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Effects of Bi doping on the electrical and thermal transport properties of Cu2SnSe3
Abstract
In the study, we deal with the effects of Bi doping on the thermoelectric properties in a series of Cu2Sn1-xBixSe3 (x = 0, 0.02, 0.04, 0.06, 0.08, 0.10) compounds. The pristine and Bi-doped Cu2SnSe3 samples are synthesized by the solid-state sintering technique. Cubic structure with F4‾3m space group is maintained for all the samples. FESEM analysis indicated that the average grain size increases with an increase in Bi concentration. It is found that the characteristics of electrical resistivity changes from semiconducting in the case of the pristine sample to metallic behavior for the doped samples. The decrease in both electrical resistivity (ρ) and the Seebeck coefficient (S) with an increase in x is attributed to the increased hole concentration. The highest power factor (PF) of ~348 μW/mK² has been achieved for the x = 0.08 sample at 350 K, which is four times larger than that of the pristine sample. The thermal conductivity (κ) of the doped samples is observed to be higher than that of the pristine Cu2SnSe3, attributed to the increased grain size and electronic thermal conductivity. As a combined effect on the values of PF (= S²/ρ) and thermal conductivity, a maximum figure-of-merit (ZT) of ~0.027 for the x = 0.08 sample is attained at 350 K, about twice that of the pristine sample.
... No impurity peaks were detected in any of the samples within experimental limits. The prominent peaks for pristine located at 26.69 • , 44.59 • , 52.93 • , 65.10 • , and 71.93 • correspond respectively to (111), (220), (311), (400), and (331) planes as indicated by JCPDS card no. as #01-089-2879 [27]. To evaluate the structural properties of synthesized materials, Rietveld analysis was performed using FULLPROF software, and crystallographic information was obtained. ...
... With an increase in Y concentration above x = 0.02, lattice contraction occurs. The change in trend might be due to the off-stoichiometry induced by the increase in point defects such as Cu and/or Sn vacancies [27]. Substitution of Y 3+ is less likely to substitute Cu + in Cu 2 SnSe 3 because Cu and Y have different valence states (Cu + vs. Y 3+ ). ...
... The electrical resistivity of the pristine sample showed a reduction in the resistivity with an increase in the temperature up to 303 K, indicating the non-degenerate semiconducting nature of the sample. Above 303 K, a small positive temperature coefficient is observed due to the electronphonon interaction [27]. The addition of yttrium at the Sn site does not change the trend till the dopant concentration x = 0.04. ...
n the present study, we report the effect of Y doping at the Sn-site on structural, electrical, and low-temperature
(10–350 K) thermoelectric properties of the Cu2SnSe3 system. The bulk polycrystalline samples of Cu2Sn1-xYxSe3
(0 ≤ x ≤ 0.12) are synthesized by the conventional solid-state reaction route method. Rietveld analysis of room
temperature XRD data confirmed that the studied samples possess cubic crystal structure with F43m space group.
Temperature-dependent electrical resistivity of the samples with x ≤ 0.04 shows that they exhibit semi�conducting behavior; while samples with x > 0.04 exhibit metallic behavior. In the studied temperature range,
the Seebeck coefficient for all the samples is found to be positive, indicating that the majority of charge carriers
are holes. A significant reduction in the electrical resistivity leads to an enhancement in the power factor,
attaining a value of ~253 μW/mK2 for the sample with x = 0.12 at 350 K. At high temperatures, the thermal
conductivity of the samples decreases with temperature which is attributed to the phonon-phonon scattering. The
systematic evolution of thermoelectric properties with Y incorporation over the range (0 ≤ x ≤ 0.12) shows the
highest ZT of 0.024 at 350 K for the sample with x = 0.10, which is about six times higher than that of the
pristine Cu2SnSe3. The results demonstrate that Y-doped compounds Cu2Sn1− xYxSe3 are potential candidates for
thermoelectric applications.
... Cu-based ternary chalcogenides have gained attention lately due to their adjustable transport characteristics, making them attractive lead-free TE materials. Additionally, this category of materials offers the benefits of high elemental abundance and moderate toxicity, making them regarded as economical and environmentally beneficial options for TE applications [14]. ...
... Despite of these properties, Cu 2 SnSe 3 (CTSe) is tetrahedrally bonded semiconductor having a direct bandgap of about 1.0 eV [7]. In the CTSe crystal, Cu-Se bond network forms a three-dimensional framework for electrical conductivity, while the Sn atoms function as carriers to modify the carrier concentrations [12,14]. These materials are more accessible to electrical and other physical modification because of their phonon-glass electron crystal (PGEC) like behaviour [7]. ...
