Thin-Film Thermoelectric Modules for Power Generation Using Focused Solar Light

We demonstrated the fabrication of thin-film thermoelectric generators and evaluated their generation properties using solar light as a thermal source. Thin-film elements of Bi_0.5Sb_1.5Te₃ (p-type) and Bi₂Te_2.7Se_0.3 (n-type), which were patterned using the lift-off technique, were deposited on glass substrates using radiofrequency magnetron sputtering. After annealing at 300°C, the average Seebeck coefficients of p- and n-type films were 150???V/K and ?104???V/K, respectively, at 50°C to 75°C. A cylindrical lens was used to focus solar light to a line shape onto the hot side of the thin-film thermoelectric module with 15 p?Cn junctions. The minimum width of line-shaped solar light was 0.8?mm with solar concentration of 12.5 suns. We studied the properties of thermoelectric modules with different-sized p?Cn junctions on the hot side, and obtained maximum open voltage and power values of 140?mV and 0.7???W, respectively, for a module with 0.5-mm p?Cn junctions. The conversion efficiency was 8.75?×?10^?4%, which was approximately equal to the value estimated by the finite-element method.     

The Effect of Annealing in Controlled Vapor Pressure on the Thermoelectric Properties of RF-Sputtered Bi2Te3 Film

To investigate the effect of annealing in controlled atmosphere on the thermoelectric properties of Bi-Te film, Te-deficient Bi-Te film was deposited by sputtering, and then annealed with various Bi-Te alloy powders with different Te concentrations in a closed system at 250°C for 24?h. Bi-Te phases other than Bi₂Te₃ in the as-deposited film could be removed when the film was annealed with Bi-Te source powder containing 62?at.% or higher content of Te. At the same time, the values of Seebeck coefficient and carrier concentration of the films approach ?105???V/K and 3?×?10¹⁹?cm^?3 to 6?×?10¹⁹?cm^?3, respectively. This result indicates that mass transport of Te to the film takes place, resulting in the formation of Bi₂Te₃ phase and reduction of the amount of p-type carriers due to compositional change of the film from Te-deficient to stoichiometric. Annealing in controlled Te-vapor atmosphere is an effective method to improve the thermoelectric properties of Bi-Te film by changing the composition and phase of Te-deficient film to stoichiometric Bi₂Te₃ film.     

Nanostructure, Excitations, and Thermoelectric Properties of Bi2Te3-Based Nanomaterials

The effect of dimensionality and nanostructure on thermoelectric properties in Bi₂Te₃-based nanomaterials is summarized. Stoichiometric, single-crystalline Bi₂Te₃ nanowires were prepared by potential-pulsed electrochemical deposition in a nanostructured Al₂O₃ matrix, yielding transport in the basal plane. Polycrystalline, textured Sb₂Te₃ and Bi₂Te₃ thin films were grown at room temperature using molecular beam epitaxy and subsequently annealed at 250°C. Sb₂Te₃ films revealed low charge carrier density of 2.6?×?10¹⁹?cm^?3, large thermopower of 130???V?K^?1, and large charge carrier mobility of 402?cm²?V^?1?s^?1. Bi₂(Te_0.91Se_0.09)₃ and (Bi_0.26Sb_0.74)₂Te₃ nanostructured bulk samples were prepared from as-cast materials by ball milling and subsequent spark plasma sintering, yielding grain sizes of 50?nm and thermal diffusivities reduced by 60%. Structure, chemical composition, as well as electronic and phononic excitations were investigated by x-ray and electron diffraction, nuclear resonance scattering, and analytical energy-filtered transmission electron microscopy. Ab?initio calculations yielded point defect energies, excitation spectra, and band structure. Mechanisms limiting the thermoelectric figure of merit ZT for Bi₂Te₃ nanomaterials are discussed.     

