# Novel Thermal Diffusion Temperature Engineering Leading to High Thermoelectric Performance in Bi2Te3-Based Flexible Thin-Films

Dong-Wei Ao, Wei-Di Liu, Yue-Xing Chen, Meng Wei, Bushra Jabar, Fu Li, Xiao-Lei Shi, Zhuang-Hao Zheng, Guang-Xing Liang, Xianghua Zhang, et al.

► To cite this version:

Dong-Wei Ao, Wei-Di Liu, Yue-Xing Chen, Meng Wei, Bushra Jabar, et al. Novel Thermal Diffusion Temperature Engineering Leading to High Thermoelectric Performance in Bi2Te3-Based Flexible Thin-Films. Advanced Science, 2022, 9 (5), pp.2103547. 10.1002/advs.202103547. hal-03512509

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# Novel Thermal Diffusion Temperature Engineering Leading to High Thermoelectric Performance in Bi₂Te₃-Based Flexible Thin-Films

Dong-Wei Ao, Wei-Di Liu, Yue-Xing Chen, Meng Wei, Bushra Jabar, Fu Li, Xiao-Lei Shi, Zhuang-Hao Zheng,* Guang-Xing Liang, Xiang-Hua Zhang, Ping Fan, and Zhi-Gang Chen

Flexible Bi₂Te₃-based thermoelectric devices can function as power generators for powering wearable electronics or chip-sensors for internet-of-things. However, the unsatisfied performance of n-type Bi₂Te₃ flexible thin films significantly limits their wide application. In this study, a novel thermal diffusion method is employed to fabricate n-type Te-embedded Bi₂Te₃ flexible thin films on flexible polyimide substrates, where Te embeddings can be achieved by tuning the thermal diffusion temperature and correspondingly result in an energy filtering effect at the Bi₂Te₃/Te interfaces. The energy filtering effect can lead to a high Seebeck coefficient ≈160 µV K⁻¹ as well as high carrier mobility of ≈200 cm² V⁻¹ s⁻¹ at room-temperature. Consequently, an ultrahigh room-temperature power factor of 14.65 µW cm⁻¹ K⁻² can be observed in the Te-embedded Bi₂Te₃ flexible thin films prepared at the diffusion temperature of 623 K. A thermoelectric sensor is also assembled through integrating the n-type Bi₂Te₃ flexible thin films with p-type Sb₂Te₃ counterparts, which can fast reflect finger-touch status and demonstrate the applicability of as-prepared Te-embedded Bi₂Te₃ flexible thin films. This study indicates that the thermal diffusion method is an effective way to fabricate high-performance and applicable flexible Te-embedded Bi₂Te₃-based thin films.

## 1. Introduction

Thermoelectrics, as an emission-free power generation technology,[¹⁻³] can provide eco-friendly power supply for the portable and wearable electronic devices, such as chip-sensors,[⁴] electrocardiographic systems,[⁵] and implantable electronic devices.[⁶] The conversion efficiency of a thermoelectric device is determined by the performance of composing thermoelectric materials, which is evaluated by dimensionless figure-of-merit ZT (ZT = S²σT/κ, where S, σ, S²σ, T, and κ are the Seebeck coefficient, electrical conductivity, power factor, absolute temperature, and thermal conductivity, respectively).[⁷,⁸] Regardless of high-performance thermoelectric materials, such as SnSe,[⁹] Cu₂Se,[¹⁰] CoSb₃,[¹¹] GeTe,[¹²] and Mg₃Sb₂,[¹³] traditional Bi₂Te₃-based thermoelectric materials are still widely studied because of their superior performance at room-temperature.[¹⁴⁻¹⁶]

With potentially high flexibility, thermoelectric thin films are more suitable

D.-W. Ao, Y.-X. Chen, M. Wei, B. Jabar, F. Li, Z.-H. Zheng, G.-X. Liang, P. Fan

Shenzhen Key Laboratory of Advanced Thin Films and Applications
Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and Guangdong Province