Cu2SnSe3 has drawn enough attention as a potential thermoelectric material due to its unique electronic and thermal properties. We present the impact of Sm doping on the Cu2SnSe3 system’s Sn site. The polycrystalline samples of Cu2Sn1-x Sm x Se3 (0 ≤ x ≤ 0.08) were prepared using the solid-state reaction technique followed by conventional sintering. The crystal structure was characterized using XRD and the results reveal that the samples have a diamond cubic structure with a space group of F4̄3m. Scanning Electron Microscopy (SEM) analysis indicates a uniform surface homogeneity within the sample. Furthermore, the introduction of Sm causes a reduction in porosity. The electrical transport characteristics were studied in the mid temperature range of 300–650 K. The Seebeck coefficient of all the samples were found to be positive within the temperature range under study, suggesting that holes constitute the majority charge carriers. This is also confirmed by the Hall measurements as the carrier concentration was positive for all the samples. The inclusion of Sm has led to a reduction of electrical resistivity and Seebeck coefficient and hence power factor of ~539 μW mK⁻² for x = 0.08 at 630 K which is ten times greater as compared to x = 0 whose power factor is ~56 μW mK⁻² at 630 K is achieved which makes it suitable for thermoelectric applications.
... The incorporation of Bi atoms may induce lattice expansion due to the substitution of the larger radius of Bi atoms (0.103 nm) at a site of smaller radius of Sn atoms (0.017 nm), as reported previously. 36,55 Figure 2b reveals no observable variations in the XRD pattern, indicating that the sintering process primarily caused the complete sublimation of hexamine and no impurities or distortion in the structure lattice was left behind. Overall, the XRD data confirm that no secondary phases were formed, resulting in the formation of pure Cu 2 SnSe 3 phase. ...
... This improvement in electrical conductivity of the BiI 3 samples can be attributed to the presence of trace amounts of Bi that persisted during the sublimation process, as discussed in XRD analysis. 36,55 These trace amounts of Bi act as a dopant, which increases the carrier concentration and mobility of porous-Cu 2 SnSe 3 compared with hexamine samples, which results in the enhancement of overall electrical conductivity. ...
... Thomas et al studied the effect of Bi doping in Cu 2 SnSe 3 compounds and observed a distinct knee at ∼50 K resulting from electronphonon interactions. This extra current contribution dominates in low-temperature regions as high energy phonon carriers scatter the low energy ones at high temperatures, causing the elimination of phonon drag effects [135]. The diffusive term of thermopower can be explained from Mott's formula based on the single parabolic model (equation (37)), whereas a term from phonon drag is given as: ...
The continuous depletion of fossil fuels and the increasing demand for eco-friendly and sustainable energy sources have prompted researchers to look for alternative energy sources. The loss of thermal energy in heat engines (100-350 ºC), coal-based thermal plants (150-700 ºC), heated water pumping in the geothermal process (150-700 ºC), and burning of petrol in the automobiles (150-250 ºC) in form of untapped waste-heat can be directly and/or reversibly converted into usable electricity by means of charge carriers (electrons or holes) as moving fluids using thermoelectric (TE) technology, which works based on typical Seebeck effect. The enhancement in TE conversion efficiency has been a key challenge because of the coupled relation between thermal and electrical transport of charge carriers in a given material. In this review, we have deliberated the physical concepts governing the materials to device performance as well as key challenges for enhancing the TE performance. Moreover, the role of crystal structure in the form of chemical bonding, crystal symmetry, order-disorder and phase transition on charge carrier transport in the material has been explored. Further, this review has also emphasized some insights on various approaches employed recently to improve the TE performance, such as, (i). carrier engineering via band engineering, low dimensional effects, and energy filtering effects and (ii). Phonon engineering via doping/alloying, nano-structuring, embedding secondary phases in the matrix and microstructural engineering. Wehave also briefed the importance of magnetic elements on thermoelectric properties of the selected materials and spin Seebeck effect. Furthermore, the design and fabrication of TE modules and their major challenges are also discussed. As, thermoelectric figure of merit, zT does not have any theoretical limitation, an ideal high performance thermoelectric device should consist of low-cost, eco-friendly, efficient, n- or p-type materials that operate at wide- temperature range and similar coefficients of thermal expansion, suitable contact materials, less electrical/ thermal losses and constant source of thermal energy. Overall, this review provides the recent physical concepts adopted and fabrication procedures of TE materials and device so as to improve the fundamental understanding and to develop a promising TE device.
... Secondary particles were often noticed on the surface of the sample of (Bi 0.98 In 0,02 ) 2 Se 2.7 Te 0 /BiTe 3 of 85%-15%, which is triggered by covering of the atomic layer of tellurium or selenium (Fig. 5c). The smooth surface of (Bi 0.98 In 0,02 ) 2 Se 2.7 Te 0 /BiTe 3 with 80%-20% shows the homogeneous distribution of the composites in the grains (Fig. 5d) [17,18]. ...
Polycrystalline composite samples of (Bi0.98In0.02)2Se2.7Te0.3/Bi2Te3 with different concentrations of Bi2Te3 such as 5%,10%,15% and 20% were prepared by the solid-state reaction technique. The X-Ray diffraction analysis has shown the hexagonal composite crystal structure with space group of R3¯m. Field emission scanning electronic microscope shows secondary particles on the surface of the samples. All the samples have shown the usual semi-conducting behaviour throughout the temperature range. It is observed that bismuth has been co-ordinated with 6 selenium atoms in (Bi0.98In0,02)2Se2.7Te0.3 compound and it has enormous selenium vacancies. The electrical resistivity represents the noteworthy result of the grain boundaries leading to the higher content of scattering centres in the polycrystalline composite samples. It is found that the electronegativity differences of In and Te, In and Se are less than Bi and Se, Bi and Te is the reason for the decrease in Seebeck coefficient in the compound containing 15% and 20% of Bi2Te3.