Thermoelectric Properties of Bi2Te2Se Compensated by Native Defects and Sn Doping

In Bi₂Te₂Se the defect chemistry involves native defects that compete such that they can either exchange dominance or else significantly compensate each other. Here we show how the net carrier concentration, n ? p, which depends on the relative amounts of these defects and is readily obtained from Hall data, can be used as a fundamental materials parameter to describe the varied behavior of the thermoelectric properties as a function of compensation. We report the effects of tuning this parameter over multiple orders of magnitude by hole-doping the n-type material Bi₂Te₂Se_0.995, which is already significantly compensated because of its Se deficiency. Crystals with different levels of hole doping were achieved by two separate approaches, namely by selecting pieces from different locations in an undoped crystal in which a systematic carrier concentration gradient had been induced by its growth conditions, and alternatively by doping with Sn for Bi. The thermoelectric power factors for Bi_2?x Sn _x Te₂Se_0.995 for x = 0, 0.002, 0.005, 0.010, and 0.040 are reported, and the dependence of the transport properties on the extent of compensation is discussed.     

Sb2Te3 and Bi2Te3 Thin Films Grown by Room-Temperature MBE

Sb₂Te₃ and Bi₂Te₃ thin films were grown on SiO₂ and BaF₂ substrates at room temperature using molecular beam epitaxy. Metallic layers with thicknesses of 0.2?nm were alternately deposited at room temperature, and the films were subsequently annealed at 250°C for 2?h. x-Ray diffraction and energy-filtered transmission electron microscopy (TEM) combined with high-accuracy energy-dispersive x-ray spectrometry revealed stoichiometric films, grain sizes of less than 500?nm, and a texture. High-quality in-plane thermoelectric properties were obtained for Sb₂Te₃ films at room temperature, i.e., low charge carrier density (2.6?×?10¹⁹?cm^?3), large thermopower (130???V?K^?1), large charge carrier mobility (402?cm²?V^?1?s^?1), and resulting large power factor (29???W?cm^?1?K^?2). Bi₂Te₃ films also showed low charge carrier density (2.7?×?10¹⁹?cm^?3), moderate thermopower (?153???V?K^?1), but very low charge carrier mobility (80?cm²?V^?1?s^?1), yielding low power factor (8???W?cm^?1?K^?2). The low mobilities were attributed to Bi-rich grain boundary phases identified by analytical energy-filtered TEM.     

Fabrication of a Flexible Bismuth Telluride Power Generation Module Using Microporous Polyimide Films as Substrates

In this study, we investigated the effect of the structure of microporous p-type (Bi_0.4Te₃Sb_1.6) and n-type (Bi_2.0Te_2.7Se_0.3) BiTe-based thin films on their thermoelectric performance. High-aspect-ratio porous thin films with pore depth greater than 1 μm and pore diameter ranging from 300 nm to 500 nm were prepared by oxygen plasma etching of polyimide (PI) layers capped with a heat-resistant block copolymer, which acted as the template. The cross-plane thermal conductivities of the porous p- and n-type thin films were 0.4 W m^?1 K^?1 and 0.42 W m^?1 K^?1, respectively, and the dimensionless figures of merit, ZT, of the p- and n-type BiTe films were estimated as 1.0 and 1.0, respectively, at room temperature. A prototype thermoelectric module consisting of 20 pairs of p- and n-type strips over an area of 3 cm × 5 cm was fabricated on the porous PI substrate. This module produced an output power of 0.1 mW and an output voltage of 0.6 V for a temperature difference of 130°C. The output power of the submicrostructured module was 1.5 times greater than that of a module based on smooth BiTe-based thin films. Thus, the thermoelectric performance of the thin films was improved owing to their submicroscale structure.     

Synthesis of Bi0.5Sb1.5Te3 Thermoelectric Powder Using an Oxide-Reduction Process

The present study focused on synthesis of Bi_0.5Sb_1.5Te₃ thermoelectric powder using an oxide-reduction process. The phase structure and particle size of the synthesized powders were analyzed using x-ray diffractometry and scanning electron microscopy. The synthesized powder was sintered using the spark plasma sintering method. The thermoelectric properties of the sintered body were evaluated by measuring the Seebeck coefficient, electrical resistivity, and thermal conductivity. Bi_0.5Sb_1.5Te₃ powder was synthesized using a combination of mechanical milling, calcination, and reduction processes, using a mixture of Bi₂O₃, Sb₂O₃, and TeO₂ powders. The sintered body of the oxide-reduction-synthesized Bi_0.5Sb_1.5Te₃ powder showed p-type thermoelectric characteristics. The thermoelectric properties of the sintered bodies depended on the reduction time. After being reduced for 2 h at 663 K, the sintered body of the Bi_0.5Sb_1.5Te₃ powder showed a figure of merit of approximately 1.0 at room temperature.     

