A facile in-situ reaction method for preparing flexible Sb2Te3 thermoelectric thin films
Abstract
Inorganic p-type Sb2Te3 flexible thin films (f-TFs) with eco-friendly and high thermoelectric performance have attracted wide research interest and potential for commercial applications. This study employs a facile in-situ reaction method to prepare flexible Sb2Te3 thin films by rationally adjusting the synthesized temperature. The prepared thin films show good crystallinity, which enhances the electrical conductivity of ~1,440 S·cm-1 due to the weakened carrier scattering. Simultaneously, the optimized carrier concentration, through adjusting the synthesis temperature, causes the intermediate Seebeck coefficient. Consequently, a high-power factor (16.0 μW·cm-1·K-2 at 300 K) is achieved for Sb2Te3 f-TFs prepared at 623 K. Besides, the f-TFs also exhibit good flexibility due to the slight change in resistance after bending. This study specifies that the in-situ reaction method is an effective route to prepare Sb2Te3 f-TFs with high thermoelectric performance.
Keywords
INTRODUCTION
Thermoelectric (TE) technology can achieve direct conversion between thermal energy and electrical energy, which has significant applications in power generation and refrigeration[1-5]. With an increasing demand for micro-electromechanical systems of chip-sensors, wearable electronics, and implantable electronic devices, the TE flexible thin films (f-TFs) have attracted extensive interest due to their high adaptability to various conditions with high TE performance[6-10]. The TE performance of f-TFs can be accessed via power factor (S2σ)[11], where σ and S represent the electric conductivity and Seebeck coefficient, respectively. Herein, σ is defined as σ = nheμ, where nh, e, and μ represent carrier concentration, elementary charge, and carrier mobility, respectively[12,13]. The S can be evaluated by Mott formula[14,15]. The increase of S can be achieved by the decreased nh and increased effective mass (m*). However, it is a significant challenge to simultaneously increase the S and σ due to their coupled relationship. Typically, f-TFs are composed of organic f-TFs and inorganic f-TFs[16,17]. For typical organic f-TFs, such as 3-hexylthiophene-2, 5-diyl
Among inorganic TE f-TFs, p-type Sb2Te3 f-TFs with a narrow bandgap of ~0.3 eV possess good TE performance at near room temperature[26]. So far, numerous methods have been employed to synthesize
In the present work, we employed a thermal diffusion method to prepare p-type Sb2Te3 f-TFs on a flexible polyimide (PI) substrate. The Sb and Te precursor films were deposited by thermal evaporation, as shown in Figure 1A. Pure Sb and Te f-TFs were obtained separately. The schematic diagram of the thermal diffusion process and the optical image of as-prepared Sb2Te3 f-TFs are shown in Figure 1B. The copper mold consists of a convex mold on the top and a concave mold placed below. Cu molds at both the top and bottom can enhance heat conduction and improve the uniformity of heat distribution during the thermal diffusion process. The Sb2Te3 f-TFs were synthesized by thermal diffusion methods using Te and Sb pure precursor films. The schematic diagram of the reaction process of Sb and Te during the thermal diffusion process is shown in Figure 1C. Through tuning the thermal diffusion temperature (Tdiff), the Sb2Te3 f-TFs with standard stoichiometric ratios were obtained. Moreover, the moderate Seebeck coefficient of > 95 μV·K-1 was achieved at room temperature. Simultaneously, the μ and σ increased with increasing Tdiff due to the weakened carrier scattering. Correspondingly, the highest value of S2σ of 16.0 μW·cm-1·K-2 at Tdiff = 623 K has been achieved. Besides, our prepared Sb2Te3 f-TFs approach good bending resistance.
Figure 1. (A) The schematic diagram of the Sb and Te f-TFs prepared by thermal evaporation; (B) The schematic diagram of the thermal diffusion process. The Inset shows the optical image; (C) The schematic diagram of Sb2Te3 f-TF preparation through the thermal diffusion process. f-TFs: Flexible thin films.
