Advancements in thin film thermoelectric: progress and perspective on materials and devices

1 Introduction

Research on alternative energy sources began in the late 90s, when the world began to experience the shock of rising oil prices and concerns over environmental issue which leads to extensive research on renewable energy resources and ways to harvest them (Owusu and Asumadu-Sarkodie, 2016). They are considered a key solution to mitigating the negative impacts of climate change, reducing greenhouse gas emissions, and creating a more sustainable future. The development and adoption of renewable energy technologies have been growing in recent years, driven by a combination of technological advances, cost reduction, and increasing awareness of the need to transition to a low-carbon economy.

Thermoelectric (TE) phenomenon has been attracting considerable amount of research interest regarding its ability to convert thermal energy into electricity. Thermoelectric generators (TEGs) are devices that utilises this phenomenon, able to generate electricity when given a temperature difference (Seebeck effect) and vice versa (Peltier effect) (Drebushchak, 2008). This specific capacity opens possibility of TEG application in recycling waste heat energy from industry or transportation into electricity, improving system efficiency and lowering expenses and pollutants from the environment (Zoui et al., 2020). Besides having the benefits of long life, no moving parts, low maintenance, and high reliability, TEGs are able to operate independently without interrupting the process of the integrated system.

TEGs are composed of numerous thermopiles having multiple thermocouples that are connected in series electrically and in parallel thermally (Zoui et al., 2020; Olabi et al., 2022), as shown in Figure 1. The direction of heat flow in TEGs depends on how the thermocouples are set up on the substrate. If they’re placed vertically between the heat source and sink, heat will flow vertically. When thermocouples are placed horizontally on the substrate surface, heat flows horizontally. A combination of both arrangements involves placing thermocouples horizontally on the substrate but with small cavities aiding vertical heat flow (Olabi et al., 2022). Although having high efficiency and provide greater stability, the bulkiness and rigidity of the conventional TEG limits its application, especially on uneven surfaces.

Overtime, researchers have started exploring thin film structures for a better performance and flexible applications. It is reported that thin film TE materials have the advantages over their bulk counterpart such as higher thermoelectric figure of merit, zT, due to their low thermal conductivity (Ferrando-Villalba et al., 2019; Karthikeyan et al., 2020), less amount of material, and easier integration. Thin-film micro-TE devices have increased TE figure of merit and higher efficiency resulted from several effects and dependance of TE properties on the crystal structure, including quantum and classical size effects of electron and phonons, generation of superlattice structures, crystal grain size, and composition in TE materials, where it is difficult to control all these properties uniformly in bulk material (Haidar et al., 2021).

Despite these developments, a number of obstacles must be addressed before thin film thermoelectric devices may be extensively applied in industrial settings. The dimensionless figure of merit (zT), which measures the efficiency of thermoelectric devices, continues to be a significant limiting issue. Even though recent research has shown high zT values in lab settings (Ryu et al., 2023; Wei et al., 2020), it is still challenging to consistently achieve excellent performance in real-world, large-scale applications (d’Angelo et al., 2023). Significant obstacles are also presented by issues like scalability (Liu, 2022), long-term stability (Schwab et al., 2023), and material costs (Syafiq et al., 2022). Additionally, a lot of promising thin-film thermoelectric materials need hazardous or rare components (Singh et al., 2024), which may prevent them from being widely used.

This review offers a thorough analysis of the recent state of thin film thermoelectric devices, emphasizing advancements in device fabrication, performance improvement, and materials research. We hope to provide light on the future direction of thin film thermoelectric by examining current developments in nanoscale thermoelectric materials and innovative device topologies. Thin film thermoelectric devices may be essential to the global shift to sustainable energy sources as we progress toward a more energy-efficient future. By highlighting significant advancements and offering ideas for further study, this paper aims to encourage the continuous advancement of thermoelectric science and technology.

2 Classification of thin film TE material as a function of operating temperature

The classification of thin-film thermoelectric (TE) materials as a function of operating temperature involves systematically exploring materials optimized for different thermal regimes. This categorization typically considers the temperature-dependent thermoelectric properties, including the Seebeck coefficient (S), electrical conductivity (σ), and thermal conductivity (κ), which collectively influence the figure of merit (zT). Thermoelectric materials can be classified based on their optimal operating temperature ranges; low-temperature (up to ∼300 K) such as bismuth telluride, mid-temperature (∼300–700K) such as skutterudites, and high-temperature (above ∼700 K) such as silicon-germanium alloys. This approach enables targeted material selection and design for specific applications, ensuring optimal performance and efficiency. While thermoelectric materials are typically classified according to bulk temperature regimes, thin film thermoelectric materials require additional considerations due to reduced dimensionality, substrate interaction, interface density and strain effects. These parameters have a considerable impact on phonon and carrier transport mechanisms when compared to bulk counterpart (Kazemian et al., 2025). As a result, the temperature-dependent categorization in thin films must be considered alongside geometric confinement and interfacial transport phenomenon.

2.1 Low temperature

Low-temperature thermoelectric materials, typically operating below 300 K, are crucial for applications like cooling, refrigeration, and energy harvesting in ambient or cryogenic environments. For thin film TE, these materials particularly relevant for micro-cooling, flexible electronics and low-grade heat harvesting. These materials must exhibit high σ, a significant S, and low κ to maximize their thermoelectric efficiency. Bismuth telluride (Bi₂Te₃) and its alloys are the most prominent materials in this category, with wide potential for further development.

2.1.1 Bismuth telluride, Bi₂Te₃

Bismuth Telluride, Bi₂Te₃ is a narrow bandgap semiconductor having trigonal (R3m) structure (Figure 2) with a density of 7.7 g/cm³ and high melting point (585 °C) (Arora et al., 2017). It is one of the most promising materials with the highest known efficiency near room temperature (Witting et al., 2019) due to its inherently low lattice κ and high electronic weighted mobility (Witting et al., 2020). This material exhibits highly anisotropic thermoelectric properties and a narrow bandgap of ∼0.1 eV (Witting et al., 2019; Cao et al., 2023), making it particularly sensitive to thickness, strain and interface density in thin films (Parashchuk et al., 2022). The small bandgap results in limited performance caused by deleterious effects of thermally generated minority carriers (Witting et al., 2020). However, doping modification and bandgap tuning can partially mitigate this effect (Witting et al., 2019). Alloying with antimony telluride (Sb₂Te₃) for p-type and bismuth selenide (Bi₂Se₃) for n-type properties will increase their bandgap and reduce bipolar effects (Witting et al., 2020).

In 2020, Wu et al. (2020), conducted a study to enhance the thermoelectric performance of mixed-phase Bi₂Te₃ thin films by fine-tuning the deposition rates of Bi and Bi₂Te₃. In the deposition process, two targets (Bi₂Te₃ and Bi targets) were used to produce thin films instead of a single-target evaporation. In the article, they reported that films with a composition dominated by Te, rather than Bi₂Te₃, were produced when a single Bi₂Te₃ target was directly evaporated. This outcome was attributed to the significant difference in the vapor pressures of Bi and Te. Therefore, adjusting the Te/Bi ratio, film thickness, and composition of the mixed phases plays a crucial role in influencing the overall thermoelectric performance. Films composed of Bi₂Te₃, Bi₃Te₄, and Te with Te/Bi ratio of 1.5 were the result of the optimization process. The study demonstrated that films with a thickness of 100 nm achieved remarkably low κ of 0.59 W/mK and electrical resistivity of 3.17 × 10⁻³ Ω·cm. These optimized properties were obtained by setting the Bi₂Te₃ deposition rate to 3.0/s and the Bi deposition rate to 2.0/s. They concluded the low κ was contributed by interfaces between the various phases present in the films serve as obstacles that substantially suppress phonon propagation.

