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    SPECIAL TOPIC — Organic and hybrid thermoelectrics

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    Thermoelectric transport in conductive poly(3,4-ethylenedioxythiophene)
    Meng Li(李萌), Zuzhi Bai(柏祖志), Xiao Chen(陈晓), Cong-Cong Liu(刘聪聪), Jing-Kun Xu(徐景坤), Xiao-Qi Lan(蓝小琪), and Feng-Xing Jiang(蒋丰兴)
    Chin. Phys. B, 2022, 31 (2): 027201.   DOI: 10.1088/1674-1056/ac4230
    Abstract247)   HTML7)    PDF (3804KB)(107)      
    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 advances in organic, inorganic, and hybrid thermoelectric aerogels
    Lirong Liang(梁丽荣), Xiaodong Wang(王晓东), Zhuoxin Liu(刘卓鑫), Guoxing Sun(孙国星), and Guangming Chen(陈光明)
    Chin. Phys. B, 2022, 31 (2): 027903.   DOI: 10.1088/1674-1056/ac2802
    Abstract258)   HTML5)    PDF (2370KB)(117)      
    The thermoelectric (TE) materials and corresponding TE devices can achieve direct heat-to-electricity conversion, thus have wide applications in heat energy harvesting (power generator), wearable electronics and local cooling. In recent years, aerogel-based TE materials have received considerable attention and have made remarkable progress because of their unique structural, electrical and thermal properties. In this review, the recent progress in both organic, inorganic, and composite/hybrid TE aerogels is systematically summarized, including the main constituents, preparation method, TE performance, as well as factors affecting the TE performance and the corresponding mechanism. Moreover, two typical aerogel-based TE devices/generators are compared and analyzed in terms of assembly modes and output performance. Finally, the present challenges and some tentative suggestions for future research prospects are provided in conclusion.
    Recent progress in design of conductive polymers to improve the thermoelectric performance
    Zhen Xu (徐真), Hui Li (李慧), and Lidong Chen(陈立东)
    Chin. Phys. B, 2022, 31 (2): 028203.   DOI: 10.1088/1674-1056/ac22a4
    Abstract232)   HTML4)    PDF (4924KB)(105)      
    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
    Qi Zhang(张奇), Hengda Sun(孙恒达), and Meifang Zhu(朱美芳)
    Chin. Phys. B, 2022, 31 (2): 028506.   DOI: 10.1088/1674-1056/ac3a64
    Abstract179)   HTML2)    PDF (3007KB)(58)      
    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 p-type 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.
    Enhanced thermoelectric performance of PEDOT: PSS films via ionic liquid post-treatment
    Jiaji Yang(杨家霁), Xuejing Li(李雪晶), Yanhua Jia(贾艳华), Jiang Zhang(张弜), and Qinglin Jiang(蒋庆林)
    Chin. Phys. B, 2022, 31 (2): 027302.   DOI: 10.1088/1674-1056/ac2487
    Abstract219)   HTML0)    PDF (1088KB)(53)      
    Thermoelectric (TE) energy harvesting can effectively convert waste heat into electricity, which is a crucial technology to solve energy concerns. As a promising candidate for energy conversion, poly(3,4-ethylenedioxythiophene):poly (styrene sulfonate) (PEDOT:PSS) has gained significant attention owing to its easy doping, high transparency, and solution processability. However, the TE performance of PEDOT:PSS still needs to be further enhanced. Herein, different approaches have been applied for tuning the TE properties:(i) direct dipping PEDOT:PSS thin films in ionic liquid; (ii) post-treatment of the films with concentrated sulfuric acid (H2SO4), and then dipping in ionic liquid. Besides, the same bis(trifluoromethanesulfonyl)amide (TFSI) anion and different cation salts, including 1-ethyl-3-methylimidazolium (EMIM+) and lithium (Li+), are selected to study the influence of varying cation types on the TE properties of PEDOT:PSS. The Seebeck coefficient and electrical conductivity of the PEDOT:PSS film treated with H2SO4EMIM:TFSI increase simultaneously, and the resulting maximum power factor is 46.7 μW·m-1·K-2, which may be attributed to the ionic liquid facilitating the rearrangement of the molecular chain of PEDOT. The work provides a reference for the development of organic films with high TE properties.
