In Situ Synthesis of High Thermoelectric Performance Bi₂Te₃ Flexible Thin Films through Thermal Diffusion Engineering

Abstract

Bi₂Te₃-based materials are promising candidates for near-room-temperature applications due to their high thermoelectric performance and low cost. Here, an innovative thermal diffusion strategy combined with magnetron sputtering and thermal evaporation methods was employed to fabricate Bi₂Te₃ flexible thin films (f-TFs) on a flexible polyimide substrate. An in situ synthesis of Bi₂Te₃ f-TFs with good crystallinity was obtained using a straightforward thermal diffusion method through diffusion of Te into a Bi precursor under low vacuum conditions (1 × 10⁵ Pa). This method offers easy preparation, low cost, and a large-area film preparation for industrialization. The electrical conductivity increases with increasing thermal diffusion temperatures. A high room temperature carrier mobility of ~28.7 cm⁻² V⁻¹ S⁻¹ and an electrical conductivity of ~995.6 S cm⁻¹ can be achieved. Then, a moderate room temperature Seebeck coefficient >100 μV K⁻¹ was obtained due to the chemical stoichiometry being close to the standard by optimizing the thermal diffusion temperature. Consequently, a maximum room temperature PF of ~11.6 μW cm⁻¹ K⁻¹ was observed in Bi₂Te₃ f-TFs prepared using a thermal diffusion temperature of 653 K. The thermal diffusion strategy applied in the thin film preparation represents an effective approach for the preparation of high thermoelectric performance Bi₂Te₃ f-TFs, offering a promising route for future thermoelectric applications.

1. Introduction

Thermoelectric technology can convert thermal and electrical energy, which is expected to be used in portable and wearable electronic devices [1,2,3,4,5]. The efficiency of thermoelectric materials is quantitatively evaluated by the dimensionless figure of merit (ZT) = S²σT/κ [6], where S, σ, κ, and T represent the Seebeck coefficient, the electrical conductivity, the thermal conductivity, and the absolute temperature, respectively [7,8]. Owing to the challenges inherent in measuring the κ of thin films, the power factor (PF) = S²σ has been used to evaluate the electric performance [9]. Materials such as SnSe (12.8 μW cm⁻¹ K⁻¹, ≈350 K) [10], Cu2Se (≈17 μW cm⁻¹ K⁻¹, 400 K) [11], AgSbTe2 (≈19 μW cm⁻¹ K⁻¹, 550 K) [11,12], Ag2Se (19.36 μW cm⁻¹ K⁻¹, 300 K) [13], PbS (20.58 μW cm⁻¹ K⁻¹, 473 K) [14], Sb2Te3 (21 μW cm⁻¹ K⁻¹, 300 K) [15], GeTe (40.8 μW cm⁻¹ K⁻¹, 623 K) [16], and STN (54 μW cm⁻¹ K⁻¹, ≈750 K) [17] are well known for their excellent thermoelectric properties. Bi₂Te₃ materials remain ideal candidates for room-temperature applications due to their excellent room-temperature properties and low cost [18,19,20,21,22,23,24,25,26].

In recent research, various material engineering strategies, such as defect engineering [27,28], energy band engineering [29], nanoengineering [30,31], and texturing engineering [32], have significantly enhanced the thermoelectric efficiency of thin films. For instance, Feng et al. [27] improved the thermoelectric properties of Bi₂Te₃ thin films by incorporating excess Te through defect engineering, and the PF improved significantly, from approximately 1.8 to 4.65 μW cm⁻¹ K⁻². Lan et al. [28] applied a pulsed electric field to C-doped Bi₂Te₃ films, improving their defect structures and thus increasing the PF from approximately ~15 to 22.5 μW cm⁻¹ K⁻². Chen et al. [29] effectively reduced the energy band gap through Se doping of Bi₂Te₃ and improved the S to −160 μV/K. Additionally, the PF increased to 15 μW cm⁻¹ K⁻². Yabuki et al. [30] synthesized integrated nanocomposite films by embedding single-walled carbon nanotubes into Bi₂Te₃ nanoflakes, and the PF notably increased to 1.38 μW cm⁻¹ K⁻².