College of Physics and Optoelectronic Engineering

Shenzhen University

Shenzhen 518060, P. R. China

E-mail: zhengzh@szu.edu.cn

W.-D. Liu, X.-L. Shi, Z.-G. Chen

Centre for Future Materials University of Southern Queensland

Springfield Central

Brisbane, QLD 4300, Australia

W.-D. Liu, X.-L. Shi, Z.-G. Chen

School of Mechanical and Mining Engineering

The University of Queensland

St Lucia, QLD 4072, Australia

X.-H. Zhang

Univ Rennes

CNRS

ISCR (Institut des Sciences Chimiques de Rennes) UMR6226

Rennes F-35000, France

<figure><img src="image_3.png" /></figure>

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/advs.202103547

© 2021 The Authors. Advanced Science published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

DOI: 10.1002/advs.202103547
for wearable electronic design. Particularly, n-type Bi₂Te₃-
based and p-type Bi₀.₅Sb₁.₆Te₃-based thermoelectric materials
have attracted extensive attentions.[¹⁵⁻²⁰] Various material en-
gineering strategies, such as defect-engineering,[²¹,²²] texturing
engineering,[²³] nanoengineering,[²⁴⁻²⁶] have been used to boost
thermoelectric performance of Bi₂Te₃-based flexible thin films.
Till now, significant enhancements have been observed in p-
type Bi₀.₅Sb₁.₅Te₃-based thin films. Shang et al.[²⁷] employed a
magnetron sputtering method to fabricate the p-type Ag-doped
Bi₀.₅Sb₁.₅Te₃ films with an optimal S²σ of ≈¹⁴.⁰ µW cm⁻¹ K⁻² at
420 K. Varghese et al.[²⁸] used a scalable screen-printing method
to prepare p-type BiSbTe flexible film with a S²σ of ≈³⁰ µW
cm⁻¹ K⁻² at room temperature. However, high-performance n-
type Bi₂Te₃-based thermoelectric thin films need further devel-
opment due to the low S²σ (≈¹⁴ µW cm⁻¹ K⁻²[²⁸⁻²⁹]) comparing
with its p-type counterparts (the S²σ of p-type Bi₀.₅Sb₁.₅Te₃-based
thermoelectric thin films can approach as high as ≈²⁰ µW cm⁻¹
K⁻²[²⁶]).

To date, numerous methods have been adopted to prepare
high-performance n-type Bi₂Te₃-based flexible thin films. Bi₂Te₃
nanowire-based flexible thin films prepared by solution phase
printing can approach the S²σ of ≈1.10 µW cm⁻¹ K⁻² at 400 K.[³⁰]
Hybridizing Bi₂Te₃ with graphene oxide and form flexible thin
films by vacuum filtration and annealing also approach the S²σ
of ≈1.08 µW cm⁻¹ K⁻² at ≈297 K.[³¹] Se-doping in Bi₂Te₃ achieved
by mechanical alloying in combination with dispenser printing to
form flexible thin films can achieve the S²σ to ≈2.65 µW cm⁻¹ K⁻²
at ≈297 K.[¹⁹] Through hybridizing Bi₂Te₃ with polyimide via dis-
penser printing, Jung et al.[³²] successfully prepared a Bi₂Te₃ flex-
ible thin film with the high S²σ of ≈3.43 µW cm⁻¹ K⁻² at ≈297 K.
Hybridizing carbon nanotubes with Bi₂Te₃ and form flexible thin
films by in situ solution method successfully enhanced the S²σ to
≈7.4 µW cm⁻¹ K⁻² at ≈297 K.[³³] Additionally, a glass fabric-based
Bi₂Te₃ flexible film prepared by screen printing has approached
the S²σ of ≈13.32 µW cm⁻¹ K⁻² at ≈297 K.[³⁴] Most of these meth-
ods are suffering from poor crystal growth, which has limited the
carrier transport properties and correspondingly limited the ther-
moelectric performance of as-prepared Bi₂Te₃-based flexible thin
films.

