Ge-Doping Impact on the Power Factor of Sb₆₅Se₃₅ Nanocrystalline Bulks

As a result of resource depletion and ecological disasters caused by the use of fossil fuels, researchers are increasingly favoring thermoelectric (TE) materials, which allow for direct thermal-to-electric energy conversion without dangerous liquids, moving parts, or greenhouse emissions.

Even though there has been a substantial improvement in TE performance, the observed conversion efficiency is much lower than the values available in literature. The higher conversion efficiency, on the other hand, generally requires a strong Seebeck coefficient, as well as high electrical conductivity and a low thermal conductivity. Recently, researchers have developed new tactics, such as band structure engineering, multi-scale phonon scattering, and the implementation of resonant levels, to successfully decouple conflicting transport properties and optimize the TE properties of various materials [1-17].

In recent years, researchers have used nanostructures to achieve further declines in lattice conductivity. Even though the goal was to improve the power factor by using quantum confinement effects [5], it looks like phonon scattering on the edges of nano-sized grains was what really made a difference [6,7]. One of the most complicated issues to resolve is how to enhance the electrical conductivity and Seebeck coefficient of inorganic nanoparticles. We can alloy nanostructures with the right elements to improve their electrical conductivity and Seebeck coefficient.

Nanoscale materials may have low thermal conductivity, a high-power factor, improved mechanical characteristics, and greater isotropy [18]. Pulverizing crystallographic nanometric specimens can be a low-cost way of producing the huge volume of material required for thermoelectric technology to gain widespread use [19].

The present work examines the impact of partial Ge replacement at the Se position on the material properties of Sb-Se, including changes in the microstructure of the material, electronic transitions, and energy band gap. Because Se’s valence state in the eutectic Sb-Se is Se³⁺, adding a quadruple element, Ge⁴⁺, can change the environment. This can change the large and small features of the eutectic Sb-Se and give us a better understanding of its TE properties. The rapid implications of this insertion, such as charge compensation impacts and distortion in the crystalline structure of Sb₆₅Se₃₅, may lead to the formation of antisite defects (Sb′_Se and V′_Se), which could significantly impact the electrical and thermoelectric characteristics of the treated Sb₆₅Se₃₅ samples. The focus of this research is to determine the influence of Ge doping in Sb₆₅Se₃₅, as well as to clarify and interpret the observed behaviors.

Melting and cold pressing were used to create nanocrystalline Sb₆₅Se_35-xGe_x (x = 0, 5, 10, 15, and 20) bulks. Under vacuum, stoichiometric mixes of high-purity Ge (99%), Sb (99.999%), and Se (99.9999%) granules were packed in graphite-coated tubes and burned to 1123K. After soaking the tubes at 1123K for approximately 5 hours, we quenched them with cold water. The ingots were then crushed into a powder and cold pressed at 5 ton/cm² for 3min. A PANalytical X’ Pert PRO diffractometer (XRD) and a (JEOL JSM-7600F) FESEM were used to determine the phase structures and microstructure, respectively. The transport properties were measured using pellets with a relative density of 98 percent or higher. A two-probe technique and a temperature gradient method were used to measure electrical conductivity and the Seebeck coefficient, respectively. Using a Perkin Elmer LAMBDA 950 spectrophotometer, reflectance spectroscopy was used to evaluate the band gap of samples. The Archimedes method was used to determine the volume densities (d) of the bulk samples. The electrical properties were all measured perpendicular to the press direction in the current experiments.

Microstructure of the synthesized samples

The EDS analysis was used to determine the elemental composition of the fabricated Ge-doped Sb₆₅Se₃₅ alloys, and the results are presented in Figure 1. The EDS spectrum revealed several well-defined peaks related to Sb, Se, and Ge, indicating the presence of antimony, selenium, and germanium in the samples. The spectrum showed no other impurity-related peaks, suggesting that the synthesized nanoparticles are Ge-doped Sb₆₅Se₃₅. The EDS data shows that each element’s estimated average deviation from ideal stoichiometry is less than 1%.

Figure 1:EDS spectra corresponding to the specimens.

