheless, these two polymer isomers exhibit substantially different backbone geometries. Ni-btt possesses a linear backbone, whereas Ni-ibtt exhibits a more undulating, zig-zag-like structure. Consequently, Ni-ibtt demonstrates slightly higher solubility and an increased bandgap in comparison to Ni-btt. The most noteworthy dissimilarity, however, manifests in their thermoelectric properties. While Ni-btt exhibits p-type behavior, Ni-ibtt demonstrates n-type carrier characteristics. This intriguing divergence prompted further investigation into the influence of OMCP backbone geometry on the electronic structure and, particularly, the thermoelectric properties of these materials.

Neutral Antiaromatic Bis(1,2-dithiolene)-Chelated Nickel Complexes Bearing Multiradical Characters

Metal-organic complexes with radical characteristics are unique species attracting immense attention in recent years due to their peculiar properties and promising applicability in a wide variety of innovative research fields. However, the reported complexes typically do not exceed diradicality. This study systematically investigates a series of square planar neutral Ni-bis(1,2-dithiolene) and Ni-bis(1,2-dioxolene) complexes with linear, branched, and macrocyclic configurations via ab initio calculations. The linear Ni-complexes display strong singlet diradical characters, while their branched counterparts can also exhibit moderate singlet multiradical characters. Importantly, the macrocyclic Ni-complexes can possess extremely strong singlet multiradical characters up to dodeca-radicality along with their global antiaromaticity and hence strong induced ring current in the presence of an external magnetic field, ascribed to the localization of unpaired α and β electrons residing in the highest few molecular orbitals at different molecular sites, minimizing their coupling and annihilation. Our work represents the first indication in the rational design of novel multiradical neutral antiaromatic macrocyclic complexes for potential applications in molecular machines and electronic devices.

Electronic transport descriptors for the rapid screening of thermoelectric materials

The discovery of novel materials for thermoelectric energy conversion has potential to be accelerated by data-driven screening combined with high-throughput calculations. One way to increase the efficacy of successfully choosing a candidate material is through its evaluation using transport descriptors. Using a data-driven screening, we selected 12 potential candidates in the trigonal ABX₂ family, followed by charge transport property simulations from first principles. The results suggest that carrier scattering processes in these materials are dominated by ionised impurities and polar optical phonons, contrary to the oft-assumed acoustic-phonon-dominated scattering. Using these data, we further derive ground-state transport descriptors for the carrier mobility and the thermoelectric powerfactor. In addition to low carrier mass, high dielectric constant was found to be an important factor towards high carrier mobility. A quadratic correlation between dielectric constant and transport performance was established and further validated with literature. Looking ahead, dielectric constant can potentially be exploited as an independent criterion towards improved thermoelectric performance. Combined with calculations of thermal conductivity including Peierls and inter-branch coherent contributions, we conclude that the trigonal ABX₂ family has potential as high performance thermoelectrics in the intermediate temperature range for low grade waste heat harvesting.

Effect of the Linking Group on the Thermoelectric Properties of Poly(Schiff Base)s and Their Metallopolymers

As polymer-based thermoelectric (TE) materials possess attractive features such as light weight, flexibility, low toxicity and ease of processibility, an increasing number of conducting polymers and their composites with high TE performances have been developed in recent years. Up to date, however, the research focusing on the structure-performance relationship remains rare. In this paper, two series of poly(Schiff base)s with either C=C or C≡C linker and their metallopolymers were synthesized and doped with single-walled carbon nanotubes to evaluate how the linking groups affected the TE properties of the resulting composites. Apart from the effect exerted by the morphology, experimental results suggested that the linkers played a key role in determining the band gaps, preferred molecular conformation and extent of conjugation of the polymers, which became key factors that influenced the TE properties of the resulting materials. Additionally, upon coordination with transition metal ions, the TE properties could be tuned readily.

