Recent Strategies for Improving Thermoelectric Efficiency of Bi₂Te₃-Based Thin Films

Abstract

This chapter provides a concise summary of recent strategies for enhancing the thermoelectric (TE) efficiency of Bi₂Te₃-based thin films. In the introduction, a concise overview of thermoelectricity, its advantages over other technologies, its market value, and its potential future applications will be presented. Next, the preparation methods for Bi₂Te₃-based thin films will be described under the heading of thin film preparation methods. Then, contemporary strategies for enhancing the TE characterizations of Bi₂Te₃-based thin films will be discussed. Various strategies, such as the thin film fabrication methods and post-thermal annealing dependent TE properties of Bi₂Te₃-based thin films, have been discussed. The thin films prepared via vacuum techniques followed by thermal annealing showed high thermoelectric efficiency.

1. Introduction

In recent times, emerging industries have exhibited a strong demand for miniaturized refrigeration and power generation systems. These systems rely on advanced room-temperature thermoelectric (TE) materials. Notably, sectors such as 5G communications, the “Internet of Things” (IoT), and wearable electronics have been dependent on TE materials [1, 2, 3, 4, 5, 6, 7, 8]. In light of the ongoing energy crisis, the significance of TE materials capable of directly converting heat into electricity and vice versa is important [1, 2, 3, 4, 5, 6, 7]. Although this technology has captured the attention of numerous researchers, its everyday practical applicability remains limited due to its suboptimal conversion efficiency. Two crucial metrics, namely the dimensionless figure of merit (ZT = S²σT/κ) and the power factor (PF = S²σ), are indicators of the efficiency of TE materials [9, 10]. Here, the Seebeck coefficient (S), electrical conductivity (σ), thermal conductivity (κ), and absolute temperature (T) are key parameters. Researchers commonly utilize these metrics to evaluate and describe the performance of TE materials. Notably, higher ZT values have been correlated with superior performance of semiconductor materials [11, 12, 13].

The typical conventional bulk TE materials include inorganic substances such as Bi₂Te₃, Sb₂Te₃, PbTe, SiGe, skutterudite, and half-Heusler alloys [14, 15, 16]. However, owing to their elevated cost and limited efficiency, the practical applications of these materials remain circumscribed. As of now, the highest ZT value on record remains below 3, a factor that confines their utility to specific niche applications. To open up more versatile applications, the quest for greater ZT values has become paramount [17, 18, 19, 20, 21]. Moreover, the practical deployment of TE devices has encountered hindrances due to the obstacles, include constraints in shaping possibilities and a plateauing of performance. In contrast, thin film thermoelectric materials introduce a promising results. They exhibit qualities such as lightweight, mechanical flexibility, easy fabrication process, and cost-effectiveness [22, 23, 24, 25]. These attributes lay the groundwork for pioneering new thermoelectric devices, offering substantial potential for upcoming electronic advancements and miniature components. Notably, recent evidence underscores that the design of thin films can enhance TE efficiency through more proficient scattering of phonons, leading to a significant reduction in the lattice thermal conductivity [26].

Irrespective of the advancements in high-performance thermoelectric (TE) materials like SnSe, Cu₂Se, CoSb₃, GeTe, and Mg₃Sb₂, the traditional Bi₂Te₃-based materials continue to be extensively investigated due to their exceptional performance at room temperature [27, 28, 29]. Over the past two decades, the outstanding TE capabilities exhibited by thin films of these materials have garnered global attention. This is primarily attributed to their application in miniature and flexible TE power generators for various electronic devices, as well as their efficacy in TE cooling systems. Through composition and microstructure optimization, the ZT values of both n-type and p-type Bi₂Te₃-based bulk materials have shown consistent improvement, currently achieving ranges of 1.3–1.8 and 1.0–1.4, respectively. These high-performance bulk materials have established themselves as the benchmark for efficient device operation. Significantly, researchers and engineers in the field of thermoelectricity have been closely focusing on thin film versions of Bi₂Te₃-based materials [30, 31]. The thin films derived from Bi₂Te₃ are particularly attractive and well-suited for novel applications in miniature and flexible electronic devices [1, 3, 8]. The recent publications concerning both n-type and p-type Bi₂Te₃-based thin films are summarized in Figure 1. Within this chapter, we delve into the crucial and recent advancements that have been made to enhance the thermoelectric efficiency of these thin films.

