Transparent and hybrid multilayer films with improved thermoelectric

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Transparent and hybrid multilayer films with improved thermoelectric performance by chalcogenide-interlayer-induced transport enhancement Hyun Ju, and Jooheon Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b13389 • Publication Date (Web): 30 Aug 2019 Downloaded from pubs.acs.org on August 30, 2019

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ACS Applied Materials & Interfaces

Transparent and hybrid multilayer films with improved thermoelectric performance by chalcogenide-interlayer-induced transport enhancement

Hyun Ju and Jooheon Kim* School of Chemical Engineering & Materials Science, Chung-Ang University, Seoul 06974, Republic of Korea

Keywords: Thermoelectric; Thin films; Multilayer; Conductive polymer; Chalcogenide

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ABSTRACT Transparent thermoelectric thin films for efficient heat energy conversion can be extensively applied in healthcare and small electronics. We report a strategy for the fabrication of organic/inorganic hybrid multilayer films with improved thermoelectric performances. As an organic layer, a highly conductive poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is deposited after a solvent treatment. An inorganic interlayer consisting of PEDOT-coated SnSeTe nanosheets (PEDOT-SnSeTe nanosheets) is introduced between the PEDOT:PSS layers to fabricate an alternately deposited PEDOT:PSS/PEDOT-SnSeTe nanosheets/PEDOT:PSS (PSP) multilayer film. To demonstrate the interlayer-induced thermoelectric behaviors, the thermoelectric properties and corresponding carrier transport properties of single-layer, bilayer, and multilayer films are investigated. The inorganic layer influences the intrinsic conduction toward a favorable value for an improved thermoelectric transport. A considerably increased thermoelectric power factor of 110 μW·m–1·K–2 is achieved for a PSP multilayer film fabricated at 4000 rpm, greatly higher than that of the PEDOT:PSS and solution-mixed SnSeTe/PEDOT:PSS composite film. The multilayer strategy proposed in this study is promising for the fabrication of transparent and hybrid thin-film thermoelectrics with high thermoelectric performances.

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INTRODUCTION Heat energy generated in the industry or by various energy sources can be removed naturalistically. It can be also converted to useful electrical energy using the thermoelectric energy harvesting technology. Particularly, thin-film thermoelectric devices for the conversion of body heat to electrical energy can be extensively employed in healthcare and small electronic devices. The conversion efficiency of electrical energy of a thermoelectric material can be calculated by ZT = (S2T)/, where , S, , and T are the intrinsic electrical conductivity, Seebeck coefficient, thermal conductivity, and absolute temperature of the material, respectively. Recently, conductive polymers with high  values have been considered profitable thermoelectric materials because they can be easily processed and are inexpensive and eco-friendly.1–4 However, mainly inorganic thermoelectric materials with excellent performances have been studied.5–10 Poly(3,4-ethylenedioxythiophene) (PEDOT), one of the polythiophene-based conductive polymers, has been extensively investigated by modulating doping or oxidation levels to obtain high  values.11–15 The optimized oxidation level of the PEDOT conductive polymer was 22% providing a considerably improved thermoelectric power factor (σS2).11 Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) films mixed with dimethyl sulfoxide (DMSO) with engineered doping levels exhibited considerably improved σS2 values.15 However, the modulation of the oxidation or doping levels of a conductive polymer generally leads to a low σS2 because the Fermi energy level (EF) of the material shifts to the conduction band edge, yielding a reduced S.16 σS2 of pristine PEDOT can be improved by introducing inorganic nanomaterials with excellent S values into PEDOT. The σS2 values of inorganic filler/PEDOT:PSS composites in 3



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recent literatures are listed in Table 1. Small amounts of inorganic tellurides or selenides (Sb2Te3, Bi2Te3, Cu2SnSe3, etc.) can considerably improve the S and thus σS2 values of the PEDOT-based materials.17–20 Bi2Te3/PEDOT:PSS composites with improved S and σS2 values compared to those of the pristine PEDOT:PSS were reported.21 Ge et al. demonstrated that Cu2SnSe3 nanoparticles introduced into PEDOT:PSS provided improved S and σS2 values.22 However, the composite strategy for inorganic materials with PEDOT also leads to a trade-off relation between the  and S values. PEDOT with introduced inorganic nanomaterials exhibited a high S, which contributes to the significant reduction in . Another potential method considered in this study is a multilayer film strategy based on alternate stacking of organic and inorganic layers. Chain stretching of conductive polymers and energy filtering effect at the nanointerfaces are expected to occur in the organic/inorganic multilayer films.16 S2· Materials

