Benzothienobenzothiophene-Based Molecular Conductors: High Conductivity, Large Thermoelectric Power Factor, and One-Dimensional Instability

Revealing the Enhanced Thermoelectric Properties of Controllably Doped Donor-Acceptor Copolymer: The Impact of Regioregularity

Albeit considerable attention to the fast-developing organic thermoelectric (OTE) materials due to their flexibility and non-toxic features, it is still challenging to design an OTE polymer with superior thermoelectric properties. In this work, two "isomorphic" donor-acceptor (D-A) conjugated polymers are studied as the semiconductor in OTE devices, revealing for the first time the internal mechanism of regioregularity on thermoelectric performances in D-A type polymers. A higher molecular structure regularity can lead to higher crystalline order and mobility, higher doping efficiency, order of energy state, and thermoelectric (TE) performance. As a result, the regioregular P2F exhibits a maximum power factor (PF) of up to 113.27 µW m^-1  K^-2 , more than three times that of the regiorandom PRF (35.35 µW m^-1  K^-2 ). However, the regular backbone also implies lower miscibility with a dopant, negatively affecting TE performance. Therefore, the trade-off between doping efficiency and miscibility plays a vital role in OTE materials, and this work sheds light on the molecular design strategy of OTE polymers with state-of-the-art performances.

Recent Advances and Prospects of Small Molecular Organic Thermoelectric Materials

Thermoelectric (TE) materials possess unique energy conversion capabilities between heat and electrical energy. Small organic semiconductors have aroused widespread attention for the fabrication of TE devices due to their advantages of low toxicity, large area, light weight, and easy fabrication. However, the low TE properties hinder their large-scale commercial application. Herein, the basic knowledge about TE materials, including parameters affecting the TE performance and the remaining challenges of the organic thermoelectric (OTE) materials, are initially summarized in detail. Second, the optimization strategies of power factor, including the selection and design of dopants and structural modification of the dope-host are introduced. Third, some achievements of p- and n-type small molecular OTE materials are highlighted to briefly provide their future developing trend; finally, insights on the future development of OTE materials are also provided in this study.

Modular synthesis of unsymmetrical [1]benzothieno[3,2-b][1]benzothiophene molecular semiconductors for organic transistors

A modular approach to underexplored, unsymmetrical [1]benzothieno[3,2-b][1]benzothiophene (BTBT) scaffolds delivers a library of BTBT materials from readily available coupling partners by combining a transition-metal free Pummerer CH-CH-type cross-coupling and a Newman-Kwart reaction. This effective approach to unsymmetrical BTBT materials has allowed their properties to be studied. In particular, tuning the functional groups on the BTBT scaffold allows the solid-state assembly and molecular orbital energy levels to be modulated. Investigation of the charge transport properties of BTBT-containing small-molecule:polymer blends revealed the importance of molecular ordering during phase segregation and matching the highest occupied molecular orbital energy level with that of the semiconducting polymer binder, polyindacenodithiophene-benzothiadiazole (PIDTBT). The hole mobilities extracted from transistors fabricated using blends of PIDTBT with phenyl or methoxy functionalized unsymmetrical BTBTs were double those measured for devices fabricated using pristine PIDTBT. This study underscores the value of the synthetic methodology in providing a platform from which to study structure-property relationships in an underrepresented family of unsymmetrical BTBT molecular semiconductors.

Leaving-group controlled N- or C-cyclization of azadienes via domino sequences: synthesis, mechanism, and chemoselectivity

We reported a leaving-group controlled N- or C-cyclization of azadienes bearing a benzothiophene moiety. Density functional calculations revealed that the competition of two cyclizations was governed by the nature of the leaving group on the partner.

Unconventional Thermoelectric Materials for Energy Harvesting and Sensing Applications

Heat is an abundant but often wasted source of energy. Thus, harvesting just a portion of this tremendous amount of energy holds significant promise for a more sustainable society. While traditional solid-state inorganic semiconductors have dominated the research stage on thermal-to-electrical energy conversion, carbon-based semiconductors have recently attracted a great deal of attention as potential thermoelectric materials for low-temperature energy harvesting, primarily driven by the high abundance of their atomic elements, ease of processing/manufacturing, and intrinsically low thermal conductivity. This quest for new materials has resulted in the discovery of several new kinds of thermoelectric materials and concepts capable of converting a heat flux into an electrical current by means of various types of particles transporting the electric charge: (i) electrons, (ii) ions, and (iii) redox molecules. This has contributed to expanding the applications envisaged for thermoelectric materials far beyond simple conversion of heat into electricity. This is the motivation behind this review. This work is divided in three sections. In the first section, we present the basic principle of the thermoelectric effects when the particles transporting the electric charge are electrons, ions, and redox molecules and describe the conceptual differences between the three thermodiffusion phenomena. In the second section, we review the efforts made on developing devices exploiting these three effects and give a thorough understanding of what limits their performance. In the third section, we review the state-of-the-art thermoelectric materials investigated so far and provide a comprehensive understanding of what limits charge and energy transport in each of these classes of materials.