... Furthermore, these materials are considered as eco-friendly and cost-effective candidates for thermoelectric (TE) applications because of their low toxicity and high elemental abundance. The Cu-based thermoelectric materials, the diamond-like ternary selenide, Cu 2 SnSe 3 , belonging to the I 2 -IV-VI 3 Cu 2 SnSe 3 possesses the phonon-glass-electron-crystal (PGEC) behavior, the glass-like thermal conductivity [7]. ...
In this study, polycrystalline Cu 2 SnSe 3 (CTSe) has synthesized using the solvothermal method. The characterization done by powder X-Ray diffraction (XRD), Raman-spectroscopic and X-ray photoelectron spectroscopic (XPS) study are confirming the presence of Cu 2 SnSe 3 nanocrystals. The crystalline structure and crystallite size are investigated through an X-ray diffractometer (XRD). The XRD results show that the cubic structure with the crystallite size was found to be 37 nm. Scanning Electron Microscope (SEM) image proved the agglomerated nanosheet-like structure of polycrystalline Cu2SnSe3. Finally, the thermoelectric properties of the Cu 2 SnSe 3 nanocrystals are investigated at different temperatures in addition calculated physical and transport parameters tabulated for deep understanding of the prepared material.
The crystallization of a local p-type ZnSb material can be deterministically tuned by doping with the n-type topological insulator Bi2Te3. Upon annealing, pure ZnSb shows a transition across three states, namely, high resistance, intermediate resistance, and low resistance. The metastable intermediate ZnSb phase has p-type conduction. Here, we find that the metastable ZnSb phase of the pure ZnSb film is significantly suppressed by Bi2Te3 doping, and the underlying conduction is changed into n-type conduction because Bi2Te3 doping induces a change in crystallization and simultaneously changes the type of crystalline phases. Nanoprecipitate formation and crystallization transition are the driving forces of the composition-dependent p–n conduction conversion. The X-ray diffraction, X-ray photoelectron and Raman spectroscopy results of the crystalline thin films suggest that the local bonding arrangement around Sb atoms changes. The Bi–Sb topological insulators are thus mainly responsible for the p–n transition and ultralow resistance shift shown by ZnSb–Bi2Te3 alloys.
New thermoelectric compounds Cu2Sn1−xFexSe3 (0 ≤ x ≤ 0.2) have been investigated, and the results indicate that the electrical conductivity of Cu2Sn1−xFexSe3(x > 0) is obviously enhanced by Fe doping due to the increased carriers concentration, and then the significant improved power factor is obtained. As a result, the figure of merit reaches 1.1 at ~820 K for Fe-doped compounds Cu2Sn1−xFexSe3 with x = 0.05 and 0.1 due to the higher power factor and moderate thermal conductivity, which is about 2.4 times that of pure Cu2SnSe3 at the same temperature. Our results indicate that Fe-doped compounds Cu2Sn1−xFexSe3 can be hoped to utilize as a potential environmental thermoelectric materials.
ᅟ: We report on the successful preparation of Bi-doped n-type polycrystalline SnSe by hot-press method. We observed anisotropic transport properties due to the (h00) preferred orientation of grains along the pressing direction. The electrical conductivity perpendicular to the pressing direction is higher than that parallel to the pressing direction, 12.85 and 6.46 S cm-1 at 773 K for SnSe:Bi 8% sample, respectively, while thermal conductivity perpendicular to the pressing direction is higher than that parallel to the pressing direction, 0.81 and 0.60 W m-1 K-1 at 773 K for SnSe:Bi 8% sample, respectively. We observed a bipolar conducting mechanism in our samples leading to n- to p-type transition, whose transition temperature increases with Bi concentration. Our work addressed a possibility to dope polycrystalline SnSe by a hot-pressing process, which may be applied to module applications.
Highlights:
1. We have successfully achieved Bi-doped n-type polycrystalline SnSe by the hot-press method. 2. We observed anisotropic transport properties due to the [h00] preferred orientation of grains along pressing direction. 3. We observed a bipolar conducting mechanism in our samples leading to n- to p-type transition.
In this study, we report low and mid temperature range thermoelectric properties of Sb-substituted Cu2SnSe3 compounds. The Cu2Sn1−xSbxSe3 (0 ≤ x ≤ 0.04) alloys were prepared using conventional solid-state reaction followed by spark plasma sintering. The crystal structure was characterized using XRD and it reveals that all the samples exhibit cubic structure with space group . The electrical transport characteristics indicate degenerate semiconducting behavior. Electrical resistivity was found to follow small polaron hopping (SPH) model in the entire temperature range of investigation. The Seebeck coefficient data reveals that the majority of charge carriers are holes and the analysis of Seebeck coefficient data gives negative values of Fermi energy indicating that the Fermi energy is below the edge of valence band. The electronic contribution (κe) for total thermal conductivity is found to be less than 1%. The maximum ZT value of 0.64 is observed for the sample with x = 0.03 (at 700 K) which is approximately 2.3 times that of the pristine sample.