Figure 1.

Year-wise published articles of TE thin film of p-type, n-type, device, and review articles.

2. Crystal structure

As depicted in Figure 2, compounds that are derived from Bi₂Te₃ all exhibit crystal structure of tetradymite type and crystallizing into rhombohedral structure with space group of R-3 m. To illustrate using Bi₂Te₃ as a case study, its quintuple layers (QLs) are arranged in the crystallographic c-axis direction, featuring two distinct Te lattice sites: Te(1) and Te(2). The stacking sequence within these QLs follows a Te(1)-Bi-Te(2)-Bi-Te(1) pattern. The bonds between adjacent QLs are of the weak van der Waals (vdW) type, specifically Te(1)-Te(1) bonds, while the Te-Bi bonds exhibit a significant covalent nature. The layered architecture of Bi₂Te₃, including these feeble Te(1)-Te(1) bonds, leads to its anisotropic electrical and thermal transport properties [32, 33]. The electrical conductivity within the plane (σ||) substantially surpasses the conductivity perpendicular to the plane (σ⊥) due to the noticeably stronger carrier scattering that occurs along the c-axis (⊥) as opposed to within the plane (||) of the layers. Several factors contribute to the diminished lattice thermal conductivity (κL) are the presence of heavy atoms, intricate chemical bonding, the periodic structure of QLs, and the fragility of the vdW bonds. Both n-type and p-type variants of Bi₂Te₃ typically exhibit anisotropy ratios of σ||/σ⊥ ranging from 2 to 7, while the corresponding anisotropy ratios for thermal conductivity, κ||/κ⊥, are approximately in the range of 2–2.5 [34, 35, 36]. Consequently, in all Bi₂Te₃-based materials, establishing correlations between crystal orientation and thermoelectric characteristics holds paramount importance.

Figure 2.

Crystal structure of Bi₂Te₃.

3. Preparation methods of Bi₂Te₃-based thermoelectric thin films

Several researchers have recently explored the topic of the synthesis of bulk materials and their characteristics. However, the development of nanomaterials is necessary for the miniaturization of electronics in the present period of research. The current energy situation compels scientists to do in-depth research on thermoelectric materials. A few years ago, the importance of thermoelectric materials began to receive increasing attention. Nowadays, more research is being done on the creation of thermoelectric thin films, among other types of nanoparticles.

3.1 Physical methods

Thin film preparation methods are techniques used to deposit thin layers of material onto substrates with precise control over thickness, composition, and structure. Thin films are widely used in various applications, such as electronics, optics, energy storage, and coatings. The physical preparation methods of thin films can be broadly categorized into several techniques. Here are some common ones:

3.1.1 Thermal evaporation technique

Thermal evaporation is a method employed for the deposition of thin films onto a substrate. This involves heating the material within a vacuum chamber to the point of evaporation, after which the evaporated material condenses onto the substrate. The basic steps involved in thermal evaporation are:

• Preparation of the substrate: The substrate, which is the surface onto which the material is deposited, is prepared by cleaning and drying it to remove any contaminants.

• Loading the material: The material to be deposited is loaded into a boat or crucible made of a refractory metal such as tungsten, which can withstand high temperatures.

• Evaporation: The chamber is evacuated to a high vacuum to eliminate any gas molecules that might interfere with the deposition process. The boat or crucible containing the material is then heated to a high temperature by electrical methods or electron beam, typically between 1000°C and 1500°C, causing the material to evaporate.

• Deposition: The evaporated material condenses onto the substrate, forming a thin film.

The film’s thickness is regulated by factors such as the quantity of material placed in the boat or crucible, the temperature at which the boat operates, and the separation distance between the boat and the substrate. By controlling these parameters, it is possible to deposit films with precise thicknesses and uniformity. However, it is not suitable for depositing materials that decompose at high temperatures or that have a high vapor pressure at room temperature. Also, it has some limitations, such as lower deposition rates compared to other deposition techniques like sputtering, and sensitivity to the volatility and thermal stability of the source material. In general, thermal evaporation stands as a versatile and extensively applied method for depositing thin films across a range of diverse applications.