Maksymuk et al. (2021) demonstrated that TE properties of thin films can be notably enhanced by refining the fabrication conditions and applying appropriate post-deposition annealing. In thin film systems, annealing not only activates dopants but also reduces grain boundary density and residual strain introduced during deposition (Peng et al., 2023). Films with composition of Bi_2-xSb_xTe₃ and Bi₂Te_3-ySe_y were studied, with measured S value of 190 μV/K and −220 μV/K at 300 K, indicating p-type and n-type properties for the films respectively. Along the measurement in temperature range of 300–600 K, increment in Sb and Se concentration were observed, causing the S value to decrease because the influence of intrinsic carriers, typical of narrow bandgap materials. However, in thin films, substrate heat spreading and strain-induced structure modification may shift the effective temperature at which bipolar conduction becomes significant (Wang et al., 2015). The maximum power factors (PF) recorded were approximately 50 μW/cm·K² for p-type films and 41 μW/cm·K² for n-type films, comparable to those of the highest-performing bulk samples with similar chemical compositions. A flexible thin film thermoelectric microinverter (FTEM) was constructed using Bi₂Te₃-based films with the best energy conversion efficiency obtained from this work, which are Bi_0.5Sb_1.5Te₃ alloy as p-type leg and Bi₂Te_2.9Se_0.1 alloy as n-type leg with zT value of 3.1 and 2.8, respectively, at T = 300 K. The FTEM achieved TE efficiency η_max ≈ 3.4% at ΔT of 100 K. By leveraging innovative deposition techniques, structural optimizations, and material compositions, these studies highlight the versatility and efficiency of Bi₂Te₃ thermoelectric films for sustainable energy generation.

2.1.2 Antimony telluride, Sb₂Te₃

Antimony Telluride, Sb₂Te₃ is another well-established thermoelectric material, suited for use in power generation and electronic cooling due to its remarkable ability to convert heat into energy at comparatively low temperatures. Sb₂Te₃ is a narrow bandgap semiconductor that works especially well as p-type thermoelectric device because of its advantageous thermoelectric characteristics, which include a high S, low κ, and high σ. Its layered crystal structure (same as Bi₂Te₃ in Figure 2) facilitates strong anisotropic bonding, allowing for the tailoring of material properties to further enhance thermoelectric efficiency. However, this layered structure includes weak van de Waals interactions, causing it to have inferior mechanical properties despite possessing high zT value of ∼1 (Soleimani et al., 2020) at room temperature (Zhao et al., 2021). Additionally, having poor mechanical qualities may cause the material to cleave along the van der Waals gap during cutting, causing significant particle damage and wasting bulk materials (Ma et al., 2024). Nevertheless, low-dimensional fabrication process makes it comparatively simpler to regulate and optimize material preparation conditions (Ahmed and Han, 2020). In thin film Sb₂Te₃, the layered van der Waals structure enables easier strain accommodation when deposited on mismatched substrates. However, residual stress and lattice mismatch may alter band dispersion and carrier mobility, particularly at elevated temperatures (Ren et al., 2022). Ferrer-Argemi et al. (2019) studied Ag-doped Sb₂Te₃ with varying silver content and annealing temperatures to improve its thermoelectric properties. The result shows that electrodeposited AgSbTe₂ have lower κ than Sb₃₇Te₆₃ films due to the presence of silver dopant, which impeded thermal transport across the films by inhibiting nucleation (Ferrer-Argemi et al., 2019; Shang et al., 2020). Annealing process further reduced the κ of the films, leading to impressive zT value of 0.95 and 0.93 for Ag_3.9Sb_33.6Te_62.5 and AgSbTe₂, respectively, with respective κ values of 0.59 W/mK and 0.22 W/mK. They concluded that the silver content in the films delays the Sb₂Te₃ crystal’s formation but has minimal effect on κ—as opposed to the significant improvement on electrical characteristics.

Adam et al. (2021) also studied the behavior of highly doped Sb₂Te₃ deposited on ceramic substrates via vacuum thermal evaporation technique, over various temperature and Bi dopant content. As a function of temperature, the σ behavior demonstrated a noticeable shift from metallic to semiconducting behavior. The films behave like metals in conducting electricity until they reach a specific transition temperature at around 400 K. This σ depends on factors like carrier mobility and scattering from phonons and impurities, typical of metals and highly doped semiconductors. The observed transition may be further influenced by thin film specific effects such as substrate-induced strain and grain boundary scattering, which modify carrier transport pathways compared to bulk Sb₂Te₃ (Mertens et al., 2024). However, past this temperature, there’s a notable increase in σ, indicating a shift towards semiconducting behavior due to intrinsic conduction mechanisms which is common in chalcogenide-based alloys (Zelenina et al., 2021). On the other hand, Bi-doping significantly affected σ, where the maximum σ value of un-doped Sb₂Te₃ is found to be 55 Ω^-1cm^-1, while Sb_1.85Bi_0.15Te₃ was 51.35 Ω^-1cm^-1. Both works of Ferrer-Argemi et al. and Adam et al. demonstrate the potential of Sb₂Te₃ thin films to be an outstanding thermoelectric material, with proper doping and annealing conditions.

2.1.3 Polymer-based

Polymer-based TE materials represent a promising class of energy conversion materials which leverage the unique properties of conductive polymers to generate electricity for low-temperature application ranges. The conductive polymers have intrinsically high σ due to the conjugated π bonds, making them appealing to practical TE applications (Li et al., 2022a). Polymer thin films inherently exhibit strong thickness-dependent transport behavior due to their anisotropic molecular ordering and phase segregation (Yousefian et al., 2024). In such system, reduced dimensionality enhances carrier percolation pathways while limiting phonon transport through structural disorder. Among these, poly (3,4-ethylenedioxythiophene):poly (styrene sulfonate) (PEDOT:PSS) has become a prominent contender due to its unusual structure, which enables adjustment of its S and σ through doping (Song et al., 2024), post-treatment (Wu et al., 2022), or structural alterations (Feng et al., 2025). PEDOT:PSS is a stable, processable aqueous dispersion made up of a conjugated polymer (PEDOT) and a sulfonated polyanion (PSS) (Figure 3). To date, PEDOT:PSS has been one of polymer-based TE material with highest zT of 0.42, along with PEDOT:Bi₂Te₃ at 0.58 (Xu et al., 2021).

Maeda et al. (2019) developed a technique to fabricate PEDOT:PSS thermoelectric films via gel-film formation process, where thickness can be adjusted within the range of 1–30 µm by modifying the amount of PEDOT:PSS water dispersion used, setting this method apart from direct film formation techniques like spin-coating and drop-casting. Ethanol was used to introduce the PEDOT:PSS water dispersion, enabling the elimination of excess PSS owing to its hydrophilic properties. Rozhkova et al. (2022) also stated that alcohol solvent promotes the development of more homogeneous films by preventing the formation of PSS agglomerates. This approach streamlined the film preparation process by eliminating the need for pre- and post-treatment steps. A variety of film thickness were studied, showing that the σ value increased with decreasing thickness, while the S was independent of thickness. The observed increase in σ suggests enhanced chain alignment and reduced bulk-like disorder (Tounakti et al., 2023), highlighting how confinement in thin films can significantly modify transport mechanisms compared to bulk polymer systems. An S value of 16.0 μV/K, σ value of 1,070 Ω^-1cm^-1, and PF value of 0.28 μW/cm·K² was achieved for thickness of 1.1 µm. It is anticipated that the gels produced by this method can be enhanced by incorporating secondary dopants or inorganic materials.

Standard formulation of PEDOT:PSS possesses relatively low σ due to its core-shell structure, where the hydrophobic PEDOT core is encased in a hydrophilic PSS shell, creating localized conductive PEDOT regions surrounded by insulating PSS areas. In an attempt to counter this problem, researches on solvent used (Dong et al., 2020), method of deposition (Dong et al., 2021), and postdeposition treatment (Liu et al., 2023) have been extensive. In 2021, Hinckley et al. (2021) conducted a study to achieve films with metallic-like conductivities via a blend of solution-shearing deposition and solvent treatments applied in a specific direction. The solution shear deposition technique induces an anisotropic structure in the thin film, leading to enhanced σ along the deposition direction (Hinckley et al., 2021). Hence, Hinckley et al. utilized this approach as a post-treatment for solution-sheared PEDOT:PSS films, achieving significant improvement in electrical performance, with a record-high σ of 8,500 ± 400 Ω^-1cm^-1and a PF of approximately 8 μW/cm·K². The recent developments on PEDOT:PSS properties will definitely extend its already large applicative potential, with further improvements are still to be achieved.

2.2 Medium temperature

Medium temperature thermoelectric materials, designed to operate in the range of 300–700 K, play a vital role in applications like industrial waste heat recovery and automotive exhaust systems. To attain the best thermoelectric performance, these materials need a well-balanced combination of low κ and high σ. The most notable options in this range are skutterudites, half-Heusler alloys, and specific complex oxides, which provide robustness and tunability for better efficiency.