References (μW/(m·K2))

MoS2/PEDOT:PSS

45.6

23

Exfoliated graphene/PEDOT:PSS

53.3

24

MoS2/PEDOT:PSS layer-by-layer film

41.6

25

Single-walled CNT/PEDOT:PSS

105

26

Te nanowires/PEDOT:PSS

28.5

27

Te nanowires/PEDOT:PSS aerogel

11.3

28

Table 1. The power factor values of inorganic filler/PEDOT:PSS composite materials reported in recent literatures. In this study, transparent organic/inorganic multilayer films were fabricated and thermoelectric enhancement was demonstrated in the multilayer film structure. The 4


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multilayered films were fabricated by alternate stacking of PEDOT-coated SnSe0.97Te0.03 (PEDOT-SnSeTe) nanosheets and PEDOT:PSS. SnSe is a promising material with an outstanding S (~520 μV/K).29 However, SnSeTe was chosen because the SnSeTe/polymer composite exhibited an improved thermoelectric performance compared to that of SnSe/polymer.30 SnSeTe nanosheets were fabricated by the hydrothermal exfoliation reaction method.4,31 The conductive PEDOT was introduced on the individual SnSeTe surface for both outstanding distribution to form an even inorganic interlayer and excellent interaction with the adjacent PEDOT:PSS. The multilayer films including the single-layer PEDOT:PSS were characterized. The thermoelectric properties of the samples with different compositions and spin-coating speed of the layers were discussed. The σS2 values were used to evaluate the thermoelectric performances of the conductive-polymer-based materials, as the strong charge– lattice coupling contribution to the lattice  dominates the electronic contribution. Therefore, the σ and electronic  values were independent.32 To the best of our knowledge, the proposed organic/inorganic multilayer film strategy has not been previously employed. The obtained results are valuable for the development of transparent thermoelectric thin films.

RESULTS AND DISCUSSION A schematic of the fabrication of the transparent PSP multilayer films is shown in Figure 1. The conductive PEDOT:PSS and PEDOT-SnSeTe nanosheets were alternately stacked for the successful fabrication of the PSP multilayered film structures, and then the fabricated films were characterized. A digital photograph of the single-layer PEDOT:PSS film fabricated at 2000 rpm is presented in Figure 2a, showing its transparent thin-film structure. Figure 2b presents X-ray photoelectron spectroscopy (XPS) survey spectrum of the PEDOT:PSS film, 5



which shows peaks at the C, O, and S atomic positions, originated from the C–O, C–S, and O–S bondings in the PEDOT:PSS. The S2p core spectrum of the PEDOT:PSS film in Figure 2c shows the binding energy peaks near the 168 eV, corresponded to the S signal from PSS, while the two energy bands at nearly 164 eV originate from the PEDOT. This result is well agreement with the previously reported papers for characterization of the PEDOT:PSS.33–35 The thermoelectric properties of the PEDOT:PSS base film are shown in Figure 2d. Outstanding σ and S values of ~650 S·cm-1 and ~25 μV·K-1 are obtained, in good agreement with the previously reported values.36–38

Figure 1. Schematic of the fabrication of the transparent PSP multilayer films.

Figure 2. (a) Digital photograph, (b) XPS survey and (c) S2p core spectra of the single-layer 6


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PEDOT:PSS film fabricated at 2000 rpm. (d) Thermoelectric properties of the single-layer PEDOT:PSS film measured at 300 K. , S, and σS2 are the electrical conductivity, Seebeck coefficient, and power factor values of the material, respectively. To confirm the successful fabrication of the chalcogenide interlayer between the PEDOT:PSS layers, the PEDOT-SnSeTe nanosheets were analyzed. Figure 3a shows X-ray diffraction (XRD) patterns of the SnSeTe nanosheets, PEDOT-SnSeTe nanosheets, and pristine PEDOT for comparison. The XRD pattern of the PEDOT coating on the surface of the SnSeTe nanosheets in the PEDOT-SnSeTe nanosheet structure was not observed because the organic PEDOT exhibited a lower intensity than that of the SnSeTe nanosheets. Therefore, the PEDOT coating layer was further identified by the thermogravimetric analysis (TGA) and electron microscopy analyses. Figure 3b shows the TGA results for the SnSeTe nanosheets, PEDOTSnSeTe nanosheets, and pristine PEDOT. The SnSeTe nanosheets exhibited an excellent thermal stability up to the high temperature of 900 K. In the case of the PEDOT-SnSeTe nanosheets, the absorbed moisture in the PEDOT is removed at the temperature of ~350 K, and the main thermal decomposition of the PEDOT is observed at ~570 K. The amount of PEDOT coating is ~11 wt%, obtained by estimating the decomposed PEDOT in the PEDOT-SnSeTe nanosheets. The field-emission scanning electron microscopy (FE-SEM) image and corresponding energy-dispersive X-ray spectroscopy (EDS) maps of the PEDOT-SnSeTe nanosheets in Figure 3c demonstrate the even distributions of Sn, Se, Te, and S atoms. The FESEM image and corresponding EDS maps of the SnSeTe nanosheets (Figure S1) indicate the absence of S atoms originating from the PEDOT coating. Field-emission transmission electron microscopy (FE-TEM) could visually confirm the successful fabrication of the PEDOT coating on the surface of the SnSeTe nanosheets. Figure 3d and 3e show low- and high-magnification 7



