Research Progress on Thermoelectric Properties of Doped SnSe Thin Films

Abstract

With the continuous advancement of science and technology, SnSe thin films are widely used in various fields such as solar cells, energy harvesting, and flexible devices. The importance of SnSe thin films continues to be highlighted, from solar cells to flexible devices. With the continuous improvement of performance requirements for SnSe thin films in different fields, research on the properties of SnSe thin films has gradually become a hot topic. As an environmentally friendly and green material, SnSe thin films are more in line with modern semiconductor technology compared to crystalline materials, and they have unique advantages in the construction and application of thermoelectric micro/nano devices. This article first analyzes the characteristics of SnSe materials and then compares and analyzes PVD technologies and CVD technologies on doped SnSe thin films. In particular, it summarizes the research progress of CVD technologies on doped SnSe thin films, such as vacuum evaporation, magnetron sputtering, and pulse laser deposition, and it summarizes the research progress of PVD technologies on doped SnSe thin films, such as dual-temperature-zone CVD, the solution process method, and electrochemical deposition technology. It analyzes the performance of doped SnSe thin films prepared by different techniques. Finally, the preparation technology for the optimal thermoelectric properties of doped SnSe thin films and the approaches for potential research direction of future researchers were discussed, in the context of providing better performance SnSe thin films for the fields of solar cells, energy harvesting, and flexible devices.

1. Introduction

Against a contemporary backdrop of rapid technological development, issues such as energy scarcity and ecological pollution are becoming increasingly prominent. Under the trend of deeply integrating the concept of sustainable development into various fields of research, the development of clean energy has become a core issue of concern in academia. At present, renewable energy sources such as solar energy, wind energy, hydro energy, biomass energy, geothermal energy, and hydrogen energy have gradually been applied and transformed. Among them, thermoelectric materials with the potential to replace traditional fossil fuels have shown unique application advantages. As functional materials that can directly convert thermal energy into electrical energy, thermoelectric materials have attracted much attention due to their environmentally friendly properties. The temperature difference power generation technology supported by these materials can efficiently convert industrial waste heat into usable electricity, significantly improving the comprehensive energy utilization efficiency and playing a key role in multiple industrial fields. Against the backdrop of the ongoing global energy crisis, the development of new thermoelectric materials has important practical value. For example, the use of low-dimensional structure-controlled thermoelectric material thin films can not only optimize the thermoelectric properties of materials but also achieve good compatibility with existing semiconductor manufacturing processes, demonstrating broad application potential. In addition, the energy conversion efficiency of thermoelectric power generation devices based on the Seebeck effect, Peltier effect, and Thomson effect is positively correlated with the thermoelectric properties of the material. The thermoelectric efficiency is qualified by the dimensionless figure-of-merit ZT.

The definition of ZT is as follows:

$$\mathrm{Z}\mathrm{T}={\mathrm{S}}^2\mathsf{\sigma }{\displaystyle \frac{\mathrm{T}}{\mathsf{\kappa }}}$$

In the equation, S, σ, κ, and T represent the Seebeck coefficient, electrical conductivity, thermal conductivity, and operating temperature of the material, respectively. S²σ is the power factor (PF). By measuring the Seebeck coefficient, electrical conductivity, thermal conductivity, and absolute temperature of a material, the ZT of the material can be calculated. From the formula, we can also conclude that thermoelectric materials with excellent performance often have high PF and low thermal conductivity. We compared and analyzed the application of SnSe, Bi₂Te₃, PbTe, and Cu₂Se materials, as shown in

Table 1. Comparison of SnSe, Bi₂Te₃, PbTe, and Cu₂Se materials in application.

Index	SnSe	Bi2Te3	PbTe	Cu2Se
ZT	3.1 (783 K) [1]	1.4 (300–400 K) [2]	1.55 (723 K) [3]	1.1 (873 K) [4]
Material cost	the cheapest	the most expensive	expensive	cheaper
Toxicity	non-toxic	highly toxic	highly toxic	low toxicity
Technical maturity	high	medium	low	medium

.

In Table 1, we compare the ZT, toxicity, material cost, and technical maturity of SnSe, Bi₂Te₃, PbTe, and Cu₂Se materials. It is concluded that SnSe materials exhibit significant advantages in the medium- and high-temperature thermoelectric field. SnSe materials not only have high thermoelectric properties but also have advantages in cost and toxicity compared to other materials. More importantly, SnSe materials can be prepared on a large scale through the solution method, with high process maturity. In the future, researchers will further promote the application of SnSe materials by controlling doping elements and grain size.

Therefore, the SnSe material was chosen as the research object of this paper. In IV–VI group compounds have attracted much attention due to their unique physical properties. This type of material exhibits comprehensive advantages such as a moderate bandgap range (1.0–2.0 eV), strong visible light absorption ability, excellent chemical stability, and high abundance of constituent elements. Among them, SnSe, as a representative semiconductor material of this group, has an orthorhombic crystal structure that endows it with extremely low lattice thermal conductivity and an excellent Seebeck coefficient and conductivity combination, thus demonstrating significant potential in the field of thermoelectric energy conversion [5]. The research shows that in 2014, Zhao L et al. [6] achieved a record breaking ZT of 2.6 for SnSe single crystals prepared by the Bridgman method at 923 K, highlighting its efficient thermoelectric conversion capability. However, single-crystal materials have defects such as insufficient mechanical strength and high preparation costs, which limit their practical applications. Although polycrystalline materials theoretically have greater engineering potential, their thermoelectric properties are significantly weaker than those of single-crystal forms. This property gap has prompted the academic community to focus on the thermoelectric optimization research of polycrystalline SnSe. It is worth noting that the unique band structure of SnSe also endows it with excellent photoelectric conversion properties, making it one of the key directions in current thermoelectric material research due to its comprehensive advantages.

Meanwhile, compared with bulk materials, thin-film materials are more in line with modern semiconductor technology and have unique advantages in the construction and application of thermoelectric micro/nano devices. Therefore, in recent years, researchers have extensively explored the high energy harvesting efficiency, green properties, and low cost of SnSe. Researchers have enhanced the thermoelectric properties of SnSe thin films by doping various elements, which has become the simplest and most effective strategy to maximize the thermoelectric properties of SnSe thin-film materials. Through nearly a decade of development, compared to other cell options, thin-film cells have shown significant advantages over traditional cells due to their volumetric energy density and gravimetric energy density, as shown in Figure 1.

However, this advantage has not yet been fully realized in current research. Therefore, in this paper, the thermoelectric properties of doped SnSe thin films obtained by researchers in different preparation technologies are systematically and comprehensively reviewed, in order to comprehensively understand the current research status and progress of the thermoelectric properties of doped SnSe thin films and provide reference and direction for future research.

2. SnSe Thin-Film Material

SnSe, as a group IV–VI semiconductor compound, has outstanding physical and chemical properties. The material is not only environmentally friendly, non-toxic, and harmless but also shows good chemical stability. These characteristics make it show broad application prospects in the field of green energy technology and sustainable development, which is highly consistent with the research trend of modern materials science [8]. From the crystallographic point of view, single-crystal SnSe presents layered orthogonal symmetry and belongs to the Pnma space group. When the external pressure reaches about 12.6 GPa, the structure of the material will change from orthorhombic to monoclinic, accompanied by the evolution of the electronic state into the semimetallic state. This phase transition process is significantly related to the reconstruction of the electronic energy band and the lattice distortion [9]. Although single-crystal morphology is of great value in basic research, its harsh growth conditions and poor mechanical properties seriously restrict its practical application. In contrast, polycrystalline SnSe not only has a relatively simplified preparation process but also shows better mechanical properties. This balance of machinability and practicability creates favorable conditions for its engineering application.

At the same time, due to the trend of miniaturization, the latest progress of self-powered wearable devices and the Internet of things (IOT) needs to provide reliable and continuous power in a limited space. Thermoelectric thin films may meet these requirements [10], yet most studies on thin films show that their thermoelectric properties are low. However, new parameters, such as interfaces [11], the choice of substrates [12], the thickness of films [13], post-processing heat-treatment of films [14], etc., have paved the way for improving the thermoelectric properties of structured films. The development of these films on flexible substrates widens the use of these thermoelectric generators in devices like wearable clothes, smartwatches, and integrated sensor technology. The fabrication of SnSe thin films has also engrossed researchers’ interest due to their wide variety of applications.

Therefore, with the rapid development and wide application of solar cells, energy collection, and flexible devices in various fields, doped SnSe thin films have attracted much attention in recent years, and researchers have gradually realized the importance and urgency of improving the thermoelectric properties of SnSe thin films. Many scholars have conducted in-depth research in the field of thin-film doping. Although significant progress has been made in the research of different preparation methods and element doping, there are still many unsolved problems. If the thermoelectric properties are improved more efficiently, further research is needed.

