Enhancement of the Seebeck Coefficient of Organic Thermoelectric Materials via Energy Filtering of Charge Carriers

Open AccessCCS ChemistryMINI REVIEW1 Oct 2021Enhancement of the Seebeck Coefficient of Organic Thermoelectric Materials via Energy Filtering of Charge Carriers Xin Guan and Jianyong Ouyang Xin Guan International Studies College, National University of Defense Technology, Nanjing, Jiangsu 210012 and Jianyong Ouyang *Corresponding author: E-mail Address: email protected Department of Materials Science and Engineering, National University of Singapore, Singapore 117574 https://doi.org/10.31635/ccschem.021.202101069 SectionsAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Recently, organic materials have emerged as next-generation thermoelectric (TE) materials because of their unique advantages including low cost, high mechanical flexibility, low or no toxicity, and low intrinsic thermal conductivity over inorganic TE materials. However, the Seebeck coefficient of organic materials with high TE properties is remarkably lower than that of its inorganic counterparts. Therefore, it is important to improve their Seebeck coefficient and thus, overall TE properties. A high-performance TE material should have high Seebeck coefficient and high conductivity, but there is a trade-off relationship between these two parameters. Lowering the charge carrier concentration can increase the Seebeck coefficient while decreasing the conductivity. A solution is to adopt energy filtering that can remarkably increase the Seebeck coefficient while only slightly lowering the conductivity. In this article, we review energy filtering methods to enhance the Seebeck coefficient and power factor of organic TE materials, including the composite formation with inorganic TE nanomaterials or carbon nanomaterials, composition formation of p- and n-type materials, and surface energy filtering induced by the dipole moment or ionic accumulations. Finally, a prospect is provided for the development of high-performance organic TE materials by energy filtering. Download figure Download PowerPoint Introduction Heat has been the primary energy source on earth since the beginning of civilization. However, heat utilization efficiency is merely around 30%, that is, the majority of the heat is dissipated to the environment as waste. Hence, it is of significance to develop novel technology to efficiently harvest heat.1,2 Among the heat-to-electricity conversion technologies, thermoelectric (TE) generators (TEGs) utilizing the Seebeck effect of TE materials is considered as a feasible technology, particularly for the harvesting of low-grade heat with temperatures below 200 °C, which is about 2/3 of the total waste heat on earth.3 TEGs have merits of no moving parts, no circulating liquid or gas and a very long life span.4 Their TE conversion efficiency depends on the figure of merit (ZT), ZT = S2σT/κ, with S as the Seebeck coefficient, σ the conductivity, T the absolute temperature, and κ the thermal conductivity.3,5S2σ is called the power factor (PF).6 A high-performance TE material should have high PF and low thermal conductivity. Conventional TE materials are inorganic semiconductors or semimetals such as Bi2Te3, Sb2Te3, and their derivatives. Although they can exhibit a high ZT value, they have problems like high cost, toxic or scarce element(s), high thermal conductivity, and poor mechanical properties.7 Organic TE materials have attracted great attention due to their advantages including low cost, good mechanical flexibility, low or no toxicity, and low intrinsic thermal conductivity.8,9 However, the TE properties of organic materials are remarkably lower than that of their inorganic counterparts.10 In particular, the Seebeck coefficients of organic materials with high TE properties are lower than inorganic TE materials by about one order of magnitude. The value of the Seebeck coefficient is proportional to the difference between the mean energy (EJ) of the charge carriers and the Fermi level (EF) of the TE material, S = − E J − E F k T with k as the Boltzmann constant.11,12 Metals usually show low Seebeck coefficient because the Fermi level is quite close to the mean energy of the charge carriers. For a nondegenerated semiconductor, the Seebeck coefficient is related to the position of the Fermi level that is in the middle of the conduction band (CB) and valence band (VB). Figure 1 schematically illustrates the VB, CB, and