FE-TEM images of the SnSeTe nanosheets, respectively; ~500-nm sheet-like shapes originating from the intrinsic layered structure are observed (Figure S2). The lattice spacing of ~0.3 nm corresponds to the (001) planes of SnSeTe. A selected-area electron diffraction pattern of the SnSeTe nanosheet is shown in Figure 3e and S3. The ~5 nm PEDOT coating is observed in the FE-TEM images in Figure 3f and 3g, distinguished from the inorganic SnSeTe region.

Figure 3. (a) XRD patterns and (b) TGA results for the SnSeTe nanosheets, PEDOT-SnSeTe nanosheets, and pristine PEDOT. FE-SEM images and corresponding EDS maps of the (c) PEDOT-SnSeTe nanosheets. Low- and high-magnification FE-TEM images of the (d,e) SnSeTe nanosheets and (f,g) PEDOT-SnSeTe nanosheets, respectively. The successfully fabricated PEDOT-SnSeTe nanosheets could form an even inorganic interlayer through the outstanding distribution of nanosheets. This layer was introduced between the PEDOT:PSS layers to fabricate the PSP multilayer film. The PEDOT-SnSeTe nanosheet layer was expected to interact with the adjacent PEDOT:PSS layers, improving the thermoelectric transport. Digital photographs of the PEDOT:PSS layer, PEDOT:PSS/PEDOTSnSeTe nanosheet (PS) bilayer, and PSP multilayer are shown in Figure S4. The film consisting 8


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of a higher number of layers exhibits a darker image. Figure 4 shows XPS depth profiles with the atomic concentrations (Sn, S, and Si) and cross-sectional FE-SEM images of the PEDOT:PSS (Figure 4a and 4d), PS bilayer (Figure 4b and 4e), and PSP multilayer (Figure 4c and 4f) films fabricated at 2000 rpm. In the XPS depth profiles, the atomic concentrations of Sn, S, and Si originate from the SnSeTe nanosheets, pristine PEDOT-based materials, and glass substrate, respectively. The atomic concentrations of Si in all samples are almost zero at the film region, but are considerably increased at the end of the film depth, indicating the region of the glass substrate. The XPS depth profiles of the pristine PEDOT:PSS (Figure 4a) show a high concentration only for the S atoms in the film region and maintained concentration of Sn atoms (0%). The depth profiles of the PS bilayer and PSP multilayer in Figure 4c and 4e show the variations in Sn and S concentrations in the film region. As shown in Figure 4c, the high percentage of Sn atoms considerably decreases when the film depth passes through the interface between the PEDOT-SnSeTe nanosheets and PEDOT:PSS, owing to the absence of SnSeTe nanosheets in the PEDOT:PSS layer. The S atomic concentration in the PS bilayer exhibits the opposite trend to that of the Sn atoms. The atomic percentages of Sn and S atoms exhibit the trade-off behavior in the PSP multilayer film (Figure 4e). A high content of Sn atoms exists in the PEDOT-SnSeTe nanosheet layer, while a high concentration of S atoms is observed in the PEDOT:PSS layer. Cross-sectional FE-SEM images of the PEDOT:PSS, PS bilayer, and PSP multilayer films and corresponding XPS depth profiles are shown in Figure 4b, 4d, and 4f, respectively, which demonstrate the distinct layers with a height of each layer of ~90 nm, consistent with the XPS depth results.