Surface Doping of Organic Single-Crystal Semiconductors to Produce Strain-Sensitive Conductive Nanosheets

A highly periodic electrostatic potential, even though established in van der Waals bonded organic crystals, is essential for the realization of a coherent band electron system. While impurity doping is an effective chemical operation that can precisely tune the energy of an electronic system, it always faces an unavoidable difficulty in molecular crystals because the introduction of a relatively high density of dopants inevitably destroys the highly ordered molecular framework. In striking contrast, a versatile strategy is presented to create coherent 2D electronic carriers at the surface of organic semiconductor crystals with their precise molecular structures preserved perfectly. The formation of an assembly of redox-active molecular dopants via a simple one-shot solution process on a molecularly flat crystalline surface allows efficient chemical doping and results in a relatively high carrier density of 1013 cm-2 at room temperature. Structural and magnetotransport analyses comprehensively reveal that excellent carrier transport and piezoresistive effects can be obtained that are similar to those in bulk crystals.

Organic co-crystal semiconductors: a crystal engineering perspective

Organic semiconductors are being pursued with vigor for the development of efficient and smart electronics. As a brief tutorial account, we traverse the fundamentals and advancements in the area and provide a crystal engineering perspective.

Tantalum Recycling by Solvent Extraction: Chloride Is Better than Fluoride

The recycling of tantalum (Ta) is becoming increasingly important due to the criticality of its supply from a conflict mineral. It is used extensively in modern electronics, such as in capacitors, and so electronic waste is a potentially valuable secondary source of this metal. However, the recycling of Ta is difficult, not least because of the challenges of its leaching and subsequent separation from other metals. In this work, we show that Ta(V) halides, such as TaCl5 and TaF5, which can potentially be accessed from Ta metal upon acid halide leaching, can be recovered by solvent extraction using a simple primary amide reagent. The need for high halide concentrations in the aqueous phase implies the formation of the hexahalide salts [TaX6]− (X = F, Cl) and that an anion-swing mechanism operates. While extraction of the fluorides is poor (up to 45%), excellent extraction under chloride conditions is found (>99%) and presents an alternative route to Ta recycling.

Two-dimensional radical–cationic Mott insulator based on an electron donor containing neither a tetrathiafulvalene nor tetrathiapentalene skeleton

We have succeeded in developing a two-dimensional radical–cationic Mott insulator that does not contain a 1,3-dithiol-2-ylidene moiety.

Selenium-Substituted Diketopyrrolopyrrole Polymer for High-Performance p-Type Organic Thermoelectric Materials

Development of high-performance organic thermoelectric (TE) materials is of vital importance for flexible power generation and solid-cooling applications. Demonstrated here is the significant enhancement in TE performance of selenium-substituted diketopyrrolopyrrole (DPP) derivatives. Along with strong intermolecular interactions and high Hall mobilities of 1.0-2.3 cm²  V^-1  s^-1 in doping-states for polymers, PDPPSe-12 exhibits a maximum power factor and ZT of up to 364 μW m^-1  K^-2 and 0.25, respectively. The performance is more than twice that of the sulfur-based DPP derivative and represents the highest value for p-type organic thermoelectric materials based on high-mobility polymers. These results reveal that selenium substitution can serve as a powerful strategy towards rationally designed thermoelectric polymers with state-of-the-art performances.

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.

New π-extended catecholato complexes of Pt(ii) and Pd(ii) containing a benzothienobenzothiophene (BTBT) moiety: synthesis, electrochemical behavior and charge transfer properties

Benzothienobenzothiophene (BTBT) and derivatives have received increasing attention as organic field-effect transistor materials and molecular conductors. This report presents the first synthesis of metal complexes involving a BTBT moiety, which was achieved by complexation of 2,2'-bipyridyl complexes of Pt(ii) and Pd(ii) with dihydroxy-substituted BTBT (1) as a new π-extended catecholato ligand (^tBu₂Bpy = 4,4'-di-tert-butyl-2,2'-dipyridyl). The resulting complexes M(^tBu₂Bpy)(O₂BTBT) (M = Pt (3Pt) and Pd (3Pd)) were characterized by UV-vis spectroscopy, density functional theory (DFT) calculations, and cyclic voltammetry. The electron donating ability of BTBT was substantially enhanced upon including two oxygen substituents followed by metal coordination. This enabled chemical oxidation of 3Pt and 3Pd with a mild chemical oxidant (ferrocenium hexafluorophosphate) and formation of the one-electron-oxidized state. While 3Pt and 3Pd exhibited an absorption band originating from a catecholate → Bpy ligand-to-ligand charge transfer transition typical of this class of catecholato complexes, the radical cations exhibited a unique π-π* intramolecular charge transfer (ICT) transition absorption in which the π and π* orbitals were the newly incorporated benzothienothiophene-based donor and semiquinonato-based acceptor, respectively. The BTBT⁺ skeleton was electronically divided into two sites by the present chemical modification. The ICT properties of the complexes were found to be modulated by varying the metal ions. These findings offer a new approach to molecular design for (semi)conducting materials using optical properties.