EXPERIMENTAL
Sb2Te3 f-TF preparation process
The p-type Sb2Te3 f-TFs were prepared on a flexible PI substrate by a thermal diffusion method. First, Sb and Te films were deposited on PI substrates by using thermal evaporation. Te content is higher than a standard stoichiometric ratio of Sb:Te (2:3) due to the facile volatilization of Te induced by high saturation vapor pressure. The purity Sb (99.99 %) and Te (99.99 %) powders were weighed for 0.7025 and 1.5217 g, respectively. The Sb and Te evaporation parameters were as follows: the evaporation power of 18 and 20 W, the evaporation time of 13 and 16 min, and the evaporation pressure of 5 × 10-5 Torr, respectively, and the thickness of the Sb film and Te film were ~220 and ~520 nm. Secondly, the Te and Sb f-TFs were pressurized in the copper molds placed on heating equipment with a pressure of 1 N·mm-2. The Tdiff was set as 573, 603, 623, and 643 K, respectively. The corresponding heating and cooling rates are ~20 and 2 K/min, respectively. The sample was held at the target temperature for 30 min, and the pressurized sample was kept under vacuum for about 2 h to cool to room temperature. The as-prepared p-type Sb2Te3 f-TFs are shown in the inset of Figure 1B. The thickness of the as-prepared Sb2Te3 thin film is ~700 nm.
Characterization of the Sb2Te3 f-TFs
X-ray diffraction (XRD, D/max 2500 Rigaku Corporation, CuKα radiation) was employed to investigate crystal structures. Scanning electron microscopy (SEM) (Zeiss spra 55) and SEM with energy dispersive
RESULTS AND DISCUSSION
The crystal structure of Sb2Te3 f-TFs was investigated by XRD technology [Figure 2A]. All XRD peaks can be indexed to Sb2Te3 (PDF#15-0874), and no obvious impurity peaks were observed. The (015), (1010), and (110) diffraction peaks are the three main peaks. The highest peak is (015), indicating (015) preferred orientation of Sb2Te3 f-TFs. The enlarged (015) peaks are plotted in the inset I of Figure 2A, and the (015) peaks increased with increasing the Tdiff. Further, the corresponding calculated crystallinity increased with increasing Tdiff [Figure 2B]. Figure 2C depicts the calculated lattice parameters a and c, which clearly indicate the absence of any displacement in crystal structures. The valence states of Sb and Te in the Sb2Te3 films were investigated by XPS [Figure 2D-F]. Figure 2D-F presents the full XPS spectra and XPS spectra of Sb and Te, respectively. The presence of oxidized Sb2Te3 (indicated by peaks at 539.35 and 530.19 eV) and oxygen (evident in the O1s peak) is observed in Figure 2E due to the unencapsulated Sb2Te3 f-TFs used in an atmospheric environment. The binding energies at 528.46 and 537.77 eV were related to Sb 3d5/2 and Sb 3d3/2, respectively [Figure 2E], and the corresponding valance state of Sb was +3. The 3d core level of Te with two peaks at 586.54 and 576.99 eV was related to the oxidized
Figure 2. (A) The XRD spectra of as-prepared Sb2Te3 f-TFs; (B) The enlarged (015) peaks and the corresponding calculated crystallinity; (C) The lattice parameters; (D) The full XPS spectra; (E) and (F) XPS spectra of Sb and Te, respectively. f-TFs: Flexible thin films; XPS: X-ray photoelectron spectroscopy; XRD: X-ray diffraction.
The crystal morphology and chemical composition of the as-prepared Sb2Te3 f-TFs were investigated through SEM and SEM-EDS technology [Figure 3]. Figure 3A shows the SEM surface morphology of Sb2Te3 f-TFs. All the films depict a morphology characterized by large particles, suggesting a typical dense polycrystalline structure. Figure 3B presents the EDS spectrum and atomic content of Sb2Te3 f-TFs prepared at Tdiff = 623 K. As can be seen, the chemical stoichiometry of Sb:Te was ~2.0:3.0. The corresponding EDS maps are shown in Figure 3C. Uniformly distributed Sb and Te elements were obtained, and no obvious enrichment in Te and Sb was detected. Figure 3C shows the atomic content of Sb2Te3 f-TFs, where the chemical stoichiometry of Sb:Te was closed to 2.0:3.0 for Sb2Te3 films prepared at Tdiff from 573 to 623 K. When the Tdiff reaches at 643 K, the chemical stoichiometry of Sb:Te was 2.0:2.8. It is suggested that the Te content slightly decreases due to the evaporation of Te at high temperatures.
