DOI 10.1007/s13233-020-8116-y www.springer.com/13233 pISSN 1598-5032 eISSN 2092-7673 Macromolecular Research Review Macromol. Res., 28(6), 531-552 (2020) 531 © The Polymer Society of Korea and Springer 2020 Recent Progress in Organic Thermoelectric Materials and Devices Abstract: In recent years, thermoelectric (TE) devices have attracted a growing attention due to their promising ability to convert waste heat into readily available electric energy. Compared to inorganic counterparts, organic TE devices emerged as the potential candidates for room-temperature and flexible (even wearable) TE power generation. During last few decades, extensive studies have been performed on the p- and n-type materials and devices to build up the inter-relationship among the TE parameters ( i.e ., electrical conductivity, Seebeck coefficient, thermal conductivity and power factors), demonstrating a great potential of organic TEs. In this review, recent progresses in the organic TE materials and devices, dopants and doping method, charge transport models and flexible TE device applications are summarized and the key strategies and future prospects to further optimize TE performance are discussed. Keywords: organic thermoelectrics, waste heat, Seebeck coefficient, conductivity, power factor. 1. Introduction The use of renewable energy sources is becoming imperative owing to global warming and the shortage of fossil fuel reserves. 1 Heat is an abundant source of energy, but is often wasted irre- spective of whether it originates from natural sources like solar and geothermal sources or artificial sources like motor vehi- cles and the industrial sector. 2 If heat from one of these sources is brought into contact with certain conductors or semiconductors, an electric current can be generated as a result of the potential difference, which develops because of temperature gradient. This phenomenon was discovered by Thomas J. Seebeck in 1821 and is termed the Seebeck effect. 3 The conversion of electrical energy to thermal energy is known as the Peltier effect, which was discovered by Jean Peltier (1834), 4 and can be employed for spot cooling or heating applications. Therefore, thermoelectric (TE) devices are attractive candidates for solving the energy problem by means of harvesting waste heat and solar thermal energy and are also useful for health monitoring sensors that employ body heat. The performance of TE materials is generally expressed by a dimensionless thermoelectric figure of merit ZT, 5-7 (1) where α is the Seebeck coefficient, σ is electrical conductivity, κ is thermal conductivity, and T is the temperature. The power factor (PF), expressed as PF = α 2 σ, is often used to compare the energy conversion capabilities among different materials in organic TEs. However, the possibility of improving the PF is lim- ited owing to the interdependence and coupling between the two major parameters, the α and σ. As α and σ are strongly and oppositely dependent on the carrier concentration associated with the electronic structure of the materials, these two param- eters need to be fine-modulated to optimize the resulting PF. So far, inorganic materials like bismuth telluride (Bi 2 Te 3 ), tin selenide (SnSe), lead telluride (PbTe), and silicon-germanium (SiGe) have been investigated for their TE properties. 8-11 These materials have been employed in TE devices with a high figure of merit (ZT>2), 12 and successfully commercialized. Despite their high performance, these materials have several disadvantages such as their high cost, toxicity, high thermal conductivity much larger than 1W m -1 K -1 , 13 and processing difficulties. Conversely, organic materials offer more cost-effective high-throughput fabrication using solution-based printing techniques with the additional advantages such as facile tunability of molecular struc- tures to tailor the properties, low temperature energy harvest- ing and temperature sensing, etc. Organic materials are flexible and robust with low weight and low thermal conductivity (0.1- 0.8W m -1 K -1 ). 14 Therefore, organic TE materials have been identi- fied as an alternative to find complementary applications of inor- ganic TE materials and considerable efforts have been devoted Soonyong Lee †,1 Soohyun Kim †,2 Ambika Pathak 1 Ayushi Tripathi 1 Tian Qiao 2 Yeran Lee 1 Hyunjung Lee* ,2 Han Young Woo* ,1 Department of Chemistry, College of Science, Korea University, 145 Anam-ro, Seongbuk-gu, Seoul 02841, Korea School of Advanced Materials Engineering, Kookmin Univ., Jeongneung-ro 77, Seongbuk-gu, Seoul 02707, Korea Received March 20, 2020 / Revised April 23, 2020 / Accepted April 28, 2020 ZT α 2 σT k ------------ = Acknowledgments: This work was supported by the National Research Foundation (NRF) of Korea (Grants NRF-2019R1A2C2085290, 2019- R1A6A1A11044070, 2017R1A2B2010552, and 2015R1A5A7037615). *Corresponding Authors: Hyunjung Lee (hyunjung@kookmin.ac.kr), Han Young Woo (hywoo@korea.ac.kr) S. Lee and S. Kim contributed equally to this work.