Fermi level of an n-type semiconductor, the mean energy of the electrons in the CB is higher than the Fermi level, and it thus has a negative Seebeck coefficient. The Fermi level of a semiconductor depends on the doping level. For an n-type semiconductor, the Fermi level shifts down with decreasing doping levels while the mean energy of the electrons in the CB hardly changes. As a result, the value of |EJ–EF| increases, leading to an increase in the absolute Seebeck coefficient. Figure 1 | Schematic illustration of the energy band structure of an n-type semiconductor. Decreasing the doping level can shift the Fermi level down and thus increase the Seebeck coefficient. Download figure Download PowerPoint Although lowering the charge carrier density can increase the Seebeck coefficient, it decreases the conductivity, which is proportional to the charge carrier density.13 A solution to saliently increasing the Seebeck coefficient while not remarkably lowering the conductivity is through energy filtering.14–20 Energy filtering can block the charge carriers with low energy and, in turn, increase the mean energy and Seebeck coefficient. It has been extensively studied for inorganic TE materials, and it has also become a popular strategy to improve the Seebeck coefficient of organic TE materials. In this article, organic TE materials particularly those with high TE properties are briefly and initially introduced. Then, it covers the fundamental principle of energy filtering and its application to various organic TE materials. Finally, we will provide an update on the development of high-performance organic TE materials by exploiting energy filtering. Methods introducing energy filtering to organic TE materials can be very different from inorganic TE materials due to their different structures and properties. Organic TE Materials Organic TE materials are conjugated organic compounds or conjugated polymers in a doped state that possess high conductivity. They can be classified into p- and n-type materials in terms of charge carriers. Figures 2a–2g show the chemical structures of some representative organic TE materials with high TE properties. Their application is related to processability. Poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS) can be dispersed in water, and it can be processed into high-quality films through solution processing techniques. Polyaniline (PANi) doped with some bulky anions can be dispersed only in some toxic organic solvents, while other doped conducting polymers cannot be dispersed in any solvent. Because of the high conductivity and aqueous solution processability, PEDOT:PSS has become the most popular TE material today. Since organic materials usually have a low intrinsic thermal conductivity of below 1 W m−1 K−1, research has been focused on developing organic TE materials with high PF. Table 1 lists the Seebeck coefficient of some representative inorganic and organic TE materials with high TE properties. The absolute Seebeck coefficients of organic TE materials with high TE properties are usually 101 μV K−1, while those for inorganic TE materials are 102–103 μV K−1. Figure 2 | Chemical structures of some representative TE polymers with high TE properties. (a) PEDOT:PSS, (b) PEDOT:Tos and PEDOT:OTf, (c) polyN-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole) (PCDTBT), (d) polythiophene and its derivatives with side group at the β site, (e) PANi, (f) PPy, and (g) poly(Ni-ett). Download figure Download PowerPoint Table 1 | Seebeck Coefficients of Some Representative Inorganic and Organic TE Materials with High TE Properties Semiconductor S (μV/K) Organic Materials S (μV/K) Se +900 PEDOT:PSS +14–20 Te +500 PEDOT:Tos +30 Sb2Te3 +185 PCDTBT +30–55 Pb3Ge39Se58 +1670 PPy +10 Pb15Ge37Se58 −1990 PANi +10–20 Bi2Te3 −230 Poly(Ni-ett) −125 The Seebeck coefficient and conductivity of organic TE materials depend on the doping level. Lowering the doping level can increase the Seebeck coefficient while decreasing the conductivity. Thus, there is an optimal doping level in terms of the PF. For example, Crispin et al.21 investigated the effects of doping level on the conductivity, Seebeck coefficient, PF, and ZT of PEDOT:tosylate (PEDOT:Tos) and found that the optimal doping level is about 0.22 in terms of the PF or ZT value of PEDOT:Tos (Figures 3a and 3b). Dedoping a TE polymer can be achieved by using an alkaline or reducing agent. Yao et al.22 studied the dedoping of