Cu2Se Nanowire

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Functional Nanostructured Materials (including low-D carbon)

Good Performance and Flexible PEDOT:PSS/ CuSe Nanowire Thermoelectric Composite Films 2

Yao Lu, Yufei Ding, Yang Qiu, Kefeng Cai, Qin Yao, Haijun Song, Liang Tong, Jiaqing He, and Lidong Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b01718 • Publication Date (Web): 18 Mar 2019 Downloaded from http://pubs.acs.org on March 19, 2019

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is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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ACS Applied Materials & Interfaces

Good Performance and Flexible PEDOT:PSS/Cu2Se Nanowire Thermoelectric Composite Films Yao Lua, Yufei Dinga, Yang Qiub, Kefeng Caia*, Qin Yaoc, Haijun Songa, Liang Tonga, Jiaqing Heb*, Lidong Chenc* a

Key Laboratory of Advanced Civil Engineering Materials, Ministry of Education,

School of Materials Science & Engineering, Tongji University, 4800 Caoan Road, Shanghai 201804, China b

Materials Characterization and Preparation Center and Department of Physics,

Southern University of Science and Technology, Shenzhen 518055, China c State

Key Laboratory of High Performance Ceramics and Superfine Microstructure,

Shanghai Institute of Ceramics, Chinese Academy of Science, Shanghai, 200050, China

KEYWORDS: PEDOT:PSS; copper selenide; nanowire; composite; thermoelectric; flexible.

ABSTRACT

Herein,

Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)

(PEDOT:PSS)

coated CuxSey (PC-CuxSey) nanowires are prepared by a wet-chemical

method, and PEDOT:PSS/CuxSey nanocomposite films on flexible nylon membrane are fabricated by vacuum assisted filtration and then cold-pressing. XRD analysis reveals that the CuxSey with different compositions can be obtained by adjusting the nominal Cu/Se molar ratios of their sources. For the composite film starting from Cu/Se nominal molar ratio of 3, an optimized power factor ~ 270.3 W/mK2 is obtained at 300 K. Moreover, the film exhibits a superior flexibility with 85% of the original power factor retention after bending for 1000 cycles around a rod with diameter of 5 mm. TEM and 1

ACS Paragon Plus Environment

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STEM observations of the focused ion beam (FIB) prepared sample reveal that it is mainly attributed to a synergetic effect of the nylon membrane and the composite film with nanoporous structure formed by the intertwined nanowires, besides the intrinsic flexibility of nylon. Finally, a thermoelectric prototype composed of 9-leg of the optimized hybrid film generates a voltage and a maximum power of 15 mV and 320 nW, respectively, at a temperature gradient of 30 K. This work offers an effective approach for high TE performance inorganic/polymer composite film for flexible TE devices.

2


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1. INTRODUCTION Nowadays, the explosive growth of wearable electronic devices and urgent demand for sustainable energy sources stimulate researcher’s interests in lightweight and flexible thermoelectrics (TEs) 1-2. Flexible TE materials can recover the waste heat into electricity, which can realize the self-power for wearable devices from the temperature difference between the skin and the surrounding environment 3. The evaluation of a TE material is quantified by the dimensionless figure of merit, ZT = S2σT/κ, here S, σ, κ and T denote the Seebeck coefficient, electrical conductivity, thermal conductivity, and the absolute temperature, respectively 4-5. Hence, high power factor (PF=S2σ) and low κ are necessary for a good TE material. Until now, the most studied TE materials are inorganic semiconductors, especially chalcogenides, such as Pb-Te alloys 6, Bi-Te alloys 7, Ag2Se 8-9, and SnSe 10. They can reach the best TE performance in a wide range of operating temperature

11.

Among

chalcogenides, copper selenides as p-type TE materials have recently obtained much attention 12-16. They have complex crystal structures (e.g. tetragonal, orthorhombic, and cubic structures) and variable compositions (e.g. CuSe, Cu3Se2, Cu2-xSe, and Cu2Se) with structural transition occurring at the temperature ∼ 400 K 17-19. It was reported that Cu2-xSe and Cu2Se possess excellent TE performance at high temperatures. For example, spark plasma sintered Cu2-xSe and Cu2Se bulks reached ZTs of 1.5-2.1 at 1000 K 12, 14-15. Most recently, Nunna et al. 16 first in-situ synthesized Cu2Se on the surface of multi-walled carbon nanotubes (MWCNTs), and then spark plasma sintered Cu2Se/0.75 wt% MWCNT hybrid material with a record ZT of 2.4 at 1000 K. In fact, 3



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bulk Cu2-xSe and Cu2Se also show excellent TE performance near room temperature (RT), with PF around 600-850 W/mK2, suggesting that they are good candidates for preparing inorganic/organic TE nanocomposites. Like the bulk Cu2Se, Cu2Se films have also attracted much attention, and various fabrication methods have been employed, such as galvanic synthesis 20, solvothermal synthesis

21,

pulsed laser deposition

22

and wet-chemical synthesis

23.

