Advances in Organic Thermoelectric Devices for Multiple Applications

Organic thermoelectric (OTE) devices composed of π‐conjugated molecules are the basic building blocks for self‐powered integrated organic electronics. In addition to molecular design and doping strategies, the highly tunable energy conversion process in OTE devices has drawn significant research interest. Specifically, the diverse physical properties of organic semiconductors, novel device geometry design, and advanced fabrication techniques combined enable the OTE device to be a powerful multiscale platform from single‐molecular scale to thin films for modulating the TE performance, studying the fundamental charge transport mechanism, exploring novel energy conversion phenomenon, and realizing various functionalities. Here, the authors comprehensively review the recent experimental and theoretical advances in related topics of OTE devices, including multifunctional, external physical fields, and temperature modulated, as well as quantum OTE devices. The remaining issues and perspectives toward future OTE device research are also discussed at the end.


Introduction
Organic thermoelectric (OTE) materials and devices are important candidates for solving the general issues of energy and environment, which could directly convert wasted heat into electricity in a green manner.Their unique advantages of excellent intrinsic flexibility, solution processability, abundant source elements, and prominent room temperature TE performance are particularly promising for low-cost large area distributed power supply systems.As a result, OTEs have drawn significant research interests over the past decade, which has promoted a rapid development of the entire field from the aspects of molecule to device.To date, both state-of-the-art p-and n-type OTE materials exhibit a TE DOI: 10.1002/apxr.202300027figure of merit ZT over 0.3 at or near room temperature, [1,2] which significantly enhanced the energy conversion efficiency.Further by combining with advanced processing techniques, such as inkjet printing and roll-to-roll printing, various prototype functional OTE devices have been demonstrated, including power generators, [3] sensors, [4] and even Peltier coolers. [5]n spite of these achievements, OTEs are still far away from satisfactory for future practical applications such as selfpowered internet of things (IoTs) in terms of both performance and functionalities.Previously, the OTE study has been mainly focused on molecular design to enhance the material ZT values.For further developments, as illustrated in Figure 1, to bridge the gap between basic OTE materials to applications, the OTE device studies are also indispensable.Specifically, owing to the rich physical properties of -conjugated polymers and diverse device geometry of organic electronics, one promising direction is to manipulate the TE conversion process in various OTE devices from single molecular scale to macroscopic thin films, via temperature and external physical fields including electrical, light, and magnetic field, to not only tune the TE performance, but also to study the fundamental charge transport mechanism and explore exotic TE effects.Another important direction would be to endow the OTE devices with new and multiple functionalities, such as stretchability, self-healing, and comfortability, toward wearable and smart electronics, by developing novel functional materials, device geometry design, and fabrication strategies.
For the fields of OTEs, previous reviews have nicely covered the topics of organic semiconductors, [6] p-and n-type OTE materials, [7][8][9] molecular design and theoretical modeling, [10,11] doping strategies, [12] OTE applications, [13,14] etc.However, considering the rapid development of OTE devices, it is important to comprehensively review recent advances regarding this special topic regarding the two above mentioned directions: 1) multiscale field-modulated TE conversion in OTE devices, and 2) multifunctional OTE devices.Herein, after a brief introduction of OTE device principle, categories, and parameters, we provide a systematic summarization and discussion covering recent representative experimental as well as theoretical progresses in related fields.Copyright 2022, Wiley-VCH.Panel 4, row one reproduced with permission. [4]Copyright 2015, Springer Nature.Bottom image, rightmost panel, top row reproduced with permission. [52]Copyright 2020, Springer Nature.

Fundamentals of OTE Device
"Current TE devices are mainly based on three basic longitudinal effects including the Seebeck effect, the Peltier effect, and the Thomson effect as well as the magnetic field or magnetism induced the transverse Nernst-Ettingshausen and related effects.Fundamentally, these different physical phenomena are originated from the diffusion transport of charge carriers in devices, which is resulting from the applied temperature or potential gradient induced energy level variation acrossing the devices.Specifically, for the Seebeck effect under a longitudinal temperature gradient ∇T, charge carriers will diffuse from the device high temperature end to the low temperature end, leading to an electromotive force E emf and a measurable potential difference ∆V between the two ends.On the other hand, for the Peltier effect which would exploit electricity to pump heat, due to the transport entropy carried by flowing charges, a temperature difference will be generated over the devices with an electrical current passing through.The Thompson effect can be analogously considered as a continuous version of Peltier effect for homogenous materials.While for the Nernst effect under both a longitudinal temperature gradient and a perpendicular magnetic field, the diffusive charge carriers along the temperature gradient direction will deflect due to the Lorentz force, leading to a transverse potential difference." By exploiting various TE effects, OTE devices can be mainly separated into two categories: the Seebeck effect-based OTE generators (OTEG) for energy harvesting and the Peltier effect-based OTE coolers (OTEC).Figure 2a,b presents one protype Π-shape OTEG and OTEC unit, respectively, both with a pair of p/n bulk legs of TE materials.In addition to alternating p/n legs, the OTE devices can also be realized by using only p-or n-type materials (Figure 2c).Typically, the electrodes of unipolar device are designed to connect the hot and cold ends of adjacent TE units, which is difficult to maintain a large temperature gradient.However, by utilizing novel TE effects as represented by the transverse Nernst effect, unipolar TE devices have been demonstrated to be particularly desired for flexible TE applications with simpler structure as compared to traditional Π geometry.
Practical TE devices often connect multiple OTEG or OTEC units in series either vertically or horizontally to increase the output power.For common Π-shape TE modules, the vertical structure has been widely applied in inorganic TE devices (Figure 2d), in which the heat flow direction is perpendicular to the substrate.However, the film thickness of OTE materials is usually thinner than 1 mm, which is difficult for establishing large vertical temperature gradients.Therefore, many OTE devices adopt the horizontal structure instead (Figure 2e).In this way, a highly integrated and flexible module with a larger temperature difference can be achieved.
The performance of OTE devices is determined by the energy conversion efficiency ƞ, which theoretically is dominated by the figure of merit ZT values of the TE materials: Realistically, the efficiency will also be affected by many other factors such as Joule heating, heat loss, the electrical and thermal contact resistance, and etc.As LeBlanc et al. reported that there will be a 30%-60% efficiency decrease from the calculated maximum value for various practical daily and industrial TE applications. [15,16]For OTEG, the output power is another key parameter to evaluate the performance.When the OTEG is connected to an external load R L , the output power can be written as: where I is the current, V L is the output voltage, S pn and R OTEG represent the OTEG Seebeck coefficient and internal resistance, re-spectively (Figure 2f).Considering a constant S pn , the P output will increase with a larger ∆T or a smaller R OTEG , which indicates both the material properties and geometry of OTEG should be well designed and balanced for optimized performance, as shorter TE legs could reduce R OTEG while simultaneously enhance the heat loss between the hot and cold ends.Theoretically, when R L = R OTEG , the OTEG can achieve the maximum P output (Figure 2g).Similar to ƞ, the experimental values of P output are also generally smaller than predictions (Figure 2h).

Representative Advances
In this section we will thoroughly summarize and discuss the recent experimental and theoretical advances under the broad  [15,[48][49][50][51][52][53][54][55] concept of OTE devices, including flexible and stretchable OTEGs, fabric and self-healable OTE devices, organic Peltier coolers, ionic OTE devices, multifunctional OTE sensors, external physical fields (electrical, light, magnetic) and temperature modulated OTE devices, and single molecular TE devices, from the perspective of performance and functions for practical applications as well as the fundamental TE conversion mechanism in -conjugated polymers.

Functional OTE Devices
The diverse molecular design combined with innovative physical properties of OTE materials and devices are particularly promising for functional applications.In contrast to most inorganic TE materials which are mechanically rigid and exhibit the optimum ZT values at high temperature (600-900 K), the unique advantages of inherent flexibility and maximum TE performance around room temperature of OTE materials are extremely desired for self-powered wearable and smart devices such as electronic skins and IoTs.Moreover, wearable devices typically have a relatively low power consumption in the range of 0.0001-10 mW, and to date due to the performance boosting of OTE materials in the past decade, several groups have developed flexible OTEG with large power output over 1 mW, which could satisfy the basic requirements in driving these low consumption electronics, as summarized in Figure 3. Therefore, other than keep improving the OTE device power output by either enhancing the materials ZT values or optimizing the device geometry and integration via advanced logic circuit design, future and ongoing studies should focus on endowing the OTE devices with different functionalities, as represented by the flexibility and sensing abilities, via compositing functional materials, incorporating soft substrates, novel device geometries and processing methods, etc.In this part, we will review and discuss the recent advances in functional OTE devices to illustrate their diversity for future self-powered electronics.