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Figure 4. XPS depth profiles of Sn, S, and Si atoms in the (a) PEDOT:PSS, (c) PS bilayer, and (e) PSP multilayer films fabricated at 2000 rpm. Cross-sectional FE-SEM images and corresponding XPS depth profiles of the (b) PEDOT:PSS, (d) PS bilayer, and (f) PSP multilayer films. To demonstrate the chalcogenide-interlayer-induced thermoelectric transport behavior of the multilayer film, the thermoelectric properties of the PEDOT:PSS, PS bilayer, and PSP multilayer films were investigated. Figure 5 shows the thermoelectric properties of the 10


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fabricated films. The results are compared to those of the solution-mixed SnSeTe/PEDOT:PSS composite with a similar content of SnSeTe particles. σ is decreased after the deposition of the PEDOT-SnSeTe nanosheet layer on the PEDOT:PSS layer (PS bilayer), while S is increased (Figure 5a and 5b). This can be explained as the introduced inorganic layer of PEDOT-SnSeTe nanosheets affects the intrinsic conduction toward the less electrically conductive network, but is more favorable for the thermoelectric enhancement, demonstrated by the σS2 value in Figure 5c. In general, the introduction of the PEDOT:PSS to a material leads to simultaneous improvement in σ and significant reduction in S, originated from the charge characteristics of the PEDOT:PSS. However, a considerable improvement in σ without a significant reduction in S is observed after the additional PEDOT:PSS layer is coated on the surface of the PS layer (PSP multilayer), yielding the highest σS2 among those of the three types of films and higher than that of a SnSeTe/PEDOT:PSS composite film with a similar SnSeTe content. Therefore, the carrier transport properties were investigated based on the following equations to evaluate the crucial factor for the changes in σ and S:5 𝜎 = 𝑛𝑒𝜇, 2

2

𝑆=

8𝑘𝐵2𝜋 3ℎ2𝑒

(1)

∗

𝜋

3

𝑚 𝑇(3𝑛) ,

(2)

where n, e, μ, h, kB, and m* are the charge concentration of carriers, charge per carrier, charge mobility of the carriers, Planck constant, Boltzmann constant, and effective mass of the carrier, respectively. The μ and n values of the film materials are strongly related to σ and S, according to the equations. S is inversely proportional to n, while σ is directly proportional to both μ and n. Table S1 shows the n and μ values of the PEDOT:PSS, PS bilayer, and PSP multilayer films. The σ values of the films are affected mainly by the behavior of n. However, the significantly high μ of the PSP multilayer film contributes to the further improvement in σ. This value is 11



also higher than that of the solution-mixed SnSeTe/PEDOT:PSS composite with similar content of SnSeTe, as the PEDOT:PSS chains in the spin-coated film can be stretched and arrayed, while the chains in the bulk PEDOT:PSS are clustered.39 This enhances the charge hopping and interchain interactions with adjacent PEDOT:PSS chains in the layers of PEDOT:PSS and PEDOT-SnSeTe nanosheets, yielding the formation of considerably electrically conductive paths. The S values of the films are in good agreement with the inverse relationship with the n values. Consequently, the maximum σS2 of 83 μW·m–1·K–2 is achieved for the PSP multilayer film, which is dramatically higher than that of the pristine PEDOT:PSS and also higher than that of the SnSeTe/PEDOT:PSS composite (Figure 5c).

Figure 5. (a) Electrical conductivities, (b) Seebeck coefficients, and (c) power factors of the PEDOT:PSS, PS bilayer, and PSP multilayer films fabricated at 2000 rpm and those of the solution-mixed SnSeTe/PEDOT:PSS composite with a similar SnSeTe content, presented for comparison. Another experiment was carried out to optimize the conditions for the multilayer film deposition. Figure 6a shows digital photographs of the PSP multilayer films obtained at different speeds of spin coating. All film samples exhibit transparent thin-film characteristics and become brighter with the increase in spin speed. The thermoelectric properties of the 12


