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A Simple Layer-by-Layer Assembly Method for Simultaneously Enhanced Electrical Conductivity and Thermopower of PEDOT:PSS/ce-MoS2 Heterostructure Films Xiaodong Wang, Fanling Meng, Qinglin Jiang, Weiqiang Zhou, Fengxing Jiang, Tongzhou Wang, Xia Li, Si Li, Yuancheng Lin, and Jingkun Xu ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00315 • Publication Date (Web): 13 Jun 2018 Downloaded from http://pubs.acs.org on June 14, 2018

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ACS Applied Energy Materials

A Simple Layer-by-Layer Assembly Method for Simultaneously Enhanced

Electrical

Conductivity

and

Thermopower

of

PEDOT:PSS/ce-MoS2 Heterostructure Films Xiaodong Wang,†,‡ Fanling Meng,‡ Qinglin Jiang,† Weiqiang Zhou,† Fengxing Jiang,*,† Tongzhou Wang,† Xia Li,† Si Li,‡ Yuancheng Lin,† and Jingkun Xu*,† †

Department of Physics, Jiangxi Science and Technology Normal University, Nanchang

330013, China. ‡

Key Laboratory of Automobile Materials, Ministry of Education, State Key Laboratory of

Inorganic Synthesis and Preparative Chemistry, College of Materials Science and Engineering, Jilin University, Changchun, 130012, China *The corresponding authors, email: [email protected] (F. Jiang); [email protected] and [email protected] (J. Xu) Abstract The organic/inorganic composites are considered as a promising strategy to gain high thermoelectric (TE) performance. Although many efforts have been focused on composites, complicated methods are real hindrance to the development of TE materials. Here, we demonstrate a potential TE thin film comprised of highly conductive poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and chemically exfoliated MoS2 (ce-MoS2) nanosheets by layer-by-layer (LbL) assembly method. This work achieves the effective integration of composite, treatment, and electron transfer based on the advantages of PEDOT:PSS and ce-MoS2. On the one hand, the negative

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charged ce-MoS2 nanosheets facilitate the formation of heterostructure TE films with positive charged PEDOT and reduce the oxidation level of PEDOT, which would favor an enhanced thermopower (21.9 µV K-1). On the other hand, the simultaneous enhancement in the electrical conductivity (867 S cm-1) of PEDOT:PSS/ce-MoS2 composite film is caused by dimethyl sulfoxide (DMSO) worked as the dispersion of ce-MoS2, which corresponds to the removal of the excess non-conductive PSS and the molecular conformation arrangement of PEDOT:PSS. This LbL assembly method incorporating rich-electron ce-MoS2 and DMSO has been confirmed to be an effective strategy to yield a simultaneous enhancement of electrical conductivity and thermopower. The optimized power factor is achieved to be 41.6 µW m-1 K-2 at the layer number of 4, which surpass that of the single layer PEDOT:PSS film by a factor of 5. This work may provide a fundamental understanding of and design principles on how to build PEDOT:PSS-based composite films with highly enhanced TE performance, which can be potentially used in TE energy harvesting systems. Key words: PEDOT:PSS, MoS2, layer-by-layer assembly, heterostructure, thermoelectric thin film

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Introduction Organic/inorganic composites have attracted wide attention in the field of new generation

energy

harvesting

technology

inclusive

of

super-capacitors,1

electrochromics,2 solar cells,3 thermoelectric (TE) conversion4. TE materials have played an important role in recovering low-grade waste heat into useful electrical energy based on Seebeck effect. The energy conversion efficiency of a TE material is generally estimated by the figure of merit, ZT = S2·σ· T/κ, where S, σ, κ, and T are the thermopower (Seebeck coefficient), electrical conductivity, thermal conductivity, and absolute temperature. As promising TE materials, organic semiconductors show high electrical conductivity, low thermal conductivity, and rich resources, while inorganic semiconductors often exhibit high thermopower.5 The organic/inorganic composites can integrate the bilateral benefits of the two individual components and offset their disadvantages.6-9 For obtaining a good composite, material and method are of important choices for the optimization of TE performance. Organic semiconductors served as alternative TE candidates have been widely recognized since the discovery of conducting polymers, such as polyaniline,10 polypyrrole,11

polythiophene,12-13

and

their

derivatives14.