Preparation and Characterization of Bi0.4Sb1.6Te3 Bulk Thermoelectric Materials

This work focused on the preparation of p-type Bi_0.4Sb_1.6Te₃ bulk materials by combining mechanical alloying (MA) and hot extrusion, with emphasis on grain refinement and preferred grain orientation. Pure Bi, Sb, and Te powders were mechanically alloyed then hot extruded in the temperature range 360–450°C. Bi_0.4Sb_1.6Te₃ bulk materials were successfully prepared by MA and hot extrusion. All the samples had sound appearance, with single phases and high densities. The hot-extruded samples had small grain sizes, and the lower the extrusion temperature, the smaller the grain sizes. The results indicated that the extrudates had preferred orientation. The basal plane was predominantly oriented parallel to the direction of extrusion. Similar Seebeck coefficients were obtained when extrusion temperature was in the range 380–420°C. Electrical resistivity decreased with increasing extrusion temperature. Thermal conductivity was relatively low, even if the extrusion temperature was 450°C. As a result, a ZT value of 1.2 was obtained at room temperature for the sample extruded at 400°C. Therefore, combination of MA and hot extrusion results in significant improvement of both the thermoelectric and mechanical performance of Bi_0.4Sb_1.6Te₃ bulk materials.     

Interface Microstructure and Performance of Sb Contacts in Bismuth Telluride-Based Thermoelectric Elements

A thermoelectric joint composed of p-type Bi_0.5Sb_1.5Te₃ (BiSbTe) material and an antimony (Sb) interlayer was fabricated by spark plasma sintering. The reliability of the thermoelectric joints was investigated using electron probe microanalysis for samples with different accelerated isothermal aging time. After aging for 30 days at 300°C in vacuum, the thickness of the diffusion layer at the BiSbTe/Sb interface was about 30 μm, and Sb₂Te₃ was identified to be the major interfacial compound by element analysis. The contact resistivity was 3 × 10^?6 ohm cm² before aging and increased to 8.5 × 10^?6 ohm cm² after aging for 30 days at 300°C, an increase associated with the thickness of the interfacial compound. This contact resistivity is very small compared with that of samples with solder alloys as the interlayer. In addition, we have also investigated the interface behavior of Sb layers integrated with n-type Bi₂Se_0.3Te_2.7 (BiSeTe) material, and obtained similar results as for the p-type semiconductor. The present study suggests that Sb may be useful as a new interlayer material for bismuth telluride-based power generation devices.     

p-Type Bismuth Telluride-Based Composite Thermoelectric Materials Produced by Mechanical Alloying and Hot Extrusion

We produced six different composites of p-type bismuth antimony telluride alloys and studied their structure and thermoelectric properties. The components of the composites were obtained in powder form by mechanical alloying. Mixed powders of two different compositions were consolidated by hot extrusion to obtain each bulk composite. The minimum grain size of bulk composites as revealed by scanning electron microscopy shows a 50% reduction compared with the conventional (Bi_0.2Sb_0.8)₂Te₃. X-ray diffraction (XRD) analysis only shows peak broadening with no clear indication of separate phases, and indicates a systematic decrease of crystallite size in the composite materials. Scattering mechanisms of charge carriers were evaluated by Hall-effect measurements. The thermoelectric properties were investigated via the Harman method from 300 K up to 460 K. The composites show no significant degradation of the power factor and high peak ZT values ranging from 0.86 to 1.04. The thermal conductivity of the composites slightly increases with respect to the conventional alloy. This unexpected behavior can be attributed to two factors: (1) the composites do not yet contain a significant number of grains whose sizes are sufficiently small to increase phonon scattering, and (2) each of the combined components of the composites corresponds to a phase with thermal conductivity higher than the minimum value corresponding to the (Bi_0.2Sb_0.8)₂Te₃ alloy.     