3. Comparison of Performance Indexes Between PVD and CVD

SnSe thin films have attracted much attention because of their important application value in the fields of electronic devices, thermoelectric conversion, and other fields. Its thermoelectric properties are directly affected by the preparation technology, in which physical vapor deposition (PVD) and chemical vapor deposition (CVD) are two mainstream technologies. In this section, the effects of the two technologies on the thermoelectric properties of SnSe thin films are analyzed from the aspects of material properties, technology comparison, and application fields.

3.1. Electrical Performance Comparison

In terms of electrical properties, PVD uses physical methods to vaporize the SnSe materials and deposit them on the substrate. The PVD process is usually carried out in a high-vacuum environment, which has the characteristics of slow film growth rate and high crystallization quality. It is precisely because it results in a reduced influence of impurities and relatively controllable resistivity in the preparation process that it is suitable for high-precision devices [15]. However, the preparation of thin films is limited by the process pressure of physical preparation, and subsequent annealing optimization is required. CVD is the deposition of SnSe thin films on substrates by chemical reactions. CVD can achieve large-area uniform deposition and can be flexibly controlled by doping, which is suitable for applications requiring a wide range of carrier concentration [16]. However, there are many defects in the chemical reaction process, and the initial mobility may be lower than that of a film prepared by PVD and may need to be improved by subsequent treatment such as liquid phase ligand exchange.

In the application field of SnSe thin films, the electrical properties of SnSe thin films are widely used. In the aspect of phase-change memory, SnSe thin film prepared by PVD is more suitable for phase-change memory applications because of its high resistance ratio and stable conductivity. In terms of thermoelectric materials and flexible electronic devices, the thin films prepared by CVD have the characteristics of adjustable carrier concentration and large-area deposition, which make them more advantageous in the field of thermoelectric conversion and flexible substrates. We compared and analyzed electrical performance indexes between PVD and CVD, as shown in

Table 2. Comparison of electrical performance indexes between PVD and CVD.

Electrical Performance Index	PVD	CVD
Conductivity	High and stable, suitable for high-resistance ratio devices	Wide adjustable range, suitable for flexible electronics
Carrier mobility	High initial value, depending on crystallization quality	Can be optimized by post-processing
Process adaptability	Suitable for small-area and high-precision devices	Suitable for large-area and low-cost production

. PVD has advantages in crystal quality and initial mobility, while CVD is more prominent in conductivity tunability and process scalability. Therefore, the choice of technology needs to be combined with specific application requirements, such as high-precision equipment preferring PVD, while large-area or adjustable performance scenarios are more suitable for CVD. In the future, research can further explore the composite process of the two technologies to optimize comprehensive performance.

3.2. Thermal Performance Comparison

PVD transfers SnSe materials from the source target to the substrate by physical methods without involving chemical reactions [17]. Its advantages lie in the strong controllability of the process and the high purity of the film, which is suitable for large-scale preparation. However, the crystallinity and orientation of the film may be limited by the target quality and process parameters. CVD generates SnSe thin films on the substrate surface through the chemical reaction of precursor gas. Its advantage is that it can achieve atomic-level precise control and is suitable for the preparation of complex structure films. However, the process cost is high and impurities may be introduced, so its use is still limited.

In terms of thermal properties, thermal conductivity is the key index to measure the thermoelectric properties of SnSe thin films. SnSe thin films prepared by PVD are prone to introduce lattice defects and interface scattering, which reduce the phonon transmission efficiency. Therefore, SnSe thin films prepared by PVD usually have low thermal conductivity. In the future, researchers can further reduce the phonon thermal conductivity by adjusting the sputtering power or annealing process. The thermal conductivity of SnSe thin films prepared by CVD is relatively high, but the PF can be optimized by doping or layer structure design to indirectly improve the thermoelectric properties. In the future, researchers can control the stoichiometric ratio of thin films by using the ratio of precursors and growth temperature to reduce the scattering of impurities.

In the application field of SnSe thin films, the thermal properties of SnSe thin films are widely used. SnSe thin films prepared by PVD have the characteristics of geothermal conductivity, which is an important index to improve the ZT, making them suitable for applications in the field of thermoelectric converter devices. SnSe thin films prepared by CVD have the characteristics of high crystallinity and uniformity, and they can provide more stable thermal management ability, making them suitable for applications in the field of electronic heat dissipation materials. We compared and analyzed the thermal performance indexes between PVD and CVD, as shown in

Table 3. Comparison of thermal performance indexes between PVD and CVD.

Thermal Performance Index	PVD	CVD
Phonon scattering	The thermal conductivity of the prepared films is more favorable for reduction due to more defects	The prepared films rely on chemical control to achieve performance balance
Application adaptation	Thin films having low thermal conductivity are suitable for thermoelectric refrigeration devices	Highly crystalline films for high power density scenarios

. PVD is good at reducing thermal conductivity through defect control, while CVD is more suitable for high-precision structural design. Future research can combine the advantages of PVD nanostructures and CVD chemical control techniques to further optimize the thermoelectric properties of SnSe thin films.

3.3. Performance Comparison of the Same Doping Elements

We compared and analyzed Bi-doped SnSe thin film prepared by PVD and CVD, as shown in

Table 4. Comparison of Bi-doped SnSe thin film prepared by PVD and CVD.

Index	PVD (573 K) [18]	CVD (700 K) [19]
substrate	SrTiO3	Si
Bi content	5.7%	2%
the Seebeck coefficient	−385 μV/K	−659 μV/K
PF	0.3 μW·cm−1·K−2	0.6 µWcm−1·K−2
ZT	0.034	0.074

. In terms of electrical transport performance indicators, the Seebeck coefficient of the PVD material is −385 μV/K, while that of the CVD material is −659 μV/K. The latter has a larger absolute value, indicating that the carrier concentration or mobility characteristics of CVD materials are more favorable for thermoelectric conversion. The PF reflects the ability of materials to convert thermal energy into electrical energy. Using PVD, it is 0.3 μW·cm⁻¹·K⁻², and using CVD, it is increased to 0.6 µW·cm⁻¹·K⁻², reflecting the significant advantages of CVD technology in electrical transport performance, which may be related to its higher Seebeck coefficient and optimized carrier control. The comprehensive thermoelectric properties index ZT shows that the ZT of the CVD material is 0.074, which is about 2.2 times that of the PVD material, indicating that CVD technology performs better in thermoelectric properties and ultimately has higher thermoelectric conversion efficiency.

In summary, we know that Bi-doped SnSe thin films prepared by PVD can achieve higher Bi content, and therefore PVD is more suitable for preparing high extraction epitaxial films. CVD is based on the solution method, so it can be prepared in large quantities at low temperatures. Dual-temperature-zone CVD significantly improves the thermoelectric property indicators at high temperatures, highlighting the potential of CVD technology to enhance material thermoelectric properties in high-temperature environments.

4. Research Progress on Doped SnSe Thin Films

As is well known, thin-film manufacturing is very important for modern micro/nano devices, because thin-film materials can be easily combined into complex structures for various applications. Therefore, doped SnSe thin films have been widely studied and show great potential in the field of miniaturization of optoelectronic, photovoltaic, and thermoelectric devices. This is precisely because the physical properties of SnSe thin films are very sensitive to the growth conditions and the deposition technology used. Therefore, the deposition process is very important for the preparation of high-quality thin films [20]. In the past century, SnSe thin films have been grown by various physical and chemical methods. All these methods have advantages and disadvantages in the preparation of uniform and defect-free films. Their purpose is to change their electronic and thermal properties by adjusting the synthesis conditions and doping other elements. In the following section, we review the different physical and chemical methods used by different researchers to prepare doped SnSe thin films in the past, and we discuss the relationship between these deposition methods and the thermoelectric properties of doped SnSe thin films.

4.1. Research Progress of PVD on Doped SnSe Thin Films

PVD is a commonly used thin-film preparation technology. It uses physical processes to sublimate solid materials into a gas state and deposit them on the substrate surface in a vacuum to form the required thin-film structure. PVD includes many different methods, such as vacuum evaporation, magnetron sputtering, and pulsed laser deposition (PLD).

4.1.1. Vacuum Evaporation

Vacuum evaporation is a technology that uses an electron beam, cathode material, a high-frequency power supply and other equipment to heat and evaporate the cathode material in a high-vacuum environment and then deposit it on the surface of the substrate to form a thin film. The core process of vacuum evaporation is as follows: the raw material is gasified and converted into steam, and then the steam mixture is transferred to the substrate in a vacuum environment. When the steam contacts the substrate, surface adsorption occurs, and the thin-film structure is finally formed through the nucleation and crystallization process. At present, the main vacuum coating processes include thermal evaporation, electron beam evaporation, and plasma-enhanced evaporation.