Among

them,

poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) is one of the most fascinating polymers owing to its low thermal conductivity, good environmental stability, easy processing as well as adjustable electronic structure.15-16 Numerous approaches have been proposed to improve the TE performance of PEDOT:PSS. Kim et

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al.17 firstly reported an enhanced electrical conductivity (80 S cm-1) of PEDOT:PSS accompanied with a low thermopower of ~10 µV K-1 by adding dimethyl sulfoxide (DMSO). Xiong et al.18 found that the organic solvents dilution filtration could improve the electrical conductivity of PEDOT:PSS (~1500 S cm-1) by three orders of magnitude with a changeless thermopower (~15 µV K-1). It has been proved that the addition of high boiling organic solvents such as DMSO19, EG20, and IL21 enable the molecular conformation rearrangement, and therefore significantly improve the electrical conductivity of PEDOT:PSS. Additionally, the post-treatment processing with concentrated H2SO4,22 H3PO4,23 DMSO,24 and EG25 can also effectively remove the insulated PSS chains from the surface of PEDOT:PSS film, giving rise to a highly enhanced electrical conductivity. Unfortunately, many effects focused on the unilateral improvement of electrical conductivity and overlooked the crucial thermopower for the development of high performance PEDOT:PSS TE film. For the improvement of thermopower, one of the most common methods is to incorporate with inorganic high thermopower nano-materials, such as Bi2Te3,26 Te,27 Ca3Co4O928 and so on. Du et al.29 fabricated PEDOT:PSS/Bi2Te3 composite and obtained an enhanced thermopower from 9.5 to 15.8 μV K-1. Ju et al.30 reported that the thermopower of the composite films can be significantly improved to 110 μV K-1 by adding 20% SnSe nanosheets into the PEDOT:PSS matrix. Nevertheless, the electrical conductivity dramatically decreased to be 320 S cm-1, which is significantly lower than the high value of 1500 S cm-1. It is worth noting that the optimization of oxidation level is also a common method to tune the thermopower of PEDOT:PSS by reducing agent,

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including N2H4,31-32 NaBH4,33 TDAE34 and so on. Although both of the methods can effectively improve the thermopower of PEDOT:PSS, they also greatly reduce the electrical conductivity leading to an increased Joule heat loss.35 Factually, it is difficult to realize the simultaneous improvement of thermopower and electrical conductivity by a single processing or composite method. Generally, the conflict limited the improvement of power factor (P = σ·S2) is that the electrical conductivity and thermopower inversely depend on the carrier concentration.36 Therefore, it is desired to find a good material and an effective composite strategy for the optimization of TE performance of organic/inorganic composites. In this work, we proposed a simple strategy combining the composite, solvent treatment, and electron optimization by layer-by-layer (LbL) assembly technology to address the simultaneous improvement of electrical conductivity and thermopower of PEDOT:PSS film. The chemically exfoliated MoS2 (ce-MoS2) nanosheets were prepared by lithium intercalation method and homogeneously dispersed in DMSO solution. A series of PEDOT:PSS/ce-MoS2 heterostructure TE films have been fabricated based on interfacial electrostatic adsorption. The effect of the electron-rich ce-MoS2 and the solvent treatment on the TE enhancement of PEDOT:PSS has been systemically analyzed. Experimental Materials PEDOT:PSS aqueous solution (Clevios PH 1000) with the PSS to PEDOT weight ratio about 2.5 were purchased from HC Stark (Germany). Hexane, concentrated sulfuric acid (H2SO4), hydrogen peroxide (H2O2), and several kinds of organic solvents such as

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methanol (MeOH), ethanol (EtOH), ethylene glycol (EG), and dimethyl sulfoxide (DMSO) were purchased from Sinopharm Chemical Reagent Co., Ltd. MoS2 powder and n-butyllithium solution (1.6 M in hexanes) were obtained from Sigma-Aldrich. All chemicals were directly used without any purification. The exfoliation of MoS2 nanosheets The chemically exfoliated MoS2 (abbreviated ce-MoS2) nanosheets were obtained based on the previously literature.37-38 In brief, the Lithium intercalation that 3.0 g MoS2 powders were mixed with 20 mL of 1.6 M butyllithium solution in hexane. This mixture was kept at 80 oC for two days under the nitrogen gas protection. After removing the excess lithium and organic residues by centrifugation and washing with hexane, the as-prepared [LixMoS2] was dispersed into deionized water assisted by ultrasonic for about 1.0 h immediately. The as-obtained aqueous was centrifugation at 8000rpm for several times to separate the byproducts formed such as LiOH and the un-exfoliated sample, until the aqueous dispersion turned neutral. LbL assembly PEDOT:PSS/ce-MoS2 thin films PEDOT:PSS (Clevios PH1000) solution with 5.0 vol.% EG was stirred for 24 h at room temperature to obtain a homogeneous suspension. While the ce-MoS2 nanosheets were dispersed in DMSO (ce-MoS2/DMSO) or H2O (ce-MoS2/H2O) for obtaining a stable and uniform 0.1 mg/mL suspension solution. The PEDOT:PSS/ce-MoS2 layer-by-layer (LbL) assembly films were prepared by alternative spin-coating PEDOT:PSS and ce-MoS2 under the spin speed of 1500 rpm for 30 s onto the 1.5×1.5 cm2 glass substrates, which were pre-treated by Piranha solution (the volume ratio of H2SO4:H2O2 is 7:3). All intermediate

