N-type Organic Thermoelectrics Research Articles

High-Performance n-Type Organic Thermoelectrics Enabled by Synergistically Achieving High Electron Mobility and Doping Efficiency.

n-Doped polymers with high electrical conductivity (σ) are still very scarce in organic thermoelectrics (OTEs), which limits the development of efficient organic thermoelectric generators. A series of fused bithiophene imide dimer-based polymers, PO8, PO12, and PO16, incorporating distinct oligo(ethylene glycol) side-chain to optimize σ is reported here. Three polymers show a monotonic electron mobility decrease as side-chain size increasing due to the gradually lowered film crystallinity and change of backbone orientation. Interestingly, polymer PO12 with a moderate side-chain size delivers a champion σ up to 92.0 S cm-1 and a power factor (PF) as high as 94.3 µW m-1 K-2 in the series when applied in OTE devices. The PF value is among the highest ones for the solution-processing n-doped polymers. In-depth morphology studies unravel that the moderate crystallinity and the formation of 3D conduction channel derived from bimodal orientation synergistically contribute to high doping efficiency and large charge carrier mobility, thus resulting in high performance for the PO12-based OTEs. The results demonstrate the great power of simple tuning of side chain in developing n-type polymers with substantial σ for improving organic thermoelectric performance.

Cyano-Functionalized Fused Bithiophene Imide Dimer-Based n-Type polymers for High-Performance Organic Thermoelectrics.

Doped n-type polymers usually exhibit low electrical conductivities and thermoelectric power factors (PFs), restricting the development of high-performance p-n junction-based organic thermoelectrics (OTEs). Herein we report the design and synthesis of a new cyano-functionalized fused bithiophene imide dimer (f-BTI2), CNI2, which synergistically combines the advantages of both cyano and imide functionalities, thus leading to substantially higher electron deficiency than the parent f-BTI2. On the basis of this novel building block, a series of n-type donor-acceptor and acceptor-acceptor polymers were successfully synthesized, all of which show good solubility, deep-lying frontier molecular orbital levels, and favorable polymer chain orientation. Among them, the acceptor-acceptor polymer PCNI2-BTI delivers an excellent electrcial conductivity up to 150.2 S cm-1 and a highest PF of 110.3 μW m-1 K-2 in n-type OTEs, attributed to the optimized polymer electronic properties and film morphology with improved molecular packing and higher crystallinity assisted by solution-shearing technology. The PF value is the record of n-type polymers for OTEs to date. This work demonstrates a facile approach to designing high-performance n-type polymers and fabricating high-quality films for OTE applications. This article is protected by copyright. All rights reserved.

An n-Type Polythiophene Derivative with Excellent Thermoelectric Performance.

Typical n-type conjugated polymers are based on fused-ring electron-accepting building blocks. Herein, we report a non-fused-ring strategy to design n-type conjugated polymers, i.e. introducing electron-withdrawing imide or cyano groups to each thiophene unit of a non-fused-ring polythiophene backbone. The resulting polymer, n-PT1, shows low LUMO/HOMO energy levels of -3.91 eV/-6.22 eV, high electron mobility of 0.39 cm2 V-1 s-1 and high crystallinity in thin film. After n-doping, n-PT1 exhibits excellent thermoelectric performance with an electrical conductivity of 61.2 S cm-1 and a power factor (PF) of 141.7 μW m-1 K-2 . This PF is the highest value reported so far for n-type conjugated polymers and this is the first time for polythiophene derivatives to be used in n-type organic thermoelectrics. The excellent thermoelectric performance of n-PT1 is due to its superior tolerance to doping. This work indicates that polythiophene derivatives without fused rings are low-cost and high-performance n-type conjugated polymers.

Acceptor-acceptor type polymers based on cyano-substituted benzochalcogenadiazole and diketopyrrolopyrrole for high-efficiency n-type organic thermoelectrics

Acceptor-acceptor type polymers based on cyano-substituted benzochalcogenadiazole and diketopyrrolopyrrole for high-efficiency n-type organic thermoelectrics

Effects of Conformation on Doping Efficiency in π-Extended Bipyranylidene Molecules: Relationship between Molecular Structure and Electron-Doping Ability for Developing n-Type Organic Thermoelectrics