The main ideas in the theory of thermoelectrics are discussed. We discuss power generation, thermoelectric cooling, transport theory, the Seebeck coefficient, and phonon drag.
In analyzing zT improvements due to lattice thermal conductivity (κL ) reduction, electrical conductivity (σ) and total thermal conductivity (κTotal ) are often used to estimate the electronic component of the thermal conductivity (κE ) and in turn κL from κL = ∼ κTotal − LσT. The Wiedemann-Franz law, κE = LσT, where L is Lorenz number, is widely used to estimate κE from σ measurements. It is a common practice to treat L as a universal factor with 2.44 × 10−8 WΩK−2 (degenerate limit). However, significant deviations from the degenerate limit (approximately 40% or more for Kane bands) are known to occur for non-degenerate semiconductors where L converges to 1.5 × 10−8 WΩK−2 for acoustic phonon scattering. The decrease in L is correlated with an increase in thermopower (absolute value of Seebeck coefficient (S)). Thus, a first order correction to the degenerate limit of L can be based on the measured thermopower, |S|, independent of temperature or doping. We propose the equation: L = 1 . 5 + exp − | S | 116 (where L is in 10−8 WΩK−2 and S in μV/K) as a satisfactory approximation for L. This equation is accurate within 5% for single parabolic band/acoustic phonon scattering assumption and within 20% for PbSe, PbS, PbTe, Si0.8 Ge 0.2 where more complexity is introduced, such as non-parabolic Kane bands, multiple bands, and/or alternate scattering mechanisms. The use of this equation for L rather than a constant value (when detailed band structure and scattering mechanism is not known) will significantly improve the estimation of lattice thermal conductivity.
Thermoelectric properties of alkaline-earth-metal disilicides are strongly dependent on their electronic band structure in the vicinity of the Fermi level. In particular, the strontium disilicide, SrSi2 with a narrow band gap of about few tens of meV is composed of non-toxic, naturally abundant elements, and its thermoelectric properties are very sensitive to the substitution/alloying with third elements. In this article, we summarize the thermoelectric performance of substituted and Sr-deficient/Sr-rich SrSi2 alloys to realize the high thermoelectric figure-of-merit (ZT) for practical applications in the electronic and thermoelectric aspects, and also to explore the alternative routes to further improve its ZT value.
The field of thermoelectrics has long been recognized as a potentially transformative power generation technology and the field is now growing steadily due to their ability to convert heat directly into electricity and to develop cost-effective, pollution-free forms of energy conversion. Of various types of thermoelectric materials, nanostructured materials have shown the most promise for commercial use because of their extraordinary thermoelectric performances. This article aims to summarize the present progress of nanostructured thermoelectrics and intends to understand and explain the underpinnings of the innovative breakthroughs in the last decade or so. We believed that recent achievements will augur the possibility for thermoelectric power generation and cooling, and discuss several future directions which could lead to new exciting next generation of nanostructured thermoelectrics.
X-ray powder diffraction by p-type Cu2SnSe3, prepared by the vertical Bridgman–Stockbarger technique, shows that this material crystallizes in a monoclinic structure, space group Cc, with unit cell parameters a=6.5936(1) Å, b=12.1593(4) Å, c=6.6084(3) Å, and β=108.56(2)°. The temperature variation of the hole concentration p obtained from the Hall effect and electrical resistivity measurements from about 160 to 300 K, is explained as due to the thermal activation of an acceptor level with an ionization energy of 0.067 eV, whereas below 100 K, the conduction in the impurity band dominates the electrical transport process. From the analysis of the p vs T data, the density-of-states effective mass of the holes is estimated to be nearly of the same magnitude as the free electron mass. In the valence band, the temperature variation of the hole mobility is analyzed by taking into account the scattering of charge carriers by ionized and neutral impurities, and acoustic phonons. In the impurity band, the mobility is explained as due to the thermally activated hopping transport. From the analysis of the optical absorption spectra at room temperature, the fundamental energy gap was determined to be 0.843 eV. The photoconductivity spectra show the presence of a narrow band gap whose main peak is observed at 0.771 eV. This band is attributed to a free-to-bound transition from the defect acceptor level to the conduction band. The origin of this acceptor state, consistent with the chemical composition of the samples and screening effects, is tentatively attributed to selenium interstitials. © 2001 American Institute of Physics.
▪ Abstract Recently there has been a resurgence of research efforts related to the investigation of new and novel materials for small-scale thermoelectric refrigeration and power generation applications. These materials need to couple and optimize a variety of properties in order to exhibit the necessary figure of merit, i.e. the numerical expression that is commonly used to compare one potential thermoelectric material with another. The figure of merit is related to the coefficient of performance or efficiency of a particular device made from a material. The best thermoelectric material should possess thermal properties similar to that of a glass and electrical properties similar to that of a perfect single-crystal material, i.e. a poor thermal conductor and a good electrical conductor. Skutterudites are materials that appear to have the potential to fulfill such criteria. These materials exhibit many types of interesting properties. For example, skutterudites are members of a family of compounds we call...