As a typical example, the thermal evaporation method uses the principle of high-temperature vaporization to realize the thin-film deposition of bulk SnSe material on the substrate surface. This technology can adapt to a variety of substrate materials, including but not limited to glass substrate, mica substrate, and sapphire substrate. By accurately controlling the process parameters, the vacuum coating technology can achieve micron- to nanometer-scale coating. The thin film has good bonding strength with the substrate, compact structure, excellent mechanical properties, and environmental erosion resistance. The advantage of this method is that thin film prepared in a high-vacuum environment is pure because there is less substrate pollution. The heat energy is transferred to the source material, evaporated, and deposited on the substrate after condensation.

In 2020, Liu S et al. [21] prepared Co-doped SnSe thin films during the deposition of vacuum thermal evaporation on single-crystal Al₂O₃ substrates, to tune the nanosheet surface morphology. At 300 °C, the Co-doping amount is 2.60%, and the Co-doped SnSe thin film has the highest PF, which is 32.32 μWm⁻¹K⁻², as shown in Figure 2. This study presented a method to fabricate polycrystalline SnSe thin films with smooth surface morphology and also improved PF.

In 2021, Ibraheem S H [22] prepared doped SnSe thin films on glass substrates by flash evaporation technology. It was found that different Sb doping levels will lead to a change of morphology, and the conductivity increases with the increase of the percentage. When the Sb doping amount is 2.5% and the light intensity is 1500 lux, the conductivity value is the highest. This study shows the photoconductivity of thin film and provides a direction for the subsequent research of doped SnSe thin films.

In 2022, Kumar M et al. [23] designed a scheme to optimize the carrier concentration by doped Bi, so as to improve the thermoelectric properties of doped SnSe thin films. The researchers deposited SnSe/Bi composite films on Si/SiO₂ substrates by thermal evaporation technology. The PF of 20 wt% Bi and 30 wt% Bi showed a stable increase with the increase in temperature, and 25 wt% Bi showed a significant jump at a temperature close to 450 K, increasing exponentially with further increase in temperature. In addition, the PF of 20 wt% Bi and 30 wt% Bi remained almost unchanged, and the maximum values obtained were 180 μWm⁻¹K⁻² and 130 μWm⁻¹K⁻². The PF of 25 wt% Bi at 580 K was 800 μWm⁻¹K⁻². Finally, the maximum PF of 800 μWm⁻¹K⁻² was observed at 580K, which was equivalent to that of single-crystal and polycrystalline SnSe. In the same year, Zhao F et al. [24] conducted research in the field of SnSe solar cells. The researchers prepared Zn-doped SnSe thin films by co-evaporation on glass substrates and molybdenum-coated soda–lime glass substrates. The researchers explored the differences in crystallinity and grain size of the SnSe thin films by changing the doping amount of Zn. These differences were due to the influence of grain boundaries on thermal transport by phonon scattering. Zn-doped thin films enhanced thermal conductivity, and this benefited heat dissipation in solar cells under operating conditions. Ultimately, the SnSe thin films doped with 0.285% Zn exhibited better crystallinity and larger grain size. Direct Eg between 0.9 eV and 1.1 eV and indirect bandgap values between 0.86 eV and 0.98 eV were obtained for the SnSe thin films. At the same time, all the thin films exhibited p-type conductivity with a low resistivity of 1.32 Ωcm and a maximum electron mobility of 32.69 cm²V⁻¹s⁻¹. This enabled the manufactured material to exhibit a high power conversion efficiency of 0.55% when applied to SnSe solar cells.

In 2023, Bektas T et al. [25] studied the effect of annealing on Eg. The researchers deposited Sn-Sb-Se thin films from SnSbSe bulk crystals grown by the vertical Bridgman method using the thermal evaporation method, and they deposited them on sodium–calcium glass substrates cleaned by chemical process and ultrasonic vibration at room temperature. The bandgap and activation energy of the grown sample were found to be 1.59 eV and 106.1 meV, respectively. In addition, the direct bandgap of the grown sample was 1.59 eV, and the absorption coefficient was in the range of ~10⁴ cm⁻¹.

In 2024, Jabeen M et al. [26] prepared SnSe thin films having a thickness of 500 nm by thermal evaporation treatment of tin ingots and selenium powder as raw materials, followed by annealing at 250 °C in an inert Ar atmosphere. The transmittance of SnSe thin films in the visible light region is relatively low, absorbing almost all the photons in the visible spectrum. The absorption coefficient of the thin film increased with the increase in annealing temperature, and for the sample annealed at 250 °C, its absorption coefficient was about 10⁴ cm⁻¹. The Eg of SnSe thin films in the deposited and annealed states are 1.36 and 1.28 eV, respectively. In the same year, Sarkar P et al. [27] proposed a solution for producing inexpensive, reliable, and particularly efficient thin-film solar cells. The researchers compared the deposition of pure SnSe thin films and Te-doped SnSe thin films on glass substrates using the resistance thermal evaporation method. The direct Eg of SnSe_1−xTe_x thin film was displayed. Due to the decrease in grain size, the optical Eg of the SnSe_0.98Te_0.02 sample increased with further increase in the Te dopant concentration compared to undoped SnSe. This also indicated that the Eg caused by Te-doping varied between 1.75 and 1.89 eV. Therefore, the variation of Eg of SnSe_1−xTe_x thin film with Te dopant concentration was obtained, confirming the successful doping of Te into the SnSe matrix. Adding an appropriate amount of Te to SnSe_1−xTe_x thin film can improve its electrical and optical properties, providing a reference for the subsequent production of cheap and efficient thin-film solar cells.

4.1.2. Magnetron Sputtering

Sputtering methods can be divided into direct current (DC) sputtering, radio frequency (RF) sputtering, magnetron sputtering, reactive sputtering, intermediate frequency sputtering, and pulse sputtering according to their characteristics. A working principle diagram is shown in Figure 3. The combination of multiple sputtering methods can achieve various sputtering methods such as DC magnetron sputtering, RF magnetron sputtering, and reactive magnetron sputtering.

Magnetron sputtering [28] is a technique that uses high-energy ions to bombard the surface of a target material to generate metal ions, which are deposited on a substrate to prepare thin films. The principle of magnetron sputtering thin film preparation technology is mainly as follows: Ar gas is introduced into the vacuum chamber as the discharge medium, the deposited material is used as the cathode target, and the coating chamber wall is used as the anode. When a high-voltage electric field and orthogonal magnetic field are applied between the anode and cathode, a plasma glow discharge phenomenon is formed inside the cavity. During this process, the Ar molecules are ionized into Ar⁺ ions and free electrons, and these charged particles undergo directional motion under the combined action of electromagnetic fields. High-energy Ar⁺ ions bombard the surface of the target material under the acceleration of an electric field, triggering a cascade collision effect of target atoms through momentum transfer, ultimately causing neutral atoms to break free from lattice constraints and migrate to the substrate surface to deposit a thin film. Therefore, magnetron sputtering can accurately control the concentration of Se vacancies, ensure uniform deposition of SnSe thin films, and form dense thin films on the substrate.

Figure 3. Working principle diagram of magnetron sputtering [29].

Figure 3. Working principle diagram of magnetron sputtering [29].

Sputtering power, deposition time, and substrate temperature all affect the quality and doping efficiency of the thin film in magnetron sputtering. When the sputtering power is greater than 200 W, the high power increases the plasma density and promotes the Sn/Se atomic sputtering rate. But when the power exceeds 300 W, Ar gas or lattice distortion will be introduced, reducing the density of the film. Therefore, selecting an appropriate sputtering power can suppress defects and improve carrier mobility. The deposition time controls the film thickness and vacancy concentration. If the time is too short, it will lead to the formation of an amorphous structure, and if the time is too long, it will increase grain boundary scattering. The temperature of the substrate also affects the film. In low-temperature environments, amorphous films are formed on the substrate, while in high-temperature environments, columnar crystal growth is promoted on the substrate. In summary, in order to better apply magnetron sputtering, we need to optimize the parameters of the sputtering power, deposition time, and substrate temperature to improve the quality and doping efficiency of the doped SnSe thin films.

Compared to traditional thermoelectric thin-film preparation processes, magnetron sputtering exhibits significant advantages in the preparation of SnSe thin films:

First, the process parameters have flexible and adjustable wide-range characteristics;

Second, it can achieve precise control of sedimentation rate and film thickness;

Third, it has a wide range of material adaptability.

