Low-Cost and Sustainable Organic Thermoelectrics Based on Low-Dimensional Molecular Metals

A Thermoelectric Polymer Field-Effect Transistor via Iodine-Doped P3HT

Doping can alter certain electronics, including the thermoelectric properties of an organic semiconductor. These alterations may enable viable tunable devices that could be useful in temperature sensing for autonomous controls. Here, we demonstrate a dual-modulation organic field-effect transistor (OFET) where temperature can modulate the current-voltage characteristics of the OFET and gate voltage can modulate the thermoelectric properties of the active layer in the same device. Specifically, Poly(3-hexylthiophene-2,5-diyl) (P3HT) was utilized as the host p-type semiconducting polymer, and iodine was utilized as the thermoelectric minority dopant. The finished devices were characterized with a semiconductor analyzer system with temperature controlled using two thermoelectric cooling plates. The FETs with iodine doping levels in the range of 0.25% to 0.5% mole ratio with respect to the P3HT exhibit the greatest on/off ratios. This study also observed that P3HT thin film samples with an intermediate iodine doping concentration of 0.25% mole ratio exhibit an optimal thermoelectric power factor (PF).

Soft Organic Thermoelectric Materials: Principles, Current State of the Art and Applications

The enormous demand for waste heat utilization and burgeoning eco-friendly wearable materials has triggered huge interest in the development of thermoelectric materials that can harvest low-cost energy resources by converting waste heat to electricity efficiently. In particular, due to their high flexibility, nontoxicity, cost-effectivity, and promising applicability in various fields, organic thermoelectric materials are drawing more attention compared with their toxic, expensive, heavy, and brittle inorganic counterparts. Organic thermoelectric materials are approaching the figure of merit of the inorganic ones via the construction and optimization of unique transport pathways and device geometries. This review presents the recent development of the interdependence and decoupling principles of the thermoelectric efficiency parameters as well as the new achievements of high performance organic thermoelectric materials. Moreover, this review also discusses the advances in the thermoelectric devices with emphasis on their energy-related applications. It is believed that organic thermoelectric materials are emerging as green energy alternatives rivaling their conventional inorganic counterparts in the efficient and pure electricity harvesting from waste heat and solar thermal energy.

Significantly Enhanced Thermoelectric Properties of Copper Phthalocyanine/Single-Walled Carbon Nanotube Hybrids by Iodine Doping

The copper phthalocyanine/single-walled carbon nanotube (CuPcI/SWCNT) hybrids were fabricated through doping the CuPc/SWCNT mixture using iodine vapor. It was found that both CuPc and SWCNTs were oxidized by iodine vapor resulting in great increase in carrier concentration. Moreover, the strong π-π conjugation interactions between CuPcI- and I-doped SWCNTs make the CuPcI molecules to assemble on the surface of SWCNTs in an ordered face-on packing, which benefits decreasing the carrier transport barrier across the CuPcI/SWCNT interfaces. The combination of iodine bidoping and the ordered face-on packing of CuPcI on the SWCNT surface realizes the synergetic enhancement of carrier concentration and carrier mobility and therefore the great improvement of electrical conductivity. The maximum electrical conductivity (6281 S cm^-1) and thermoelectric power factor (∼304 μW m^-1 K^-2) at room temperature were obtained at a composition of 60 wt % SWCNTs. The power factor value is 3 orders of magnitude higher than the pure CuPcI and 1 order of magnitude higher than SWCNTs. Consequently, the highest ZT value of CuPc/SWCNT hybrids is up to 0.03, which is among the highest value of organic small-molecule complexes.