In this study, to render sufficient energy for Bi₂Te₃ crystal
growth, we use an advanced thermal diffusion method to fabri-
cate n-type Bi₂Te₃ flexible thin films on flexible polyimide (PI)
substrates (Figure 1a). Our prepared Bi₂Te₃ flexible thin films
(use the film prepared under the diffusion temperature of 623
K as an example) also possess high stability after bending test
under various bending radius and bending cycles as evidenced by
the minor change of resistance (ΔR/R₀) presented in Figure 1b,c,
respectively. Through tuning the diffusion temperature (Tₐ diff), Te-
embedding can form and lead to Bi₂Te₃/Te heterostructures (Fig-
ure 1d). It can induce energy filtering effect at the Bi₂Te₃/Te in-
terfaces due to an energy offset (ΔE₉) between the Te and Bi₂Te₃
conduction band minimums as shown in Figure 1e, which is cal-
culated by density functional theory (DFT), and in turn lead to a
high Seebeck coefficient ≈160 µV K⁻¹ as well as high carrier mo-
bility of ≈200 cm² V⁻¹ s⁻¹. Correspondingly a high maximum S²σ
of 14.65 µW cm⁻¹ K⁻² can be achieved in the Te-embedded Bi₂Te₃
flexible thin film prepared under the diffusion temperature of
623 K, which is comparable with other Bi₂Te₃-based flexible thin
films (Figure 1f). Applicability of as-prepared n-type Bi₂Te₃ flexi-

ble thin films are further demonstrated by an assembled thermo-
electric sensor, which can fast reflect the finger touch status.

**2. Results and Discussion**

To understand the crystal structure of the as-deposited Bi₂Te₃-
based flexible thin films, we first performed XRD analyses and
their XRD patterns are shown in **Figure 2a**. As can be seen,
when the thermal diffusion temperature is 523 K, the Bi-Te re-
action is insufficient and large amount of Bi (JCPDS 44-1246)
can be observed as well as the formed Bi₂Te₃ (JCPDS 15-0863).
With increasing the thermal diffusion temperature to 548 K, the
as-prepared thin film is mainly composed of Bi₂Te₃ phase. Fur-
ther increasing the temperature to above 573 K, Te precipitates
can be observed. When the temperature is above 648 K, almost
pure phase Bi₂Te₃ can be observed. To clarify the crystal struc-
ture of Bi₂Te₃-based thin film prepared by our thermal diffusion
method, the enlarged (102) peak was plotted in Figure 2b, where
the appearance of Te at the deposition temperature of 573-623
K can be clearly observed. Unobvious Te is observed in the XRD
pattern, indicating that there is fewer Te phase above 648 K. Fig-
ure 2c schematically shows the thermal diffusion reaction pro-
cess under different diffusion temperatures. With high satura-
tion vapor pressure and diffusion energy,[³⁷,³⁸] Te atoms can easily
sublimate and diffuse into the Bi precursor film and form Bi₂Te₃
under a high diffusion temperature and high gas pressure. At 523
K, poor Te diffusion leads to poor formation of Bi₂Te₃. With in-
creasing the diffusion temperature to 548 K, Te sublimates and
reacts with Bi to form Bi₂Te₃ more sufficiently. With further in-
creasing the diffusion temperature to 573 K, Te fully sublimates
with insufficient Bi₂Te₃ formation due to Te over-sublimation.
When the diffusion temperature is as high as 648 K, the Bi pre-
cursor film and sublimated Te can fully react to form Bi₂Te₃. To
render a more detailed semiquantitative analysis of the Te con-
tent, the chemical compositions of the Bi₂Te₃ thin films prepared
at different diffusion temperatures are summarized in Table S1
(Supporting Information). As can be seen, due to insufficient Te
diffusion at 523 K, the film is in lack of Te. With the diffusion
temperature increased to over 548 K, the Te content increased.
Optical images of the Te precursor films after thermal diffusion
reaction are shown in Figure S1 (Supporting Information) which
demonstrates Te remains under the diffusion temperature of 548
K and fully sublimates at 573 K. Figure 2d-f shows the scanning
electron microscope (SEM) images of as-prepared Bi₂Te₃ flexible
thin films which are prepared at the diffusion temperature of 573,
623, and 648 K, respectively. The average grain sizes of the Bi₂Te₃-
based films prepared at 573, 623, and 648 K are ≈210, ≈285, and
≈310 nm, respectively (detailed size distribution was shown in
Figure S2a-c, Supporting Information). The Bi₂Te₃-based flexi-
ble thin films are growing denser with increasing diffusion tem-
perature. The increased grain size and density with increasing
diffusion temperature should be attributed to faster grain growth
rate and element diffusion rate at higher temperature.