Thermoelectric materials, which can generate electricity from waste heat or be used as solid-state Peltier coolers, could play an important role in a global sustainable energy solution. Such a development is contingent on identifying materials with higher thermoelectric efficiency than available at present, which is a challenge owing to the conflicting combination of material traits that are required. Nevertheless, because of modern synthesis and characterization techniques, particularly for nanoscale materials, a new era of complex thermoelectric materials is approaching. We review recent advances in the field, highlighting the strategies used to improve the thermopower and reduce the thermal conductivity.
To investigate the thermoelectric properties, we have attempted to tune selenium content in Cu2SnSe3-δ (−0.04 ≤ δ ≤ 0.1) compounds synthesized via the conventional solid-state reaction. Electrical and thermal properties are presented in the low and near room temperature regime (10–350 K). The lowest electrical resistivity is observed for the compound with δ = 0.08. The electron-phonon coupling and the presence of small polarons are experimentally validated from temperature-dependent electrical and thermal transport data. A decrease in the Seebeck coefficient along with carrier concentration is observed, verifying the linear electron band dispersion. A downward shift of the Fermi level deep into the valence band is realized in the case of non-stoichiometric samples, thereby reducing the Seebeck coefficient. A maximum power factor of ~50 μWmK⁻² is achieved for the sample with δ = 0.04, which is about twice that of the pristine compound. The highest ZT of ~0.01 is obtained for the compound with δ = 0.08. The observed results suggest that stoichiometry plays a vital role in tuning the thermoelectric properties in the Cu2SnSe3 compound.
Thermoelectric properties of BiCuSeO have been finely tuned previously by doping cations on bismuth sites. Here thermoelectric properties of BiCuSeO with isoelectronic indium doping were investigated by experimental methods detailedly. We found that electrical conductivity was remarkably enhanced by isoelectronic indium doping in BiCuSeO system, which resulted from about sixfold increase of carrier mobility from 5.6 cm² v⁻¹ s⁻¹ to 31 cm² v⁻¹ s⁻¹ at room temperature. Furthermore, we obtained a maximum power factor up to 4.6 μW cm⁻¹ K⁻² in isoelectronic indium doping BiCuSeO at 800 K. Finally, the dimensionless thermoelectric figure of merit reached ∼ 0.6 at 800 K in Bi0.925In0.075CuSeO.
Cu-based chalcogenides have received increasing attention as promising thermoelectric materials due to their high efficiency, tunable transport properties, high elemental abundance and low toxicity. In this review, we summarize the recent research progress on this large family compounds covering diamond-like chalcogenides and liquid-like Cu2X (X=S, Se, Te) binary compounds as well as their multinary derivatives. These materials have the general features of two sublattices to decouple electron and phonon transport properties. On the one hand, the complex crystal structure and the disordered or even liquid-like sublattice bring about an intrinsically low lattice thermal conductivity. On the other hand, the rigid sublattice constitutes the charge-transport network, maintaining a decent electrical performance. For specific material systems, we demonstrate their unique structural features and outline the structure-performance correlation. Various design strategies including doping, alloying, band engineering and nanostructure architecture, covering nearly all the material scale, are also presented. Finally, the potential of the application of Cu-based chalcogenides as high-performance thermoelectric materials is briefly discussed from material design to device development.
Thermoelectric energy conversion system has great appeal in term of its silence, simplicity and reliability as compared with traditional power generator and refrigerator. The past two decades witnessed a significantly increased academic activities and industrial interests in thermoelectric materials. One of the most important impetuses for this boost is the concept of “nano”, which could trace back to the pioneer works of Mildred S. Dresselhaus at 1990s. Although the pioneer passed away, the story about the nano thermoelectric materials is still continuous. In this perspective, we will review the main mile stones along the concept of thermoelectric nanocomposites, and then discuss some new trends, strategies and opportunities.
A series of Cu2Sn1-xPbxSe3 (0 ≤ x ≤ 0.04) compounds was prepared by solid state synthesis technique. The electrical resistivity (ρ) decreased with increase in Pb content up to x = 0.01, thereafter it increased with further increase in x (till x =0.03). However, the lowest value of electrical resistivity is observed for Cu2Sn0.96Pb0.04Se3. Analysis of electrical resistivity of all the samples suggests that small poloron hoping model is operative in the high temperature regime while variable range hopping is effective in the low temperature regime. The positive Seebeck coefficient (S) for pristine and doped samples in the entire temperature range indicates that the majority charge carriers are holes. The electronic thermal conductivity (κe) of the Cu2Sn1-xPbxSe3 compounds was estimated by the Wiedemann-Franz law and found that the contribution from κe is less than 1% of the total thermal conductivity (κ). The highest ZT ~ 0.013 was achieved at 400 K for the sample Cu2Sn0.98Pb0.02Se3, about 30% enhancement as compared to the pristine sample.