Synergistically Optimized Electrical and Thermal Transport Properties in Copper Phthalocyanine-Based Organic Small Molecule with Nanoscale Phase Separations

A series of copper phthalocyanine (CuPc)-based organic small molecules were prepared through vapor-phase reaction. Nanoscale phase separation was observed with tunable CuPc and copper phthalocyaninato iodide (CuPcI) phase content by changing the iodine ratio. The Seebeck coefficient of the samples was significantly enhanced, which is considered to be attributed to the enhanced surface polarization effect due to the formation of a great number of nanoscale interfaces between the CuPc phase and the CuPcI phase. In addition, these nanointerfaces also gave rise to increased phonon scattering and therefore significantly reduced the lattice thermal conductivity of the small-molecule samples. As a result of the combination of the synergistically optimized electrical and thermal transport properties, the maximum ZT value reaches 3.0 × 10^-2 at room temperature, which is among the highest values for small-molecule charge-transfer complex reported so far. Our results shed light on optimizing the thermoelectric performance of organic small molecules by introducing nanoscale phase separations and tailoring the nanoscale interfaces.

Paper Thermoelectrics by a Solvent-Free Drawing Method of All Carbon-Based Materials

As practical interest in the flexible or wearable thermoelectric generators (TEGs) has increased, the demand for the high-performance TEGs based on ecofriendly, mechanically resilient, and economically viable TEGs as alternatives to the brittle inorganic materials is growing. Organic or hybrid thermoelectric (TE) materials have been employed in flexible TEGs; however, their fabrication is normally carried out using wet processing such as spin-coating or screen printing. These techniques require materials dissolved or dispersed in solvents; thus, they limit the substrate choice. Herein, we have rationally designed solvent-free, all carbon-based TEGs dry-drawn on a regular office paper using few-layered graphene (FLG). This technique showed very good TE parameters, yielding a power factor of 97 μW m-1 K-2 at low temperatures. The p-type only device exhibited an output power of up to ∼19.48 nW. As a proof of concept, all carbon-based p-n TEGs were created on paper with the addition of HB pencil traces. The HB pencil exhibited low Seebeck coefficients (-7 μV K-1), and the traces were highly resistive compared to FLG traces, which resulted in significantly lower output power compared to the p-type only TEG. The demonstration of all carbon-based TEGs drawn on paper highlights the potential for future low-cost, flexible, and almost instantaneously created TEGs for low-power applications.

Lightweight and Bulk Organic Thermoelectric Generators Employing Novel P-Type Few-Layered Graphene Nanoflakes

Graphene exhibits both high electrical conductivity and large elastic modulus, which makes it an ideal material candidate for many electronic devices. At present not much work has been conducted on using graphene to construct thermoelectric devices, particularly due to its high thermal conductivity and lack of bulk fabrication. Films of graphene-based materials, however, and their nanocomposites have been shown to be promising candidates for thermoelectric energy generation. Exploring methods to enhance the thermoelectric performance of graphene and produce bulk samples can significantly widen its application in thermoelectrics. Realization of bulk organic materials in the thermoelectric community is highly desired to develop cheap, Earth-abundant, light, and nontoxic thermoelectric generators. In this context, this work reports a new approach using pressed pellets bars of few-layered graphene (FLG) nanoflakes employed in thermoelectric generators (TEGs). First, FLG nanoflakes were produced by a novel dry physical grinding technique followed by graphene nanoflake liberation using plasma treatment. The resultant material is highly pure with very low defects, possessing 3 to 5-layer stacks as proved by Raman spectroscopy, X-ray diffraction measurement, and scanning electron microscopy. The thermal and electronic properties confirm the anisotropy of the material and hence the varied performance characteristics parallel to and perpendicular to the pressing direction of the pellets. The full thermoelectric properties were characterized both parallel and perpendicular to the pressing direction, and the proof-of-concept thermoelectric generators were fabricated with variable amounts of legs.

