Effect of Sputtering Power and Post-Deposition Annealing on Thermoelectric Performance of Ag₂Se Flexible Thin Films

Abstract

Ag₂Se has attracted significant attention as a promising alternative to Bi₂Te₃ for near-room-temperature thermoelectric (TE) applications. In this study, flexible Ag₂Se thin films were fabricated via magnetron sputtering under different sputtering power settings, followed by post-deposition annealing to optimize their TE properties. Structural and compositional analyses confirmed the successful synthesis of Ag₂Se films with high crystallinity. Additionally, tuning the sputtering power and annealing temperatures can effectively enhance the electrical conductivity, Seebeck coefficient, and overall power factor. A significant power factor of ~17.4 µW·cm⁻¹·K⁻² at 100 °C was achieved in the 30 W sputtering power and 300 °C annealing sample, pointing out the huge potential of Ag₂Se thin films as self-powered flexible devices.

1. Introduction

Thermoelectric (TE) materials, as a series of functional materials, can convert temperature differences into electrical potentials, or vice versa [1,2,3]. Thus, devices fabricated from TE materials are widely investigated by researchers to solve the issues like heat recovery, room-temperature power generation, or self-charging power sources [4,5,6]. Typically, the efficiency of the TE devices is determined by the intrinsic properties of the TE materials, which are evaluated by the dimensionless figure of merit, zT (zT = S²σT∕κ, where S, σ, T, and κ are the Seebeck coefficient, electrical conductivity, absolute temperature, and total thermal conductivity, respectively) [7,8]. To fabricate TE devices for the applications of room-temperature power generation or self-charging power sources, forming TE materials into thin films [9] and employing flexible substrates, like polyimide (PI) [10], etc., are two effective strategies for researchers to enhance flexibility, though sometimes leading to a slight reduction in thermoelectric performance [9]. However, considering that the thermal conductivity (κ) values of thin films are difficult to measure accurately, the power factor (PF = S²σ) is commonly used to assess their TE performance potential [11]. The literature reports that intrinsic Ag₂Se thin films typically exhibit κ values around 1.20 to 1.35 W·m⁻¹·K⁻¹, located in the range of approximately 1.9 to 2.2 of Ag/Se ratio [12]. In addition, S and σ are coupled with the carrier concentration (n), which shows that the increase in n leads to the increase in σ and the decrease in S [13,14]. Thus, the simultaneous increase in S and σ is also a challenge for researchers to enhance the TE performance of flexible thin films.

Over the past several decades, near-room-temperature TE materials, such as Bi₂Te₃ and Sb₂Te₃, have been extensively studied. In recent years, considerable efforts have been devoted to exploring new material systems with high TE performance near room temperature, with particular attention given to silver and silver–copper chalcogenides (Ag₂Q and AgCuQ, where Q = S, Se, and Te) [15,16]. Specifically, Ag₂Se thin films exhibit comparable n-type TE performance to that of mature Bi₂Te₃, showing high σ and low κ near room temperature [17]. Moreover, the abundance of Ag₂Se in the Earth’s crust and its environmental compatibility make it a promising alternative to Bi₂Te₃ [18]. Furthermore, Ag₂Se possesses a relatively narrow band gap (~0.02 eV to 0.22 eV), demonstrating huge potential for the enhancement of TE performance [15]. Although Ag₂Se possesses a relatively stable orthorhombic structure near room temperature, a phase transition occurs when the temperature reaches approximately 130 °C [18]. Thus, Ag₂Se thin films are typically measured in the temperature range of room temperature to 100 °C. According to various reports on Ag₂Se TE thin films, multiple methods have been developed for the fabrication of high-performance Ag₂Se thin films, such as adjusting the Ag/Se ratio (e.g., Ag_1.8Se [19] and Ag_2.06Se [20]), texturing (e.g., (00l)-oriented [21] and (013)-oriented texturing [22]), and doping (e.g., Te [21], Ga [23], and S [24]).

In this work, we synthesized Ag₂Se thin film samples by a widely utilized magnetron sputtering method under different sputtering power settings, followed by a post-deposition annealing process at different annealing temperatures. Magnetron sputtering was selected due to its suitability for precise parameter control, high film uniformity, and compatibility with flexible substrates. In addition, post-deposition annealing was performed to enhance the crystallinity and phase purity of the Ag₂Se thin films, addressing how sputtering power and annealing temperature affected TE performance. The schematic diagrams of the magnetron sputtering process and the post-deposition annealing process are shown in Figure 1a and 1b, respectively. The aim of this work was to synthesize high-performance flexible Ag₂Se thin films by magnetron sputtering followed by post-deposition annealing. Moreover, the influence of sputtering power and annealing temperature on TE performance was systematically investigated based on the crystal structure and microstructural analysis.