Thermoelectric materials are crucial in renewable energy conversion technologies to solve the global energy crisis. They have been proven to be suitable for high-end technological applications such as missiles and spacecraft. The thermoelectric performance of devices depends primarily on the type of materials used and their properties such as their Seebeck coefficient, electrical conductivity, thermal conductivity, and thermal stability. Classic inorganic materials have become important due to their enhanced thermoelectric responses compared with organic materials. In this review, we focus on the physical and chemical properties of various thermoelectric materials. Newly emerging materials such as carbon nanomaterials, electronically conducting polymers, and their nanocomposites are also briefly discussed. Strategies for improving the thermoelectric performance of materials are proposed, along with an insight into semiconductor physics. Approaches such as nanostructuring, nanocomposites, and doping are found to enhance thermoelectric responses by simultaneously tuning various properties within a material. A recent trend in thermoelectric research shows that high-performance thermoelectric materials such as inorganic materials and carbon nanomaterials/electronically conducting polymer nanocomposites may be suitable for power generation and energy sustainability in the near future.
The improvement of the thermoelectric performance of semiconducting chalcogenides is an important task. The principal challenge in this field is to tailor the thermoelectric properties, such as electrical conductivity, Seebeck coefficient, and thermal conductivity, to improve the figure of merit (ZT). Herein, we present an improvement in the thermoelectric properties of In-III-doped p-type Cu3SbSe4 thin films deposited by using a microwave-assisted technique. The deposited thin films were characterized for their optical, structural, morphological, compositional, and electrical transport properties to show the applicability of the materials in thermoelectric energy conversion. The improvement in ZT is achieved with In-III doping because of the enhancement in the charge carrier density of Cu3SbSe4 thin films. A maximum ZT of 0.183 was achieved for Cu-3(Sb0.94In0.06)Se-4 at 300K. Our results show that this deposition strategy and doping process can provide an effective solution to fabricate Cu3SbSe4-based materials for thermoelectric applications.
In this review, we discuss some of the representative strategies of phonon engineering by categorizing them into the methods affecting each component of phonon thermal conductivity, i.e., specific heat, phonon group velocity, and mean free path. In terms of specific heat, a large unit cell is beneficial in that it can minimize the fraction of thermal energy that can be transported since most of the energy is stored in the optical branches. In an artificial structure such as the superlattice, phonon bandgaps can be created through constructive interference by Bragg reflection, which reduces phonon group velocity. We further categorize the mean free path, i.e., scattering processes, into grain boundary scattering, impurity scattering, and phonon–phonon scattering. Rough-surfaced grains, nano-sized grains, and coated grains are discussed for enhancement of the grain boundary scattering. Alloy atoms, vacancies, nanoparticles, and nano-sized holes are treated as impurities, which limit the phonon mean free path. Lone pair electrons and acoustical-to-optical scattering are suggested for manipulating phonon–phonon scattering. We also briefly mention the limitation and temperature range in which the Wiedemann–Franz law is valid in order to achieve a better estimation of electronic thermal conductivity. This paper provides an organized view of phonon engineering so that this concept can be implemented synergistically with power factor enhancement approaches for design of thermoelectric materials.
Thermoelectric materials can provide sources of clean energy and increase the efficiency of existing processes. Solar energy, waste heat recovery, and climate control are examples of applications that could benefit from the direct conversion between thermal and electrical energy provided by a thermoelectric device. The widespread use of thermoelectric devices has been prevented by their lack of efficiency, and thus the search for high-efficiency thermoelectric materials is ongoing. Here we describe our initial efforts studying copper-containing ternary compounds for use as high-efficiency thermoelectric materials that could provide low-cost alternatives to their silver-containing counterparts. The compounds of interest are semiconductors that crystallize in structures that are variants of binary zincblende structure compounds. Two examples are the compounds Cu2SnSe3 and Cu3SbSe4, for which we present here preliminary thermoelectric characterization data.
Zn doped ternary compounds Cu2ZnxSn1-xSe3 (x = 0, 0.025, 0.05, 0.075) were prepared by solid state synthesis. The undoped compound showed a monoclinic crystal structure as a major phase, while the doped compounds showed a cubic crystal structure confirmed by powder XRD (X-Ray Diffraction). The surface morphology and elemental composition analysis for all the samples were studied by SEM (Scanning Electron Microscopy) and EPMA (Electron Probe Micro Analyzer), respectively. SEM micrographs of the hot pressed samples showed the presence of continuous and homogeneous grains confirming sufficient densification. Elemental composition of all the samples revealed an off-stoichiometry, which was determined by EPMA. Transport properties were measured between 324 K and 773 K. The electrical resistivity decreased up to the samples with Zn content x = 0.05 in Cu2ZnxSn1-xSe3, and slightly increased in the sample Cu2Zn0.075Sn0.925Se3. This behavior is consistent with the changes in the carrier concentration confirmed by room temperature Hall coefficient data. Temperature dependent electrical resistivity of all samples showed heavily doped semiconductor behavior. All the samples exhibit positive Seebeck coefficient (S) and Hall coefficient indicating that the majority of the carriers are holes. A linear increase in Seebeck coefficient with increase in temperature indicates the degenerate semiconductor behavior. The total thermal conductivity of the doped samples increased with a higher amount of doping, due to the increase in the carrier contribution. The total and lattice thermal conductivity of all samples showed 1/1 dependence, which points toward the dominance of phonon scattering at high temperatures. The maximum 1/TZF = 0.48 at 773 K was obtained for the sample Cu2SnSe3 due to a low thermal conductivity compared to the doped samples.