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fabricated PSP multilayer films as a function of the spin-coating speed are shown in Figure 6b– d. σ of the PSP multilayer film gradually decreases with the increase in spin-coating speed (Figure 6b), while S continuously improves (Figure 6c). This is attributed to the ratio change of PEDOT:PSS in the PSP multilayer film. The thicknesses of the PEDOT:PSS and PEDOTSnSeTe nanosheet layers are similar at a low spin-coating speed, whereas the PEDOT:PSS layer is thinner than the inorganic PEDOT-SnSeTe nanosheet layer at a high rotation speed because the PEDOT:PSS polymer has a lower weight than that of the inorganic PEDOTSnSeTe nanosheets, which can easily form a thin layer. Therefore, the content of PEDOT:PSS in the PSP multilayer film decreases with the increase in spin-coating speed, which contributes to the low σ and high S values of the multilayer films. This is well supported by the data for the carrier transport properties of the PSP multilayer films obtained at different spin-coating speeds (Table S2), which reveal the trade-off relation of the μ and n values. n of the multilayer film decreases with the decrease in spin-coating speed, which is directly proportional to the content of PEDOT:PSS in the film, while μ is inversely related to the PEDOT:PSS content. σS2 (Figure 6d) increases with the spin-coating speed, reaching a peak value, and then decreases with the further increase in speed. The maximum σS2 of ~110 μW·m-1·K-2 was observed for the PSP multilayer film spin-coated at 4000 rpm, which is 1.8 times higher than that of the SnSeTe/PEDOT:PSS composite.

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Figure 6. (a) Digital photographs of the transparent PSP multilayer films obtained at different spin speeds. (b) Electrical conductivity, (c) Seebeck coefficient, and (d) power factor of the PSP multilayer film as a function of the spin-coating speed [rpm].

CONCLUSION Transparent organic/inorganic multilayer films were fabricated by alternately stacking the PEDOT:PSS and PEDOT-SnSeTe nanosheets. The PEDOT-SnSeTe nanosheets were fabricated by the hydrothermal exfoliation reaction of the bulk powder and solutionprocessable surface coating of the PEDOT nanolayer on the surface of the SnSeTe nanosheets. The ~500-nm SnSeTe nanosheets and PEDOT coating (~5 nm, ~11 wt%) on the nanosheets were observed. The PEDOT-SnSeTe nanosheets formed an inorganic interlayer introduced 14


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between the PEDOT:PSS layers to fabricate the PSP multilayer film. The XPS depth profile analysis was carried out to identify the compositions and provide depth information for the multilayer films. The thermoelectric properties of the PEDOT:PSS, PS bilayer, and PSP multilayer films were investigated to demonstrate the interlayer-induced thermoelectric behavior of the multilayer film. σ was decreased after the deposition of the PEDOT-SnSeTe nanosheet layer on the PEDOT:PSS layer, while S was increased, as the inorganic layer affected the intrinsic conduction toward the less electrically conductive network, but was favorable for the increase in thermoelectric performance. After the additional deposition of the PEDOT:PSS layer on the surface of the PS layer, a significant improvement in σ was observed without a substantial reduction in S, providing the highest σS2. This was attributed to the charge hopping and interchain interaction with adjacent chains in the layers of PEDOT:PSS and PEDOT-SnSeTe nanosheets, yielding the formation of highly electrically conductive paths. The maximum σS2 of 110 μW·m–1·K–2 was observed for the PSP multilayer film spin-coated at 4000 rpm, which is 2.74 times higher than that of the pristine PEDOT:PSS and also 1.8 times higher than that of the solution-mixed SnSeTe/PEDOT:PSS composite film. The proposed strategy for the fabrication of hybrid multilayer films can be used to combine the inorganic and organic materials for high-performance thin-film thermoelectrics.

ASSOCIATED CONTENT Supporting Information. Additional ﬁgures, tables, and discussion are included in the Supporting Information, as follows: Experimental methods contain fabrication procedures of PEDOT-SnSeTe nanosheets and multilayer films. Characterization part is also included; FESEM image and corresponding EDS maps of the SnSeTe nanosheets (Figure S1); FE-SEM 15



image of the SnSeTe bulk material, revealing the intrinsic layered structure (Figure S2); Selected-area electron diffraction pattern of the SnSeTe nanosheet (Figure S3); Digital photographs of the (a) glass substrate, (b) PEDOT:PSS layer, (c) PEDOT:PSS/PEDOT-SnSeTe nanosheet (PS) bilayer, and (d) PSP multilayer (Figure S4); Carrier concentration and mobility values of the PEDOT:PSS, PS bilayer, and PSP multilayer films (Table S1); Carrier transport properties of the PSP multilayer films obtained at different spin-coating speeds (rpm) (Table S2).

AUTHOR INFORMATION Corresponding Author * Tel: +82-2-820-5763, Fax: +82-2-812-3495, E-mail address: [email protected] (J. Kim) Notes The authors declare no competing financial interest.

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Graphical abstract 339x180mm (96 x 96 DPI)


Transparent and hybrid multilayer films with improved thermoelectric

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