2.2.1 Tin selenide, SnSe

The use of toxic and rare materials such as lead and tellurium has driven the search for more sustainable and cost-effective materials with similar or better thermoelectric performance. A major breakthrough came in 2014 when the Kanatzidis group introduced the tin selenide, SnSe thermoelectric system, marking a significant milestone in the field of thermoelectrics (Chung, 2022). SnSe is a binary compound with layered crystal structure (Figure 4), possessing remarkably low κ and exceptional electronic transport properties (Bai et al., 2023). The intrinsically low κ originates from its layered orthorhombic crystal structure and strong anharmonic lattice vibrations, which enhance phonon-phonon scattering. In thin film form, additional boundary scattering further suppresses lattice thermal transport (Nandihalli, 2023). Additionally, SnSe is an environmentally friendly substitute to conventional thermoelectric materials due to its non-toxic property and abundance. By simultaneously modifying structural and chemical aspects, the TE properties of SnSe can be optimized over a wide phase space made up of intricate interactions between the lattice, charge, and orbital degrees of freedom (Xie et al., 2021).

Burton et al. (2020) successfully electrodeposited SnSe thin films onto a gold (Au) substrate with thickness of ∼8 µm. The S of the as-deposited films were measured to be similar as its bulk counterpart. However, the σ of the untreated sample is much lower at ∼0.01 Ω^-1cm^-1, compared to single crystal SnSe at ∼1.5 Ω^-1cm^-1. This is because the as-deposited films appear to have high disorder and porosity, visible through XRD. A notable increase in σ occurs at 450 K due to annealing, which makes the film denser and more crystalline. This annealing process boosts the σ of electro-deposited SnSe to 0.53 Ω^-1cm^-1 at 570 K, close to the σ of the a-axis of single crystal SnSe at 572 K (0.86 Ω^-1cm^-1). Despite this improvement, the σ of these films generally remains lower than that of single crystal SnSe due to two main reasons. First, the films have some porosity, as observed in low-resolution SEM, which reduces σ. Second, during deposition, delamination from the substrate causes cracks across the film, forming grain boundaries. Nevertheless, their findings represent an important step in developing thin film thermoelectric via scalable methods.

Recently, Ahmad et al. (2024) explored deposition of SnSe via cost-effective successive ionic layer adsorption and reaction (SILAR) method, producing high quality thin film. The characterization results showed the prepared film exhibits orthorhombic structure of SnSe crystals and regularly dispersed on the glass substrate. FTIR spectra also revealed absorption modes corresponding to chalcogenides bonding of SnSe. The thermoelectric properties of the thin films were observed from room temperature up to ∼600 K, which showed the σ generally decreased with temperature, indicating semi-metallic behaviour, with higher σ indicating increased charge carrier concentration and mobility in the thin film. The temperature-dependent σ reflects semiconducting behavior dominated by intrinsic carrier activation at elevated temperatures. Conversely, S values, in line with literature, increased with temperature, suggesting improved carrier concentration and mobility as prime factors for enhanced σ and superior PF values. In short, both of these studies highlight the potential of SnSe thin films for efficient thermoelectric applications across a wide temperature range.

2.2.2 Copper Zinc Tin Sulfide, Cu₂ZnSnS₄ (CZTS)

Copper Zinc Tin Sulfide, Cu₂ZnSnS₄ (CZTS) is another emerging eco-friendly thermoelectric material comprising of earth-abundant elements. The formation of the kesterite structure (Figure 5) resulted from replacing trivalent elements (In or Ga) with bivalent Zn and adding Sn to the chalcopyrite copper indium gallium selenide (CIGS) structure (Gupta et al., 2024). The complex crystal structure of the quaternary compounds promotes strong phonon scattering, leading to low κ, while its larger band gap (approximately 1.4–1.6 eV) reduces the bipolar effect, which typically present in narrow band gapTE material, consequently enhance its TE properties (Gupta et al., 2024; Ali et al., 2021). In order to increase CZTS’s PF and overall thermoelectric performance, recent research focus on enhancing it through doping, defect engineering, and nanostructuring. These developments make CZTS a strong contender for application in sustainable power generation and energy harvesting.

Gunavathy et al. (2020) carried out fabrication of CZTS thin film through an economical green chemical route, employing nebulizer assisted spray pyrolysis (NSP) technique. The properties of the films were studied at varying substrates temperature ranging from 300 °C to 375 °C. Results showed increasing substrate temperature enhances crystallinity and grain size but introduces traces of the Cu₂S phase. Raman excitation at 488 nm indicated a pure CZTS thin film at 300 °C. EDX analysis revealed nonstoichiometric compositions, with Cu/(Zn + Sn) ratios of 0.78 for 300 °C films and 0.84 for 350 °C films with Cu₂S phase. They stated that Cu/(Zn + Sn) and sulphur content can be optimized to suppress Cu₂S phase by adjusting the starting solution’s composition. Other than that, the carrier concentration increased with temperature, and 350 °C films exhibited high S and PF. Future work may focus on optimizing precursor composition and annealing in different environments. Nonetheless, this study affirms the feasibility of low-cost CZTS thin film preparation via NSP, with substrate temperature significantly affecting its properties, particularly thermoelectric properties.

Isotta et al. (2022) utilized CZTS as their p-type thermoelectric material for device fabrication. The films were fabricated via physical vapor deposition methods, producing good purity kesterite phases with promising PF. Sequential deposition was done to fabricate films with targeted nominal stoichiometry Cu_2.125Zn_0.875SnS₄ (CZTS) and Cu_2.125Zn_0.875SnSe₄ (CZTSe). Using direct current magnetron sputtering, a Cu/Sn/Cu/ZN metallic precursor stack was deposited on soda lime glass, targeting particular thickness to obtain Cu-rich composition with nominal value of [Cu]/([Zn] + [Sn]) = 1.133 and [Zn]/[Sn] = 0.875. Then, chalcogenization step was introduced using tube annealing with S, Se or Sn powder in a graphite box. The XRD spectrum of the samples correlates with the respective I-4 kesterite phase while the EDXS results show similar composition as targeted nominal stoichiometry. The thermoelectric characterization of CZTS and CZTSe films confirmed a p-type behavior, enabling them to pursue device fabrication with AZO as n-type thermoelectric material. The fabricated CZTSe-based devices achieved peak power density of 279 nW cm^-2. The performance attained is stated to be more than an order of magnitude higher when compared to other sustainable candidates, suggesting the materials and design selected represent a promising path for environmentally friendly thin film devices.

2.2.3 Magnesium silicide, Mg₂Si

Magnesium silicide Mg₂Si, known for its environmentally friendly composition and low cost, is a promising n-type thermoelectric material with a reverse fluorite structure (Figure 6) (Horiba et al., 2020; Kurokawa et al., 2023). Composed of abundant elements, this material is particularly appealing for mid-to-high-temperature applications within the 300 °C–600 °C (Kurokawa et al., 2023) range. Other than that, Mg-based materials have advantage in specific figure of merit (zT/ρ) due to its low density (ρ = 1.98–2.76 g·cm–3), which is essential for applications where weight is a factor. However, the fact that Mg’s boiling point (1,095 °C) is lower than Si’s (1,414 °C), Mg’s high vapor pressure and reactivity make it challenging to directly synthesize Mg₂Si thin films through vapor deposition, which is the most straightforward approach. Nevertheless, formation of Mg₂Si thin films has been reported using molecular beam epitaxy (MBE), pulsed laser deposition (PLD), and sputtering method (Horiba et al., 2020). With its sustainable attributes and tunable properties, Mg₂Si is emerging as a leading candidate for advanced thermoelectric applications.

Kurokawa et al. (Yi and Yu, 2021) examined the thermoelectric properties of Mg₂Si thin films that were deposited through thermal evaporation of magnesium followed by face-to-face annealing (FTFA). Measurements using SEM-EDX, XRD, and Raman techniques showed that the FTFA reduces the oxidation of Mg₂Si thin films. At an annealing temperature of TA = 923 K, the oxygen concentration decreased by 28 at%. Their primary findings included the carrier concentration of Mg₂Si thin films being within the range of 10¹⁵–10¹⁷ cm^-3, the Raman shift approaching 257 cm^-1 as TA decreased, indicating reduced strain and enhanced crystallinity, the κ value of Mg₂Si thin films being diminished due to interfacial thermal resistance, and a relatively high zT of 0.68 at 712 K. This high zT was achieved through a significant increase in σ, a |S| value of 235 μV/K, and a very low κ value ranging from 1.4 to 1.7 W/mK. The findings indicate that zT can be further improved by optimizing the doping concentration and controlling carrier concentration through the use of Mg vacancies (V_Mg) and doping levels. The study highlights FTFA’s potential for producing high-quality thin films, advancing Mg₂Si as a sustainable and efficient thermoelectric material for mid-temperature applications.