To further characterize the formation of Te-embeddings,
**Figure 3a** presents the XPS spectrum of Bi₂Te₃ thin film prepared
at the diffusion temperature of 623 K. The binding energy corre-
sponding to Bi and Te are clearly detected. Figure 3b shows the
observed peaks at 583.18 and 572.88 eV, which can be mainly as-
cribed to the binding energy of Te 3d<sub>3/2</sub> and Te 3d<sub>5/2</sub> deriving from
<figure><img src="image_4.png" /><figcaption>**Figure 1.** a) Optical images of as-prepared Bi₂Te₃ flexible thin film under (a1) flat and (a2) bending statuses, respectively. b) The ΔR/R₀ of the Bi₂Te₃ flexible thin film prepared under the diffusion temperature of 623 K as a function of bending radius at the bending cycles of 800 times. c) The ΔR/R₀ of the Bi₂Te₃ flexible thin film prepared under the diffusion temperature of 623 K as a function bending cycles at the bending radius of 8 mm. d) Schematic structure of Bi₂Te₃ and Te. e) DFT calculated electronic band structures of Te and Bi₂Te₃, respectively. f) Comparison of the maximum room-temperature S²σ between our Bi₂Te₃-based flexible thin films and other state-of-art n-type Bi₂Te₃-based flexible thin films prepared by different methods, including thermal evaporation (2011),[35] mechanically alloyed (MA) and dispenser printing (2012),[19] screen print (2014),[34] disperser printing (2017),[32] solution phase printing (2019),[30] vacuum filtration and annealing (2019),[31] in situ solution (2019),[33] magnetron sputtering (2020),[36] and this work.</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>**Figure 2.** a) XRD patterns of Bi₂Te₃-based flexible thin films prepared under different diffusion temperature and b) the enlarged (102) peak of Te. c) Schematic illustration of Bi₂Te₃ flexible thin film preparation through thermal diffusion process. SEM images of the Bi₂Te₃-based flexible thin films prepared at the thermal diffusion temperature of d) 573, e) 623, and f) 648 K, respectively.</figcaption></figure>

Te²⁻ of Bi₂Te₃, respectively.[³⁶] The deviation between the detected Te spectrum and the Te²⁻ standard spectrum of Bi₂Te₃ is induced by Te⁰ (blue area), which proves the existence of Te. The binding energy detected at 162.48 and 157.08 eV should be attributed to the Bi 4f<sub>5/2</sub> and Bi 4f<sub>7/2</sub> (Figure 3c), respectively, which is consistent with that of Bi₂Te₃.[³⁹] Raman spectroscopy ranging from 90 to 210 cm⁻¹ is also employed to study the composition changes of the Bi₂Te₃ thin films prepared at the diffusion temperature range of 573–648 K as shown in Figure 3d. The peaks located at 103.0 and 134.3 cm⁻¹ are attributed to the E²g and A¹g modes, respectively, which are in agreement with the rhombohedra structure of

Bi₂Te₃.[⁴⁰] The E²g (121.6 cm⁻¹) and A¹g (140.2 cm⁻¹) peaks are assigned to the phonon mode of Te phase.[⁴⁰] With increasing diffusion temperature, the E²g and A¹g peaks gradually disappeared at the diffusion temperature of 648 K indicating disappeared Te. Figure 3e-i shows the low- and high-magnification SEM images with corresponding energy-dispersive spectrometer (EDS) maps and line scan spectra of the Bi₂Te₃ film prepared at the diffusion temperature of 623 K. As can be seen, the observed Te precipitate in the size of ≈400 nm.