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films were annealed at 80 oC for 15 min, before spin-coating the next layer. The resulting composite films were vacuum dried under the 80 oC for 1.0 h and 120 oC for 15 min. Characterization X-ray diffraction (XRD) analysis were carried out by DX–2700 B X-ray diffractometer (Dandong Haoyuan Inc.) with Cu-Kα radiation (λ=0.15418 nm) at 40 kV and 40 mA (2θ=4° ~20°) at a scanning rate of 2° min-1. X-ray photoelectron spectroscopy (XPS) analysis were detected by using an ESCALAB 250XI photoelectron spectrometer (Thermo Fisher Scientific, USA). Atomic force microscopy (AFM) was obtained by the Bruker Dimension icon ScanAsyst (Veeco Multimode, Plainview, NY, USA). Raman spectra were recorded on a Lab RAM HR Evolution Raman spectrometer (HORIBA Scientific, France) with a 532 nm laser. UV-vis adsorption spectra were recorded using a Shimadzu UV-3600 spectrophotometer. ESR spectra were performed by using a JES-FA200 ESR Spectrometry (JEOL, Japan). Zeta potential measurement was taken by using a SZ-100 nanoparticle series instrument (HORIBA Scientific, France). The thicknesses of single layer PEDOT:PSS and ce-MoS2 are 108±8 nm and 15±4 nm according to the measurement of profilometer (DektakXT Surface Profiler, Bruker) with a tip radius of 2.5 μm, and the thicknesses of the PEDOT:PSS/ce-MoS2 thin films are step-wise evolution accordingly. Measurements For the measurement of electrical conductivity, a standard four-point probe technique with a Keithley 2700 meter was used to measure the resistance (R). The electrical conductivity (σ) is calculated by the σ=L/(R×A), which L, A are the length and the cross-section area of the thin film sample. All the samples were tailored into the

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required size (12.0×5.0 mm2) for TE measurement. For the measurement of thermopower, the sample was glued to the two cross-linked glass substrates by silver paste. A temperature difference ∆T between the two ends of the sample was produced by a 1000 Ohm resistive heater, which was controlled by a keithley 2401 source meter. And a pair of platinum thermometers (Pt100) was fixed at the sample by silver paste to detect the temperature difference ∆T. For reducing the influence of heat radiation on Pt100, the heater was fixed at the end of the hot side. The induced thermoelectric voltage ∆V was extracted by a pair of copper wires glued to the sample by the silver paste. The thermopower of the copper wires was ignored during the measurement. The ∆T and ∆V were measured by a Keithley 2700 meter, which is controlled by a computer equipped with a LabVIEW software. A temperature different ∆T was kept at 5.0±0.5 K by adjusting the output power of the heater. Finally, the thermopower is defined as S = -∆V/∆T. Results and discussion LbL assembly PEDOT:PSS/ce-MoS2 multilayer films

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Scheme 1. The conformation evolution of PEDOT:PSS caused by chemical treatment (a) pristine PEDOT:PSS, (b) solvent added PEDOT:PSS, and (c) DMSO post-treated PEDOT:PSS; The exfoliation process of MoS2 (d) 2H phase MoS2, (e) LixMoS2, and (f) 1T phase MoS2; (g) The sketch of the spin-coating setup and the diagram of the PEDOT:PSS/ce-MoS2 heterostructure TE films. As we know, the pristine PEDOT:PSS solution are comprised of the positive charged PEDOT chains and negative charged PSS chains.39 And the electrical conductivity of PEDOT:PSS is strongly dependent on the molecular conformation. The reason for the

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intrinsic low electric transport property of PEDOT:PSS is that the conducting PEDOT cores are surrounded by the non-conducting PSS shells, resulting in a coil conformation arrangement as shown in Scheme 1a.40-41 It has been validated that the addition of organic solvent can drive molecular conformation arrangement in PEDOT:PSS film (see Scheme 1b), leading to an increased interaction between PEDOT inter-chains.17 Therefore, four kinds of organic solvents such as MeOH, EtOH, EG, and DMSO were selected as additives for a comparison with the same volume fraction of 5%. Scheme 1c presents that more positive charged PEDOT chains can be exposed outside after solvent post-treatment, resulting from the selective removal of the non-conducting PSS chains from the PEDOT:PSS surface.24 Scheme 1d-f present the preparation procedure of ce-MoS2 by lithium-intercalation exfoliated method. The methods commonly applied for producing high-quality MoS2 nanosheets include liquid exfoliation,36,

