Recent Progress in Thermoelectric Materials Based on Conjugated Polymers

polymers

Review Recent Progress in Thermoelectric Materials Based on Conjugated Polymers

Chang-Jiang Yao 1, Hao-Li Zhang 2,* and Qichun Zhang 1,*

1 School of Materials Science and Engineering, Nanyang Technological University (Singapore), Singapore 639798, Singapore; [email protected] 2 State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Tianshui Southern Road 222, Lanzhou 730000, China * Correspondence: [email protected] (Q.Z.); [email protected] (H.-L.Z.)



Received: 4 December 2018; Accepted: 2 January 2019; Published: 9 January 2019


Abstract: Organic thermoelectric (TE) materials can directly convert heat to electricity, and they are emerging as new materials for energy harvesting and cooling technologies. The performance of TE materials mainly depends on the properties of materials, including the Seebeck coefficient, electrical conductivity, thermal conductivity, and thermal stability. Traditional TE materials are mostly based on low-bandgap inorganic compounds, such as bismuth chalcogenide, lead telluride, and tin selenide, while organic materials as promising TE materials are attracting more and more attention because of their intrinsic advantages, including cost-effectiveness, easy processing, low density, low thermal conductivity, and high flexibility. However, to meet the requirements of practical applications, the performance of organic TE materials needs much improvement. A variety of efforts have been made to enhance the performance of organic TE materials, including the modification of molecular structure, and chemical or electrochemical doping. In this review, we summarize recent progress in organic TE materials, and discuss the feasible strategies for enhancing the properties of organic TE materials for future energy-harvesting applications.

Keywords: thermoelectric; organic polymer; Seebeck coefficient; power factor; conductivity

1. Introduction About 90% of the world’s power is produced by heat engines that use fossil combustion as energy sources. However, fossil-based energy technologies cannot meet the fast-growing worldwide demand on electricity. Meanwhile, the development of new energy conversion technologies to address this issue is very urgent and highly desirable. Unfortunately, the maximum conversion efficiencies of most electricity-generation devices are limited at ca. 40% or lower, and a large amount of energy is lost as waste heat to the environment. Therefore, developing new technologies with improved energy conversion efficiency and reduced waste heat are urgent for addressing global environmental concerns, and they would provide new opportunities for the utilization of renewable energy resources. Thermoelectric (TE) and photovoltaics are two promising clean conversion technologies for solving the energy problem from an environmental–sustainable viewpoint (Figure1). Compared to the great success in the development of photovoltaic materials in the last decade, the progress in the exploration of TE materials lags behind. TE materials utilize the temperature difference between the hot end and its surroundings to generate power [1], which can directly convert low-quality waste heat into the usable electric energy. Therefore, TE devices are promising candidates for making full use of waste heat and solar thermal energy [3].

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Figure 1. Electricity generated from thermoelectric and photovoltaics materials. Figure 1. Electricity generated from t thermoelectrichermoelectric and photovoltaicsphotovoltaics materials.materials. Conventional TE devices are typically based on inorganic compounds, as they generally show betterConventional TE performance TE devices and arehigher typically stability basedbase compd on ared inorganic with compounds,compoundsorganic materials., as they Some generally examples, show betterincluding TE TE performance performancePbTe, Bi2Te and3, andSiGe, higher higher and stability Skutterudite, stability compared compared are with widelyorganic with expl organic materials.ored, and materials. Some the ZT examples, value Some (a examples including figure of, PbTe,includingmerit) Bi can2Te PbTe, 3reach, SiGe, Bi as2Te and high3, SiGe Skutterudite, as ,2.6 and [4-6] Skutterudite (Figure are widely 2),. areHowever, explored, widely inorganic andexplored, the ZT semicoand value thenductor (aZT figure value materials of (a merit) figure suffer can of reachmerit)some as caninherent high reach as 2.6 disadvantages,as [high4–6] (Figureas 2.6 [4 2including).-6] However, (Figure rarity, 2) inorganic. However, toxicity, semiconductor inorganic poor processability, semiconductor materials suffer and materials somea high inherent costsuffer of disadvantages,somemanufacturing. inherent including disadvantages Besides, rarity,the ideal, toxicity,including operation poor rarity, processability, temperature toxicity, andpoorfor most a highprocessability high cost ofperformance manufacturing., and a highinorganic Besides, cost ofTE themanufacturingmaterials ideal operation are generally. Besides, temperature abovethe ideal 200 for omostdegree,peration high which performancetemperature cannot meet inorganicfor m theost increasinghigh TE materials performance demand are generally inorganicfor collecting above TE 200materialsthe degree,waste are heat which generally generated cannot above at meet a temperature 200 the degree, increasing belowwhich demand 150cannot degree for meetcollecting [7]. the Organic increasing the semiconductors, waste demand heat generated for which collecting have at a temperaturethebeen waste overlooked heat below generated in 150 the degreepast at a decadestemperature [7]. Organic due belowto semiconductors, their 150 low degree energy [7] which conversion. Organic have semiconductors beenefficiency overlooked and possible, which in the have pastpoor decadesbeenthermal overlooked duestability, to their in thehowever, low past energy decades may conversion dueoffer to solutionstheir efficiency low energyto and the possible conversion above-mentioned poor efficiency thermal problems. stability,and possible however, Organic poor maythermalsemiconductors, offer stability, solutions such however, to the as above-mentionedconducting may offer polymers, solutions problems. ar e topromising Organic the above semiconductors, candidates-mentioned for problems. suchconverting as conducting Organiclow-end polymers,semiconductors,thermal energy are promising into such useful as candidatesconducting electricity for [8]], polymers converting and they, are low-endmight promising enable thermal candidatesthe energyconstruction intofor usefulconvert of sustainable, electricitying low- low-end [8], andthermaltoxicity, they energy larger-area, might into enable useful highly the electricity flexible construction [8]thermoelectric], and of sustainable,they might devices.[9] enable low-toxicity, Inthe the construction last larger-area, decade, of greatsustainable, highly efforts flexible lowhave- thermoelectrictoxicity,been input larger to devices- area,improve highly [9]. the In flexible theperformance last decade,thermoelectric of great organi efforts devicesc TE have materials,.[9] been In the input including last to decade, improve the greatexplosion the performanceefforts of have new ofbeenmolecular organic input TE designto materials, improve strategy including the and performance the the fabrication explosion of organic of of nanocomposites new TE molecular materials, designconsisting including strategy of the conducting and explosion the fabrication polymers of new ofmolecularand nanocomposites various design nanomaterials. strategy consisting and [10] of the conducting Thefabrication polymers polymers of nanocompositesapplied and in various these consisting researches nanomaterials. of are conducting mainly [10] The conducting polymerspolymers appliedandpolymers various in these(e.g., nanomaterials. researchesPoly(3,4-ethylenedioxythiophene are [10] mainly The polymers conducting applied polymers(PEDOT), in these (e.g., Polyaniline researches Poly(3,4-ethylenedioxythiophene (PANI), are mainly and conductingPolypyrrole (PEDOT),polymers(PPy)) (Figure Polyaniline (e.g., 3).Poly(3,4 (PANI),-ethylenedioxythiophene and Polypyrrole (PPy)) (PEDOT (Figure), 3Polyaniline). (PANI), and Polypyrrole (PPy)) (Figure 3).

FigureFigure 2.2. SchematicSchematic showingshowing thethe variousvarious materialsmaterials usedused inin TETE research,research, andand theirtheir individual individual contributions.Figurecontributions. 2. Schematic Reproduced Reproduced show withing with the permission permission various from from materials [ 10[10].]. used in TE research, and their individual contributions. Reproduced with permission from [10].