Emergence of Metallic Conductivity in Ordered One-Dimensional Coordination Polymer Thin Films upon Reductive Doping

The prospect of introducing tunable electric conductivity in metal-organic coordination polymers is of high interest for nanoelectronic applications. As the electronic properties of these materials are strongly dependent on their microstructure, the assembly of coordination polymers into thin films with well-controlled growth direction and thickness is crucial for practical devices. Here, we report the deposition of one-dimensional (1D) coordination polymer thin films of N,N'-dimethyl dithiooxamidato-copper by atomic/molecular layer deposition. High out-of-plane ordering is observed in the resulting thin films suggesting the formation of a well-ordered secondary structure by the parallel alignment of the 1D polymer chains. We show that the electrical conductivity of the thin films is highly dependent on their oxidation state. The as-deposited films are nearly insulating with an electrical conductivity below 10^-10 S cm^-1 with semiconductor-like temperature dependency. Partial reduction with H₂ at elevated temperature leads to an increase in the electrical conductivity by 8 orders of magnitude. In the high-conductance state, metallic behavior is observed over the temperature range of 2-300 K. Density functional theory calculations indicate that the metallic behavior originates from the formation of a half-filled energy band intersecting the Fermi level with the conduction pathway formed by the Cu-S-Cu interaction between neighboring polymer chains.

Copper(I)-Based Flexible Organic-Inorganic Coordination Polymer and Analogues: High-Power Factor Thermoelectrics

We investigated the transport properties of the single layer and bulk copper(I) 4-hydroxythiophenolate (Cu(SC₆H₄OH)) coordination polymers and their analogues Cu(SeC₆H₄OH) and Cu(TeC₆H₄OH) on the basis of density functional calculations. We found that the bulk phases show superior power factors when compared with single-layer phases. This performance is comparable to the reported best organic thermoelectric candidates p-type poly(3.4-ethylenedioxythiophene) (PEDOT) and n-type poly[K_x(Ni-ett)] (ett = ethylenetetrathiolate). The non-parabolic conduction band minimum of Cu(SeC₆H₄OH) along the x direction can decouple the transport quantities, Seebeck coefficient, and electrical conductivity to achieve the highest power factor among all the candidates.

Significantly Reduced Thermal-Activation Energy for Hole Transport via Simple Donor Engineering: Understanding the Role of Molecular Parameters for Thermoelectric Behaviors

Thermal activation energy for charge transfer (E_act,σ) plays a crucial role in determining the electrical properties of organic semiconductors, which are largely dominated by the Coulomb binding energy (E_coul,ICTC) or static energy disorder (σ_ICTC) of the formed integer charge transfer complexes at low or high doping concentration, respectively. Herein, we provide two typical donor-acceptor type polymers with distinct donors to disclose the role of molecular parameters in response for their corresponding thermoelectric (TE) behaviors. Noticeably, both the E_coul,ICTC and σ_ICTC of the polymers can be effectively restrained by varying the initial carbazole (CZ) donor to the dithieno[3,2-b:2',3'-d]pyrrole (DTP) moiety, which contributes to the remarkably decreased E_act,σ values of the PDTP-DPP than that of PCZ-DPP. Accordingly, the optimized power factors (PF) for PDTP-DPP (10.8 μW m^-1 K^-2) is almost 5 times higher than the primary PCZ-DPP (1.8 μW m^-1 K^-2) at ambient condition. In addition, a further improved PF over 85.5 μW m^-1 K^-2 can be achieved by PDTP-DPP at 488 K due to the synergy of thermal-induced dedoping and thermal-activated semiconducting behavior. Ultraviolet photoelectron and X-ray photoelectron spectroscopy measurements confirm the lower thermal activation energy for efficient p-doping of PDTP-DPP than that of PCZ-DPP.