For instance,

Ghosh et al. 20 prepared Cu2-xSe thin film by a galvanic deposition method, and the film showed a PF of 173 W/mK2 at 300 K. Nevertheless, to further improve and stabilize the TE performance, high-temperature post-treatment is always necessary. Therefore, although the Cu2-xSe film exhibits an excellent TE performance, the energy-intensive and high-temperature processing hampers low-cost and large scale production severely. Moreover, the preparation of flexible copper selenide films is still lacking. Hence, the rational design and fabrication of flexible Cu2Se materials with excellent TE performance at the aim of energy saving remain a great challenge. Inorganic materials are inherently brittle and rigid, so it is hardly to prepare flexible (bendable and/or foldable) inorganic films. In order to realize the flexibility, recently, flexible substrates, such as polyvinylidene fluoride (PVDF) and polyimide (PI), have been used to support inorganic TE materials 24. By depositing Cu2Se on PI using pulsed reactive magnetron sputtering, the Cu2Se film reached a high PF of 1100 W/mK2 at RT 25. However, because of the weak bonding strength between Cu2Se and PI substrate, it was difficult to form compact and flexible composite film. Another approach to realize flexibility is to form inorganic/polymer composite. For example, 4


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Pammi et al.

26

fabricated Cu2-xSe NW/PVDF composite flexible film by vacuum

filtration and then cold pressing. The film showed good mechanical durability and flexibility. However, because of the insulating PVDF, the composite showed a relatively low TE performance (PF of 105.3 W/mK2 at 303 K). Inorganic-nanostructure/conducting polymer (CP) composite materials can combine the high PF of the inorganic filler and simultaneously low κ, relatively high σ, as

well

as

good

flexibility

of

the

CP

27.

Poly(3,4-

ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) has high electrical conductivity (103 S/cm), water solubility and excellent stability; hence, it is the most studied CP

28-29.

Since inorganic TE component usually shows higher TE properties

than CP, the more the inorganic TE nanostructure component in the composite, the better of the properties1. The common methods for preparing inorganic/organic composite materials include physical mixing synthesis

33-34

30,

in-situ polymerization

and layer-by-layer self-assembling

35.

31-32,

in-situ

Amon them, in-situ synthesis

technique is an effective method to adequately disperse inorganic nanoparticles in a CP matrix, enhancing wettability and making the two phases robustly tethered 36. In 2010, See et al.

37

reported PEDOT:PSS/Te composite film by an in-situ synthesis method

combined with drop casting. The composite film achieved a PF of 70 W/mK2 because of the Te nanorods with a high S and PEDOT:PSS with a high σ. Our group recently extended this method and did a series of work. At first, we in-situ prepared PC-Te nanorods, then integrated PC-Te with PEDOT:PSS

38.

By using PVDF porous

membranes as substrate, PC-Te/PEDOT:PSS flexible films, with optimized PF of 51.4 5



W/mK2, were fabricated through vacuum assisted filtration. Subsequently, we prepared PC-Te/PEDOT:PSS and PC-Te/SWCNT composite films, respectively, then post-treated with acid

39-40.

The optimized PFs of 141.9 W/mK2 and 104 W/mK2

were obtained for the two composite films. Most recently, we 41 further extended this technique to PEDOT:PSS coated chalcogenides: we synthesized PC-Cu7Te4 nanorods using the PC-Te nanorods as templates, then Te/PEDOT:PSS/Cu7Te4 flexible composite films were fabricated via physical mixing and then drop casting. The flexible ternary composite film exhibited a PF of 65.3 W/mK2 due to a synergistic effect of high electrical conducting PC-Cu7Te4, high S of the PC-Te and double interfacial energy filtering effect. This work suggests a successful approach for fabrication of onedimensional (1D) chalcogenides/PEDOT:PSS flexible composite films. Compared with CuxTey, CuxSey is cheaper and lower toxic. Moreover, it is known from literature that CuxSey has better TE performance than CuxTey. Te and Se are in the same family, like Te nanorods, Se nanowires (NWs) can also be used as templates to assemble 1D selenides 42 43. Herein, PEDOT:PSS coated Se (PC-Se) NWs were first in situ synthesized; then PEDOT:PSS coated polycrystal CuxSey (PC-CuxSey) NWs were synthesized using the PC-Se NWs as templates; finally, highly flexible PC-CuxSey composite films were prepared on nylon porous membrane by vacuum assisted filtration and then cold-pressing. For the composite film starting from Cu/Se nominal molar ratio of 3, a maximum PF of 270.3 W/mK2 at 300 K was obtained. Besides, flexible TE generators were assembled using the optimal film and their output performance were studied. 6


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2. EXPERIMENTAL SECTION 2.1 Materials PEDOT:PSS (Clevios PH1000) was bought from Wuhan Zhuoxin Technology Co., Ltd. Ethanol, ethylene glycol (EG), ammonia hydroxide, L-ascorbic acid, selenium dioxide (SeO2) and copper sulfate pentahydrate (CuSO4·5H2O) were bought from Sinopharm Chemical Reagent Co., Ltd. β-cyclodextrin was bought from Aladdin Industrial Corporation. Porous nylon membrane (the average diameter of the pores ~ 0.22 m) was bought from Haiyan Taoyuan Group. Silver paste (SPI# 04998-AB) was used as electrodes. All the materials were directly used. 2.2 Synthesis of PC-Se NWs SeO2 (0.5 g), β-cyclodextrin (0.5 g) and 2 ml PEDOT:PSS were added into 100 ml distilled (DI) water in a glass beaker with stirring to form solution I. 2 g L-ascorbic acid was added into 100 ml distilled water in another glass beaker with stirring to form solution II. The solution I was slowly dropped into the solution II with continuous stirring. The color of the mixed solution rapidly changed from colorless into yellow, and then immediately turn into brick-red. After reaction for ~4 h, the precipitate was collected by centrifugation at 11000 rpm for 10 min and alternatively washed with DI water and ethanol for ten times. The final precipitate was stored in 100 ml absolute ethanol. Subsequently, 2.5 L ammonium hydroxide was dropped into the solution, and the solution was placed for 48 h statically at room temperature, with a color varying from brick-red to dark-red during the process, and flocculated precipitate (PC-Se NWs) was formed. The precipitate was collected by centrifugation, and re-dispersed into 100 7