Co-evaporation technique: The co-evaporation technique is similar to thermal evaporation. In this technique, thin film can be fabricated with controlled compositions by simultaneously evaporating multiple source materials onto a substrate. Within the co-evaporation method, a vacuum chamber hosts two or more source materials, often in solid pellet or rod form, positioned in close proximity. A notable advantage of this technique is the capacity to manipulate the thin film’s composition by fine-tuning the relative evaporation rates of the distinct source materials. However, the co-evaporation technique also presents challenges, such as the need to carefully control the temperature and pressure during deposition to avoid segregation or phase separation of the different components, as well as the need to ensure uniform deposition across the entire substrate surface. Careful monitoring and optimization of process parameters are required to achieve desired film properties and ensure reproducibility. In summary, the co-evaporation technique is a versatile method for fabricating thin films with controlled compositions, offering opportunities for tailoring material properties for a wide range of applications.

Flash evaporation technique: Flash evaporation is also similar to thermal evaporation technique and is a widely used technique for the production of high-purity materials by evaporating solids and liquids from a heated source. The technique involves evaporating the material at a high temperature and then rapidly cooling it. Flash evaporation is an efficient and cost-effective way to produce high-quality films and materials [37].

3.1.2 Sputtering techniques

Within the realm of thin film deposition methods, sputtering emerges as a physical vapor deposition (PVD) approach that facilitates the application of diverse thin films onto a substrate. In this process, energetic ions bombard a target material, causing the ejection of atoms or molecules from the target. These expelled particles then settle on a substrate, forming a thin film [38]. Sputtering holds several advantages over alternative deposition techniques, encompassing swift deposition rates, uniform film quality, and the capability to apply a broad spectrum of materials. Furthermore, its potential for seamless upscaling to accommodate mass production renders it a favored choice for industrial applications. Below, we outline some prevalent sputtering techniques:

Direct current (DC) sputtering: In this procedure, high-energy ions, usually of argon, are directed at a target material, inducing the expulsion of atoms from the target’s surface. These expelled atoms subsequently traverse the chamber and settle onto the substrate, culminating in the creation of a thin film. The process involves applying a DC voltage between the target and substrate, which creates a plasma discharge in the chamber. The positive argon ions in the plasma are attracted to the negatively charged target, and the resulting collisions between the ions and target atoms cause the ejection of material from the target surface.

Radio frequency (RF) sputtering: In radio frequency (RF) sputtering, an RF power supply is used to create a high-frequency alternating current (AC) electric field. This field ionizes the gas in the sputtering chamber, creating plasma, which then accelerates ions toward the target, causing sputtering.

Magnetron sputtering: Magnetron sputtering employs a magnetic field to confine the plasma close to the target, resulting in heightened sputtering rates and enhanced uniformity of the film. Magnetron sputtering is typically done with DC or RF power supplies and is widely used for various applications due to its high deposition rate and good film quality.

Ion beam sputtering (IBS): Unlike magnetron sputtering techniques, ion beam sputtering uses an ion beam to sputter material from a target and deposit it onto a substrate. Ion beam sputtering involves generating a stream of ions within a vacuum chamber and aiming it at the target material. The ions collide with the target’s surface, leading to the expulsion of atoms that subsequently settle on a substrate. The ion beam can be generated using a range of ion sources, including radio frequency (RF) ion sources, Kaufman-type ion sources, and Bernas-type ion sources. It is particularly useful for depositing complex materials, such as multilayer structures and alloys, and for depositing films with high uniformity and low defect densities.

3.1.3 Molecular beam epitaxy (MBE)

Molecular beam epitaxy (MBE) is a type of thin film deposition technique used to create high-purity, single-crystalline materials on a substrate. It involves the use of a vacuum environment and beams of molecules to grow thin films in a highly controlled manner. The molecules are usually deposited in a layer-by-layer fashion, allowing for the creation of complex multi-layered structures.

3.1.4 Pulsed laser deposition (PLD)

It is a highly versatile process, and it utilizes the energy released from a single laser pulse to create a plasma of excited atoms and molecules that are then deposited onto a substrate. The laser pulse is typically in the nanosecond range (10⁻⁹ seconds) and is focused onto the target material, which is then ablated. The ablated material is then condensed onto a substrate to form a thin film. PLD is a highly controllable technique and can be used to produce a variety of film morphologies, ranging from uniform, smooth films to highly textured films with a high degree of crystallinity [39].