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O O H N n n N n S n H S n Polyacetylene Polyaniline Polypyrrole Polythiophene Poly(3,4- (PA) (PANI) (PPy) (PT) ethylenedioxythiophene) (PEDOT) C8H17 C8H17 C6H13 C H 6 13 N S n TB n S N n N C6H13 N S Poly(phenylenevinylene) Poly(2,7-carbazolylenevinylene) poly[N-9'-heptadecanyl-2,7-carbazole-alt-5,5'-(4',7'- PPV di-2-thienyl-2',1',3'-benzothiadiazole]PCDTBT Figure 3. Chemical structures of representative p-typep-type polymers for TE applications. 2. Theory of TE Devices 2. Theory of TE Devices The TE effect involves any transport phenomenon that implicates the relationship between heat The TE effect involves any transport phenomenon that implicates the relationship between heat and electrical potential energy. There are three reversible TE effects [11]: Seebeck for TE generation, and electrical potential energy. There are three reversible TE effects [11]: Seebeck for TE generation, Peltier effects for electronic refrigeration, and the Thomson effect. Although the non-zero TE effect Peltier effects for electronic refrigeration, and the Thomson effect. Although the non-zero TE effect might exist in all materials, the performance of a real TE device is lower than the Carnot efficiency, might exist in all materials, the performance of a real TE device is lower than the Carnot efficiency, because of two irreversible processes: Joule heating and thermal conduction [12]. because of two irreversible processes: Joule heating and thermal conduction [12]. The performances of thermoelectric generators (TEGs) are mostly evaluated by the thermoelectric The performances of thermoelectric generators (TEGs) are mostly evaluated by the conversion efficiency E and the power output P. They are associated with the size of the TEGs, and TE thermoelectric conversion efficiency E and the power output P. They are associated with the size of materials and morphologies. The calculated thermoelectric conversion efficiency E is derived from the TEGs, and TE materials and morphologies. The calculated thermoelectric conversion efficiency E thermodynamic theory (Equation (1)) [13–15]: is derived from thermodynamic theory (Equation (1)) [13-15]:

(T푇1 −−T푇22))·R푅L퐿 퐸 E== 2 (1) (푇 (T−1−푇T2))·R푅 ((R푅++RL푅퐿) )2 (1) 푇 T(푅1(+R +푅퐿R) L−) − 1 2 ++ 1 22 ZR푍푅

T1: HotT1: Hot side side temperature; temperature;

T2: ColdT2: Cold side side temperature; temperature; k: Thermalk: Thermal Conductivity; Conductivity;

RL: LoadRL: Load resistance; resistance; R: InternalR: Internal resistance resistance of the of semiconductor. the semiconductor. The Thomson effect is ignored for simplicity; thus, the Seebeck coefficient S is independent of The Thomson effect is ignored for simplicity; thus, the Seebeck coefficient S is independent of temperature. The maximum conversion efficiency Emax can be obtained when R/+ R （p1 ZT） temperature. The maximum conversion efficiency Emax can be obtained when RLL/R = (1 + ZT) (Equation (2))(2)) and the maximum power outputoutput PPmaxmax cancan be be achieved achieved under under RRLL == RR (Equation(Equation (3 (3)).)). p√1 + 푍푇 − 1 1 + ZT − 1 퐸Emax푚푎푥 = ((푇T1 − T푇2)) (2(2)) 1 2 푇 p√1 + 푍푇 + 푇 T11 1 + ZT + T22 푃 = 푆2(푇 − 푇 )2/(4푅) (3) 푚푎푥 2 1 22 Pmax = S (T1 − T2) /(4R) (3) Since most materials show very small TE effects, and only a few semiconducting materials have potentialSince in most future materials practical show applications. very small Also, TEeffects, to fulfil and the onlyfunction a few of semiconducting the TE devices ( materialsFigure 4), haveboth potentialp-type and in n future-type practicalsemiconducting applications. materials Also, are to simultaneously fulfil the function required of the TEin one devices TE device (Figure, 4and), both it is p-typevital to andchoose n-type the semiconductingproper materials materialsto match the are generator simultaneously voltage required and current. in one TE device, and it is vital to choose the proper materials to match the generator voltage and current.

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FigureFigure 4. 4.The The illustration illustration of the of TE the generator: TE generator: Different temperatureDifferent temperature operated across operated the thermoelectric across the couple,thermoelectric and electrical couple power, and electrical generated. power Reproduced generated. with Reproduced permission with from permission [16]. from [16].

InIn early early 1820, 1820, Thomas Thomas Johann Johann Seebeck discovered the firstfirst TE TE effecteffect in in his his experimental experimental investigationinvestigation on on the the relationship relationship between between electricity electricity and and heat heat [17] [.17 A]. dimensionless A dimensionless TE figure TE figure of merit of merit(ZT) is (ZT usually) is usually adopted adopted to evaluate to evaluate the efficiency the efficiency of TE of TEenergy energy conversion. conversion. ZTZT is definedis defined as as ZTZT = =S2SσT2σ/Tk,/ wherek, where S isS is the the Seebeck Seebeck coefficient, coefficient, σσ isis the the electrical electrical conductivity (contributed from both both phononsphonons (or(or latticelattice conductivity)conductivity) and electrons), k isis thethe thermalthermal conductivity,conductivity, andand TT isis thethe absoluteabsolute temperaturetemperature (Equation(Equation (4)).(4)). Hence, a higher TE performance c canan be achieved by larger S, higherhigher σ, andand lower lowerk .k Unfortunately,. Unfortunately, in most in most materials, materials these, these three three variable variable parameters parameters (S, σ, and (S, kσ), are and strongly k) are interdependent,strongly interdependent, and materials and withmaterials intrinsic with high intrinsicZT are high rare [ZT18]. are Recent rare investigation[18]. Recent suggestsinvestigation that thesuggest engineerings that the of engineer micro-/nanostructuresing of micro-/nanostructures in composite materials in composite could materials be an efficient could way be an to efficient address thisway issue, to address and to this reach issue a higher, and toZT reachvalue a (Figurehigher 5ZT), which value could(Figure open 5), newwhich venues could for open the new search venues and thefor design the search of TE and materials, the design especially of TE of composites materials, especiallyconsisting ofof organiccomposite semiconductors.s consisting of organic semiconductors. S2σT ZT = 푆2휎푇 (4) 푍푇 = k (4) 푘 S: SeebeckS: Seebeck Coefficient; Coefficient; σ: Conductivity;σ: Conductivity; T: Temperature;T: Temperature; k: Thermalk: Thermal Conductivity. Conductivity.

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Figure 5.5. Schematic showing the coupled dependence of of the the various various TE TE properties properties on on carrier carrier density. density. The shape of each individual curve was extractedextracted from actual data on single-walledsingle-walled carbon nanotube networks. Figure reproduced with permission [[19]19]..

