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 theoretical calculations of organic thermoelectric materials

Organic thermoelectric(OTE)materials and devices have garnered significant attention in the past decade for flexible and wearable electronics.Due to the numerous combinations of different backbones,side chains,and functional groups for polymer molecules,further efficient developments of high perfor-mance OTEs rely on reverse and rational molecular design as well as fundamental understanding to the structure-property relationship,which both require precise theoretical input.Recently,many theo-retical efforts and progresses have been made to predict TE properties and develop high performance OTE materials.Here,we present first the general methods and principles for OTE theoretical calculations.Subsequently,the latest theoretical advances regarding the effects of molecular design,chemical dop-ing,ambipolar charge transport etc.,to TE conversion are carefully reviewed.These theoretical advances not only significantly deepen the fundamental understanding of OTEs,but also provide precise guidance to the molecular design of OTE materials.Finally,we propose several perspectives for future theoretical investigations of OTEs.

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.

Fabricate organic thermoelectric modules use modified PCBM and PEDOT：PSS materials

In this paper, we fabricated an organic thermo- electric （TE） device with modified [6,6]-phenyl-C61- butyric acid methyl ester （PCBM） and poly（3,4-ethylene- dioxythiophene） polystyrene sulfonate （PEDOT：PSS）; the device showed good stability in air condition. For n-leg, PCBM were doped with acridine orange base （3,6-bis （dimethylamino）acridine） （AOB） and 1,3-dimethyl-2,3- dihydro- 1H-benzoimidazole （N-DMBI）. Co-doped PCBM utilizes synergistic effects of AOB and N-DMBI, resulting in excellent electrical conductivity and Seebeck coefficient values reaching 2 S/cm and -500 μV/K, respectively, at room temperature with dopant molar ratio of 0.11. P-type leg used modified PEDOT：PSS. Based on modified PCBM and PEDOT：PSS materials, we fabricated a TE module device with 48 p-type and n-type thermocouple and tested their output voltage, short current, and power. Output voltage measured -0.82 V, and generated power reached almost 945 μW with 75 K temperature gradient at 453 K hot-side temperature. These promising results showed potential of modified PEDOT and PCBM as TE materials for application in device optimization.

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.

Molecular Insights into n-Type Organic Thermoelectrics

Organic thermoelectrics(OTEs)have been recently intensively investigated as they hold promise for flexible,large-area,and low-cost energy generation or heating–cooling devices for appealing applications,for example,wearable energy harvesting.In the past 7 years,n-type OTEs have witnessed a sharp increase in their performance thanks to significant progress in developing and understanding the fundamental physical properties of n-type OTE materials as well as the working principle and physical processes of the TE devices.

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.

Structure design for high performance n-type polymer thermoelectric materials

Organic thermoelectric(OTE)materials have been regarded as a potential candidate to harvest waste heat from complex,low temperature surfaces of objects and convert it into electricity.Recently,n-type conjugated polymers as organic thermoelectric materials have aroused intensive research in order to improve their performance to match up with their ptype counterpart.In this review,we discuss aspects that affect the performance of n-type OTEs,and further focus on the effect of planarity of backbone on the doping efficiency and eventually the TE performance.We then summarize strategies such as implementing rigid n-type polymer backbone or modifying conventional polymer building blocks for more planar conformation.In the outlook part,we conclude forementioned devotions and point out new possibility that may promote the future development of this field.

Thermoelectric Properties of an Indandione-Terminated Quinoidal Compound:Effect of the n-Type Dopants

The investigation of n-type doping holds a significant interest for the application of thermoelectrics.Herein,the doping of an indandione-terminated compound Q-4F with a singlet open-shell ground state was studied using two n-dopants N-DMBI and LCV.Both of these two dopants can effectively dope Q-4F due to the large offset between the singly occupied molecular orbital(SOMO)of dopants and the lowest unoccupied molecular orbital(LUMO)of Q-4F.N-DMBI has a higher doping ability than LCV as demonstrated by the UV-vis-NIR and EPR measurements.However,in comparison to N-DMBI doped Q-4F,LCV doped system exhibits much higher electrical conductivity and power factor due to its unperturbed molecular packing and favorable morphology after doping.The optimal conductivity of LCV doped Q-4F is 7.16×10^(-2)±0.16 S·cm^(-1) and the highest power factor reaches 12.3±0.85μW·m–1·K^(-2).These results demonstrate that the modulation of n-dopants is a powerful strategy to balance the doping efficiency and microstructure toward a maximum thermoelectric performance.

Quinoidal conjugated materials:Design strategies and thermoelectric applications

The growing demand for waste heat energy recovery from electronic devices,solar energy,and industrial production has led to increased attention on thermoelectric materials.In the past decades,significant progress has been achieved in inorganic thermoelectric materials.Moreover,flexible,lightweight,and bio-friendly organic thermoelectric(OTE)materials have emerged as promising candidates for thermoelectric devices.In particular,quinoidal conjugated small molecules and polymers with high mobility are suitable for thermoelectric conversion.Such kind of materials have gained increasing research interest due to their unique structural features and characteristics of polarons’delocalization.Concurrently,quinoidal materials with high mobility and conductivity have been developed,and their use for thermoelectric conversion has been increasingly reported.This perspective summarizes the recent advancements in the design and synthesis of quinoidal conjugated small molecules and polymers,their advantages for thermoelectric conversion,and the latest reports on their charge carrier transport mechanisms.Moreover,to further enhance the TE performances of quinoidal materials,the existing challenges are discussed and the future developments are also outlooked.

Enhanced n-Type Thermoelectric Performance of Conjugated Polymers Based on an Indandione-Terminated Quinoidal Unit through Comonomer Optimization

Comprehensive Summary Conjugated polymers(CPs)containing quinoidal units are promising in n-type organic thermoelectric materials because of their deep-positioned lowest unoccupied molecular orbital(LUMO)energy levels and planar conjugated backbones.Herein,three CPs have been synthesized by copolymerizing an indandione-terminated quinoidal unit with bithiophene derivatives.Owning to the high electron affinity of the indandione-terminated quinoidal unit,all polymers showed deep LUMO energy levels below-4.10 eV.Incorporating electron-withdrawing substituents(F or CN)on the bithiophene comonomer can further downshift the LUMO energy levels.As a result,a more efficient n-doping process can be realized when employing 4-(2,3-dihydro-1,3-dimethyl-1H-benzimidazol-2-yl)-N,N-dimethylbenzenamine(N-DMBI)as the dopant.Ultimately,the polymer with CN substituents delivered the best thermoelectric performance with a power factor of up to 2.14μW·m^(−1)·K^(−2),because it possessed the lowest LUMO energy level among the three CPs.This work highlights that the modulation of LUMO energy level is an effective strategy to optimize the thermoelectric performance of CPs.

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.