Crystalline Structure-Dependent Mechanical and Thermoelectric Performance in Ag2Se1-x S x System

Self-powered wearable electronics require thermoelectric materials simultaneously with a high dimensionless figure of merit (zT) and good flexibility to convert the heat discharged by the human body into electricity. Ag₂(S,Se)-based semiconducting materials can well satisfy these requirements, and thus, they are attracting great attention in thermoelectric society recently. Ag₂(S,Se) crystalizes in an orthorhombic structure or monoclinic structure, depending on the detailed S/Se atomic ratio, but the relationship between its crystalline structure and mechanical/thermoelectric performance is still unclear to date. In this study, a series of Ag₂Se_1-x S _x (x = 0, 0.1, 0.2, 0.3, 0.4, and 0.45) samples were prepared and their mechanical and thermoelectric performance dependence on the crystalline structure was systematically investigated. x = 0.3 in the Ag₂Se_1-x S _x system was found to be the transition boundary between orthorhombic and monoclinic structures. Mechanical property measurement shows that the orthorhombic Ag₂Se_1-x S _x samples are brittle while the monoclinic Ag₂Se_1-x S _x samples are ductile and flexible. In addition, the orthorhombic Ag₂Se_1-x S _x samples show better electrical transport performance and higher zT than the monoclinic samples under a comparable carrier concentration, most likely due to their weaker electron-phonon interactions. This study sheds light on the further development of flexible inorganic TE materials.

Poly(3,4-Ethylenedioxythiophene) Nanoparticles as Building Blocks for Hybrid Thermoelectric Flexible Films

Hybrid thermoelectric flexible films based on poly(3,4-ethylenedioxythiophene) (PEDOT) nanoparticles and carbon nanotubes were prepared by using layer-by-layer (LbL) assembly. The employed PEDOT nanoparticles were synthesized by oxidative miniemulsion polymerization by using iron(III) p-toluenesulfonate hexahydrate (FeTos) as an oxidant and poly(diallyldimethylammonium chloride) (PDADMAC) as stabilizer. Sodium deoxycholate (DOC) was used as a stabilizer to prepare the aqueous dispersions of the carbon nanotubes. Hybrid thermoelectric films were finally prepared with different monomer/oxidant molar ratios and different types of carbon nanotubes, aiming to maximize the power factor (PF). The use of single-wall (SWCNT), double-wall (DWCNT), and multiwall (MWCNT) carbon nanotubes was compared. The Seebeck coefficient was measured by applying a temperature difference between the ends of the film and the electrical conductivity was measured by the Van der Pauw method. The best hybrid film in this study exhibited a PF of 72 µW m−1K−2. These films are prepared from aqueous dispersions with relatively low-cost materials and, due to lightweight and flexible properties, they are potentially good candidates to recover waste heat in wearable electronic applications.

Ultrafast Pathways of the Photoinduced Insulator-Metal Transition in a Low-Dimensional Organic Conductor

Among functional organic materials, low-dimensional molecular crystals represent an intriguing class of solids due to their tunable electronic, magnetic, and structural ground states. This work investigates Cu(Me,Br-dicyanoquinonediimine)₂ single crystals, a charge transfer radical ion salt which exhibits a Peierls insulator-to-metal transition at low temperatures. The ultrafast electron diffraction experiments observe collective atomic motions at the photoinduced phase transition with a temporal resolution of 1 ps. These measurements reveal the photoinduced lifting of the insulating phase to happen within 2 ps in the entire crystal volume with an external quantum efficiency of conduction band electrons per absorbed photon of larger than 20. This huge cooperativity of the system, directly monitored during the phase transition, is accompanied by specific intramolecular motions. However, only an additional internal volume expansion, corresponding to a pressure relief, allows the metallic state for long times to be optically locked. The identification of the microscopic molecular pathways that optically drive the structural Peierls transition in Cu(DCNQI)₂ highlights the tailored response to external stimuli available in these complex functional materials, a feature enabling high-speed optical sensing and switching with outstanding signal responsivity.

