Organic Thermoelectric Devices Research Articles

Flexible Organic Thermoelectric Composites and Devices with Enhanced Performances through Fine-Tuning of Molecular Energy Levels

Flexible Organic Thermoelectric Composites and Devices with Enhanced Performances through Fine-Tuning of Molecular Energy Levels

Organic Thermoelectric Devices for Energy Harvesting and Sensing Applications

AbstractThe rise of artificial intelligence and the Internet of Things have spurred an increasing demand for wearable, sustainable, and maintenance‐free power sources. Organic thermoelectric (OTE) devices are emerged as promising candidates because they are capable of converting heat energy into electricity directly without the need of moving parts, capable of flexible and seamless integration with multifunctional miniaturized electronics. In addition, OTE devices can perform straightforward as various self‐powered sensors, boosting their applications in the field of intelligent interactions. This review focuses on recent advances in OTE materials and devices for applications in energy harvesting and sensing. The basic knowledge and key parameters of OTE materials and devices are presented, followed by detailed introduction of recent progress of OTE generators, sensors, and other OTE‐integrated devices. Next, several aspects of optimizing OTE devices toward multifunctional applications are highlighted. Finally, an overview of the current challenges and future research directions of OTE‐based devices is addressed. It is hoped that this review can pave the way for speeding up a bright future for the development and practical applications of OTE devices.

Theoretical Perspective of Enhancing Order in n-Doped Thermoelectric Polymers through Side Chain Engineering: The Interplay of Counterion-Backbone Interaction and Side Chain Steric Hindrance.

Donor-acceptor (D-A) copolymers doped with n-type dopants are widely sought after for their potential in organic thermoelectric devices. However, the existing structural disorder significantly hampers their charge transport and thermoelectric performance. In this Letter, we propose a mechanism to mitigate this disorder through side chain engineering. Utilizing molecular dynamics simulations, we demonstrate that strong Coulomb interactions between counterions and charged polymer backbones induce a transition in the stacking arrangement of the polymer backbones from a slipped to a vertical configuration. However, the presence of side chain steric hindrance impedes the formation of closely packed and ordered vertical stacking arrangements, resulting in greater distances between adjacent backbones and a higher level of structural disorder in the doped films. Therefore, we propose minimizing side chain steric hindrance to enhance the structural order in doped films. Our findings provide essential insights for advancing high-performance thermoelectric polymers.

Cyclopentadithiophene-based Conjugated Polymers for Organic Thermoelectric Devices and Other Applications

The 4H-cyclopenta[2,1-b:3,4-b’]dithiophene (CPDT)-based conjugated polymers (CPs) have garnered significant attention in various fields of organic electronics due to their strong electron-donating properties, extended π-plane, and rigid, planar chemical structure. These unique features enable CPDT-based CPs to be highly advantageous for use in a range of organic semiconductor devices. While CPDT-based CPs have been extensively investigated and utilized as electron donors in various organic semiconductor devices, there is limited literature discussing the electrochemical properties of CPDT building blocks and the representative examples of CPDT-based CPs. In this mini-review, the authors outline the electrochemical properties of the CPDT building block, which stem from its rigid and planar chemical structure, facilitating the use of CPDT derivative materials in the field of organic semiconductors, such as organic photovoltaics (OPVs), organic thin film transistors (OTFTs), and organic photodetectors (OPDs). Furthermore, the authors highlight the advantages of CPDT-based CPs, particularly, for organic thermoelectric applications (OTEs) such as strong electron-donating properties and extended π-conjugation, which lead to facile p-type doping characteristics in CPDT-based CPs. The authors discuss the basic working principles of OTEs, including several key parameters of OTE devices such as the Seebeck coefficient (S) and power factor (PF). Additionally, the authors address the main challenge in OTEs: the trade-off relationship between electrical conductivity and the Seebeck coefficient. The review presents several strategies to overcome these trade-off limitations, focusing on CPDT and other CPs for OTE applications.

Thermally-induced charge carrier population control on graphene nanoribbons

Organic thermoelectric devices allow the conversion of heat into electricity in a sustainable way, making them strong candidates to solve the present energy crisis.