3.2 Chemical methods

3.2.1 Spin coating

Spin coating represents a straightforward and extensively employed approach in thin film fabrication. This technique involves applying a liquid precursor solution onto a substrate and subsequently spinning it at high speeds. This spinning action ensures the even dispersion of the solution across the substrate, leading to the creation of a thin film as the solvent evaporates.

3.2.2 Spray pyrolysis

In this process, a liquid precursor solution is deposited onto a hot substrate, forming a thin film as the solvent evaporates. The advantage of spray pyrolysis is its ability to deposit films at lower temperatures than other methods. This allows for the deposition of films with lower melting points. The process is also relatively inexpensive.

3.2.3 Chemical vapor deposition (CVD)

Chemical Vapor Deposition is a well-established approach in thin film production. This method entails introducing a volatile precursor into a reaction chamber, where it undergoes decomposition or reactions to yield a solid thin film on a substrate. The precursor is typically in the form of a gas or vapor, and the deposition process is controlled by adjusting the temperature, pressure, and gas flow rates in the reaction chamber.

3.2.4 Electrochemical deposition

Electrochemical deposition, also known as electrodeposition or electroplating, is a method for thin film preparation that involves the use of an electric current to drive the deposition of ions from a solution onto a substrate, forming a thin film. Electrochemical deposition allows for precise control over the deposition rate, thickness, and composition of the thin film by adjusting the electrochemical parameters such as voltage, current density, and deposition time [40].

The above mentioned techniques are important for making high quality thin films. The process chosen will depend on the particular material, the thin film’s desired qualities, and the application. Each method has its advantages and limitations, and careful consideration should be given to the processing conditions and parameters to obtain thin films with desired properties.

4. Strategies

4.1 Deposition techniques and conditions

The kinetic progression of growing thin films is influenced by rate-limiting steps, which exert a substantial influence on the growth mode, morphology, microstructure, and chemical composition of the resulting films. For precise control over crystallinity, orientation, atomic defects, and ultimately, the thermoelectric (TE) performance of thin films, meticulous management of the film growth parameters proves indispensable. The selection of deposition method and conditions stands as the chief determinant of thin film growth. Advanced n-type Bi₂Te₃ and p-type Sb₂Te₃ thin films, designed for thermoelectric applications, have been successfully crafted using an array of techniques encompassing both physical and chemical deposition processes. The growth dynamics of thin films are notably swayed by deposition variables, including factors such as deposition time, pressure, temperature, and others. The orientation, microstructure, and TE characteristics of the film are significantly influenced by these conditions. Table 1 summarizes recent deposition methodologies utilized to prepare TE thin films.

Material	Process	Conditions	Substrate	Thickness of film (μm)	Orientation	PF@T	Ref.
Physical processes
n-Type thin films
Bi2Te3	DC magnetron sputtering	Target: Bi2Te3; Target-substrate dist.: 6 cm; Target power: 0.25 kW; Working pressure: 4.1 × 10^−4 torr; Working temp.: RT; Deposition time: 2 min	PET	0.055	(1010)	4.1 μWcm^−1 K^−2 at RT	[41]
Bi2Te3	Thermal evaporation	Target: Bi2Te3; Deposition current: 70 A; Working pressure: 1 × 10^−6 torr	SiO2	0.1	(015)	0.02 μWcm^−1 K^−2 at 420 K	[42]
Bi2Te3	Thermal co-evaporation	Target: Bi & Te; Evaporation rates: Bi (1.3 Å/s) & Te (3.0 Å/s); Substrate rotation: 4 rpm; Working pressure: 5 × 10^−6 torr; Working temp.: 250°C; Deposition time: 40 min	Si/SiO2	1.0	(1010)	28 μWcm^−1 K^−2 at RT	[43]
p-type thin films
Sb2Te3	Thermal evaporation	Target: Sb2Te3; Working pressure: 15 torr; Working temp.: 350°C; Deposition time: 15 min	Al2O3(0001)	0.5	(00 l)	33 μWcm^−1 K^−2 at 300 K	[44]
Chemical processes
n-type thin films
Bi2Te3	Aqueous Electrodeposition	Electrolyte: 7.5 mM Bi powder and 10 mM of TeO2 in 1 M HNO3; Potential: −0.1 V vs. SCE	ITO coated quartz	1.2	(015)	0.05 μWcm^−1 K^−2 at 350 K	[45]
Bi2Te3	Nonaqueous pulsed electrodeposition	Electrolyte: 2.25 mM [N^nBu4]-[BiCl4], 3 mM [N^nBu4]2[TeCl6], and 0.1 M [N^nBu4]Cl in anhydrous CH2Cl2; Potential: −0.6 V vs. SCE with an initial nucleation pulse to −1.0 V vs. SCE	TiN	0.5	(015)	0.89 μWcm^−1 K^−2 at 520 K	[46]

Table 1.