These technical features provide an ideal process platform for the construction of multi-dimensional structures (including low-dimensional structures and multilayer heterojunctions), as well as carrier concentration control and doping optimization of SnSe thin films. Practice has shown that this technology can prepare large-area SnSe thin film materials having excellent uniformity. However, the limitation of magnetron sputtering is that the sputtering power can easily be too high, leading to lattice distortion. At the same time, it needs to be carried out in a high-vacuum environment, which requires much higher maintenance costs compared to the solution method. Due to the low sedimentation rate, it is limited in industrial applications.

In 2019, Chen Z J et al. [30] conducted a systematic study on the microstructure of SnSe and the effect of MoSe₂-SnSe heterojunction on TE properties. The researchers used multi-step magnetron sputtering to deposit Mo-SnSe multilayer films on glass substrates. By annealing the sputtered Mo-SnSe thin films, the temperature dependence of the PF of all thin films increased. At 576 K, the maximum PF of SnSe thin films doped with 2.6 at.% Mo was 0.44 μW·cm⁻¹·K⁻², and the Seebeck coefficient was approximately 230 μV/K. Its PF was higher than that of SnSe thin films deposited under the same conditions.

In 2023, Li Y F et al. [31] explored the concentration of selenium vacancies. The researchers synthesized SnSe thin films on silicon and fused silica substrates by magnetron sputtering, and they controlled the concentration of Se vacancies simply by controlling the sputtering time. The researchers found that due to strong vacancy scattering, the thermal conductivity of the vacancy structures decreased by an average of 33.8% compared to the original SnSe thin films. This was due to the enhanced carrier mobility and preserved Seebeck coefficient by vacancy scattering. At the same time, the direct Eg was relatively large, and the Seebeck coefficient remained above 300 μV/K. At 700 K, the S-130 thin-film sample with a Sn:Se ratio of 1.08 ultimately achieved a high ZT of 0.6, which was 50% higher than the original SnSe thin films, and it also achieved a high PF of 2.01 μW·cm⁻¹·K⁻², as shown in Figure 4. This study indicates that the optimal concentration of selenium vacancies can not only achieve ideal band structures but also synergistically optimize thermoelectric properties.

4.1.3. PLD

PLD is a technique that utilizes high-energy-density laser pulses to evaporate and deposit target materials onto a substrate to form a thin film. As a typical representative of physical vapor deposition technology, it has important application value in the fields of semiconductor and optoelectronic thin-film preparation. PLD uses a high-energy laser to evaporate the target materials, and the parameters directly affect the plasma plume characteristics and doping dissociation degree. When the laser energy density is low, a low-density plume will be generated, resulting in the formation of porous thin films. When the laser energy density is too high, it can cause excessive sputtering, resulting in uneven dissociation of the elements that need to be doped. Therefore, it is necessary to optimize the energy density to reduce the defect density of the film; in order to obtain films with better quality and doping efficiency, it is necessary to optimize the parameters of the laser energy density to guide the industrial production of high-performance doped SnSe thin films.

As shown in Figure 5, the controllable pulsed laser deposition core process, which is particularly suitable for doping SnSe thin films, includes three characteristic stages:

Firstly, a high-energy pulsed laser focuses on the surface of a composite target material composed of SnSe and doped elements (such as Mn, Fe, Pb, etc.), inducing the instantaneous gasification of the target material to form high-density plasma.

Subsequently, the plasma plume undergoes isothermal adiabatic expansion and transport in a vacuum environment, during which the particles achieve kinetic energy balance through collisions.

Finally, the energy-modulated particle beam interacts with the substrate surface and forms a thin-film structure having predetermined doping characteristics through a heteroepitaxial growth mechanism.

The core advantage of PLD compared to conventional deposition processes is the ability to achieve heterogeneous integration of high-quality thin films at room temperature, which is of decisive significance for the preparation of SnSe thin films on thermally sensitive substrates [32]. The fluctuation of the PLD laser energy density leads to differences in plasma plume density, resulting in uneven film thickness. Meanwhile, there is a high cost of equipment and maintenance expenses.

Figure 5. Working principle diagram of pulsed laser deposition [33].

Figure 5. Working principle diagram of pulsed laser deposition [33].

In terms of doping control, PLD exhibits unique technological advantages: by adjusting the parameters such as laser energy density and pulse frequency, the dissociation degree and transport characteristics of the doping elements in the plasma plume can be accurately controlled. By combining the composition design of multi-component composite targets, in situ control of the doping concentration in thin films can be achieved, providing an ideal experimental platform for studying the effect of doping efficiency on the thermoelectric properties of SnSe thin films. In addition, by measuring the number of deposition pulses and real-time monitoring of film growth rate, nanometer-level precision film thickness control can be achieved, which enables PLD to meet the differentiated requirements of thin-film structures in different application scenarios [34].

In 2021, Horide T et al. [18] conducted research on improving the thermoelectric properties of doped SnSe by doping Bi. The researchers used PLD to prepare highly oriented SnSe thin films doped with Bi. In the experiment, the researchers replaced Sn²⁺ with Bi³⁺ to eliminate intrinsic holes at low Bi content and doped electrons at high Bi content. Intrinsic holes can enhance phonon scattering and compensate carrier concentration, thus improving the n-type Hall resistance and n-type Seebeck coefficient. Making the SnSe thin film doped with Bi with a target Bi content of 5.7% exhibits n-type Hall resistivity and an n-type Seebeck coefficient. At a temperature of approximately 300 °C, the Seebeck coefficient was −385 μV/K, as shown in Figure 6. The PF was 0.3 μW·cm⁻¹·K⁻², conservatively obtaining a ZT of 0.034 at 300 °C.

In 2023, Xue Y et al. [35] effectively optimized the carrier concentration of SnSe-based materials using post-selenization technology. In the study, the researchers prepared epitaxial SnSe thin films with a-axis orientation using PLD. They obtained the PF value within the high plane. Self-hole doped SnSe thin films were prepared by the post-selenization method, and the carrier concentration was increased. By analyzing the P-type self-hole doped SnSe thin films, it was found that the PF value reached 5.9 μW·cm⁻¹·K⁻² at 600 K in film samples selenized for 20 min, which was 40% higher that of the unselenized sample, concurrent with the results on numerous other studies, as shown in Figure 7. By increasing the selenization time, the carrier concentration increased from 2.65 × 10¹⁷ to 8.8 × 10¹⁸ cm⁻³, while producing a relatively high ZT (1.16 at 600 K). Through research, it has been found that the thermoelectric properties of SnSe thin films after post-selenization treatment have been significantly optimized, but related studies are still in the preliminary stage. Therefore, self-hole doped SnSe thin films with adjustable carrier concentration achieved through selenization have broad application prospects in the field of thermoelectric devices. In the same year, Horide T et al. [36] deposited SnSe thin films on single-crystal SrTiO₃ (STO) substrates using PLD. The researchers obtained the Seebeck coefficient values of SnSe_1.3 and Bi_0.06SnSe through testing, which were 340–480 and −250–−895 μV/K, respectively. As the Seebeck coefficient of SnSe decreases with increasing temperature, the open-circuit voltage of the module decreases with increasing temperature. Finally, the researchers found the conductivity of the Bi_0.06SnSe at 573K to be 0.44–1.5 S/cm, with a Seebeck coefficient ranging from −385 to −607 μV/K.

In 2024, Yamaguchi K et al. [37] prepared doped SnSe thin films on SrTiO₃ single-crystal substrates by PLD. The researchers found that the maximum Seebeck coefficients were 254 μV/K at x = 0.3 and 150 °C and 288 μV/K at x = 0.5 and 250 °C, which were lower than those of the SnSe thin films (550−620 μV/K), as shown in Figure 8. This is reasonable because a high carrier concentration results in a low Seebeck coefficient. Despite the trade-off between electrical conductivity and the Seebeck coefficient, the PF was improved with an increasing Te content. The thin film with x = 0.5 had the highest PF among the Sn (Se, Te) films (x = 0−0.5). The PF of the present Sn (Se, Te) films was 0.17 μW·cm⁻¹·K⁻² at room temperature and 1.1 μW·cm⁻¹·K⁻² at 300 °C.

In 2025, Liu Z et al. [38] used a fully light-controlled artificial synaptic device that utilized SnSe thin film successfully fabricated on the silicon substrate by PLD. Then the researchers used the photoinduced doping effect, which plays a critical role in negative persistent photoconductivity, and the learning-experience behaviors of human beings were successfully simulated by applying 430 and 255 nm pulses. The device displayed remarkable capability for building a three-layered artificial neural network. The recognition accuracy reached 95.33% for the dataset of the Modified National Institute of Standards and Technology (MNIST) after 20 training epochs and 84.78% for the face database of Yale after 40 training epochs. This work demonstrates the potential of 2D-layered materials for developing neuromorphic computing and simulating biological behaviors without additional treatment. Furthermore, the one-step method for preparation is highly adaptable and expected to realize large-area growth and integration of SnSe-based devices.