Stretchable and Self-Healing OTEG
Recently, the integration of the bionic characteristics with OTE devices has become one of the most promising directions toward wearable and implantable biomedical electronics for human body with long-term operation stability, to satisfy the increasing requirements of healthcare and lifestyle.These multifunctional OTE devices are therefore demanded to be mechanically stretchable (>50%), self-healable, recyclable, and simultaneously realize efficient TE conversion to power the electrical circuit.For device flexibility, deposition OTEG onto mechanically soft substrates, including insulating polymers, [17][18][19][20][21] paper, [22,23] rubber, [24,25] liquid metal, [26,27] and fibers, [28][29][30] is the most widely applied strategy.[33][34] As compared to flexibility, device stretchability typically indicates the capability of bearing larger strains without failure with superior interfacial contact, which should be able to be repeatably stretched, bent, twisted, and compressed.As summarized in Figure 4, to realize excellent device stretchability, other than deposition onto elastomeric soft substrates, novel chemical and assembly design of dynamically interconnected OTE materials is necessary.This can be achieved by either developing polymers with intrinsic stretchability via introducing multivalent assembly of flexible conjugate backbones, alkyl side chain engineering, and modulating the hydrogen bonding interactions, [35][36][37] or composite materials by blending uniformly high performance OTE polymers with various stretchable components and dopants which are compatible with the OTE materials without forming secondary phases.
By blending polyethylene glycol (PEG) into PEDOT: PSS solution, [38] Jeong and Choi et al. reported an OTE device with optimized stretchability up to 50%.Müller et al. also developed a stretchable OTE material which yields a large tensile strain of 100%, composed by a semiconductor/insulator blend. [39]In Figure 4. General strategies including intrinsic stretchability, composite materials, and utilizing of elastic substrates to achieve stretchable OTE materials and devices.All elements except "Composites" panel reproduced with permission. [3]Copyright 2022, Wiley-VCH."Composites" panel reproduced with permission. [41]Copyright 2019, Wiley-VCH.2020, Crispin and Tybrandt et al. demonstrated an elastomer composite by mixing polyurethane (PU) and different ionic liquid into PEDOT:PSS solution (Figure 5a,b). [40]The electrostatic repulsion between PEDOT:PSS ions prevents the aggregation within the composite films and ensures the stability of the dispersion.The ionic liquids could tune the composite electrical properties while the mechanical properties can be precisely manipulated by the percentage of PU copolymer.With the simultaneously existed hard immobile, soft mobile components, and the phase separation in between, the optimized free-standing films exhibited high electrical conductivity over 140 S cm −1 , small Young's modulus less than 7 MPa, and most importantly outstanding stretchability over 600% (Figure 5c).With a nearly constant Seebeck coefficient under external strains (Figure 5d), the resulting OTE modules can generate the maximum power of 25 nW when it was stretched vertically and parallel to a temperature gradient of 30 K (Figure 5e-g).
Self-healing is another essential function for wearable devices to operate stably over a long period of time under ambient environments with large external strains.However, common flexible or stretchable OTEGs are often subject to unrecoverable mechanical damages such as cracks.[43] For example, Baran et al. reported a 3D-printed self-healable OTEG with prominent deformability and repeatable cutting-healing ability, which is resulting from the complicated material interconnections. [43]hang, Xiao, and Yang et al. demonstrated a self-healable, recyclable, and stretchable Lego-like multifunctional high performance OTEG module (Figure 6a,b), [44] by employing dynamic covalent polyimine network as well as flowable liquid-metal wiring in a novel geometry concept of "soft motherboard-rigid plugin modules."After the device surface is damaged (Figure 6c,d), the covalent bond exchange interaction within the polyimine will generate new bonds, and thus healing the device.In particular, a high open-circuit voltage reaches up to 1 V cm −2 with a temperature gradient of 95 K is achieved (Figure 6e,f).It should be notice that, with similar design concept, the performance of stretchable and self-healable OTEG can be continuously optimized by various material choices and doping methods, increasing the integration density, novel structural design, and advanced fabrication techniques, which paves the route for future adaptable and wearable energy-harvesting electronics with long term operation stability.

Fabric OTEG
One promising approach for OTEG-based wearable devices is to combine with textile fabric, which could not only efficiently power the electronics, but also endow the devices with the extra functionalities and advantages of fabrics including ) Molecular structure of the elastomer composition and schematic of its fabrication process, respectively.c) Demonstration of the good stretchability.d) Thermopower under external strains.e) Photograph of OTE module under 30% strain and its power output with different temperature gradients.f,g) Vertically and horizontally stretched OTE module power output versus strains with a constant temperature gradient of 30 K, respectively.Reproduced with permission. [40]Copyright 2020, Springer Nature.Reproduced with permission. [44]Copyright 2021, AAAS.comfortability, nontoxic, permeability, and washability.The fabric OTEG is typically categorized by different dimensions based on the integration and woven schemes, [45,46] in which the 1D fabric OTEG is simply composed by series of segmented p/n OTE yarns or fibers connected by electrodes.Via different stitching methods, 1D wires can form various 2D fabrics, which build up the final 3D textile fabric OTEG.
For 1D fabric, [47] Nakamura et al. reported a PEG and carbon nanotube (CNT) composite OTE fiber prepared by wet-spinning, exhibiting an optimized power factor of 90 μW m −1 K −2 .Lin et al. developed a breathable OTE fabric by uniformly coating the PE-DOT:PSS onto the fiber surface via soaking. [30]Based on the temperature gradient between the ambient environment and skin, the resulting fiber OTEG produced a maximum power output of 388.7 μW.For functional OTE fabrics, Müller's group demonstrated robust washability of the conducting yarns prepared by soaking the silk with the PEDOT:PSS complex. [48]The PEDOT: PSS layer remained on the yarns even after 4 days of immersing in water.Moreover, the fabrics can stand the hand-wash program of a typical washing machine, and exhibited no obvious change in electrical conductivity (14 S cm −1 ) within four washing cycles.Such water-proof flexible fabric OTE devices will presumably be extremely desired for future self-powered and smart clothing applications.
As discussed above, one crucial limitation to the power output of flexible thin-film OTEGs is to build large temperature gradient, while 3D fabric structures with numerous internal air holes could in principle reduce the heat conduction and enhance the vertical temperature difference.For example, Hu et al. demonstrated a 3D fabric OTEG by coating the locknit spacing fabric substrate with polyurethane/CNT composite, [49] which leads to a more effective vertical TE conversion.Park and Kim et al. reported a fully CNT yarn-based fabric OTEG without any metal contacts. [50]The yarns were prepared by soaking into the CNT solution, and subsequent chemical doping with FeCl 3 or polyethyleneimine as p-and n-type dopants, respectively, which resulting in a high electrical conductivity reach up to 3147 S cm −1 .The integrated OTEG of 60 p-n pairs produced a maximum power density of 10.85 μW g −1 with a temperature difference of 5 K, which was able to drive various wearable devices by utilizing the body heat only.In 2019, Di and Zhang et al. reported a large area 3D spacer fabric OTEG textile via industrial textile processing scheme. [51]They further investigated the performance of fabric OTEGs with different architectures by both finite element simulation and experimental studies, emphasizing the structural effect of fabric OTEG and leading to an optimized power density of 51.5 mW m −2 with a temperature gradient of 47.5 K.
Wang, Jiang, and Snyder et al. further demonstrated a "true" OTE fabric textile without the requirement of substrates, [52] by incorporating the CNT yarns into Π-shape OTEG modules via advanced fabrication techniques, novel doping methods, and most importantly the unique 3D patterned interlocking scheme (Figure 7a,b).As compared to common woven architecture, the free-standing fabric OTEG modules exhibited an excellent stretchability and compatibility with body movement, as well as a record high power density of 70 mW m −2 with a temperature difference of 44 K (Figure 7c-f), which demonstrates its application prospects toward "real" OTEG clothing instead of embedding the OTEG into cloth.It should be notice that such unique woven scheme can also be applied for other fabric OTE materials and devices.

