Recent Advances in Molecular Design of Organic Thermoelectric Materials

Open AccessCCS ChemistryMINI REVIEW1 Oct 2021Recent Advances in Molecular Design of Organic Thermoelectric Materials Dongyang Wang, Liyao Liu, Xike Gao, Chong-an Di and Daoben Zhu Dongyang Wang Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 School of Chemical Sciences, University of Chinese Academy of Sciences, Beijing 100049, Liyao Liu Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, Xike Gao Key Laboratory of Synthetic and Self-Assembly Chemistry for Organic Functional Molecules, Center for Excellence in Molecular Synthesis, Shanghai Institute of Organic Chemistry, University of Chinese Academy of Sciences, Chinese Academy of Sciences, Shanghai 200032, Chong-an Di *Corresponding author: E-mail Address: email protected Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 and Daoben Zhu Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 https: //doi. org/10. 31635/ccschem. 021. 202101076 SectionsAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Organic thermoelectric (OTE) materials have gained widespread attention because of their potential for wearable power generators and solid cooling elements. Nevertheless, the development of state-of-the-art OTE materials still suffers from limited molecular categories because of the rarity of molecular design strategies, which limits further development of this emerging field. Recently, many efforts have been devoted to developing molecular design concepts for high performance OTE materials. In this mini review, we present the fundamentals of the thermoelectric (TE) effect and the basic guidelines for designing OTE materials based on doped semiconductors. Subsequently, we provide an overview of the molecular design of conjugated backbones and side chains for enabling efficient TE conversion, including the effects of the molecular framework, heteroatomic substitution, and side chain length and polarity. Finally, we emphasize the developing tendency of molecular engineering towards high-performance and multifunctional OTE materials. Download figure Download PowerPoint Introduction Ever-growing wasted heat causes severe environmental issues and a global energy dilemma. Exploring eco-friendly energy technologies with sustainable electricity is of vital significance in alleviating the energy crisis. Thermoelectric (TE) materials can directly convert renewable heat into electricity via the Seebeck effect and have been considered as promising systems to meet this challenge. 1–7 Additionally, TE materials can convert electricity to thermal energy, which could be also of considerable interest for spot-cooling devices via the Peltier effect. In the past 200 years, tremendous efforts have been made to improve the TE performance. Recently, TE materials have become increasingly important in satisfying the rising requirements for distributed energy used in emerging fields, such as health monitoring sensors and the Internet of Things (IoT). In the past decade, organic thermoelectric (OTE) materials have received increasing attention, not only because of their low-cost, mechanical process flexibility, molecular diversity, large-area processing methods, but also their intrinsically low thermal conductivity and excellent performance at room temperature. 6, 8–13 The TE performance is generally evaluated by the dimensionless figure of merit, ZT = S2σT/κ, which is composed of the Seebeck coefficient (S), electrical conductivity (σ), thermal conductivity (κ), and absolute temperature (T). 14 Therefore, a promising OTE candidate should possess high σ, high S, and low κ, and thus be a good electrical conductor but a poor thermal conductor. These benchmarks imply that OTE materials can benefit from the molecular design of organic semiconductors (OSCs) such as organic photovoltaics (OPVs), organic light-emitting diodes (OLEDs), and organic thin-film transistors (OTFTs). 15–28 Despite early investigations of their thermopower properties before 1990, OTE materials experienced a slow development until 2010. 29, 30 Significant development in OTE materials has resulted from investigation of two conducting polymers, poly (3, 4-ethlenedioxythiophene) (PEDOT) and poly (nickel 1, 1, 2, 2-ethenetetrathiolate) poly (Ni-ett), which exhibited maximum ZT values greater than 0. 1. 31–34 In the past five years, increasing attention has also been paid to doped OSCs with high mobility. For instance, two diketopyrrolopyrrole (DPP) derivatives, namely selenium-substituted diketopyrrolopyrrole (PDPPSe-12) and aromatic-dicyanovinyl-dipyrrolo3, 4-cpyrrole-1, 4-diylidene) bis (thieno3, 2-bthiophene (A-DCV-DPPTT) DPPTT = dipyrrolo[3, 4-cpyrrole-1, 4-diylidenebis (thieno3, 2-bthiophene) ], have been reported to possess mobilities over 0. 5 cm2 V−1 s−1 and exhibit a maximum ZT greater than 0. 2. 