Electronic structure engineering in organic thermoelectric materials

Electronic structures, which play a key role in determining electrical and optical properties of π-conjugated organic materials, have attracted tremendous interest. Efficient thermoelectric (TE) conversion of organic materials has rigorous requirements on electronic structures. Recently, the rational design and precise modulation of electronic structures have exhibited great potential in exploring state-of-the-art organic TE materials. This review focuses on the regulation of electronic structures of organic materials toward efficient TE conversion. First, we present the basic knowledge regarding electronic structures and the requirements for efficient TE conversion of organic materials, followed by a brief introduction of commonly used methods for electronic structure characterization. Next, we highlight the key strategies of electronic structure engineering for high-performance organic TE materials. Finally, an overview of the electronic structure engineering of organic TE materials, along with current challenges and future research directions, are provided.

Advances in organic thermoelectric materials and devices for smart applications

Organic thermoelectric(OTE)materials have been considered to be promising candidates for large area and low‐cost wearable devices owing to their tailorable molecular structure,intrinsic flexibility,and prominent solution processability.More importantly,OTE materials offer direct energy conversion from the human body,solid‐state cooling at low electric consumption,and diversified functions.Herein,we summarize recent developments of OTE materials and devices for smart applications.We first review the fundamentals of OTE materials from the viewpoint of thermoelectric performance,mechanical properties and bionic functions.Second,we describe OTE devices in flexible generators,photothermoelectric detectors,self‐powered sensors,and ultra‐thin cooling elements.Finally,we present the challenges and perspectives on OTE materials as well as devices in wearable electronics and fascinating applications in the Internet of Things.

Recent Advances in Molecular Design of Organic Thermoelectric Materials

Organic thermoelectric(OTE)materials have gained widespread attention because of their potential for wearable power generators and solid cooling elements.Nevertheless,the development of state-ofthe-art OTE materials still suffers from limited molecular categories because of the rarity ofmolecular design strategies,which limits further development of this emerging field.Recently,many efforts have been devoted to developing molecular design concepts for high performance OTE materials.

Thermoelectric transport in conductive poly(3,4-ethylenedioxythiophene)

Poly(3,4-ethylenedioxythiophene)(PEDOT)has proved its quite competitive thermoelectric properties in flexible electronics with its excellent electrical and mechanical properties.Since the early discovery of PEDOT,considerable experimental progress has been achieved in optimizing and improving the thermoelectric properties as a promising organic thermoelectric material(OTE).Among them,theoretical research has made significant contributions to its development.Here the basic physics of conductive PEDOT are reviewed based on the combination of theory and experiment.The purpose is to provide a new insight into the development of PEDOT,so as to effectively design and preparation of advanced thermoelectric PEDOT material in the future.

Recent progress in design of conductive polymers to improve the thermoelectric performance

Organic semiconductors,especially polymer semiconductors,have attracted extensive attention as organic thermoelectric materials due to their capabilities for flexibility,low-cost fabrication,solution processability and low thermal conductivity.However,it is challenging to obtain high-performance organic thermoelectric materials because of the low intrinsic carrier concentration of organic semiconductors.The main method to control the carrier concentration of polymers is the chemical doping process by charge transfer between polymer and dopant.Therefore,the deep understanding of doping mechanisms from the point view of chemical structure has been highly desired to overcome the bottlenecks in polymeric thermoelectrics.In this contribution,we will briefly review the recently emerging progress for discovering the structure–property relationship of organic thermoelectric materials with high performance.Highlights include some achievements about doping strategies to effectively modulate the carrier concentration,the design rules of building blocks and side chains to enhance charge transport and improve the doping efficiency.Finally,we will give our viewpoints on the challenges and opportunities in the field of polymer thermoelectric materials.

Optimization of the thermoelectric properties of poly(nickel-ethylenetetrathiolate) synthesized via potentiostatic deposition

The coordination polymer poly（nickel-ethylenetetrathiolate） （poly（Ni-ett））, formed by nickel（Ⅱ） and 1,1,2,2-ethenetetrathiolate （ett）, is the most promising N-type organic thermoelectric material ever reported; it is synthesized via potentiostatic deposition, and the effect of different applied potentials on the optimal performance of the polymers is investigated. The optimal thermoelectric property ofpoly（Ni-ett） synthesized at 0.6 V is remarkably greater than that of the polymers synthesized at 1 and 1.6 V, exhibiting a maximum power factor of up to 131.6μW/mK2 at 360 K. Furthermore, the structure-property correlation ofpoly（Ni-ett） is also extensively investigated. X-ray diffraction （XRD） and X-ray photoelectron spectroscopy （XPS） analyses revealed that the larger size of crystalline domains and the higher oxidation state of poly（Ni-ett） synthesized at 0.6 V possibly results in the higher bulk mobility and carrier concentration in the polymer chains, respectively, accounting for the enhanced power factor.

The enhanced power factor of multi-walled carbon nanotube/R stabilized polyacrylonitrile composites due to the partially conjugated structure

The studies of organic thermoelectric(TE)materials mainly focus on conductive polymers due to their conjugated molecular structures and high intrinsic electrical conductivity.When the conductive polymer is mixed with certain insulating polymers,the power factor was found enhanced.It is doubtful that the partially conjugated molecular structure is beneficial to the TE performance.Polyacrylonitrile(PAN)is an insulating polymer with a non-conjugated structure in its backbone,however,it has a partially conjugated structure after thermal treatment.In this work,a composite of PAN and multi-walled carbon nanotubes(MWCNT)was made and thermally treated in order to study the partially conjugated structure on the improvement of the power factor.By controlling the PAN content and the temperature of thermal treatments,a maximum power factor of 22 mW/mK2 was obtained from the MWCNT/PAN composite with 45%PAN content after thermally treated at 300℃in air,which is 300%and 80%higher than that without PAN and before thermal treatment,respectively.It is demonstrated that the partially conjugated polymers play an important role in TE performance and they are promising candidates for high-efficient organic TE materials.