Peltier Devices
As introduced previously, the Peltier effect-based OTEC can achieve solid-state cooling and heating, which has drawn increasing interests since the discovery of remarkable refrigeration effect in bulk Bi and Bi 2 Ti 3 in 1950s.Nowadays, commercial inorganic solid-state Peltier coolers have been widely applied in daily life such as semiconductor air conditioning, medical and lab instruments.
For OTEC, the unique advantages of OTE materials including intrinsically low thermal conductivity, light weight, and thin film (<1 mm) phase, are particularly promising for flexible and compact solid-state refrigeration application.However, even with the rapid developments of ZT values and a deeper understanding in the fundamental charge transport process, the first OTEC device was only reported until 2005 with conducting polymer polypyrrole (PPy) by Kaynak et al.Even so, the built-up temperature gradient across the PPy OTEC device is very small, [53] mainly due to the relatively low and unstable material TE performance.In particular, the low electrical conductivity will induce large Joule heating effect through the thin-film OTEC device, and therefore hindered the observation of robust Peltier refrigeration effect.Since then, possibly due to the above difficulties, the reported progress in Peltier effect-based OTEC is extremely limited for over a decade.
In 2018, Di and Zhu et al. overcome the above limitations by fabricating thermally suspended Peltier devices combined with advanced interlocked infrared imaging technique. [5]In their study, a high performance poly(Ni-ett) film was transferred onto a specially designed suspended parylene substrate with prepatterned electrodes (Figure 8a,b).The heat insulated substrate would significantly reduce the thermal loss to the environment and maintain the temperature gradient produced by Peltier cooling, especially in the high vacuum environment.With an alternating direction rectangular current input (Figure 8c,d), the Peltier cooling effect is successfully observed by separating it from the Joule heating effect and the Thomson relations in OTE materials is further verified, via the real-time mapping of the spatially-resolved temperature distribution over the Peltier device (Figure 8e,f).In particular, a large temperature difference reaches up to 41 K between the two electrode ends is achieved.
However, the cooling performance of OTEC is still far from satisfied for practical flexible refrigeration devices which could achieve local fast heat management.Other than keep boosting the material TE performance, the improved electrode contacts with high electrical conductivity, reduced interfacial thermal conduction, and optimized device structure as well as integration density are also crucial for future enhancement of OTEC performance.Di and Zhu et al. estimated the cooling performance of the ultrathin poly(Ni-ett) OTEC devices with improved contacts and optimized device structure, which is significantly improved and even comparable to the commercial Bi 2 Ti 3 TEC modules.Therefore, even with limited studies, the current results from simulations and experiments already indicate the bright future for OTEC applications.to other reported fabric OTEGs in terms of the power output.Reproduced with permission. [52]Copyright 2020, Springer Nature.

Multifunctional Sensors
In Seebeck effect, the output voltage is originated from the temperature gradient across the device.Therefore, when the temperature at one end of the electrode is fixed, the real-time temperature at the other end can then be calculated by ΔT = ΔV/S.[56] As the rapid development of smart devices and IoTs which often contain multiple highly integrated microelectronics, beyond typical temperature sensing or power generation, more requirements are pro-posed for self-powered sensors in terms of the effective acquisition and sensing of new and multiple physical and/or chemical stimuli by utilizing various functional OTE components.
Benefiting from the efficient light absorption, OTE materials have been developed for photodetectors and photoelectric generators via converting the photons into electrical signals. [57]For example, Kim et al. developed a TE material EDOS-C6 for light detection, [58] which NIR absorption in the doped state will induce a temperature increase and thus generating a TE voltage output due to the Seebeck effect.Recently, the OTE materials have been applied in mid-NIR and terahertz (THz) wave detectors, [59][60][61][62] due  [5] Copyright 2018, Springer Nature.
to their sensitive interfacial photo response, which is desired for non-invasive inspection applications such as portable and smart monitoring of medical release.Di, Xu, and Zhu et al. reported a maximum TE voltage when the NIR laser spot is located at the interface of electrode and active layer in a poly[Cu x (Cu-ett)]based OTE photodetector, [63] which could respond to NIR light with an intensity ranging from 1.7 mW cm −2 to 17 W cm −2 in 30 μs.These excellent sensing performances enable the detection of randomly moving NIR laser at a distance of 1.5 m away from the device.Zhang and Yao et al. fabricated a sensitive NIR and THz photodetector which could stably and reproducibly respond to the 1064 nm and 2.54 THz irradiation at room temperature, based on a CH 3 NH 3 PBI 3 (MAPbI 3 )/PEDOT:PSS composite with good electrical properties, wide absorption range, and large absorption coefficients. [64]Oda and Kawano et al. reported a flexible and wearable THz scanner based on macroscopic CNT films, which could achieve spatially resolved mapping of THz signals in a simple, non-contact and non-damaged manner, due to its effective and broad absorption of THz wave. [60]n 2015, Di and Zhu et al. realized the simultaneous temperature as well as pressure sensing via a novel concept of microstructure-frame-supported OTE (MFSOTE) materials. [4]he temperature sensing is achieved via the Seebeck effect while the pressure is calculated from the change in the MFSOTE piezo resistance (Figure 9a).Both remarkable pressure and tempera-ture sensitivity, such as sensing the pressure difference of a series of gentle finger taps or generating electricity from the temperature gradient between the finger and environment, are achieved without external power supply.By utilizing the inkjet printing techniques, flexible, highly integrated, and sensitive MFSOTE arrays are constructed with total 1350 pixels distributed in a 2 × 3 cm 2 area, enabling the simultaneous acquisition of spatially resolved pressure mapping as well as temperature distribution (Figure 9b), which is highly desired for future smart robots and electronic skins.Subsequently, Di and Zhu et al. developed a wearable OTEG incorporated gas sensors by printing thin-film OTE legs on a paper substrate, [56] which can output a voltage of 52.3 mV for a skin-attached OTEG array (Figure 9c).More importantly, the wearable OTEG device can realize a long-term sensitive sensing of ammonia (Figure 9d), facilitating the biosensing application.
Owing to the significantly larger thermopower reach up to 10 mV K −1 of ionic OTE materials than it of conducting polymers or organic semiconductors, ionic OTEG is able to power the ion-gated effect transistor and achieve a remarkable sensing amplification simultaneously with only single TE leg.Combined with the unique advantages of flexible OTEG, ionic OTE devices have become a new and promising direction toward multifunctional applications.By integrating an ionic OTEG module, Fabiano and Crispin et al. demonstrated a self-powered b) Optical image and temperature as well as pressure sensing mappings of the printed e-skin which is held by a human hand.Reproduced with permission. [4]Copyright 2015, Springer Nature.c) Device structure of the OTE gas sensor.d) Demonstration of the device sensing ability to the NH 3 gas.Reproduced with permission. [56]Copyright 2019, Wiley-VCH.organic electrochemical transistor (OECT). [65]Due to the large capacitance, OECT is able to operate with a relatively small voltage while the giant ionic thermopower could generate high output voltage of 0.3 V with a temperature gradient of 30 K for only one unit module.A heat-gated logic inverter is eventually achieved based on the ionic OTEG.PEDOT: PSS could support both ionic and carrier transport.However, its ionic transport and TE properties, can be significantly affected by ambient humidity. [66]By ingeniously incorporating the MOF-801/hydrogel self-humidifying bilayer which could control the humidity via modulating the absorption amount of moisture, Kim et al. reported a long-term stable PEDOT:PSS-based OTEG which could continuously operate over 72 h and simultaneously maintain the 90% constant relative humidity in ambient environments.These results demonstrated the great potential of ionic TE materials and devices toward more fascinating multifunctional applications by powering more complicated electrical circuits in the fields of wearable and smart biomedical applications.