Figure 2 illustrates the microstructure of the virgin and Gedoped compounds produced at five different germanium content levels. The particle was small and exhibited a uniform morphology, as demonstrated. Moreover, FESEM images show well-defined particles with a thin and appreciably fine flake-like structure. Besides this, increasing the concentration of doping tends to reduce the average particle size, signaling that containing high levels of doping is a vital strategy for crystalline refinement. The extremely small particle sizes result in a variety of grain boundaries, which could further improve the thermoelectric properties [20].

Figure 2:FE-SEM images of Ge-incorporated Sb₆₅Se₃₅ (0 ≤ x ≤ 20) samples.

Figure 3 displays the XRD patterns of pure and Ge-doped samples at room temperature. The observed reflection peaks confirm the single phasic nature of the investigated materials crystallizing in rhombohedral Sb₂Se [21]. The disappearance of secondary phases corresponding to unreacted Ge, Sb, or Se confirms the prepared samples’ purity. With the addition of Ge ions, the diffraction peaks shift in the higher-angle direction, revealing that the product tends to accumulate in a comparable but more sophisticated form [22,23]. As the dopant concentration went up, the diffraction peaks got wider and moved toward higher angles. At the same time, some dominant peaks got smaller. This suggests that the rhombohedral structure of Sb₂Se changed without changing the crystal structure. Vegard’s law [24] states that the removal of the host atoms substitutes atoms with smaller ionic radii [25]. As the concentration of germanium increases, the strong (200) peak shows a slight red shift and amplitude decreases (except for x = 5), signifying the suppression of crystallization and a decrease in particle size. One used Sherrer’s equation to calculate the average crystallite size (Cs) in Table 1 after correcting for instrumental broadening [26]. With different ratios of germanium, particle size decreases. Furthermore, as particle size is reduced, the dislocation density increases. Proposals suggest that a relatively high dislocation density leads to an increase in the band gap of the semiconductor material, as the dislocations significantly isolate from the interatomic distance [21]. As a result, raising the dislocation density makes the materials more resistant to fracture initiation, and high-density high dislocation plays an essential role in strengthening the strength and micro-hardness of the treated materials, as has been proven in earlier work [27].

Figure 3:XRD patterns of polycrystalline Ge-doped Sb_{65Se35 (0 ≤ x ≤ 20) alloys.}

Table 1:EDS, XRD and optical band gap results of the synthesized alloys.

Electrical transport and optical properties

Electrical conductivity (𝜎) is extremely important and required for the applicability of synthesized SSG compounds to TE devices. Figure 4 shows how the electrical conductivity of Sb_{65Se35 bulks changes with temperature from 100K to 400K when different amounts of Ge are added. It is seen that the conductivity decreases monotonically with increasing temperature, which indicates a degenerate semiconductor characteristic. Other works [28-30], possibly due to the distinct Fermi surfaces of the ferromagnetic and paramagnetic states, have reported a similar semiconductor nature.}

Figure 4:Electrical conductivity (𝜎) as a function of temperature for various compounds.

The pure sample has the weakest electrical conductivity. The electrical conductivity increases in a non-monotonic manner as the germanium content rises. Moreover, the 𝜎 of doped samples is higher than the undoped ones. Electrical conductivity is given by σ = neμ where n, e and 𝜇 are the carrier concentration, the electron charge and the carrier mobility, respectively [31]. This implies that the simultaneous variation of n and 𝜇 should influence 𝜎. Moreover, the increase in electrical conductivity when Ge concentration in Sb_{65Se35 samples grows could be due to two factors. In the beginning, when there are more Ge ions in the Sb65Se35 lattice, the Ge ions give the lattice freer electrons, which makes it more electrically conductive. It is believed that the Ge ions in the Sb65Se35 lattice function as both charge carriers and acceptor impurities. Second, when the concentration of Ge increases, the Ge atoms make more neck contact with Se sites, speeding charge carrier movement and thereby boosting conductivity.}