Effect of Se Substitution on Structural and Electrical Transport Properties of Bi0.4Sb1.6Se3x Te3(1−x) Hexagonal Rods

In the current study, novel hexagonal rods based on Bi_0.4Sb_1.6Te₃ ingots dispersed with x amount of Se (x = 0.0, 0.2, 0.4, 0.6, 0.8, and 1.0) in the form Bi_0.4Sb_1.6Se_3x Te_3(1?x) were synthesized via a standard solid-state microwave route. The morphologies of these rods were explored using field-emission scanning electron microscopy (FESEM). The crystal structure of the powders was examined by x-ray diffraction (XRD) analysis, which showed that powders of the 0.0 ≤ x ≤ 0.8 samples could be indexed to the rhombohedral phase, whereas the sample with x = 1.0 had an orthorhombic phase structure. The influence of variations in the Se content on the thermoelectric properties was studied in the temperature range from 300 K to 523 K. Alloying of Se into Bi_0.4Sb_1.6Te₃ effectively caused a decrease in the hole concentration and, thus, a decrease in the electrical conductivity and an increase in the Seebeck coefficient. The maximal power factor measured in the present work was 7.47 mW/mK² at 373 K for the x = 0.8 sample.     

Composition-Dependent Thermoelectric Properties of n-Type Bi2Te2.7Se0.3 Doped with In4Se3

We present the effects of In₄Se₃ addition on thermoelectric properties of n-type Bi₂Te_2.7Se_0.3. In this study, polycrystalline (In₄Se₃) _x -(Bi₂Te_2.7Se_0.3)_1?x pellets were prepared by mechanical alloying followed by spark plasma sintering (SPS). The thermoelectric properties such as Seebeck coefficient and electrical and thermal conductivities were measured in the temperature range of 300 K to 500 K. Addition of In₄Se₃ into Bi₂Te_2.7Se_0.3 resulted in segregation of In₄Se₃ phase within Bi₂Te_2.7Se_0.3 matrix. The Seebeck coefficient of the (In₄Se₃) _x -(Bi₂Te_2.7Se_0.3)_1?x samples exhibited lower values compared with that of pure Bi₂Te_2.7Se_0.3 phase. This reduction of Seebeck coefficient in n-type (In₄Se₃) _x -(Bi₂Te_2.7Se_0.3)_1?x is attributed to the formation of unwanted p-type phases by interdiffusion through the interface between (In₄Se₃) _x and (Bi₂Te_2.7Se_0.3)_1?x as well as consequently formed Te-deficient matrix. However, the decrease in electrical resistivity and thermal conductivity with addition of In₄Se₃ leads to an enhanced thermoelectric figure of merit (ZT) at a temperature range over 450 K: a maximum ZT of 1.0 is achieved for the n-type (In₄Se₃)_0.03-(Bi₂Te_2.7Se_0.3)_0.97 sample at 500 K.     

Thermoelectric Properties of n-Type Bi2Te3/PbSe0.5Te0.5 Segmented Thermoelectric Material

To investigate the effects of segmentation of thermoelectric materials on performance levels, n-type segmented Bi₂Te₃/PbSe_0.5Te_0.5 thermoelectric material was fabricated, and its output power was measured and compared with those of Bi₂Te₃ and PbSe_0.5Te_0.5. The two materials were bonded by diffusion bonding with a diffusion layer that was ～18 μm thick. The electrical conductivity, Seebeck coefficient, and power factor of the segmented Bi₂Te₃/PbSe_0.5Te_0.5 sample were close to the average of the values for Bi₂Te₃ and PbSe_0.5Te_0.5. The output power of Bi₂Te₃ was higher than those of PbSe_0.5Te_0.5 and the segmented sample for small ΔT (300 K to 400 K and 300 K to 500 K), but that of the segmented sample was higher than those of Bi₂Te₃ and PbSe_0.5Te_0.5 when ΔT exceeded 300 K (300 K to 600 K and 300 K to 700 K). The output power of the segmented sample was about 15% and 73% higher than those of the Bi₂Te₃ and PbSe_0.5Te_0.5 samples, respectively, when ΔT was 400 K (300 K to 700 K). The efficiency of thermoelectric materials for large temperature differences can be enhanced by segmenting materials with high performance in different temperature ranges.     