35, 36 Regarding the structure–property relationship, an ideal OTE material should have tunable charge carrier concentration (n) and high charge carrier mobility (μ), which are highly relevant to the interaction between host molecules and dopants, position of the energy levels, and intermolecular packing order. The energy levels are directly bound to the molecular backbone structure, whereas the miscibility and self-assembly order are associated with side chain properties. Therefore, the molecular design of novel backbones and side chains is indispensable for fine-tuning the TE performance. Benefiting from increasing investigation of molecular design and chemical doping, tremendous progress has been made in this area. In fact, several insightful reviews of OTE materials have been published regarding material categories, processing techniques, and chemical doping. 37–40 In this mini review, we summarize the recent molecular design strategies of small molecular and polymeric semiconductors for OTE applications by focusing on molecular backbone and side chain engineering. In the first section, we identify the critical role of molecular design in determining TE performance and highlight the principal requirements of the conjugated backbone and side-chain. In the second section, we focus on the conjugated backbone modulation for improving the TE performance. In the following section, we indicate the influence of side chains on the TE performance of OSCs. Finally, we highlight the current challenges and outlook for the future developments of OTE materials. Critical Role of Molecular Design in Determining TE Performance Fundamentals for efficient TE conversion The power generation of TE materials is realized by utilizing the Seebeck effect, which is induced by a balance between the diffusion of chemical potential and electrostatic repulsion arising from the buildup of charge. The resulting performance is typically evaluated by the figure of merit ZT and power factor PF as follows: ZT = S 2 σ κ T (1) PF = S 2 σ (2) in which σ, κ, and S can be described as: σ = n e μ (3) κ = κ C + κ L = L σ T + 1 3 C V ν l (4) S = k B e ∫ E − E F k B T σ (E) σ d E (5) For metals or degenerate semiconductors with parabolic band, S can be written as a function of n and effective mass (m*) in a form appropriate for an energy-independent scattering approximation. S = 8 π 2 k B 2 3 e h 2 m * T (π 3 n) 2 / 3 (6) where kB is the Boltzmann constant, EF is the Fermi level, h is the Planck constant, L is the Lorentz factor, Cv is the constant-volume specific heat, l is the scattering phonon mean free path, v is the phonon speed, and κ is composed of charge carriers transport heat (κC), and lattice phonons transport heat (κL). 41 To maximize the ZT value of an OTE material, high S, high σ, and low κ are required. A high n can contribute to an increase in σ and κ. However, as shown in eqs 2 and 3, the increase in n usually results in an inevitably low S. The decoupling of these interrelated parameters is relatively nontrivial. The σ of OSCs is commonly governed by both n and their self-assembly structural order from the molecular orientation to the molecular alignment. Because charge transport can be strongly anisotropic due to the electrical coupling differences among the molecules, the π-electron coupling of closer intermolecular contacts for efficient charge transfer is required. Moreover, conjugated units with planar and rigid structures that stack with each other for efficient interchain electronic coupling can enhance charge transport. In contrast, S reflects the average entropy per charge carrier and directly relates to the density of states (DOS). Therefore, the S can be manipulated by tuning the shape of the DOS. For instance, narrowing the DOS is favorable to raising the asymmetry of the Fermi energy and enhancing S. Although the κ of OTE materials is relatively low, the mechanism of how κ changes with the diverse doping concentration and different morphology is still unclear. Therefore, it remains vital to acquire the limits of κ and its dependence on molecular properties. Basic rules for designing OTE materials Based on the aforementioned discussion, it is essential to manipulate interrelated properties at the molecular level (Figure 1). The molecular design of a conjugated backbone is powerful for tuning the energy level by incorporating diverse aromatic cores. Of particular note, the molecular structure is also crucial in affecting transition from single molecule to microscale and macroscale thin films. Consequently, ordered self-assembly has been widely employed for achieving efficient charge transport and high TE performance. Additionally, it is highly desirable to introduce novel variations to enable multifunctional OTE applications. In this context, we summarize three design rules for developing OTE materials. Figure 1 | Illustration of backbone design in OSCs toward TE applications. Download figure Download PowerPoint The frontier molecular orbitals (FMOs) in the conjugated molecules play vital roles in affecting intramolecular and intermolecular charge transport, doping efficiency, and stability. For OSCs, these are the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). In principle, the energy of the HOMO directly depends on the electron density and delocalization of the π electrons throughout a conjugated backbone. 