Quinoid-Resonant Conducting Polymers Achieve High Electrical Conductivity over 4000 S cm-1 for Thermoelectrics

New conducting polymers polythieno[3,4-b]thiophene-Tosylate (PTbT-Tos) are prepared by solution casting polymerization. Through tuning the alkyl group of TbT, the electrical conductivity can be effectively enhanced from 0.0001 to 450 S cm-1. Interestingly, the electrical conductivity of PTbT-C1-Tos increases significantly from 450 S cm-1 at room temperature to 4444 S cm-1 at 370 K, which is disparate from polyethylenedioxythiophene-Tos exhibiting metallic conducting behavior. Quasi-reversible phase transformation with temperature from 3D crystallites to lamellar-stacking coincides with the increasing electrical conductivity of PTbT-C1-Tos with heating. Methyl-substituted PTbT-Tos with the best electrical property is further utilized for thermoelectrics and a power factor as high as 263 µW m-1 K-1 is obtained. It is believed that PTbT-Tos will be a promising family of conducting polymers for solution-processed organic electronics.

Experimental and theoretical evidence of a supercritical-like transition in an organic semiconductor presenting colossal uniaxial negative thermal expansion

Thermal expansion coefficients of most materials are usually small, typically up to 50 parts per million per kelvin, and positive, i.e. materials expand when heated. Some materials show an atypical shrinking behavior in one or more crystallographic directions when heated. Here we show that a high mobility thiophene-based organic semiconductor, BHH-BTBT, has an exceptionally large negative expansion between 95 and 295 K (-216 < α2 = αb < -333 MK-1), being compensated by an even larger positive expansion in the perpendicular direction (287 < α1 < 634 MK-1). It is shown that these anomalous expansivities are completely absent in C8-BTBT, a much studied organic semiconductor with a closely related molecular formula and 3D crystallographic structure. Complete theoretical characterization of BHH-BTBT using ab initio molecular dynamics shows that below ∼200 K two different α and β domains exist of which one is dominant but which dynamically exchange around and above 210 K. A supercritical-like transition from an α dominated phase to a β dominated phase is observed using DSC measurements, UV-VIS spectroscopy, and X-ray diffraction. The origin of the extreme negative and positive thermal expansion is related to steric hindrance between adjacent tilted thiophene units and strongly enhanced by attractive S···S and S···C interactions within the highly anharmonic mixed-domain phase. This material could trigger the tailoring of optoelectronic devices highly sensitive to strain and temperature.

Carrier Charge Polarity in Mixed-Stack Charge-Transfer Crystals Containing Dithienobenzodithiophene

Dithieno[2,3- d;2'3'- d']benzo[1,2- b;4,5- b']dithiophene forms mixed-stack charge-transfer complexes with fluorinated tetracyanoquinodimethanes (F _nTCNQs, n = 0, 2, and 4) and dimethyldicyanoquinonediimine (DMDCNQI). The single-crystal transistors of the F _nTCNQ complexes exhibit electron transport, whereas the DMDCNQI complex shows hole transport as well. The dominance of electron transport is explained by the superexchange mechanism, where transfers corresponding to the acceptor-to-acceptor hopping ( t_e^eff) are more than 10 times larger than the donor-to-donor hopping ( t_h^eff). This is because the donor orbital next to the highest occupied molecular orbital makes a large contribution to the electron transport owing to the symmetry matching. Like this, inherently asymmetrical electron and hole transport in alternating stacks is understood by analyzing bridge orbitals other than the transport orbitals.

Evidence for Itinerant Carriers in an Anisotropic Narrow-Gap Semiconductor by Angle-Resolved Photoemission Spectroscopy

The ability to accurately determine the electronic structure of solids has become a key prerequisite for modern functional materials. For example, the precise determination of the electronic structure helps to balance the three thermoelectric parameters, which is the biggest challenge to design high-performance thermoelectric materials. Herein, by high-resolution, angle-resolved photoemission spectroscopy (ARPES), the itinerant carriers in CsBi₄ Te₆ (CBT) are revealed for the first time. CBT is a typical anisotropic, narrow-gap semiconductor used as a practical candidate for low-temperature thermoelectric applications, and p-doped CBT series show superconductivity at relatively low carrier concentrations. The ARPES results show a significantly larger bandwidth near the Fermi surface than calculations, which means the carriers transport anisotropically and itinerantly in CBT. It is reasonable to believe that these newly discovered features of carriers in narrow-gap semiconductors are promising for designing optimal thermoelectric materials and superconductors.