2. Materials and Methods

Thin film fabrication: Firstly, the polyimide (PI) substrates were cut into chips with the size of 20 mm × 20 mm × 0.12 mm and ultrasonically and sequentially cleaned in deionized water, ethanol, and acetone, with 20 min for each step. The PI substrates were chosen for their high mechanical and thermal stability, which enabled post-deposition annealing without degradation. Moreover, the flexibility of the PI substrate is outstanding, making it widely used in flexible TE device fabrication [21,22]. Secondly, Ag₂Se thin films were deposited on PI substrates using the magnetron sputtering technique. As shown in Figure 1a, the Ag₂Se target material (99.99% purity, Zhongnuo Advanced Material Co., Ltd., Beijing, China) was loaded into the vacuum chamber, which was evacuated to a pressure below 3.0 × 10⁻³ Pa. During the deposition process of Ag₂Se thin films, the working pressure was maintained at 0.5 Pa with an argon (Ar) flow rate of 40 sccm. The sputtering power of this process was set to 10 W, 20 W, 30 W, 40 W, and 50 W to further investigate the effect of sputtering power on TE performance.

Post-deposition annealing: The synthesized 30 W sample of Ag₂Se thin films were sandwiched between the Cu substrates and placed on the heater, as shown in Figure 1b. The annealing environment is in vacuum with a glove box at the annealing temperature for 30 min, followed by another 30 min to maintain the temperature, and an automatic decrease to room temperature. The annealing temperatures of the thin film samples were set to 150 °C, 200 °C, 250 °C, 300 °C, and 350 °C to investigate the effect of annealing temperature on TE performance.

Characterization: X-ray diffraction (XRD, D/max-2500, Rigaku Corporation, Tokyo, Japan) was used to characterize the crystal structure of the synthesized samples with a scanning range of 2θ = 10° to 50° and a scanning speed of 10°/min. The distribution and analysis of the valence states of elements were detected by an X-ray photoelectron spectroscope (XPS; ESCALAB Xi+, Thermo Fisher, Waltham, MA, USA). The surface and cross-sectional morphology of the synthesized samples were observed by field-emission scanning electron microscopy (SEM, Supra 55 Sapphire, Zeiss, Oberkochen, Germany), and the elemental composition was analyzed by the equipped energy dispersive X-ray spectrometer (EDS, BRUKER QUANTAX 200, Zeiss, Oberkochen, Germany). The high-resolution nanostructure of the synthesized samples was characterized by a spherical aberration-corrected transmission electron microscope (AC-TEM, Titan3 Themis G2, FEI, Hillsboro, CA, USA). The S and σ of the synthesized samples were measured using an SBA apparatus (SBA458, Netzsch, Selb, Germany). The carrier concentration (n) and carrier mobility (μ) at room temperature were measured by a Hall measurement system (HL5500PC, Nanometrics, Milpitas, CA, USA). The thickness of the synthesized thin films was measured by a surface-profile measurement system (Dektak XT, Bruker, Billerica, MA, USA).

3. Results and Discussions

Figure 1d shows the X-ray diffraction (XRD) patterns of Ag₂Se TE thin films with different sputtering power settings of 10 W, 20 W, 30 W, 40 W, and 50 W from 2θ = 20° to 50°, respectively. All the diffraction peaks of the samples can be well indexed to the standard PDF card (PDF#24-1041) of the orthorhombic β-Ag₂Se phase without impurity peaks detected, confirming the high purity in the phases and crystallinity of all the synthesized samples. Although all the samples exhibit the (013) diffraction peak, its intensity increases significantly at intermediate sputtering powers (particularly at 30 W and 40 W), suggesting a progressive enhancement of preferential orientation along the (013) plane rather than a uniform growth behavior across all conditions [22]. In addition, the 30 W sample shows the strongest (013) characteristic peak, which is associated with the higher crystallinity and μ [18,25]. Figure 1d shows the enlarged XRD patterns of the (112) and (121) characteristic peaks of the synthesized samples from 32° to 36°. Both the (112) and (121) characteristic peaks shift to the lower angle, pointing out the expansion of the lattice in the synthesized samples. The calculated lattice parameters of the synthesized samples are listed in Table S1. According to the lattice parameters and the lattice volume, the expansion of the lattice can be observed, which agrees with the conclusion by the XRD patterns.