Abstract Electron doping is an essential process for developing n-type organic thermoelectric materials, and thus the search for efficient n-type dopants is critically important. By replacing the central 1-methylpyrrole ring in 2,5-bis((2,6-diphenyl-4H-pyran-4-ylidene)methyl)-1-methylpyrrole (1) with electron-rich 3,4-ethylenedioxythiophene and 2,2′-bis(3,4-ethylenedioxythiophene) moieties, we synthesized new candidate molecules (2 and 3, respectively) as n-type dopants. The single-crystal X-ray analyses of 1 and 3 elucidated that 3 has a totally planar π-conjugated structure over the whole molecule, whereas 1 has a non-planar structure. Although the energy levels of the highest occupied molecular orbitals of 1–3 evaluated by the electrochemical measurement in solution were not significantly different, the work function of 3 thin film evaluated by the Kelvin probe method was slightly higher than those of 1 and 2. Furthermore, 3 was capable of electron-doping to an n-type semiconducting polymer, poly(benzimidazobenzophenanthroline) (BBL), and the resulting doped BBL showed decent thermoelectric characteristics with the power factor of 1.25 × 10−3 µW m−1 K−2, which was higher by one order of magnitude than those of 1- and 2-doped BBL thin films. These results imply that the high planarity of 3 can contribute to electron-doping ability, which could be useful information for further development of n-type dopants for organic thermoelectric applications.

Molecular engineering accelerated polarity switching enabling high-performance n-type organic thermoelectrics

Molecular engineering of indaceno[1,2-b:5,6-b′]dithiophene-based copolymers enables a robust approach to accelerate polarity switching during p-doping, affording a high power factor of over 20 μW m−1 K−2 (n-type).

Lactone Backbone Density in Rigid Electron-Deficient Semiconducting Polymers Enabling High n-type Organic Thermoelectric Performance.

Three lactone-based rigid semiconducting polymers were designed to overcome major limitations in the development of n-type organic thermoelectrics, namely electrical conductivity and air stability. Experimental and theoretical investigations demonstrated that increasing the lactone group density by increasing the benzene content from 0 % benzene (P-0), to 50 % (P-50), and 75 % (P-75) resulted in progressively larger electron affinities (up to 4.37 eV), suggesting a more favorable doping process, when employing (N-DMBI) as the dopant. Larger polaron delocalization was also evident, due to the more planarized conformation, which is proposed to lead to a lower hopping energy barrier. As a consequence, the electrical conductivity increased by three orders of magnitude, to achieve values of up to 12 S cm and Power factors of 13.2 μWm-1 K-2 were thereby enabled. These findings present new insights into material design guidelines for the future development of air stable n-type organic thermoelectrics.

Corrigendum to “De novo design of polymers embedded with platinum acetylides towards n-type organic thermoelectrics” [Chem. Eng. J. 405 (2021) 126692

Corrigendum to “De novo design of polymers embedded with platinum acetylides towards n-type organic thermoelectrics” [Chem. Eng. J. 405 (2021) 126692

N-type organic thermoelectrics: demonstration of ZT\u2009>\u20090.3

The ‘phonon-glass electron-crystal’ concept has triggered most of the progress that has been achieved in inorganic thermoelectrics in the past two decades. Organic thermoelectric materials, unlike their inorganic counterparts, exhibit molecular diversity, flexible mechanical properties and easy fabrication, and are mostly ‘phonon glasses’. However, the thermoelectric performances of these organic materials are largely limited by low molecular order and they are therefore far from being ‘electron crystals’. Here, we report a molecularly n-doped fullerene derivative with meticulous design of the side chain that approaches an organic ‘PGEC’ thermoelectric material. This thermoelectric material exhibits an excellent electrical conductivity of >10 S cm−1 and an ultralow thermal conductivity of <0.1 Wm−1K−1, leading to the best figure of merit ZT = 0.34 (at 120 °C) among all reported single-host n-type organic thermoelectric materials. The key factor to achieving the record performance is to use ‘arm-shaped’ double-triethylene-glycol-type side chains, which not only offer excellent doping efficiency (~60%) but also induce a disorder-to-order transition upon thermal annealing. This study illustrates the vast potential of organic semiconductors as thermoelectric materials.

High Thermoelectric Performance in n-Type Perylene Bisimide Induced by the Soret Effect.

