Preparation and thermoelectric properties of screen-printable rGO/Sb2Te3/SV4/PEDOT:PSS composite thermoelectric film

Because of the advantages of facile and fast preparation process, screen printing technology shows great potentials in the prospective large-scale production of thermoelectric materials. Herein, rGO/Sb2Te3 composite powders have been prepared by a hydrothermal process, and then flexible rGO/Sb2Te3/SV4/PEDOT:PSS composite films with different weights of rGO/Sb2Te3 composite powders have been prepared via a screen printing process. The effects of the contents of rGO/Sb2Te3 composite powders on thermoelectric properties of the rGO/Sb2Te3/SV4/PEDOT:PSS composite films have been studied. The Seebeck coefficients of the achieved composite films was basically unchanged with the content of rGO/Sb2Te3 composite powders increasing, whereas the electrical conductivities decrease, resulting in a maximal power factor of 2.96 μW/mK2 at 375 K for the composite film containing 85 wt.% rGO/Sb2Te3 composite powders. The cold pressing combining annealing process has been employed to improve the thermoelectric properties of the composite films. After the treatment, the electrical conductivity of the composite film with 85 wt.% rGO/Sb2Te3 powders has been significantly improved, while the corresponding Seebeck coefficient has slightly decreased. An optimal power factor of 14.13 μW/mK2 has been acquired at 375 K, which is ∼ 5 times higher when compared to the untreated composite film (2.96 μW/mK2 at 375 K).


Introduction
Thermoelectric (TE) technology can convert the low-grade heat into useful electrical power without causing additional pollutions [1,2]. Nowadays, TE materials have aroused great interests [3,4], mainly because of their TE performance significantly effecting conversion efficiencies of the assembled TE generators. The TE performance of a material is normally estimated by the dimensionless figure of merit ZT (=S 2 σT/κ, where S represents the Seebeck coefficient, σ is the electrical conductivity, κ stands for the thermal conductivity of the materials, and T is the absolute temperature) [5,6]. As a traditional TE material, antimony telluride (Sb 2 Te 3 ) has a great potential for the application in the TE areas [7]. For instance, Sb 2 Te 3 bulk materials were prepared via a hot press sintering approach, and a ZT=0.57 was acquired at 565 K [8]. Sb 2 Te 3 bulk samples were prepared via a spark plasma sintering (SPS) process, and a ZT=0.58 was achieved at 420 K [9]. Graphene possesses unique advantages, such as high values of specific surface area, carrier mobility, mechanical and electrical properties [10]. Normally, graphene can be used as fillers for Sb 2 Te 3 matrixes to prepare the corresponding composites [11].
Poly (3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS, PP) is a typical studied conducting polymer TE materials, on account of its low thermal conductivity, solution processability, good pliability and environmental stability [12]. However, the TE properties of PP are still far behind that of inorganic TE materials [13]. Mixing PP with inorganic TE materials is a valid approach to improve its ZT value. One of the reasons is that the inorganic TE materials have good TE performance [14][15][16] achieved at room temperature (RT) by decoupling the interdependent σ and S. Moreover, the energy filtration effect might be formed between inorganic and PP components [17,18], which is beneficial to enhance the ZT value of PP. For example, Kim et al [18] fabricated PP/Bi 2 Te 3 nanowire composite film, and a PF of∼ 205±18 μW/mK 2 was obtained at RT by adjusting the work function of PP, and the barrier energy between Bi 2 Te 3 nanowires and PP. Sb 2 Te 3 /PP composites have been previously prepared mainly in the form of bulk materials, for instance, Sb 2 Te 3 bulk samples were prepared by a SPS process [19], and then soaked the prepared Sb 2 Te 3 bulks into PP solution, consequently a ZT=1.18 was gained at 523 K for the Sb 2 Te 3 /PP composite bulk material. Recently, various methods were attempted to fabricate the flexible Sb 2 Te 3 /PP composite films, which are facile for flexible wearable TE generators. For example, Sb 2 Te 3 /PP composite films were prepared via an aerosol jet printing method [20], and a PF of ∼ 28.3 μW/mK 2 was acquired at RT for the Sb 2 Te 3 /PP sample with 85 wt.% Sb 2 Te 3 nanoflakes. Sb 2 Te 3 films on the polyimide substrate were prepared by a screen printing technology (SPT) [21], and then annealed it under N 2 atmosphere at 450°C, after that the annealing treated samples were covered with PP solution, and a ZT=0.2 ± 0.02 was received for the Sb 2 Te 3 /PP composite film at RT.
Considering the economy and the requirements of scale production, the cheap and high-throughput SPT has emerged as a promising fabrication route for preparation of flexible and wearable TE power generations [21][22][23][24][25]. While researches on the screen-printed rGO/Sb 2 Te 3 /SV4/PP (SV4 is an ink containing 1.5 wt.% PP) composite TE materials have not yet been reported. Herein, rGO/Sb 2 Te 3 composite powders are synthesized via a hydrothermal method (HM), and then flexible rGO/Sb 2 Te 3 /SV4/PP composite films are prepared via a SPT. The cold pressing combining annealing treatment process is applied to enhance the TE properties of the asprepared samples. The effects of the contents of rGO/Sb 2 Te 3 composite powders as well as post-treatment process on the microstructures and TE properties of the rGO/Sb 2 Te 3 /SV4/PP composite films are also investigated.