    Facile fabrication of highly flexible, porous PEDOT: PSS/SWCNTs films for thermoelectric applications
    Fu-Wei Liu(刘福伟), Fei Zhong(钟飞), Shi-Chao Wang(王世超), Wen-He Xie(谢文合), Xue Chen(陈雪), Ya-Ge Hu(胡亚歌), Yu-Ying Ge(葛钰莹), Yuan Gao(郜源), Lei Wang(王雷), and Zi-Qi Liang(梁子骐)
    Chin. Phys. B, 2022, 31 (2): 027303.   DOI: 10.1088/1674-1056/ac3502
    Abstract210)   HTML0)    PDF (1976KB)(61)      
    High-performance organic composite thermoelectric (TE) materials are considered as a promising alternative for harvesting heat energy. Herein, composite films of poly (3,4-ethyienedioxythiophene):poly(styrene sulfonate)/single-walled carbon nanotubes (PEDOT:PSS/SWCNTs) were fabricated by utilizing a convenient solution mixing method. Thereafter, the as-prepared hybrid films were treated using sulfuric acid (H2SO4) to further optimize the TE performance. Film morphological studies revealed that the sulfuric acid treated PEDOT:PSS/SWCNTs composite samples all possessed porous structures. Due to the successful fabrication of highly conductive networks, the porous nano-architecture also exhibited much more excellent TE properties when compared with the dense structure of the pristine samples. For the post-treated sample, a high power factor of 156.43 μW· m-1· K-2 can be achieved by adjusting the content of CNTs, which is approximately 3 orders of magnitude higher than that of the corresponding untreated samples (0.23 μW· m-1· K-2). Besides, the obtained films also showed excellent mechanical flexibility, owing to the porous nanostructure and the strong π-π interactions between the two components. This work indicates that the H2SO4 treatment could be a promising strategy for fabricating highly-flexible and porous PEDOT:PSS/SWCNTs films with high TE performances.
    Thermoelectric characteristics of flexible reduced graphene oxide/silver selenide nanowire composites prepared by a facile vacuum filtration process
    Zuo Xiao(肖佐), Yong Du(杜永), Qiufeng Meng(孟秋风), and Lei Wang(王磊)
    Chin. Phys. B, 2022, 31 (2): 028103.   DOI: 10.1088/1674-1056/ac447e
    Abstract186)   HTML0)    PDF (2384KB)(60)      
    The reduced graphene oxide/silver selenide nanowire (rGO/Ag2Se NW) composite powders were fabricated via a wet chemical approach, and then flexible rGO/Ag2Se NW composite film was prepared by a facile vacuum filtration method combined with cold-pressing treatment. A highest power factor of 228.88 μW·m-1·K-2 was obtained at 331 K for the cold-pressed rGO/Ag2Se NW composite film with 0.01 wt% rGO. The rGO/Ag2Se NW composite film revealed superior flexibility as the power factor retained 94.62% after bending for 500 times with a bending radius of 4 mm, which might be due to the interwoven network structures of Ag2Se NWs and pliability of rGO as well as nylon membrane. These results demonstrated that the GO/Ag2Se NW composite film has a potential for preparation of flexible thermoelectric devices.
    Enhancing the thermoelectric performance through the mutual interaction between conjugated polyelectrolytes and single-walled carbon nanotubes
    Shuxun Wan(万树勋), Zhongming Chen(陈忠明), Liping Hao(郝丽苹), Shichao Wang(王世超), Benzhang Li(李本章), Xiao Li(黎潇), Chengjun Pan(潘成军), and Lei Wang(王雷)
    Chin. Phys. B, 2022, 31 (2): 028104.   DOI: 10.1088/1674-1056/ac48f9
    Abstract181)   HTML3)    PDF (1723KB)(40)      
    We present a method of constructing composites composed of conjugated polyelectrolytes (CPEs) and single-walled carbon nanotubes (SWCNTs) to obtain a high-performing flexible thermoelectric generator. In this approach, three kinds of polymers, namely, poly[(1,4-(2,5-didodecyloxybenzene)-alt-2,5-thiophene] (P1), poly[(1,4-(2,5-bis-sodium butoxysulfonate-phenylene)-alt-2,5-thiophene] (P2), and poly[(1,4-(2,5-bis-acid butoxysulfonic-phenylene)-alt-2,5-thiophene] (P3) are designed, synthesized and complexed with SWCNTs as thermoelectric composites. The electrical conductivities of the CPEs/SWCNTs (P2/SWCNTs, and P3/SWCNTs) nanocomposites are much higher than those of non-CPEs/SWCNTs (P1/SWCNTs) nanocomposites. Among them, the electrical conductivity of P2/SWCNTs with a ratio of 1:4 reaches 3686 S·cm-1, which is 12.4 times that of P1/SWCNTs at the same SWCNT mass ratio. Moreover, CPEs/SWCNTs composites (P2/SWCNTs) display remarkably improved thermoelectric properties with the highest power factor (PF) of 163 μW·m-1·K-2. In addition, a thermoelectric generator is fabricated with P2/SWCNTs composite films, and the output power and power density of this generator reach 1.37 μW and 1.4 W·m-2 (cross-section) at ΔT=70 K. This result is over three times that of the thermoelectric generator composed of non-CPEs/SWCNTs composite films (P1/SWCNTs, 0.37 μW). The remarkably improved electrical conductivities and thermoelectric properties of the CPEs/SWCNTs composites (P2/SWCNTs) are attributed to the enhanced interaction. This method for constructing CPEs/SWCNTs composites can be applied to produce thermoelectric materials and devices.