To further characterize the crystal structure of as-prepared Bi₂Te₃ thin films, TEM image of a typical sample prepared
<figure><img src="image_6.png" /><figcaption>**Figure 3.** a) XPS full spectra survey, b) the Te 3d spectra, and c) the Bi 4f spectra of the Bi₂Te₃-based thin film prepared at the diffusion temperature of 648 K. d) Raman spectra of the Bi₂Te₃ films prepared at the diffusion temperature of 573, 598, 623, and 648 K, respectively. e) High magnification and f) low magnification SEM images of the Bi₂Te₃ film prepared at the diffusion temperature of 623 K with corresponding EDS maps of g) Bi and h) Te, and i) EDS line scan spectra.</figcaption></figure>

under the diffusion temperature of 623 K is shown in **Figure 4a**, which clearly shows two typical grains of Te and Bi₂Te₃, respectively. Figure 4b shows the high-resolution TEM (HRTEM) image of the right part of Figure 4a, which can well-superimpose with the crystal structure of Te along [2\bar{2}1] zone axis. Corresponding HRTEM image of the left part of Figure 4a is presented in Figure 4c and can well-superimpose with the crystal structure of Bi₂Te₃ along [010\bar{1}]. This indicates the left and right part of Figure 4a are highly crystallized Te and Bi₂Te₃, respectively. Figure 4d–f shows corresponding EDS maps. Te can be observed

across the whole area. Bi can only be characterized on the left part. EDS line scan further clarified this point as shown in Figure 4g. This suggests the left part of Figure 4a is composed of both Bi and Te and should be Bi₂Te₃, and the right part of Figure 4a is composed of only Te (should be Te), which are consistent with the HRTEM analyses. In addition, Te-rich Bi₂Te₃ with dense precipitates can make the grain boundaries and interfaces dense, introduce additional dislocations and point defects, and correspondingly strengthen phonon scattering leading to improved thermoelectric performance.[20]
<figure><img src="image_7.png" /><figcaption>**Figure 4.** TEM analysis of Bi₂Te₃ thin films prepared at 623 K. a) High magnification TEM image. HRTEM of b) the blue dotted frame and c) the red dotted frame. d) EDS maps for d) Bi, e) Te, and f) total elements of the Bi₂Te₃ films taken from a). g) EDS scan lines for Bi and Te of the Bi₂Te₃ films along the red line taken from a).</figcaption></figure>

Room-temperature thermoelectric performance of as-
prepared Bi₂Te₃ thin films is shown in **Figure 5**. Figure 5a
shows the σ of the Bi₂Te₃ thin films as a function of diffusion
temperature, which increases with increasing the diffusion
temperature. The σ value increases with increasing the diffusion
temperature, which approaches as high as 567.69 S cm⁻¹ at the
diffusion temperature of 648 K. To understand the evolution of
σ with increasing diffusion temperature, Figure 5b presents the
n_e-dependent µ of as-prepared Bi₂Te₃-based thin films in com-
parison with the single parabolic band (SPB) model calculated
values. The calculation process of the SPB model is shown in
Equations (S1)–(S5) of the Supporting Information. As can be
seen, the µ dramatically increases while the n_e stays around 2–4
× 10¹⁹ cm⁻³, which indicates the increasing µ is not induced
by the reduced n_e. The µ approaches the maximum of ≈200
cm² V⁻¹ s⁻¹ at the diffusion temperature of ≈623 K. Figure 5c
displays the calculated deformation potential coefficient (E_def)
as a function of diffusion temperature. The E_def decreases with
increasing temperature and also minimizes at the diffusion
temperature of ≈623 K, which is consistent with the maximized
µ at the same diffusion temperature. This should be attributed
to the Bi₂Te₃/Te heterostructure, which induced energy filtering
effect and correspondingly increased carrier energy and µ.[41,42]