ml EG to form suspension solution for further chemical reactions. 2.3 Synthesis of PC-CuxSey NWs First, a certain amount of CuSO4 was dissolved in 10 ml EG. Second, L-ascorbic acid was dissolved in 10 ml distilled water. Third, the CuSO4 solution was poured into the PC-Se NWs contained solution with vigorously stirring. Subsequently, a certain amount of the L-ascorbic acid solution (the molar ratio of the L-ascorbic acid: CuSO4 being 3:1) was dropped into the above dispersion. After continuously stirring for 4 h at room temperature (RT) to allow fully reaction, the product was centrifuged at 4000 rpm and washed with DI water and ethanol in sequence for six times. To study the effect of different Cu:Se molar ratios of the precursors on composition of the product, the amount of CuSO4 addition was adjusted. The molar ratios of Cu:Se precursors were 1:2, 1:1, 3:2, 2:1, 3:1 and 4:1, respectively, and the corresponding products were named as PC-Cu1Se2, PC-Cu1Se1, PC-Cu3Se2, PCCu2Se1, PC-Cu3Se1 and PC-Cu4Se1, respectively. 2.4 Preparation of PC-CuxSey composite films The as-prepared PC-CuxSey NWs were dispersed in 100 ml ethanol by sonication for 20 min, then PC-CuxSey film on a porous nylon membrane was obtained by vacuum assisted filtration. The sample was dried in vacuum at 50 °C for ~12 h, and then coldpressed at 20 MPa for 3 min. For comparison, the Cu3Se1 film was fabricated by the same procedure as that for PC-Cu3Se1 film but without using PEDOT:PSS. Figure 1 depicts the fabrication process of the PC-CuxSey composite films, and illustrates the interaction between Se NWs and PEDOT:PSS. Through the in-situ synthesis method, 8


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the organic PEDOT:PSS and inorganic NWs can be robustly tethered to one another.

Figure 1. Schematic demonstrating the preparation of PC-CuxSey composite films (a) and the growth process of PEDOT:PSS coated Se nanowire (b).

2.5 Thermoelectric device fabrication The optimized film was cut into strips (25 mm × 5 mm) and pasted on the PI substrate with an interval of ~ 5 mm. Then, these strips were connected in series using silver paste to fabricate a TE prototype. 2.6 Measurements and characterizations 9



Electrical conductivity at RT was measured using a steady-state four-probe technique by Ecopia HMS-3000. The Seebeck coefficient at RT was determined by the slope of the linear relationship between the thermal electromotive force and temperature difference (~ 10 K) between two ends of a film. The temperature dependence of TE properties was measured by Sinkuriko ZEM-3 in He atmosphere, with the instrument test error of ±5% for both σ and S. The Hall measurement was performed using the Van der Pauw method (The Lake Shore 8400 Series). The film thickness was measured by a thickness meter (Shanghai Liu Ling Instrument Factory) combined with fieldemission scanning electron microscope (FESEM) observation. The bending test was performed using a home-made apparatus around rods with different diameters. The phase composition of the samples was examined by X-ray diffraction (XRD) (D/MAX 2550VB3+/PCII). X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi) was employed to detect the bonding energy of the film. The morphology of the samples was characterized by a FESEM (FEI Nova NanoSEM 450) with energy dispersive X-ray spectroscopy (EDS). The morphology of the PC-CuxSey NWs was observed by transmission electron microscope (TEM) and high resolution TEM (HRTEM, JEM-2100F). The internal nanostructure of the cold-pressed film was observed by double-aberration corrected transmission electron microscope (TEM, FEI Titan @300kV in both TEM and STEM mode), and the TEM sample was prepared by Dualbeam system with scanning electron microscope and focused ion beam (SEM/FIB, FEI Helios600i) with the in-situ lift-out technique. The performance of TE device was measured by a home-made apparatus, and the 10


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details of the principle can be seen in our previous work.41 As shown in Figure S1, the hot side of the TE generator was heated by a heating element, whereas the cold side was connected with a cold metal. The TE device was connected into a circuit with a rheostat and a microammeter in series. Under a particular temperature gradient, through modulating the external load resistance, the output voltage and current were collected by a digital multimeter (Agilent 34970) and a microammeter, respectively.

3. RESULTS AND DISCUSSION

Figure 2. (a) XRD patterns of the (a) PC-Cu1Se2, PC-Cu1Se1, PC-Cu3Se2 films and (b) PCCu2Se1, PC-Cu3Se1, PC-Cu4Se1 films.

