Elsevier

Synthetic Metals

Volume 271, January 2021, 116628
Synthetic Metals

Morphological modulation to improve thermoelectric performances of PEDOT:PSS films by DMSO vapor post-treatment

https://doi.org/10.1016/j.synthmet.2020.116628Get rights and content

Highlights

  • DMSO vapor post-treatment was employed to promote the micro-structural order of PEDOT:PSS with 5% DMSO films.

  • The enhancement of micro-structural order could avoid the trade-offs between electrical conductivity and Seebeck coefficient.

  • Carrier mobility plays a significant role in the thermoelectric performance of organic materials.

Abstract

The thermoelectric performance of poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) films are optimized by dimethyl sulfoxide (DMSO) vapor post-treatment on the basis of DMSO addition in PEDOT:PSS solution. Compared with pristine films, the electrical conductivity of PEDOT:PSS with 5% DMSO films is increased by two orders of magnitude. When the PEDOT:PSS with 5% DMSO films are subjected to DMSO vapor post-treatment, the electrical conductivity and Seebeck coefficient are further enhanced. As a result, the power factor of 60 min treated PEDOT:PSS with 5% DMSO films is twice larger than the one without DMSO vapor treatment. Microscopical investigations reveal the morphological evolution of the DMSO vapor treated films. After certain of treating time, quite even and uniformed film morphology is developed, which indicates that film micro-structural order is improved. Through the study of carrier concentration by optical absorption spectra and XPS, it is confirmed that the improvements of thermoelectric performances are attributed to the enhancement of carrier mobility.

Introduction

Energy shortage and air pollution are considered as crucial issues for human being’s long-term development. Consensus has been reached that renewable and clean energy should be used in priority. Thermoelectric (TE) device, which enables direct conversion between heat and electricity via Seebeck and Peltier effects, is a promising candidate for new energy technology mainly attributed to the abundant waste heat and pollution-free conversion process [1], [2], [3]. Apart from those advantages, thermoelectric device is lightweight, compact, quiet, reliable and working without moving parts. Based on the above merits, extensive attention is being paid on TE technology [4], [5], [6]. Currently, the high performance TE materials are mainly inorganic semiconductors, such as alloys of Bi2Te3 and Sb2Te3 [7], [8]. However, their disadvantages, such as containing rare and toxic elements, high cost and processing difficulty, limit their extensive application. Conducting polymers (CPs) materials, characterized by abundant component elements in earth, low cost, mechanical flexibility, easy processing and light weight, are drawing increasing interest to develop the TE devices [9], [10], [11], [12], [13].

The energy conversion efficiency of TE devices is evaluated by a dimensionless factor called figure of merit, ZT, which is defined as:ZT=S2σκTwhere S, σ, κ, T are Seebeck coefficient, electrical conductivity, thermal conductivity and absolute temperature, respectively. Although considerable improvements of ZT based on CPs have been achieved, from 10−3 to 10−1, the efficiency of organic TE devices is still not high enough to meet the demand for practical commercialization [14], [15], [16]. Great efforts were put in the star molecule of PEDOT to improve its thermoelectric performance, including morphology and structure control, optimizing the doping level, nanocomposite strategy and surface energy filtering [17], [18], [19]. In contrast to the strategy for the improvement of ZT based on inorganic TE materials, which is aimed at reducing thermal conductivity [20], [21], [22], [23], the strategy for CPs focuses on enhancing Seebeck coefficient (S) and electrical conductivity (σ) to improve the power factor (S2σ) since CPs have intrinsically low thermal conductivity on the scale of 0.1–0.5 W m−1 K−1 [24], [25], [26]. Chemical [15], [27], [28] or electrochemical doping [29], [30] is the main method to optimize the power factor of CPs. On the one hand, doping enhances electrical conductivity by the increase of carrier concentration. On the other hand, however, it causes the Femi level to approach valence band (p-type), which decreases Seebeck coefficient since S is dependent on the energy difference between Fermi level and valence band edge [31]. Therefore, doping level needs to be controlled finely to achieve the balance between electrical conductivity and Seebeck coefficient in order to obtain the optimal power factor. Thus, control of doping level is limited for further enhancement of ZT due to the opposite dependence of electrical conductivity and Seebeck coefficient. Striving for decoupling the relationship between electrical conductivity and Seebeck coefficient is a more effective way to enhance power factor. Several reports have demonstrated the simultaneous increases in electrical conductivity and Seebeck coefficient, such as by adding ionic liquids [32] or urea [33], and physical dedoping [16]. In these cases, improvement in power factor is attributed to the enhancement of micro-structural order, which is beneficial for the delocalization of electron cloud and hopping of charge carriers between adjacent polymer chains. As a result, carrier mobility and electrical conductivity are improved. As far as Seebeck coefficient is concerned, owing to the formation of bipolaron network when the micro-structural order is enhanced [34], the density of states changes sharply around Fermi level, which promotes the enhancement of Seebeck coefficient according to the Mott equation [35]. Theory and experiments [36] have shown that enhancement of micro-structural order is a promising way to improve power factor of CPs. Solvent vapor post-treatment has been proved to be an effective way to control the morphology of organic semi-conductive films, especially in organic solar cells [37] and organic thin film transistors [38]. There are also some reports about the effect of polar-solvent vapor annealing on organic thermoelectric performance [39], [40]. However, the synergistic effect of secondary doping [41] and solvent vapor annealing on PEDOT:PSS films is rarely studied, and the influence of annealing time on thermoelectric property also need be researched further.