Low-cost, non-toxic, abundant organic thermoelectric materials are currently under investigation for use as potential alternatives for the production of electricity from waste heat. While organic conductors reach electrical conductivities as high as their inorganic counterparts, they suffer from an overall low thermoelectric figure of merit (ZT) due to their small Seebeck coefficient. Moreover, the lack of efficient n-type organic materials still represents a major challenge when trying to fabricate efficient organic thermoelectric modules. Here, a novel strategy is proposed both to increase the Seebeck coefficient and achieve the highest thermoelectric efficiency for n-type organic thermoelectrics to date. An organic mixed ion-electron n-type conductor based on highly crystalline and reduced perylene bisimide is developed. Quasi-frozen ionic carriers yield a large ionic Seebeck coefficient of -3021 μV K-1 , while the electronic carriers dominate the electrical conductivity which is as high as 0.18 S cm-1 at 60% relative humidity. The overall power factor is remarkably high (165 μW m-1 K-2 ), with a ZT = 0.23 at room temperature. The resulting single leg thermoelectric generators display a high quasi-constant power output. This work paves the way for the design and development of efficient organic thermoelectrics by the rational control of the mobility of the electronic and ionic carriers.

De novo design of polymers embedded with platinum acetylides towards n-type organic thermoelectrics

To improve the performance of n-type organic thermoelectric (TE) materials still remains great challenge due to the electron trapping and inefficient n-doping. Conducting polymers with dual electronic-ionic transport are promising for TE generators owing to the potential of both large thermoelectric responses and favorable conductivities. However, current progresses are mainly limited to the scanty available p-type polyelectrolytes with inferior electrical properties, which restrict the breakthrough of TE devices. Herein, three π-conjugated polymers with or without the incorporation of platinum acetylides are elaborately designed. Remarkably, the embedded heavy metal atoms can effectively sharpen their density of states nearby the Fermi levels as well as strengthening their through-bond coupling among the metal d-orbitals and the neighboring π-orbitals synchronously. Meanwhile, a simple interfacial modification by using trifluoromethanesulfonic acid is introduced to offer them dual electronic-ionic transport feature. Therefore, a remarkably high thermopower of over −3150 μV K−1 and an enhanced conductivity of 17.1 S m−1 can be achieved by P(TBT-Pt), which is significantly superior to the P(TBTC6) without platinum acetylides. In addition, all these platinum acetylenes exhibit low κ values of around 0.5 W m−1 K−1.

A thermally activated and highly miscible dopant for n-type organic thermoelectrics

N-doping plays an irreplaceable role in controlling the electron concentration of organic semiconductors thus to improve performance of organic semiconductor devices. However, compared with many mature p-doping methods, n-doping of organic semiconductor is still of challenges. In particular, dopant stability/processability, counterion-semiconductor immiscibility and doping induced microstructure non-uniformity have restricted the application of n-doping in high-performance devices. Here, we report a computer-assisted screening approach to rationally design of a triaminomethane-type dopant, which exhibit extremely high stability and strong hydride donating property due to its thermally activated doping mechanism. This triaminomethane derivative shows excellent counterion-semiconductor miscibility (counter cations stay with the polymer side chains), high doping efficiency and uniformity. By using triaminomethane, we realize a record n-type conductivity of up to 21 S cm−1 and power factors as high as 51 μW m−1 K−2 even in films with thicknesses over 10 μm, and we demonstrate the first reported all-polymer thermoelectric generator.

The effect of aromatic ring size in electron deficient semiconducting polymers for n-type organic thermoelectrics

N-type semiconducting polymers have been recently utilized in thermoelectric devices, however they have typically exhibited low electrical conductivities and poor device stability, in contrast to p-type semiconductors, which have been much higher performing.

Pyrazine-Flanked Diketopyrrolopyrrole (DPP): A New Polymer Building Block for High-Performance n-Type Organic Thermoelectrics.

n-Doped conjugated polymers usually show low electrical conductivities and low thermoelectric power factors, limiting their applications in n-type organic thermoelectrics. Here, we report the synthesis of a new diketopyrrolopyrrole (DPP) derivative, pyrazine-flanked DPP (PzDPP), with the deepest LUMO level in all the reported DPP derivatives. Based on PzDPP, a donor-acceptor copolymer, P(PzDPP-CT2), is synthesized. The polymer displays a deep LUMO energy level and strong interchain interaction with a short π-π stacking distance of 3.38 Å. When doped with n-dopant N-DMBI, P(PzDPP-CT2) exhibits high n-type electrical conductivities of up to 8.4 S cm-1 and power factors of up to 57.3 μW m-1 K-2. These values are much higher than previously reported n-doped DPP polymers, and the power factor also ranks the highest in solution-processable n-doped conjugated polymers. These results suggest that PzDPP is a promising high-performance building block for n-type organic thermoelectrics and also highlight that, without sacrificing polymer interchain interactions, efficient n-doping can be realized in conjugated polymers with careful molecular engineering.