Yi et al. (Kurokawa et al., 2023) systematically investigated Mg₂Si_xSn_1-x solid solutions, focusing on optimizing zT through alloying, valley convergence and phonon-alloy scattering by controlling the mixing and annealing conditions. Solid solutions of Mg₂Si–Mg₂Sn whose silicide to stannide ratio is 3:7, which is known to exhibit the best zT (Liu et al., 2012). Results showed PF ranging from 0.044 μW/cm·K² – 0.032 μW/cm·K² at 573 K and effective mass from 1.1 m₀–0.8 m₀. The κ also varied depending on the degree of homogeneity of Mg₂Si_0.3Sn_0.7 due to the alteration of phonon-alloy scattering, resulting in a large swing of zT between 1.33 (the well-mixed Mg₂Si_0.3Sn_0.7 solid solution) and 0.8 at 673K. The PF improvement is mainly due to the thermopower enhancement which is achieved by valley convergence the conduction bands. The κ reduction is due to alloying of Si and Sn, which suppresses phonons and electronic transport. The findings emphasize the importance of stoichiometric precision and microstructural uniformity, contributing valuable insights for designing high-performance thermoelectric materials.

2.3 High temperature

Energy harvesting in harsh conditions, such power plants and space travel, requires high-temperature thermoelectric materials that can operate beyond 700 K. These materials need to have outstanding thermoelectric qualities while withstanding significant mechanical and thermal stresses. Thin films particularly must withstand thermal expansion mismatch and mechanical stress arising from substrate constraints. Strain-induced band structure modifications and interfacial diffusion become critical factors determining long-term stability and optimal operating temperature. The benchmark for this class is silicon-germanium (SiGe) alloys, which provide efficiency and stability at high temperatures. The potential of other materials, such as oxides and nitrides, to provide high-performance and sustainable solutions is also being studied.

2.3.1 Silicon-germanium, SiGe

Silicon-Germanium, SiGe has been a star member of thermoelectric materials particularly for high temperature application such as industrial waste heat recovery and space exploration. Initially, both Si and Ge are deemed not good TE materials due to their high κ although possessing large PF (Chen et al., 2021). However, balanced thermoelectric properties were achieved by combining the enhanced electronic properties of germanium (Ashburn and Bagnall, 2006; Cariou et al., 2015) with the favorable thermal stability of silicon (Cook, 2022). The alloy (Figure 7) scattering effect results in low κ (Duran et al., 2022), while its electrical properties can be further optimized through doping and nanostructuring. Its robustness under extreme conditions and the abundance of silicon makes SiGe an attractive choice for sustainable, high-efficiency thermoelectric devices. Its qualities are being improved by ongoing research, guaranteeing its relevance in cutting-edge thermoelectric applications.

Xie and Gupta (2020) presented experimental work to investigate nanograin thermoelectric Si₈₀Ge₂₀ thin films. To prepare the films, a unique ball milling technique was applied to generate nanoparticles, which then sintered using a near-infrared laser with quasi-continuous wave. The study focuses on the effect of different laser sintering powers to the electrical and thermal properties of the Si₈₀Ge₂₀ + 2% phosphorus thin films. The EDS result showed the as-deposited films contain two at% phosphorus, but reduced after laser sintering due to dopant evaporation, which became negligible to the composition of Si₈₀Ge₂₀. The samples were sintered at three different powers of 20%, 30%, and 40%, with 30% having more percolation structure, hence produced higher σ at 80.9 Ω^-1cm^-1 with S value of −120.2 μV/K at room temperature. Lower laser power than 30% leads to lower dopant activation and resulted in low carrier concentration, thus leading to lower σ, while higher power will cause thermal evaporation loss. However, κ of 20% sample shows lower value due to higher porosity. They conclude that samples with higher porosity have higher S and lower κ, but the improvement was not sufficient to compensate for the loss of σ.

Taniguchi et al. (2020) leads an experimental investigation of Si_1-xGe_x epitaxial films on Si substrates in terms of thermoelectric properties due to its compatibility with Si technology. Si_1-xGe_x epitaxial films were fabricated with various controlled x values and strains using various growth methods. PF higher than SiGe alloy-based bulk at room temperature was reported for Ge epitaxial films without strains at ∼47 μW/cm·K². On the other hand, ultralow κ of ∼2 W/mK was recorded for strained Si_1-xGe_x epitaxial films even without nanostructures. The ultralow κ indicates the suitability of Si_1-xGe_x epitaxial films application in thin film thermoelectric generator fabrication under various ΔT situations, while Ge epitaxial films with high PF are also feasible with little modification of nano-structuring. The observed behavior highlights the strong influence of strain engineering in thin films. Unlike bulk alloys, epitaxial strain can modify phonon dispersion and suppress thermal transport even without extensive nanostructuring (Krahl et al., 2020). With improvements in nanostructuring, doping, and epitaxial growth enabling optimized PF and κ, the studies on SiGe thin films demonstrate their potential for high-temperature thermoelectric applications and ensuring their continued relevance in advanced thermoelectric technologies.

2.3.2 Zinc oxide, ZnO

Oxide-based thermoelectric materials have been an alternative to conventional TE materials because of their cheap resources. Zinc Oxide (ZnO) has garnered attention for its exceptional properties, including high thermal stability, environmental friendliness, and cost-effectiveness. ZnO-based films are especially useful for the integration of TE technologies, as they address the increasing need for self-powered wireless sensing networks (WSN) and the Internet of Things (IoT) in multiple fields, such as medical diagnostics (Rehman et al., 2021), appliance control (Rehman et al., 2022), sports performance tracking (Rehman et al., 2023) and aerospace applications (Rehman et al., 2020). The n-type ZnO have wide bandgap of 3.37 eV and high exciton binding energy (60 meV), but being largely studied as TE materials because of its thermal stability at elevated temperature, making it useful in high-temperature ranges (Rehman et al., 2021). ZnO crystallizes in the wurtzite structure (Figure 8) with lattice parameters a = 3.25 A^o and c = 5.20 A^o and is found to be a piezoelectric material (Wang and Song, 2006). Its thermoelectric performance can be significantly enhanced through nanostructuring to reduce κ as well as doping, although this process often involves a trade-off between σ and S (Ong et al., 2011). In oxide thin films, grain boundary density and dopant segregation at interfaces significantly affect carrier concentration and mobility (Ghosh et al., 1991). Deposition temperature controls not only crystallinity but also residual stress, influencing high-temperature thermoelectric stability.

Mahesh and Kumar (2020) reported the synergistic effects of aluminium (Al) and indium (In) co-doping on the physical properties of ZnO thin films, prepared via the spray pyrolysis technique with varied dopant concentration (0.5%–2% equimolar), comparing co-doped AIZO films with single doped AZO and IZO films. The FESEM images showed variation of the grain in morphology according to the preferential orientation from flake-like to pyramidal grains with increasing doping percentages. The transmittance spectra showed an increase in transmittance with doping percentages while the doped films showing interference fringes due to high uniformity and smoothness. In short, the co-doping led to enhanced crystallinity, improved transparency (>90% transmittance) and reduced resistivity (3.67 × 10⁻² Ω cm for AIZO 1.5). The co-doped AIZO films tend to behave like IZO at higher doping amounts and like AZO at lower doping percentages. The study provides a comprehensive understanding of the physical properties of AIZO thin films and their comparison with AZO and IZO thin films.

Pham et al. (2022) investigates the thermoelectric properties of antimony (Sb)-doped ZnO (SZNO) thin film, focusing on the effects of deposition temperature on their performance. Using DC magnetron sputtering, the films were deposited across temperature ranging from ambient temperature to 873 K. The study revealed that the optimal deposition temperature of 673 K resulted in films exhibitng n-type conductivity with the highest thermoelectric PF of 4.06 μW/cm·K² at 773 K and a good optical transparency (∼82%). XRD and photoluminescence (PL) analyses showed that Sb doping improved the crystallinity and grain size of the ZnO films. By increasing the density-of-state effective mass, which raises the n_e (carrier concentration) of the films produced in the T_d zone of RT – 673 K, the replacement of Sb⁵⁺ ions for Zn²⁺ greatly improved the S and thermoelectric PF. Substitution of Sb⁵⁺ ions for Zn²⁺ introduces three additional electrons per dopant atom, requiring charge compensation that alters carrier concentration and transport behavior. The high |S| of the films formed at higher T_d than 673 K may be caused by the reduction of n_e. The study concludes that SZO thin film prepared at the optimal deposition temperature of 673 K exhibit enhanced thermoelectric properties and high transparency, making them promising candidates for transparent thermoelectric applications.