Since the stoichiometry of the CuxSey material has a great effect on TE performance 25, we adjust the nominal molar ratio of Cu/Se to study the composition effect on the TE properties of PC-CuxSey films. At first, the composition of the films (PC-Cu1Se2, PC-Cu1Se1, PC-Cu3Se2, PC-Cu2Se1, PC-Cu3Se1 and PC-Cu4Se1) were analyzed by XRD. XRD spectra of the PC-Cu1Se2, PC-Cu1Se1 and PC-Cu3Se2 11



Page 12 of 35

films are shown in Figure 2 (a). For the PC-Cu1Se2 film, the majority product is orthorhombic CuSe (JCPDS No. 27-0184), whereas two peaks for hexagonal Se (JCPDS No. 06-0362) are detected, which is unsurprising since Se source is excessive. For the PC-Cu1Se1 film, all the XRD peaks can be indexed to orthorhombic CuSe (JCPDS No. 27-0184), and the characteristic peaks of the PC-Cu3Se2 film can be indexed to tetragonal Cu3Se2 phase (JCPDS No. 27-0184), besides a few weak peaks of CuSe (JCPDS No. 27-0184). XRD spectra of the PC-Cu2Se1, PC-Cu3Se1 and PC-Cu4Se1 films with higher Cu/Se nominal ratios are present in Figure 2(b). As seen, both cubic Cu2-xSe (JCPDS No. 06-0680) phase and tetragonal Cu2Se (JCPDS No. 29-0575) phase are detected in the PC-Cu2Se1 film. For the PC-Cu3Se1 and PC-Cu4Se1 films, besides the cubic Cu2xSe

and tetragonal Cu2Se, cubic Cu (JCPDS No. 04-0836) is also detected. Moreover,

with the Cu/Se nominal ratio increase, the peaks for Cu phase become stronger. The XRD peaks for PEDOT:PSS are not detected in the samples owing to the amorphous characteristics and low content. According to the XRD results, the chemical reactions involved in the process are deduced as follows: SeO2 + 2C6H8O6 → Se + 2C6H6O6 + 2H2O

(1)

Cu2+ + C6H8O6 → Cu + C6H6O6 + 2H+

(2)

xCu + ySe → CuxSey

(3)

XPS analysis is performed to investigate the chemical composition of the PCCu1Se1 and PC-Cu2Se1 films (Figure S2). Peaks of Cu 2p, Se 3d and S 2p are identified 12


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in the survey spectra (Figure S2(a)). The low intensity of S 2p peaks detected in PCCu1Se1 and PC-Cu2Se1 films implies a small amount coating of PEDOT:PSS (Figure S2(b)). For the PC-Cu1Se1 sample, the binding energy values of Cu 2p3/2 and Cu 2p1/2 are 932.1, 952.0 eV, demonstrating that the valence of Cu is +2 (Figure S2(c)) 44. The binding energy of Se 3d is 54.1 eV, which corresponds to that of Se2- (Figure S2(d)) 45. Besides, the peak integral area ratio of the Cu 2p to Se 3d is 1.02, which is in accordance with the EDS result of the PC-Cu1Se1 film (Figure S3(a)). For the PC-Cu2Se1 sample, the binding energies of Cu 2p3/2 and Cu 2p1/2 are at 931.9, 952.0 eV, respectively (Figure S2(c)). In addition, the peaks at 934.0 and 942.2 eV are detected, which corresponds to Cu+ 46. Consequently, both Cu+ and Cu2+ exist in the PC-Cu2Se1 sample. In the chemical reactions, Se atoms are apt to obtain electrons from Cu atoms, resulting in a decrease of the binding energy of Se 3d. Therefore, the characteristic peak of Se 3d shifts to a lower energy (54.0 eV) with the increase of Cu ion concentration (Figure S2(d)). The Cu/Se ratio calculated from the XPS spectrum is 1.98, which is in agreement with EDS result recorded on the PCCu2Se1 film (Figure S3(b)). Meanwhile, it is reported that the valence states of Cu in Cu2-xSe are Cu+ and Cu2+ 47. Hence, these verify a copper deficiency, Cu2-xSe, in the PC-Cu2Se1 NWs.

13



Figure 3. High-magnification (a-f) and low-magnification (g-i) FESEM images of the PC-Cu1Se1 film (a, d, g), PC-Cu2Se1 film (b, e, h), PC-Cu3Se1 film (c, f, i) before (a-c) and after (d-i) coldpressing, (j) schematic diagram of the PC-CuxSey nanowires stacking before and after cold-pressing.

Figure 3 shows typical surface FESEM images of the PC-Cu1Se1, PC-Cu2Se1 and PC-Cu3Se1 films. The films before cold-pressing (Figure 3 (a-c)) consist of uniformly nanowires loosely intertwined with each other with high porosity. The average diameter of the PC-CuxSey NWs is 80±5 nm (see more clearly in Figure S4) and the length is a few microns. The difference is that the PC-Cu1Se1 NWs are approximately straight (Figure 3 (a)), while the PC-Cu2Se1 NWs and PC-Cu3Se1 NWs are curving and twisty (Figure 3 (b, c)). After cold-pressing, PC-CuxSey NWs contact together closely (Figure 3 (d-f)). The cold-pressed films show dense and flat surface (Figure 3 (g-i)). Figure 3 (j) depicts the stacking situation of the PC-CuxSey NWs before and after cold-pressing. Before cold-pressing, they are loosely stacked, whereas the NWs closely contact after cold-pressing, which is evidenced by a high-angle annular dark-field (HAADF) image and elements mapping in Figure S5.

