Improved thermoelectric properties of α-phase Cu₂Se thin films through multiphase nanostructuring

Abstract

Copper selenide (Cu₂Se) has been extensively studied due to its promising thermoelectric properties in bulk form. However, the miniaturization of thermoelectric devices using thin films is highly desired for smart applications. To date, there are few reports on composite thin films of Cu₂Se for thermoelectric applications, primarily due to their lower conversion efficiency. In the present work, Cu₂Se-based multiphase nanocomposites are presented to demonstrate enhanced conversion efficiency. The detailed structural characterization reveals that thermally evaporated Te-doped Cu₂Se thin films have multiphase compositions. The electrical conductivity decreases after Te-doping, due to enormous scattering of carriers against secondary phases and lattice defects. However, upon further increasing Te-doping concentration, both the electrical conductivity and Seebeck coefficient start increasing simultaneously, due to the formation of Cu₂Te nanoclusters and Te–Se solid solution, in the matrix of Cu₂Se. We emphasize the power factor, with the highest value reaching 234.0 μW mK⁻² at 400 K, as a key indicator of thermoelectric performance. A slightly overestimated value of dimensionless figure-of-merit (ZT) of 0.2 was obtained using the power factor and merely the electronic part of the thermal conductivity. The current synthesis route synergizes the effects of a multiphase system in thin film research to enhance the thermoelectric efficiency of Cu₂Se and related materials classes.

Copper selenide (Cu₂Se) has been extensively studied due to its promising thermoelectric properties in bulk form.

Introduction

The increasing demand for green energy arises from critical concerns about global warming and the heavy dependence on fossil fuels. A significant challenge that remains is improving the efficiency of energy conversion, as a large portion of energy is dissipated as waste heat. Thermoelectric devices have gained considerable attention for their ability in converting heat directly into electrical energy and vice versa. These thermoelectric devices are comprised of p-type and n-type material legs connected electrically in series and thermally parallel.¹ The conversion efficiency of a single leg material can be evaluated using a dimensionless figure-of-merit: ; where, σS² is explicitly defined as power factor (PF), S, σ, k, and T are the Seebeck coefficient (thermopower), the electrical conductivity, the thermal conductivity, and the working temperature, respectively. The simultaneous suppression of thermal conductivity and enhancement of PF yields significant improvement in the performance of thermoelectric materials. Nonetheless, the intrinsic limitation of low conversion efficiency continues to impede their integration into practical thermoelectric (TE) applications.

Over the past decades, researchers have continued to explore a wide range of materials to enhance thermoelectric performance, from common metals to complex semiconductors.^2–5 Several classes of materials are being investigated in search of promising thermoelectric properties including but not limited to oxides,^4,6–12 skutterudites,^13,14 and chalcogenides.^15–25 They aim to enhance their thermoelectric properties through various strategies such as doping,^26–29 nano structuring,^{14,20,23,30–34} and material hybridization.³⁵ Despite considerable progress, developing efficient, cost-effective, and environmentally friendly thermoelectric materials, remains a persistent challenge. Zhao et al.³⁶ recently reported an impressive ZT value of 2.6 in single-crystalline SnSe at 923 K due to the combined effects of ultralow thermal conductivity and high PF.