Kirigami-inspired organic and inorganic film-based flexible thermoelectric devices with built-in heat sink

Thermoelectric (TE) devices can convert heat to electricity directly, which offers a unique opportunity to realize waste heat recovery. However, conventional TE devices inevitably use heat sinks, which are bulky, rigid and heavy, limiting practical applications. Herein, we propose a fully integrated film-based TE device with intrinsically built-in fins as heat sink in a hexagonal honeycomb device structure, that simultaneously achieves high TE performance and conformability, as confirmed by experiments and modelling. A flexible Kapton substrate with copper electrodes, integrating either carbon nanotube (CNT) veils or bismuth telluride (Bi2Te3) TE ‘legs’, both of n- and p-type, achieved a remarkable specific power of 185.4 nW K−2 for a Bi2Te3-based device and 53.1 nW K−2 for a CNT-based device, thanks to the heat dissipation effect granted by the built-in fins. Besides, the addition of oriented polymer films interconnects, contracting when above their glass transition temperature, allowed a single substrate two-dimensional (2D) TE device to self-fold into a three-dimensional (3D) hexagonal honeycomb structure, with built-in fins, contactlessly and autonomously. The demonstrated shape-programmed kirigami-inspired scalable TE device paves the way for realising self-powered applications comprising hundreds of TE legs with both inorganic (e.g., Bi2Te3) and organic (e.g. CNT veils) TE materials and integrated heat sinks.

Modulation of the morphotropic phase boundary for high-performance ductile thermoelectric materials

The flexible thermoelectric technique, which can convert heat from the human body to electricity via the Seebeck effect, is expected to provide a peerless solution for the power supply of wearables. The recent discovery of ductile semiconductors has opened a new avenue for flexible thermoelectric technology, but their power factor and figure-of-merit values are still much lower than those of classic thermoelectric materials. Herein, we demonstrate the presence of morphotropic phase boundary in Ag2Se-Ag2S pseudobinary compounds. The morphotropic phase boundary can be freely tuned by adjusting the material thermal treatment processes. High-performance ductile thermoelectric materials with excellent power factor (22 μWcm−1 K−2) and figure-of-merit (0.61) values are realized near the morphotropic phase boundary at 300 K. These materials perform better than all existing ductile inorganic semiconductors and organic materials. Furthermore, the in-plane flexible thermoelectric device based on these high-performance thermoelectric materials demonstrates a normalized maximum power density reaching 0.26 Wm−1 under a temperature gradient of 20 K, which is at least two orders of magnitude higher than those of flexible organic thermoelectric devices. This work can greatly accelerate the development of flexible thermoelectric technology.

Advances in flexible inorganic thermoelectrics

AbstractSolid‐state bismuth telluride‐based thermoelectric devices enable the generation of electricity from temperature differences and have been commercially applied in various fields. However, in many scenarios, the surface of the heat source is not flat. Therefore, it is crucial to develop flexible thermoelectric materials and devices to efficiently utilize heat sources and expand their applications. Compared with organic thermoelectric materials and devices, inorganic flexible thermoelectric materials and devices have much higher thermoelectric performance and stability. Considering the rapid development in this research field, we carefully summarize the design principles, structures, and thermoelectric properties of inorganic flexible materials and their devices reported in the recent 3 years, including sulfides, selenides, tellurides, and composite materials designed based on these inorganics. The structural designs of flexible thermoelectric devices based on micro‐sized bulk materials are also carefully summarized. Additionally, we overview the mechanical stability and methods for reducing internal resistance for designs of inorganic flexible thermoelectric devices. In the end, we provide outlooks on future research directions for inorganic flexible thermoelectric materials and devices. This review will help guide thermoelectric researchers, beginners, and students.

Advances in Organic Thermoelectric Devices for Multiple Applications

AbstractOrganic thermoelectric (OTE) devices composed of π‐conjugated molecules are the basic building blocks for self‐powered integrated organic electronics. In addition to molecular design and doping strategies, the highly tunable energy conversion process in OTE devices has drawn significant research interest. Specifically, the diverse physical properties of organic semiconductors, novel device geometry design, and advanced fabrication techniques combined enable the OTE device to be a powerful multiscale platform from single‐molecular scale to thin films for modulating the TE performance, studying the fundamental charge transport mechanism, exploring novel energy conversion phenomenon, and realizing various functionalities. Here, the authors comprehensively review the recent experimental and theoretical advances in related topics of OTE devices, including multifunctional, external physical fields, and temperature modulated, as well as quantum OTE devices. The remaining issues and perspectives toward future OTE device research are also discussed at the end.