4.2. Research Progress of CVD on Doped SnSe Thin Films

CVD, as a mature thin-film preparation technology, is widely used in the preparation of SnSe thin films due to its precise and controllable composition, dense structure, and good adhesion to substrates. The basic process of CVD, illustrated in Figure 9, mainly consists of three steps: the vapor transport zone, the reaction zone, and the by-product removal area. In the process of preparing SnSe thin films by CVD, the precursor flow includes a Sn precursor and a Se precursor. It needs to be mixed with carrier gas (Ar/N₂) and enter the system through the vapor transport zone. CVD has high controllability and scalability and can prepare thin films with good uniformity over a large area. In addition, it has strong adjustability for the thickness, morphology, and crystal structure of the thin film.

At present, CVD has been applied to the purification of substances, the development of new crystals, and the preparation of various single-crystal, polycrystalline, and amorphous thin films. Compared to PVD, although PVD is mainly suitable for preparing precision devices that require high-temperature environments and high target-material losses, the solution process method is much cheaper than PVD and can be used for large-area preparation in low-temperature environments. Thus, CVD is currently the most widely used thin-film material preparation technology in integrated circuits, and it can be used to prepare most metal and alloy thin-film materials. However, CVD generally has the problems of impurity contamination and toxicity risks, and it is necessary to pay more attention to the residual solvents and the treatment of waste liquids and gases.

4.2.1. Dual-Temperature-Zone CVD

Dual-temperature-zone CVD is a thin film preparation technique that achieves material-oriented growth and component control by precisely controlling the temperature in different regions. The core of this method is to divide the reaction system into two independent temperature zones, a high-temperature source zone (HT Zone) and a low-temperature deposition zone (LT Zone), as illustrated in Figure 10, and drive the directional transport and controllable deposition of gas-phase precursors through temperature differences. Heating the precursor into gaseous reactants occurs in the HT Zone; gaseous substances are transported to the substrate surface in the LT Zone to prepare the required thin film materials. What is more, heating the precursor into gaseous reactants is a necessary step for dual-temperature-zone CVD. It not only heats the precursor into gaseous reactants but also promotes chemical reactions in the subsequent deposition process through the HT environment. Therefore, both PVD and CVD involve “gas-phase transport”, but PVD is the direct transport of physical phase transition, while CVD is the transport and driving of chemical reactions. This is the core distinguishing factor between the two.

Dual-temperature-zone CVD deposits doped SnSe thin films through chemical reactions, with key parameters affecting precursor decomposition and doping element binding. Among them, temperature gradient is the most important. When the temperature gradient is too large, it will accelerate the reaction, resulting in uneven composition and affecting the uniformity of the film. Therefore, optimizing the gradient will greatly improve the uniformity of the film, making it easier to achieve a high PF.

In 2021, Pang J et al. [19] investigated the thermoelectric applications of Bi-doped SnSe thin films and prepared Bi-doped n-type SnSe thin films using CVD. Compared with undoped SnSe thin films, the Seebeck coefficient of the Bi-doped SnSe thin films was greatly improved. The Seebeck coefficient of Sn_0.99Bi_0.01Se thin film at 600K was −905.8 μV/K, which was much higher than the 285.5 μV/K of the undoped SnSe thin films. Due to the significant increase in the Seebeck coefficient, the PF of the Sn_0.99Bi_0.01Se and Sn_0.98Bi_0.02Se thin films increased. When the bismuth doping concentration was 2%, the PF reached its maximum value of 0.6 µW·cm⁻¹·K⁻² at 700 K. It was attributed to the increase in PF that the maximum ZT of the Sn_0.98Bi_0.02Se at 700 K was 0.074 compared to the original SnSe. This study also demonstrates the potential of Bi-doped SnSe thin films in thermoelectric applications.

In 2025, Rani S et al. [41] used SnSe thin films with varying concentrations of Ni dopants synthesized using a single-step chemical vapor deposition method in Si/SiO₂ substrates. The researchers found that Ni doping in SnSe enhanced the hydrogen evolution reaction (HER) performance by reducing the overpotential value required to achieve a current density of 100 mA cm⁻² from 231 ± 24 to just 89 ± 35 mV versus the reversible hydrogen electrode, which was attributed to an increased number of active sites and lower semiconductor/electrolyte barrier height. Ni doping also induces a transition in p-type SnSe to n-type by substituting Sn sites and occupying Sn vacancies, which facilitates enhanced HER kinetics of about three times the current density. External electric fields and photoirradiation further modulate HER kinetics, highlighting the potential for tuning SnSe and similar 2D materials for electrocatalytic applications.

4.2.2. Solution Process Method

The solution process method in thin-film deposition technology belongs to the category of chemical solution preparation technology. Its core is to use precursor materials and solvents to form a uniform solution and then deposit it on the substrate through specific processes. This process achieves film deposition through a three-stage mechanism of dissolution coating conversion: first, a specific precursor is dissolved in an appropriate solvent to form a homogeneous solution; then the coating on the substrate surface is prepared by spin coating, spraying process, dip coating, or sol gel technology; and finally, the target thin-film material is formed through pyrolysis or chemical reaction.

Compared to traditional PVD, the solution process method exhibits significant technological and economic advantages, and it has wide applications in industrial fields such as flexible electronic devices, energy storage devices (such as lithium-ion cell electrodes), and optical functional thin films (such as anti-reflective coatings). Its process has strong compatibility and can be adapted to various substrate materials such as glass, polymers, and metals. It can also achieve precise control of thin-film thickness from nanometer to micrometer levels through solution concentration regulation.

Despite process defects such as insufficient film density and residual solvents, the solution method remains one of the preferred technological paths for laboratory research and industrial production due to its core advantages of simple equipment, controllable process, and low raw material costs. The current research hotspots focus on the development of nanocomposite precursors, optimization of rapid curing processes, and alternative applications of environmentally friendly solvents. These technological innovations will drive breakthrough progress in emerging fields such as flexible photovoltaic devices, microelectromechanical systems, and biomedical materials using solution process methods.

In 2016, Nelson M [42] conducted research on the application of photovoltaic cells, to prepare Ni-doped SnSe thin films. He deposited Sn_xSe_y-SnO₂:Ni films on a 375 °C glass substrate by spray pyrolysis and observed that the optical bandgap of the deposited SnSe films varied between 1.39 and 2.23 eV. The lowest transmittance in the visible to near-infrared region was about 44.3%. These properties are applicable to the window layer and absorption layer in photovoltaic cell applications, providing reference for the subsequent application of photovoltaic cells.

In 2019, Heo S H et al. [43] designed a solution process method to prepare highly textured and hole-doped SnSe thin films, further enhancing their anisotropy and improving their thermoelectric properties. At 550 K, the PF of the SnSe thin films was 4.27 μW·cm⁻¹K⁻². At 750 K, the Seebeck coefficient of the sample processed for 13 min could reach about 320 μV/K, as shown in Figure 11. The PF was about 3.2 μW·cm⁻¹K⁻², and the ZT reached its maximum of 0.58.

In 2022, Heo S H et al. [44] investigated the practicality of Ag-doped SnSe thin films at low temperatures and once again utilized solution technology to prepare high-performance Ag-doped SnSe thin films at low temperatures. The Seebeck coefficient of the Ag-doped SnSe thin films was in the range of 207−217 μV/K, which was lower than that of undoped samples (259.9 μV/K). The PF was 11.97 μW·cm⁻¹K⁻². The sample doped with 2% Ag exhibited the highest ZT of 0.46 at 300 K, as shown in Figure 12. At 550 K, the highest ZT peak was observed at 0.93. The research conclusion shows that the ZT of Ag-doped SnSe thin films is significantly improved, and it also proves the practicality of Ag-doped SnSe thin films after solution processing at low temperatures. This proves the feasibility of using thin films as energy harvesters in emerging electronic systems, providing a reference for future research directions.

In 2025, Ma, J et al. [45] added SnSe powders into isopropanol to form dispersion, and SnSe nanosheets were obtained through ultrasonic peeling and centrifuging. Then, composite films were prepared by vacuum filtration of a solution of mixed SnSe nanosheets and PEDOT:PSS with 5 vol% DMSO. With the increase in SnSe content, the Seebeck coefficient of the composite films was increased from 19.03 to 30.33 μV/K, achieving an improvement of nearly 60%. The addition of the SnSe nanosheets contributed to a partial increase in the PF of the composite films compared to the initial value, and the highest PF of 25.65 μW·cm⁻¹K⁻²was achieved at a SnSe nanosheet content of 60 wt% in the composite film, as shown in Figure 13. This study provides a new strategy to improve the performance of PEDOT:PSS films, and it also demonstrates that PEDOT:PSS/SnSe-based flexible composite films have tremendous potential for the development of wearable electronic devices.