Among chalcogenides, Cu₂Se has received immense importance due to its phonon-liquid electron-crystal (PLEC) behavior.^19,35,37 The superionic Cu_2−xSe attracts the attention of thermoelectric researchers due to its distinct thermal transport properties, which are linked to a structural shift that takes place at around 400 K.³⁸ Around 400 K, the Cu_2−xSe solid state undergoes a phase change from the low-temperature α-phase to the high-temperature β-phase.^39,40 In the compound, Cu_2−xSe, selenium (Se) atoms are situated within face-centered cubic (FCC) lattice sites in both of its phases.⁴¹ However, the behavior of copper (Cu) atoms differs between the α-phase and the β-phase.^42,43 In the α-phase, Cu atoms demonstrate ordered and localized characteristics, while in the β-phase, they manifest disorder and superionic attributes. This superionic behavior arises from the movement of Cu ions under an applied electric field, leading to a decrease in their electrochemical potential.^40,44,45 The critical scattering of both phonons and charge carriers around 400 K greatly increases the ZT value, corresponding with the phonon-liquid electron-crystal (PLEG) concept.¹⁷ Only in the critical scattering region, application into practical devices is hindered due to a lack of stability in the performance. Furthermore, thermoelectric performance significantly drops at higher temperatures, even if Cu₂Se achieves an ultrahigh ZT > 2 at 410 K.⁴⁶ Cu₂Se emerges as a highly intriguing thermoelectric material that produces surprisingly high ZT values range of 1.5–2.1 at temperatures close to 1000 K. The process of chemical synthesis can provide nanoparticles with precise sizes and shapes, potentially leading to a tighter grain boundary. Cu₂Se thermoelectric materials exhibit a ZT value exceeding 2 at high temperatures (800–1000 K), which is attributed to the “liquid-like” behavior of the β-phase with an anti-fluorite structure, as shown by both theoretical and experimental studies. Additionally, bulk Cu₂Se with a monoclinic α-phase demonstrates favorable thermoelectric performance near room temperature.⁴⁷ Day et al., offer a theoretical prediction for the maximum ZT value of α-Cu₂Se, estimated to be around 1.16 at 305 K, which is comparable to that of standard thermoelectric materials.⁴⁸ Cu₂Se nanoparticles are synthesized by electrochemical procedure and their optical and electrical characterization are reported by Rong.⁴⁹ Cu_2−xSe-based chalcogenides attract significant attention as promising P-type thermoelectric materials, exhibiting a figure of merit (ZT) greater than unity.^{40,44,50–52}

Copper chalcogenide thin films garner significant interest among researchers due to their wide range of applications in different optoelectronic devices.⁵³ Thermoelectric materials have a wide range of advanced applications beyond conventional power generation and cooling. They can harvest body heat to power wearable electronics, enabling continuous health monitoring or AI-driven hand motion recognition systems, as demonstrated in recent studies.⁵⁴ In robotics, thermoelectric generators (TEGs) capture waste heat from mechanical components, providing supplementary power for sensors and processors, enhancing energy efficiency and responsiveness in AI-integrated robotic hands.⁵⁵ Moreover, thermoelectric devices play a crucial role in fire alarming systems by detecting sudden temperature changes and generating small voltages to trigger alarms, offering a reliable, battery-free solution for fire detection.⁵⁶ In the field of smart buildings, integrating thermoelectric materials into building structures allows energy harvesting from temperature gradients, such as between indoor and outdoor surfaces, to power sensors and control systems, contributing to energy-efficient and sustainable infrastructure.⁵⁷ However, only a few reports can be found over thin films of Cu₂Se for thermoelectric applications.^58–61 Lin et al. synthesize Cu₂Se thin films using a solution-based method. This technique yields a maximum power factor (PF) of 6.2 μW cm⁻¹ K⁻² on a rigid Al₂O₃ substrate and 4.6 μW cm⁻¹ K⁻² on a flexible polyimide substrate.⁶² They optimize the carrier concentration of Cu₂Se thin films using a soaking procedure in Cu⁺ ion solution which further enhances the thermoelectric properties.^62,63 However, the Cu₂Se thin films demonstrated to date typically exhibit suboptimal thermoelectric performance, primarily attributable to structural imperfections such as voids and compositional defects within the films. High-temperature post-treatment is commonly used to improve and stabilize the thermoelectric (TE) performance of thin films. However, this method often weakens the bonding strength between the thin film and the substrate, especially when using organic flexible substrates. As a result, designing and fabricating Cu₂Se thin films with enhanced TE performance without relying on high-temperature processing remains a significant challenge.⁶⁴

In this work, the thermoelectric transport properties of Te-doped Cu₂Se thin films have been systematically explored and explained with the aid of detailed structural characterizations. In our previous study, we employed X-ray absorption spectroscopy, an element-sensitive, orbital-specific, and local structural technique, to examine the atomic-scale structural influence of Te in the matrix of bulk Cu₂Se.²¹ The results demonstrated the formation of Cu₂Te nanoclusters instead of Te replacing Se sites, indicating an essential approach to improve thermoelectric performance in Cu₂Se superionic conductors. Building on this idea, thin films through thermal evaporation have been fabricated and tested to strengthen the idea of miniaturization of thermoelectric devices.