42

chemical exfoliation,43-44 chemical vapor

deposition (CVD),45 and hydrothermal synthesis46. The lithium intercalation has been regarded as one of the most common methods owing to (i) lithium intercalation method is one of the most widely efficient methods to obtain 1T phase MoS2 (1T-MoS2) (i.e. named ce-MoS2 in this work) nanosheets from 3D 2H phase MoS2 (2H-MoS2) powder (i.e. monolayers of 1T-MoS2 are metallic, whereas monolayers of 2H-MoS2 are semi-conducting);47 (ii) The lithium intercalation method exfoliated nanosheets of MoS2 containing a high concentration of metallic 1T phase ce-MoS2 show superior thermoelectric properties, with a room temperature power factor of 73.1 μWm-1K-2, which is two orders of magnitude higher than that of 2H-MoS2 (0.5 μWm-1K-2);38, 48 (iii)

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Consistent with the previous report, the Zeta potential of -50 mV proves the negatively charged property of the ce-MoS2.49 Meanwhile, Wang et al.50 reported that the ce-MoS2 also has a strong reducing property, which can directly reduce the noble metal cations such as Au3+, Ag+, Pd2+ and Pt4+. It means that ce-MoS2 can be used as an effective reducing agent. Furthermore, noted that spin-coating is a fast and easy method to generate uniform composite thin film on the substrate with a specific thickness, which has been widely concerned in new generation energy harvesting system including solar cells,51 photovoltaic devices52, field effect transistors,53 thermoelectrics,54 etc. Therefore, this piece of knowledge opens the door for a rational design of the PEDOT:PSS/ce-MoS2 heterostructure TE films, using the spin-coating assisted LbL assembly method. Since the ce-MoS2 exhibits negative charge, the contact of the ce-MoS2 and the positive charged PEDOT results in a spontaneous electrostatic adsorption, which follows the form of an initial interconnected interfaces. It is desired that ce-MoS2 could optimize the oxidation level of PEDOT, thus optimizing the thermopower of PEDOT:PSS film. In order to remove insulating PSS, ce-MoS2 were dispersed in the DMSO solvent (ce-MoS2/DMSO). During the LbL assembling process, it is equivalent to the post-treatment solvent to PEDOT:PSS. To effective combine the aforementioned two processes, we fabricated a series of PEDOT:PSS/ce-MoS2 thin films by alternative spin-coating EG added PEDOT:PSS and ce-MoS2/DMSO in Scheme 1g. Structural characterization of the PEDOT:PSS/ce-MoS2 thin films

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Figure 1. (a) XRD spectra of ce-MoS2 and MoS2 powder; The fitted curve of Mo 3d (b) and S 2p (c) XPS spectra; (d) XPS spectra of PEDOT:PSS/ce-MoS2 thin film. Figure 1a displays the XRD spectra of the ce-MoS2 and MoS2 power. The position of the (002) peak (2θ=14.4°) of ce-MoS2 is consistent with that of MoS2 power (JCPDS NO. 37-1492), which reveals an interlayer spacing of 6.15 Å.48 While the broad peaks of (002) and (001) (2θ=7.3°) can be attributed to the widening of the interlayer spacing and the randomly arrangement.44 These results indicate that the ce-MoS2 nanosheets have been successfully exfoliated by lithium intercalation method. Furthermore, Figure 1b and 1c demonstrate the fitted curves of Mo 3d and S 2p from XPS spectra.50 The exfoliation results in the transformation from semiconducting 2H phase (blue curve) with trigonal 12


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prismatic to the metallic 1T phase (red curve) with the octahedral.37, 50 Noted that the peaks corresponding to 1T phase ce-MoS2 show about 0.8 eV red shift compared with that of 2H phase. According to the area integral based on the Mo 3d and S 2p regions, the 2H/1T ratio is calculated to be 2.1. It means that the method of lithium intercalation can effective exfoliate the MoS2 powder (2H phase) to ce-MoS2 nanosheets (1T phase), and the content of 1T phase ce-MoS2 is 68%. Figure 1d confirms that the as-prepared films content the PEDOT:PSS and MoS2 according to the corresponding elements of C, O, Mo and S. The relative low intensity of Mo indicates that only a small amount of ce-MoS2 were assembled on the surface of PEDOT:PSS.

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Figure 2. 3D AFM morphology images of (a) single PEDOT:PSS layer, (b) PEDOT:PSS/ce-MoS2

bilayer,

(c)

PEDOT:PSS/ce-MoS2/PEDOT:PSS

layer

and

(d)