2.3.3 Lanthanum (III) telluride, La_3-xTe₄

As a rare-earth chalcogenides, n-type lanthanum telluride (La_3-xTe₄) (May et al., 2008) is known for its excellent thermal stability and high zT (Wood, 1988; Thiagarajan et al., 2026), operating under a large temperature range of 500–1300 K. La_3-xTe₄ possesses a Th₃P₄-type structure with space group I-42d (Figure 9). La_3-xTe₄ is composed of both light and heavy conduction bands, where the light bands facilitate electron transport, while the heavy bands at elevated energy levels help maintain a high S, even at high carrier concentrations (Pan et al., 2025). Other than that, La_3-xTe₄ exhibits intrinsically low lattice κ due to complex crystal structure (Li et al., 2022b) and large number of random vacancies (Alam and Ramakrishna, 2013). The electronic properties of La_3-xTe₄ change from metallic at x = 0 (La₃Te₄) to semi-insulating at x = 1/3 (La₂Te₃) as lanthanum vacancies are altered (Li et al., 2022b). With its high-temperature stability and tuneable properties, La_3-xTe₄ continues to attract attention as a promising candidate for advanced thermoelectric applications.

Shah et al. (2023) studied the impact of annealing temperature on the structural, optical, and electrical properties of lanthanum erbium telluride (La_0.1Er_0.2Te_0.2) nanoparticles. LaErTe nanomaterials have higher capacitance than other transition metal oxides, having good σ, large surface area, and long cycle life (Payrer, 2026). The method used in the study was hydrothermal synthesis of La_0.1Er_0.2Te_0.2 nanoparticles using mixtures of Er(NO₃)₃·5H₂O, La(NO₃)₃·6H₂O, and Te(NO₃)₄, followed by doctor blade to deposit the nanoparticles onto FTO substrates. The nanoparticles exhibited a hexagonal structure with increased crystallinity at higher annealing temperatures, as confirmed by XRD analysis. The bandgap energy decreased from 3.50 eV (unannealed) to 2.26 eV (annealed at 600 °C), reflecting enhanced optical absorption in the visible region. Electrical characterization revealed that σ decreases with increasing thickness and annealing temperature. The study concludes that the annealing temperature has a significant impact on the properties of La_0.1Er_0.2Te_0.2 nanoparticles and underscore its potential as a tunable material for optoelectronic and thermoelectric applications.

Recently, Cheikh (2023) developed innovative processing techniques for La_3-xTe₄ thin films, optimizing their thermoelectric properties for high-temperature applications, such as radioisotope thermoelectric generators (RTGs). Using electrophoretic deposition (EPD) in tetrahydrofuran as a solvent, they produced uniform films with a thickness of 10–15 µm and a green density of ∼65%. These films were then densified using ultrafast high-temperature sintering (UHS), achieving densities above 90% within 10 s at 1,450 °C. The resulting films exhibited thermoelectric performance comparable to bulk materials, a high S of −210 μV/K with a peak zT of 1.06 at 1273 K. Additionally, the UHS enabled pressure-less sintering, maintaining the phase purity of La_3-xTe₄​ and minimized oxidation, making it a viable approach for scalable, high-throughput manufacturing of thermoelectric devices. The reduced thickness also facilitates faster thermal response, which may alter effective operating temperature compared to bulk RTG. This study provides a significant leap in processing strategies, combining efficiency and performance for advanced thermoelectric applications.

2.4 Summary of thin film thermoelectric materials

The materials discussed in this review were selected based on three criteria: (i) relevance to specific temperature regimes, (ii) demonstrated thin film processability and (iii) level of technological maturity, encompassing both well-established benchmark systems and emerging material candidates. Although thin film thermoelectric materials can be broadly categorized into low, medium and high temperature regimes similar to bulk materials, their transport behavior is strongly governed by thin-film-specific effects. Reduced dimensionality enhances phonon boundary scattering, often lowering κ relative to bulk systems. However, increased grain boundary and interface scattering may also reduce carrier mobility, requiring microstructural optimization. Substrate interactions introduce strain, lattice mismatch and thermal expansion constraint, which modify band structure and influence carrier transport particularly at elevated temperature. Additionally, interfacial thermal resistance and substrate heat spreading affect the effective temperature gradient across thin films, potentially shifting the optimal working temperature compared to bulk counterparts. Therefore, classification by temperature must be interpreted alongside structural confinement, strain engineering and interface design which collectively define thermoelectric performance in thin film systems.

Table 1 presented a structured overview of key TE materials discussed above, classified based on their optimal operating temperature ranges: low, medium, and high. It highlights their significant improvements, fabrication methods, thermoelectric properties and potential applications. By consolidating insights from recent studies, this segment serves as a guide for selecting and optimizing TE materials for various energy conversion and harvesting applications.

TABLE 1. Summary of thin film thermoelectric materials.

To provide a clearer comparison of performance, scalability and technological maturity across different thin film thermoelectric system, Table 2 summarizes representative materials discussed in this review, in addition to thermoelectric metrics, the table highlights deposition methods, substrate considerations and scalability potential, offering a more application-oriented perspective.

TABLE 2. Benchmarking and technological assesment of representative thin film thermoelectric system.

Performance values are representative thin film data summarized from recent literature discussed in this review.

3 Thin film deposition methods

This section encompasses a range of advanced techniques used to fabricate thin films with precise thicknesses, compositions, and properties tailored to specific applications. Thin film thermoelectric fabrication techniques offer distinct advantages over bulk processing, including precise thickness control, compatibility with microfabrication, enhanced interface engineering and potential quantum confinement effects. These features enable tailored phonon and carrier transport that are difficult to achieve in bulk materials. The discussion begins with sputtering, a widely utilized physical vapor deposition method known for its adaptability and precise control, enabling the deposition of diverse materials for applications like catalytic films and biocompatible alloys (Garg et al., 2024). Thermal evaporation is then highlighted as a versatile technique that uses vacuum and high temperatures to achieve uniform thin films, with advancements addressing challenges in depositing high-melting-point materials like MoS₂ ⁹³. Spin coating follows as a cost-effective method praised for its simplicity and ability to create homogeneous films, with particular relevance in solar cells and functional devices (Mendhe, 2023). Finally, chemical vapor deposition (CVD) is explored for its precision and ability to synthesize advanced thin film materials, including two-dimensional conductive frameworks and transition metal chalcogenides, underscoring its significance in electronics and energy applications (Choe et al., 2022). This structured flow provides a comprehensive overview of key deposition methods, their principles, advancements, and potential applications.

3.1 Sputtering

Sputtering is an effective industrial method which has been extensively investigated for the fabrication of thermoelectric thin films. This physical vapor deposition process utilized plasma composed of (usually) Ar⁺ ions, accelerated due to magnetic field that bombards the atoms of the target materials and causing it to be deposited to the substrate. This technique is very useful for creating thin films with desired qualities because it provides excellent regulation of the thickness as well as the composition of the deposited material (Garg et al., 2024). This versatility enabled researchers to develop a diverse array of thin film materials through sputtering, including metals, metal oxides, nitrides, alloys, carbon-based materials, and ceramics, each tailored with unique properties for various applications (Garg et al., 2024). Sputtering’s adaptability over alternative deposition methods is one of its main benefits. Metals, semiconductors, and insulators are among the many materials that may be deposited using it. Recent studies further demonstrated sputtering’s potential in developing high-performance catalytic films for water purification and hydrogen production, as well as biodegradable metals and alloys with enhanced corrosion resistance and biocompatibility (Garg et al., 2024).

Among other sputtering techniques, magnetron sputtering (MS) has gained prominence due to its low deposition temperature and high sputtering rate, both of which make it ideal for producing thin films with specialized functions. The applied magnetic field confines secondary electrons near the target surface, increasing ionization efficiency and plasma density. This enhanced ion bombardment improves film adhesion and density but may also induce re-evaporation of volatile species such as Te during high temperature processing, potentially affecting stoichiometric control. Continuous advancements in MS have expanded its capabilities, especially in addressing the growing complexities of thin film synthesis. High power pulsed magnetron sputtering (HPPMS), an advanced form of MS, has proven particularly valuable for the deposition of transparent conducting oxides, such as indium tin oxide (ITO) and aluminum-doped zinc oxide (AZO), employing both ceramic target sputtering and reactive sputtering of metallic targets (Li et al., 2022c). Figure 10 shows a schematic of a top-down configuration sputter.