Coordination polymers for n-type thermoelectric applications

Coordination polymers (CPs) are potential thermoelectric (TE) materials to replace the sometimes costly, brittle and toxic heavy metal inorganic TEs for near-ambient-temperature applications. Air-stable and highly conductive p-type thermoelectric CPs are relatively well known, but the their n-type counterparts are only now emerging and both are needed for most practical applications. This perspective reviews recent advances in the development of n-type thermoelectric CPs, particularly the 1D and 2D metal bisdithiolenes, and introduces a relatively new class of guest@metal-organic framework(MOF)-based composites. Low dimensional CPs with reasonable n-type thermoelectric performance are emerging with good charge mobility and air-stability but still relatively low electrical conductivity.

Optimizing the thermoelectric performances of conjugated polymer backbones via incorporating tailored platinum(ii) acetylides

Conjugated polymers incorporated with platinum acetylides offer an effective approach to realizing both high conductivity and high Seebeck coefficient values.

Unprecedented Enhancement of Thermoelectric Power Factor Induced by Pressure in Small-Molecule Organic Semiconductors

Establishing the relationship between pressure and heat-electricity interconversion in van der Waals bonded small-molecule organic semiconductors is critical not only in designing flexible thermoelectric materials, but also in developing organic electronics. Here, based on first-principles calculations and using naphthalene as a case study, an unprecedented elevation of p-type thermoelectric power factor induced by pressure is demonstrated; and the power factor increases by 267% from 159.5 µW m^-1 K^-2 under ambient conditions to 585.8 µW m^-1 K^-2 at 2.1 GPa. The underlying mechanism is attributed to the dramatic inhibition of lattice-vibration-caused electronic scattering. Furthermore, it is revealed that both restraining low-frequency intermolecular vibrational modes and increasing intermolecular electronic coupling are two essential factors that effectively suppress the electron-phonon scattering. From the standpoint of molecular design, these two conditions can be achieved by extending the π-conjugated backbones, introducing long alkyl sidechains to the π-cores, and substituting heteroatoms in the π-cores.

Boosting the Seebeck Coefficient for Organic Coordination Polymers: Role of Doping-Induced Polaron Band Formation

Organic polymers are becoming emerging thermoelectric materials. Tremendous progress has been achieved for p-type doping, but efficient n-type organic materials are still rare. By investigating potassium-doped n-type poly(nickel-ethylenetetrathiolate) using density functional theory coupled with Boltzmann transport equation, we find that (i) formation of the electron polaron band (EPB) split from the conduction band (CB) dominates electron transport; (ii) at low doping concentration, the upper CB gets involved in transport in addition to the EPB as the temperature rises, leading to a highly elevated Seebeck coefficient and power factor; and (iii) at even higher temperature, because the CB starts to dominate, the Seebeck coefficient levels off and then decreases with temperature. Such an "exotic" nonmonotonic temperature effect has been found in experiment but has never been explained. We find that such behavior is primarily due to a polaron effect. A doping-induced polaron band can be employed to boost the Seebeck coefficient, making the organic coordination polymer a peculiar n-type thermoelectric material.

Recent Progress in Thermoelectric Materials Based on Conjugated Polymers

Organic thermoelectric (TE) materials can directly convert heat to electricity, and they are emerging as new materials for energy harvesting and cooling technologies. The performance of TE materials mainly depends on the properties of materials, including the Seebeck coefficient, electrical conductivity, thermal conductivity, and thermal stability. Traditional TE materials are mostly based on low-bandgap inorganic compounds, such as bismuth chalcogenide, lead telluride, and tin selenide, while organic materials as promising TE materials are attracting more and more attention because of their intrinsic advantages, including cost-effectiveness, easy processing, low density, low thermal conductivity, and high flexibility. However, to meet the requirements of practical applications, the performance of organic TE materials needs much improvement. A variety of efforts have been made to enhance the performance of organic TE materials, including the modification of molecular structure, and chemical or electrochemical doping. In this review, we summarize recent progress in organic TE materials, and discuss the feasible strategies for enhancing the properties of organic TE materials for future energy-harvesting applications.