A range of thin film deposition techniques, including screen printing [32], solvothermal methods [33], and magnetron sputtering [34,35], have been utilized to fabricate high-performance n-type Bi₂Te₃-based flexible thin films (f-TFs). Notably, Varghese et al. [32] synthesized Bi₂Te₃ nanocrystal inks via a microwave-assisted wet chemical approach for screen printing onto polyimide substrates. Subsequent cold pressing and sintering led to thermoelectric films that achieved a PF of approximately 5.65 at 448 K. Rani et al. [33] produced Bi₂Te₃ nanostructures through solvothermal methods, attaining a PF of 6.5 μW cm⁻¹ K⁻² at 385 K. He et al. [34] fabricated Bi₂Te₃ f-TFs through direct current magnetron sputtering with in situ annealing, achieving a PF of 8.2 μW cm⁻¹ K⁻². Furthermore, Joo et al. [35] generated Bi₂Te₃ f-TFs using radiofrequency co-sputtering at 573 K and recorded a PF of 9.7 μW cm⁻¹ K⁻². In our previously published work [36], we successfully synthesized high-quality Te-embedded Bi₂Te₃ f-TFs by using magnetron sputtering in combination with the thermal diffusion method. To further optimize the chemical content of Bi₂Te₃ f-TFs and improve its thermoelectric performance, the process can be further optimized to prepare Te through thermal evaporation, with the aim of achieving full sublimation of the Te and Bi-Te reaction.

In this study, a thermal diffusion process combined with thermal evaporation and magnetron sputtering was employed to prepare n-type Bi₂Te₃ f-TFs on polyimide (PI) substrates, as shown in Figure 1a. The thermal diffusion method can provide sufficient energy for the growth of Bi₂Te₃ crystals, significantly improving carrier transport performance [36]. The Bi film was deposited using magnetron sputtering, and the Te film was deposited using thermal evaporation methods. Evaporating Te film results in a higher preparation efficiency, which is more uniform and makes it easier to produce large-area thin films than magnetron sputtering. Additionally, the copper mold is more conducive to Te sublimation and the Bi-Te reaction. Figure 1b shows a schematic diagram of the Te diffusion into Bi to react to form Bi₂Te₃ during thermal diffusion. Thermal diffusion is the mutual diffusion reaction between Bi and Te to form Bi₂Te₃. Te sublimation and diffusion speed are faster due to the high saturated vapor pressure of Te. Bi₂Te₃ film is on the Bi precursor film after thermal diffusion, while there is no residue of materials in the Te precursor film due to the complete sublimation of Te induced by a high saturated vapor pressure. The σ increased with increasing thermal diffusion temperatures (T_diff) due to the improved carrier mobility induced by the weak carrier scattering. The maximum σ of ~995.6 S cm⁻¹ was achieved at T_diff = 653 K. By optimizing the T_diff, a S > 95 µV K⁻¹ can be obtained due to the reasonable chemical content regulation. Consequently, a room-temperature PF of ~11.6 µW cm⁻¹ K⁻² can be achieved in Bi₂Te₃ f-TFs under T_diff = 653 K. The PF value of our prepared Bi₂Te₃ f-TFs is competitive with those of films prepared by other scholars, as shown in Figure 1c. And the PF value of the prepared thin film is only slightly lower than the previously reported PF value of 14.65 µW cm ⁻¹ K⁻² [36]. The novelty thermal diffusion method has the advantages of easy preparation, low cost, and large-area film preparation. In this work, the ultra-high performance of the as-prepared intrinsic Bi₂Te₃ thin film was not achieved due to the lack of regulation. It is expected to achieve significant performance improvement through doping or structural regulation in the future.