A study by Yonezawa et al. (2020) utilized radio frequency magnetron sputtering of Bi₂Te₃ thin film, followed by thermal annealing as treatment process. The composition of the deposited film exhibited a slight deviation from the stoichiometric ratio, prompting the use of thermal annealing to adjust the Te composition by evaporating Te atoms from the thin films. The result of this study shows that at annealing temperatures below 250 °C, the composition of the films remained close to that of the as-deposited material. However, as the annealing temperature was increased further, a decrease in the Te content and a corresponding increase in Bi occurred, owing to the preferential evaporation of Te atoms. This process led to improved crystallinity and a stronger crystal orientation along the c-axis. Furthermore, the S for all samples remained negative, indicative of n-type behavior, and it became more negative as the annealing temperature was raised to 350 °C. Moreover, the σ increased as the annealing temperature increased up to 300 °C, but further temperature increment decreases the σ. In conclusion, this study presents the reliability of sputtering method in synthesizing Bi₂Te₃ thin film.

Recently, Tao et al. (2024) used a novel sputtering technique, the High Target Utilisation Sputtering technique (HiTUS) to fabricate flexible thermoelectric thin films with exceptional performance at room temperature. The study explored the complex effects of plasma power, target power, and substrate bias in HiTUS on the properties of the films. The study demonstrated that the deposited film achieved a PF of 6.5 × 10⁻⁶ μW/cm·K² with a corresponding zT of approximately 0.14 at room temperature, representing the highest values reported for room-temperature sputtered undoped bismuth telluride. Post-deposition annealing significantly improved the film’s crystallinity, making it highly polycrystalline, and enhanced the PF to 23.5 × 10⁻⁶ μW/cm·K², with a zT of about 0.45 at room temperature. These outstanding results highlight the potential of HiTUS-fabricated thermoelectric films for large-area production of thin-film thermoelectric materials and devices. Compared to conventional MS, HiTUS decouples plasma generation from target biasing, enabling independent control of ion energy and flux (Lofaj et al., 2016). This improves compositional stability and reduces target poisoning in multi-component thermoelectric systems.

In summary, sputtering has emerged as a highly adaptable and efficient technique for the fabrication of thin films, especially in thermoelectric applications. Its capability to deposit a diverse array of materials with exact regulation of both thickness and composition makes it an essential tool in contemporary materials science. Advances in sputtering techniques, such as magnetron sputtering (MS) and its variants like HPPMS and HiTUS, have further enhanced its capabilities, enabling the synthesis of high-performance films with specialized properties. Studies on materials like Bi₂Te₃ and other thermoelectric compounds highlight sputtering’s reliability and adaptability in achieving tailored film properties through process optimization and post-deposition treatments, solidifying its role in the development of next-generation thin-film technologies.

3.2 Thermal evaporation

Another popular technique for depositing thin film is thermal evaporation, in which the source material evaporates in a vacuum as a result of heating at a high temperature, allowing the vapour particles to move and reach a substrate as in Figure 11, where they again solidify (Bashir et al., 2020). This process involves heating the substance to be deposited to its evaporation or sublimation point in a high-vacuum chamber. Following their evaporation, the atoms or molecules go straight ahead towards the colder substrate, where they condense to create a homogeneous thin layer (Soonmin, 2015). Thermal evaporation is a commonly employed deposition method known for its versatility across various applications. However, Lu et al. (2024) demonstrated difficulties encountered when using conventional thermal evaporation in depositing uniform MoS₂ thin films, because of its elevated melting point of 1,375 °C, which typically results in thick and rough films. To overcome this limitation, a specialized vacuum thermal evaporation system was developed, enabling the scalable production of MoS₂ films by employing K₂MoS₄ as a precursor, this approach successfully achieves uniform polycrystalline MoS₂ thin films with precisely controlled thicknesses between 0.8 and 2.4 nm over a 50 mm × 50 mm area. Additionally, they also stated this method supports patterned and sequential deposition, allowing applications in a more advanced devices, such as field-effect transistors (FETs), memristors, and electrocatalysts for hydrogen evolution, thereby expanding the functional potential of MoS₂ films in cutting-edge technologies.

A recent study by Fan et al. (2020) successfully fabricated Bi₂Te₃ thin film using two-step single-source thermal evaporation in attempt to control the content deposition and use low-negative thermal treatment. The result of this work shows that all samples exhibit a negative S, indicating n-type conductivity, with a grain size of approximately 100 nm. Besides that, the SEM results also confirmed that the film has smooth surfaces and nano-sized particles, in ultralow κ, which is only 70% of bulk Bi₂Te₃, leading to their high zT value. The PF increased by nearly tenfold following the rapid heat treatment (RHT). These results are comparable to the highest values achieved for flexible Bi₂Te₃ thin films, demonstrating the effectiveness of this innovative approach for producing high-performance flexible thermoelectric thin films.

Ahmoum et al. (2021) examined the thermoelectric performance of nickel-doped zinc oxide (Ni-doped ZnO) using the thermal co-evaporation method, focusing on optimizing carrier concentration and structural properties. The results revealed that all the prepared samples exhibited preferential orientation along the (002) crystallographic plane. An increase in Ni content resulted in improved crystallinity and reduced surface roughness, achieving a minimum roughness of 5.9 nm at a deposition temperature of 1,250 °C. Electrical characterization revealed an electron concentration of 1.5 × 10¹⁹ cm^-3, a S value of 178 μV/K and σ of 31.75 Ω^-1cm^-1, with a maximum PF of 1.01 μW/cm·K² These findings highlight the role of Ni doping in balancing charge carriers and improving thermoelectric performance, establishing thermal co-evaporation as a viable method for developing high performance ZnO-based thermoelectric materials.

In summary, thermal evaporation is a versatile and widely employed technique for thin film deposition, enabling the creation of high-quality films for diverse applications. Despite challenges such as the difficulty in achieving uniform films for high-melting-point materials like MoS₂, advancements in specialized systems and methodologies have significantly improved the technique’s precision and scalability. Studies on materials such as Bi₂Te₃ and Ni-doped ZnO highlight the method’s ability to produce smooth, nano-sized particles with enhanced thermoelectric properties, including increased PF and low κ. However, significant vapor pressure differences between the materials in the multi-component alloys may causes preferential evaporation, leading to compositional deviation from stoichiometry. Strategies such as co-evaporation, two-step deposition and rapid heat treatment can be employed to mitigate this issue and promote uniform grain growth and dopant activation. Other than that, evaporation rates critically influence nucleation density and surface diffusion dynamics. Therefore, optimization of evaporation rate requires balancing electrical conductivity enhancement and phonon suppression. In recent development, anti-solvent-assisted or stepwise evaporation strategies have been employed to regulate nucleation kinetics and improve compositional uniformity (Majewski et al., 2025).

3.3 Spin coating

Spin coating is a desirable approach for a variety of materials, including inorganic, organic, and hybrid combinations, due to its versatility, speed of production, and accurate control over thickness (Mendhe, 2023). The ability to produce extremely homogenous and uniform layers with regulated thicknesses ranging from a few nanometres to several microns makes spin coating a popular thin film production process. Using the combined effects of surface tension and centripetal force, a precursor solution is deposited on a flat substrate and quickly spun to distribute the solution uniformly throughout the surface. This process can be seen in Figure 12a. A solid thin film with constant coverage and good film quality is created when the solvent evaporates (Figures 12b,c). ⁸⁴It is an environmentally benign method that works especially well for applications that need for precise film properties, such as those found in functional thin films (Yin, 2023). The basic ideas of spin coating are covered in this discussion, along with an assessment of film quality and the most recent advancements in the area. Special focus is paid to how spin coating may be used to optimise characteristics for high-efficiency solar cells and other useful devices.

Recently, Osman et al. (2024) studied the impact of spin coating rates on the electrical and optical properties of sol-gel-prepared perovskite strontium titanate (SrTiO₃ or STO) thin films. The thickness of the STO films, was directly correlated with spin speed when they were deposited onto glass substrates at different spin speeds. Higher spinning speed produced the most transparent films, suggesting a crucial relationship between coating speed and optical clarity. X-ray diffraction analysis confirmed that the STO films possessed a cubic crystal perovskite structure, a desirable feature for uniformity and stability. Optical properties also varied with spin speed; where the optical band gap ranged narrowly from 2.83 to 2.90 eV, while the refractive index remained between 2.4 and 2.42, suggesting minimal dispersion changes across different speeds. Electrical performance peaked at 8,000 rpm, where STO films exhibited the best σ and reduced sheet resistance, emphasizing that spin speed not only impacts film thickness and transparency but also plays a crucial role in optimizing σ for potential electronic applications. This study underscores the versatility of the spin coating method in finetuning thin film properties for specific device requirements.