PEDOT:trifluoromethanesulfonate (PEDOT:OTf) by NaOH or reducing agents like glucose and ascorbic acid, and they observed that dedoping with NaOH can give rise to a higher Seebeck coefficient and thus higher PF at the same doping level than with the reducing agents. The different dedoping effects by NaOH and reducing agents are attributed to the relationship of the Seebeck coefficient to the two different doping forms, oxidative doping, and protonic acid doping of PEDOT:OTf. Protonic acid doping can generate charge carriers with energy levels closer to the Fermi level. Thus, the removal of these low-energy holes can give rise to a more significant enhancement in the Seebeck coefficient. Figure 3 | Variations of (a) the conductivity (σ), Seebeck coefficient (α), and PF (σα2) and (b) ZT of PEDOT:Tos with respect to doping level. Adapted with permission from ref 21. Copyright 2011 Nature Publishing Group. Download figure Download PowerPoint High conductivity is also required for TE materials. The conductivity of organic TE materials is related to their chemical structure, doping level, and morphology. Apart from the polymerization conditions, a postpolymerization treatment can affect the morphology and thus the conductivity of the conducting polymers. Particular examples include the secondary doping of solution-processable PEDOT:PSS and PANi doped with bulky anions. For example, the conductivity of PEDOT:PSS can be enhanced by 3–4 orders of magnitude through a treatment with polar organic solvents like dimethyl sulfoxide (DMSO) or ethylene glycol (EG), acids, co-solvents, solutions, surfactants, or ionic liquids.23–34 The conductivity enhancement is ascribed to the removal of some poly(4-styrenesulfonic acid) (PSSH) from PEDOT:PSS and conformational change of the PEDOT chains from the coil to the expanded-coil or linear structure. Although secondary doping can significantly enhance the conductivity of PEDOT:PSS, they only slightly affect the Seebeck coefficient. As mentioned earlier, dedoping can increase the Seebeck coefficient, and secondary doping can enhance the conductivity of TE polymers. The combination of these two strategies can give rise to an organic TE material with high Seebeck coefficient and high conductivity. For instance, enhancements in both the Seebeck coefficient and conductivity were observed by treating PEDOT:PSS with a polar organic solvent consisting of an alkaline or reducing agent like hydrazine.6 These can be achieved through sequential treatments of secondary doping and then dedoping as well.35,36 For example, Fan et al.35 studied the sequential treatments of PEDOT:PSS with H2SO4 and NaOH. The acid treatment can enhance the conductivity but it does not remarkably affect the Seebeck coefficient, while the base treatment can enhance the Seebeck coefficient but lowers the conductivity. These sequential treatments can improve the Seebeck coefficient of PEDOT:PSS from ∼16 to ∼40 μV K−1, and the optimal PF of PEDOT:PSS is 332 μW m−1 K−2. Conjugated organic materials are n-type conductors when in the reduced state. However, it is difficult to reduce conjugated organic materials because of their high CB. Thus, there are much less n-type organic TE materials than p-type organic TE materials in literature, and the TE properties of n-type organic TE materials are usually much poorer than that of p-type organic TE materials.37 A breakthrough on n-type organic TE materials was made by Zhu et al.38 They found that the incorporation of some metal atoms into the conjugated backbone can notably improve the stability of n-type TE polymers. They observed a conductivity of <200 S/cm and Seebeck coefficient of −125 μV/K on poly(Ni-ett). Probably due to the poor stability of n-type organic materials, there is almost no reports on the post treatment of n-type TE polymers for the enhancement of their Seebeck coefficient and conductivity. The Seebeck coefficient and conductivity of organic TE materials are interdependent. Russ et al.39 studied the variations of the Seebeck coefficient and PF of a variety of p- and n-type organic materials and composites with respect to their conductivity (Figures 4a and 4b). They found that the Seebeck coefficient decreases with increasing conductivity in the relationship S ∝ σ − 1 / 4 , and the PF increases with increasing conductivity by PF = σ S 2 ∝ σ 1 / 2 . Figure 4 | Variations of (a) the Seebeck coefficient and (b) PF with the conductivity of organic TE materials and