15 The spatial distribution of molecular orbital relies on efficient π-orbital overlap between two or more conjugated units. Moreover, the charge transfers mostly take place by removing electrons from donor molecules to the LUMO of n-type materials or accepting electrons from the HOMO of p-type materials to acceptor molecules. Therefore, modulation of FMOs is considered the most efficient strategy to achieve a high doping efficiency between host and dopant molecules. Looking beyond TE performance, ambient stability is also a basic requirement for doped OTE materials, which is mainly determined by the electron affinity (EA) values divided by ionization energy (IA). 37 For example, in n-type materials, molecules with low EA values usually suffer from chemical instability due to the influence of water, oxygen, and other trap states; therefore, lowering the LUMO level is a general strategy to enhance their air stability. These restrictive relationships demonstrate that appropriate FMOs are crucial for OSCs to ensure efficient TE conversion. Previous studies performed on high mobility OSCs reveal that high crystallinity endows materials with efficient charge transport ability, which offers a basic guideline for designing OTE materials. 42 In most cases, planar conjugated molecules contribute to a high degree of intrachain order and crystalline microstructures with intimate intermolecular packing modes. 23, 43 In principle, intimate intermolecular contacts can satisfy the need of coupled π-electronic levels and thus lead to efficient intermolecular charge transfer. Concerning chemical doping, the incorporation of specific side chains can improve miscibility between dopant and host molecules to enhance the doping efficiency. However, the additional side chains are generally highly insulating and prevent electronic communication between molecules. Consequently, a remaining issue for molecular design relies on the balance between the high-degree oriented backbone and side chains that are likely to prevent electronic contact. To develop multifunctional OTE materials, side chains with specific groups such as tetraethylene glycol and ether chains in the conjugated backbone are required to endow the devices with sensing, photosensitive, and stretchable behaviors. 25, 44–46 These materials can hardly be realized by using conventional molecular design strategy. In stretchable materials for example, the typical design principles, such as adding flexible conjugation breakers and incorporating longer and softer side chains, can lead to decreased crystallinity of films. Thus, it remains challenging to maintain good charge transport properties upon reduced interchain transport efficiency. In other words, the balance between functionality and OTE performance should be evaluated when taking advantage of novel side chains. Molecular Design of Conjugated Backbone The energy level determines the charge transport because the carriers with energy close to the chemical potential can participate in transport. Generally, the band gap of OSCs ranges from 1 to 3 eV, and the introduction of carriers can also affect the HOMO and LUMO. Therefore, the energy level and doping efficiency are critical in determining σ. Moreover, σ is affected by the π-electronic coupling level, which requires close intermolecular contacts to facilitate efficient charge transfer. Therefore, the conjugated framework with planar structures allows more efficient electronic coupling. In principle, the conjugated backbone strongly affects the electronic structure, energy level offset, and planarity of conjugated molecules (Figure 2), which is crucial for enhancing the n, intermolecular interactions, and charge transfer between the host materials and dopants. Evidently, the rational design of conjugated backbones is essential for exploring high-performance OTE materials. Figure 2 | Key factors for designing conjugated backbones for OTE materials: (1) electronic structure, (2) energy level offset, (3) planarity. Download figure Download PowerPoint Framework effect Conjugated frameworks are decorated with acceptors and donors according to the electron-donating or -withdrawing role. Various types of molecules, such as DPPs, imides, and amides, have acted as acceptors. Moreover, the conjugated framework is a key factor in determining the energy level, doping efficiency with dopants, and planarity of conjugated molecules. For instance, the specific molecular modulation from incorporating electron-rich or -deficient units can achieve the targeted n- and p-type DPPs-based OTE materials (Figures 3a and 3b). As a result, an in-depth understanding of the conjugated framework effects is considerably important. 