4.2.3. Electrochemical Deposition

Electrochemical deposition is a wet thin-film preparation technique. As shown in Figure 14, the main working principle of electrochemical deposition is as follows. Under the action of an external electric field, a coating is formed by the migration of positive and negative ions in the electrolyte solution and the oxidation-reduction reaction of electron gain and loss on the electrode. In the process of electrochemical deposition, the thickness of the film layer can be accurately controlled by changing deposition conditions such as current, potential, electrolytic concentration, electrolyte pH value, temperature, deposition time [46], etc. Due to its advantages of low equipment cost, simple operation, and high production efficiency, the electrochemical deposition method is widely used in industrial production for the preparation of thin films.

In 2020, Jalalian-Larki B et al. [48] used the electrochemical deposition method to prepare indium-doped SnSe thin films in order to further enhance their application potential in solar cells. The researchers found that using the electrochemical deposition method for indium doping can significantly affect the physical properties of SnSe nanoparticle thin films. The SnSe nanostructured thin films were grown by electrochemical deposition, and the optical and photovoltaic properties were altered by adding different amounts of indium dopants. The conductivity of the doped samples was improved in electrical properties, and compared with the undoped samples, the solar cell efficiency of the sample having the highest doping concentration was 0.36%. Therefore, by adjusting the doping concentration of indium through electrochemical deposition, the thermoelectric properties of doped SnSe can be optimized, thereby improving the application potential of the material and providing reference for the subsequent application of doped SnSe thin films in the field of solar cells.

In 2022, Ikhioya I L et al. [49] doped Mo into SnSe thin films through the electrochemical deposition method, which improved the conductivity of Mo-doped SnSe thin films and further verified the effectiveness of element doping in balancing optoelectronic properties. Meanwhile, the researchers found that Mo-doped SnSe thin films are suitable for manufacturing buffer layers in solar cells and photovoltaic devices. As the dopant concentration increased from 0.1 to 0.3 mol%, the range of deposited materials was found to be 1.50 to 2.22 eV. Due to the influence of molybdenum dopants on the SnSe, the doped Eg decreased from 1.55 to 2.22 eV, and the SnSe acted as a substitute impurity at its lattice position. Also, due to the generation of nanoparticles, the Eg exhibited by these doped materials narrowed, making it a promising material for photovoltaic applications, especially in solar cell manufacturing.

In 2022, Udofia K I et al. [50] successfully synthesized Zr-doped SnSe nanocrystalline thin films on FTO glass substrates using the electrochemical deposition method. The researchers found that the optical Eg range of the Zr-doped SnSe thin films at a Zr doping concentration of 0.3 mol% was 1.20 eV to 1.81 eV, with a maximum Eg of 1.81 eV. At the same time, it was found that the absorbance value increased with the increase in Zr doping concentration, and the transmittance edge moved towards the lowest wavelength; the reflectivity value increased with the increase in Zr dopant concentration.

In 2024, Ben Hjal A et al. [51] successfully synthesized SnSe thin films on glass substrates using the electrochemical deposition method. When the deposition potentials on ITO and FTO substrates were −1.1 and −1 V, the element distribution was uniform. The analysis showed that the Eg of the ITO ranged from 1.25 to 2.24 eV, while the Eg of the FTO ranged from 1.46 to 2.87 eV, covering a wide spectral range. This demonstrates significant advantages for photovoltaic applications.

5. Comparative Analysis of the Electronic Structure and Preparation Techniques of Doped SnSe Thin Films on Their Thermoelectric Properties

5.1. Electronic Structure Modulation by Doping and Its Impact on Thermoelectric Properties

The thermoelectric properties of SnSe thin films are fundamentally governed by their electronic structure and the resulting charge carrier transport. Doping introduces foreign atoms into the SnSe lattice, which can profoundly alter the electronic band structure, Fermi level position, carrier concentration, and carrier type, and can introduce defect states. These modifications directly dictate the Seebeck coefficient, electrical conductivity (σ), power factor (PF), and ultimately the figure-of-merit ZT. Understanding how these mechanisms affect the thermoelectric properties of SnSe thin films is crucial for future researchers to design materials having high thermoelectric properties.

5.1.1. Carrier Concentration Mechanism and Its Impact on Properties

In doped SnSe thin films, the carrier concentration is changed by doping with elements having different valence states, which introduces excess electrons (n-type) or holes (p-type) and shifts the Fermi level to the minimum or maximum value of the conduction band. Most importantly, the Seebeck coefficient typically decreases as carrier concentration increases because the Fermi level moves deeper into the bands, reducing the average energy per carrier relative to the Fermi level. However, optimizing doping to achieve an optimal carrier concentration that balances the Seebeck coefficient and the electrical conductivity is key to maximizing PF.

For example, Bi³⁺ substituting Sn²⁺ acts as an electron donor. At low Bi content, it compensates for intrinsic holes, reducing hole concentration. At higher concentrations, it introduces excess electrons, increasing electron concentration [18]. This carrier concentration tuning is evident in the significant increase in the Seebeck coefficient magnitude (from 285.5 μV/K in undoped to −905.8 μV/K in Sn_0.99Bi_0.01Se at 600K) and the modulation of conductivity. The peak PF (0.6 µW·cm⁻¹·K⁻² at 700 K) for Sn_0.98Bi_0.02Se and ZT (0.074) are achieved at an optimal Bi concentration where the trade-off between the Seebeck coefficient and electrical conductivity is balanced [19].

5.1.2. Band Structure Engineering Mechanism and Its Impact on Properties

In doped SnSe thin films, doping can modify the bandgap (Eg) by altering the orbital interactions or introducing strain. More importantly, bandgap narrowing can promote carrier excitation, potentially increasing electrical conductivity. When the Seebeck coefficient significantly increases, it is beneficial for the improvement of PF.

For example, Te doping in SnSe (forming SnSe_1−xTe_x) is reported to tune the optical bandgap [27]. The observed increase in Eg suggests modification of the band edges. While the primary application focus for Te-doped films has been solar cells [24], the bandgap tuning mechanism is crucial. A wider bandgap can reduce bipolar conduction at high temperatures, potentially benefiting the Seebeck coefficient in thermoelectrics by suppressing minority carriers. Optimizing Te concentration for thermoelectric applications warrants further investigation focusing on band structure effects.

5.1.3. Defect State Mechanism and Its Impact on Properties

In doped SnSe thin films, doping atoms, especially if their size or electronegativity differs significantly from the host atoms, can create point defects. These act as scattering centers for charge carriers and phonons. More importantly, ionized impurity scattering from charged dopants can significantly reduce carrier mobility. Doping can also introduce specific defect states within the bandgap.

For example, introducing Se vacancies into doped SnSe thin films will result in defect states. They can effectively scatter phonons and optimize vacancy concentration, as seen in the enhanced PF (2.01 μW·cm⁻¹·K⁻²) and ZT (0.6) achieved in S-130 thin-film samples [31]. Magnetron sputtering of Mo-doped SnSe can lead to the formation of MoSe₂-SnSe heterojunction [30]. The potential barrier at interfaces can selectively scatter low-energy carriers while allowing high-energy carriers to pass through, potentially increasing the average energy per carrier and enhancing the Seebeck coefficient. The formation of such heterostructures is a distinct electronic structure modulation achieved through specific doping and processing routes.

This analysis highlights the multifaceted role of doping in modulating the electronic structure of SnSe thin films. While achieving high ZT, the mutual influence between the Seebeck coefficient, the electrical conductivity, and the thermal conductivity can be comprehensively analyzed, as shown in

Table 5. Comparative analysis of doping modulation on electronic structure and its impact on thermoelectric properties.

Doping Strategy	Doping Element	Primary Electronic Structure Modulation	Key Impact on Thermoelectric Properties
Carrier tuning [18]	Bi	Control of carrier concentration; Fermi level shift	The Seebeck coefficient indicates significant increase
Bandgap tuning [24,27]	Te	Optical bandgap increase	Primary application in solar cells; further research is needed to optimize the thermoelectric properties
Defect/ Vacancies [30,31]	Mo/Controlled Se Deficiency	Formation of MoSe2-SnSe heterojunctions; Se vacancies	Thermal conductivity and the Seebeck coefficient have been increased; electrical conductivity has been decreased; The vacancy has been optimized, indicating significant increases in PF and ZT

.

In Bi doping, we found that the carrier concentration was optimized after doping, mainly by significantly enhancing the n-type characteristics through controlling the Fermi level, which ultimately affects the thermoelectric properties of doped SnSe thin films.