Figure 5d shows the n<sub>e</sub>-dependent |S| of as-prepared n-type Bi<sub>2</sub>Te<sub>3</sub>-based thin films in comparison with the SPB model calculated values. With increasing diffusion temperature, the |S| increased from ≈140 µV K<sup>-1</sup> at the diffusion temperature of 523 K to ≈165 µV K<sup>-1</sup> at the diffusion temperature of 623 K. The high |S| at 623 K is attributed to the strong energy filtering effect at the Bi<sub>2</sub>Te<sub>3</sub>/Te interfaces with the maximum element Te content. Further increasing the diffusion temperature to 648 K oppositely reduces the |S| due to sufficiently reacted Te. The minor change of S with the diffusion temperature lower than 623 K can be mainly attributed to the change of n<sub>e</sub> regardless of the slightly varying effective mass (m<sup>*</sup>) ranging from 0.7 to 0.93 m<sub>0</sub>. Figure 5e shows the n<sub>e</sub>-dependent S<sup>2</sup>σ of as-prepared n-type Bi<sub>2</sub>Te<sub>3</sub>-based thin films in comparison with the SPB model calculated values, where the S<sup>2</sup>σ approaches as high as 14.58 µW cm<sup>-1</sup> K<sup>-2</sup> at the diffusion temperature of 623 K. This should be mainly ascribed to the energy filtering effect boosted μ while the n<sub>e</sub> is close to the optimized level. Figure 5f presents the repeatedly measured σ, |S|, and S<sup>2</sup>σ of Bi<sub>2</sub>Te<sub>3</sub> flexible thin film prepared under the diffusion temperature of 623 K. As can be seen, thermoelectric performance of as-prepared Bi<sub>2</sub>Te<sub>3</sub> flexible thin film remains closely consistent during the repeating measurement process, which indicates high stability of the film. The
<figure><img src="image_8.png" /><figcaption>**Figure 5.** a) Measured σ of as-prepared Bi₂Te₃-based flexible thin films as a function of diffusion temperature. b) Measured μ as a function of *n*_e in comparison with the SPB model calculated values. c) *E*<sub>def</sub> as a function of diffusion temperature. d) Measured |S| as a function of *n*_e in comparison with the SPB model calculated values. e) Measured S²σ as a function of *n*_e in comparison with the SPB model calculated values. f) Repeatedly measured room-temperature σ, |S| and S²σ of Bi₂Te₃ flexible thin film prepared under the diffusion temperature of 623 K. g) Optical photo of a finger contact on thin film thermoelectric sensor with corresponding h) calculated temperature distribution and i) the output voltage signals.</figcaption></figure>

evaluation of repeatability of σ, S, and S²σ of Bi₂Te₃ flexible thin film is further shown in Figure S3 (Supporting Information) also indicating high repeatability and stability in their thermoelectric performance. To demonstrate the applicability of our n-type Te-embedded Bi₂Te₃ flexible thin films, we have assembled a demonstrating sensor with corresponding p-type Sb₂Te₃ legs (thermoelectric performance is shown in Table S2, Supporting Information) as shown in Figure 5g. Touching the electrode with finger can induce a temperature difference between the center and the surrounding area as simulated in Figure 5h and correspondingly induce electrical signals. Continuous response of the thermoelectric sensor is measured for 7 cycles. The time interval between peak and valley of each pulse is ≈1 s, which

includes device response time and finger heat transfer time. The output signals of the thermoelectric sensor show a fast on-off response as shown in Figure 5i, where the state of finger touching is “on,” and the state of finger moving away is “off.”

**3. Conclusion**

In this study, we have successfully prepared n-type Bi₂Te₃ flexible thin films on flexible PI substrates by a novel thermal diffusion reaction method. Reaction control can lead to the formation of Te-embeddings and Te/Bi₂Te₃ heterostructures in the Bi₂Te₃ thin films, which theoretically can induce energy filtering effect due to the Δ*E*<sub>g</sub> between Bi₂Te₃ and Te valence bands. Experimentally, the
reaction condition and formation of Te embeddings can be engi-
neered through adjusting the diffusion temperature. An excellent
$S^2\sigma$ of 14.58 µW cm$^{-1}$ K$^{-2}$ has been achieved at room tempera-
ture due to Te-embeddings induced energy filtering effect and
corresponding ultrahigh $\mu$ of ≈200 cm$^2$ V$^{-1}$ s$^{-1}$. Our prepared n-
type Te-embedded Bi$_2$Te$_3$ flexible thin film possesses comparable
thermoelectric performance comparing with state-of-art Bi$_2$Te$_3$-
based flexible thin films. This result well proves that tunning the
diffusion temperature during the thermal diffusion process can
effectively introduce embeddings and Te/Bi$_2$Te$_3$-heterostructures
and boost the thermoelectric performance of B$_2$Te$_3$ flexible thin
films. The assembled thermoelectric sensor with fast response
characteristics further proved the applicability of as-prepared Te-
embedded Bi$_2$Te$_3$ thin films prepared by our thermal diffusion
method.