composites. Adapted with permission from ref 39. Copyright 2016 Nature Publishing Group. Download figure Download PowerPoint Fundamental Principles of Energy Filtering It is very important to significantly improve the Seebeck coefficient of TE materials. Many strategies, such as nanostructuring,40 band engineering,41 resonant state,42 convergence of electronic bands,43 dedoping, and energy filtering, have been studied for inorganic TE materials. However, only dedoping and energy filtering are effective for the improvement of the Seebeck coefficient of organic TE materials. Although a high Seebeck coefficient can be observed at low doping level, it is at the sacrifice of the conductivity. Instead, energy filtering can significantly enhance the Seebeck coefficient while not remarkably lowering the conductivity of TE materials. Since the first report in 1957, energy filtering has been extensively reported on TE materials.44–46 Figure 5 schematically illustrates the energy filtering of p-type semiconductors.12 At the absence of any energy barrier, the holes with high and low energy can accumulate at the cold end of the temperature gradient, leading to a low Seebeck coefficient. When there is an appropriate energy barrier for the charge carriers in the material, it can filter the holes with low energy that are close to the Fermi level.47 As a result, the mean energy of the accumulated holes increases, giving rise to an increase in the Seebeck coefficient. However, this energy barrier should not be too high, because an overly high energy barrier can block the holes of both high and low energies and thus significantly lower the conductivity of the material. Figure 5 | Schematic illustration of energy filtering with different energy barrier heights. The blue and red straight arrows indicate the transports of the holes with low and high energies, respectively. The return arrows stand for the filtering of the holes with low energy by the energy barrier. EVB and EVAC are for the VB and vacuum level energies, respectively. Adapted with permission from ref 12. Copyright 2017 Royal Society of Chemistry. Download figure Download PowerPoint There are a variety of ways to introduce the energy barrier into a TE material. A homogeneous TE material can have grains, and charge trapping may take place at the grain boundary because of the high energy sites due to the defects.48 The grain boundary can thus induce energy filtering of charge carriers. Defects can exist in porous structures as well, giving rise to the difference between the electron energy at the pore and at a perfect lattice.49 The energy barrier is often present at the interface between two different materials. When the two electronic materials with different Fermi levels are in contact, electron transfer can take place between them, leading to the energy band bending of one or both materials near the interface. The energy barrier due to the band bending can filter the charge carriers of low energy. For example, energy filtering can take place for the charge carriers near the interface of superlattice structure that is formed by the epitaxial growth of layer-by-layer nanostructures of different substances.50 A more general structure of energy barrier is the dispersion of a nanomaterials like nanoparticles or nanowires in a TE matrix. For instance, if a metal nanoparticle with a high Fermi level is dispersed in an n-type inorganic semiconductor with a lower Fermi level, electrons transfer from the metal nanoparticle into the semiconductor, leading to the bending down of the CB of the semiconductor near the interface (Figures 6a–6c).51 This energy barrier can scatter the electrons with low relaxation time. Since the electrons with low relaxation time have low energy, the energy barrier filters the electrons with low energy in the semiconductor and thus increases the Seebeck coefficient. Figure 6 | Illustration of the energy filtering by the energy barrier at the interface between metal nanoparticles and an n-type semiconductor. (a) Dispersion of metallic (M) particles in a semiconducting PbTe matrix. (b) Electron energy band bending at the PbTe side close to the metal/PbTe interface due to the electron accumulation. The energy barrier dispersion is determined by the potential V(r) and energy barrier VB at the interface. (c) The dependence of the relaxation time (τi) of the electrons in PbTe on the electron energy. The scattering of electrons by the energy