Contact/Noncontact‐Mode Thermoelectric Characteristics of Polytriarylamine/Lewis Acid Complex Films in Horizontal Device Geometry

Herein, the thermoelectric characteristics of polytriarylamine‐Lewis acid complex films were investigated by employing a horizontal device structure. Poly[N,N′‐bis(4‐butylphenyl)‐N,N′‐bis(phenyl)benzidine] (PolyTPD) is doped with tris(pentafluorophenyl)borane (BCF) via Lewis acid–base reactions by varying the BCF molar ratio (0–300 mol%). The resulting PolyTPD:BCF films are spun on glass substrates and silver electrodes are deposited leading to the horizontal type of organic thermoelectric devices (OTEDs). The OTEDs with the PolyTPD:BCF films are examined by varying temperature differences up to 45 K between two silver electrodes directly contacting hot/cold sources. Both device current and voltage are proportionally increased with the temperature difference, leading to higher powers at larger temperature differences, irrespective of BCF molar ratio. However, the highest current is achieved at 50 mol% owing to the highest electrical conductivity, even though the device voltage is slightly lower at 50 than 20 mol%. The origin of high electrical conductivity is assigned to the formation of radical cations in PolyTPD chains by BCF doping, which is influenced by the reaction time. The device current can be also generated by the illumination of IR radiation (noncontact mode) that is away from the OTEDs with the PolyTPD:BCF films.

Realizing p‐Type and n‐Type Doping of a Single Conjugated Polymer via Incorporation of a Thienoisatin‐Terminated Quinoidal Unit

AbstractSelective doping of a single conjugated polymer (CP) to obtain p‐type and n‐type conductive materials would be highly attractive for organic thermoelectric applications, because it will greatly reduce the time and costs of synthesizing different types of CPs. However, this strategy has rarely been investigated. In this study, two CPs are synthesized, designated PTQDPP‐T and PTQDPP‐2FT, based on a newly developed quinoidal unit with thienoisatin as the termini and a thiophene‐flanked diketopyrrolopyrrole (ThDPP) unit as the quinoidal core. The electron‐rich thiophene rings in thienoisatin and the electron delocalization induced by thienoisatin resulted in polymers with high‐lying highest occupied molecular orbital, and the electron‐deficient nature of ThDPP unit and its quinoidal backbone endowed the polymers with low‐lying lowest unoccupied molecular orbitals. As a result, both polymers can be p‐type and n‐type doped. Because of its high mobility, doped PTQDPP‐2FT performed better in organic thermoelectric devices than the doped PTQDPP‐T. After being doped with FeCl3 and N‐DMBI, PTQDPP‐2FT showed p‐type and n‐type power factors of 278.2 and 2.37 µW m−1 K−2, respectively. These are the best for bipolar (p‐type and n‐type) performances that obtained by selective doping of a single polymer.

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 113.27µWm-1 K-2 , more than three times that of the regiorandom PRF (35.35µWm-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.

One-step post-treatment boosts thermoelectric properties of PEDOT:PSS flexible thin films