In this study, the films of PEDOT:PSS were spin-coated to study their TE properties. In the process of film formation, solvent was evaporated in a short time, which was disadvantageous to the micro-structural order of the solid films. Thereby, the thermoelectric performance of PEDOT:PSS films were optimized by two successive steps in our work: (i) 5% DMSO was added in the PEDOT:PSS pristine solution; (ii) DMSO vapor post-treatment was employed to improve the relative disorder morphology. Owing to the positive effect of DMSO vapor post-treatment, the power factor of PEDOT:PSS with 5% DMSO films was enhanced from 12.11 μW m−1 K−2 to 24.59 μW m−1 K−2. The relationship between the micro-structure modified by solvent vapor treatment and TE properties was also discussed.

Section snippets

Materials and preparation

An aqueous dispersion of PEDOT:PSS was purchased from an agent of Heraeus (product Clevios PH1000). DMSO was purchased from Sigma Aldrich. Solution of PEDOT:PSS with optimal 5% DMSO in volume ratio (PEDOT:PSS-5% DMSO) was prepared by adding DMSO into PEDOT:PSS aqueous and stirring over 12 h. Pristine PEDOT:PSS and PEDOT:PSS-5% DMSO solutions were spin-coated at 4000 rpm for 60 s on 20 mm × 20 mm square glasses, which were pre-cleaned successively by detergent, deionized water, acetone and

Results and discussions

The calculated electrical conductivity of pristine PEDOT:PSS and PEDOT:PSS-5% DMSO films at room temperature were 0.5–1 S cm−1 and 673.4 S cm−1, respectively. The huge improvement in electrical conductivity indicates the transformational morphology and conformation of the PEDOT:PSS film caused by DMSO addition [42], [43], which may enhance the molecular arrangement in order and further increase the carrier mobility. While in the process of thin film formation, fast evaporation of solvent may

Conclusion

In this work, the improvements of thermoelectric property of PEDOT:PSS were conducted by two successive steps. Firstly, 5% DMSO was added into the pristine solution, which enabled the increase of electrical conductivity by two orders of magnitude. Secondly, on the basis of DMSO additive, the resulted films were post-treated by DMSO vapor for certain of time. Consequently, the electrical conductivity and Seebeck coefficient were further enhanced. The power factor of 60 min treated films doubled

CRediT authorship contribution statement

Yabo Xu wrote the main manuscript and carried out most of the experiments and data analysis. Zemei Liu and Xiaozhen Wei helped measure and collect the experimental data. Bo Zhao and Hua Wang guided the progress of experiments and manuscript writing. Jinmeng Wu, Jingyun Guo, Shaoping Chen and Yinke Dou took part in the mechanism discussions. All authors reviewed the manuscript.

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.

Acknowledgments

This work was financial supported by the National Natural Science Foundation of China (61775155 and 61605137), Natural Science Foundation of Shanxi Province (201801D121017 and 201901D111116).

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