Cu-based ternary chalcogenide semiconductors are a promising class of p-type thermoelectric materials. Here we show that S for Se substitution is a promising route for improving the thermoelectric performance of these compounds, in particular for Cu3SbSe4 and Cu2SnSe3. Phonon scattering due to the combined effects of the atomic mass and size difference between S and Se produces a significant reduction in lattice thermal conductivity over a wide temperature range, and limits the phonon mean free path to its minimum possible value at high temperature. The small electronegativity difference between S and Se is ideal for conserving carrier mobility, as demonstrated by the reasonably large hole mobility in the disordered Cu3Sb(Se1−x
Sx
)4 compounds. These effects along with the low-cost, environmentally benign nature of S, make S for Se substitution a simple yet effective method of improving the thermoelectric performance of Cu-based ternary selenides.
We report the thermoelectric properties of Mn-doped Cu2Mnx
Sn1−x
Se3 compound, with x ranging from 0.005 to 0.1 at temperature ranging from 80 K to 723 K. All samples maintain cubic zincblende-like structure, and no impurity phase was detected. The electrical resistivity decreases rapidly when Mn4+ replaces Sn2+ in the matrix. The excess Mn impurities in the x = 0.05 and x = 0.1 samples also affect the Seebeck coefficient. The total thermal conductivity is increased for Mn-doped samples except for the x = 0.005 sample. In all, both power factor and figure of merit are improved by Mn doping over the entire temperature range. The ZT value of the x = 0.02 sample reaches 0.035 at 300 K, and for x = 0.01 reaches 0.41 at 716 K, which are comparable to the best thermoelectric performance for ternary Cu-based compounds.
P-type compounds Cu2GaxSn1 − xSe3 (x = 0.025, 0.05, 0.075) with a diamond-like structure were consolidated using hot pressing sintering (HP) and spark plasma sintering (SPS) techniques. High-temperature thermoelectric properties as well as low-temperature Hall data are reported. Microstructural analysis shows that the distribution of Ga is homogeneous in the samples sintered by HP but inhomogeneous in the samples sintered by SPS, even with an electrically isolating and thermally conducting BN layer during the sintering. The Seebeck coefficients of the samples sintered by HP and SPS show similar dependence on the carrier concentration and are insensitive to the composition inhomogeneity. In contrast, the composition inhomogeneity results in lower carrier mobility and thus lower electrical conductivity in the samples sintered by SPS than those sintered by HP. Lattice thermal conductivity is further reduced through Ga doping; however, this effect is weakened by the inhomogeneous distribution of Ga. Due to their larger carrier mobility and lower lattice thermal conductivity, the samples sintered by HP exhibit 15–35% higher thermoelectric figure of merits (ZT) than those SPS samples with a high Ga doping level and without the coated BN layer, in which the composition homogeneity is worse. A ZT value of 0.43 is obtained for the HP Cu2Ga0.075Sn0.925Se3 sample at 700 K.
Cu2SnSe3 nanocrystals with a metastable zincblende and wurtzite structure have been successfully synthesized. The crystal structures were confirmed by means of X-ray diffraction and selected area electron diffraction. The lattice mismatch between Cu2SnSe3 and ZnSe is only 0.26%, thus homogeneously alloyed (ZnSe)x(Cu2SnSe3)1−x nanocrystals could be synthesized. The band gaps of nanocrystals can be tuned in a wide range of 2.75 to 0.92 eV; however the bowing parameter as high as 19.1 eV for alloyed (ZnSe)x(Cu2SnSe3)1−x nanocrystals was observed. These low cost and dispersible alloyed (ZnSe)x(Cu2SnSe3)1−x nanocrystals with a targeted band gap have a high potential in thin film solar cells.
The p-type Cu2SnX3 (X = Se, S) compounds are known experimentally to be good thermoelectric materials, although the reasons for this good performance in an adamantine-derived crystal structure are not well understood. Here, we demonstrate the existence of a three-dimensional (3D) hole conductive network in these ternary diamondlike Cu2SnX3 (X = Se, S) semiconductors using ab initio calculations, and identify the features of the electronic structure responsible for this good performance. We also provide results as a function of doping level to find the regime where the highest performance will be realized and estimate the maximum figure of merit, ZT. Our results clearly show that the strong hybridization between 3d orbitals from copper and p orbitals from selenium or sulfur at the upper valence band leads to the 3D p-type hole transport channel, mainly consisting of Cu-X and X-X networks in Cu2SnX3 (X = Se, S). The resulting heavy, but still conductive, hybridized bands of Cu d–chalcogen p character are highly favorable for thermoelectric performance. The electrical transport properties of these p-type materials are mainly determined by these bands and have been investigated by Boltzmann transport methods. The optimal doping levels of Cu2SnX3 are estimated to be around 0.1 holes per unit cell at 700 K. The theoretical figure of merit ZT has been predicted.