Figure 3. (A) SEM morphology of Sb2Te3 f-TFs prepared at Tdiff = 603, 623, and 643 K, respectively; (B) EDS spectrum and atomic content of Sb2Te3 f-TFs prepared at Tdiff = 623 K; (C) The corresponding SEM-BSE images and EDS maps; (D) The measured atomic contents of Sb2Te3 f-TFs. BSE: Backscattered electron; EDS: energy dispersive X-ray spectroscopy; f-TFs: flexible thin films; SEM: scanning electron microscopy.
To further study the changes in microstructure in detail, TEM was employed for the as-prepared Sb2Te3
Figure 4. (A) Low-resolution TEM image of Sb2Te3 f-TF prepared at Tdiff = 623 K; (B) High-resolution TEM image taken from the yellow square in Figure 4A; (C) The enlarged TEM images of the yellow square in Figure 4B; (D) The corresponding lattice strains of Figure 4C along different directions; (E) The TEM-EDS maps of Sb and Te; (F) The corresponding TEM-EDS spectrum. EDS: Energy dispersive
As can be seen, Sb2Te3 thin films possess the highest S2σ near room temperature, as shown in Supplementary Figure 1. This work focuses on analyzing room-temperature TE performance as the core research topic. The TE performance of as-prepared Sb2Te3 f-TFs at room temperature is shown in Figure 5. The room temperature σ of Sb2Te3 f-TFs as a function of Tdiff is shown in Figure 5A. The room temperature σ increased with increasing Tdiff. Moreover, the highest value of σ reaches 1,440 S·cm-1 at Tdiff = 643 K. To further study the changes of σ, we measured the μ and nh of Sb2Te3 f-TFs at room temperature. Figure 5B compares the measured and calculated [based on single parabolic band (SPB)-model] μ as a function of nh[33]. The μ increases with increasing Tdiff, while the nh varies from around 1.1 × 1019 cm-3. It can be suggested that increasing μ is not mainly caused by the changes of nh. The μ is achieved from 63.4 cm2·V-1·s-1 at Tdiff = 573 K to a high value of 79.7 cm2·V-1·s-1 at Tdiff = 643 K. In addition, the deformation potential coefficient (Edef) calculated by the SPB model roughly decreases with increasing Tdiff. It is suggested that the enhanced crystallinity leads to the weakened carrier scattering, which is the main reason for the increased σ with increasing Tdiff. Figure 5C presents the room temperature S as a function of Tdiff. The positive values of S show the typical p-type semiconductor characteristics. Similar Seebeck coefficients are achieved due to all the films with a near standard stoichiometric ratio of 2:3. The S slightly increases and then decreases in the range of 95-110 μV·K-1 with increasing Tdiff. The maximum S of ~106 μV·K-1 is achieved at Tdiff = 623 K. The high Seebeck coefficient of as-prepared Sb2Te3 f-TFs is competitive with that of some bulk Sb2Te3[34]. Figure 5D shows the comparison between the measured and calculated S (based on the SPB model) as a function of nh. As can be seen, the measured room temperature nh remains nearly constant around
Figure 5. (A) The measured room temperature σ as a function of Tdiff; (B) Comparison between the measured and SPB model calculated μ as a function of nh; (C) The measured room temperature S as a function of Tdiff; (D) Comparison between the measured and SPB model calculated S as a function of nh; (E) Comparison between the measured S2σ and the calculated S2σ based on the SPB model at room temperature; (F) The ∆R/R0 and ∆S/S0 changes of Sb2Te3 with bending cycles of 1,000 and a bending radius of 18 mm. SPB: Single parabolic band.
CONCLUSION
In this work, we successfully prepared Sb2Te3 f-TFs with high TE performance and bending resistance by the thermal diffusion method. Sb2Te3 f-TFs with standard stoichiometric ratios were achieved, which rationally tuned Tdiff and increased the crystallinity of Sb2Te3 f-TFs. With increasing Tdiff, tuning crystallinity increased σ and thus attenuated carrier scattering, achieving a high σ of ~1,440 S·cm-1 at Tdiff = 643 K. The moderate S larger than 95 μV·K-1 has been achieved due to standard stoichiometric ratios of Sb2Te3 f-TFs. Correspondingly, an excellent room temperature S2σ of 16.0 μW·cm-1·K-2 at Tdiff = 623 K has been achieved. Besides, a ΔR/R0 of < 10% is achieved after 1,000 bending cycles with a bending radius of 18 mm, indicating good bending resistance.