• High power factor of 64.4 µW m −1 K −2 is achieved in PEDOT:PSS films at 360 K. • 80 vol.% DMAC solution boosts the electrical conductivity from 5 to 964 S cm −1 . • 0.5 mol L −1 LAA improves the Seebeck coefficient from 18.7 to 25 µV K −1 . • Mott variable range hopping model explains the thermoelectric transport behavior. Developing high-performance poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) significantly widens the practical applications of flexible organic thermoelectric devices, while the water-based co-solvent dopants and/or post-treatments are still rarely studied so far. Here, we develop a one-step post-treatment to improve the power factor of PEDOT:PSS films by using a water-based solution, which is composed of co-solvent (polar solvent dimethylacetamide (DMAC) and deionized water) and organic reducing agent L-ascorbic acid (LAA). The 80 vol.% DMAC solution significantly boosts the room-temperature electrical conductivity of the films from 5 to 964 S cm −1 , while the Seebeck coefficient can be further enhanced from 18.7 to 25 µV K −1 by treating with 0.5 mol L −1 LAA, contributing to a significantly improved power factor of 55.3 µW m −1 K −2 . The boosted electrical conductivity is ascribed to the separated PEDOT and PSS phases triggered by the high dielectric constant and polarity of DMAC; while the improved Seebeck coefficient is attributed to the reduced oxidation degree of PEDOT from the reducing agent LAA, both confirmed by the comprehensive structural and morphological characterizations. Furthermore, a maximum power factor of 64.4 µW m −1 K −2 can be achieved at 360 K and the observed temperature-dependent electrical transport behavior can be well explained by the Mott variable range hopping model. Besides, a flexible thermoelectric device, assembled by the as-fabricated PEDOT:PSS films, exhibits a maximum output power of ∼23 nW at a temperature difference of 25 K, indicating the potential for applying to low-grade wearable electronics.

Gram-scale polymer-based thermoelectric module for charging Li-ion batteries

Organic thermoelectric devices are promising for charging Li-ion batteries in state-of-art sensors and communication devices. Herein, we report the design and fabrication of a gram-scale polymer-based thermoelectric module prepared by lamination of ultrathin half-dried poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT/PSS) films with Ni foils. The hot pressing of a half-dried PEDOT/PSS film increases the film electrical conductivity and decreases its contact resistance. Such devices exhibit a power density of 72 μW/cm2 at 100 °C under natural cooling conditions in the absence of a heat sink. A 5 g device is capable of fully charging a commercial Li-ion battery (Nichicon SLB03070LR35) within 2 days. The manufactured organic thermoelectric device can be used instead of a CR2032 coin cell to power commercial sensors of acceleration, geomagnetism, temperature, humidity, atmospheric pressure, and illuminance. The manufactured devices exhibit excellent stability for over 2 months under continuous operation conditions. This work highlights the potential of organic thermoelectric devices for energy harvesting.

Intermolecular coupling enhanced thermopower in single-molecule diketopyrrolopyrrole junctions

<p indent="0mm">Sorting out organic molecules with high thermopower is essential for understanding molecular thermoelectrics. The intermolecular coupling offers a unique chance to enhance the thermopower by tuning the bandgap structure of molecular devices, but the investigation of intermolecular coupling in bulk materials remains challenging. Herein, we investigated the thermopower of diketopyrrolopyrrole (DPP) cored single-molecule junctions with different coupling strengths by varying the packing density of the self-assembled monolayers (SAM) using a customized scanning tunneling microscope break junction (STM-BJ) technique. We found that the thermopower of DPP molecules could be enhanced up to one order of magnitude with increasing packing density, suggesting that the thermopower increases with larger neighboring intermolecular interactions. The combined density functional theory (DFT) calculations revealed that the closely-packed configuration brings stronger intermolecular coupling and then reduces the highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gap, leading to an enhanced thermopower. Our findings offer a new strategy for developing organic thermoelectric devices with high thermopower.

Enhancing the Performance of N-Type Thermoelectric Devices via Tuning the Crystallinity of Small Molecule Semiconductors.

In the development of high-performance organic thermoelectric devices, n-type materials, especially with small molecule semiconductors, lags far behind p-type materials. In this paper, three small molecules are synthesized based on electron-deficient naphthalene bis-isatin building blocks bearing different alkyl chains with the terminal functionalized with 3-ethylrhodanine unit and studied their aggregation and doping mechanism in detail. It is found that crystallinity plays an essential role in tuning the doping behavior of small molecules. Molecules with too strong crystallinity tend to aggregate with each other to form large crystalline domains, which cause significant performance degradation. While molecules with weak crystallinity can tolerate more dopants, most of them exhibit low mobility. By tuning the crystallinity carefully, organic thermoelectric devices based on C12NR can maintain high mobility and realize effective doping simultaneously, and a high power factor of 1.07 µW m-1 K-2 at 100°C is realized. This delicate molecular design by modulating crystallinity provides a new avenue for realizing high-performance organic thermoelectric devices.