Process-dependent TE properties of Bi₂Te₃-based thin films.

4.1.1 Physical methods

In recent times, diverse physical vacuum methods, including magnetron sputtering and thermal evaporation, have been investigated to enhance the properties of thermoelectric (TE) thin films and bolster their efficiency. Tao et al. embarked on fabricating n-type Bi₂Te₃ thin films on a PET substrate through DC magnetron sputtering, employing varying sputtering pressures spanning from 0.03 to 0.6 Pa. A peak power factor (PF) of 4.1 μWcm⁻¹ K-2 was achieved at room temperature (RT) under a sputtering pressure of 0.055 Pa. This attainment is attributed to a more balanced and stoichiometric transfer of atoms from the target to the substrate that takes place under lower pressures [41]. Bendt et al. prepared p-type Sb₂Te₃ epitaxial films on Al₂O₃(0001) by thermal evaporation at 350°C under high working pressure (2000 Pa). The obtained highest PF (33 μWcm⁻¹ K⁻² at 300 K) might be due to (00 l) preferential orientation of films under high working pressure [42]. Saberi et al. prepared n-type Bi₂Te₃ thin films on SiO₂ substrate by single source thermal evaporation and reported PF of 0.02 μWcm⁻¹ K⁻² at 420 K [43]. Also, Shen et al. prepared same thin films by thermal co-evaporation and reported one of the highest PF of 28 μWcm⁻¹ K⁻² for the 1 μm thick films at RT [44]. The high PF values for thermal co-evaporation might be due to thick film with (1010) orientation compared to 0.1 μm thick films of thermal evaporation.

4.1.2 Chemical methods

Within the realm of chemical techniques, electrodeposition has garnered recent attention due to its cost-effectiveness and straightforward nature. Jose et al. undertook the fabrication of n-type Bi₂Te₃ thin films using the electrodeposition method, reporting a power factor (PF) of 0.05 μWcm⁻¹ K⁻² at 350K [45]. This electrodeposition was conducted within an aqueous electrolyte composed of 7.5 mM Bi powder and 10 mM TeO₂ in 1 M HNO₃, with a potential of −0.1 V vs. SCE. In a separate study, Katarina et al. generated analogous thin films through pulsed electrodeposition within a nonaqueous electrolyte (comprising 2.25 mM [NnBu₄]-[BiCl₄], 3 mM [NnBu₄]₂[TeCl₆], and 0.1 M [NnBu₄]Cl in anhydrous CH₂Cl₂) with an initial nucleation pulse at −1.0 V vs. SCE and subsequent potential of −0.6 V vs. SCE. The achieved PF was 0.89 μWcm⁻¹ K⁻² at 520 K, which represents a remarkable ∼18-fold enhancement compared to the PF (0.05 μWcm⁻¹ K⁻²) obtained via aqueous electrodeposition. This notable improvement is largely attributed to the substantial reduction of film oxidation within the nonaqueous electrolyte context. The oxidation of the films causes the degradation of their TE properties [46].

Among the possible fabrication methods of Bi₂Te₃/Sb₂Te₃-based TE thin films, physical vacuum techniques showed superior TE efficiency. This might be due to the production of pore-free, defect-free, stoichiometric, and high-quality thin films in physical techniques. Due to simplicity and industrial scalability of physical methods, we have also adopted physical vacuum techniques such as DC/RF-sputtering, thermal evaporation and electron beam evaporation techniques to prepare the high-quality TE thin films and high TE efficient thin film generators for wearable energy harvesting applications.

4.2 Post-thermal annealing

Post-thermal annealing plays an important role in enhancing the TE performance of thin films by enhancing the stoichiometry, by improving the crystalline nature, by attaining the preferential orientation/texture, by evolving suitable microstructure, and by increasing the grain size/crystallite size [38, 47, 48, 49, 50, 51, 52, 53, 54, 55]. For example, the post-thermal annealing-dependent microstructure of sputtered Bi₂Te₃-based thin films is shown in Figure 3.