The power generation efficiencyefficiency at a determined temperature difference cancan be evaluated by the 2 power factorfactor ((PFPF= =S S2σσ).). Thus,Thus, ZTZT isis oftenoften replaced replaced by byPF PFto to evaluate evaluate the the thermoelectric thermoelectric performance performance of theof the organic organic polymer polymer materials, materials and their, and related their composites, related composites due to their, due small to thermal their small conductivities. thermal conductivities.The reported theoretical studies on thermoelectric transport in polymers mainly aims to deal with the relationshipThe reported between theoretical transport studies properties on thermoelectric and the strength transport of the in electron–phonon polymers mainly interaction aims to deal [20]. Severalwith the kinds relationship of models between have been transport established properties based and on polyamine the strength in the of the1990s, electron which–phonon mainly contributedinteraction [20] to the. Several explanation kinds ofof themodels behavior haveof beenS and establishedσ as a function basedof on electric polyamine field, in temperature, the 1990s, levelwhich of mainly localization, contributed etc. These to the models explanation are further of the developed behavior toof studyS and electricσ as a function transport of inelectric disordered field, semiconductors.temperature, level There of localization, are two typical etc. These models models to evaluate are further the developed thermal resistance to study electric in materials: transport the diffusein disordered mismatch semiconductors. model and the acousticThere are mismatch two typical model models [21]. Both to evaluate are phenomenological the thermal resistance approaches, in assumematerials: linear the phonon diffuse dispersions, mismatch model oversimplify and the the effects acoustic from mismatch the interfaces, model and[21] are. Both fitted are to reproducephenomenological existing experimentalapproaches, assume results. linear phonon dispersions, oversimplify the effects from the interfaces,Except and for are these fitted simplified to reproduce approaches, existing molecular experimental dynamic results. (MD) techniques are also applied to analyzeExcept the thermalfor these conductivitysimplified approaches, of polymers, molecular based on dynamic classical (MD) mechanics techniques [22]. are The also ab initioapplied is anto alternativeanalyze the method thermal to conductivity calculate the of thermal polymers properties, based on under classical the using mechanics the Onsager [22]. The relations, ab initio and is thean Landaueralternative theory method of quantumto calculate transport the thermal [23], andproperties the thermodynamics under the using theory the Onsager establishes relations linear, relations and the betweenLandauer conjugate theory of “flows”quantum and transport “driving [23] forces”, and throughthe thermodynamics proportionality theory coefficients establishes [24 ].linear The electricalrelations andbetween heat conjugate currents both “flows” generate and “driving concomitantly forces” in through a thermoelectric proportionality system, coefficients and they interact [24]. The each electrical other, and theheat two currents forces both can generate concomitantly either of two flows in a thermoelectric or both of them system at the, and same th timeey interact [25]. each other, and the two forces can generate either of two flows or both of them at the same time [25]. 3. p-Type TE-Conducting Polymers 3. p-TypeThe most TE-Conducting typical organic Polymers materials bearing TE properties are conducting polymers (e.g., PEDOT, PANIThe and mo PPy)st typical (Figure organic3), which materials possess bearing many TE properties charming are features, conducting including polymers high (e.g. electrical, PEDOT, conductivity,PANI and PPy) low (Figure bad gap 3) energy,, which environmental possess many and charming thermal stability,features, aincluding light weight high and electrical strong backbone,conductivity, and low easy bad processability. gap energy, environmental and thermal stability, a light weight and strong backbone,As one and leg easy of processability. the TE devices, p-type TE organic semiconductors (OSCs) [26–29] have beenAs extensively one leg of studied. the TE devices, For example, p-type TE PEDOT organic derivatives semiconductors can achieve (OSCs) the [26 highest-29] have power been µ · −1 factorextensivelyPF over studied. 300 ForW example,m with PEDOTZT around derivatives 0.25 [can8]. achieve The TE the performance highest power of typicalfactor PF p-type over traditional300 μW·m−1 conducting with ZT around polymers 0.25 such [8]. as The polyaniline TE performance (PANI), polypyrroleof typical p- (PPY)type traditional [30,31], polythiophene conducting (PTH),polymers poly(3,4-ethylenedioxythiophene): such as polyaniline (PANI), polypyrrole poly(styrenesulfonate)/tosylate (PPY) [30,31], polythiophene (PEDOT:PSS, (PTH), PEDOT-Tos), poly(3,4- ethylenedioxythiophene): poly(styrenesulfonate)/tosylate (PEDOT:PSS, PEDOT-Tos), polyacetylene (PA) [32], polycarbazoles (PC) [33], polyphenylenevinylene (PPV), and their derivatives have been

Polymers 2019, 11, 107 6 of 19 polyacetylene (PA) [32], polycarbazoles (PC) [33], polyphenylenevinylene (PPV), and their derivatives have been widely studied as potential TE materials, and their performances are summarized in Table1. The intrinsic conductivities of most conducting polymers are in the range of 10−16–10−5 S/cm, while their intrinsically thermal conductivities typically lie between 0.11 and 0.4 W/mK, which is beneficial for ZT value [34]. Currently, the power factors (PF) for most polymer-based TE materials are in the range of 10−6–10−10 Wm−1 K−2, which is much smaller compared with that of conventional inorganic TE materials [35–38]. However, the insulating behaviors of the organic materials in the pristine state can be depressed after doping. In the following sections, we will discuss the performance of some of p-type conjugated polymers in detail, in order to understand the relationship between the structures and TE performance.

Table 1. The S, σ, k and ZTmax values of a few typical p-type polymers [39].

2 Polymer S µV/K σ S/cm k W/mK ZTmax PFmax µW/m K Ref. PANI −16–225 10−7–320 0.02–0.542 1.1 × 10−2 at 423 K [40–45] PA −0.5–1077 1.53 × 10−3–2.85 × 104 [46–51] PT 10–100 10−2–103 0.028–0.17 2.9 × 10−2 at 250 K [52–55] PPy −1–40 0–340 0.2 3 × 10−2 at 423 K [31,56] PC 4.9–600 4.0 × 10−5–5 × 10−2 19 [33,57] PEDOT:PSS 8–888 0.06–945 0.34 1.0 × 10−2 at 300 K [43,44,58–60] PEDOT-Tos 40–780 6 × 10−2–300 0.37 0.25 at RT [8]

3.1. Polyacetylene (PA) The iodine-vapor-doped polyacetylene has electrical conductivity as high as 10,000 S·cm−1 [61,62]. Employing doped polyacetylene films as active elements for TE measurement, Na et al. found that the variation of temperature can influence the electrical conductivity, and the maximum conductivity −1 can achieve 30,000 S·cm at T = 220 K in the FeCl3-doped stretchable polyacetylene films [63]. Later on, the polyacetylene films doped with iodine and FeCl3 have been demonstrated to show PF of up to 2 × 10−3 W·m−1 K−2 [47] and 8.3 × 10−5 W·m−1 K−2 [64], respectively. However, the further development of polyacetylene as a TE material is limited by its insolubility and poor air stability.

3.2. Polyaniline (PANI) Since PANI has many attractive properties, such as high conductivity, good stability, and easy preparation/processing, it has become the most commonly studied TE material among all conducting polymers [65]. The electrical conductivity of (±)-10-camphorsulfonic acid (CSA)-doped PANI can achieve 300 S/m, and the Seebeck coefficient reaches 8–12 µV/K at 300 K. It is noted that a PANI-based multilayered film doped with CAS shows a six-fold increase in ZT, compared to that of bulk CSA-doped PANI, and the ZT value of the former one can reach 1.1 × 10−2 at 423 K. One also should notice that the dopant agents can greatly affect the performance of TE polymers; for example, the maximum ZT value of HCl-doped PANI can only give 2.67 × 10−4 at 423 K [66,67]. The excellent thermal stability and low thermal conductivity of PANI are attractive for its application in TE devices; however, its electrical conductivity is greatly associated with the morphology, molecular weight, chain arrangement, oxidation level, and doping level. Thus, further enhancing the electrical conductivity can be realized through optimizing these parameters. In addition, the cheap starting materials can be one advantage for making PANI film more promising for practical applications if its ZT value can be further improved by dopants or other methods.