**4. Experimental Section**

**Bi₂Te₃ Thin Film Preparation:** The n-type Bi₂Te₃ thin films were deposited on a flexible PI substrate using a thermal diffusion reaction method. First, Bi and Te thin films were deposited separately on PI substrates by using radiofrequency magnetron sputtering technology. The deposition parameters of the thin film are summarized in the Supporting Information. Then, as-deposited Te and Bi thin films were tightly pressurized, under the external pressure of 5 × 10⁶ Pa in a vacuum (2 × 10⁻³ Pa) chamber followed by a heating process. The diffusion temperature was increased to the target temperature within 30 min and dwelled for 2 h, the samples were cooled down to room temperature under vacuum. The thickness of the thin films is ranging ≈550 nm and the measured results by using have been added in Table S1 (Supporting Information).

**Device Fabrication:** First, Bi, Sb, and Te precursor films were deposited on the PI substrates. Subsequently, the flexible thermoelectric prototype sensor was fabricated using the thermal diffusion reaction method.

**Characterization of the Bi₂Te₃ Films:** The crystal structures as-prepared Bi₂Te₃ films were characterized by X-ray diffraction (XRD, D/max 2500 Rigaku Corporation, CuKα radiation). The surface morphology and cross-section microstructures were investigated by SEM (Zeiss supra 55). The compositions of Bi₂Te₃ films are analyzed by an EDS (Bruker Quantax 200). Samples for cross-sectional TEM analysis were fabricated by a dual-beam SEM-FIB (focused ion beam, Scios, FEI). The crystal structures of as-prepared Bi₂Te₃-based flexible thin films were also investigated by the transmission electron microscope (TEM, FEI, Titan3 Themis G2). The chemical bonding was investigated by Raman spectra measurement system (Horiba Jobin Yvon). X-ray photoelectron spectroscopy (XPS, Thermo Escalab 250XI) was used to study the valence states of the composing elements. Carrier concentration ($n_e$) and mobility ($\mu$) values were recorded from the Hall measurement system (HL5500PC, Nano metrics). The $\sigma$ and S of Bi₂Te₃ film were simultaneously measured by the SBA458 (Nezschs). The error bars of the measured data were determined from the SBA458 (5% for S, 5% for $\sigma$, and 10% $S^2\sigma$).

**Supporting Information**

Supporting Information is available from the Wiley Online Library or from
the author.

**Acknowledgements**

This work was supported by the National Natural Science Foundation of
China (Grant No. 11604212), Guangdong Basic and Applied Basic Re-
search Foundation (Nos. 2020A1515010515 and 2019A1511510107), Sci-
ence and Technology plan project of Shenzhen (No. 20200811230408001),

Australia Research Council and Innovation Centre for Sustainable Steel
Project. Z.G.C. thanks the USQ Strategic research fund and USQ start-up
grant. The authors are thankful for the assistance on STEM-HAADF ob-
servation received from the Electron Microscope Center of the Shenzhen
University.

**Conflict of Interest**

The authors declare no conflict of interest.

**Author Contributions**

D.-W.A., W.-D.L., and Y.-X.C. contributed equally to this work. D.-W.A.: In-
vestigation, Conceptualization, Methodology, Writing – original draft. W.-
D.L.: Investigation, Writing – review & editing. Y.-X.C.: Writing – review
& editing. M.W.: Investigation, Conceptualization, Methodology. B.J.: In-
vestigation, Data curation, Validation. F.L.: Data curation, Validation. X.-
L.S.: Data curation, Validation. Z.-H.Z.: Resources, Writing – review & edit-
ing, Supervision, Funding acquisition. G.L.: Resources, Writing – review &
editing. X.-H.Z.: Supervision. P.F.: Resources, Writing – review & editing,
Supervision, Funding acquisition. Z.-G.C.: Supervision, Writng – review &
editing.

**Data Availability Statement**

Research data are not shared.

**Keywords**

Bi₂Te₃, thermal diffusion method, thermoelectrics, flexible thin films

Received: August 15, 2021

Revised: November 8, 2021

Published online: December 22, 2021

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