Conjugated-Backbone Effect of Organic Small Molecules for n-Type Thermoelectric Materials with ZT over 0.2

Conjugated backbones play a fundamental role in determining the electronic properties of organic semiconductors. On the basis of two solution-processable dihydropyrrolo[3,4-c]pyrrole-1,4-diylidenebis(thieno[3,2-b]thiophene) derivatives with aromatic and quinoid structures, we have carried out a systematic study of the relationship between the conjugated-backbone structure and the thermoelectric properties. In particular, a combination of UV-vis-NIR spectra, photoemission spectroscopy, and doping optimization are utilized to probe the interplay between energy levels, chemical doping, and thermoelectric performance. We found that a moderate change in the conjugated backbone leads to varied doping mechanisms and contributes to dramatic changes in the thermoelectric performance. Notably, the chemically doped A-DCV-DPPTT, a small molecule with aromatic structure, exhibits an electrical conductivity of 5.3 S cm^-1 and a high power factor (PF_373 K) up to 236 μW m^-1 K^-2, which is 50 times higher than that of Q-DCM-DPPTT with a quinoid structure. More importantly, the low thermal conductivity enables A-DCV-DPPTT to possess a figure of merit (ZT) of 0.23 ± 0.03, which is the highest value reported to date for thermoelectric materials based on organic small molecules. These results demonstrate that the modulation of the conjugated backbone represents a powerful strategy for tuning the electronic structure and mobility of organic semiconductors toward a maximum thermoelectric performance.

Efficient Benzodithiophene/Benzothiadiazole-Based n-Channel Charge Transporters

A series of donor-acceptor (D-A) small molecules based on electron-deficient benzothiadiazole (BTD) and electron-rich benzodithiophene (BDT) featuring an A-D-A structure is presented. Exhaustive spectroscopic, electrochemical, and computational studies evidence their electroactive nature and their ability to form well-ordered thin films with broad visible absorptions and low band gaps (ca. 2 eV). Time-resolved microwave conductivity (TRMC) studies unveil unexpected n-type charge transport displaying high electron mobilities around 0.1 cm²  V^-1  s^-1 . Efficient electron transport properties are consistent with the low electron reorganization energies (0.11-0.17 eV) theoretically predicted.

Fluorene-Based Two-Dimensional Covalent Organic Framework with Thermoelectric Properties through Doping

Organic semiconductors have great potential as flexible thermoelectric materials. A fluorene-based covalent organic framework (FL-COF-1) was designed with the aim of creating an enhanced π-π interaction among the crystalline backbones. By the introduction of fluorene units into the frameworks, the FL-COF-1 had high thermal stability with a BET surface area over 1300 m² g^-1. The open frameworks were favorable for doping with iodine and followed with the improved charge carrier mobility. The compressed pellet of I₂@FL-COF-1 exhibited a high Seebeck coefficient of 2450 μV K^-1 and power factor of 0.063 μW m^-1 K^-2 at room temperature, giving the first example of COFs' potential application as thermoelectric materials.

Improved stability of a metallic state in benzothienobenzothiophene-based molecular conductors: an effective increase of dimensionality with hydrogen bonds

A dihydroxy-substituted benzothienobenzothiophene, BTBT(OH)₂, was synthesized, and its charge-transfer (CT) salt, β-[BTBT(OH)₂]₂ClO₄, was successfully obtained. Thanks to the introduced hydroxy groups, a hydrogen-bonded chain structure connecting the BTBT molecules and counter anions was formed in the CT salt, which effectively increases the dimensionality of the electronic structure and consequently leads to a stable metallic state.

Anisotropic Li+ ion conductivity in a large single crystal of a Co(iii) coordination complex

Anisotropic Li⁺ ion conductivity has been observed in a large single crystal of a novel Co(iii) coordination complex attributable to the presence of directional ion channels with a suitable size.

Structure and characterization of charge transfer complexes of benzo[1,2-b:3,4-b′:5,6-b′′]trithiophene [C3h-BTT]

Four charge-transfer complexes of C_3h-BTT (2) with the organic acceptors TCNQ, F₄TCNQ, chloranil, and fluoranil were prepared and crystallographically characterized.

Air-stable ambipolar organic transistors based on charge-transfer complexes containing dibenzopyrrolopyrrole

A charge-transfer complex of DBPP and DMDCNQI shows air-stable ambipolar transistor characteristics.

Multiple functionalization of tungsten disulfide inorganic nanotubes by covalently grafted conductive polythiophenes

Covalently grafted nanometric polythiophene adlayers have been generated towards morphologically well-defined core–shell WS₂ INTs/polymer composites achieving high charge conductivity.