TE Conversion Modulation
Previously, the research on optimizing the OTE performance has mostly focused on developing new materials, doping strategy, and molecular assembly.On the other hand, the sensitive response to external electrical, light, and magnetic field as well as operating temperature of -conjugated molecules offers a unique and straightforward route to manipulate the TE conversion in OTE devices, which theoretically would significantly affect the resulting TE performance, and even hold the promise to the discovery of unusual TE effects as well as new understanding toward the fundamental charge transport picture.

Electrical Field-Effect
By applying an electrical field in OTE device, the Fermi-level position as well as the charge carrier density in the organic semiconductor layer will be largely modified, which will significantly affect the device thermopower and conductivity, thus modulating the TE conversion.Moreover, in contrast to traditional chemical doping which is difficult to in situly control the doping level with a fixed Fermi level position, the utilization of field-effect provides a more controllable way to obtain more degrees of in situ charge transport information from both transistor characterizations and entropy measurements, [67][68][69][70][71][72][73][74][75][76][77][78][79][80][81] enabling the accurate comparison of experiments with various charge transport models for conjugated polymers including the variable range hopping, Mott mobility edge, (semi)metallic transport, etc.In particular, the electrical field-effect modulation can be realized by gating in the well investigated (organic field-effect transistor) OFET or OECT geometry which are fundamentally compatible with the OTE devices in terms of fabrication techniques and materials, while the temperature gradient is generally established along the horizontal direction by placing two parallel Peltier blocks below the insulating substrates of the vertical transistor architecture and the gate voltage tuned Seebeck signal can be measured from the source and drain.
For OTE performance optimization, by employing an OECT which could control the bulk oxidation and charge transport via gating, Crispin et al. demonstrated the modulation of PE-DOT:PSS thin film TE properties (Figure 10a-c). [77]Di and Zhu et al. reported a systematic investigation of modulating the TE properties of various organic semiconductors based on a typical OFET geometry with both electrolyte and SiO 2 dielectric layer (Figure 10d-f). [78]In particular, comparable TE performance to Reproduced with permission, [77] Copyright 2012, American Chemical Society.d) Schematic structure of the OFET-based OTE device.e,f), the resulting TE proprieties of PBTTT and P3HT with both SiO 2 and electrolyte dielectric, respectively.Reproduced with permission, [78] Copyright 2015, Wiley-VCH.g) Optical image of a Mott transistor-based BEDT-TTF OTE device and its molecular structure.h) 2D mapping of the device Seebeck coefficient versus the gate voltages and operation temperature along both a and c axes.i) Device gate voltage dependent anisotropic TE power factors measured at 100 K. Reproduced with permission. [80]Copyright 2016, American Institute of Physics.
chemically doped devices is achieved, demonstrating the potential of the field-effect OTE devices in performance modulation.In 2019, Katz et al. reported a 500% enhancement of the power factor of F4TCNQ doped DNTT by electrical field modulation with a bottom gate OFET geometry, which is due to the charging effect of the polymeric gate dielectrics. [79]Kawasugi et al. reported a strong anisotropic TE transport in an organic Mott insulator -(BEDT-TTF) 2 Cu[N(CN) 2 ]Cl with an electrical doublelayer transistor architecture (Figure 10g). [80]As the gate voltage increasing, interestingly a remarkable enhancement of both thermopower and electrical conductivity along the c-axis is simultaneously achieved (Figure 10h,i), as compared to the TE properties measured along the a-axis.This unusual effect is attributed to the pesudogap formation induced by the electron-electron coupling effect in strong correlated systems.
The field-effect modulation is also an important strategy to investigate the charge transport in OTE devices, especially by combining with further temperature dependent measurements, as initially demonstrated by Pernstich et al. with a Rubrene-based OFET. [67]Later, Venkateshvaran and Sirringhaus et al. improved the field-effect OTE device structure via a microfabricated on-ship geometry for enhanced accuracy, [68,69] and revealed the surprising disorder-free transport in amorphous IDTBT by field-effect and temperature dependent Seebeck characterizations. [70]Meanwhile, Li and Liu et al. proposed a unified physical model for the Seebeck transport in OFET based on the variable range hopping and Marcus theory, by comparing with the temperature and field dependent transport data, [71] illustrating that the energetic disorder is favorable for large thermopower and the importance of polarons to the Seebeck effect in doped polymers.
Tanaka, Ito, and Takenobu et al. reported the temperature dependent TE properties of a semicrystalline polymer PBTTT via ionic liquid electrolyte gating in a transistor geometry (Figure 11a). [72]In particular, a surprising nonmetallic to macroscopic metallic transport transition is revealed while reducing the temperature (Figure 11b), which is attributed to the interconnection of adjacent PBTTT polycrystalline domains by analyzing its S- relation (Figure 11c,d  Reproduced with permission. [72]Copyright 2020, AAAS.e) Measured (symbols) and calculated (curves) pentacene S versus gate voltages in a thin film transistor.f) Measured (symbols) and calculated (curved) S at 200 K via different transport models including the variable range hopping (S VRH ), the mobility edge (S ME ), and the hybridization of the two (S Hyb ).Reproduced with permission. [73]Copyright 2012, American Physical Society.coefficient ≈250 K in a pentacene OFET (Figure 11e). [73]By comparing the Seebeck and transistor measurements with different transport models.This intriguing phenomenon is explained by transition from low charge density variable range hopping to high charge density band-like transport during the increase of gate voltage at low temperature (Figure 11f), illustrating the critical effect of both energetic disorder and structural disorder to the Seebeck transport in -conjugated systems.
As presented, by electrical field modulation of Seebeck transport, various exciting transport phenomena and corresponding theoretical models have been discovered and proposed, which significantly advanced the fundamental understanding of charge transport in doped polymers.Further investigations may focus on the co-modulation of electrical field and other physical fields.On the other hand, the best TE performance achieved by field modulation is still only comparable to chemical doping, possibly limited by the interfacial effect nature of OFET gating, which requires an ultra-thin polymer layer with optimized interfacial properties to resolve.

Temperature Effect
In general, the performance of TE devices is highly temperature dependent, as the Seebeck coefficient, electrical conductivity, and thermal conductivity all extremely sensitive to the operation temperature.In particular, for bipolar OTE materials, the Seebeck coefficients can even intriguingly switch the signs, transiting from p-type to n-type or vice versa, by modulating the temperature.For example, as early as 1986, Zhu et al. discovered the sign change of (BEDT-TTF) 2 BrI 2 Seebeck coefficient from positive to negative while decreasing the temperature. [82][85][86][87][88] As summarized in Figure 12, The TE performance of the prototype n-type high performance OTE material family poly[A x (Mett)] developed by Zhu et al. can be clearly modulated by operating temperature. [2,83]The poly(Ni-ett) device revealed a large ZT of 0.21 at 300 K and a further enhanced ZT of 0.32 at 400 K.More interestingly, its thermopower exhibited an intriguing nonmonotonic temperature dependency, which is initially linear increasing with temperature until a sudden decrease appears ≈500 K.In 2017, Di and Zhu et al reported a high performance n-type small molecule A-DCV-DPPTTT, [84] which ZT value is significantly enhanced from maximum 0.12 at 300 K to 0.26 at 373 K.In 2019, Di and Zhang et al. reported a Selenium-substituted high performance DPP-based polymer PDPPSe-12, exhibiting a high power factor of 300 μW m −1 K −2 and a ZT value of 0.18 at room temperature, which are further optimized by temperature modulation reaching up to maximum value of 364 μW m −1 K −2 and 0.25 at 328 K, respectively. [85]In 2020, Liu and Koster et al. synthesized a series of fullerene-based small molecules for TE applications, in which the PTEG-2 reveal a highest power factor of 41 μW m −1 K −2 with a corresponding ZT value of 0.15 at room temperature.These values are significantly boosted to maximum power factor over 80 μW m −1 K −2 and ZT reaching 0.34, respectively, ≈400 K, revealing strong temperature dependency. [86]Recently, Di and Zhu et al. reported a high performance OTE device via molecular Figure 12.85][86][87] orientation engineering of polymer DPP-BTz. [87]In particular, its ZT value is largely enhanced from 0.20 at 298 K to a remarkable value of 0.40 at 323 K.
Although temperature modulation has become a general strategy to optimize the TE performance in these studies, the deep understanding of such sensitive temperature dependency is still insufficient.Theoretically, for common polymer-based OTE devices with the mobility edge or variable range hopping transport,  should monotonically increase with temperature due to the enhancement of both charge mobilities and densities in the frame of thermal activation.][91] Similarly, for the Seebeck coefficients, the existing various transport models although are physically distinct, all predict a monotonic temperature dependency either decrease or increase as temperature.However, as presented, the ZT values of OTE materials and devices often exhibit much more complicated nonmonotonic temperature dependency with optimized performance typically around room temperature, resulting from the nonmonotonic relation of Seebeck coefficient with temperature, which is beyond current theoretical expectations for single transport model.
On the theoretical side, in contrast to the one-band transport which organic molecules are commonly considered to follow, recently by ingeniously employing a multiband transport model, Shuai et al. explained the unusual experimentally revealed nonmonotonic temperature dependency of thermopower in doped poly(Ni-ett) (Figure 13a). [92,93]In addition to the first conduction band composed by empty energy levels, a half-filling dopinginduced narrow polaron band around the Fermi level is also taken into consideration (Figure 13b).Surprisingly, the polaronic states in doped poly(Ni-ett) are calculated to contribute significantly to the total thermopower, indicating their importance in the Seebeck transport (Figure 13c).Later, to investigate the intriguing temperature-induced Seebeck polarity switch phenomenon in bipolar polymers, Shuai et al. further employed a two-band model containing both conduction band and valence band by considering the electrons and holes contribute separately to the bipolar Seebeck transport (Figure 13d). [94]The results qualitatively reproduce the mysterious Seebeck sign switch behavior observed at low temperature, further justifying the multiband transport and providing new understanding of the energy level structure in doped polymers.