Doping Engineering Enables Highly Conductive and Thermally Stable n-Type Organic Thermoelectrics with High Power Factor

This work exploits the scope of doping engineering as an enabler for better-performing and thermally stable n-type organic thermoelectrics. A fullerene derivative with polar triethylene glycol type...

N-Type Organic Thermoelectrics of Donor-Acceptor Copolymers: Improved Power Factor by Molecular Tailoring of the Density of States.

It is demonstrated that the n-type thermoelectric performance of donor-acceptor (D-A) copolymers can be enhanced by a factor of >1000 by tailoring the density of states (DOS). The DOS distribution is tailored by embedding sp2 -nitrogen atoms into the donor moiety of the D-A backbone. Consequently, an electrical conductivity of 1.8 S cm-1 and a power factor of 4.5 µW m-1 K-2 are achieved. Interestingly, an unusual sign switching (from negative to positive) of the Seebeck coefficient of the unmodified D-A copolymer at moderately high dopant loading is observed. A direct measurement of the DOS shows that the DOS distributions become less broad upon modifying the backbone in both pristine and doped states. Additionally, doping-induced charge transfer complexes (CTC) states, which are energetically located below the neutral band, are observed in DOS of the doped unmodified D-A copolymer. It is proposed that charge transport through these CTC states is responsible for the positive Seebeck coefficients in this n-doped system. This is supported by numerical simulation and temperature dependence of Seebeck coefficient. The work provides a unique insight into the fundamental understanding of molecular doping and sheds light on designing efficient n-type OTE materials from a perspective of tailoring the DOS.

Side-chain effects on N-type organic thermoelectrics: A case study of fullerene derivatives

In this contribution, the two key parameters, the polarity and side chain length have been changed to study their effects on n-type organic thermoelectrics of a series of fullerene derivatives. Fullerene derivatives bearing either an alkyl side chain or ethylene glycol (EG) side chains of different lengths are used as the host molecules for molecular doping. It is found that the polar EG side chains can enable better miscibility with the polar dopant than the alkyl side chain, which leads to more than 5-fold enhancement of doping efficiency. Beyond the doping efficiency, another crucial parameter of molecular doping, the molecular order, is readily acquired by simultaneous control of the polarity and the length of the side chain. A polar side chain with an appropriate chain length can contribute to increasing Seebeck coefficients of doped fullerene derivatives more effectively than an alkyl side chain, likely due to the resultant good miscibility and high molecular order. As a result, an optimized power factor of 23.1 μW m-1 K-2 is achieved in the fullerene derivative with a tetraethylene glycol side chain. This represents one of the best n-type organic thermoelectrics. Additionally, EG side chains can improve the air stability of n-doped fullerene derivatives films as compared to an alkyl side chain. Our work sheds light on the design of side-chains in efficient n-type small molecules thermoelectric materials and contributes to the understanding of their thermoelectric properties.

N-Type Organic Thermoelectrics: Improved Power Factor by Tailoring Host-Dopant Miscibility.

In this contribution, for the first time, the polarity of fullerene derivatives is tailored to enhance the miscibility between the host and dopant molecules. A fullerene derivative with a hydrophilic triethylene glycol type side chain (PTEG-1) is used as the host and (4-(1,3-dimethyl-2,3-dihydro-1H-benzoimidazol-2-yl)phenyl)dimethylamine n-DMBI) as the dopant. Thereby, the doping efficiency can be greatly improved to around 18% (<1% for a nonpolar reference sample) with optimized electrical conductivity of 2.05 S cm-1 , which represents the best result for solution-processed fullerene derivatives. An in-depth microstructural study indicates that the PTEG-1 molecules readily form layered structures parallel to the substrate after solution processing. The fullerene cage plane is alternated by the triethylene glycol side chain plane; the n-DMBI dopants are mainly incorporated in the side chain plane without disturbing the π-π packing of PTEG-1. This new microstructure, which is rarely observed for codeposited thin films from solution, formed by PTEG-1 and n-DMBI molecules explains the increased miscibility of the host/dopant system at a nanoscale level and the high electrical conductivity. Finally, a power factor of 16.7 µW m-1 K-2 is achieved at 40% dopant concentration. This work introduces a new strategy for improving the conductivity of solution-processed n-type organic thermoelectrics.