PEDOT:PSS/ce-MoS2 quad-layer films. The morphology images of the PEDOT:PSS/ce-MoS2 composite films were obtained by atomic force microscope (AFM) and the results were presented in Figure 2 and Figure S1 (Supporting Information). It can be clearly seen from Figure 2a and Figure S1a that the single PEDOT:PSS layer prepared by spin-coating method has a smooth surface morphology with the root-mean-square (RMS) roughness value measured to be 1.19 nm, which is smaller than that of the PEDOT:PSS film prepared by dilution filtration method.18 The striking different between the bright regions (PEDOT-rich) and dark regions (PSS-rich) can be founded, which indicates the phase separation caused by EG solvent addition. This is consistent with the previous reports.55 While a more distinct phase boundaries and a rougher surface morphology can be seen from the PEDOT:PSS/ce-MoS2 bilayer (i.e. spin-coating the second layer of ce-MoS2/DMSO) in Figure 2b and Figure S1b compared with that of the single PEDOT:PSS layer. On the one hand, DMSO as the dispersion worked as a post-treatment solvent, resulting in a reduction of PSS chains and an aggregation of PEDOT chains.56 On the other hand, the ce-MoS2 nanoparticles were uniformly dispersed on the surface of PEDOT:PSS, which increases the RMS to 1.85 nm of the composite bilayer. The morphology images and the RMS roughness values of the PEDOT:PSS/ce-MoS2/PEDOT:PSS and PEDOT:PSS/ce-MoS2 quad-layer films were illustrated in Figure 2c and 2d, which is consistent with the evolution from the single PEDOT:PSS layer to the PEODT:PSS/ce-MoS2 bilayer films. As

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the third layer of PEODT:PSS is deposited onto the PEDOT:PSS/ce-MoS2 bilayer, its granular structure is lost, giving way to smooth surface. The RMS roughness value of the PEDOT:PSS/ce-MoS2/PEDOT:PSS film in Figure S1c was measured to be 0.56 nm, which suggests a good smoothness and uniformity. While this value further increased to 1.78 nm after spin-coating the fourth layer of ce-MoS2 (seen from Figure S1d). And it should be noted that the obvious small bright region seen from the Figure 2d further proves the phase separation of PEDOT:PSS and the agglomeration of PEDOT chains, which is beneficial for the electron transport property of the composite films. Thermoelectric performance of the PEDOT:PSS/ce-MoS2 thin films

Figure 3. The electrical conductivity (σ) of (a) pristine PEDOT:PSS added with different organic solvents and (b) PEDOT:PSS/ce-MoS2 thin films with error bar depending on the 15



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number of layers; (c) XPS spectra of pristine PEDOT:PSS, EG added PEDOT:PSS, PEDOT:PSS/ce-MoS2 bilayers in H2O and DMSO systems. (d) The fitted curve of XPS spectra of PEDOT:PSS/ce-MoS2 bilayers in DMSO system. Figure 3a presents the electrical conductivity of pristine PEDOT:PSS added with different kinds of organic solvents (volume fraction of 5%). It suggests that the electrical conductivity of PEDOT:PSS can be effectively improved by adding EG (647 S cm-1) or DMSO (629 S cm-1), which is consistent with previous reports.25,

57

It has been

demonstrated that polar organic solvents such as EG and DMSO can induce a screening effect to reduce Coulomb interactions between the inversed charged PEDOT and PSS inter-chains. Obviously that MeOH and EtOH possess neither two or more polar groups nor strong dipole moment show little effect on the electrical conductivity of PEDOT:PSS. Hence, we chose EG added PEDOT:PSS as organic layer for obtaining a higher electrical conductivity than other solvents. Then the evolution of electrical conductivity of PEDOT:PSS/ce-MoS2 thin films was illustrated in Figure 3b depending on the number of the layers of PEDOT:PSS/EG and ce-MoS2. The red points symbolize the ce-MoS2 dispersed in DMSO, while the blue points correspond to ce-MoS2 dispersed in H2O. Seen that the first step makes an electrical conductivity of 647 S cm-1 for a single layer PEDOT:PSS, which increases to 835 S cm-1 after spin-coating of ce-MoS2/DMSO. According to the measurement of profilometer, the average thickness of the composite films also increases from 108 nm for a PEDOT:PSS single layer to around 123 nm for a PEDOT:PSS/ce-MoS2 bilayer, i.e. PEDOT:PSS is the major component. The step-wise evolution of LbL assembly films lead