barrier lowers the relaxation time of the electrons, particularly the electrons with lower energy. Adapted with permission from ref 51. Copyright 2008 American Physical Society. Download figure Download PowerPoint Energy Filtering of Organic TE Composites with Nanofillers Because inorganic TE materials can usually exhibit a much higher Seebeck coefficient than organic TE materials, it is straightforward to blend inorganic TE materials into organic TE materials to achieve a high Seebeck coefficient. If there is no electronic interaction between the two materials, the conductivity and Seebeck coefficient of the composites can be estimated in terms of a series or parallel model. In terms of the series model, the conductivity (σc,s) and Seebeck coefficient (Sc,s) of a composite are given as below:10 1 σ c , s = 1 − χ f σ m + χ f σ f , S c , s = ( 1 − χ f ) S m κ m + χ f S f κ f / 1 − χ f κ m + χ f κ f , where σc,s, Sc,s, χf, σm, Sm, σf, Sf, κf, κm, and κf are the conductivity and Seebeck coefficient of the composite, volume fraction of the fillers, conductivity and Seebeck coefficient of the matrix, conductivity and Seebeck coefficient of the fillers, thermal conductivity of the matrix and thermal conductivity of the fillers, respectively. The conductivity (σc,p) and Seebeck coefficient (Sc,p) of a composite in terms of the parallel model are given as below:10 σ c , p = ( 1 − χ f ) σ m + χ f σ f S c , p = ( 1 − χ f ) σ m S m + χ f σ f S f / ( 1 − χ f ) σ m + χ f σ f . Composites with various inorganic TE materials and TE polymers were reported in the literature, such as Bi2Te3/poly(3-hexylthiophene) (P3HT),52,53 Bi0.5Sb1.5Te3/PEDOT:PSS,54–56 Te/PANi,57 and Te/PEDOT.58 However, it is often observed that the Seebeck coefficient of these composites follow neither the series nor the parallel model. The deviation is ascribed to the charge transfer between the two TE components in the composites because they have different Fermi levels. This charge transfer can induce an internal electric field at the interface, and it can scatter the transport of charge carriers with low energy. For example, Jo et al.56 reported that a treatment of Bi2Te3/PEDOT:PSS with DMSO can lower the work function of PEDOT:PSS and the work function decreases with the increasing annealing time with DMSO vapor (Figure 7a). As a result, the energy barrier between PEDOT:PSS and Bi2Te3 in the Bi2Te3/PEDOT:PSS composites can be estimated in terms of their work functions. The Seebeck coefficient of the composites increases with the DMSO vapor annealing time, consistent with the variation of the energy barrier between PEDOT:PSS and Bi2Te3 (Figure 7b). Figure 7 | (a) Dependence of the work function of PEDOT:PSS on the annealing time with DMSO vapor. (b) Variations of the Seebeck coefficient of Bi2Te3/PEDOT:PSS composites and energy barrier between PEDOT:PSS and Bi2Te3 with the DMSO vapor annealing time. PSVA refers to polar solvent vapor annealing. Adapted with permission from ref 56. Copyright 2020 Elsevier. Download figure Download PowerPoint Carbon nanomaterials, particularly carbon nanotubes (CNTs) and graphene, are also popular fillers of TE polymers, such as CNTs/PEDOT:PSS,59–61 CNTs/PANi,62–65 and single-walled carbon nanotubes (SWCNTs)/P3HT.66 For instance, He et al.65 blended amine-functionalized CNTs (A-CNTs) into PANi. They observed that the Seebeck coefficient of the A-CNTs/PANi composites depends on the A-CNTs loading and can be higher than that of both neat PANi and neat A-CNTs. The high Seebeck coefficient is ascribed to the energy barrier at the interface between PANi and A-CNTs (Figures 8a and 8b). Figure 8 | Variations of (a) the conductivity and Seebeck coefficient and (b) PF of A-CNTs/PANi composites with the A-CNTs loading at room temperature. Adapted with permission from ref 65. Copyright 2018 Elsevier. Download figure Download PowerPoint Graphene was also studied as a filler of TE polymer composites. For example, graphene quantum dots can enhance the Seebeck coefficient of PEDOT:PSS by 110% and the PF by 550% due to the energy filtering.67 Energy filtering was also reported for the composites of PANi or polypyrrole (PPy) with graphene.68,69 In addition, both inorganic TE materials and carbon nanomaterials were investigated as co-fillers of TE polymer composites to introduce double interfaces for energy filtering.18,70–73 For instance, Wang et al.72 prepared PANi composites with SWCNTs and Te nanorods as the fillers. The ternary