Figure 3.

SEM images of post-thermal annealing of sputtered thermoelectric films at different temperatures [38]. Image adapted with permission from [38] copyright © 2021 Elsevier. All rights reserved.

In general, the fabrication techniques produce either Te-rich or Bi/Sb-rich films due to different evaporation rates of elements. These excess elements accumulate at grain boundaries. Non-stoichiometry of thin films may cause deteriorated TE properties due to compositional defects. Post-thermal annealing is a simple method to enhance the stoichiometry of thin films by controllable evaporation of the excess composition at suitable heat treatment. Sometimes, the films fabricated by a few techniques show an amorphous nature. Amorphous nature reduces the electrical conductivity and thereby reduces the TE efficiency. Post-thermal annealing has a role in improving the crystalline nature by applying suitable heat treatment conditions. Improvement in crystalline nature due to post-thermal annealing also leads to attaining preferential orientations such as (00 l), (110), (1010), etc., and to evolve novel microstructure such as hexagonal shape plates, etc. Texture and microstructure assume a pivotal role in enhancing TE efficiency through the concurrent optimization of electrical conductivity and Seebeck coefficient. Further, post-thermal heat treatment may increase the average grain size/crystallite size, which leads to improvement in electrical conductivity due to mobility enhancement because of the joining of grain boundaries. Post-thermal annealing has various types: in situ/ex situ, rapid/non-rapid, and vacuum/inert gas. In general, in situ annealing treatment is performed in a fabrication chamber without breaking the vacuum of the deposition process by heating the sample holder, while ex situ annealing is performed in external furnace using rapid thermal processing (∼5 K/s) or non-rapid thermal processing (∼5 K/min) under vacuum or inert gas (N₂, Ar, mixed gas (95%Ar + 5%H2)). In recent times, ex situ annealing was attracted by researchers due to higher TE efficiency over in situ annealing, irrespective of the fabrication process. Table 2 shows the post-thermal annealing of various TE thin films.