Experimentation

The complete synthesis and characterization route has been demonstrated in Fig. 1. At first, pure and Te-doped Cu₂Se powdered samples were synthesized by ball milling followed by their compaction into pellets using spark plasma sintering (SPS), as the complete synthesis is given in ref. 21. A series of pure Cu₂Se and Te-doped Cu₂Se_0.95Te_0.05 (Te-5%), Cu₂Se_0.9Te_0.1 (Te-10%), and Cu₂Se_0.8Te_0.2 (Te-20%) films having a thickness of 100 nm were evaporated over glass substrates, under a vacuum environment maintained at 2.0 × 10⁻⁵ mbar through thermal evaporation technique (NANOVAK 3000TH). The bulk chunks of the materials to be evaporated were placed into a tungsten crucible and precleared glass slides were used as the substrates. The evaporation rate was fixed at 0.5 Å s⁻¹ under the current ranging from 90 to 100 A. The samples were prepared and handled under vacuum (10⁻³ torr) to prevent oxidation and contaminations. The obtained thin films were annealed at 400 C under an inert environment.

Fig. 1. Schematic of thermoelectric thin film fabrication and characterization process. Pellets are deposited via thermal evaporation to form 100 nm thin films, followed by annealing at 400 °C in a tube furnace. The films undergo structural and compositional analysis through XPS and SEM, with thermoelectric performance evaluated by a dedicated measurement system.

The Cu and Se binding energies were determined by X-ray photoelectron spectroscopy (XPS) using a Thermo Scientific Escalab-250xi equipped with monochromatic Al Kα radiation. The effect of charging was corrected from the as-acquired data with respect to the C 1s peak (284.8 eV). XPSPEAK41 software was used to deconvolute the high-resolution Cu 2p, Se 3d, and Te 3d spectra. The surface morphology and elemental composition of all series of thin films were determined using a scanning electron microscope (ZEISS Gemini SEM 300-8202017309) equipped with an energy-dispersive spectroscopy (EDS) detector. The electrical conductivity and Seebeck coefficient were measured simultaneously using a Thermoelectric Parameter Test System (Joule Yacht-NAMICRO-3L).

Results and discussion

The full survey scan for the pure Cu_2−xSe and Te-doped (Te-20%) is shown in Fig. 2(a). The presence of Cu, Se and Te is evident from different core shell contributions. A strong peak around ∼285 eV is assigned to C 1s employed for referencing to correct peak shift about surface charging as shown in Fig. 2(a). The high-resolution spectra of Cu 2p as taken from pure Cu_2−xSe and Te-20% has been deconvoluted and shown in Fig. 2(b) and (c), respectively. The detailed analysis of Cu 2p data suggests the formation of Cu_2−xSe as indicated by the presence of Cu¹⁺ and Cu²⁺. The presence of Cu²⁺ ions in Cu_2−xSe is attributed to Cu vacancies caused by the ionic mobility of Cu and it also supports the superionic behavior Cu_2−xSe, as reported earlier.¹⁹ Additionally, traces of CuO could also be detected due to surface oxidation of the films. After Te-doping, the formation of Cu₂Te was predominately detected as indicated by a synthetic peak at a lower energy, as shown in Fig. 2(c). These results are also supported by the high-resolution spectra of Se 3d as taken from pure Cu_2−xSe as shown in Fig. 2(d), and high-resolution spectra of Se 3d and Te 3d ask taken from Te-20% as shown in Fig. 2(e) and (f), respectively. It is suggested that Te is present in multiphase system comprised of Te, Se–Te and Cu₂Te. Butt et al. has also reported that Te-doping results in the formation of Cu₂Te nanoclusters in the matrix of Cu_2−xSe through the aid of X-ray absorption spectroscopy coupled with computational study.²¹ Further, Te-doping has greatly suppressed the concentration of Cu²⁺ which infers reduced mobility of Cu ions in the lattice, which results in an improved thermal stability of Cu_2−xSe. This analysis demonstrates that Te-doping alters the electronic environment and bonding, which is essential for enhancing thermal stability and thermoelectric properties of Cu_2−xSe.

Fig. 2. The full survey XPS spectra for thin film specimens of Cu₂Se and Cu₂Se_0.8Te_0.2 (Te-20%) (a), the high-resolution spectra of Cu 2p from Cu₂Se (b), Te-20% (c), the high-resolution Se 3d spectra from Cu₂Se (d), Te-20% (e), and the high-resolution Te 3d spectra Te-20% (f).