In Te doping, we found that the doped SnSe thin films changed the bandgap, which is crucial for optoelectronic applications. This makes Te-doped SnSe thin films more commonly used in the field of solar cells, and future research on suppressing bipolar conduction in thermoelectric materials can be further strengthened.

In Mo doping and selenium vacancies, doped SnSe thin films generate heterojunctions (Mo doping) and point defects (Se vacancies). These defects and changes in interface engineering result in an increase in thermal conductivity and the Seebeck coefficient of doped SnSe thin films, a decrease in electrical conductivity, optimization of vacancies, and ultimately a significant increase in PF and ZT.

Finally, we summarized the influence of electronic structure changes on the thermoelectric properties of SnSe thin films having different doping levels. We hope to make it easier for readers to understand the impact of these electronic structure changes on thermoelectric properties and provide reference for future researchers to design high-performance doped SnSe thermoelectric thin films. Future research can complement several parameters compared in the article to obtain doped SnSe thin films with higher thermoelectric properties.

5.2. Comparison of Doped SnSe Thin Films Under Different Deposition Conditions and Process Levels

5.2.1. The Influence of Substrate Type and Sedimentation Temperature Parameters

In single-crystal substrates (SrTiO₃, MgO, Al₂O₃), PLD and vacuum evaporation techniques can be used to achieve a-axis epitaxial growth of the SnSe, reduce grain boundary scattering, and improve carrier mobility. In amorphous substrates (glass, SiO₂), highly textured thin films are formed on the surface of the glass substrate using solution and magnetron sputtering methods, resulting in increased phonon scattering and decreased thermal conductivity.

In magnetron sputtering, the temperature range is often between 300 and 400 °C. At low temperatures, the microstructure is amorphous, while at high temperatures, the microstructure is columnar. Annealing can increase the ZT. In dual-temperature-zone CVD, the high temperature is often at 600 °C and the low temperature is at 400 °C. This allows for directional diffusion of precursors driven by temperature gradients, improving the uniformity of the components and enhancing the thermoelectric properties. In the solution process method, the temperature range is usually between 250 and 350 °C. At low temperatures, the microstructure is composed of nanoparticles, and at high temperatures, sintering densification occurs.

Based on the above analysis, in substrate selection, single-crystal substrates (SrTiO₃) can meet the requirements of epitaxial growth, while amorphous substrates are suitable for low-cost mass production. At the same time, future researchers can synergistically design substrates and temperatures. By selecting different substrates based on demand and utilizing AI-assisted temperature regulation, we can promote the development of thermoelectric properties.

5.2.2. Comparative Analysis of Doping Efficiency of SnSe Thin Films

By comparing the preparation methods summarized in this article, we found that CVD undergoes gas-phase chemical reactions in the reaction zone, and doping elements directly participate in film growth in atomic or molecular form, entering the lattice position almost unobstructed, thus achieving ultra-high doping efficiency close to the theoretical properties value. This gives CVD unparalleled advantages in research that requires precise control of carrier concentration. The efficiency bottleneck of the solution process lies in its chemical environment. The ligands on the surface of nanocrystals stabilize the nanocrystals, but they also become a physical barrier for doping. The key to improving the efficiency for future researchers lies in ligand engineering, which can try to select short-chain, easily removable ligands or develop new ligand exchange technologies to reduce steric hindrance. The doping efficiency and uniformity of magnetron sputtering depend on the target material. In order to overcome uniformity issues, co-sputtering and optimized target-material preparation processes are commonly used in industry and scientific research, as shown in

Table 6. Comparative analysis of doping efficiency of doped SnSe thin films in different preparation methods.

Preparation Method	Doping Efficiency	Limiting Factor	Physical Properties
CVD	>90%	Decomposition temperature of precursor	In the reaction zone, dopant atoms can directly reach the substrate surface, but the thermal decomposition of the precursor limits the doped concentration
Solution process	60–80%	Ligand exchange rate	The steric hindrance of ligand space will hinder the entry of doped ions into the lattice
Magnetron sputtering	70–85%	Uniformity of target doping	The segregation of doping elements in the target material can lead to uneven composition of the film layer

.

5.2.3. Comparative Analysis of Thermoelectric Properties of Doped SnSe Thin Films with Different Deposition Strips on the Same Substrate and Preparation Method

Using the same SiO₂ substrate and magnetron sputtering method, we compared different deposition conditions and process levels, as shown in

Table 7. Comparative analysis of doped SnSe thin films prepared by magnetron sputtering.

Preparation Method	Magnetron Sputtering [30]	Magnetron Sputtering [31]
substrate	SiO2 (576 K)	Si/SiO2 (700 K)
the Seebeck coefficient	230 μV/K	30 0 μV/K
PF	0.44 μW·cm−1·K−2	2.01 μW·cm−1·K−2
ZT	/	0.6

, and found that there were significant differences in the thermoelectric properties of doped SnSe thin films. The Seebeck coefficient increased from 230 to 300 μV/K, and the improvement in PF was significant. In the future, the exploration of magnetron sputtering for doped SnSe thin films can focus on the selection of process temperature, so as to achieve the best thermoelectric properties of doped SnSe thin films in the optimal temperature range.

We compared the thermoelectric properties of doped SnSe thin films under different deposition conditions and process levels using the same SiO₂ substrate and solution process method, as shown in

Table 8. Comparative analysis of doped SnSe thin films prepared by the solution process.

Preparation Method	Solution Process [43]	Solution Process [44]	Solution Process [45]
substrate	SiO2 (750 K)	SiO2 (300 K)	SiO2 (300 K)
the Seebeck coefficient	320 μV/K	207–217 μV/K	30.33 μV/K
PF	3.2 μW·cm−1·K−2	11.97 μWcm−1·K−2	25.65 μW·cm−1·K−2
ZT	0.58	0.46	/

. Through comparison during the process, we can see that under the same substrate conditions, the preparation temperature has shifted from high temperature (750 K) to room temperature (300 K), which is of great significance for practical applications and industrial mass production. It is worth noting that the Seebeck coefficient under high-temperature conditions decreased from 320 to 30.33 μV/K; the PF value significantly increased. This was precisely because the electrical conductivity (σ) increased by an order of magnitude, compensating for the decrease in Seebeck coefficient and overall improving the PF value. In the future, research on the solution process for doping SnSe thin films can pay more attention to changes in carrier concentration and electrical conductivity and can achieve higher thermoelectric properties, thereby improving the application of the solution process method in large-scale production and industry.

In the above three sections, we have attempted to summarize a thought process, hoping to facilitate reference for future researchers. Firstly, our starting point is high doping efficiency. This is because only when the dopant atoms efficiently and precisely enter the SnSe thin film lattice can subsequent accurate control become possible. Next, the thermoelectric properties of doped SnSe thin films are optimized using two different optimization strategies, as shown in Figure 15.

Path 1: Optimize electrical conductivity. By attending to accurate control of carrier concentration, the number of carriers involved in conductivity can be maximized, thereby directly optimizing the electrical conductivity.

Path 2: Maintain high Seebeck coefficient. By reducing lattice distortion, it is possible to avoid excessive scattering of phonons and damage to the band structure caused by lattice defects, thereby maintaining a high Seebeck coefficient.

Ultimately, the goal of maximizing PF value is achieved through the combining the two paths. This approach emphasizes the synergistic optimization of the thermoelectric properties of doped SnSe thin films in the design of deposition conditions and process levels.

5.3. The Influence of Doped SnSe Thin Films Under Various Preparation Methods

We compared and analyzed the thermoelectric parameters of doped SnSe thin films under different preparation methods, as shown in

Table 9. Thermoelectric parameters of doped SnSe thin films under various preparation methods.

Preparation Method	Substrate	Temperature	Seebeck Coefficient	PF	ZT	Year
Solution process	SiO2	750 K	320 μV/K	3.2 μW·cm−1·K−2	0.58	2019 [43]
Magnetron sputtering	SiO2	576 K	230 μV/K	0.44 μW·cm−1·K−2	/	2019 [30]
Dual- temperature-zone CVD	Si	700 K	−650 μV/K	0.6 μW·cm−1·K−2	0.074	2021 [19]
PLD	SrTiO3	573 K	−385 μV/K	0.3 μW·cm−1·K−2	0.034	2021 [18]
Solution process	SiO2	300 K	207–217 μV/K	11.97 μW·cm−1·K−2	0.46	2022 [44]
Vacuum evaporation	Si/SiO2	580 K	−440 μV/K	8.0 μW·cm−1·K−2	/	2022 [23]
PLD	MgO	600 K	332 μV/K	5.9 μW·cm−1·K−2	1.16	2023 [35]
Magnetron sputtering	Si/SiO2	700 K	300 μV/K	2.01 μW·cm−1·K−2	0.6	2023 [31]
PLD	SrTiO3	573 K	−385–−607 μV/K	/	/	2023 [36]
PLD	SrTiO3	573 K	250 μV/K	1.1 μW·cm−1·K−2	/	2024 [37]
Solution process	SiO2	300 K	30.33 μV/K	25.65 μW·cm−1·K−2		2025 [45]

.