SWCNT/Te/PANi composite films exhibit high Seebeck coefficient due to energy filtering at the two interfaces of PANI/SWCNT and PANI/Te, while they do not cause much loss of conductivity. Kim et al.18 also prepared ternary composites of PEDOT:PSS with reduced graphene oxide (rGO) and Te nanowires (Te NWs). The energy filterings arising from the energy barriers between Te NWs and PEDOT:PSS (0.24 eV) and between PEDOT:PSS and rGO (0.31 eV) can enhance the Seebeck coefficient to 202 μV K−1. The high-performance TE polymers are usually p-type, and their TE composites are usually formed with a p-type filler.14–16,74–76 In terms of the series or parallel model for the Seebeck coefficient of composites, the composites can have a low Seebeck coefficient if the charge carriers in the fillers are different from in the matrix. Hence, the TE properties of composites of a p-type polymer with an n-type filler has been rarely explored. Katz et al.54 prepared the composites of p-type PEDOT:PSS with n-type Bi2Te3 and reported that the n-type Bi2Te3 decreases the Seebeck coefficient of PEDOT:PSS. They did not mention whether there is energy filtering in these composites. We observed that the interface between PEDOT:PSS and MXene (Ti3C2Tx) is an n-type 2D material can induce energy filtering of charge carriers.77 As shown in Figure 9a, MXene loaded less than 33 wt % can significantly enhance the Seebeck coefficient of PEDOT:PSS. The Seebeck coefficient can be increased from 23 of neat PEDOT:PSS to 57.3 μV K−1, and the PF can be improved from 44.1 up to 155 μW m−1 K−2. The energy filtering arises from the electric field at the interface between MXene to PEDOT:PSS as a result of the electron transfer (Figure 9b). This is the first time to enhance the Seebeck coefficient and PF of a p-type TE polymer by adding n- type fillers. Figure 9 | (a) Variations of the Seebeck coefficient and conductivity of MXene/PEDOT:PSS with the MXene (b) Schematic for the interface between MXene and PEDOT:PSS of the composites with the MXene of and wt Adapted with permission from ref Copyright 2020 American Chemical Society. Download figure Download PowerPoint However, when the MXene loading is higher than the some MXene are and This lowers the Seebeck coefficient because MXene is an n-type TE material. When the MXene loading is than 33 wt the in the Seebeck coefficient by the n-type formation is more than the increase by energy filtering. Thus, the Seebeck coefficient a value at an MXene loading of 33 wt the Seebeck coefficient of the MXene/PEDOT:PSS composite with wt % MXene is higher than that of neat PEDOT:PSS. Energy Filtering of Organic TE The fundamental principle of energy filtering is the scattering of charge carriers. When a TE material is with a secondary material that can induce the scattering of the charge carriers at the surface of the TE this can to energy filtering of charge carriers as and improve the Seebeck coefficient of the it from energy filtering that usually has fillers dispersed in the matrix, we it as surface energy energy filtering can be observed on double structures with a TE polymer with a of ionic material or a material with high intrinsic dipole The surface energy filtering of a TE by is related to the in the secondary material. Apart from electrons and can accumulate at the cold end a temperature to the Fan et a of ionic liquid on PEDOT:PSS films and observed that the ionic liquid can remarkably enhance the Seebeck coefficient while only slightly lowering the conductivity of PEDOT:PSS. This can enhance the Seebeck coefficient up to μV and the PF up to μW m−1 K−1, which is the value for TE polymers. The improvement in Seebeck coefficient and PF depends on the ionic liquid and TE polymer of the This Seebeck coefficient enhancement is attributed to the surface energy filtering induced by the accumulated in the ionic liquid a temperature This surface energy filtering by ionic was by other It can also when the ionic liquid is with a of ionic liquid In to ionic and ionic liquid can take place in polymer temperature including structure shown in Figure and its chemical structure shown in Figure were studied for the surface energy filtering of PEDOT:PSS Because the by the can affect the and the surface energy filtering by the is to the of the a of on an PEDOT:PSS can enhance the Seebeck from to μV while lowering the conductivity from to S in an environment of (Figure The for the surface energy filtering by a polymer