Thermoelectric PEDOT:PSS Sheet/SWCNTs composites films with layered structure
Introduction
Heat and electricity are the two most common forms of energy in human life, thermoelectric (TE) devices can realize mutual conversion between the two without external force or noise, making them good candidates for alleviating the dilemma of environmental pollution and energy shortage [1,2]. The dimensionless figure-of-merit, ZT(=S2σ/κ, where S, T, σ and κ stand for Seebeck coefficient, absolute temperature, electrical conductivity and thermal conductivity, respectively.) is used to assess and compare the performance of TE materials [3]. Owing to the inherent low thermal conductivity of polymer and complexity to measure the thermal conductivity of film-type TE materials accurately, power factor (PF) S2σ can substitute for ZT [4,5]. Compared to the traditional inorganic thermoelectric materials such as Bi2Te3 [6], GeTe [7] and skutterudites [8], organic TE materials have advantages in flexibility, price and processing, having broad application prospects in wearable devices.
The most widely studied organic TE materials are Poly(3,4-ethylenedioxythiophene):poly (styrene sulfonate) (PEDOT:PSS), poly(3-hexylthiophene), polyaniline and polypyrrole. Among them, PEDOT:PSS is favored by numerous researchers due to its highest PF value so far [9]. PEDOT itself is an insoluble polymer which limits its application, the addition of PSS extremely improves its solubility in aqueous solvent [10,11]. However, the insulating PSS molecular chain wrapped in the outer layer of the core-shell structure inevitably damages the conductivity of PEDOT, certain physical or chemical approaches are adopted to remove part of the PSS. It is demonstrated that treating with polar organic solvent such as dimethyl sulfoxide (DMSO) [12], sodium hydroxide (NaOH) [13] and ethylene glycol (EG) [14] is a relatively simple and effective method to enhance the TE performance of PEDOT:PSS as well as its composites. The higher the content of conductive polymer chain PEDOT, the higher the conductivity of PEDOT:PSS based materials.
In recent years, significant efforts have been made on improving the PF value of PEDOT:PSS, incorporating carbon nanotubes into it enable the materials to maintain original flexibility, fitting the non-planar applications with low temperature gradients [15]. Moreover, there is ‘conduction bridge’ between PEDOT:PSS and intertubes which is effective in improving the electrical conductivity [16]. In conclusion, if the specific surface area of PEDOT:PSS could be increased through constructing special PEDOT:PSS structure, it is not only beneficial to the elution of PSS, but also forming special conductive network structure consisting of single wall carbon nanotubes (SWCNT) and polymer. This might be more conducive to the transmission of electrons. As previous reported for various other polymers [17], for the first time, we deform PEDOT:PSS colloidal particles into flakes under dynamic turbulent flow, as shown in Fig. 1.
Herein, we prepared pure PEDOT:PSS films and PEDOT:PSS sheet/SWCNT composite films by vacuum assisted filtration. Compared with the directly mixing of PEDOT:PSS and SWCNT, PEDOT:PSS sheet has a lager contact area with SWCNT. Interestingly, under the filtration force, the two components assemble to an ordered layered structures in which SWCNT wrapped PEDOT:PSS sheet tightly. Consequently, with further polar solvent treatment, this preparation method endowed PEDOT:PSS sheet/SWCNT thermoelectric with greatly enhanced electrical conductivity of 3085.4 S cm−1 and PF value of 224 μW m−1 K−2.
Section snippets
Materials
CLEVIOS PH1000 PEDOT:PSS aqueous solution (solid content of 1.25 wt%, PSS:PEDOT = 2.5:1) was produced by Heraeus GmbH, Germany. SWCNT (purity>90 wt%, diameter of 1–2 nm) was purchased from Shenzhen Nanotech Port Co. Ltd, China. All reagents including EG, Ethanol, H2SO4 and DMSO were AR grade (Chengdu Kelong Chemical Co. Ltd, China). Materials mentioned above were used without further treatment or purification.
Fabrication of PEDOT:PSS sheet/SWCNT composite films
PEDOT:PSS sheet/SWCNT composite films were fabricated according to the procedure shown
Results and discussion
Fig. 1a illustrates the typical preparation process for PEDOT:PSS sheet/SWCNT composites films, which includes three main stages: the pre-treatment(EG) for mixed solution of PEDOT:PSS/SWCNT & post-treatment(H2SO4/DMSO) for mixed suspension of PEDOT:PSS sheet and SWCNT, the construction process of PEDOT:PSS sheet, and the formation of PEDOT:PSS sheet/SWCNT films. Under rapid solvent-nonsolvent exchange, PEDOT:PSS particles are rather quickly precipitated from its mixed inferior solvent H2SO4
Conclusion
We fabricated high performance electrical conductive thermoelectric composite films by blending SWCNT with PEDOT:PSS sheets. These sheets were obtained by turbulent shearing PEDOT:PSS in non-solvent. Combined with thermoelectric measurements and a thorough analysis (SEM, Raman spectroscopy, optical microscope, and XPS), it is demonstrated that the obtained layered structure for the composite was well situated to charge transfer due to improved removal efficiency of PSS. With further EG and DMSO
CRediT authorship contribution statement
Xiaoyin Cao: Experiments, Data analysis, Writing – original draft, Writing – review & editing. Mao Zhang: Experiments, Data analysis, Writing – original draft, Writing – review & editing. Yan Yang: Resources, Conceptualization, Supervision. Hua Deng: Conceptualization, Methodology, Writing – review & editing, Supervision. Qiang Fu: Writing – review & editing, Supervision.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors gratefully acknowledge the financial support of this work by National Natural Science Foundation of China (51922071).
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Mao Zhang and Xiaoyin Cao contributed equally to this work.