3.3. Polypyrrole Although polypyrrole (PPy) has the merits of common conducting polymers, such as relatively easy processing, low cost, and good environment stability, there are less reports on their applications, as TE materials, compared with their usage in electronic energy devices. The major efforts lie on PPy-based nanocomposites. Since carbon nanotubes (CNTs) are quite powerful thermoelectric materials as well, Polymers 2019, 11, 107 7 of 19 they have been widely used to blend with the polymers to improve the TE performance. Polymer–CNT nanocomposites can be prepared by many methods, such as in situ polymerization, dissolving polymers into organic solvents containing multiwall carbon nanotube (MWNT) suspensions, melt mixing, grafting macromolecules onto CNT, and interfacial polymerization. Zhang et al. studied the performance of PPy-nanotube films with varied sizes of CNTs [68]. In their research, they fabricated two flexible and freestanding polypyrrole (PPy) nanotube films (PPy-1 and PPy-2), and carefully studied their structures, morphologies, and thermoelectric properties. The PPy film with shorter PPy nanotubes gave a Seebeck coefficient of 16.5 µV·K−1, electrical conductivity of 3.43 S·cm−1, and PF of 0.09 µW·m−1·K−2, while the longer PPy nanotubes showed an electrical conductivity, Seebeck coefficient, PF at 9.81 S·cm−1, 17.68 µV·K−1, 0.31 µW·m−1·K−2, respectively. These results indicate that the longer length and smaller sizes of the PPy nanotubes were helpful for enhancing the electrical conductivity and the Seebeck coefficient, while size and length have less effects on thermal conductivity. The nanoscale structures of the PPy-based materials also affect their TE properties. Guo et al. found that, among the different morphologies of the as-prepared PPy nanostructures such as nanoparticles and nanowires, and the nanowires with largest aspect ratios exhibited the highest PF of (22.6 ± 3.6) × 10−3 µW·m−1·K−2 [69]. This result suggests that the development of novel organic thermoelectric materials can be achieved by morphology-controlling strategy.

3.4. Polythiophene-Based Derivatives Generally, polythiophene (PT) has a low Seebeck coefficient and a low electrical conductivity, and its TE performance is strongly affected by the sizes of the side chains and the main chain structure. For example, the electropolymerized freestanding polythiophene and poly(3-methylthiophene) (PMeT) nanofilms can exhibit high Seebeck coefficients (130 and 76 µV/K), decent electrical conductivities (47 and 73 S/cm), and low thermal conductivities (0.17 and 0.15 W/mK) at room temperature, compared with those made by other method [39]. Due to the good rearrangement of polymer chains in the electropolymerized films, PT and PMeT could display much better TE performances than those obtained in pressed pellet samples. The polyethylenedioxythiophene (PEDOT) family is the most popular TE polymers because of their excellent electrical conductivity, power factor, and its high stability toward moisture and oxygen. The reported research have already indicated that TE devices based on PEDOT can have very good performances [58,70–72]. The first PEDOT-based TE device was reported in 1988 [73], and its Seebeck coefficient and electrical conductivity can be optimized by controlling the dopant concentration and the oxidation level during the polymerization. Normally, there are two common methods to fabricate conductive PEDOT films. One is in situ polymerization, namely, the production of PEDOT during film formation. For example, the PEDOT:tos (tos: tosylate) film can be in situ prepared by using Fe(tos)3 as an oxidizing agent, and the highest electrical conductivity of the as-prepared thin film can reach 4300 S/cm [59,74]. The other method is to use a stable PEDOT dispersion solution to cast a film. However, this method is only limited to the PEDOT:PSS system. PEDOT dispersion solution is cheap enough to fabricate large-area devices, and its doped state can also be easily processed through solution-processing techniques. The TE performance of PEDOT/PSS(poly(4-styrenesulfonate)) has been extensively studied by many groups [71]. The effects of dielectric solvents (e.g., DMSO) and the ratio between PEDOT and PSS in the polymeric films were carefully investigated [59]. The authors found that the Seebeck coefficient can keep unchanged within certain ranges of DMSO and PSS concentrations, while the electrical conductivity becomes the major factor for controlling the value of PF. Katz et al. modified the structure of p-type poly(bisdodecylquaterthiophene) (PQT12) (Figure6) to realize high and predominantly nonionic conductivity. [75] The highest conductivities of up to −1 −1 350 S·cm and 140 S·cm were achieved for NOBF4-doped PQTS12 and F4TCNQ-doped PDTDE12, respectively. Tighter π–π stacking, efficient charge transfer, and strong intermolecular coupling have major contribution to the conductivity. The conductivities of these polymers are stable in air, without Polymers 2019, 11, 107 8 of 19 extrinsic ion contributions. These results may suggest that further improving the ZT value could be

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FigureFigure 6. Chemical 6. Chemical structures structures of p ofolymers polymers and and dopants dopants and and their their Highest Highest Occupied Occupied Molecular Molecular Orbital (HOMO(HOMO)) and Lowest and Lowest Unoccupied Unoccupied Molecular Molecular Orbital Orbital (LUMO (LUMO)) energy energy levels, levels, respectively. respectively. Reproduced Reproduced with permissionwith permission from [75]. from [75].

3.5. Other p p-Type-Type TE Polymers Poly(2,7Poly(2,7-carbazole)-carbazole) (PC) (PC) and and their their derivatives derivatives were were also also investigated investigated as p-type as p TE-type materials TE materials[76,77]. Although[76,77]. Alth polycarbazoleough polycarbazole derivatives derivatives usually showusually high show Seebeck high coefficients;Seebeck coefficients however,; however, their electrical their electricalconductivities conductivities are small [ are57]. small The conductivity [57]. The conductivity of polycarbazole of polycarbazole (PC) derivatives (PC) can derivatives be significantly can be significantlyenhanced by enhanced the introduction by the ofintroduction vinylene and of vinylene electron-donating and electron groups-donating such as groups thiophene such asor −1 thiophenebis(3,4-ethylenedioxythiophene) or bis(3,4-ethylenedioxythiophene) [37]. A Seebeck coefficient[37]. A Seebeck of 34 µ V coefficient·K , an electroconductivity of 34 μV·K−1, an of −1 −1 −2 160electroconductivity S·cm , and a corresponding of 160 S·cm−1PF, andof 19 a µ correspondingW m ·K were PF of obtained 19 μW for m a−1· polycarbazoleK−2 were obtained derivative, for a 0 0 0 0 0 0 0 poly[polycarbazoleN-9 -heptadecanyl-2,7-carbazole-alt-5,5 derivative, poly[N-9′-heptadecanyl-(4 ,7 -di-2-thienyl-2-2,7-carbazole,1-alt,3 -benzothiadiazole]-5,5′-(4′,7′-di-2-thienyl (PCDTBT)-2′,1′,3′- (Figurebenzothiadiazole]3)[57]. (PCDTBT) (Figure 3) [57]. Some other other conducting conducting polymers polymers such such as polyparaphenylene, as polyparaphenylene, polyparaphenylene polyparaphenylene vinylene, vinylene, and poly(2,5and poly(2,5-dimethoxphenylenevinylene)-dimethoxphenylenevinylene) (PMeOPV) (PMeOPV),, and and a series a series of of copolymers copolymers have have also also been investigated as TE materialmaterial [[39]39].. Howeve However,r, their performance needs further improvement.improvement.