2. Methods and Experimental Design

2.1. Thin Film Preparation

A thermal diffusion method combined with thermal evaporation methods was employed to synthesize n-type Bi₂Te₃ thin films on a flexible PI substrate. High purity tellurium power (99.99%, Macklin, Shanghai, China) weighing 0.6 g was used to deposit the Te film on the PI substrate using thermal evaporation methods, as shown in section I of Figure 1a. The thermal evaporation parameters were as follows: a thermal evaporation power of 20 W, an evaporation time of 13 min, and a thermal evaporation pressure of 5 × 10⁻⁵ Torr. The thickness of the as-deposited Te film was ~500 nm. A high purity Bismuth target (99.99%) was used to deposit the Bi precursor film on the PI substrate using magnetron sputtering methods, as shown in section II of Figure 1a. The magnetron sputtering parameters were as follows: a radio frequency power of 25 W, a duration of 30 min, and a working pressure of 1 Pa. The thickness of the Bi film was ~150 nm. Subsequently, both the Te and Bi films were pressurized in a copper mold placed on the heating equipment in a glove box (1 × 10⁵ Pa), as shown in section III of Figure 1a. Compared to the high vacuum experimental conditions used in our previous work [36], this experimental method is conducive to large-scale production. The T_diff was set to 608 K, 623 K, 638 K, and 653 K, respectively. The as-deposited n-type Bi₂Te₃ f-TFs is shown in the inset of Figure 1c.

2.2. Characterization of the Thin Film

The crystalline structure of the sample was investigated using X-ray diffraction (XRD, D/max 2500, Rigaku Corporation, Tokyo, Japan, utilizing CuKα radiation). Scanning electron microscopy (SEM, Zeiss-spra 55, Oberkochen, Baden-Württemberg, Germany) and SEM coupled with energy dispersive spectroscopy (SEM-EDS, Bruker Quantax 200, Billerica, MA, USA) were used to analyze the surface morphology and chemical composition. The Hall properties were measured using a Hall measurement system (HL5500PC, Nanometrics, Ottawa, ON, Canada), while the S and σ values were measured using a SBA458 system (Nezsch, Selb, Bavaria, Germany).

3. Results and Discussion

To elucidate the crystalline structure of the Bi₂Te₃ f-TFs prepared through thermal diffusion, an XRD analysis was employed, and XRD spectra are shown in Figure 2a. All the XRD peaks can be indexed to Bi₂Te₃ (PDF #15-0863). It can be seen that the three main diffraction peaks correspond to the (006), (015), and (0010) planes of Bi₂Te₃. The strongest (015) peaks suggest a preferred orientation, as shown in Figure 2a and Section I of Figure 2a. As well, the intensity of the (015) diffraction peaks increased with increasing T_diff, indicating the increase in crystallinity. Te peaks (PDF #36-1452) can be observed in Figure 2a. The enlarged (012) and (110) Te peaks in Section II of Figure 2a further confirm the existence of Te. The obvious Te peak appears in the Bi₂Te₃ f-TFs prepared at T_diff > 623 K. More Te atoms can be sublimated and diffused into the Bi precursor thin film at a high T_diff. The over-sublimated Te remained in the Bi precursor during cooling, leading to the formation of Te.

To further understand the Te content evolution, a detailed semi-quantitative investigation of Te can be measured in Figure 2b. As can be seen, the Te content increases and Bi content decreases with increasing T_diff. At T_diff = 608 K, the low Te content of Bi₂Te₃ is attributed to the poor diffusion reaction. When the T_diff increased to 623 K, the chemical content of Bi and Te was close to the standard chemical content of Bi₂Te₃ (40% and 60%) due to the fully thermal diffusion process. When T_diff increased to over 638 K, the Te content was above 60% due to the Te over-sublimation. This Te-rich content is consistent with the XRD results. To further characterize the valence states of Sb and Te in the Bi₂Te₃ f-TFs, XPS studies were employed, as shown in Figure 2c–e. The typical binding energy of Bi and Te can be observed in the XPS full spectra, as shown in Figure 2c. The binding energies at 157.0 eV and 162.3 eV correspond to Bi 4f_7/2 and Bi 4f_5/2, respectively, as shown in Figure 2d, indicating a Bi valence state of +3. The binding energies at 572.9 eV and 583.3 eV correspond to Te3d_5/2 and Te3d_7/2, respectively, as shown in Figure 2d, suggesting a Te valence state of −2. The XPS spectra further prove the formation of Bi₂Te₃.