Okpozo et al. (2024) investigated the effects of magnetic-spin coating on the assembly morphology of gold-Pickering ferrofluid droplets, focusing on how varying the magnetic inertia energy ratio influences droplet arrangement during deposition. Using a combination of magnetic fields and spin coating, the study found that higher magnetic-inertia ratios resulted in more uniform droplet size, improved alignment, and increased spacing consistency among the gold assemblies within the ferrofluid. Analysis revealed that when the magnetic-inertia ratio was increased, droplets exhibited a reduced tendency to coalesce, producing a controlled pattern with minimal clustering—an improvement over traditional spin coating alone, which often yields less structured assemblies. This enhanced control over droplet morphology, enabled by magnetic assistance, provides valuable insights for applications requiring precise nano-patterning, such as in catalytic surfaces and advanced sensing materials. The study highlights the potential of magnetic spin coating to advance thin film fabrication by introducing a versatile parameter—the magnetic inertia ratio—that can be tuned to achieve desired morphological and functional properties.

Spin-spray coating combines aspects of traditional spin coating and spray deposition, offering enhanced scalability and material utilization for thin film fabrication. This technique optimizes the deposition process by employing a spray mechanism to achieve more uniform and continuous thin films, especially for solution-processed materials like perovskites. As outlined in studies such as that by Habibi et al. (2017), spin-spray coating facilitates better control over film morphology, coverage, and conversion efficiency, particularly under optimized parameters like spray pass count, substrate temperature, and solution concentration. These advancements make spin-spray coating a promising method for scalable production of high-performance thin films, particularly in applications like photovoltaics.

In conclusion, spin coating remains a highly effective and versatile technique for thin film deposition, offering precise control over film thickness, uniformity, and quality. Its ability to produce homogeneous layers with consistent properties makes it ideal for a wide range of applications, from solar cells to advanced sensors. Recent advancements, including studies on the influence of spin speed, magnetic fields, and spin-spray coating, have further demonstrated its potential to optimize film properties for specific device requirements. As research continues to explore innovative variations of the technique, spin coating remains a cornerstone in the fabrication of high-performance thin films.

3.4 Chemical vapor deposition

CVD is a widely used method for creating thin films and coatings on various substrates. Figure 13 shows the CVD system and flows of the material deposition. This method allows for high precision, purity, and controlled deposition. It enables the synthesis of various thin film materials, including metal-organic frameworks (MOF) and transition metal chalcogenides, with promising applications in electronics, energy generation, and catalysis (Choe et al., 2022). Besides, the creation of thin-film microdevices with exceptional σ has been made possible by recent developments in CVD, which have made it possible to create two-dimensional conductive MOF thin films, such as Cu₃(C₆O₆)₂, with high crystallinity and edge-on orientation (Mbam et al., 2019). Conventional CVD methods have been improved to address issues with thin film tension, growth modes, and micro/nano-level measurements. Understanding the stability and quality of CVD thin films is crucial for customizing films for specific performance needs. Moreover, the synthesis of transition metal chalcogenide thin films with desired optical and electrical characteristics has been made easier using AdditVD (Barde and Lewis, 2023). AdditVD is an advancement in CVD that enables precise control over deposition, enhancing the synthesis of transition metal chalcogenide thin films with tailored properties. By incorporating additives, it improves film quality and uniformity, enabling customization for applications in electronics and energy devices. This overview explains the fundamentals, current developments, and uses of CVD, emphasising their expanding use in the production of high-performance thin films for a range of technical requirements.

Ding et al. (2024) investigated plasma-enhanced chemical vapor deposition (PECVD) for applying silicon oxide (SiO_x) thin films to Portuguese limestone, evaluating PECVD as a potential preservation method for heritage materials. PECVD produced uniform SiOx layers on porous limestone with smooth, crack-free coverage, demonstrating effective conformal coating. The controlled deposition allowed for precise tuning of film thickness and protective properties, which enhanced the limestone’s durability against environmental factors like humidity and pollution. The study’s findings highlight PECVD’s value in conservation science, presenting it as a robust, minimally invasive option for heritage preservation that maintains the limestone’s aesthetic and structural qualities while adding a protective layer. This application underscores PECVD’s adaptability beyond its traditional uses in microelectronics and optics (Cheikh, 2023).

A recent study by Poche et al. (2024) investigated the use of cyclohexane as a precursor in PECVD to create polymer-like hydrogenated amorphous carbon (a-C:H) thin films. This method produced flexible, hydrogen-rich films with characteristics akin to those of polymers, which bodes well for uses like flexible electronics and protective coatings. The team was able to regulate the films’ surface shape, mechanical characteristics, and hydrogen concentration by altering the PECVD settings. High-quality deposition was indicated by the smooth, uniform film surfaces with few flaws that were discovered by scanning electron microscopy (SEM) examination. The films’ strong mechanical durability, flexibility, and stickiness were further characterised. These findings demonstrate PECVD’s versatility in creating robust thin films for a range of applications by highlighting its potential to create polymer-like a-C:H films using cyclohexane that provide a balance between structural stability and flexibility.

In conclusion, CVD is a versatile and highly effective technique for producing high-quality thin films and coatings with precise control over material properties. Its adaptability to various materials, including transition metal chalcogenides, makes it a promising method for thermoelectric (TE) material deposition. By enabling the customization of films for specific electrical and optical characteristics, CVD shows great potential in enhancing the performance of TE materials for energy harvesting and other advanced applications. With continuous improvements in deposition control, CVD will likely play a key role in advancing the development of high-performance TE devices.

3.5 Summary of thin film deposition methods

Table 3 summarizes the discussed deposition methods commonly used for fabricating and optimizing TE materials. The table outlines each method’s key features as well as its advantages, challenges, and applications. It also covers new developments in these methods, highlighting how they can improve thermoelectric qualities. This section offers insightful information about how deposition techniques contribute to the development of TE technology for energy harvesting and conversion applications.

TABLE 3. Summary of various deposition technique utilized in TE research.

4 Device fabrication

Traditional TEGs are primarily built with bulk p-type and n-type legs assembled in a two-dimensional array over a ceramic substrate equipped with electrical contacts (Siouane et al., 2017). In this common structure, the heat travel cross-plane where the temperature gradient is perpendicular to the substrate, enabling thermal energy conversion across the TE legs, as shown in Figure 14a. However, the configuration of thin film enables heat to travel in-plane, where the temperature gradient runs parallel to the substrate as can be seen in Figure 14b. Due to their larger dimensions in the direction of the heat flow, in-plane geometries typically enable larger temperature gradients, leading to an increased power output (Ferrando-Villalba et al., 2019). This section will examine the development of device production and provide a look ahead at the possibilities of thin film thermoelectric, taking account new materials and production techniques as discussed, that could make it possible to create the next wave of TE devices that are both incredibly efficient and reasonably priced.

4.1 Cross-plane thin film TEG

Cross-plane thin-film thermoelectric generators (TEGs) are designed to operate with heat flow perpendicular to the thin film surface, similar to the conventional bulk TEG. By using thin-film materials with optimized thermoelectric properties, cross-plane TEGs can be effectively used in energy harvesting from small-scale heat sources such as body heat (Karthikeyan et al., 2020).

Haidar et al. (2021) demonstrated the fabrication of micro-scale TEG with vertical architecture, feasible for cross-plane application using separate n-type and p-type wafers. After optimization, n-type Bi₂Te₃ and p-type Sb₂Te₃ were deposited at room-temperature, followed by annealing at 250 °C. During device fabrication, gold (Au) interconnect layer was electroplated on the substrate and etched using standard Au etching recipe. Afterward, lift-off photoresist was deposited to pattern the wafers. The n-type and p-type materials were then sputtered, followed by sputtering of Chromium (Cr) and Bi layer on top to promote adhesion and form bonding layer respectively. After deposition, the films were patterned using a lift-off process with AZ9620 photoresist and diced into individual dies (Figure 15a). Wafer-to-wafer bonding was performed using thermo-compression bonding at 350 °C and 44 MPa under nitrogen flow, forming eutectic bonds between Bi and Au layers. Fully constructed µTEG are shown in Figure 15b, having PF of 0.007 and 0.013 μW/cm·K² for Bi₂Te₃ and Sb₂Te₃, respectively. This work demonstrates the potential for effective and scalable solutions in depositing thin films for cross-plane application, highlighting important developments in the deposition and optimization of Bi₂Te₃ and Sb₂Te₃ thin films for microscale thermoelectric energy generation.