The usual theory of thermoelectric power fails to account for the marked rise in this quantity which has been found recently for some semiconductors as the temperature is lowered below room temperature. This paper develops the recently suggested explanation that the thermoelectric power Q is the sum of the usual electronic term Qe, resulting from the spontaneous tendency of the charge carriers to diffuse from hot to cold, and a phonon term Qp, resulting from the drag on the charge carriers exerted by the phonons streaming from hot to cold in thermal conduction. An equivalent description of the term Qp can be given in terms of the contribution to the Peltier heat flux in an isothermal specimen, due to phonons dragged along by the electric current. As a prelude to the discussion of Qp, the existing theory of Qe is first subjected to some refinements required by recent developments in semiconductor theory. The theory of Qp is then formulated in a simple but general way by making use of an approximate proportionality between heat current and crystal momentum in the phonon system. Using recently-derived results on the probability that a very low-frequency phonon will be scattered by other phonons, an explicit expression for Qp(T) is derived, which should be valid in the range of moderately low temperatures and low carrier concentrations. At lower temperatures, but still far above the range where the thermal conductivity is appreciably size-dependent, Qp is dominated by the scattering of phonons from the boundaries of the specimen; the theory of this effect is worked out in detail. Although Qp is independent of carrier concentration when the latter is low, a considerable decrease is predicted at high carrier concentrations, or at very low temperatures, because of a saturation effect. The effect of Fermi degeneracy on all these phenomena is discussed. Available data on p germanium show all these effects and can be fitted by the theory. The comparison indicates that a large proportion of the low-temperature lattice scattering of holes in p germanium is by shear modes. Although n germanium seems less suited for quantitative comparison, it, also, shows all the predicted effects.
The thermoelectric properties of the p-type (Bi0.25Sb0.75)2Te3 doped with 8 wt. % excess Te and the n-type Bi2(Te0.94Se0.06)3 doped substantially with 0.07 wt. % I, 0.02 wt. % Te, and 0.03 wt. % CuBr which were grown by the Bridgman method at a rate of 6 cm/h were measured before and after annealing, where annealing was done in the temperature range from 473 up to 673 K for 2–5 h in a vacuum and a hydrogen stream. No annealing effect on the power factor was observed for the p-type specimen, but the as-grown p-type specimen exhibited a large power factor of 5.53×10−3 W/mK2 at 308 K and a low thermal conductivity of 1.21 W/mK. When the n-type specimen was annealed at 473 K for 2 h in a hydrogen stream, however, the power factor at 308 K increased significantly from 3.26×10−3 to 4.73×10−3 K−1 but its thermal conductivity then increased by about 3% from 1.26 to 1.30 W/mK. As a result, the maximum thermoelectric figure of merits Z at 308 K for the as-grown p- and annealed n-type specimens reached surprisingly great values of 4.57×10−3 K−1 and 3.67×10−3 K−1, respectively, with corresponding ZT values of 1.41 and 1.13. The present materials are sure to belong to the highest class in the Z and ZT values as bismuth telluride bulk compounds. © 2003 American Institute of Physics.
Efficient solid state energy conversion based on the Peltier effect for cooling and the Seebeck effect for power generation calls for materials with high electrical conductivity σ, high Seebeck coefficient S, and low thermal conductivity k. Identifying materials with a high thermoelectric figure of merit Z(= S
2σ/k) has proven to be an extremely challenging task. After 30 years of slow progress, thermoelectric materials research experienced a resurgence, inspired by the developments of new concepts and theories to engineer electron and phonon transport in both nanostructures and bulk materials. This review provides a critical summary of some recent developments of new concepts and new materials. In nanostructures, quantum and classical size effects provide opportunities to tailor the electron and phonon transport through structural engineering. Quantum wells, superlattices, quantum wires, and quantum dots have been employed to change the band structure, energy levels, and density of states of electrons, and have led to improved energy conversion capability of charged carriers compared to those of their bulk counterparts. Interface reflection and the scattering of phonons in these nanostructures have been utilised to reduce the heat conduction loss. Increases in the thermoelectric figure of merit based on size effects for either electrons or phonons have been demonstrated. In bulk materials, new synthetic routes have led to engineered complex crystal structures with the desired phonon-glass electron-crystal behaviour. Recent studies on new materials have shown that dimensionless figure of merit (Z ×temperature) values close to 1·5 could be obtained at elevated temperatures. These results have led to intensified scientific efforts to identify, design, engineer and characterise novel materials with a high potential for achieving ZT much greater than 1 near room temperature.
The ternary diamond-like compound Cu2SnSe3 was studied, which displays the primary bond network in the material in real space. Although the compositions or structures of the compounds among copper-containing diamond like family, especially for the multication contained systems, are very complicated, the underlying relationship of structure/function was observed to be similar. The partial carrier density distribution in CZTS also shows a similar Cu-Se charge accumulation, whereas Zn and Sn atoms have little contribution to the partial charge density close to valence band maximum but donate electrons to the whole system. The electrical conductivity is greatly enhanced after In/Sn substitution, changing from 12600 S m-1 to 93200 S m-1 at room temperature, which results in the improvement of power factor within the whole temperature range. The matrix and the doped compounds exhibit extremely low thermal conductivity, especially at high temperature, as well as tunable electrical property.