4. TE Properties of n-Type Polymers 4. TE Properties of n-Type Polymers Compared to p p-type-type polymer TE materials, their n-typen-type counterparts exhibit mu muchch poor poorerer TE behaviors, due to inefficient doping and much lower conductivity. Also, the preparation of n-type behaviors, due to inefficient doping and much lower conductivity. Also, the preparation of n-type polymer TE materials is also hindered by their air air-sensitive-sensitive character, which makes their research lag far behind. It is well-known that the stability of the n-type polymers is strongly related to the LUMO far behind. It is well-known that the stability of the n-type polymers is strongly related to the LUMO levellevel,, and the deeper LUMO level of the the n n-type-type polymers polymers is is a prerequisite prerequisite for ensur ensuringing the the realization realization of high conductivityconductivity [78] [78] (Table 2 2)).. HighlyHighly stablestable n-typen-type thermoelectricthermoelectric materialsmaterials cancan bebe realizedrealized byby mixmixinging carbon carbon nanotubes nanotubes with with conjugated conjugated polymers, polymers, and these and novel these n-type novel thermoelectric n-type thermoelectric composites N possesscomposites longer possess operation longer lifetimes operation and lifetimes air stabilities. and air Typically, stabilities-alkyl. Typically, substituted N-alkyl 1H-benzimidazoles substituted 1H- benzimidazolesare employed as are n-dopants employed for as organic n-dopant thermoelectrics for organic materials thermoelectric (Figure ma7).terials (Figure 7).

Table 2. Summary of the TE performance of typical n-type polymers [78].

Polymer Dopant S [μV·K−1] σ [S·cm−1] k [W·m−1·K −1] PF [μW·m−1·K −2] ZT Ref

PA Bu4N −43.5 5 - 1 - [61] C60 Cr2(hpp)4 −175 4 12 [79] FBDPPV (Figure 9) DMBI −141 14 - 28 - [80] A-DCV-DPPTT (Figure 9) 105 0.11 [81] FBDPPV (Figure 9) DMBI −210 6.2 - 25.5 - [82] P(NDIOD-T2) N-DMBI −850 0.008 - 0.6 - [83] P(NDIOD-T2) N-DPBI −770 0.004 - 0.2 - [83] KxC70 −22.5 550 28 [84] BBL TDAE −101 0.42 - 0.43 - [85] Poly[KxNi-ett] N. A −122 40 −0.2 66 −0.1 [86] KxNi-ett N. A −90 210 0.4–0.5 170 0.30 [87] CuBHT N. A −4 to −10 750–1580 - N. A. - [88]

P3HT Doped with PF6 39 0.843 0.14 [89]

Polymers 2019, 11, 107 9 of 19

Table 2. Summary of the TE performance of typical n-type polymers [78].

Polymer Dopant S [µV·K−1] σ [S·cm−1] k [W·m−1·K−1] PF [µW·m−1·K−2] ZT Ref

PA Bu4N −43.5 5 - 1 - [61] C60 Cr2(hpp)4 −175 4 12 [79] FBDPPV (Figure 9) DMBI −141 14 - 28 - [80] A-DCV-DPPTT (Figure 9) 105 0.11 [81] FBDPPV (Figure 9) DMBI −210 6.2 - 25.5 - [82] P(NDIOD-T2) N-DMBI −850 0.008 - 0.6 - [83] P(NDIOD-T2) N-DPBI −770 0.004 - 0.2 - [83] KxC70 −22.5 550 28 [84] BBL TDAE −101 0.42 - 0.43 - [85] Poly[KxNi-ett] N. A −122 40 −0.2 66 −0.1 [86] KxNi-ett N. A −90 210 0.4–0.5 170 0.30 [87] CuBHT N. A −4 to −10 750–1580 - N. A. - [88] Polymers 2019, 11, x FOR PEER REVIEW 9 of 18 Polymers 2019P3,HT 11, x FOR PEERDoped REVIEW with PF 6 39 0.843 0.14 [989 of] 18

N N N N N N N N N N N N N N N N N N

DMBI DEtBI DPrBI DMBI DEtBI DPrBI

N N N N N N N N N N N N N N N N N N

DBuBI DiPrBI DiBuBI DBuBI DiPrBI DiBuBI Figure 7. Molecular structures of well-known n-dopants: N-alkyl-substituted 1H-benzimidazoles. FigureFigure 7 7.. MolecularMolecular structu structuresres of of well well-known-known n n-dopants:-dopants: NN--alkyl-substitutedalkyl-substituted 1H 1H-benzimidazoles.-benzimidazoles. 4.1.4.1. Building Units Units for for n-Type n-Type Polymers Polymers 4.1. Building Units for n-Type Polymers Perylene bisimide bisimide [[90]90],, n naphthalenetetracarboxylicaphthalenetetracarboxylic dianhydride dianhydride [91] [91],, Perylene bisimide [90], naphthalenetetracarboxylic dianhydride [91], naphthodithiophenediimide [92] [92, ],benzotriazole benzotriazole [93], [ 93and], fullerenes and fullerenes [79] have [79 been] have widely been used widely as n- naphthodithiophenediimide [92], benzotriazole [93], and fullerenes [79] have been widely used as n- usedtype as organic n-type small organic molecules small molecules for organic for organic semiconductor semiconductor devices devices (Figure (Figure 8). The8). The LUMO LUMO level level of type organic small molecules for organic semiconductor devices (Figure 8). The LUMO level of ofpolymers polymers based based on on these these building building units units can can be be adjusted adjusted by by introducing introducing diff differenterent electron electron-donating-donating polymers based on these building units can be adjusted by introducing different electron-donating units into these polymer structures. units into these polymer structures.

R R O N O O N O R R R R R R R R O N O O N O O N O O N O O N O O N O O N O O N O S S Ar Ar S S Ar Ar S S O N O O N O O N O O N O O N O O N O O N O O N O R R R R O N O R R R R O N O NDI cNDI NDTI NTI R NDI cNDI NDTI NTI RPDI Ar: benzene, thiophene, PDI Athr:ia bzeonlez,e bnen, zthoi[obp]thheionpeh, ene, tbheianzzoole[b, ]bpeyrnrzoole[b, ]qthuiionpohxeanlinee, benzo[b]pyrrole, quinoxaline Figure 8.8. The bu buildingilding b blockslocks forfor n-typen-type polymers. Figure 8. The building blocks for n-type polymers. 0 Naphtho[2,3-b:6,7-bNaphtho[2,3-b:6,7-b′]dithiophenediimide]dithiophenediimide (NDTI) (NDTI) and and naphtha[2naphtha[2,3-b]thiophene,3-b]thiophene diimide (NTI) Naphtho[2,3-b:6,7-b′]dithiophenediimide (NDTI) and naphtha[2,3-b]thiophene diimide (NTI) featured with with low low-lying-lying energy energy levels levels of ofLUMO LUMO (3.8 (3.8–4.1–4.1 eV) eV)and andthe easy the functionalizability easy functionalizability of the α of- featured with low-lying energy levels of LUMO (3.8–4.1 eV) and the easy functionalizability of the α- thepositionsα-positions in thiophene, in thiophene, which which allow allow the derivatives the derivatives and polymers and polymers to conjugate to conjugate efficie efficientlyntly with positions in thiophene, which allow the derivatives and polymers to conjugate efficiently with withadditional additional π- andπ- and comonomer comonomer units. units. Recently, Recently, various various air air-stable-stable doped doped n n-type-type semiconductors semiconductors additional π- and comonomer units. Recently, various air-stable doped n-type semiconductors derived from the NDTI and NTI are applicable to n-typen-type TE materials with high electron mobility derived from2 − 1the− 1NDTI and NTI are applicable to n-type TE materials with high electron mobility (~0.8 cm 2··VV−1·s−1·s) [94,95])[94,95. ]. (~0.8 cm2·V−1·s−1) [94,95]. Facchetti et al.al. investigated the morphological, structural, charge transport, and thermoelectric Facchetti et al. investigated the morphological, structural, charge transport, and thermoelectric properties of the the films, films, basedbased on on naphthalenediimide naphthalenediimide (NDI)-bithiazole (NDI)-bithiazole (Tz2)-based(Tz2)-based polymer properties of the films, based on naphthalenediimide (NDI)-bithiazole (Tz2)-based polymer [P(NDI2OD-Tz2)][P(NDI2OD-Tz2)] [96] [96] (Figure 99).). TheyThey foundfound thatthat P(NDI2OD-Tz2)P(NDI2OD-Tz2) possesspossess aa moremore planarplanar andand rigidrigid [P(NDI2OD-Tz2)] [96] (Figure 9). They found that P(NDI2OD-Tz2) possess a more planar and rigid backbone, and exhibited enhancing π–π stacking, as well as intermolecular interactions, compared backbone, and exhibited enhancing π–π stacking, as well as intermolecular interactions, compared with the widely-investigated parent polymer P(NDI2OD-T2). Furthermore, the electron affinity of with the widely-investigated parent polymer P(NDI2OD-T2). Furthermore, the electron affinity of the as-prepared polymer was improved, due to the electron-deficient nature of Tz2, thus decreasing the as-prepared polymer was improved, due to the electron-deficient nature of Tz2, thus decreasing the polymer donor–acceptor character. Decent electrical conductivity (≈0.1 S·cm−1) and a reasonable the polymer donor–acceptor character. Decent electrical conductivity (≈0.1 S·cm−1) and a reasonable power factor (1.5 µW m−1·K−2) were achieved for P(NDI2OD-Tz2) after amine doping. These data are power factor (1.5 µW m−1·K−2) were achieved for P(NDI2OD-Tz2) after amine doping. These data are higher than those of the undoped P(NDI2OD-T2) (0.003 S·cm−1 and 0.012 µW m−1·K−2, respectively). higher than those of the undoped P(NDI2OD-T2) (0.003 S·cm−1 and 0.012 µW m−1·K−2, respectively). These results suggest that reducing donor–acceptor character of planarized NDI-based polymers can These results suggest that reducing donor–acceptor character of planarized NDI-based polymers can lead to substantial electrical conductivity and thermoelectric response. lead to substantial electrical conductivity and thermoelectric response.