14


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Figure 4. TE parameters of the PC-CuxSey films starting from different Cu/Se nominal molar ratios at RT

Figure 4 exhibits the TE properties of the composite films at RT (film thickness being about 8 m (see Figure S6 and Table S1)). The blue area corresponds to the PCCu1Se2 and PC-Cu1Se1 films, with lower S and higher . According to the XRD results (Figure 2 (a)), these samples mainly consist of orthorhombic CuSe phase. The PC-Cu1Se1 film reaches an excellent electrical conductivity of ~8114 S/cm (with the S of 7.5 V/K), which is close to the value of the bulk CuSe in ref. [48]. In order to test the stability of electrical conductivity, the PC-Cu1Se1 film was exposed to air with 50% relative humidity for several days at 300 K. The time dependence of the electrical conductivity is shown in Figure S7 (a). The electrical conductivity remains steady within 30 days, then reduces by ~ 20% compared with the initial value after 60 days (Figure S7 (a)), indicating good stability. As the PEDOT:PSS layer is only a few-nanometer thick, the effect of moisture on it can be neglected, and 15



moreover, it can protect the CuSe NWs from oxidization to some extent. The yellow region corresponds to the performance of the PC-Cu2Se1, PC-Cu3Se1, PC-Cu4Se1 films, with higher Seebeck coefficient and lower electrical conductivity. In this zone, the highest PF value (270.3 W/mK2) is obtained for the PC-Cu3Se1 film, with the Seebeck coefficient and electrical conductivity of 50.8 V/K and of 1047.1 S/cm. According to the XRD result, the film consists of Cu2-xSe and Cu2Se phases, besides a small amount of Cu phase. The enhanced TE properties of the PC-Cu3Se1 film is mainly caused by the intrinsic high S of Cu2Se. For the PC-Cu4Se1 film, the Seebeck coefficient is lower than that of PC-Cu3Se1, because it contains more impurity Cu.

Figure 5. Temperature dependence of electrical conductivity, Seebeck coefficient, and power factor (a), carrier concentration and mobility (b) for the PC-Cu3Se1 composite film.

Figure 5 (a) shows the temperature dependence of the TE properties of the PCCu3Se1 composite film. As temperature increases, the S the film first increases and then decreases rapidly when the temperature rises from 340 to 380 K, and after that it slowly increases again. Moreover, the  of the film decreases with the temperature increasing, and then increases rapidly when the temperature changes from 360 to 418 16


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K. Thus, a phase transition occurs at the temperature range of 360-380 K

13, 49.

The

change tendency of both the S and  is similar to that of the bulk Cu2-xSe 50. Liu et al.13 reported that Cu2Se undergoes a structural transition from α-phase to the cubic β-phase at ~ 400 K, and they found a sharp TE transport behavior by measuring the TE properties at a small temperature step (~0.5 K). However, such behavior is not shown in Fig. 5(a). This is because we measured the properties every 20 K, a sharp change must have been missed. If we also measure the properties in a smaller temperature step, a higher power factor could be obtained. Although such sharp thermoelectric transport behavior around the phase transient temperature is an interesting issue, practically the peak value in a very narrow temperature range may have not much meaning. Hence, we did not deliberately measure the properties in a very small temperature step. Consequently, the PF value shows a similar tendency to that of Seebeck coefficient: raises from 278 to 366.7 W/mK2, then rapidly decreases to 268.1 W/mK2 at ~370 K, finally goes up to 389.7 W/mK2 at ~418 K. Figure 5(b) shows the temperature dependence of carrier concentration and carrier mobility. For the composite materials, the S is expressed from the weighted average of the difference between the Fermi level (EF) and the energy of the localized states (E), as given by 51: S = (𝑘𝐵/𝑒)(𝐸 ― 𝐸𝐹)/𝑘𝐵𝑇

(4),

where kB is the Boltzmann constant, e is the electron charge. As seen, the carrier concentration (n) reduces from 3.52×1021 to 3.13×1021 cm-3, then rises up to 7.01×1021 cm-3 when the temperature increases from 340 to 400 K, finally drops to 5.82×1021 cm-3. 17



Page 18 of 35

The decreased n will push EF far away the conduction band, thus leading to the increase of the Seebeck coefficient. As a result, the tendency of the S is inversely proportional to that of the n. Obviously, the carrier concentration of our prepared PC-Cu3Se1 composite is much higher than the optimized carrier concentration for most highperformance TE materials (1019-1020 cm-3) 52. The change tendency of the carrier concentration is just opposite to that of the carrier mobility, as a result, the electrical conductivity decreases with temperature first and then increases due to the relation as follows: σ=neμ

(5).

Table 1 lists the preparation methods and TE properties of some representative inorganic nanostructure/CP composite films. As shown in Table 1, our PC-Cu3Se1 flexible film possess a relatively high TE performance, which is only lower than that of the Bi2Te3/PEDOT composite films in ref. [31]. However, the Bi2Te3/PEDOT films were prepared by a complicated process, including nanosphere lithography and reactive ion etching for nanohole arrays template, ﬁlling Bi2Te3 into the template by thermal evaporation, removing the template, and forming composite of Bi2Te3 nanoparticle arrays with PEDOT by vapor-phase polymerization process. Whereas our method is much simpler.

Table 1. Comparison of the room-temperature TE performances of the present inorganic/conducting polymer composite films a. Materials

σ

S

PF

(S/cm)

(V/K)

(W/mK2)

1295

15.8

32.3

Methods b

κ (W/mK)

ZT

Ref.