47–51 Figure 3 | The molecular design based on DPP units for (a) n-type and (b) p-type OTE materials. (c) Charge-transfer mechanism and (d) PF at diverse dopant concentrations of N-DMBI doped A-DCV-DPPTT and Q-DCM-DPPTT. Reprinted with permission from ref 36. Copyright 2017 American Chemical Society. Download figure Download PowerPoint The conjugated units are linked by two types of structures: one is the aromatic mode form with a single bond between the monomers, and the other is the quinoid form with a double bond connection. Despite the similar molecular geometry, the two diverse connection modes can lead to significant differences in electronic distribution along the framework, resulting in differences in energy levels and doping efficiency. For instance, Huang et al. 36 reported two solution-processable DPPTT derivatives, A-DCV-DPPTT and quinoid-dicyanomethylene-dipyrrolo3, 4-cpyrrole-1, 4-diylidene) bis (thieno3, 2-bthiophene (Q-DCV-DPPTT), for studying conjugated framework-dependent TE properties. The A-DCV-DPPTT and Q-DCV-DPPTT had diverse DPPTT units and different HOMO and LUMO values, in which A-DCV-DPPTT had a higher LUMO value of −3. 9 eV than that of Q-DCV-DPPTT (−4. 5 eV). In the film state, molecular interactions of A-DCV-DPPTT and Q-DCV-DPPTT with the dopant are different, and the singly occupied molecular orbital (SOMO) of the 4- (1, 3-dimethyl-2, 3-dihydro-1H-benzoimidazol-2-yl) phenyl) dimethylamine (N-DMBI) ·was −2. 4 eV, enabling efficient electron transfer between N-DMBI and A-DCV-DPPTT (Figure 3c). In addition, the electron transfer from N-DMBI• SOMO to the LUMO level of Q-DCV-DPPTT was difficult because the charge transfer occurred in the solution state before spin-coating of the film, resulting in lower doping efficiency. Accordingly, the high electron mobility and appropriate energy levels of the A-DCV-DPPTT incorporated aromatic structures for doped films had an σ of 5. 3 S cm−1 and a high ZT value of up to 0. 26. Notably, the performance was more than one order of magnitude higher than those of Q-DCV-DPPTT with a quinone structure (Figure 3d). Theoretical studies have shown that polarons are predominantly localized on the polymer chains. Therefore, the polaron delocalization length is important in macroscopic conductivity. 52, 53 Generally, ladder-type polymers typically exhibit two-dimensional geometry and thus have limited conformational freedom. Benefiting from these planar building blocks, the electron delocalization length in conjugated polymers can be improved significantly. Wang et al. 54 studied polybenzimidazobenzophenanthroline (BBL) and polyN, N′-bis (2-octyldodecyl) -naphthalene-1, 4, 5, 8-bis (dicarboximide) -2, 6-diyl-alt-5, 5′- (2, 2′-bithiophene) P (NDI2OD-T2) (Figure 4a) ; the former had a linear-"torsion-free"-ladder-type framework and the latter a distorted donor–acceptor structure. In comparison with P (NDI2OD-Tz), BBL presented a larger calculated polaron and spin delocalization lengths, leading to an easier intramolecular transfer and higher mobility. Consequently, BBL doped with tetrakis (dimethylamino) ethylene (TDAE) achieved n-type conductivity of up to 2. 4 S cm−1, which was three orders of magnitude higher than that of P (NDI2OD-Tz) (Figure 4b). Sometimes, conjugated polymers with a rigid planar backbone instead of a disordered crystalline structure can also exhibit unexpected structural tolerance and good miscibility with dopants, resulting in both high n and μ. 51 Figure 4 | The backbone effect for high-performance OTE materials. (a) Chemical structures of BBL and P (NDI2OD-T2) and (b) σ and S versus TDAE doping time for BBL and P (NDI2OD-T2). Reprinted with permission from ref 54. Copyright 2016 Wiley-VCH. (c) Chemical structures of BDPPV derivatives and (d) PF at different doping concentrations of doped BDPPV derivatives. Reprinted with permission from ref 56. Copyright 2015 American Chemical Society. (e) Chemical structures of PDPH and PDPF and (f) PF at different doping concentrations of doped PDPH and PDPF. Reprinted with permission from ref 57. Copyright 2018 Wiley-VCH. Download figure Download PowerPoint Substituent effect The electronic structure can be fine-tuned by introducing electron-withdrawing elements, such as halogen atoms on electron-rich moieties, leading to a deeper LUMO level and higher n-type doping efficiency. 55 For instance, fluorine atoms can enhance the electron withdrawing ability via electrostatic and sigma inductive effects. Furthermore, the incorporation of fluorine atoms not only endows stability for doped materials, but also imparts high rigidity, extremely low surface energy, and various self-assembly properties in the solid state. For these reasons, the fluorine substitution effect has been considerably studied. Shi et al. 56 reported three benzodifurandione-based poly (p-phenylene vinylene) (BDPPV) derivatives, BDPPV, ClBDPPV, and FBDPPV, which had varied LUMO levels and electron mobilities from the diverse halogen atoms (Figure 4c). BDPPV derivatives have low-lying LUMO levels, which makes them the most electron-deficient conjugated polymers. As the feasibility of chemical doping is inevitably accompanied by hydride or H atom transfer, the introduction of halogen atoms (Cl and F) efficiently