The surface morphology and phase distribution of pure Cu_2−xSe film are shown in Fig. 3. The annealed films have shown a homogeneous and uniformly distributed grain structure with densely packed and well-connected grains as revealed by the secondary electron (SE) image, as shown in Fig. 3(a). Further, the image taken using in-lens detector has ruled out the presentence of any detectable secondary phase, as shown in Fig. 3(b). This homogeneity may provide better crystallinity leading towards better transport properties.⁶⁵ The average particle size was calculated approximately 140 nm, as shown in Fig. 3(c). The uniform distribution of Cu_2−xSe grains can also be confirmed by EDS mapping, as shown in Fig. 3(d)–(f). The EDS of pure Cu_2−xSe shows both Cu and Se are evenly distributed which indicates the uniformity of single phase Cu_2−xSe. The elemental analysis of pure and Te-20% thin film samples are given in Table 1. After Te-doping, significant changes in surface morphology have been observed as indicated by SEM analysis of Te-20%. Some smaller sized clusters of Cu₂Te can be seen as shown in Fig. 4(a), which could further be confirmed by in-lens image as shown in Fig. 4(b). The average particle size has been reduced due to the formation of nanoclusters of Cu₂Te in the matrix of Cu_2−xSe. The formation of Cu₂Te nanoclusters could further be confirmed from EDS mapping, as shown in Fig. 4(d)–(g). The EDS micrographs of the Te-20% thin film reveals even more pronounced changes in surface morphology. The grains appear irregular and uneven, with substantial agglomeration observed in Fig. 4(d)–(g). The surface becomes rougher and less homogeneous, likely due to the formation of Cu₂Te nanoclusters. The chemical state analysis has also suggests the formation of Cu₂Te phase which has been confirmed by SEM coupled with EDS analysis. The formation of secondary phases offer enormous scattering sites for charge carriers and phonons which may serve the purpose of energy filters.²²

Fig. 3. Scanning electron microscopic (SEM) images of pure Cu₂Se thin film using (a) secondary electrons, (b) in-lens detector, (c) particle size, (d) SEM image used for elemental mapping of (e) Cu, (f) Se.

Table 1. Atomic percentages of Cu, Se, and, Te in pure and Te-doped (20%) samples, showing elemental composition changes after tellurium doping.

Sample	Elements	Atomic (%)
Pure	Cu	62.8
	Se	37.2
Te-20%	Cu	74.5
	Se	23.2
	Te	2.3

Fig. 4. Scanning electron microscopic (SEM) images of Te-20% doped thin film using (a) secondary electron, (b) in-lenz detector, (c) particle size, (d) SEM image used for elemental mapping of (e) Cu, (f) Se and (g) Te.

The temperature dependence of TE properties of all the series of pure and Te-doped Cu_2−xSe thin films has been measured in α-phase (below 410 K). The electrical conductivity (σ) of the pure Cu_2−xSe is found to be the highest among all the compositions. However, after Te-doping, the electrical conductivity decreases as compared to the pure Cu_2−xSe, due increased carrier scattering against the defects introduced by Te-doping. Upon further increasing the Te content, the electrical conductivity begins increasing again. This increase is associated with the formation of Cu₂Te nanoclusters, as confirmed by the by XPS results as well. The higher intrinsic electrical conductivity of Cu₂Te phase offers electrical pathways in the matrix of Cu_2−xSe, which is also in good agreement with that of the previous reports.²¹ The electrical conductivity of Te-20% has been observed to be highest among all the series of Te-doped samples.

Further, in order to inquire conduction mechanism, activation energy (E_g) was determined using Arrhenius equation, as given in eqn (1).

1

where, σ, K_B, T and E_g are the electrical conductivity, Boltzmann constant and absolute temperature. The E_g values of all the samples were determined by plotting the logarithmic dependence of electrical conductivity vs. inverse of temperature (1/T), as shown in the inset of Fig. 5(b). Understanding this behavior is critical for optimizing thermoelectric performance. Upon Te-doping, the E_g value was initially increased due to increased defects, however, upon further increasing Te-contents, E_g was subsequently decreased due to higher intrinsic conductivity of Cu₂Te, as discussed earlier.^22,66

Fig. 5. Thermoelectric properties of Cu₂Se and Te-doped (5%, 10% and 20%) Cu₂Se thin films as a function of temperature: (a) electrical conductivity, (b) activation energy (E_g) for different compositions with an inset showing ln(σT) vs. 1000/T, (c) Seebeck coefficient, and (d) power factor (PF).