PTE Devices
An important feature of organic semiconductors is their excellent photosensitivity, which facilitates the performance modulation of many organic electronics via tuning the fundamental charge carrier transport process by light irradiation.From the view point of energy harvesting, current investigations of optoelectronics are mainly focused on organic photovoltaics (OPV) which directly convert solar energy into electricity by generating photo-induced electron-hole pairs.Meanwhile, to satisfy the increasing demand in wearable and smart electronics, light also provides a unique method to enhance the energy conversion efficiency in OTE devices via photo-thermo-electric (P-T-E) and photo-thermoelectric (P-TE) effects. [57]In the P-T-E effect, light irradiation can induce additional heat via the photothermal effect which will enhance Reproduced with permission. [92]Copyright 2019, American Chemical Society.b) Schematic drawing of the multiband structure on the empty states side which contributed to the Seebeck transport, including the half-filling polaron band at the Fermi level and the first as well as second conduction band.c) Corresponding calculated temperature dependent multiband contribution to the thermopower and electrical conductivity.Reproduced with permission. [93]Copyright 2021, American Institute of Physics.d) Schematic illustration of the difference in temperature dependent Seebeck calculations of poly(Ni-ttftt) molecule between one-band and two-band model.Reproduced with permission. [94]Copyright 2021, Chinese Chemical Society.
the OTE device power output by usual TE conversion.With a different physical mechanism, the P-TE effect could directly tune the carrier concentration of the active OTE materials and may affect the redox reaction in chemical doping via photo-induced excitation, thus manipulating the device TE properties.
In the past few years, by exploiting the P-T-E effect, various novel OTE devices for sensing as previously discussed photodetectors and for energy harvesting have been developed.For example, Ren and Chen et al. reported a flat-panel solar OTE device. [95]he innovative device design leads to spectrally-selective solar light absorption and the establishment of temperature difference, realizing a cost-effective solar energy harvesting into electricity.On the other hand, although a few studies reported P-TE effect modulated OTE device performance, the optimized power factors are relatively low, typically smaller than 0.1 μW m −1 K −2 , mainly limited by the insufficient photo-induced charge density (<10 [17] cm −3 ) for effective charge transport in OTE devices, which requires new strategies to enhance the separation efficiency of excitons.
In 2020, Di and Zhu et al. resolve the above issue by incorporating an organic phototransistor and realizing the TE performance modulation via gating assisted P-TE effect in a NDI(2OD)(4tBuPh)-DTYM2 device (Figure 14a,b). [96]By coupling the field-effect and the P-TE effect, a remarkable one order of magnitude enhancement in charge carrier density and over 500% improvement in power factor reaching a record high value of 11.2 μW m −1 K −2 are achieved (Figure 14c).Detailed characterizations revealed the significant enhancement of charge density is due to electrical field assisted exciton separation and charge screening as well as recombination (Figure 14d,e).More interestingly, both the electrical conductivity and the Seebeck coefficients are simultaneously enhanced upon light illumination, originating from light-modified density of states (DOS) around Fermi level with increased energetic disorder, as revealed by theoretical investigations.These results establish a fundamental relation between field modulated photo-induced carrier separation and performance in OTE devices, and pave the way for future advanced photodoping.
Subsequently in 2022, Zou, Di, and Zhu et al. developed an improved photodoping method inspired from OPVs by incorporating an unique hierarchical heterojunction, which is consisted of a bulk heterojunction photoactive IDTBT:PC61BM layer and a separate PDPP4T charge transport layer (Figure 14f). [97]Due to the well-designed energy level structure, the P-TE device achieved effective photodoping with power factors reach up to 13.7 μW m −1 K −2 (Figure 14g).A prototype photodetector array and flexible P-TE power generator is fabricated (Figure 14h,i), proving the potential of photodoping for applications in low power consumption wearable devices.Although the power factor is still rather limited, the presented studies demonstrated the diverse possibilities of P-TE effect coupling with other physical fields as well as various organic optoelectronics toward novel charge carrier dynamics in OTE devices.

MTE Devices
In the Seebeck effect, by applying an external perpendicular magnetic field to the temperature gradient, similar to the Hall effect, the charge carriers in OTE devices will deflect due to the Lorentz force, leading to the magneto-thermoelectric (MTE) effect with a modified longitudinal Seebeck signal (the magneto-Seebeck effect) as well as an accumulated transverse voltage (the Nernst-Ettingshausen effect).The Nernst effect along with the anomalous Nernst effect have attracted dramatic research interests recently, as they support a new category of transverse TE devices e) The calculated exciton separation efficiency versus electrical field strength.Reproduced with permission. [96]Copyright 2020, Wiley-VCH.f) Schematic drawing of the PDPP4T/IDTBT:PC61BM hierarchical heterojunction structure and photodoping mechanisms, which can be applied for photodetectors and P-TE devices.g) Measured TE properties versus light intensity of the HHJ-based P-TE device.h) Schematic and the sensing image of a photodetector array with a "PTE" masking.i) Prototype flexible P-TE power generator and it power output curves in dark environment as well as under light irradiation.Reproduced with permission. [97]Copyright 2022, American Chemical Society.
which are particularly suitable for large area flexible electronics with various advantages over conventional Seebeck effect-based longitudinal device geometry, including simpler device structure with single unipolar material, higher energy conversion efficiency even with same ZT value, effective utilization of material anisotropy and ambipolarity, etc.For OTE devices, although their inherent ultra-flexibility in thin-film phase is particularly desired for Nernst effect-based transverse device geometry, so far, the exploration of OTEs has solely focused on the Seebeck effectbased longitudinal TE conversion, while the Nernst effect-based transverse TE conversion in polymeric semiconductors has even not been observed yet.This is possibly due to the relatively low charge mobilities in OTE materials, as theoretically the magni-tude of material Nernst effect is proportional to its mobility, thus hindering the accurate Nernst measurements.
Previously, there are a few groups reported the observation of the magneto-Seebeck effect in polymeric OTE devices.Hu et al. reported the magnetic field modulation effect to the Seebeck coefficient in a vertical OTE device of ITO/PEDOT:PSS/Au. [98]In particular, the size of modulation, although not strictly proportional to, is positively correlated with the magnetic field strength, which is partially consistent with general MTE effects.A relatively large enhancement of the Seebeck coefficient up to 9% is achieved with a small magnetic field of 450 mT.In 2021, Kim et al. reported similar phenomenon of the magneto-Seebeck effect in oriented PEDOT:TOS/graphene hybrid devices (Figure 15a,b). [99]igure 15.a) Schematic drawing of the PEDOT:TOS-based photo-MTE device under a perpendicular magnetic field.The horizontal temperature gradient can be established by a NIR laser via the P-TE effect.b) Schematic geometry of the end-on PEDOT on modified graphene substrate (EPG).c) Cycles of magnetic field modulation from 0 to 200 mT to the Seebeck coefficient (MS) and power factor (MPF) in percentage change of MTE devices with three differently oriented PEDOT derivatives.d) 3D contour plot of the photo-MTE effect versus the light induced temperature gradient, magnetic field strength, and laser power.Reproduced with permission. [99]Copyright 2021, Wiley-VCH.
Interestingly, the size of MTE effect is discovered to be highly dependent on the PEDOT:TOS orientation, reaching up to a giant modulation of the Seebeck coefficients up to 45% with only a 200 mT magnetic field in the end-on configuration (Figure 15c).Moreover, the MTE effect can be further coupled with the P-TE effect by establishing the temperature gradient via light irradiation, demonstrating the photo-MTE effect with larger MTE response as compared to the dark situation, which opens new directions for OTE device performance modulation.
The revealed magneto-Seebeck effect although cannot be directly applied for transverse TE conversion and potentially require more control experiments to further prove the nature of MTE response, physically it has the same mechanism with the Nernst effect.Hypothetically, the failure of Nernst effect observation in the above studies might be due to the small applied magnetic fields, low material mobilities, and unoptimized device structure for transverse measurements, which makes the Nernst signals small and extremely difficult to observe.As a result, by combining strong superconducting magnets with optimized device geometry, fabrication process, and appropriate material choices and treatments, the demonstration of Nernst effect in OTE devices can be expected in the near future.An intriguing point which the current magneto-Seebeck studies have already suggested is, the revealed MTE effect in polymers is unexpectedly large by considering they are nonmagnetic with orders of magnitudes lower mobilities than their inorganic counterparts.This fact potentially indicates that the magnetic field modulated TE conversion process in OTE device may fundamentally different from existing MTE transport models for inorganic materials, and therefore holds the promise for high-performance transverse OTE devices.