Polymers 2019, 11, 107 10 of 19 backbone, and exhibited enhancing π–π stacking, as well as intermolecular interactions, compared with the widely-investigated parent polymer P(NDI2OD-T2). Furthermore, the electron affinity of the as-prepared polymer was improved, due to the electron-deficient nature of Tz2, thus decreasing the polymer donor–acceptor character. Decent electrical conductivity (≈0.1 S·cm−1) and a reasonable power factor (1.5 µW m−1·K−2) were achieved for P(NDI2OD-Tz2) after amine doping. These data are higher than those of the undoped P(NDI2OD-T2) (0.003 S·cm−1 and 0.012 µW m−1·K−2, respectively). These results suggest that reducing donor–acceptor character of planarized NDI-based polymers can leadPolymers to 2019 substantial, 11, x FOR electrical PEER REVIEW conductivity and thermoelectric response. 10 of 18

3 O

2OD C8H17 OC12H25 C H O N O O N O 10 21 S S O N S n S n N S S C12H25O N N n O O N O O N O C H 2OD 10 21 C8H17 P(NDI2OD-T2) PNDI2TEG-2T PDPH O

2OD = 2-octyldodecyl 3

3 O C8H17 C10H21 2OD F F OC8H17 O O N O O N O N S N S N N S S S N n S N n N N n C8H17O O F F C H O N O O N O 10 21 C8H17 2OD P(NDI2OD-Tz2) PNDI2TEG-2Tz PDPF O

2OD = 2-octyldodecyl 3

C18H37 C8H17

C H C10H21 18 37 CN NC S N O F O O O N S S

O N S CN n NC N O O F O C10H21

C8H17 C18H37 FBDPPV A-DCV-DPPTT C18H37 FFigureigure 9 9.. Chemical structures of some representative n-typen-type polymers.

The chemical structure of the polymers,polymers, and the solid-statesolid-state packing of interactinginteracting conjugated segments cancan largelylargely affect affect charge charge transport transport in in polymer polymer films films [97 [97]]. Bertarelli. Bertarelli et al.et haveal. have studied studied how how the alkylthe alkyl substitution substitution on the on backbones the backbones of air-s H-benzimidazole-based of air-s H-benzimidazole dopants,-based to dopants affect the, to morphology affect the andmorphology electrical and properties electrical of properties the copolymer of the P(NDI2OD-T2) copolymer P(NDI2OD [98] after-T2) blending [98] after (Table blending1). It turns(Table out 1). thatIt turns higher out thatσ values higher can σ bevalues obtained can be from obtained the dopants from the with dopants longer with linear longer alkyl linear chains. alkyl For chains. example, For −3 −1 Nexample,-butyl-substituted N-butyl-substituted dopants can dopants reach four can timesreach higherfour times conductivities higher conductivities (4.1 × 10 S (4.1·cm × )10 than−3 S· thesecm−1) inthan the these methyl-substituted in the methyl-substituted reference ones. reference Compared ones. Compared to pristine to P(NDI2OD-T2) pristine P(NDI2OD films,- theT2) structuralfilms, the analysisstructural suggests analysis that suggests an edge-on that orientation an edge-on of orientation polymer crystallites, of polymer a decrease crystallites, in the a lamellar decrease stacking in the lamellar stacking distance, and a disruption of backbone and π-stacking order were induced by the dopant molecules. The introduction of the soluble chains in small molecule-based dopants is an effective method for controlling and enhance the miscibility of dopants and polymers. This method allows to improve the conductivity of polymers due to the improved doping efficiency. Till now, there are two different proposed models to describe the molecular doping of organic semiconductors: the molecular orbital hybridization (MOH) model and the inter-charge transfer (ICT) model [99]. Koster et al. found that power factor of n-type organic thermoelectric donor– acceptor (D–A) copolymers can be increased by a factor of > 1000 through tailoring of the density of states (DOS) [100,101]. The DOS distribution of PNDI2TEG-2T is tailored by the embedded sp2-

Polymers 2019, 11, 107 11 of 19 distance, and a disruption of backbone and π-stacking order were induced by the dopant molecules. The introduction of the soluble chains in small molecule-based dopants is an effective method for controlling and enhance the miscibility of dopants and polymers. This method allows to improve the conductivity of polymers due to the improved doping efficiency. Till now, there are two different proposed models to describe the molecular doping of organic semiconductors: the molecular orbital hybridization (MOH) model and the inter-charge transfer (ICT) model [99]. Koster et al. found that power factor of n-type organic thermoelectric donor–acceptor