To better understand the surface morphology and crystallinity of the synthesized samples, scanning electron microscopy (SEM) was performed, as shown in Figure 2a–e. The observed SEM images, supported by the EDS results in Figure 2f, indicate the uniformly distributed elemental consistent of the Ag₂Se phase. Specifically, the 30 W sample shows a denser and more uniform microstructure than the other samples with high crystallinity. Moreover, the 10 W and 20 W samples show Ag-rich clusters. In contrast, the surfaces of the 40 W and 50 W samples become dense and continuous. These results suggest that the increase in sputtering power initially suppresses the Ag-rich clusters and promotes the formation of a dense Ag₂Se phase but leads to surface disorder beyond 30 W. In addition, the increase in sputtering power also strengthens the preferential growth of (013), resulting in a denser Ag₂Se phase up to 30 W. Notably, the 30 W sample has the closest Ag/Se atomic ratio to 2.02. Beyond this power, the Ag/Se atomic ratio of all the samples, listed in Table S2, decreased with increasing sputtering power, indicating the consistent tendency with the SEM observations. The cross-sectional SEM morphology of the representative Ag₂Se thin film is shown in Figure S1. The average thickness values of the synthesized Ag₂Se thin films are listed in Table S3.

Figure 3 shows the temperature-dependent σ (a), S (b), n and μ (c), PF (d) of all the synthesized Ag₂Se thin film samples at room temperature. In Figure 3a, σ first increased with increasing sputtering power, reaching a maximum value of 30 W, and then decreased. In Figure 3b, the absolute value of S first increased with increasing sputtering power up to 40 W, followed by a decrease at 50 W. All the S values shown in the figure are negative, confirming the successful fabrication of n-type Ag₂Se thin films. In Figure 3c, the carrier concentration (n) and carrier mobility (μ) correlated well with the tendency of S and σ, which also confirms the XRD results of the better μ in the 30 W sample. Figure 3d demonstrates the PF value with different sputtering power settings. The maximum PF value of 7.0 µW·cm⁻¹·K⁻² at room temperature is achieved by the 30 W sample. Thus, the simultaneous enhancement of S and σ can be achieved by tuning the sputtering power up to 30 W, pointing out the successful decoupling of these two parameters. Moreover, TE performance results are also consistent with the SEM observations and Ag/Se ratio results, further confirming the reliable tendency and results in TE performance. However, the PF values are low even compared to our previous work [20,21,22]. Thus, we further conducted post-deposition annealing to enhance the TE performance of Ag₂Se thin films.

After identifying the thin film sample with the highest PF value of different sputtering power settings, the 30 W sample was then subjected to post-deposition annealing treatment to improve its microstructure and further affect the TE performance [19,21]. The annealing temperatures were set to 150 °C, 200 °C, 250 °C, 300 °C, and 350 °C. Figure 4a shows the XRD patterns of the annealed Ag₂Se thin film samples from 2θ = 20° to 50°. All the observed diffraction peaks can be well indexed to the standard PDF card (PDF#24-1041) of Ag₂Se without significant impurity peaks, pointing out the relatively pure phase of the orthorhombic β- Ag₂Se phase possessed by the annealed samples. Compared to the samples without annealing shown in Figure 1d, (002), (112), and (113) characteristic peaks appeared with significantly high intensity.

In addition to XRD analysis, X-ray photoelectron spectroscopy (XPS) analysis was employed to further investigate the elemental valence states in the 30 W sample annealed at 300 °C. According to the spectrum in Figure 4c, the Ag 3d spectrum exhibits two distinct peaks of Ag 3d_3/2 and Ag 3d_5/2 located at approximately ~374.8 eV and ~368.8 eV, respectively, indicating the presence of the Ag⁺ oxidation state without the detectable metallic Ag state. In Figure 4d, the Se 3d spectrum reveals two peaks of Se 3d_3/2 and Se 3d_5/2 located at ~55.2 eV and ~54.4 eV, respectively, pointing out the existence of Se^2- species. These results confirm the formation of Ag₂Se with high purity and stable elemental valence states.

