A facile in-situ reaction method for preparing flexible Sb₂Te₃ thermoelectric thin films

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 = n_heμ, where n_h, 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 n_h 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 (FeCl₃-doped P3HT, S²σ = ~0.2 μW·cm^-1·K^-2 at 340 K)^[18], carbon nanotubes/polyaniline (CNTs/PANI, S²σ = ~4.07 μW·cm^-1·K^-2 at room temperature)^[19], and poly (3,4-ethyl enedioxythiophene) polystyrene sulfonate (PEDOT/PSS, S²σ = ~3.34 μW·cm^-1·K^-2 at room temperature)^[20], a room temperature S²σ is lower than 5.0 μW·cm^-1·K^-2. Inorganic f-TFs, such as Ag₂Se (S²σ = ~18.6 μW·cm^-1·K^-2 at room temperature)^[21], SnSe (S²σ = ~3.5 μW·cm^-1·K^-2 at 300 K)^[22], Cu₂Se (S²σ = ~11.12 μW·cm^-1·K^-2 at 549 K)^[23], n-type Bi₂Te₃(S²σ = ~14.65 μW·cm^-1·K^-2 at room temperature)^[24], p-type Sb₂Te₃ (S²σ = ~21.0 μW·cm^-1·K^-2 at room temperature)^[25], have been attracted great attention from the TE research community. It is evident that inorganic f-TFs approaches exhibited higher S²σ than that of the organic f-TFs, depicting good potential for commercial applications.

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 p-type Sb₂Te₃ f-TFs^[27-32], such as thermal evaporation^[29], magnetron sputtering^[30], solution printing^[31], and thermal diffusion^[32]. Vieira et al. realized a high σ of ~320 S·cm^-1 and S²σ of ~12.0 μW·cm^-1·K^-2 at 298 K for Sb₂Te₃ f-TFs prepared by thermal evaporation^[29]. Shang et al. successfully prepared (00l)-preferential orientation p-type Sb₂Te₃ f-TFs by magnetron sputtering methods and approached the σ of ~740 S·cm^-1 and S²σ of ~12.4 μW·cm^-1·K^-2 at 300 K. Sb₂Te₃ f-TFs prepared by screen-printing technology can approach σ of ~250 S·cm^-1 and S²σ of ~14.3 μW·cm^-1·K^-2 at room temperature^[30,31]. It is concluded that the σ of Sb₂Te₃ f-TFs prepared by most of the preparation methods is lower than 1,000 S·cm^-1 due to poor crystal growth. Consequently, the carrier transport properties were suppressed, and the corresponding TE performance of Sb₂Te₃ f-TFs is also limited.

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 (T_diff), 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 T_diff due to the weakened carrier scattering. Correspondingly, the highest value of S²σ of 16.0 μW·cm^-1·K^-2 at T_diff = 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

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 T_diff. Further, the corresponding calculated crystallinity increased with increasing T_diff [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 3d_5/2 and Sb 3d_3/2, 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 Sb₂Te₃. Moreover, the binding energies at 583.03 and 572.82 eV are associated with Te 3d_3/2 and Te 3d_5/2, respectively [Figure 2F], and the corresponding valance state of Te is -2.

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 T_diff = 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 T_diff from 573 to 623 K. When the T_diff 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 T_diff = 603, 623, and 643 K, respectively; (B) EDS spectrum and atomic content of Sb₂Te₃ f-TFs prepared at T_diff = 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 Sb₂Te₃f-TFs [Figure 4]. The low-resolution [Figure 4A] and high-resolution images [Figure 4B] for Sb₂Te₃ prepared at T_diff = 623 K show a neat arranged of lattices and the corresponding clear diffraction spots, which indicates good crystallinity of Sb₂Te₃ f-TFs. Figure 4C presents the enlarged image of the yellow square in Figure 4B. It can be seen that the measured value of the crystal spacing of ~3.2 Å can be indexed as the (015) plane of Sb₂Te₃, which was consistent with the XRD results. The lattice strains of Figure 4C are shown in Figure 4D and present no obvious lattice strain concentration along different directions due to the fewer lattice defects and lattice mismatches. In Figure 4E, the TEM-EDS maps depict that the Sb and Te are uniformly distributed throughout the Sb₂Te₃ nanoparticles. From the TEM-EDS [Figure 4F], it can be predicted that the chemical stoichiometry of Sb:Te is also close to 2.0:3.0 (at the nano-scale range).

Figure 4. (A) Low-resolution TEM image of Sb₂Te₃ f-TF prepared at T_diff = 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 X-ray spectroscopy; f-TFs: flexible thin films; TEM: transmission electron microscopy.

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 T_diff is shown in Figure 5A. The room temperature σ increased with increasing T_diff. Moreover, the highest value of σ reaches 1,440 S·cm^-1 at T_diff = 643 K. To further study the changes of σ, we measured the μ and n_h 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 n_h^[33]. The μ increases with increasing T_diff, while the n_h varies from around 1.1 × 10¹⁹ cm^-3. It can be suggested that increasing μ is not mainly caused by the changes of n_h. The μ is achieved from 63.4 cm²·V^-1·s^-1 at T_diff = 573 K to a high value of 79.7 cm²·V^-1·s^-1 at T_diff = 643 K. In addition, the deformation potential coefficient (E_def) calculated by the SPB model roughly decreases with increasing T_diff. It is suggested that the enhanced crystallinity leads to the weakened carrier scattering, which is the main reason for the increased σ with increasing T_diff. Figure 5C presents the room temperature S as a function of T_diff. 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 T_diff. The maximum S of ~106 μV·K^-1 is achieved at T_diff = 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 n_h. As can be seen, the measured room temperature n_h remains nearly constant around 1.1 × 10²⁰ cm^-3 with increasing T_diff. Furthermore, the m^* calculated by the SPB model changed in the range from 1.26 to 1.37 m₀. Besides, the m^* increases and then decreases with increasing T_diff. Correspondingly, the trend of S with increasing T_diff is the same as the trend of m^*. The changes of S can be attributed to the changes of m^*. Figure 5E compares the measured S²σ and the calculated S²σ based on the SPB model at room temperature. All the S²σ value of Sb₂Te₃ f-TFs at room temperature is higher than 11 μW·cm^-1·K^-2. The maximum value of S²σ approaches as high as 16.0 μW·cm^-1·K^-2 at T_diff = 623 K due to the high σ and moderate S. The high S²σ of our prepared Sb₂Te₃ f-TFs is competitive with that of the reported Sb₂Te₃ f-TFs [Supplementary Table 1]. Herein, the prepared Sb₂Te₃ f-TFs possess high bending resistance. The high bending resistance of Sb₂Te₃ thin film is mainly attributed to the PI flexible substrate and Nano-scale thickness Sb₂Te₃^[35,36]. Both the resistance change (∆R/R₀) and Seebeck coefficient change (∆S/S₀) are lower than 10% under the bending cycles of 1,000 and bending radius of 18 mm [Figure 5F].

Figure 5. (A) The measured room temperature σ as a function of T_diff; (B) Comparison between the measured and SPB model calculated μ as a function of n_h; (C) The measured room temperature S as a function of T_diff; (D) Comparison between the measured and SPB model calculated S as a function of n_h; (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 T_diff and increased the crystallinity of Sb₂Te₃ f-TFs. With increasing T_diff, tuning crystallinity increased σ and thus attenuated carrier scattering, achieving a high σ of ~1,440 S·cm^-1 at T_diff = 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 T_diff = 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.