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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]. Polymers 2019, 11, 107; doi:10.3390/polym11010107 www.mdpi.com/journal/polymers PolymersPolymers2019 2019, 11, 11, 107, x FOR PEER REVIEW 2 ofof 1918 Polymers 2019, 11, x FOR PEER REVIEW 2 of 18 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 TETE 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, valueSome (aexamples 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 includingdisadvantages 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 suchhowever, to the as above-mentionedconducting may offer polymers, solutions problems. ar e topromising Organicthe above semiconductors, candidates-mentioned for problems. suchconverting as conducting Organiclow-end polymers,semiconductors,thermal energy are promising intosuch 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 includingthe and performance the the fabrication explosion of organic of of nanocomposites new TE molecular materials, designconsisting including strategy of the conducting andexplosion 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 frommaterials [ 10[10].]. used in TE research, and their individual contributions. Reproduced with permission from [10]. Polymers 2019, 11, 107 3 of 19 Polymers 2019, 11, x FOR PEER REVIEW 3 of 18 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
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