-

-

30

Hydrothermal Bi2Te3

and physical

/PEDOT:PSS mixing methods 18


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Bi2Te3

Lithography and

/PEDOT:Tos

VPP methods

467

170

1350

0.7

0.58

31

3113

20.3

128.3

0.23

0.17

53

17

220

83

-

-

54

35

202

143

-

-

35

348

54

101

0.3

0.1

55

1047

50.8

270.3

0.25-0.3

~0.3

DilutionSiC filtration and /PEDOT:PSS post-treatment Te/Cu1.75Te

Wet-chemical

/PEDOT:PSS

process We-chemical

rGO/Te and physical /PEDOT:PSS mixing methods PANi/CNT/T e Cu2Se /PEDOT:PSS a

In situ polymerization Wet-chemical

This

process

work

Some parameters are estimated from the graphs reported in the references. b VPP = vapor phase

polymerization

19



Figure 6. Internal microstructure characterization of the PC-Cu3Se1 film, (a) a TEM image containing a few cross sections of NWs, (b) a HRTEM image of a cross section of a NW containing three different-orientation crystals, (c) a HRTEM image of a cross section of a well-crystallized NW, (d) enlarged image of the square marked in (c), inset in (d) is a FFT image, (e) a HRTEM image

of a cross section of a NW containing four different-orientation crystals, (f) a HAADF image of the square marked in (e), light bright dots marked by red dotted lines in (f) refer to Se atoms, the blue area with weak contrast among Se layers refers to Cu atoms, and the colorless zone between interlayer at another side of Se layers is Cu deficiency.

Figure S4 shows HRTEM images of a PC-Cu1Se1 NW and a PC-Cu3Se1 NW. As 20


Page 20 of 35

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shown in Figure S4(c), a distinct crystal lattice fringe with lattice spacing of 0.319 nm is observed, which matches well with the (112) plane of the orthorhombic CuSe. Besides, a thin PEDOT:PSS layer (~ 2 nm) well coated on the PC-Cu1Se1 NW is observed. Figure S4 (d) shows a part of a PC-Cu3Se1 NW with an obvious interface due to two different-orientation crystals with lattice spacing of 1.763 and 1.873 Å, corresponding to the (533) and (532) planes of the tetragonal Cu2Se, respectively. This misaligned crystal orientations cause a curving characteristic of the nanowire, which is coincided with the SEM observation result (see Figure 3(b, c)). The growth process of the CuxSey should be similar to that of Cu7Te4 in PC-Cu7Te4 NWs reported in ref. [41]. As the PC-Cu3Se1 film exhibits a better TE performance, its internal microstructure was studied using HADDF-STEM. Figure 6(a) is a typical TEM image of the composite film, which shows a cross-sectional image of a few closely contacted nanowires. Figure 6(b) is a HRTEM image showing a cross-sectional of a PC-Cu3Se1 NW containing three different-orientation crystals. The lattice distance of the crystals is 6.8, 2.32, and 2.26 Å, corresponding to the (111), (403), and (510) planes of the tetragonal Cu2Se, respectively. Figure 6(c) displays the cross-sectional HRTEM image of a well-crystalized crystal of a PC-Cu3Se1 NW. The interplanar distances of 1.91, 1.52 and 1.76 Å correspond to the lattice spacing of (116), (703) and (533) planes (Figure 6(d)). The angle between (116) and (533) planes is 50.2o, whereas the angle between (116) and (703) planes is 58.1o. Figure 6(e) shows a HRTEM image of a nanowire consisting of four different- orientation crystals. Crystals A and B contain numerous stacking faults. The HAADF image (Figure 6(f)) more clearly shows the 21



microstructure, which is due to Cu deficiency, since gathering of Cu into one Se interlayer while leaving interlayer at its either side empty of Cu (the colorless zone between the two red dotted lines) 13. It indicates the existence of Cu2-xSe phase in the PC-Cu3Se1 composite film, which is in agreement with XRD result (Figure 2(b)). Hence, the PC-Cu3Se1 composite film consists of Cu2Se and Cu2-xSe polycrystals.

Figure 7. (a) A cross-sectional TEM image of the PC-Cu3Se1 film, (b) side view of three closely contacted PC-Cu3Se1 NWs, (c) schematic illustration of heat scattering and carriers transport across a NW-PEDOT:PSS-NW junction.

Figure 7(a) is a TEM image of the PC-Cu3Se1 film, which consists of the side view of a few nanowires and cross-sectional view of several nanowires. The NWs intertwine to form a network with nanopores (marked by red dotted lines) from ~20200 nm in size. In addition, the belt-like stuff at the low part of the image is nylon (most of the nylon membrane was broke off during the FIB process), which well combines

22


Page 22 of 35

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with the film (more clearly shown in Figure S5). Figure 7(b) shows a HRTEM image of three NWs, which closely contact each other, forming NW-PEDOT:PSS-NW junctions 56. Figure 7(c) illustrates the carries and phonons transport in the film. As the vacuum filtration process can remove excessive PSS, the PEDOT:PSS in the composite film is with good electrical conductivity

38;

hence, although there is a barrier at the

PEDOT:PSS/Cu2Se interface, high-energy carriers can pass through the junction, whereas the low energy carriers cannot. This energy-filtering effect is verified by the comparison of the Hall measurement results of the PC-Cu3Se1 and Cu3Se1 films (Figure S8). For the PC-Cu3Se1 film, the carrier concentration is lower than that of Cu3Se1, but the mobility is higher than that of Cu3Se1. We believe that the thermal conductivity of the PC-Cu3Se1 film around RT is rather low for the following reasons. (1) Cu2-xSe and Cu2Se have very low thermal conductivity values (around or less than 1 W/mK) 12; (2) The film consists of NWs and pores (20-200 nm in size), which can scatter phonons of a wide range of wavelengths; (3) It has numerous NW/PEDOT:PSS interfaces, film/nylon interfaces and NWPEDOT:PSS-NW junctions (see Figure 7(c)), which can also scatter heat-carrying phonons; (4) PEDOT:PSS nanolayer possesses an intrinsic low thermal conductivity 57. Hence, the nanocomposite film may have a similar thermal conductivity as that (0.250.3 W/mK) of the pure PEDOT:PSS 36, which leads to room-temperature ZT value of ∼ 0.3 for the nanocomposite film.