DECLARATIONS
Authors’ contributions
Made substantial contributions to the conceptualization and design of methodology and writing - original draft: Ao D, Wu B, Sun B, Yang D, Zhong Y
Performed data acquisition and technical work and provided supervision for writing - review and editing: Bushra J, Zheng Z
Availability of data and materials
Not applicable.
Financial support and sponsorship
This work was supported by Natural Science Foundation of China (12204355 and 52272210), Natural Science Fundations of Shandong Province (ZR2022QA018 and ZR2023ME001), the China Postdoctoral Science Foundation(2023M732609), and Doctoral Research Initiation Fund of Weifang University (2023BS01).
Conflicts of interest
All authors declared that there are no conflicts of interest.
Ethical approval and consent to participate
Not applicable.
Consent for publication
Not applicable.
Copyright
© The Author(s) 2024.
Supplementary Materials
REFERENCES
1. Liu D, Wang D, Hong T, et al. Lattice plainification advances highly effective SnSe crystalline thermoelectrics. Science 2023;380:841-6.
2. Wu X, Han Z, Zhu Y, et al. A general design strategy for thermoelectric interface materials in n-type Mg3Sb1.5Bi0.5 single leg used in TEGs. Acta Mater 2022;226:117616.
3. Li Y, Lou Q, Yang J, et al. Exceptionally high power factor Ag2Se/Se/polypyrrole composite films for flexible thermoelectric generators. Adv Funct Mater 2022;32:2106902.
4. Chen YX, Zhang JZ, Nisar M, et al. Realizing high thermoelectric performance in n-type Bi2Te3 based thin films via post-selenization diffusion. J Materiomics 2023;9:618-25.
5. Li L, Liu WD, Liu Q, Chen ZG. Multifunctional wearable thermoelectrics for personal thermal management. Adv Funct Mater 2022;32:2200548.
6. Shi XL, Zou J, Chen ZG. Advanced thermoelectric design: from materials and structures to devices. Chem Rev 2020;120:7399-515.
7. Zhang L, Shi XL, Yang YL, Chen ZG. Flexible thermoelectric materials and devices: from materials to applications. Mater Today 2021;46:62-108.
8. Wang Y, Yang L, Shi XL, et al. Flexible thermoelectric materials and generators: challenges and innovations. Adv Mater 2019;31:1807916.
9. Chi C, Liu G, An M, et al. Reversible bipolar thermopower of ionic thermoelectric polymer composite for cyclic energy generation. Nat Commun 2023;14:306.
10. Hu B, Shi XL, Zou J, Chen ZG. Thermoelectrics for medical applications: progress, challenges, and perspectives. Chem Eng J 2022;437:135268.
11. Ao DW, Liu WD, Zheng ZH, et al. Assembly-free fabrication of high-performance flexible inorganic thin-film thermoelectric device prepared by a thermal diffusion. Adv Energy Mater 2022;12:2202731.
12. Zhang R, Pei J, Han ZJ, Wu Y, Zhao Z, Zhang BP. Optimal performance of Cu1.8S1-xTex thermoelectric materials fabricated via high-pressure process at room temperature. J Adv Ceram 2020;9:535-43.
13. Shi X, Chen H, Hao F, et al. Room-temperature ductile inorganic semiconductor. Nat Mater 2018;17:421-6.
14. Hashizume M, Yokouchi T, Nakagawa K, Shiomi Y. Anisotropic magneto-Seebeck effect in the antiferromagnetic semimetal FeGe2. Phys Rev B 2021;104:115109.
15. Hong M, Zou J, Chen ZG. Thermoelectric GeTe with diverse degrees of freedom having secured superhigh performance. Adv Mater 2019;31:1807071.
16. Deng L, Liu Y, Zhang Y, Wang S, Gao P. Organic thermoelectric materials: niche harvester of thermal energy. Adv Funct Mater 2023;33:2210770.
17. Wu Z, Zhang S, Liu Z, Mu E, Hu Z. Thermoelectric converter: strategies from materials to device application. Nano Energy 2022;91:106692.
18. Wu L, Li H, Chai H, Xu Q, Chen Y, Chen L. Anion-dependent molecular doping and charge transport in ferric salt-doped P3HT for thermoelectric application. ACS Appl Electron Mater 2021;3:1252-9.
19. Li H, Liu Y, Li P, Liu S, Du F, He C. Enhanced thermoelectric performance of carbon nanotubes/polyaniline composites by multiple interface engineering. ACS Appl Mater Interfaces 2021;13:6650-8.
