2104922 (1 of 26) © 2021 Wiley-VCH GmbH www.small-journal.com REVIEW Soft Organic Thermoelectric Materials: Principles, Current State of the Art and Applications Yinhang Zhang, Wei Wang, Fei Zhang, Kun Dai, Chuanbing Li, Yuan Fan, Guangming Chen,* and Qingbin Zheng* Y. Zhang, W. Wang, F. Zhang, K. Dai, C. Li, Q. Zheng School of Science and Engineering The Chinese University of Hong Kong Shenzhen 518172, P. R. China E-mail: zhengqingbin@cuhk.edu.cn Y. Zhang, F. Zhang Department of Materials Science and Engineering University of Science and Technology of China Hefei, Anhui 230026, P. R. China Y. Fan, G. Chen College of Materials Science and Engineering Shenzhen University Shenzhen 518055, P. R. China E-mail: chengm@szu.edu.cn The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/smll.202104922. DOI: 10.1002/smll.202104922 heat engines and transforming the as- generated mechanical energy into elec- tricity. [2–5] Compared with traditional unsustainable method, thermoelectric materials, which can transform the tem- perature gradient from waste heat or solar thermal energy into electricity through the Seebeck effect, have triggered great interest in the development of wearable cooling/heating devices and low-temper- ature energy generators. [6–17] Inorganic thermoelectric materials exhibit superior thermoelectric figure of merit than the organic ones, but their low abundance, high-cost, heaviness, and brittleness usu- ally prevents their applications for flexible and/or nontoxic devices. [18–24] In contrast, organic thermoelectric materials present intrinsically low thermal conductivity and high mechanical flexibility, making them versatile in green energy har- vesting and thermoelectric refrigeration (Figure 1). [25–42] The energy conversion efficiency of the thermoelectric materials can be estimated by the dimen- sionless figure of merit ZT = S 2 σT/κ, where S is the thermo- power or Seebeck coefficient, σ is electrical conductivity of materials, T is the absolute temperature, and κ represents the thermal conductivity. [43–51] Superficially, increasing S and σ, and decreasing κ can infinitely raise the value of ZT. However, the strong interdependence among these three parameters is extremely challenging. [52–56] Over the past years, scientists have managed to increase the ZT values of organic thermoelectric materials via molecular design, phonon, and electron transport decoupling, and production of hybrid composites incorpo- rating high thermoelectric particles. [57–69] In this review, the thermoelectric performances of the organic single-component materials, hybrid composites, and novel ionogels developed over the last several years are dis- cussed separately in terms of the respective parameter tuning and pathway optimization. The current state of the art of organic thermoelectric materials is primarily highlighted based on their structure–property relationship. In particular, the marvelous progress that has allowed organic thermoelectric materials to compete with traditional inorganic compounds is described. Finally, organic thermoelectric generators containing different legs with variousoutput powers and their multiple applications are briefly summarized. The enormous demand for waste heat utilization and burgeoning eco- friendly wearable materials has triggered huge interest in the development of thermoelectric materials that can harvest low-cost energy resources by converting waste heat to electricity efficiently. In particular, due to their high flexibility, nontoxicity, cost-effectivity, and promising applicability in various fields, organic thermoelectric materials are drawing more attention compared with their toxic, expensive, heavy, and brittle inorganic counterparts. Organic thermoelectric materials are approaching the figure of merit of the inorganic ones via the construction and optimization of unique transport pathways and device geometries. This review presents the recent development of the interdependence and decoupling principles of the thermoelectric efficiency parameters as well as the new achievements of high performance organic thermoelectric materials. Moreover, this review also discusses the advances in the thermoelectric devices with emphasis on their energy- related applications. It is believed that organic thermoelectric materials are emerging as green energy alternatives rivaling their conventional inorganic counterparts in the efficient and pure electricity harvesting from waste heat and solar thermal energy. 1. Introduction The current energy crisis, which is related to the geopolitical conflicts and environmental degradation, is a thorny and inevitable challenge that humanity must face. [1] At present, energy is still mainly obtained by combusting fossil fuels in Small 2022, 18, 2104922