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to an alternating value of electrical conductivity. Furthermore, after the third step (the spin-coating of PEDOT:PSS/EG), the electrical conductivity slightly decreases to 703 S cm-1. It was founded that by using the ce-MoS2/DMSO to assemble the PEDOT:PSS/ce-MoS2 thin film, its electrical conductivity shows a significant increase with the increasing number of layers. When the number of layers exceeds 6, the electrical conductivity trends to be stable, attaining approximately at 900 S cm-1. Similar regular can be founded in Figure 3b, where H2O was used as the dispersion of ce-MoS2. However, when using the ce-MoS2/H2O as inorganic layer, the electrical conductivity of PEDOT:PSS/ce-MoS2 thin films presents a dramatically decreasing trend, which is much lower than that assembled by ce-MoS2/DMSO. In order to clarify the mechanism of the enhancement of electrical conductivity, we investigated the XPS analysis and the results were shown in Figure 3c. Based on the distinguished signal between sulfonate moiety of PSS (ranging from 166 to 172 eV) and thiophene ring of PEDOT (ranging from 162 to 166 eV), the sulfonate/thiophene (RS/T) calculation reflects the corresponding ratio of PSS to PEDOT.27 The XPS spectra studies indicate the surface ratio RS/T of 2.52 ± 0.06, 1.24 ± 0.06, 1.21 ± 0.04, 1.06 ± 0.03 for pristine PEDOT:PSS, EG added PEDOT:PSS (PEDOT:PSS-EG), PEDOT:PSS/ce-MoS2 bilayers in H2O and DMSO systems, respectively. Noted that the pristine PEDOT:PSS is full of the non-conducting PSS chains, which seriously affect the electronic transport property.18 The decreasing ratio of RS/T after EG addition can be attributed to the organic effects drive the molecular conformation arrangement of PEDOT.57 A similar result can be observed, when spin-coating ce-MoS2 in H2O as second layer. The RS/T exhibits a slightly

17



decrease from 1.24 ± 0.06 to 1.21 ± 0.04, which means that ce-MoS2 in H2O has little impact on the removal of PSS chains. The corresponding decrease in electrical conductivity were mainly caused by the poor electronic transport property of ce-MoS2. According to the fitted curve of the XPS spectra (Figure 3d) of PEDOT:PSS/ce-MoS2 bilayers in DMSO system, the RS/T further decreased to 1.06 ± 0.03, indicating a further removal of PSS chains after spin-coating the ce-MoS2/DMSO.24 Additional, these results demonstrate that the use of ce-MoS2/DMSO to assemble the PEDOT:PSS/ce-MoS2 thin films can effectively remove part of the non-conducting PSS chains, giving rise to an enhanced electrical conductivity. The main reason for the remarkable increase in electrical conductivity is the solvent effect to the PEDOT:PSS.

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Figure 4. (a) The thermopower of the PEDOT:PSS/ce-MoS2 thin films with the error bar depending on the number of layers; the band diagrams of the ce-MoS2 (grey region) and PEDOT:PSS (blue region) layers before (b) and after (c) the alignment at the heterojunction, the enlarge imaging in Figure 4c is the diagram of energy filtering process. Figure 4a presents the thermopower of the PEDOT:PSS/ce-MoS2 thin films depending on the number of layers. It can be seen that the EG added PEDOT:PSS as first layer exhibits a low thermopower of 11.4 µV K-1 similar to that of pristine PEDOT:PSS (14.1 µV K-1), indicating that EG does not impact the oxidation level of PEDOT.58 When ce-MoS2/DMSO as second step, the thermopower of the PEDOT:PSS/ce-MoS2 thin films shows a sharp increase to 16.2 µV K-1. While with increasing the number of layers, the thermopower of the PEDOT:PSS/ce-MoS2 thin films maintains at a high level and reaches the maximum value of 21.9 µV K-1 at the layer number of 4. This enhancement of thermopower is slightly higher than that of the directly exfoliated 2H-MoS2 composite with PEDOT:PSS thin film.36 Additionally, when ce-MoS2/H2O was used, the thermopower shows a slightly increase, which is similar to the previous reported value of 19.5 µV K-1. The increase in thermopower of the PEDOT:PSS/ce-MoS2 thin films can be attributed to two reasons. Firstly, the pure ce-MoS2 thin films show a high Seebeck coefficient of 80 µV K-1, which is agreement with the previous reports.59 Such a high thermopower of ce-MoS2 makes it a potential candidate to optimize the thermopower of PEDOT:PSS. The highly enhanced thermopower of the composite film is mainly due to the energy filtering effect.35, 60 Figure 4b reveals the theoretical calculation of energy band for

19



ce-MoS261 and PEDOT:PSS62. Noted that the different Fermi level will cause an energy mismatch between ce-MoS2 and PEDOT:PSS, which is conductive to the formation of potential energy barrier.63 According to the Anderson’s rule, the electrons transfer at the heterojunction results in an aligned Fermi level as shown in Figure 4c.62 The presence of the energy potential barrier will cause the energy filtering process in which the high energy carriers preferentially participates in the transport, while the low energy charge carriers are prevented by the energy barrier, resulting in an enhanced thermopower.64 Secondly, we found that the PEDOT:PSS/ce-MoS2 exhibits a higher thermopower than that of the previous reported PEDOT:PSS/2H-MoS2. It means that the negative charged 1T-MoS2 must play an important role on the improvement of thermopower of PEDOT:PSS than 2H-MoS2. The mechanism was proposed in Figure 5a. On the one hand, the removal of PSS by DMSO post-treatment must play an important role in the formation of interface electrostatic interaction, during which part of the PSS- may be replaced by ce-MoS2- (proved by XPS analysis in Figure 3c). On the other hand, the strong reducing property of the negative charged ce-MoS2 may facilitate the electrons transfer, reducing the oxidation level of PEDOT towards a neutral state.31 We believe that the essential way to effectively improve the TE performance of PEDOT:PSS is to replace the position of non-conductive PSS with inorganic high thermopower nanomaterials, resulting in a simultaneously enhanced electrical conductivity and thermopower.

