Organic and Hybrid Thermoelectrics

Thermoelectric (TE) materials can directly convert heat into electricity based on the Seebeck effect, offering high durability and reliability with no fluids or moving parts. TE energy conversion is therefore considered to be one of the most promising transformative technologies for the reduction of global energy consumption. The past two decades have witnessed the rapid growth of thermoelectric research, including optimization of existing TE materials and exploitation of new materials. In the past five years, according to Web of Science statistics from June 2019, approximately 1500−2000 articles in TEs were published annually, in which the number of publications in organic and hybrid TEs rose significantly from 200 in 2015 to 366 in 2018. Despite this increasing research thrust, to the best of our knowledge there has so far been no specific organic and hybrid TE special issue published in a primary research journal. Motivated to fill this gap, we invited international leading experts in prime areas to present a range of research articles, from Reviews and Progress Reports to Full Papers and Communications, that summarize recent advances and offer insights into resolving current challenges. This themed issue covers organic and hybrid TE materials and applications, comprising chemical solution methods, nanoscale fabrication techniques, morphological investigation, computational studies of band structures, electrical and phonon transportation, and TE module applications. Conventional inorganic TE materials, among which more than ten families exhibit figure-of-merit zT values exceeding 1, have been extensively studied. Nevertheless, it has been difficult to realize large-scale commercial applications with inorganic TEs due to their intrinsic brittleness and toxicity. Furthermore, exciting breakthroughs in organic TEs have recently been made. They exhibit unique advantages over their inorganic counterparts, such as low weight, mechanical flexibility, rational design of molecular structures, versatile chemical doping with controllability, solution processing over large area, and intrinsically low thermal conductivity. This special issue begins with organic TE materials, among which p-type poly(3,4-ethylenedioxythiophene):poly(styrenensulfonate) (PEDOT:PSS) has been widely investigated due to its water processability, thermal stability, and outstanding electrical conductivity. Recent breakthroughs and strategies for achieving high-performance PEDOT:PSS-based TE materials, and the corresponding underlying mechanisms, are reviewed by both Zeng Fan and Jianyong Ouyang (article number 1800769) and Xavier Crispin and co-workers (1800918). Arguably, a fine understanding of the charge-transport process in organic semiconductors (OSCs) is urgently needed. The research paper written by Michael Chabinyc and co-workers (1800915) is committed to resolve this issue and clarify the interplay between the TE performance of doped OSCs and the thin film morphology that directs charge transport. Nonetheless, the strongly interdependent relationship of thermoelectric parameters remains a bottleneck to enhance TE performance, especially in organic-polymer-based TE materials. In the paper presented by Howard Katz and co-workers (1800618), a compositional range over which there is a decoupling of electrical conductivity and Seebeck coefficient is investigated. They find that blending and doping polymers with closely spaced energy levels and morphologies designed to promote carrier mobility favor increased power factors through experiment, calculation, and simulation. Martijn Kemerink and co-workers further present doped OSC blends for TE applications in a Progress Report (1800821), in which several experimental strategies to improve TE performance are demonstrated and the lack of a comprehensive model for OTEs is highlighted. In addition, past research attempts to eludicate the fundamental TE properties of organic and hybrid molecular junctions are reviewed by Bin Hu and co-workers (1800877), encompassing molecular to thin-film and bulk designs and stressing the enhanced performance obtained through interfacial effects. Both p- and n-type organic TE materials with outstanding TE properties are necessary to realize flexible thermoelectric generators (TEGs) with high conversion efficiency. However, the comparatively worse TE performance of n-type OTE materials is caused by the limited number of air-stable n-dopants and low doping efficiencies. Nickel-coordination polymers are a promising class of n-type conducting polymers that do not require extrinsic doping, thus overcoming the above-mentioned problems. Shannon Yee and co-workers give an overview of synthesis of two coordination polymers—Ni-ethenetetrathiolate (NiETT) and Nitetrathiooxalate (NiTTO)—in a Progress Report (1800884), and a Full Paper (1900066) further explores the differences between these two polymers and reveals the potential of NiTTO. The communication presented by L. Jan Anton Koster and co-workers (1800959) studies the impact of electrostatic interaction on the n-doping efficiency of fullerene derivatives. The benchmark organic acceptor, [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), displays a very limited doping efficiency while a [60]fulleropyrrolidine with a more polarizable triethylene glycol type side chain (PTEG-1) can reach ≈100% efficiency. Finally, the Review contributed by Wei Xu, Daoben Zhu, and co-workers (1800825) summarizes research progress on n-type organic TE materials and TEGs and provides useful guidelines for further exploration. Carbon nanotubes (CNTs), as novel TE materials, have been widely studied in recent years. In a Full Paper by Jaime Grunlan and co-workers (1800465), the deposition of double-walled carbon nanotubes (DWCNTs) stabilized with polyethylenimine (PEI) and graphene oxide (GO) in a layer-by-layer structure from aqueous solutions to fabricate n-type TENCs that display excellent air stability and enable efficient TE devices on flexible substrates (e.g., clothing) is reported. In another Full Paper by Jeffrey Blackburn, Andrew Ferguson, and co-workers (1800910), the dependences of electrical conductivity, Seebeck coefficient, and power factor were studied on carrier density and temperature in semiconducting single-walled carbon nanotube (s-SWCNT) networks. Regardless of all this, due to their high doping threshold, the low electrical conductivity of organic-semiconductor-based TEs largely constrains any further improvement of their power factor or zT. Due to this, organic–inorganic thermoelectric nanocomposites (TENCs) are attracting enormous research interest because they combine the high electrical conductivity of the inorganic component with the low thermal conductivity and excellent flexibility afforded by the organic matrix. The Review article by Kefeng Cai and co-workers (1800822) is mainly focused on p-type hybrid TE materials composed of PEDOT:PSS and inorganic nanocrystals (NCs). Meanwhile, Ziqi Liang, Lidong Chen, Guillermo Bazan, and co-workers (1800943) review recent advances in n-type TENCs that primarily comprise CNT and inorganic NC-based hybrids and also give a perspective for possible future improvements. In a Communication by Chunlei Wan and co-workers (1800842), a new strategy is put forward to fabricate an inorganic–organic TE superlattice of Bi2Se3hexylammonium0.11dimethylsulfoxide0.06 (Bi2Se3HA0.11DMSO0.06) through a series of chemical reactions, including lithium intercalation, ionic exchange, and organic exchange. This research could stimulate exploration of novel high-performance flexible TE materials through the hybridization inorganic and organic components at the atomic scale. In addition, the Progress Report by Jeffrey Urban and co-workers (1800823) emphasizes soft TE materials consisting of conjugated polymers and organic–inorganic hybrids. The unique processing and mechanical advantages of soft materials open the door to a suite of new TE applications, including power generation for biomedical implants and the internet of things, and wearable heating and cooling devices. More recently, some emerging hybrid semiconductor materials such as metallic halide perovskites have not only exhibited superior photovoltaic performance but also demonstrated potential for applications in TEs. In a Full Paper, Ling Xu, Bin Hu, and co-workers (1800759) explore MAPbBr3 single crystals and find enhanced Seebeck effects produced by an organic and a metal modified layer. Computational modeling aids in elucidating microscopic physical processes and establishing fundamental structure−property relations, which will in turn guides the design of favorable molecular and electronic structures towards high-performance TEs, as discussed in the Review article by Zhigang Shuai and co-workers (1800882). Critically, there is not an authoritative calibration agency in TE measurements, leading to difficulties in comparing TE performance between different research groups. In particular, the methods used to measure thermal conductivity of organic and hybrid TE materials, which play a crucial role in evaluating TE performance, are controversial. The Review article contributed by Hanfu Wang and co-workers (1800783) therefore summarizes commonly used and newly developed techniques for thermal conductivity measurement, including steady-state methods, time-domain methods, and frequency-domain methods. The operating principles, merits and limitations, technical issues, and application examples for each technique are also discussed. We hope that this Special Issue will help readers to gain a better understanding of organic and hybrid TE materials. We also hope it will inspire scientists to generate new research ideas to broaden the comprehension of this field. Finally, we would like to thank all the authors for their excellent contributions as well as the handling editor, Dr. Jovia Jiang, for her great efforts to make this issue happen. Ziqi Liang, Lidong Chen and Guillermo Bazan Ziqi Liang obtained his Ph.D. in polymer science at the Department of Materials Science and Engineering at Pennsylvania State University in March 2006. He then pursued postdoctoral work at the University of Cambridge from May 2006 to 2008. In June 2008, he joined the National Research Energy Laboratory as a postdoctoral researcher, later to become Scientist III. In September 2012, he joined the Department of Materials Science at Fudan University as a professor. Currently, Prof. Liang's group conducts research on organic and perovskite solar cells, and organic and hybrid thermoelectrics. Lidong Chen received his Ph.D. degree in materials science from Tohoku University in 1990 and joined Shanghai Institute of Ceramics, Chinese Academy of Sciences as a professor in 2001. His research activity mainly focuses on the design, synthesis, and characterization of thermoelectric materials, and the integration of thermoelectric devices. Guillermo C. Bazan received his B.S. degree in chemistry from University of Ottawa (1986), and Ph.D. in inorganic chemistry from Massachusetts Institute of Technology (1991). After one year of postdoctoral research at the California Institute of Technology, he joined the Department of Chemistry at the University of Rochester. He moved to the University of California, Santa Barbara in 1998 and is currently a professor in Departments of Chemistry and Biochemistry, and Materials, and the Director of the Center for Polymers and Organic Solids. His research interests include the synthesis of conjugated oligo/poly electrolytes for their antimicrobal and bioelectronic applications.

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