The mobility of carriers (𝜇) is limited by imperfections such as antisite defects, vacancies, and interface defects, as well as by potential barriers at the grain boundaries that are caused by the smaller particle size [32]. However, chemical bonds can significantly influence carrier mobility. As compared to ionic components, covalent bonds are generally more suitable for carrier transport [33]. The bond polarity contributes to the formation of antisite defects in Sb_{65Se35-based crystals. The more the polarity of the bond between positively and negatively charged atoms decreases, the more likely an antisite defect will form. Despite the fact that antisite defects contain holes and have the ability to counteract electrons, the interaction between vacancies and antisite defects becomes the primary factor, leading to the formation of additional electrons [34]. As a consequence, the carrier concentration rises as the particle size decreases. Furthermore, as particle size decreases, grain boundary, antisite defect, vacancy, and interface defects increase, resulting in greater carrier scattering. Consequently, carrier mobility experiences a limit [35].}

The location of the Fermi energy level throughout the component and the type of dispersion process that the charge carriers experience determines the Seebeck coefficient (S) [36]. Then, any alteration of the Fermi level in the substance has the greatest impact on TE power. Furthermore, it is well recognized that S is highly susceptible to the imbalance of the electronic density of states (DOS) at the Fermi level (EF), with a slight shift in the DOS reflecting a change in its magnitude, which is connected to the charge carrier density (n). The introduction of barriers at interfaces and grain boundaries acts as a filter for low-energy charge carriers [19]. Figure 5 shows S versus temperature for the prepared samples in the measured temperature range. All Ge-doped Sb_{65Se35 samples (x = 0.0-20) showed a negative Seebeck coefficient over the entire temperature range, confirming the dominance of electron-type carriers in thermoelectric transport. The negative values of S across the entire temperature range imply that all of the materials are n-type conductive, with electrons as the predominant charge carrier, as stated previously [37,38]. All samples exhibit a crossover from metallic to semiconducting trend, as indicated by S (T) [39]. In other words, the Ge content x strongly influences the absolute value of the Seebeck coefficient, which first increases and then peaks at a certain point. With the dopant content, the onset of intrinsic conduction (the maxima of the curves) in such samples moves to a different temperature. Moreover, close to the saturation temperature Tsat, the magnitude of S reaches its maximum value, after which thermally excited carriers reduce it by increasing the temperature [40]. Excitation across the energy gap forms electron-hole pairs, leading to an increase in bipolar effects as the temperature rises [41]. Thermal excitation causes minority carriers (holes in this case) to emerge as the temperature rises, leading to the creation of the bipolar effect. This presence of electrons and holes is believed to be crucial for S saturation at high temperatures [42]. The bipolar effect, which occurs when the temperature rises and the minority carriers increase, could explain this behavior [43,44,45]. The Goldsmid-Sharp formula can estimate the band gap of the studied samples at a saturation temperature, as it directly correlates the peak of the Seebeck coefficient to the specimen band gap, , where e represents the elementary charge, is the maximum Seebeck coefficient for the bipolar effect sample, and Tsat signifies the absolute temperature at which Smax appears. Eg values for various samples (x = 0, 5, 10, 15, and 20) are roughly 0.0119, 0.0091, 0.0081, 0.0092, and 0.0076eV, respectively. To put it differently, the smaller the particle size, the greater the band gaps, and hence the higher the intrinsic excitation temperature.}

Figure 5:Seebeck coefficient versus temperature for all compositions.

The Seebeck coefficient changes when Ge is added, which can be linked to changes in the concentration of carriers as [48]:,where h is the Planck constant and k_B is the Boltzmann constant. A drop in the Seebeck coefficient causes the concentration of carriers to rise. It signifies that a lower value of S corresponds to a larger carrier concentration (n).

The Sb_{65Se35-based alloys’ carrier concentration, on the other hand, is vulnerable to lattice imperfections caused by mechanical deformation. Therefore, the deformation-induced donor-like effect, which introduces an excess of negative carriers in the lattice, is responsible for the simultaneous increase in S. As a consequence, the new point defects introduced into the lattice as a result of the plastic deformation will alter the material’s transport behavior, as previously stated [49,50]. The combination of antisite defects and vacancies creates a donor-like effect during the grinding and heat treatment process, which can lead to an increase in electron production [51]. More mechanical stress accumulates as the particle size of crystals decreases, resulting in more antisite defects and vacancies. Antisite defects are simpler to arise when the difference in electronegativity between the components that make up the thermoelectric material is minimal [52]. An increase in the size mismatch between the cation and anion atoms promotes the creation of anion vacancies [53].}

Nevertheless, it is worthwhile noting that the samples with germanium content exhibit relatively lower Seebeck coefficients owing to an increase in carrier concentration [54]. Moreover, thermally induced boundary resistance between particles, resulting in a thermal gradient over the interfaces, can diminish the Seebeck coefficient in nanostructured materials, or electronfiltering processes at the connections can increase it [55].