20


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Figure 5. (a) Scheme of the de-doping of PEDOT by ce-MoS2; (b) Raman spectra of the pristine, EG added, PEDOT:PSS/ce-MoS2 bilayers in H2O and DMSO systems; (c) UV-vis spectra of the EG added PEDOT:PSS, DMSO, PEDOT:PSS/ce-MoS2 bilayers in H2O and DMSO systems; (d) ESR spectra of the pristine PEDOT:PSS, EG added PEDOT:PSS and PEDOT:PSS/ce-MoS2 bilayers in DMSO system. Raman spectra show the corresponding evidences in Figure 5b. Noted that the Cα=Cβ stretching vibration of benzenoid thiophene ring shows a slightly shift to red after adding EG solvent, which means the conformation of PEDOT changed from coil to linear structure caused by the phase separation between PEDOT and PSS chains.20 Furthermore, it shifts to a higher frequency Raman band of 1420 cm-1 after combining

21



the ce-MoS2, and thus validate that a decreasing doping level of PEDOT from bipolaron to polaron or neutral state.32-33 Noted that the post-treated by ce-MoS2/DMSO means the fabrication of PEDOT:PSS/ce-MoS2 bilayer film. The removal of PSS chains by DMSO may promote the dedoping process of ce-MoS2, suggesting that ce-MoS2/DMSO exhibits a superior effect on the improvement of thermopower of PEDOT:PSS than ce-MoS2/H2O. UV-vis spectra further confirm the removal of the PSS and the reduction of the oxidation level of PEDOT. The decreased intensity of the absorption bands around 190 and 230 nm corresponding to the aromatic ring of PSS indicates that both DMSO and ce-MoS2 can effectively induce the removal of the PSS molecules.36 Furthermore, the lowest intensity referred to PEDOT:PSS/ce-MoS2 bilayer illustrates a lowest PSS content, indicating the synergistic effect between DMSO and ce-MoS2 on the removal of PSS. This result is consistent with the enhanced electrical conductivity of the bilayer thin film. The visible region at 600 nm is corresponding to neutral state of PEDOT, while the absorption band around 900 nm are caused by the polaron and bipolaron of PEDOT.33 It can be seen from the inset of Figure 5c, the intensity of the broad IR absorption background shows a significant decrease. The formation of PEDOT:PSS/ce-MoS2 bilayer makes a significantly decrease intensity at 900 nm and a new optical transition at 600 nm. These signals might indicate the electron transfer happened, or even a direct reduction of the positive charged PEDOT chains by the ce-MoS2/DMSO, which corresponds well with the Raman analysis. In addition, the ESR spectra were depicted in Figure 5d. Noted that the polarion of PEDOT is in favor of forming the S=1/2 magnetic spins, while the bipoarion and neutral state of PEDOT is preferred to exhibit the spinless.65 The pristine PEDOT:PSS

22


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shows a low ESR signal, indicating the existence of polarons and bipolarons segments.66 The intensity of ESR signal increased with the addition of EG into PEDOT:PSS, owing to the change of conformation.67 While the ESR signal of PEDOT:PSS/ce-MoS2 composite film dropped down approximately 42% compared with EG-added PEDOT:PSS. This result also confirms the ce-MoS2 can induce the conformation arrangement described above. The highly enhanced thermopower of the PEDOT:PSS/ce-MoS2 thin film can be explained by the evolution of the spineless neutral state of PEDOT. This result is in good agreement with the results of Raman and UV-Vis, indicating a direct charge transfer from the negative charged ce-MoS2 to the positive charged PEDOT chains during LbL assembly process.

Figure 6. The power factor (P) of the PEDOT:PSS/ce-MoS2 thin films with error bar depending on the number of layers. Based on the LbL assembly method, we prepared PEDOT:PSS/ce-MoS2 heterostructure TE films, and effectively integrated the solvent post-treatment and