lowered the LUMO levels of molecules and resulted in stronger C–H bond acceptors in ClBDPPV and FBDPPV. Therefore, the doping levels of the three derivatives could be tuned according to the different reactivities of the dopant molecules and the polymers substituted with halogen atoms. In addition, the doping efficiencies of ClBDPPV and FBDPPV were almost identical to those of BDPPV. Clear shifts of the Fermi level toward higher binding energies were observed, resulting in a dramatic enhancement of σ, of which the highest conductivity of FBDPPV reached 14 S cm−1. The enhancement of σ was mainly caused by the increase in doping levels, whereas ordering of polymers was not likely to be disturbed by the dopant. Consequently, FBDPPV exhibit a high PF of up to 28 μW m−1 K−2 (Figure 4d). Other studies have demonstrated that electron-withdrawing modification of the donor units can increase the EA of molecules and affect TE performance. Pei et al. 57 reported two DPP-based donor–acceptor (D–A) copolymers, PDPH and PDPF, in which the former did not have electron-withdrawing modification of the donor moiety and the latter had halogen atoms as the electron-withdrawing group (Figure 4e). DPP is a widely investigated building block in OTE materials. After introducing the fluorine atoms on the donor moiety, PDPF presented not only lower HOMO and LUMO levels, but also multiple-polymer packing orientations. It thus can introduce diverse transition regions to accommodate more dopants and dopant cations, leading to enhanced doping efficiency and avoiding phase separation. Moreover, the fluorine substitution contributes to the of a between resulting in a with Benefiting from these the magnitude of both σ and PF of PDPF was enhanced by three orders of magnitude in comparison to In addition, the design of building can the of both and to such acceptors good excellent planar and high substitution substitution efficient intermolecular orbital and increase the thus affecting the intermolecular interactions and energy Although and are in the has more electrons and is larger than The HOMO levels are higher than those of the Therefore, substitution is a key strategy for fine-tuning the conjugated A of the substitution effect on TE performance was reported by et based on a DPP In comparison with exhibited a higher HOMO to facilitate p-type Although the introduction of a atom contributes to an a stronger intermolecular interaction in was achieved because of the larger of the atom than that of Therefore, the more favorable intermolecular interaction and relatively the dopant to the the packing order. Consequently, doped exhibited an excellent mobility cm2 V−1 s−1 and maximum PF of μW m−1 leading to a ZT of In addition, substitution can also contribute to a stronger and In comparison with the exhibited a considerably higher degree of spin delocalization that was to the lower of the As a result, the in the of was leading to a significant enhancement in σ with a value of S cm−1. These studies demonstrate the significance of heteroatomic substitution in enabling OTE materials. of OTE Materials Conjugated molecules usually have side chains, which can enhance their in organic However, the side chain engineering not only but also affects the intermolecular packing and the properties. In fact, the side chain effects on the properties have been widely investigated regarding side chain charge and chain In this section, we summarize side chain engineering including the length and of side chains and which affects the self-assembly packing and properties (Figure Figure | Key factors for side chain engineering of OTE materials: (1) miscibility between host and (2) self-assembly and (3) Download figure Download PowerPoint chain length are two types of chains and for OSCs. the appropriate side chains can enhance interchain The side chains generally interchain due to their chains can endow In addition, the of the of chains can in dramatic differences in carrier mobilities with changes in molecular strategy can the intermolecular Thus, the length and of side chains not only have a influence on the and but also affect the packing order and charge transport properties of OSCs. To the effect of position on OTE performance, Wang and demonstrated three copolymers, composed of and units (Figure and are by the different of the side chains, in which the side chain is from to In exhibited improved packing order and resulted in a higher μ of cm2 V−1 Furthermore, the two polymers exhibited different molecular (Figure The charge carriers of orientation can the due to the of and of the which are generally In addition, the orientation of dopant molecules with almost packing order and Therefore, the fine-tuned the with high σ of S cm−1 and PF of μW m−1 K−2 (Figure In addition, with different presented tunable σ from to S cm−1 at room The electrical of was different from that of in which the phase occurred from to these results that the length and of chains are key to affecting the TE performance. Figure | (a) Chemical structures of and