Single Molecular-Scale Device
The investigation of single-molecular TE (SM-TE) devices has drawn significant interests over the past decades, as it could not only probe the fundamental structure-property relationship of OTE materials on the ultimate scale of single molecule, but also open up new routes for boosting the TE performance.For bulk OTE materials, the thermopower, electrical conductivity, and thermal conductivity, are typically inversely correlated, which hinders the enhancement of TE performance.In contrast, for SM-TE devices, these TE parameters can be simultaneously improved by utilizing various exotic quantum effects.Therefore, the SM-TE devices potentially could achieve extremely high ZT values much large than 1, as predicted by theoretical calculations (Figure 16), [100,101] which beyond the current limits of any bulk TE materials and devices.In this section, we will review the latest representative experimental advances in the related topics of SM-TE devices.

Molecular Seebeck and Doping Effect
The molecular scale Seebeck coefficient measurements not only could reveal the TE performance of the SM-TE device, but also are critical for investigating the microscopic charge transport in OTE device. [102]To measure the single molecular scale TE properties, a nanoscale temperature gradient must be accurately established and calibrated across the molecular junction, while simultaneously measuring the electrical signals.Toward this goal, various scanning probe microscopy and break-junction-based techniques have been invented in the past decades.  In 1998,udoph  [100] Copyright 2010, American Chemical Society.b) CSW-479 molecule structure which could rotate around the middle C─C bond and its predicted ZT values versus rotation angles and temperatures. For bth molecules, the theoretical predicted maximum ZT values are orders of magnitudes larger than 1.Reproduced with permission.[101] Copyright 2009, American Physical Society.
and Ruitenbeek achieved the first measurement of the Seebeck coefficient in an atomic metal-metal junction using a modified mechanically controlled break junction (MCBJ) setup. [110]In 2007, Reddy et al. reported the first study of single molecular thermoelectricity by measuring the Seebeck coefficients of a series of Au-BDT-Au molecular junctions statistically using a modified scanning tunneling microscope (STM) setup. [104]Subsequently, by utilizing the scanning tunneling microscope-break junction (STM-BJ) technique, Cuevas and Agrait et al. measured Seebeck coefficient in atomic Pt and Au junction. [124]These experimental progresses combined with theoretical developments significantly advance the field of SM-TE device studies.
Similar to the bulk OTE materials and devices, the doping effect on single molecular scale is also critical to modulating the TE performance and have drawn intense research interests.Different from the bulk situation, on molecular scale, doping is typically achieved by incorporating metal ions into conjugated backbone, which could significantly tune the molecular density of states and resulting in surprising charge transport phenomenon, and therefore change the molecular TE properties.Following this strategy, with a Sc 3 N inside the fullerene cage, single endohedral fullerene Sc 3 N@C80 molecule has demonstrated intriguing TE properties. [125]Its Seebeck coefficient sign and magnitude can be tuned via controlling the tip-molecule distance or changing the molecular orientation, originating from molecular density of states variations induced by different tip pressure.In another report, Naher et al. fabricated a series of Ru or Pt centered conjugated molecular wires, [126] and systematically investigated their TE properties from the view point of orbital mixing between the organic ligand and the central metal dopant.From which, they proposed a novel strategy of metal center doping to enhance the single-molecular TE performance.

Quantum Interference Effect
[129][130][131] In general, the destructive quantum interference (DQI) effect is signatured by a sharp anti-resonance dip around the Fermi level in the transmission curves, which will produce a giant derivative value and thus resulting in a large Seebeck coefficient.For example, Meyhofer, Reddy, and Linke et al. reported the Seebeck coefficients of two OPE3 derivatives.One features a para-connected central phenyl ring (para-OPE3) and the other features a meta-connected central ring (meta-OPE3) (Figure 17a,b). [131]In contrast to the para-OPE3 molecule, the meta-OPE3 molecule exhibits a DQI effect as indicated by the signature dip in the transmission functions (Figure 17c,d), which resulting in that the measured Seebeck coefficient of the meta-OPE3 molecule is almost twice of the para-OPE3 molecule (Figure 17e,f).
Furthermore, by utilizing the destructive -interference effect, Garner et al. achieved a very high Seebeck coefficient reaching 1 mV K −1 in the SM-TE device of a saturated Si-based molecule with a bicyclo[2.2.2]octasilane moiety. [130]This unique moiety locks the Si-Si bonds into eclipsed configurations in a bicyclic molecular framework (Figure 17g), which induces a DQI effect in the -channel with a much sharper dip around the Fermi level in the transmission functions as compared to the -channel DQI effect (Figure 17h-j), leading to the revealed giant Seebeck coefficient (Figure 17k).

Field-Effect in Molecular Junction
Three-terminal SM-TE devices including source, drain, and gate could modulate the single-molecular charge transport via the electrostatic field-effect, which will modify the molecular energy level alignment with the Fermi level and therefore tune the TE properties.Reddy et al. demonstrated the first three-terminal SM-TE devices in 2014 (Figure 18a). [117]By employing the gating effect, they investigated the TE properties of two representative molecules the fullerene (C60) and the biphenyl-4,4′ -dithiol (BPDT) (Figure 18b,c), which revealed the correlation of the energy level alignments with the molecular TE properties, and more  [131] Copyright 2018, American Chemical Society.g) Structure of molecule Si4 and Si2-Si222-Si2.h) Schematic diagram of different channels of DQI.i) Conductance comparison among different molecules.j) Transmission functions of the  and  channel.k) Thermopower measurements of the Si2-Si222-Si2 junction with -channel DQI.Reproduced with permission. [130]Copyright 2018, Springer Nature.
importantly provide a general method to precisely modulate the TE performance in single-molecular junctions.
Later, Gehring et al. developed a new three-terminal SM-TE device measurement setup which could simultaneously acquire the Seebeck coefficient as well as the electrical conductance as a function of the gate bias (V G ) and the source-drain voltages (V SD ). [132]For molecule [Gd(tpy-SH) 2 (SCN) 3 ], they achieved the comprehensive V G and V SD dependent 2D mapping of the SM-TE device properties (Figure 18d-f), which exhibited a V G symmetry with signature of excited molecular density of states.Eventually, via field-effect modulation, a large Seebeck coefficient of 414 μV K −1 of the SM-TE device is achieved.