To further investigate the nanostructure of Ag₂Se thin films, the 30 W sample annealed at 300 °C was then studied by a transmission electron microscope (TEM). Figure 5a shows a low-resolution TEM image of the 30 W sample annealed at 300 °C, with a well-crystallized structure. Figure 5b is the high-resolution, high-angle annular dark field (HR-HAADF) image of the red circled area in Figure 5a. Figure 5c,d present the EDS mapping of the observed area in Figure 5b with Ag and Se elemental analyses, respectively. As the figures show, the annealed sample exhibits a highly ordered Ag₂Se phase without any impurity phases or clusters. Figure 5e shows the atomic-scale HR-TEM image of the rectangular area in Figure 5b with the inset of the fast Fourier transform (FFT) image. Figure 5f presents the observed image of the red rectangle area in Figure 5e. As the figure shows, the atomic distributions are uniform and neatly organized. The d-spacing of the observed area was calculated as 0.2393 nm, which corresponds to the (031) plane in standard Ag₂Se. Post-deposition annealing can increase the crystallinity by suppressing the formation of Ag-rich clusters and re-organizing the nanostructure to the dense and pure Ag₂Se phase. In summary, these results demonstrate the successful synthesis of high-quality Ag₂Se thin films by magnetron sputtering and post-deposition annealing, with a well-ordered nanostructure and high crystallinity at the atomic scale.

Finally, the annealed Ag₂Se thin film samples were measured by the ZEM-3 and LFA instruments to analyze TE properties. Figure 6 shows the temperature-dependent σ (a), S (b), n, and μ (c), and PF (d) at different annealing temperatures. Since the phase transition happened at around 130 °C, the diagrams can be divided into two temperature regions: from room temperature to 100 °C (orthorhombic Ag₂Se) and from 150 °C to 250 °C (cubic Ag₂Se). Most of the reports only consider the orthorhombic temperature region since TE performance decreases sharply after the phase transition [21,22]. In the orthorhombic Ag₂Se temperature area in Figure 6a, σ increases with the increasing temperature. The 30 W sample annealed at 300 °C exhibits the highest σ at 100 °C among all the annealed samples. The samples annealed at 300 °C and 350 °C exhibit relatively high σ in the orthorhombic Ag₂Se temperature area but decreased sharply in the cubic Ag₂Se temperature region. In Figure 6b, the absolute value of S in most of the annealed samples decreased during the phase transition and then increased with further temperature rises. In addition, the negative S values also indicate n-type Ag₂Se after post-deposition annealing. Figure 6c shows the n and μ of the annealed samples at room temperature. These trends are consistent with the variations in σ and S observed in Figure 6a and 6b, respectively. Figure 6d illustrates that the PF values of the annealed samples are temperature-dependent. All the PF values also decreased sharply after the phase transition temperature, showing that the PF values in orthorhombic Ag₂Se consistently exhibit higher TE performance than cubic Ag₂Se. The 30 W sample annealed at 300 °C possesses the highest PF values of around ~17.4 µW·cm⁻¹·K⁻² at 100 °C. Finally, the 30 W sample annealed at 300 °C was served as n-leg, with p-leg Sb₂Te₃ used to fabricate a flexible TE generator. The measured output voltage (U) and output power (P) of this device as a function of current (I) with the temperature difference ΔT (set as 5, 15, 25, 35, 45, and 55 K) are shown in Figure S2. The maximum U and P are ~140.3 mV and ~6530.0 nW under the ΔT of 55 K. Moreover, the calculated power density is around 181.4 W·m⁻² in the fabricated f-TEG.

4. Conclusions

In summary, flexible Ag₂Se thin films were successfully synthesized by magnetron sputtering under various sputtering powers, followed by post-deposition annealing at various temperatures to further optimize their TE performance. XRD, SEM, XPS, and TEM analyses confirmed the formation of phase-pure, highly crystalline Ag₂Se thin films. The sputtering power significantly influenced the film’s morphology, carrier concentration, and mobility, with the 30 W sample exhibiting the optimal thermoelectric properties. Further annealing treatment, particularly at 300 °C, enhanced its crystallinity and carrier mobility, leading to a peak power factor of ~17.4 μW·cm⁻¹·K⁻² at 100 °C. These findings demonstrate the effectiveness of tuning the sputtering and annealing parameters in enhancing the thermoelectric performance of flexible Ag₂Se films, providing valuable guidance for the design of high-performance flexible thermoelectric devices.