Thiazole Imide-Based All-Acceptor Homopolymer with Branched Ethylene Glycol Side Chains for Organic Thermoelectrics.

n-Type semiconducting polymers with high thermoelectric performance remain challenging due to the scarcity of molecular design strategy, limiting their applications in organic thermoelectric (OTE) devices. Herein, we provide a new approach to enhance the OTE performance of n-doped polymers by introducing acceptor-acceptor (A-A) type backbone bearing branched ethylene glycol (EG) side chains. When doped with 4-(2,3-dihydro-1,3-dimethyl-1H-benzimidazol-2-yl)-N,N-dimethylbenzenamine (N-DMBI), the A-A homopolymer PDTzTI-TEG exhibits n-type electrical conductivity (σ) up to 34 S cm-1 and power factor value of 15.7 μW m-1 K-2 . The OTE performance of PDTzTI-TEG is far greater than that of homopolymer PBTI-TEG (σ=0.27 S cm-1 ), indicating that introducing electron-deficient thiazole units in the backbone further improves the n-doping efficiency. These results demonstrate that developing A-A type polymers with EG side chains is an effective strategy to enhance n-type OTE performance.

Thiazole Imide‐Based All‐Acceptor Homopolymer with Branched Ethylene Glycol Side Chains for Organic Thermoelectrics

Abstractn‐Type semiconducting polymers with high thermoelectric performance remain challenging due to the scarcity of molecular design strategy, limiting their applications in organic thermoelectric (OTE) devices. Herein, we provide a new approach to enhance the OTE performance of n‐doped polymers by introducing acceptor‐acceptor (A‐A) type backbone bearing branched ethylene glycol (EG) side chains. When doped with 4‐(2,3‐dihydro‐1,3‐dimethyl‐1H‐benzimidazol‐2‐yl)‐N,N‐dimethylbenzenamine (N‐DMBI), the A‐A homopolymer PDTzTI‐TEG exhibits n‐type electrical conductivity (σ) up to 34 S cm−1and power factor value of 15.7 μW m−1 K−2. The OTE performance of PDTzTI‐TEG is far greater than that of homopolymer PBTI‐TEG (σ=0.27 S cm−1), indicating that introducing electron‐deficient thiazole units in the backbone further improves the n‐doping efficiency. These results demonstrate that developing A‐A type polymers with EG side chains is an effective strategy to enhance n‐type OTE performance.

Large scale graphene thermoelectric device with high power factor using gradient doping profile

The performance of a thermoelectric device fabricated with centimeter-scale monolayer graphene (active size, ∼7 × 1 cm2) was investigated in this study. The carrier type and junction profile of the active graphene layer were modulated by chemical doping. After device optimization, improvements in carrier concentration of at least 200%, which led to enhancements in power factor of at least 600%, were obtained. Under optimal performance conditions, a maximum Seebeck coefficient of ∼350 μV/K and power factor of ∼14000 μW/mK2 could be achieved under a temperature difference of 18 K. These thermoelectric parameters are at least three times higher than the best values reported for organic or other graphene-based thermoelectric devices.

Understanding the Solution‐State Doping of Donor–Acceptor Polymers Through Tailored Side Chain Engineering for Thermoelectrics

AbstractAmong the molecular doping methods widely utilized to control charge carriers in organic thermoelectric (TE) devices, solution mixing is attractive for the preparation of uniform and thick TE films to obtain high output powers. However, the effects of doping behavior in solution‐state polymers on their film properties are still not well understood, particularly for donor–acceptor (D‐A) polymers, key materials for high‐performance (opto)electronic devices. Here, by preparing three D‐A polymers with engineered side chains, the aggregation effects of D‐A polymers on the doping behavior in the solution and film states are demonstrated. In a less aggregated polymer, mixing the solution with a p‐dopant leads to the Lewis complex formation, which reduces doping efficiency and power factor. However, solution mixing of aggregated polymers induces the formation of ion pairs with the dopant, resulting in power factors that are two orders of magnitude higher at 23.7 µW m−1 K−2. Doping in the film state where aggregation is induced facilitates ion‐pair formation, resulting in smaller deviation in TE properties (power factors = 9.0–16.4 µW m−1 K−2) despite differences in their ordering structures. These results provide an in‐depth understanding of efficient solution‐state doping of D‐A polymers for TEs.