The Investigation of the Seebeck Effect of the Poly(3,4-Ethylenedioxythiophene)-Tosylate with the Various Concentrations of an Oxidant

Poly(3,4-ethylenedioxythiophene)-tosylate (PEDOT-Tos) can be synthesized through an in situ polymerization and doping process with iron(III) p-toluenesulfonate hexahydrate as an oxidant. Both the Seebeck coefficient and the electrical conductivity were modified by varying the concentration of the oxidant. We investigated the effects of varying the concentration of the oxidant on the particle sizes and doping (oxidation) levels of PEDOT-Tos for thermoelectric applications. We demonstrated that an increase in the oxidant enabled an expansion of the particle sizes and the doping levels of the PEDOT-Tos. The modification of the doping levels by the concentration of the oxidant can provide another approach for having an optimal power factor for thermoelectric applications. De-doping of PEDOTs by reduction agents has been generally investigated for changing its oxidation levels. In this study, we investigated the effect of the concentration of the oxidant of PEDOT-Tos on the oxidation levels, the electrical conductivities and the Seebeck coefficients. As loading the oxidant of PEDOT-Tos, the Seebeck coefficient was compromised, while the electrical conductivity increased.

Benzoquinone-Bridged Heterocyclic Zwitterions as Building Blocks for Molecular Semiconductors and Metals

In pursuit of closed-shell building blocks for single-component organic semiconductors and metals, we have prepared benzoquino-bis-1,2,3-thiaselenazole QS, a heterocyclic selenium-based zwitterion with a small gap (λ_max = 729 nm) between its highest occupied and lowest unoccupied molecular orbitals. In the solid state, QS exists in two crystalline phases and one nanocrystalline phase. The structures of the crystalline phases (space groups R3 c and P2₁/ c) have been determined by high-resolution powder X-ray diffraction methods at ambient and elevated pressures (0-15 GPa), and their crystal packing patterns have been compared with that of the related all-sulfur zwitterion benzoquino-bis-1,2,3-dithiazole QT (space group Cmc2₁). Structural differences between the S- and Se-based materials are interpreted in terms of local intermolecular S/Se···N'/O' secondary bonding interactions, the strength of which varies with the nature of the chalcogen (S vs Se). While the perfectly two-dimensional "brick-wall" packing pattern associated with the Cmc2₁ phase of QT is not found for QS, all three phases of QS are nonetheless small band gap semiconductors, with σ_RT ranging from 10^-5 S cm^-1 for the P2₁/ c phase to 10^-3 S cm^-1 for the R3 c phase. The bandwidths of the valence and conduction bands increase with applied pressure, leading to an increase in conductivity and a decrease in thermal activation energy E_act. For the R3 c phase, band gap closure to yield an organic molecular metal with a σ_RT of ∼10² S cm^-1 occurs at 6 GPa. Band gaps estimated from density functional theory band structure calculations on the ambient- and high-pressure crystal structures of QT and QS correlate well with those obtained experimentally.

Straightforward Synthesis of 2- and 2,8-Substituted Tetracenes

A simple regiospecific route to otherwise problematic substituted tetracenes is described. The diverse cores (E)-1,2-Ar¹ CH₂ (HOCH₂ )C=C(CH₂ OH)I (Ar¹ =Ph, 4-MePh, 4-MeOPh, 4-FPh) and (E)-1,2-I(HOCH₂ )C=C(CH₂ OH)I, accessed from ultra-low cost HOCH₂ C≡CCH₂ OH at multi-gram scales, allow the synthesis of diol libraries (E)-1,2-Ar¹ CH₂ (HOCH₂ )C=C(CH₂ OH)CH₂ Ar² (Ar² =Ph, 4-MePh, 4-iPrPh, 4-MeOPh, 4-FPh, 4-BrPh, 4-biphenyl, 4-styryl; 14 examples) by efficient Negishi coupling. Copper-catalysed aerobic oxidation cleanly provides dialdehydes (E)-1,2-Ar¹ CH₂ (CHO)C=C(CHO)CH₂ Ar² , which in many cases undergo titanium(IV) chloride-induced double Bradsher closure, providing a convenient method for the synthesis of regiochemically and analytically pure tetracenes (12 examples). The sequence is typically chromatography-free, scalable, efficient and technically simple to carry out.