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. 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(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