23



Page 24 of 35

Figure 8. Flexibility of the PC-Cu3Se1 film and evaluation of the film/nylon heterointerface. The electrical conductivity, Seebeck coefficient (a) and the ratio of power factor before and after bending (b) as a function of bending cycles, inset in (a) is an image demonstrating the bending test, where the bending diameter is 5 mm, (c) a HRTEM image showing good combination between the film and nylon membrane, inset is the enlarged image with some lattice fringes, (d) from left to right: HAADF image of a heterointerface between the film (upper part) and nylon (lower part), EDS images of elemental Cu, Se, C, S and N.

The reliability of the flexible TE films is very important for practical applications. Figure 8 (a, b) demonstrates the TE property of the PC-Cu3Se1 film varying with bending cycles (bending diameter of ∼ 5 mm). The power factor only decreases by about 15% after bending for 1000 cycles, which is due to the gradually decreased  (∼ 10% decrease) and the almost stable S. Compared with the Bi2Te3/SWCNT

2

and

Bi2Te3/PEDOT 31 hybrid flexible films, our hybrid film exhibits a better flexibility (see Table S2). Besides, the flexibility of our hybrid film is even comparable to that of the Cu2-xSe NW/PVDF film 26. This demonstrates a great potential for application of the PC-Cu3Se1/nylon hybrid film. In order to explain the reasons for the good flexibility of the hybrid film, detailed 24


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microstructure near the hetero-interface between the Cu2Se/PEDOT:PSS film and the nylon membrane was examined. Figure 8(c) shows that the Cu2Se NWs are well combined with the amorphous (nylon membrane). It is of interest that some lattice fringes were observed in nylon membrane under HRTEM (see inset in Figure 8(c)). As seen in Figure 8(d), Se signals are detected in the nylon membrane (the C, N elements are ascribed to the CONH group of nylon). This should be because the nylon membrane is porous (pore size ~ 200 nm), some tips of the PC-Cu3Se1 NWs penetrate into the pores during the filtration and they bind together during the cold pressing. Figure S5 also demonstrates the good combination between the two phases. As a result, reasons for the good flexibility of the hybrid film are as follows. First, nylon membrane possesses an intrinsic superior flexibility. Second, the nanoporous structure can accommodate the bending of the film 58, which makes the film itself have a certain flexibility. Third, the PEDOT:PSS nanolayer coated on the Cu2Se NWs may reduce the stress on inorganic nanofiller

59

and enhance the adhesion of wire-to-wire

and wire-to-nylon membrane. Moreover, cold-pressing process reinforces the NWPEDOT:PSS-NW junctions and the film-nylon heterointerface 26.

25



Figure 9. Performance of the TE device fabricated with the PC-Cu3Se1 composite film, (a) the relationship between output voltage and temperature gradient (inset is a digital photo of the 9leg TE device), (b) the output voltage and power versus current at different temperature differences. Demonstration of power generation by a 25-leg TE device, (c) a digital photo of the device, (d) a photo of 4.5 mV voltage created due to the temperature difference between an arm and the ambient, (e) a photo of 15.4 mV voltage created when the tea water was poured into the 500 ml beaker till the liquid level reached the lower edge of the device. The device is winded around the arm or the beaker, one side of the device was attached to an arm skin or the beaker and the other side is exposed to the air by using a bubble film as a thermal insulator. Insets in (d) and (e) are infrared thermal images showing the temperature differences between the arm and the ambient (~ 3.5 K), and between the out surface of the beaker and the ambient (~ 12 K), respectively.

Benefits from the flexibility of the composite films, a ﬂexible TE generator consisting of 9 legs (25 mm × 5 mm × 8 m) is fabricated (Figure 9(a)). Every leg is pasted on a PI substrate and connected in series using a silver paste. Figure 9(a) shows the open circuit voltage (VO) versus temperature gradient (T). When T is 18 and 30 26


Page 26 of 35

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K, VO of the device is 9.5 and 16 mV, respectively. The VO is defined as: 60 (6),

𝑉𝑂 = 𝑁𝑆∆𝑇

where N represents the number of TE legs. For T = 30 K, note that the calculated output voltage of one leg per 1 K T is ~ 59.2 V, which is larger than the value ~ 51 V measured at 300 K. This is mainly owing to the enhanced S value of each leg with increasing temperature during the test, as shown by the temperature dependence of S in Figure 5(a). As shown in Figure 8(b), the output voltage is inversely proportional to the output current. The output power (P) is calculated by the expression: P = (𝑈/(Rin + 𝑅𝑙𝑜𝑎𝑑))2𝑅𝑙𝑜𝑎𝑑

(7),

where Rload and Rin represent the external load resistance and the inner resistance of the device. When Rload = Rin, the maximum output power is obtained. At the T of 30 K, the Pmax of ~ 328 nW is generated when the Rload ~ 150 Ω (the voltage is about 10.7 mV and the current is about 42.9 A). By dividing the cross-sectional area for heat flow and N, the maximum power density ~ 91 W/cm2 is obtained

61,

verifying that the

composite film possesses a good TE performance. The power density of our module is superior to those of the reported TE devices based on organic/inorganic composites under the similar temperature difference.