    N-type core-shell heterostructured Bi2S3@Bi nanorods/polyaniline hybrids for stretchable thermoelectric generator
    Lu Yang(杨璐), Chenghao Liu(刘程浩), Yalong Wang(王亚龙), Pengcheng Zhu(朱鹏程), Yao Wang(王瑶), and Yuan Deng(邓元)
    Chin. Phys. B, 2022, 31 (2): 028204.   DOI: 10.1088/1674-1056/ac272e
    Abstract178)   HTML0)    PDF (3324KB)(34)      
    With the growing need on distributed power supply for portable electronics, energy harvesting from environment becomes a promising solution. Organic thermoelectric (TE) materials have advantages in intrinsic flexibility and low thermal conductivity, thus hold great prospect in applications as a flexible power generator from dissipated heat. Nevertheless, the weak electrical transport behaviors of organic TE materials have severely impeded their development. Moreover, compared with p-type organic TE materials, stable and high-performance n-type counterparts are more difficult to obtain. Here, we developed a n-type polyaniline-based hybrid with core-shell heterostructured Bi2S3@Bi nanorods as fillers, showing a Seebeck coefficient -159.4 μV/K at room temperature. Further, a couple of n/p legs from the PANI-based hybrids were integrated into an elastomer substrate forming a stretchable thermoelectric generator (TEG), whose function to output stable voltages responding to temperature differences has been demonstrated. The in situ output performance of the TEG under stretching could withstand up to 75% elongation, and stability test showed little degradation over a one-month period in the air. This study provides a promising strategy to develop stable and high thermopower organic TEGs harvesting heat from environment as long-term power supply.
    Donor-acceptor conjugated copolymer with high thermoelectric performance: A case study of the oxidation process within chemical doping
    Liangjun Chen(陈凉君), Wei Wang(王维), Shengqiang Xiao(肖生强), and Xinfeng Tang(唐新峰)
    Chin. Phys. B, 2022, 31 (2): 028507.   DOI: 10.1088/1674-1056/ac3504
    Abstract181)   HTML2)    PDF (1377KB)(48)      
    The doping process and thermoelectric properties of donor-acceptor (D-A) type copolymers are investigated with the representative poly([2,6'-4,8-di(5-ethylhexylthienyl)benzo[1,2-b;3,3-b] dithiophene]3-fluoro-2-[(2-ethylhexyl)-carbonyl]thieno[3,4-b]thiophenediyl)) (PTB7-Th). The PTB7-Th is doped by FeCl3 and only polarons are induced in its doped films. The results reveal that the electron-rich donor units within PTB7-Th lose electrons preferentially at the initial stage of the oxidation and then the acceptor units begin to be oxidized at a high doping concentration. The energy levels of polarons and the Fermi level of the doped PTB7-Th remain almost unchange with different doping levels. However, the morphology of the PTB7-Th films could be deteriorated as the doping levels are improved, which is one of the main reasons for the decrease of electrical conductivity at the later stage of doping. The best electrical conductivity and power factor are obtained to be 42.3 S·cm-1 and 33.9 μW·mK-2, respectively, in the doped PTB7-Th film at room temperature. The power factor is further improved to 38.3 μW·mK-2 at 75℃. This work may provide meaningful experience for development of D-A type thermoelectric copolymers and may further improve the doping efficiency.