20. Fan Z, Li P, Du D, Ouyang J. Significantly enhanced thermoelectric properties of PEDOT:PSS films through sequential post-treatments with common acids and bases. Adv Energy Mater 2017;7:1602116.
21. Gao Q, Wang W, Lu Y, et al. High power factor Ag/Ag2Se composite films for flexible thermoelectric generators. ACS Appl Mater Interfaces 2021;13:14327-33.
22. Rongione NA, Li M, Wu H, et al. High-performance solution-processable flexible snse nanosheet films for lower grade waste heat recovery. Adv Elect Mater 2019;5:1800774.
23. Zheng ZH, Zhang DL, Jabar B, et al. Realizing high thermoelectric performance in highly (0l0)-textured flexible Cu2Se thin film for wearable energy harvesting. Mater Today Phys 2022;24:100659.
24. Ao DW, Liu WD, Chen YX, et al. Novel thermal diffusion temperature engineering leading to high thermoelectric performance in
25. Wei M, Shi XL, Zheng ZH, et al. Directional thermal diffusion realizing inorganic Sb2Te3/Te hybrid thin films with high thermoelectric performance and flexibility. Adv Funct Mater 2022;32:2207903.
26. Liu H, Li D, Ma C, et al. Van der Waals epitaxial growth of vertically stacked Sb2Te3/MoS2 p-n heterojunctions for high performance optoelectronics. Nano Energy 2019;59:66-74.
27. Ma F, Ao D, Liu X, Liu WD. Ti-doping inducing high-performance flexible p-type Bi0.5Sb1.5Te3-based thin film. Ceram Int 2023;49:18584-91.
28. Shen S, Zhu W, Deng Y, Zhao H, Peng Y, Wang C. Enhancing thermoelectric properties of Sb2Te3 flexible thin film through microstructure control and crystal preferential orientation engineering. Appl Surf Sci 2017;414:197-204.
29. Vieira EMF, Figueira J, Pires AL, et al. Enhanced thermoelectric properties of Sb2Te3 and Bi2Te3 films for flexible thermal sensors. J Alloys Compd 2019;774:1102-16.
30. Shang H, Li T, Luo D, et al. High-performance Ag-modified Bi0.5Sb1.5Te3 films for the flexible thermoelectric generator. ACS Appl Mater Interfaces 2020;12:7358-65.
31. Chang PS, Liao CN. Screen-printed flexible thermoelectric generator with directional heat collection design. J Alloys Compd 2020;836:155471.
32. Zheng ZH, Shi XL, Ao DW, et al. Harvesting waste heat with flexible Bi2Te3 thermoelectric thin film. Nat Sustain 2023;6:180-91.
33. Liu WD, Chen ZG, Zou J. Eco-friendly higher manganese silicide thermoelectric materials: progress and future challenges. Adv Energy Mater 2018;8:1800056.
34. Shi J, Chen X, Wang W, Chen H. A new rapid synthesis of thermoelectric Sb2Te3 ingots using selective laser melting 3D printing. Mater Sci Semicond Process 2021;123:105551.
35. Yang Q, Yang S, Qiu P, et al. Flexible thermoelectrics based on ductile semiconductors. Science 2022;377:854-8.
Cite This Article
How to Cite
Ao, D.; Wu B.; Bushra J.; Sun B.; Yang D.; Zhong Y.; Zheng Z. A facile in-situ reaction method for preparing flexible Sb2Te3 thermoelectric thin films. Soft. Sci. 2024, 4, 3. http://dx.doi.org/10.20517/ss.2023.34
Download Citation
Export Citation File:
Type of Import
Tips on Downloading Citation
Citation Manager File Format
Type of Import
Direct Import: When the Direct Import option is selected (the default state), a dialogue box will give you the option to Save or Open the downloaded citation data. Choosing Open will either launch your citation manager or give you a choice of applications with which to use the metadata. The Save option saves the file locally for later use.
Indirect Import: When the Indirect Import option is selected, the metadata is displayed and may be copied and pasted as needed.
Comments
Comments must be written in English. Spam, offensive content, impersonation, and private information will not be permitted. If any comment is reported and identified as inappropriate content by OAE staff, the comment will be removed without notice. If you have any queries or need any help, please contact us at support@oaepublish.com.