The surface and cross-section morphology of the Bi₂Te₃ f-TFs was analyzed using SEM technology, as shown in Figure 3. The uniformly distributed nanoparticles of the Bi₂Te₃ f-TFs prepared at T_diff = 608 K, 623 K, 638 K, and 653 K can be observed in Figure 3a–d, respectively. This well-crystallized structure is consistent with the XRD results. The size of the Bi₂Te₃ particles increased with increasing T_diff. Finally, the large flake-like particles could be obtained at T_diff > 638 K, as shown in Figure 3d, indicating a high crystallinity. The cross-section SEM images of Bi₂Te₃ f-TFs are shown in Figure 3e. Highly dense crystals could be observed in all the Bi₂Te₃ f-TFs. And the thicknesses of the Bi₂Te₃ f-TFs prepared at T_diff = 608 K, 623 K, 638 K, and 653 K were ~293 nm, ~386 nm, ~300 nm, and ~280 nm, respectively. Figure 3f exhibits the SEM-BSE of the Bi₂Te₃ f-TFs prepared at T_diff = 653 K and the corresponding EDS mapping. As can be seen, most of the Bi and Te elements were uniformly distributed. Minor Te-enriched regions can be observed, as shown the marked white circle, resulting from over-sublimation of Te in Bi₂Te₃. The EDS maps further validated the Te-embedded Bi₂Te₃ composite film.

Figure 4 shows the electrical performances of the as-prepared Bi₂Te₃ f-TFs synthesized at different T_diff. The temperature-dependent σ of Bi₂Te₃ f-TFs is shown in Figure 4a. As can be seen, σ decreased with increasing T, and the maximum σ was achieved at room temperature. When T_diff increased from 608 K to 653 K, the room-temperature σ significantly increased from ~359.19 S cm⁻¹ to ~995.6 S cm⁻¹. To further understand the electric performance evolution, the Hall performance of Bi₂Te₃ f-TFs was investigated. The single parabolic band (SPB) model was employed to analyze the electrical performance. The n_e-dependent μ curves calculated using the SPB model are shown in Figure 4b. The μ increased with increasing T_diff, leading to an increase in σ. Correspondingly, the deformation potential coefficient (E_def) decreased with increasing T_diff, as shown in Figure 4c. This is consistent with the increase in μ, potentially resulting from reduced carrier scattering in the Te-embedded Bi₂Te₃ heterostructure. Compared to the highest σ of 567.69 S cm⁻¹ in our previous work [36], the high σ in this work is due to the decreased energy filtration effect caused by the composition approaching the standard stoichiometric ratio. Figure 4d plots the T-dependent S, and the negative S values confirm the traditional n-type semiconductor behavior. Additionally, S exhibited a decreasing trend with increasing T_diff values from 608 K to 653 K. All the room-temperature S values were above 100 μV K⁻¹ due to the near standard stoichiometric ratio, and the maximum S of −151.95 μV K⁻¹ in Bi₂Te₃ f-TFs prepared at T_diff = 608 K was obtained. n_e decreased with increasing T from 608 K to 638 K, and then n_e slightly increased. It can be seen that the change in S is attributed to the changes in n_e, regardless of the effective mass (m^*) evolution. Figure 4f shows the T-dependent PF, and the largest PF values were obtained at room temperature. The PF value of Bi₂Te₃ f-TFs prepared at T_diff = 653 K was obviously higher than that of T_diff < 638 K. The largest PF of ~11.6 μW cm⁻¹ K⁻² was achieved at T_diff = 653 K due to the corresponding high σ and S. As T_diff further increased, the thermoelectric performance decreased due to severe chemical content segregation, as shown in the Figure S1 and Table S1.

4. Conclusions

In this research, we successfully synthesized Bi₂Te₃ f-TFs with good crystallinity using a specialized thermal diffusion strategy combined with magnetron sputtering and thermal evaporation methods. The σ increased with increasing T due to the increase in μ induced by the decrease in E_def. The maximum room-temperature σ of ~995.6 S cm⁻¹ was obtained at T_diff = 653 K. By optimizing T_diff, a near-standard stoichiometric ratio can be achieved, leading to a moderate S > 100 μV K⁻¹. Finally, a high room-temperature PF reached ~11.6 μW cm⁻¹ K⁻² due to the high σ and S. The successful preparation of thin films at low vacuum levels is particularly beneficial for large-scale production of thin films.