39, 41, 55, 62

Note that the power density is

determined by the following equation: (𝑁 ∙ 𝑆 ∙ ∆𝑇)2 𝑃𝑚𝑎𝑥

𝑃density = 𝑁 ∙ 𝐴 =

4∙N∙

𝑁∙𝑤∙𝑑

𝑙

∙𝑤∙𝑡

=

𝑆2 ∙ 𝜎 4𝑙

∙ ∆𝑇2

(7),

where A, w, d, and l are area, width, thickness and length of the legs, respectively. After considering the effect of T and the l of the legs, the power density of the present work 27



Page 28 of 35

is still higher than that of previously reported works in Table 2.

Table 2 Comparison of the output properties between the present organic-inorganic TE devices and our module. VO

Pmaxb

Power density

(mV)

(nW)

(W/cm2)

PANi/CNT/Te

7.9

1000

Te/PEDOT:PSS

13.4

Te/PEDOT:PSS/Cu7Te4

Materialsa

ΔT (K)

N

l (mm)

ref.

62.4

40

4

50

55

47.7

57.2

40

9

20

39

31.2

94.7

39.5

39.1

8

25

41

-

280

80

40

10

20

62

16

328

91

30

9

25

This work

PEDOT:PSS (p)/ TiS2[(HA)(NMF)] (n) PEDOT:PSS/Cu2Se a

HA = hexylamine; NMF = N-methylformamide; b Pmax= maximum value of output power.

In order to demonstrate the utility of our prepared PC-Cu3Se1 materials, a 25-leg flexible TE generator has been assembled and sealed with a transparent tape for protection (Figure 9 (c)). One side of the device is attached to an arm or a 500 ml beaker, and the other side is exposed to the air by using a bubble film as a thermal insulator. Flexibility of the TE device can be verified by adhering it around the curved surface of a 500 ml beaker and a human’s upper arm closely (Figure 9 (d, e)). The TE device generates an output voltage of 4.5 mV derived from the temperature difference between an upper arm and the ambient (Figure 9 (d)). The inset in Figure 9(d) is a photo taken by an infrared camera showing the ΔT is ~ 3.5 K. Hence, the ΔV/ΔT value per leg is ~ 51.4 V/K, which is similar to the Seebeck coefficient ~ 51 V measured at 300 K. In addition, a voltage difference of 15.4 mV is generated when the tea water is 28


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poured into the beaker till the tea level reaches the lower edge of the device (Figure 9 (e)). As shown in the infrared image, a ΔT of 12 K between the out surface of the beaker and the ambient produces a thermovoltage of 15.4 mV, with one leg per 1 K T about 51.3 V. These examples indicate common waste heat in the ambient environment can be recovered to electric energy by the flexible TE devices. 4. CONCLUSIONS In summary, we successfully prepared good performance and flexible PEDOT:PSS/Cu2Se nanocomposite films on nylon by a simple and facile method. A maximum power factor ~ 389.7 W/mK2 at 418K was obtained from the film starting from Cu/Se nominal molar ratio of 3. The hybrid film shows an excellent flexibility, which results from the intrinsic flexibility of nylon, nanoporous structured composite film, and the good combination between nylon and film. A prototype device consisting of 9-leg of the optimized film generates a maximum power density of ~ 91 W/cm2 at a temperature difference of 30 K. This work demonstrates an effective route to prepare high TE performance PEDOT:PSS/selenide composite flexible film and the film is very promising for application in wearable energy harvesting. ■ ASSOCIATED CONTENT Supporting Information. Schematic illustration of TE device measurement. XPS spectra of the nanocomposite films. EDS spectra of the PC-Cu1Se1 and PC-Cu2Se1 films. TEM and HRTEM images of PC-Cu1Se1 NWs and PC-Cu3Se1 NWs. HAADF cross-sectional image of some tightly contacted NWs on nylon and corresponding EDS images. Cross-sectional SEM

29



Page 30 of 35

images. The relationship between the electrical conductivity and time of the PCCu1Se1 film exposed to air. Temperature dependence of TE properties for the PCCu1Se1 film. Room temperature Hall measurement results and TE parameters of the PC-Cu3Se1 film and Cu3Se1 film. The thickness of the PC-CuxSey films. Comparison of flexibility of the flexible films. ■ AUTHOR INFORMATION Corresponding Authors * E-mail: [email protected] (K.C.). * E-mail: [email protected] (J.H.). * E-mail: [email protected] (L.C.). ORCID Yao Lu: 0000-0002-8734-1056 Kefeng Cai: 0000-0002-7543-1628 Notes The authors declare no competing financial interest. ■ ACKNOWLEDGEMENTS This work was supported by the Key Program of National Natural Science Foundation of China (51632010 and 5162005) National Natural Science Foundation of China (51702150), National Basic Research Program of China (973 Program) under Grant No.2013CB632500, and the foundation of the State Key Lab of Advanced Technology for Material Synthesis and Processing (Wuhan University of Technology). Innovation

Commission

of

Shenzhen

Municipality

30


(Grant

Nos.

Page 31 of 35 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


KQTD2016022619565991 and KQCX2015033110182370).

31



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