Synthesis of rGO/Sb 2 Te 3 composite powders
The rGO/Sb 2 Te 3 composite powders were prepared by a HM, and the details are as below. A certain weight of GO was put into deionized (DI) water (80 ml), and a solution A (GO dispersion) was obtained after sonicated for 2 h. An appropriate mass of SbCl 3 , TeO 2 , PVA, and KOH were put into the solution A and then stirred at RT for 1 h (solution B). The excessive KBH 4 was put into the solution B and stirred to form a uniform dispersion (solution C). The solution C was then transferred to a Teflon-lined autoclave (100 ml) and heated to 180°C and kept for 24 h. Afterwards, the autoclave was slowly cooled to RT, and the achieved precipitates were alternately washed for several times by absolute ethanol and DI water. In the end, the samples were dried in vacuum for 8 h at 60°C. The mass ratio of rGO was 0.4 wt.% in the rGO/Sb 2 Te 3 composite powders.
2.3. Preparation and post-treatment of rGO/Sb 2 Te 3 /SV4/PP composite films 0.6 ml SV4 slurry and 0.4 ml PP solution (with 5 wt.% DMSO) were put to a 10 ml beaker and then stirred for 1 h to form paste A. Afterwards, different mass ratio of rGO/Sb 2 Te 3 composite powders were put into the paste A,

Characterization and measurement
The X-ray diffraction (XRD, Bruker D8 Advance, Karlsruhe) was used to study the compositions of samples. The morphologies, thicknesses and energy-dispersive spectroscopy (EDS) mapping of the as-prepared films were characterized by scanning electron microscopy (SEM, Tescan Mira 3 XH, equiped with Oxford EDS AZtec X-MaxN 80). The σ and S of the samples were performed by an MRS-3 TE test system produced by Wuhan Giant Instrument Technology Co., Ltd. from 315 K to 375 K under low-vacuum ( 40 Pa), and the instrument test errors for the S is 7% and the σ is 5%. Before the measurement, the standard sample nickel tape was taken to calibrate the equipment to guarantee that the equipment is in normal working condition. The resistance of the rGO/Sb 2 Te 3 /SV4/PP composite films after being bent for different times was performed by a digital multimeter (VC9807A+).  characteristics of PP [27]. Besides, as the contents of the inorganic component rGO/Sb 2 Te 3 composite powders increased, the peak intensities of Sb 2 Te 3 for the composite films also enhanced. Figures 2(a)-(c) are the surface SEM images of rGO/Sb 2 Te 3 /SV4/PP composite films with different contents of rGO/Sb 2 Te 3 composite powders. With the loadings of rGO/Sb 2 Te 3 powders increased, the surface morphologies of the composite films became rougher, which might be due to the poor film-forming properties of rGO/Sb 2 Te 3 powders. This phenomenon is adverse to enhance the σ of the composite films. Figures 2(d)-(i) present the SEM image and SEM-EDS mappings of Sb, Te, C, O, and S elements for the SP85 composite film, manifesting that Sb 2 Te 3 and rGO are evenly distributed in the rGO/Sb 2 Te 3 /SV4/PP composite film. Figures 3(a)-(c) display the relationship between temperature and TE properties of rGO/Sb 2 Te 3 /SV4/PP composite films from 315 K to 375 K. As the mass fraction of rGO/Sb 2 Te 3 composite powders enhanced from 85 wt.% to 95 wt.%, the σ decreased from 24.3 S/ cm to 16.6 S/ cm at 315 K, whereas the S of the composite films was basically unchanged (e. g. the values of the S were in the ranges of 29.7 μV/K-32.2 μV/K at 315 K). The PF was decreased with the contents of rGO/Sb 2 Te 3 composite powders increasing, and a PF of 2.15 μW/mK 2 was obtained at 315 K for the SP85 sample. When the testing temperaturerosefrom 315 K to 375 K, the σ was not significantly changed, while the S increased constantly. For example, the S of SP85 sample increased from 29.7 μV/K to 35.1 μV/K when the temperature increasing from 315 K to 375 K, and a maximal PF of 2.96 μW/mK 2 was achieved at 375 K for the SP85. Figure 3(d) shows the relationship between resistance change rate and bending times of SP85 sample. The SP85 was cut into a rectangle shape (25×5 mm 2 ) and bent for several times with a radius of 11 mm. When the bending times increased from 100 to 400, the corresponding resistance change rate was slightly increased from 0.62% to 5.0%, indicating good flexibility of the composite film.