(D–A)Polymers copolymers2019, 11, x FOR can PEER be REVIEW increased by a factor of > 1000 through tailoring of the density of11 states of 18 (DOS) [100,101]. The DOS distribution of PNDI2TEG-2T is tailored by the embedded sp2-nitrogen atomsnitrogen at aatoms donor at unit a donor of the unit D-A of copolymer the D-A (Figurecopolymer9). Consequently, (Figure 9). Cons PNDI2TEG-2Tzequently, PNDI2TEG-2Tz achieved an −1 −1 −2 electricalachieved conductivityan electrical conductivity of 1.8 S·cm of, and 1.8 S·cm a power−1, and factor a power of 4.5 factorµW·m of 4.5K μW·m. −1 K−2. Pei andand ZhuZhu etet al.al. reportedreported that that the the n-doping n-doping efficiency efficiency of of a D–Aa D– polymerA polymer can can be enhancedbe enhanced by theby introductionthe introduction of electron-deficiency of electron-deficiency F atoms F atoms into donorinto donor units units of the of polymer the polymer backbone backbone (PDPF). (PDPF). A high A −1 −1 −2 n-typehigh n-type electrical electrical conductivity conductivity of 1.30 of S1.30·cm S·cmand−1 and an an excellent excellent power power factor factor (PF (PF) of) of 4.65 4.65µW μ·W·mm −K1K−2 were obtained,obtained, which which were were enhanced enhanced for threefor three orders orders of magnitude, of magnitude, comparing comparing to the non-engineered to the non- polymerengineered (PDPH) polymer [96 ].(PDPH) They suggested [96]. They that suggested there are th threeat there advantages are three ofadvantages the electron-withdrawing of the electron- modificationwithdrawing ofmodification the donor of moiety the donor in a D–Amoiety conjugated in a D–A conjugated polymer: (1) polymer: intramolecular (1) intramolecular H-bonds areH- formed,bonds are and formed, both the and HOMO both andthe LUMOHOMO levelsand LUMO of the polymer levels of are the lowered; polymer (2) are the lowered; improvement (2) the of theimprovement n-doping efficiency of the n-doping and the efficiency avoid of phase and the separation; avoid of andphase (3) separation; the lowering and of the(3) carrierthe lowering hopping of barrierthe carrier while hopping the mobility barrier remains while the unaffected, mobility remains or even greatlyunaffected, enhanced. or even greatly enhanced. Nakano and Takimiya et al.al. have developed several new π-conjugated-conjugated polymerspolymers PNDTI-BBTs 0 (Figure(Figure 10)10 )with with strong strong electron electron affinity, affinity, including including naphtho[2 naphtho[2–b:6,7-b–b:6,7-b′]dithiophenediimide]dithiophenediimide (NDTI) 0 (NDTI)and benzo[1,2-c:4,5-c and benzo[1,2-c:4,5-c′]-bis[1,2,5]thiadiazole]-bis[1,2,5]thiadiazole (BBT) units [102]. (BBT) The units low-lying [102]. LUMO The energy low-lying levels LUMO(~−4.4 eV) energy of PNDTI-BBTs levels (~make−4.4 them eV) readily of PNDTI-BBTs doped by N,N make-dimethyl-2-phenyl-2,3-dihydro-1H- them readily doped by Nbenzoimidazole,N-dimethyl-2-phenyl-2,3-dihydro-1H-benzoimidazole (N-DMBI), providing the doped polymer (N-DMBI), films, providingwith relatively the doped high polymerSeebeck films,coefficients with relativelyand conductivities. high Seebeck The coefficientsconductivity and and conductivities. the power factor The of conductivity the doped thin and films the power(~0.18 −1 −1 −2 −1 factorS·cm−1 ofand the ~0.6 doped μW·m thin−1·K films−2 for (~0.18 PNDTI-BBT-DT; S·cm and ~ ~0.65.0 S·cmµW·m−1 and·K ~14for μW·m PNDTI-BBT-DT;−1·K−2 for PNDTI-BBTDP) ~5.0 S·cm − − andwere ~14 dramaticallyµW·m 1·K changed2 for PNDTI-BBTDP) by the usage of were soluble dramatically alkyl groups changed such as by 2-decyltetradecyl, the usage of soluble PNDTI- alkyl groupsBBT-DT, such or 3-decylpentadecyl as 2-decyltetradecyl, groups, PNDTI-BBT-DT, and PNDTI- orBBT-DP. 3-decylpentadecyl The better crystalline groups, and nature PNDTI-BBT-DP. of PNDTI- TheBBT-DP better than crystalline PNDTI-BBT-DT nature of in PNDTI-BBT-DPthe thin film was than the main PNDTI-BBT-DT reason for causing in the thin the differences film was the in maintheir reasonelectric forproperties. causing the differences in their electric properties.

Figure 10.10. Naphthodithiophenediimide-benzobisthiadiazole-basedNaphthodithiophenediimide-benzobisthiadiazole-based polymers.polymers. Reprinted Reprinted with permission from [[102].102].

Hummelen et al. reported reported a promising n-type doping system [103], [103], which is constructed by introducing polar triethylene glycol (TEG) side chainchain onto both fullerene host (PTEG-1) and dopant (TEG-DMBI)(TEG-DMBI) materialsmaterials (Figure(Figure 1111).). The improved miscibilitymiscibility of solution-processedsolution-processed TEG-DMBI-doped PTEG-1 films films displayed displayed a higher a higher carrier carrier density density and mobility and mobility compared compared with N-DMBI-doped with N-DMBI-doped films. A record power factor of 19.1 mW·m−1·K−2 and an electrical conductivity of 1.81 S·cm−1 were achieved.

Polymers 2019, 11, x FOR PEER REVIEW 11 of 18

nitrogen atoms at a donor unit of the D-A copolymer (Figure 9). Consequently, PNDI2TEG-2Tz achieved an electrical conductivity of 1.8 S·cm−1, and a power factor of 4.5 µW·m−1 K−2. Pei and Zhu et al. reported that the n-doping efficiency of a D–A polymer can be enhanced by the introduction of electron-deficiency F atoms into donor units of the polymer backbone (PDPF). A high n-type electrical conductivity of 1.30 S·cm−1 and an excellent power factor (PF) of 4.65 µW·m−1K−2 were obtained, which were enhanced for three orders of magnitude, comparing to the non- engineered polymer (PDPH) [96]. They suggested that there are three advantages of the electron- withdrawing modification of the donor moiety in a D–A conjugated polymer: (1) intramolecular H- bonds are formed, and both the HOMO and LUMO levels of the polymer are lowered; (2) the improvement of the n-doping efficiency and the avoid of phase separation; and (3) the lowering of the carrier hopping barrier while the mobility remains unaffected, or even greatly enhanced. Nakano and Takimiya et al. have developed several new π-conjugated polymers PNDTI-BBTs (Figure 10) with strong electron affinity, including naphtho[2–b:6,7-b′]dithiophenediimide (NDTI) and benzo[1,2-c:4,5-c′]-bis[1,2,5]thiadiazole (BBT) units [102]. The low-lying LUMO energy levels (~−4.4 eV) of PNDTI-BBTs make them readily doped by N,N-dimethyl-2-phenyl-2,3-dihydro-1H- benzoimidazole (N-DMBI), providing the doped polymer films, with relatively high Seebeck coefficients and conductivities. The conductivity and the power factor of the doped thin films (~0.18 S·cm−1 and ~0.6 μW·m−1·K−2 for PNDTI-BBT-DT; ~5.0 S·cm−1 and ~14 μW·m−1·K−2 for PNDTI-BBTDP) were dramatically changed by the usage of soluble alkyl groups such as 2-decyltetradecyl, PNDTI- BBT-DT, or 3-decylpentadecyl groups, and PNDTI-BBT-DP. The better crystalline nature of PNDTI- BBT-DP than PNDTI-BBT-DT in the thin film was the main reason for causing the differences in their electric properties.

Figure 10. Naphthodithiophenediimide-benzobisthiadiazole-based polymers. Reprinted with permission from [102].

Hummelen et al. reported a promising n-type doping system [103], which is constructed by Polymers 2019, 11, 107 12 of 19 introducing polar triethylene glycol (TEG) side chain onto both fullerene host (PTEG-1) and dopant (TEG-DMBI) materials (Figure 11). The improved miscibility of solution-processed TEG-DMBI-doped films.PTEG-1 A fi recordlms displayed power a factor higher of carrier 19.1 mW density·m− 1and·K− mobility2 and an compared electrical with conductivity N-DMBI- ofdoped 1.81 fi Slms.·cm− A1 wererecord achieved. power factor of 19.1 mW·m−1·K −2 and an electrical conductivity of 1.81 S·cm−1 were achieved.