4.2 In-plane thin film TEG

In-plane thin-film thermoelectric generators (TEGs) utilize heat flow parallel to the film’s surface, offering distinct advantages in applications where planar integration and flexibility are critical. This configuration allows for efficient energy harvesting from lateral temperature gradients.

In 2022, Syafiq et al. (2022) demonstrated the fabrication of in-plane, thin film TEG using Cu/Zn/Sn-based chalcogenides as p-type materials. The ink for the films was synthesized either via hot injection or from ball-milled powders. The synthesized inks were deposited on glass substrate by spin coating, while the n-type AZO was deposited via sputtering. The thermoelectric properties of the films confirmed the compatibility of both film types to be coupled. The couples were arranged in series and connected by silver metal contact in a zigzag pattern as shown in Figure 16. Voc vs. ΔT measurement of the fabricated TEGs showed a linear plot, with maximum voltage of ∼87, ∼118, ∼46, and ∼98 mV, respectively for CZTS/AZO, CZTSe/AZO, CZTSSe/AZO, and CTS/AZO TEGs at 160 K ΔT. TEGs performance were evaluated through I-V-P measurements, showing the maximum power per unit active planar area of ∼43, ∼188, ∼23, and ∼59 nW/cm² for CZTS/AZO, CZTSe/AZO, CZTSSe/AZO, and CTS/AZO TEGs, respectively. The study shows an exceptional process to understand how in-plane thin film TEG works, opening wider potential in various applications.

4.3 Multilayer TEG

Instead of the traditional TEG structure or the in-plane structure of thin film, Chen et al. (2020) explored a new design based on multilayer-structure module. The thin films were stacked, layer-by-layer structure, oriented perpendicular to the thin film surface. Specifically, a three-layer structure is created by sequentially depositing a p-type TE layer, an insulating layer, and an n-type TE layer onto a substrate as shown in Figure 17. Through selective shielding, one end of the TE layers is connected to form p-n junctions to be stacked in series. This method enables multiple three-layer p-n junctions to be stacked, resulting in a compact design in a small volume by simply adjusting the number of layers. This multilayer structure allows high integration of TE elements, producing a high output voltage. Consequently, this design simplifies the manufacturing process and reduces the need for a larger device footprint.

In the experiments, NiCu and NiCr materials were used as p-type and n-type, respectively, with SiO₂ as insulating layer. A single p-n junction of said materials yields approximately 45 μV/K. Beyond material optimization, thin film thermoelectric device performance is strongly governed by architectural and interfacial factors. Increasing film thickness reduces internal electrical resistance and enhances current output, however, excessive thickness may introduce residual stress due to thermal expansion mismatch between the thermoelectric layer and substrate, leading to cracking or delamination. In addition, contact resistance at metal-semiconductor interfaces (e.g., Ag-chalcogenide or oxide-chalcogenide junctions) can significantly contribute to Joule losses in microscale devices. Furthermore, planar thin film configurations are susceptible to thermal shunting through substrate, which reduces the effective temperature gradient across thermoelectric legs and limits achievable output voltage. This effect becomes increasingly significant as device dimensions shrink, limiting achievable open-circuit voltage despite high material zT. Further studies were done using n-type Bi and Bi₂Te₃, and p-type Sb and Sb₂Te₃ to investigate the reliability of the multilayer structure. The open circuit voltage of the device as a function of temperature difference exhibits a linear trend, with maximum output voltage of approximately 3, 40, 80 and 150 mV for Ni-based double junctions, Bi-Sb single junction, Bi₂Te₃-Sb₂Te₃ single junction and Bi₂Te₃-Sb₂Te₃ double junctions, respectively, at ΔT 50 K. These results indicate that the multilayer device have good stability and suggests promising alternatives for preparing high integration devices.

Compared to conventional bulk modules, which typically achieve power densities in the mW/cm² range under ΔT >100 K, thin film TEGs generally operate in the nW - µW/cm² regime due to higher internal resistance and limited leg thickness. Flexible polymer-based devices offer mechanical compliance but often exhibit lower power factors, resulting in comparable or slightly lower output power. Nevertheless, thin film TEGs offer superior integration capability with microelectronic platforms and are more suitable for low-power IoT sensors, wearable electronics and microsystems operating under small temperature gradients (<20 K). Despite significant progress, the practical deployment of thin film TEGs remains limited by low absolute power output, substrate-related heat losses and interfacial resistance. Addressing these challenges is essential for translating material-level advances into device-level performance gains.

5 Future prospects and research

When analysing the research and future prospects of thin-film thermoelectric materials, it is critical to examine the most recent advances in material science, nanotechnology, and manufacturing techniques that attempt to overcome thermoelectric’s current limitations and broaden its scope of applications. Despite significant advances in thin film thermoelectric materials, translating high material-level performance into practical device output remains challenging. Future research should prioritize interface and contact engineering, substrate-induced strain and microstructure control will be essential in reduced-dimensional systems. At device level, minimizing thermal shunting and optimizing leg geometry are critical to improve effective temperature gradients and power density. While thin film TEGs generally produce lower absolute power output than bulk modules, their compatibility and microfabrication and flexible platforms make them promising for low-power IoT and wearable applications operating under small temperature differences.

Other than that, in general, the search for new materials with high thermoelectric efficiency is at the forefront to reduce cost and enhance performance. One of the primary research objectives is to increase zT values of thin film thermoelectric materials. Researchers aim to achieve zT values above 3, which will make thermoelectrics competitive with standard power generation technologies. Materials such as skutterudites, layered oxide and chalcogenides are emerging as viable alternatives in thermoelectric applications. Other than that, manipulation technique such as nanostructuring presented a breakthrough that shows great promise in maximizing thermoelectric efficiency by tuning electrical and thermal properties independently. Furthermore, it is also crucial to develop cost-effective and scalable production methods to make thermoelectric films commercially viable. Roll-to-roll printing and inkjet printing emerged as attractive approaches to large-area, low-cost thermoelectric device manufacturing. These advancements could result in significant improvement in efficiency, allowing thermoelectrics to play a larger role in power generation.

One of the popular thermoelectric potential applications is its integration into renewable energy systems such as solar photovoltaic (PV) systems, to capture waste heat and convert into electricity. The heat absorbed by the panel increases its temperature, causing irreversible degradation of electric power output (Shaker et al., 2024). Hence, researchers studied how TEG can improve the efficiency of the PV system by absorbing and utilizing the heat accumulated on the panel. However, integrating the conventional bulk TEG added inconvenient extra weight to the integrated system. The conventional TEG module has been manufactured with approximately 30 g per piece (Rashid et al., 2021). Recent study done by Nasser ABBAS et al. (2023) integrates 50 modules per panel for it to achieve satisfactory output, which accumulates around 1.5 kg additional weight, which is not ideal especially in building integrated applications. Therefore, our future work will revolve around solving this matter by employing improved TEG structure that will suit the application in solar PV system.

6 Conclusion

6.1 In summary

• This work emphasizes the classification of thermoelectric materials based on their optimal operating temperature ranges. The materials can be classified into low temperature (up to 300 K) such as Bi₂Te₃, Sb₂Te₃ and PEDOT:PSS, medium temperature (300–700 K) such as SnSe, CZTS and Mg₂Si, and high temperature (more than 700 K) such as SiGe, ZnO and La₂Te₃. This classification helps in selecting the appropriate material based on the temperature range of the application, ensuring optimal performance and efficiency.
• This work also highlights recent studies that have significantly improved thermoelectric performance using unique fabrication technique and novel design structures, opening up new opportunities for waste heat recovery, low power devices and sustainable energy applications.
• Despite these strides, challenges remain, particularly in improving the long-term stability, scalability, and cost-effectiveness of these devices. Addressing concerns such as material availability and environmental impact will be crucial in pushing thin film thermoelectric devices from laboratory proof-of-concepts to wider practical uses.
• Continued research and cross-disciplinary collaboration will be crucial to overcome these limitations and realize the full potential of thin film thermoelectric in contributing to a sustainable energy future.

Overall, the potential of thin film thermoelectric is limitless, providing tremendous prospects to improve energy harvesting and allowing sustainable, low-power applications across diverse industries.

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Footnotes

Edited by: Vanira Trifiletti, University of Milano Bicocca, Italy

Reviewed by: Anoop Gopinathan, Amrita Vishwa Vidyapeetham University, India

Gizem Durak Yüzüak, Munzur University, Türkiye