Results and discussion
The cold pressing combining with annealing treatment was introduced to improve TE properties of the rGO/Sb 2 Te 3 /SV4/PP composite films. The XRD patterns of the post-processed samples manifested that the peaks' position of Sb 2 Te 3 in the composite films was not obviously changed, while the peaks' intensities of Sb 2 Te 3 slightly increased. Figures 4(a)-(c) reveal the surface SEM images of rGO/Sb 2 Te 3 /SV4/PP composite films after being cold pressing combining with annealing treatment. Compared with the pristine composite films, there is no significantly variation for the morphology of the post-processed composite films. Figures 4(d) and (e) are the cross-section SEM images of the CASP series samples, revealing that the thicknesses of CASP series samples are lower than 1 μm. Figures 5(a)-(c) show the temperature dependence of TE parameters of the post-treated composite films. Compared with the untreated composite films, the σ was significantly increased, while the S was not significantly changed. A highest σ of the CASP85 sample was 298.46 S/ cm at 375 K, which is∼12 times higher compared to the untreated SP85 (24.03 S/ cm at 375 K). This phenomenon may be due to the dense structure of the treated films with the help of the cold pressing combining annealing treatment. As the test temperature rose from 315 K to 375 K, the σ remained unchanged, whereas the S showed an upward trend, and a maximum PF of 14.13 μW/mK 2 was gained at 375 K for CASP85, which is∼ 5 times higher compared to the SP85 (2.96 μW/mK 2 at 375 K). This value is also greater than the value of the (Bi,Sb) 2 (Te,Se) 3 -based film prepared by a SPT (∼ 6 μW/mK 2 at RT) [28], but lower than the values of the screen printable PP layer (34 μW/mK 2 at 200°C) [29], and the Sb 1.5 Bi 0.5 Te 3 film (∼ 77 μW/mK 2 at 75°C) prepared by a inkjet printing technology [30]. Table 1 compared TE properties of CASP85 with those of previously reported PP-based materials. Figure 5(d) presents the relationship between resistance change rate and bending times of CASP85. When the bending times were increased from 100 to 400, the resistance change rate of the post-treated CASP85 increased from 1.45% to 5.0%, indicating that the flexibility of the composite films is not obviously changed when compared with the untreated samples ( figure 3(d)).

Conclusions
In this work, rGO/Sb 2 Te 3 /SV4/PEDOT:PSS composite films have been prepared by a screen printing technique, and the effects of the content of rGO/Sb 2 Te 3 composite powders and post-treatment on the microstructures and TE properties of the composite films have been studied. As the mass fraction of rGO/Sb 2 Te 3 component increases from 85 wt.% to 95 wt.%, the electrical conductivity of the composite films decreases from 24.42 S/ cm to 15.92 S/ cm, while the Seebeck coefficients do not change much. When the content of rGO/Sb 2 Te 3 is 85 wt.%, a power factor of 2.96 μW/mK 2 has been acquired at 375 K. After the cold pressing combining with annealing treatment, the electrical conductivity increased to 298.46 S/cm for the  a) Some values were estimated as per the raw data in the corresponding References; b) CSA=camphorsulfonic acid; c) GQDs=graphene quantum dots.
composite film with 85 wt.% rGO/Sb 2 Te 3 . A power factor of 14.13 μW/mK 2 has been achieved at 375 K. This is a simple and easy-to-operate method for the preparation of thermoelectric composite films.