Thermal Transport Effect in Single-Molecule Junction
As compared to the electrical conductance or Seebeck coefficient characterizations in SM-TE devices, the thermal conductance is particularly challenging to measure, as it requires measuring the extremely small heat flow in the picowatt range across one molecule.In 2017, Reddy et al. reported the first thermal conductance measurement in Au and Pt single atomic junctions by developing the calorimetric scanning thermal microscopy (C-SThM) technique, [113] which features a picowatt-resolution custom-built calorimetric probe.They revealed that at room tem-perature the thermal conductance of the Au atomic junctions is quantized, and more importantly verified the Wiedemann-Franz law on the single-atomic scale.
Subsequently, Reddy et al. further optimized the C-SThM technique and measured the thermal conductance of a series of thiol terminated alkanes with different chain length (C2, C4, C6, C8, C10) (Figure 19a-e).In contrast to the exponential molecular length dependency of electrical conductance, the measured thermal conductance is almost independent with chain length (Figure 19f), [114] which is consistent with theoretical prediction and prove the heat transport in SM-TE devices is ballistic.Potentially, the C-SThM technique can be applied to other ultrasensitive thermal transport studies.
By employing a different setup, Mosso et al. at IBM-Zurich observed similar quantized thermal conductance in Au atomic junctions which further verified the Wiedemann-Franz law at the atomic limit, [115] and also successfully measured the thermal conductance in single-molecule junctions. [116]Specifically, a STM tip was combined with a unique suspended micro electromechanical system (MEMS), which could insulate the sample from the chip substrate and is featured with a four-point contact electrical measurement system as well as a Pt microheater for temperature controlling, allowing the simultaneous characterizations of atomic and molecular scale charge and heat transport.molecular junctions, respectively.Reproduced with permission. [117]Copyright 2014, Springer Nature.d) Gd(tpy-SH) 2 (SCN) 3 molecular structure.e,f) 2D mapping of its molecular junction thermopower as well as power factor versus the gate and source drain voltages, respectively.Reproduced with permission. [132]Copyright 2019, Springer Nature.The TE and charge transmission within the molecular junction for various cooling and heating effects.e) Measured and fitted cooling power versus bias for the three different molecular junctions.Reproduced with permission. [135]Copyright 2018, Springer Nature.

Molecular Peltier effect
In contrast to the Seebeck effect, the Peltier effect in molecular junctions is rarely explored, owing to the difficulties in accurate measurement of picowatt-level cooling power within the molecular junctions, [133,134] similar to molecular thermoconductance measurements.In 2018, Reddy et al. reported an atomic force microscopy (AFM)-based experimental setup for measuring the nanoscale heating and cooling power as well as the electrical conductance and the Seebeck coefficients within the molecular junctions (Figure 20a-c). [135]By applying a voltage through an AFM tip, the current flow across the junction produces Joule heat dissipation and Peltier cooling effect (Figure 20d), resulting in a temperature variation which can be further measured by an ultrasensitive calorimetric microdevice with a temperature resolution smaller than 0.1 mK.Based on this setup, the Peltier cooling effect is successfully observed in various molecular junctions.For the HOMO-dominated BPDT molecular junctions, an extracted cooling power of 300 pW with a negative bias ≈3 mV is acquired, while for the LUMO-dominated 4,4′-bipyridine molecular junctions, the Peltier cooling is observed with a positive bias, which overall exhibit a good consistency with the theoretical predictions (Figure 20e).
Although significant progress has been made during the last decade for the field of SM-TE devices, there is still a large discrepancy between theoretically predicated molecular TE performance and the experimental results.In addition to technical challenges in precisely measuring the molecular TE properties as well as establishing and calibrating the temperature gradient across one molecule, fundamental process of charge transport through a single molecule is still unclear, limited by the insufficient control of single-molecular junction transmission functions.Therefore, more endeavors are required to systematically investigate the molecular structural and geometry as well as external fieldeffects to the single-molecular transmissions and TE properties.

Conclusion and Perspective
In conclusion, OTE devices from single molecular scale to integrated thin-film-based modules have all developed rapidly in the past decade in terms of transport mechanism, performance, and functions, which significantly accelerates the realization of nextgeneration energy-harvesting wearable, implantable, and intelligent electronics into a foreseeable future.Not limited to these dreaming applications, we put forward the following perspectives to guide future investigations in OTE devices.
The OTE device energy conversion efficiency can be continuously optimized by both enhancing the material ZT values and improving the OTE module architecture design.For practical applications, the ideal OTE material ZT values should reach and over 1 at room temperature.To achieve such ambitious goal, some remaining fundamental issues regarding the charge transport in OTE device need to be resolved.Particularly, the bipolar transport which generally exists in many organic semiconductors, requires a clearer understanding as well as fine modulation.Because although it is desired for fabricating p-/n-TE legs with single material, on the other hand it will also severely reduce the device thermopower and TE performance.Further investigations should also be performed for electron-phonon coupling and its induced (bi)polarons in OTE devices, as they are essential to understand and break the inverse correlation between TE parameters.Such studies in principle should carried out by combining external fields and temperature modulated transport characterizations with various spatially and time-resolved in situ structural and chemical probing methods, such as STM, TOF-SIMS, and GIWAXS, in both macroscopic thin-film OTE devices and microscopic SM-TE devices.From the perspective of OTE module architecture, controlling device anisotropy with preferred heat flow directions could benefit the TE conversion.Moreover, systematic investigations of the TE leg spacing and geometry design is also necessary to balance the trade-off relation among power output, Joule heat, and temperature difference for optimized conversion efficiency, which may be achieved by high-capacity simulations and machine learning.
Regarding future new-concept OTE devices with novel and multiple functions, such as the conformability and bionic sensing ability toward advanced wearable and smart electronics.In addition to the general strategy of cooperative developments in device materials, fabrication techniques, and geometry designs, exploring novel TE effects in OTE devices is also a promising route, as it holds the promise for realizing entirely new categories of OTE devices by utilizing fundamentally different mechanisms.
For example, the spin-Seebeck effect, by directly converting the heat into pure spin currents, is crucial for driving spintronics, which have not been discovered in OTE devices yet.However, by considering the rapid development of organic spintronics such as organic spin-valves, the organic spin caloritronics which combine the basic idea of TE and spintronic devices are urgently needed.For spin-OTE investigations, [136][137][138][139][140] conjugated molecules with robust magnetism and long spin-coherence time as well as optimized device interface spin interactions might be the key.By integrating the SM-TE technique with the spinpolarized STM, [121,122,141] Friesen et al. successfully realize the atomic-scale mapping of the spin-modulated Seebeck effect in the real space on Fe surface.Similar technique and strategy in principle should be fully suitable for studying the coupling effects between magnetism and heat in spin-OTE devices, which holds the promise self-powered low consumption quantum devices beyond the Moore's law.
Moreover, the longitude spin current generated in spin-Seebeck effect can be further converted into transverse electrical voltage signal via the inverse-spin hall effect with the need of only a tiny magnetic field, similar to the anomalous Nernst effect.This is crucial for achieving flexible transverse OTE devices, which in principle are more suitable for TE conversion than Seebeck effect-based traditional longitude devices.For SM-TE devices, due to various exotic quantum effects, integrating with other nano electronics including molecular motors, nano robots, nano transistors, and quantum bits is particularly promising for developing novel self-powered organic quantum devices which functions and performance may even beyond imagination.

Figure 1 .
Figure 1.Schematic illustration of device engineering in the development from OTE materials to applications.Image two, row two and bottom image, rightmost panel, bottom row reproduced with permission.[3]Copyright 2022, Wiley-VCH.Panel 4, row one reproduced with permission.[4]Copyright 2015, Springer Nature.Bottom image, rightmost panel, top row reproduced with permission.[52]Copyright 2020, Springer Nature.