Figure 6:Wavelength dependence of the reflectance for all compositions.

Finally, optical properties serve only as a tool to gain insight into the contributions of the electronic band structure, which in turn shape the electronic transport properties. Understanding the nature of the band structure has paved the way for future band engineering research involving the proper alloying of the system, which could result in significant TE performance enhancements. To study the effect of Ge substitution on the band gap of Sb_{65Se35, Diffuse Reflectance Spectroscopy (DRS) was performed on the specimens at 300K. Figure 6 shows the UV–vis reflectance of all samples as a function of wavelength at room temperature. Raising the wavelengths significantly increased the diffused reflectance of both pure and doped samples. In addition, the incorporation of Ge dopant into the host Sb65Se35 reduces diffused reflectance. Pure, 5, 10, 15, and 20 Ge-doped Sb65Se35 have absorption edges that begin at 394, 404, 413, 380, and 369nm, respectively. Doping causes a change in the absorption edge, which implies a change in the band structure.}

The indirect band gap of undoped and doped samples is plotted against photon energy (hν). One extracted the band gap values, as shown in Figure 7, by graphing the root square of the Kubelka- Munk (KM) function [56] versus hν and then projecting the linear section of the [F(R)hν]^0.5 plot to the energy axis. Table 1 states that an increase in Ge doping causes the E_opt to rise from 0.48 to 1.48eV. The rising trend in E_opt would result in fewer carriers and a lower carrier density, revealing an increase in the materials’ resistance. Ge-dopants will change E_opt and thus acquire resistance as a result of this observation and hypothesis [57].

Figure 7:Variation of the absorption coefficient with energy in various alloys.

The Power Factor (PF) of Sb₆₅Se₃₅ doped with Ge, which must have an average value (W m^-1 K^-2) of approximately 10^-3 for materials used in contemporary devices [58], characterizes the electrical performance of TE materials. We computed the Power Factor (PF) across the entire temperature range of the data to determine the effectiveness of using these compositions as TE generators. Figure 8 shows the temperature dependence of the PF plots. Clearly, as the temperature increases, the PF rises and then declines, reaching maxima at specific temperatures that depend on the Ge content. The power factor of Sb₆₅Ge₅Se₃₀ is 0.8 (mW m^-1 K^-2) at 240K, which is significantly lower than that of state-of-the-art TE alloys.

Figure 8:Temperature dependence of the power factor for all compositions.

These results show that Sb₆₅Se₃₅ is a good thermoelectric material that needs more attention. There are many ways to improve Sb₆₅Se₃₅’s power factor, such as alloying, nano structuring, and band modification, which could make PF even better. Moreover, the high performance of Sb₆₅Se₃₅ implies the discovery of new thermoelectric materials in systems with similar complex band structures.

Polycrystalline Sb₆₅Se₃₅ doped with Ge materials were synthesized by melt quenching method. There were no secondary phases detected in the pure rhombohedral Sb₂Se phase. All of the crystals exhibit microstructures of various shapes and sizes. The temperature dependence of electrical conductivity measurements shows that all compositions have metallic-like behavior. Electrons are the predominant charge carriers, according to Seebeck coefficient measurements. The Ge content significantly influences the electrical and TE properties of the system under investigation. For the Sb₆₅Ge₅Se₃₀ composition, the highest power factor value was 0.8 (mW m⁻¹ K⁻²) observed at T = 240K. All of these characteristics are critical in determining a material’s suitability for use in thermoelectric devices. Furthermore, the high performance of Sb₆₅Se₃₅ suggests the possibility of developing new thermoelectric materials with similar complex band structures.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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