The temperature dependent Seebeck coefficient (S) of all the samples showed positive values which confirms holes are the majority charge carriers, as shown in Fig. 5(c). At room temperature, the Seebeck coefficient of pure Cu_2−xSe thin film is 28.35 μV K⁻¹ and varies very little across the temperature range. For the Te-doped thin film with 5% Te, the Seebeck coefficient at room temperature is 30 μV K⁻¹, which is slightly higher than that of the pure sample. The highest Seebeck value of 64 μV K⁻¹ at 400 K has been observed for Te-5%, which is about 100% larger than that of the pure sample at the same temperature. However, upon further increase in Te concentration results in the formation of Cu₂Te nanoclusters, which provide additional pathways for charge carriers. As stated above, Cu₂Te exhibits higher intrinsic electrical conductivity and a lower Seebeck coefficient compared to Cu_2−xSe.⁶⁷ The embedded Cu₂Te nanoclusters dominate carrier transport, resulting in a reduction of the Seebeck coefficient for all the samples having Te contents larger than 5%, which suggests that Te-5% is the optimal composition for maintaining a balance between electrical conductivity and the Seebeck coefficient. The striking increase in the Seebeck coefficient for the Te-5% sample at 400 K is attributed to the phase transition of α-Cu₂Se to β-Cu₂Se occurring at this temperature. This transition enhances carrier dynamics, amplifying the Seebeck effect.⁶⁸ Our highest Seebeck coefficient of 64 μV K⁻¹. In contrast, a study on polyaniline-Sb₂Te₃, Bi₂Te₃ thin films achieved a maximum Seebeck coefficient of approximately 30 μV K⁻¹.⁶⁹ This indicates that our film's Seebeck coefficient is over twice as high. The power factor (PF), derived from the Seebeck coefficient (S) and electrical conductivity (σ) is a critical parameter for evaluating thermoelectric performance. Fig. 5(d) shows the temperature dependent PF values of all the series. At 300 K, the (PF) of pure thin film is 97 μW mK⁻² reaching its highest value of 166 μW mK⁻² at 400 K. In the Te-doped series, (PF) value of Te-5% is 30 μW mK⁻², rising to a maximum of 234 μW mK⁻² at 400 K, which is larger than that of the pure thin film. This significant improvement highlights the role of the Seebeck coefficient in optimizing PF. The highest PF of 234.0 μW m⁻¹ K⁻², is lower than that of the α-Cu₂Se thin film's reported value.⁷⁰ Further increasing film's thickness (beyond 100 nm) or tuning annealing conditions could higher (PF), but we improve the power factor and Seebeck coefficient as compared to pure Cu₂Se thin films.

The experimental determination of total thermal conductivity (K_t) of thin films is challenging,³³ especially the lattice thermal conductivity (K_l). However, the electronic thermal conductivity (K_e) can be determined using the Wiedemann–Franz law: K_e = L_oσT, where L_o is the Lorentz constant, (σ) is electrical conductivity, and T is the absolute temperature. For metallic and degenerate materials, L_o ∼2.44 × 10⁻⁸ W Ω K⁻² is the constant value.⁷¹ However, the true Lorentz number values are calculated using single parabolic band coupled with acoustic phonon scattering.^67,71,72 The combined effect of nanoclusters, along with other defects, reduced K_e, as shown in Fig. 6(a), which results in an increased value for figure of merit (ZT) of the doped samples, as shown in Fig. 6(b). The ZT values were estimated by merely relying only on K_e while ignoring K_l due to the complexity and unavailability of methods to measure K_t of the thin film samples.²² Thus, the estimated ZT is slightly overestimated, but it suggests a roadmap for the importance of thin films in the field of thermoelectric device miniaturization.

Fig. 6. Temperature dependence of (a) electronic thermal conductivity (K_e), (b) over-estimated figure of merit (ZT).

Conclusions

This work demonstrates that Te-doped Cu₂Se multiphase nanocomposite thin films exhibit substantial improvements in thermoelectric transport properties, exceeding pristine Cu₂Se thin films. The incorporation of Te promotes the formation of secondary phases, such as Cu₂Te nanoclusters and Te–Se solid solutions. Initially, Te-doping results in a decrease in electrical conductivity due to enormous carrier scattering against defects, but further increase in Te-doping enhances thermoelectric power factor through the synergy of optimized electrical conductivity and Seebeck coefficient. This composition, characterized by reduced electronic thermal conductivity and an enhanced power factor, attains a ZT of 0.2, reinforcing its applicability in thermoelectric systems. This study opens up new avenues for the incorporation of thin films into miniaturization of thermoelectric devices.

Data availability

Data will be provided upon request.

Conflicts of interest

All the authors declare that there is no conflict of interest.