Material	Process	Conditions	Substrate	Annealing conditions	Thickness (μm)	Orientation	Remarks	PF@T	Ref.
Bi2Te3	Single source thermal evaporation	Target: Bi2Te3; Deposition current: 90 A – 120 A; Substrate rotation: 5 rpm; Working pressure: 1 × 10^−3 Pa; Deposition time: 10 min	PI	Ex situ; Rapid; Vacuum; 500 K @ 4 K/s	0.24	(015)	Stoichiometric Crystallinity Preferred orientation (00 l) Average grain size	5.3 μWcm^−1 K^−2 at 478 K	[47]
Bi2Te3	Single source thermal evaporation	Target: Bi2Te3; Deposition current: 90 A – 120 A; Substrate rotation: 5 rpm; Working pressure: 1 × 10^−3 Pa; Deposition time: 10 min	PI	Ex situ; Rapid; Ar gas; 498 K @ 4 K/s	0.24	(015)	Stoichiometric Crystallinity Average grain size	5.3 μWcm^−1 K^−2 at 503 K	[48]
Bi2Te3	RF-magnetron sputtering	Target: Bi2Te3; Power: 50 W; Substrate rotation: 30 rpm; Working pressure: 3 × 10^−3 Torr	SiO2/Si	Ex situ; Slow; N2 gas; 543 K @ 1 K/min	0.25	(00 l)	Preferred orientation (00 l) Stable phase Microstructure	13.7 μWcm^−1 K^−2 at RT	[49]
Bi2Te3	RF-magnetron sputtering	Target: Bi2Te3; Target-substrate dist.: 14 cm; Power: 200 W; Working pressure: 1.0 Pa; Working temp.: RT; Deposition time: 1 h	PI	Ex situ; Slow; Mixed gas (95%Ar + 5%H2); 523 K @ 1 h	1.0	(00 l)	Stoichiometric Crystallite size Preferred orientation (00 l)	16.1 μWcm^−1 K^−2 at RT	[50]
Bi2Te3	RF-magnetron co-sputtering	Target: Bi2Te3 & Te; Target-substrate dist.: 20 cm; Power: 60 W (Bi2Te3) & 10 W (Te); Substrate rotation: 100 rpm; Working pressure: 0.4 Pa; Working temp.: RT	SiO2/Si	Ex situ; Slow; N2 gas; 523 K @ 1 h	10	(015)	Stoichiometric Crystallinity Preferred orientation (00 l)	7 μWcm^−1 K^−2 at RT	[38]
Bi2Te3	DC magnetron sputtering	Target: Bi2Te3; Target-substrate dist.: 14 cm; Power: 200 W; Substrate rotation: 100 rpm; Working pressure: 1 Pa; Deposition time: 40 min	Glass	Ex situ; slow; mixed gas (95%Ar + 5%H2); 573 K @ 2 h	1.0	(00 l)	Crystallinity Preferred orientation (00 l) Average crystallite size	27.3 μWcm^−1 K^−2 at RT	[51]
Bi2Te3	Sputtered seed electrodeposition	Electrolyte: 2.0 mM Bi(NO3)3, 3.0 mM TeO2 and 0.78 M HNO3 in 285 ml DI water; Potential: −0.01 V vs. SCE; Working temp: 290 K; Deposition time: 60 min	Al2O3	Ex situ; Slow; mixed gas (95%Ar + 5%H2); 573 k @ 1 h	3.8	(110)	Stoichiometric Preferred orientation (110)	13.9 μWcm^−1 K^−2 at 300 K	[52]
Bi2Te3	Inkjet printing	Conc.: 2.5 mg/mL; Viscosity: 1.62 cP; Working temp.: 45°C; Droplet size: 1.5 μL	PI	Ex situ; Slow; mixed gas (95%Ar + 5%H2); 723 K @ 10 min	0.9	(1010)	Average grain size	1.10 μWcm^−1 K^−2 at 400 K	[53]
Bi2Te3	Drop-casting	Solvent: Methanol; Conc.: 10 mg/mL	PI	Ex situ; Slow; mixed gas (95%Ar + 5%H2); 523 K @ 4 K/min @ 1 h	40	(015)	Stoichiometric Microstructure Average grain size	3.5 μWcm^−1 K^−2 at RT	[54]
Bi2Te3	Drop-casting & electrodeposition	Drop-casting; Solvent: Methanol; Conc.: 0.01 g/mL; &; Electrodeposition; Electrolyte: Bi(NO3)3; (0.002 mol/L), TeO2 (0.003 mol/L), and HCl (5.0 mL, diluted with; deionized water); Potential: −0.01 V vs. SCE; Deposition time: 20 min	ITO	Ex situ; Slow; Mixed gas (Ar: 95% + H2: 5% gas); 523 K @ 1 h	10	(015)	Average grain size Crystalline nature	1.28 μWcm^−1 K^−2 at RT	[55]
Sb2Te3	RF-magnetron sputtering	Target: Sb2Te3; Target-substrate dist.: 20 cm; Power: 60 W; Substrate rotation: 100 rpm; Working pressure: 0.4 Pa; Working temp.: RT	SiO2/Si	Ex situ; Slow; N2 gas; 523 K @ 1 h	10	(015)	Stoichiometric Crystallinity	13 μWcm^−1 K^−2 at RT	[38]

Table 2.

Post-thermal annealing dependent TE properties of Bi₂Te₃-based thin films.

5. Conclusion

In the field of thermoelectricity, as well as in the provision of power for the upcoming low-power electronics and in quantum computation, Bi₂Te₃-based thin films have shown extraordinary physical characteristics and highly significant application prospects. For instance, the development of flexible and miniature electronic devices depends heavily on thin film thermoelectric (TE) devices, which are based on high-performance Bi₂Te₃-based films. These devices require self-powered power sources, as well as very effective site-specific and on-demand active cooling. The thin film devices prepared using vacuum methods and post-thermal annealing showed high thermoelectric efficiency. Despite the significant advancements made over the past few years, further research into novel physical mechanisms and applied technologies pertaining to Bi₂Te₃-based thin films is still required in order to unlock new insights into their fascinating physical characteristics and open the door for their successful commercial applications.

Acknowledgments

The authors are grateful to the Science and Engineering Research Board (SERB), Department of Science and Technology, Government of India (SRG/2022/000398).

Conflict of interest

The authors declare no conflict of interest.