Figure 2 .
Figure 2. a,b) Schematic drawing of one OTEG and Peltier-cooling OTEC device unit with alternating p/n legs, respectively.c) Unipolar OTE device geometry.Schematic illustration of Π-shape d) vertically and e) horizontally integrated OTEG module.f) Schematic plot of the OTEG output power versus the output current under different temperature gradients (∆T 1 >∆T 2 ).g) Illustration of the maximum achievable output power when the load resistance is equal to the OTEG internal resistance.h) Comparison of simulated power output with the experimental values at different temperature gradients.

Figure 5 .
Figure 5. a,b) Molecular structure of the elastomer composition and schematic of its fabrication process, respectively.c) Demonstration of the good stretchability.d) Thermopower under external strains.e) Photograph of OTE module under 30% strain and its power output with different temperature gradients.f,g) Vertically and horizontally stretched OTE module power output versus strains with a constant temperature gradient of 30 K, respectively.Reproduced with permission.[40]Copyright 2020, Springer Nature.

Figure 6 .
Figure 6.a) The Lego-like OTEG structure.b) Demonstration of stretchability.c) Schematic illustration of the self-healing mechanism and process.d) Demonstration of the device self-healing ability.e,f the OTEG module power output and open circuit voltages versus temperature gradients, respectively.Reproduced with permission.[44]Copyright 2021, AAAS.

Figure 7 .
Figure 7. a) Schematic fabrication process of the 1D OTE fibers.b) Schematic structure of the 1D OTE fiber (top) and the final free-standing 3D fabric textile with the unique interlocking woven scheme (bottom).c,b) Demonstration of the 3D textile fabric OTE device twistability and stretchability, respectively.e) Elbow movements with the wearable fabric OTEG.f) Output voltage of the fabric OTEG with different temperature gradients.g) Comparisonto other reported fabric OTEGs in terms of the power output.Reproduced with permission.[52]Copyright 2020, Springer Nature.

Figure 8 .
Figure 8. a,b) Schematic and optical illustration of the poly(Ni-ett)-based suspended Peltier device structure, respectively.c,d) Schematic of different temperature effects in Peltier device and strategy to separate the Peltier effect with an alternating direction current input, respectively.e) Real-time infrared imaging of the temperature distribution across the Peltier device with opposite current input.f) Extracted time-resolved Peltier cooling performance versus the input current direction and density.Reproduced with permission.[5]Copyright 2018, Springer Nature.

Figure 9 .
Figure 9. a) Schematic operation mechanism of the MFSOTE device for dual-sensing of pressure and temperature, via TE and piezoresistive effects.b)Optical image and temperature as well as pressure sensing mappings of the printed e-skin which is held by a human hand.Reproduced with permission.[4]Copyright 2015, Springer Nature.c) Device structure of the OTE gas sensor.d) Demonstration of the device sensing ability to the NH 3 gas.Reproduced with permission.[56]Copyright 2019, Wiley-VCH.

Figure 10 .
Figure 10.a,b) Schematic structure of the OECT-based PEDOT:PSS OTE device and its TE parameters versus the gate voltages.c) Schematic energy level structures of the localized density of states below the transport energy level E T (left), and the conduction band as well as valence band with the in-gap localized (bi)polaronic states induced by OECT gating (right).Reproduced with permission,[77] Copyright 2012, American Chemical Society.d) Schematic structure of the OFET-based OTE device.e,f), the resulting TE proprieties of PBTTT and P3HT with both SiO 2 and electrolyte dielectric, respectively.Reproduced with permission,[78] Copyright 2015, Wiley-VCH.g) Optical image of a Mott transistor-based BEDT-TTF OTE device and its molecular structure.h) 2D mapping of the device Seebeck coefficient versus the gate voltages and operation temperature along both a and c axes.i) Device gate voltage dependent anisotropic TE power factors measured at 100 K. Reproduced with permission.[80]Copyright 2016, American Institute of Physics.
), revealing the fundamental structureproperty relation.Kemerink et al. observed a remarkable simultaneous enhancement of both charge density and Seebeck

Figure 11 .
Figure 11.a) Schematic and optical image of the field-effect PBTTT OTE Device.b) Temperature dependent conductivity characterizations with different gating voltages.c) The acquired PBTTT S- relations and comparison to other reported data with different doping strategies.d) Schematic illustration of the PBTTT ordered domains with the domain boundaries connected by tie molecules, which leads to the revealed macroscopic metallic charge transport.Reproduced with permission.[72]Copyright 2020, AAAS.e) Measured (symbols) and calculated (curves) pentacene S versus gate voltages in a thin film transistor.f) Measured (symbols) and calculated (curved) S at 200 K via different transport models including the variable range hopping (S VRH ), the mobility edge (S ME ), and the hybridization of the two (S Hyb ).Reproduced with permission.[73]Copyright 2012, American Physical Society.

Figure 13 .
Figure13.a) Molecular structure of the n-type K-doped poly(Ni-ett).Reproduced with permission.[92]Copyright 2019, American Chemical Society.b) Schematic drawing of the multiband structure on the empty states side which contributed to the Seebeck transport, including the half-filling polaron band at the Fermi level and the first as well as second conduction band.c) Corresponding calculated temperature dependent multiband contribution to the thermopower and electrical conductivity.Reproduced with permission.[93]Copyright 2021, American Institute of Physics.d) Schematic illustration of the difference in temperature dependent Seebeck calculations of poly(Ni-ttftt) molecule between one-band and two-band model.Reproduced with permission.[94]Copyright 2021, Chinese Chemical Society.

Figure 14 .
Figure 14.a,b) Schematic drawing of the P-TE transistor and molecular structure of NDI(2OD)(4tBuPh)-DTYM2, respectively.c) Gate voltage modulated TE properties versus light intensity.d) Illustration of the charge transport and exciton separation mechanism of the electrical field-modulated P-TE effect.e)The calculated exciton separation efficiency versus electrical field strength.Reproduced with permission.[96]Copyright 2020, Wiley-VCH.f) Schematic drawing of the PDPP4T/IDTBT:PC61BM hierarchical heterojunction structure and photodoping mechanisms, which can be applied for photodetectors and P-TE devices.g) Measured TE properties versus light intensity of the HHJ-based P-TE device.h) Schematic and the sensing image of a photodetector array with a "PTE" masking.i) Prototype flexible P-TE power generator and it power output curves in dark environment as well as under light irradiation.Reproduced with permission.[97]Copyright 2022, American Chemical Society.

Figure 16 .
Figure16.a) PPE molecule structure and its predicted ZT values versus unit numbers.Reproduced with permission.[100]Copyright 2010, American Chemical Society.b) CSW-479 molecule structure which could rotate around the middle C─C bond and its predicted ZT values versus rotation angles and temperatures.For both molecules, the theoretical predicted maximum ZT values are orders of magnitudes larger than 1.Reproduced with permission.[101]Copyright 2009, American Physical Society.

Figure 18 .
Figure18.a) Schematic illustration of the three-terminal device setup.b,c) Thermopower measured as a function of gate voltages for BPDT and C60 molecular junctions, respectively.Reproduced with permission.[117]Copyright 2014, Springer Nature.d) Gd(tpy-SH) 2 (SCN) 3 molecular structure.e,f) 2D mapping of its molecular junction thermopower as well as power factor versus the gate and source drain voltages, respectively.Reproduced with permission.[132]Copyright 2021, Springer Nature.

Figure 19 .
Figure 19.a-d) Schematic illustration of C-SThM set-up for measuring the heat flow within the molecular junction, molecular structure with different numbers of carbon, and the silicon nitride beam structure, respectively.e) Conductance measurements for different molecules.f) Extracted thermal and electrical conductance versus the length of molecule.Reproduced with permission.[114]Copyright 2019, Springer Nature.

Figure 20 .
Figure 20.a,b) Schematic and optical illustration of the single-molecular Peltier device set up. c) Molecular structure of the three target molecules.d)The TE and charge transmission within the molecular junction for various cooling and heating effects.e) Measured and fitted cooling power versus bias for the three different molecular junctions.Reproduced with permission.[135]Copyright 2018, Springer Nature.