According to Table 9, different preparation methods, substrates, and temperatures have significant effects on the thermoelectric properties of doped SnSe thin films. (1) In terms of thermoelectric properties, the influence of thermoelectric properties is mainly reflected in the microstructure, compositional uniformity, and electrical properties of the thin film. Vacuum evaporation and magnetron sputtering are suitable for preparing high-quality and high-performance thin films, while PLD has advantages in the preparation of complex-component thin films, and the solution process and electrochemical deposition methods have significant cost benefits in low-cost large-scale production and deposition. (2) In terms of process stability, breakthrough improvements in thermoelectric properties can also be achieved through process parameter optimization. The vacuum evaporation and solution process methods can maintain high repeatability in thin-film systems having single components, but the consistency of preparation in complex-component materials is easily affected. Although magnetron sputtering and electrochemical deposition require complex process control, they have demonstrated excellent batch reproducibility in industrial production. Although PLD can achieve precise construction of complex oxide films, its process stability is significantly constrained by fluctuations in equipment parameters.

Through comparative analysis, researchers choose appropriate methods and weigh specific application requirements and material properties before preparing thin films. Future research can further explore the optimization and combination of these methods to achieve the widespread application of SnSe thin films in the fields of optoelectronics, thermoelectrics, and energy. Meanwhile, in recent years, doped SnSe thin films prepared by magnetron sputtering have been widely used, and the prepared films have great potential in high power-output scenarios with high ZT, making them the most promising technology for preparing doped SnSe thin films.

5.4. The Influence of Different Dopants on Doped SnSe Thin Films

We compared and analyzed the influences of different dopants on the thermoelectric parameters of doped SnSe thin films, as shown in

Table 10. The influences of different dopants on the thermoelectric parameters of doped SnSe thin films.

Preparation Method (Dopant)	Concentration	Seebeck Coefficient	PF	ZT	Effect	Year
Magnetron sputtering (Mo)	2.6%	230 μV/K	0.44 μW·cm−1·K−2	/	Formation of MoSe2-SnSe heterojunction	2019 [30]
Dual- temperature-zone CVD (Bi)	2%	−650 μV/K	0.6 µW·cm−1K−2	0.074	CVD achieves high ZT	2021 [19]
PLD (Bi)	5.7%	−385 μV/K	0.3 μW·cm−1·K−2	0.034	Researchers replaced Sn2+ with Bi3+ to eliminate intrinsic holes at low Bi content and doped electrons at high Bi content	2021 [18]
Solution process (Ag)	2%	207–217 μV/K	11.97 μW·cm−1·K−2	0.46	This proves the feasibility of using thin films as energy harvesters in emerging electronic systems	2022 [44]
Vacuum evaporation (Bi)	25%	−440 μV/K	8.0 μW·cm−1·K−2	/	Equivalent to that of single-crystal and polycrystalline SnSe	2022 [23]
PLD (Bi)	6%	−385–−607 μV/K	/	/	The open circuit voltage of the module decreases with increasing temperature	2023 [36]
Vacuum evaporation (Te)	2%	/	/	/	Eg increases to 1.75–1.89 eV, and the electrical properties improve; suitable for solar cells	2024 [27]

.

Through the comparison in Table 10, it is found that Te-doped SnSe thin films mainly improve optical and electrical properties, making them more suitable for the field of solar cells. The ZT of Bi-doped SnSe thin films is highest under dual-temperature-zone CVD, highlighting the advantages of doped SnSe thin films in thermoelectric properties. Ag doping can be better applied in the field of flexible devices. In summary, future researchers can achieve the characteristics of increased conductivity and decreased thermal conductivity brought by a co-doping strategy through multi-element combination, further increasing the ZT.

6. Conclusions

This article aimed to review the progress in the thermoelectric properties of doped SnSe thin films prepared by researchers using different preparation methods in recent years. The article was divided into three parts. The first part established that the current energy crisis has become an important issue in today’s society, and it highlighted the importance of the development of thermoelectric materials. Therefore, it introduced the fact that the structural advantages and potential of SnSe materials have become a research focus, laying the foundation for the subsequent introduction of preparation methods and properties. The second part explained that doping is currently the simplest and most effective way to improve the thermoelectric properties of SnSe thin films, laying the foundation for a comparative analysis of doping performance under different preparation methods in subsequent sections. The third part introduced the workflows of vacuum evaporation, magnetron sputtering, PLD, dual-temperature-zone CVD, the solution process method, and the electrochemical deposition method. Next, we reviewed the progress made by researchers in recent years in preparing the thermoelectric properties of doped SnSe thin films using different preparation methods. Additionally, the thermoelectric properties of doped SnSe thin films materials under different preparation methods were compared, providing suggestions and directions for future researchers to prepare doped SnSe materials.

Although significant progress has been made in the production of doped SnSe thin films, there are still some challenges and issues. Future research will focus more on innovative methods for improving thermoelectric properties and the application of doped SnSe thin films in solar cells, in order to promote the further development of thin-film applications in the field of solar cells. However, at the current level of development, there is still a significant gap between the thermoelectric properties of SnSe thin films and bulk materials. How to further improve the thermoelectric properties of SnSe thin films has become an urgent problem for researchers in thin-film thermoelectric materials. Figure 16 shows the specific issues and future research directions.

The application of computational materials science to overcome the research bottleneck of SnSe thin films can be assisted by density functional theory and neural networks in first-principles computational methods for doping design. The first-principles calculation method refers to a method that does not rely on empirical parameters and is based on the fundamental principles of quantum mechanics to numerically solve physical properties. The first-principles calculation method represented by density functional theory can numerically simulate the electronic structure of material systems, thereby achieving high-throughput and high-precision prediction of material properties, and is an indispensable research tool in material design.

Despite the great success achieved so far, the development of DFT methods still faces challenges: limited by computational complexity, the application of DFT in large-scale or high-throughput material calculations is relatively restricted, which limits its wider application [52]. How to improve the efficiency of DFT while maintaining accuracy is a major challenge in the field of computational physics. The development of traditional algorithms often faces the dilemma of balancing accuracy and efficiency, where more efficient algorithms often come at the cost of sacrificing some accuracy [53]. The development of AI technology represented by neural networks as first-principles computing methods has introduced new opportunities. The purpose of first-principles calculations is to construct a mapping relationship between materials and physical properties, that is, to predict the physical properties of a given material structure. Thanks to the massive number of parameters and powerful representation capabilities of neural networks, they are expected to learn this relationship with high accuracy, thus skipping the time-consuming first-principles calculation numerical simulation process and achieving efficient prediction of physical properties [54].

At present, with the explosive development of neural network technology, the field of deep learning force fields is flourishing, and deep learning force field methods developed by researchers, such as DeepMD, NequIP, and Equiformer, have emerged [55,56]. By predicting the formation energy of materials, deep learning force fields can also assist in determining material stability, thereby accelerating material search and discovery. Representative works such as DeepMind’s GNoME framework, which combines material search and deep learning force fields, have discovered 2.2 million new stable or metastable crystal structures, greatly expanding the material database [57].

First, through performance optimization, we can balance the contradiction between conductivity and the Seebeck coefficient through multi-element co-doping or multi-scale structural design (heterojunction, superlattice), while suppressing thermal conductivity. New dopants (such as rare earth elements) can be explored to broaden the dimension of carrier control. We can use machine learning to enable AI to establish databases and assist us in screening doping schemes of different concentrations and predicting higher performance co-doping combinations.

Second, through process innovation, we can improve the stability of pulsed laser deposition to solve batch fluctuation problems; and we can attempt to combine artificial intelligence and screen the optimal doping scheme through machine learning.

Thirdly, through application expansion, we can deepen the integrated research of SnSe thin films in flexible thermoelectric devices and photovoltaic thermoelectric coupling systems and explore the impact of interface engineering on device-level thermoelectric conversion efficiency. Through machine learning, AI can assist us in designing gradient interfaces, reducing phonon transmission while maintaining electron tunneling, and improving interface structure design.

Fourth, we can make progress in interdisciplinary research. The study of doped SnSe thin films is a multidisciplinary field that involves multiple disciplines such as materials science, surface engineering, and laser technology. Future research needs to strengthen interdisciplinary collaboration to achieve innovation and breakthroughs in thin-film preparation technology.

In summary, as a highly promising thermoelectric material, the performance breakthrough of doped SnSe thin films relies on the synergistic innovation of material processing to ultimately promote the large-scale application of doped SnSe thin films in clean energy technology.