23



Page 24 of 37

electron transfer to realize the simultaneously improvement of the electrical conductivity and thermopower. Figure 6 displays the power factor of the PEDOT:PSS/ce-MoS2 thin films as a function of the number of layers. It can be seen that the power factor of the PEDOT:PSS/ce-MoS2 thin films assembled by PEDOT:PSS/EG and ce-MoS2/DMSO show a dramatically increased trend, benefiting from the simultaneously increased electrical conductivity and thermopower. However, when use of ce-MoS2/H2O, the power factor shows a slightly variation resulting from the opposite changing trend between electrical conductivity and the thermopower. Accordingly, the four-layered PEDOT:PSS/ce-MoS2 thin films presents the highest power factor, amounting to 41.6 µW m-1 K-2, which is 5 times higher than that of single-layered PEDOT:PSS. The corresponding optimized electrical conductivity and thermopower were 867 S cm-1 and 21.9 µV K-1, respectively. Table 1 presents the TE properties of PEDOT:PSS composite with different 2D inorganic nanocomposites via different film-forming methods. Although further optimization is still required such as the transformation of the composite films from glass to flexible substrate and the assembly of flexible TE devices. This method is able to provide a new insight into development of highly thermoelectric performance for composite with potential application in energy harvesting. Table 1. Thermoelectric properties of PEDOT:PSS composites with different 2D inorganic nanomaterials. σ

P

(μV K )

(S cm-1)

(μW m-1 K-2 )

S Method

System

-1

Refs.

Vacuum filtration

PEDOT:PSS/graphene

17.3

1284

38.4

7

Vacuum filtration

PEDOT:PSS/Ca3Co4O9

18.5

1240

42.4

28

24


Page 25 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Vacuum filtration

PEDOT:PSS/WS2

21.0

1025

45.2

68

Vacuum filtration

PEDOT:PSS/MoS2

19.5

1200

45.6

36

Vacuum filtration

PEDOT:PSS/BNNSs

23.2

1266

68.1

9

Drop-casting

PEDOT:PSS/rGO

16.7

1160

32.6

69

Drop-casting

PEDOT:PSS/SnSe

110

326

394.5

30

Spin-coating

PEDOT:PSS/graphene

58.9

32.1

11.1

70

Spin-coating (LbL)

PEDOT:PSS/ce-MoS2

21.9

867

41.6

This work

Conclusion In this work, we proposed a simple strategy to construct an organic/inorganic heterostructure TE film by spin-coating PEDOT:PSS/EG and ce-MoS2/DMSO alternatively. The resulting PEDOT:PSS/ce-MoS2 heterostructure TE film demonstrates a significantly improved power factor of 41.6 µW m-1 K-2 corresponding to the simultaneously enhanced thermopower and electrical conductivity. The enhanced thermopower (21.9 µV K-1) is attributed to the energy filtering effect caused by composite and optimization of the oxidation level of PEDOT produced by the negative charged ce-MoS2. While the improved electrical conductivity (867 S cm-1) is resulting from the molecular conformation rearrangement and the removal of the non-conducing PSS chains produced by organic solvent on PEDOT:PSS. Based on layer-by-layer method to realize the synergetic action of solvent effect of DMSO and intrinsic reduction of ce-MoS2 was firstly proposed to be the key point for optimizing the TE performance of PEDOT:PSS. This work may open up a tremendous opportunity to realize the simultaneously enhanced electrical conductivity and thermopower of PEDOT:PSS. Furthermore, we have proved that the spin-coating assisted LbL method can efficiently realize the optimization of the composite films, providing a reference for preparing high TE

25



performance films. In practical applications, the optimized composite films with highly enhanced TE performance can be directly or transferred to flexible substrate used for TE energy harvesting systems. Supporting Information The Supporting Information is available free of charge on the ACS Publications website. 2D AFM characteristic (PDF). Acknowledgements This work was supported by the National Natural Science Foundation of China (51762018, 51463008, 51572117 and 51662012), the Natural Science Foundation of Jiangxi Province (20161BAB216129), and Innovation Driven "5511" Project of Jiangxi Province (20165BCB18016). References (1) Zhang, Y. Z.; Cheng, T.; Wang, Y.; Lai, W. Y.; Pang, H.; Huang, W. A Simple Approach to Boost Capacitance: Flexible Supercapacitors Based on Manganese Oxides@MOFs via Chemically Induced in situ Self-Transformation. Adv. Mater. 2016, 28, 5242-5248. (2) Barile, C. J.; Slotcavage, D. J.; McGehee, M. D. Polymer–Nanoparticle Electrochromic Materials That Selectively Modulate Visible and Near-Infrared Light. Chemi. Mater. 2016, 28, 1439-1445. (3) Roland, S.; Neubert, S.; Albrecht, S.; Stannowski, B.; Seger, M.; Facchetti, A.; Schlatmann, R.; Rech, B.; Neher, D. Hybrid Organic/Inorganic Thin-Film Multijunction Solar Cells Exceeding 11% Power Conversion Efficiency. Adv. Mater. 2015, 27, 1262-1267. (4) Culebras, M.; Cho, C.; Krecker, M.; Smith, R.; Song, Y.; Gomez, C. M.; Cantarero, A.;

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Free-Standing Films

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