Figure 11. Chemical structures of PTEG-1, N-DMBI, and TEG-DMBI. Reprinted with permission from [103]. 4.2. Poly(nickel-ethylenetetrathiolate) (KxNiett) Since Poleschner et al. firstly reported the poly(Ax(M-ett)) system 30 years ago, the synthesis, electrical/magnetic properties, and the structure of poly(Ax(M-ett)) system have been studied by many groups. Although these polymers have excellent conductive properties (~10−5–50 S·cm−1), there are few investigations on their TE performances. Recent research progress in the thermoelectric properties of poly(M-ett) (M = metal, ett = ethylenetetrathiolate) showed that poly(M-ett) provided the best performance of n-type organic thermoelectric materials. Yang et al. reported that the intrinsic metallic behaviors and high-performance TE power factors can coexist within dopant-free linear-backbone conducting polymers, poly(nickel-ethylenetetrathiolate) and its analogs. Based on density functional calculations, Yang and Xue et al. found that four crystalline π-d conjugated transition–metal coordination polymers such as poly(Pd-C2S4), poly(Pt-C2S4), poly(Ni-C2S4), and poly(Ni-C2Se4) showed intrinsic metallic behaviors, generating from dense intermolecular interaction networks between sulfur/selenium atoms. Moderate carrier concentrations (1019–1021·cm−3) and decent conductivities (103–104 S·cm−1) have been found in these polymers. Especially, poly(Ni-C2S4), poly(Ni-C2Se4), and poly(Pd-C2S4) exhibit high power factors (~103 µW·m−1·K−2)[104].

5. Strategies for Enhancing the ZT of Organic TE Materials According to the definition of the figure of merit, ZT = S2σT/k. A desired TE material should have the following properties: a large Seebeck coefficient to promote the energy conversion of heat to electricity, a high electrical conductivity to reduce Joule heating, and a low thermal conductivity to prevent thermal shorting. All of the current research is focused on these aspects through the following strategies.

5.1. Molecular Structure Modification An effective TE material should have an enhanced Seebeck coefficient and a high electrical conductivity simultaneously, while remaining small κ constant. Chemical functionalization is an effective way towards improving ZT through increasing the carrier mobility and keeping a constant carrier density, which leads to an improved Seebeck coefficient and an enhanced electrical conductivity simultaneously, according to the equation σ = enµ, where e is the electron charge, n is the charge carrier density, and µ is the carrier mobility. The theoretical calculation based on the existing data can conclude some useful structure–property relationships, which can instruct scientists to design efficient TE materials for real applications. Polymers 2019, 11, 107 13 of 19

5.2. Nanocomposites of Organic/Inorganic Hybridization

Compared to conventional rigid inorganic thermoelectric (e.g., PbTe or Bi2Te3), organic thermoelectric shows obvious advantages for their easy fabrication into versatile formats. However, their performance is limited by their inferior electrical conductivity, a small Seebeck coefficient, and a low power factor (PF). This issue might be solved by nanocomposited organic/inorganic hybrid films, because these hybrid films can integrate low thermal conductivities of organic TE materials and high electroconductivity/Seebeck values of inorganic counterparts together, leading to higher TE materials, such as HH alloys, skutterudites, and Si–Ge systems. Polymer/inorganic TE nanocomposites can be prepared by solution mixing, physical mixing, interfacial polymerization, or in situ chemical oxidative polymerization. Several nanocomposites, including PANI/NaFe4P12, PANI/PbTe, PEDOT/Na2TeO3, and PEDOT/Ca3Co4O9 can be fabricated by in situ chemical oxidative polymerization, while PANI/BiTe3 and PEDOT/Bi2Te3 composites can only be realized by a physical mixing method, due to the easy oxidation of Bi2Te3 during the process of in situ chemical oxidative polymerization [105]. Carbon nanotubes (CNTs) are one-dimensional carbon nanomaterials with nanoscale diameters and single or multiple walls, and they can be employed as effective conductive fillers in polymer films for achieving high ZTs, due to their high electrical conductivity, excellent mechanical properties, and high thermostability. [106] Generally, the organic/inorganic composites can exhibit a higher Seebeck coefficient, compared to those of pure polymers [107].

5.3. Organic Polymer Doped with Fillers The high electrical conductivity of the conductive polymers could be realized by the doping method, however, their Seebeck coefficient is compromised, leading to very low ZT. In order to address this issue, conjugated polymer-based nanocomposites might be a good solution, because the electrical conductivities of these materials was dramatically enhanced, while their thermal conductivity and Seebeck coefficient remain insensitive to the filler concentration. Note that the fillers of TE composites can be either inorganic or organic, while the matrix can be either insulating or intrinsically conductive.

6. Conclusions and Perspectives In this review, we summarize recent progresses in organic thermoelectric materials. In contrast to conventional rigid inorganic thermoelectrics, flexible organic thermoelectrics show obvious advantages for their easy fabrication into versatile formats. However, organic materials still suffer from poor thermoelectric properties. Thus, developing new strategies to enhance the TE performance of conjugated polymers will still be the future focus. The deeply understanding of the structure–property relationship is very important for the exploration of novel organic thermoelectric materials. The theoretical modelling of the relationships between molecular structures, energy levels, density of state, and thermoelectric performance can provide instructive indication for the further development. Since the thermoelectric performance (ZT value) greatly relies on the interdependence of S, σ, and k. The theoretical prediction work is expected to further advances in experimental activities. Also, the further understanding on the relationship between the molecular structure and property needs to be studied through the synthesis of more polymers in future. In addition, post-treatment approaches to optimize the morphology and the doping level of different dopant-doped polymers should also be developed. Apart from the classical conducting polymers such as PANI, PEDOT, and PPy, which have been broadly studied as p-type TE materials, some n-type organic TE materials based on PDI, NDI, NDTI, and NTI units have been constructed, and their TE performance have been summarized. The complete efficient TE generators compose of both p-type and n-type materials. However, the n-type organic TE materials with comparable performance are still rare, due to their poor air stability. Thus, more efficient n-type organic TE materials are greatly in demand. Polymers 2019, 11, 107 14 of 19

To make full use of the intrinsic advantages of organic thermoelectric and extend TE applications under different environments and on various surfaces, flexible and stretchable devices of polymer composites are strongly suggested. To further realize industrial TE applications, an effective way to improve the ZT of organic polymer materials is highly desirable. In fact, organic/inorganic hybrid films, which integrate mechanical flexibility, stretchability, and solution processing from the organic component and higher thermoelectric properties from the inorganic part together, would be a promising direction to achieve higher ZT.

Funding: This research was funded by AcRF Tier 1 (RG 111/17, RG 2/17, RG 114/16, RG 8/16) and Tier 2 (MOE 2017-T2-1-021 and MOE 2018-T2-1-070), Singapore, the National Key R&D Program of China (2017YFA0204903), National Natural Science Foundation of China (NSFC. 51733004, 51525303, 221702085, 21673106, 21602093, 21572086, 1522203), 111 Project and the Fundamental Research Funds for the Central Universities (lzujbky-2017-11, lzujbky-2017-109). Acknowledgments: Q.Z. acknowledges financial support from AcRF Tier 1 (RG 111/17, RG 2/17, RG 114/16, RG 8/16) and Tier 2 (MOE 2017-T2-1-021 and MOE 2018-T2-1-070), Singapore. H.Z. appreciates the financial support from the National Key R&D Program of China (2017YFA0204903), National Natural Science Foundation of China (NSFC. 51733004, 51525303, 221702085, 21673106, 21602093, 21572086, 1522203), 111 Project and the Fundamental Research Funds for the Central Universities (lzujbky-2017-11, lzujbky-2017-109). Conflicts of Interest: The authors declare no conflict of interest.

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