Effect of Surface Preparation on Mechanical Properties of Ni Contacts on Polycrystalline (Bi1?x Sb x )2(Te1?y Se y )3 Alloys

We present a comparison of different surface preparation techniques for hot-extruded (Bi_1?x Sb _x )₂(Te_1?y Se _y )₃ alloys prior to Ni plating and their consequences on adhesion. The surface preparation is carried out by a relatively fast and well-controlled electrochemical etching procedure that provides morphologies with homogeneous flat surfaces on a scale of tens of micrometers. This procedure is not optimal for Ni metallization since it does not ensure enough roughness on the surface. Applying wet chemical etching with hypochlorite and nitric acid after the electrochemical etching changes the morphology, and the surface roughness increases, as evidenced by scanning electron microscopy and atomic force microscopy. Tensile tests carried out on n-type and p-type 1-mm-thick specimens, covered with a 12-??m Ni layer followed by Sn-Pb soldered joints, confirm a significant improvement in the ultimate adhesion strength, with fracture occurring within the thermoelectric material. Use of this two-step surface treatment, after slicing and prior to metallization, leads to maximum adhesion strength of around 38?MPa for either type of thermoelectric material. These results are consistent with the morphological changes observed by scanning electron microscopy on the surface after chemical etching.     

Thermopower Enhancement of Bi2Te3 Films by Doping I Ions

The thermoelectric properties of I-doped Bi₂Te₃ films grown by metal-organic chemical vapor deposition have been studied. I-doped epitaxial (00l) Bi₂Te₃ films were successfully grown on 4° tilted GaAs (001) substrates at 360 °C. I concentration in the Bi₂Te₃ films was easily controlled by the variation in a flow rate of H₂ carrier gas for the delivery of an isopropyliodide precursor. As I ions in the as-grown Bi₂Te₃ films were not fully activated, they did not influence the carrier concentration and thermoelectric properties. However, a post-annealing process at 400 °C activated I ions as a donor, accompanied with an increase in the carrier concentration. Interestingly, the I-doped Bi₂Te₃ films after the post-annealing process also exhibited enhancement of the Seebeck coefficient at the same electron concentration compared to un-doped Bi₂Te₃ films. Through doping I ions into Bi₂Te₃, the thermopower was also enhanced in Bi₂Te₃, and a high power factor of 5 × 10^?3 W K^?2 m^?1 was achieved.     

Thermoelectric Properties of Sintered n-Type and p-Type Tellurides

Characterization of powder-metallurgically manufactured (Bi _x Sb_1?x )₂(Te _y Se_1?y )₃ thermoelectric materials is presented. The manufacturing methods were spark plasma sintering (SPS) and hot isostatic pressing (HIP). x-Ray diffraction (XRD) and density measurements as well as transport characterization and scanning electron microscopy were performed on the materials. It is shown that both sintering techniques yield reasonable thermoelectric characteristics for p-type (x = 0.2, y = 1) as well as n-type (x = 0.95, y = 0.95) materials. Insight into the underlying reasons such as the scattering processes limiting the characteristics is gained by fitting experimental transport data using a theoretical model. The limitations and further optimization issues of our approach in thermoelectric material preparation are discussed.     

Thermoelectric Power Generation Characteristics of a Thin-Film Device Consisting of Electrodeposited n-Bi2Te3 and p-Sb2Te3 Thin-Film Legs

A thermoelectric thin-film device of the cross-plane configuration was fabricated by flip-chip bonding of the top electrodes to 242 pairs of electrodeposited n-type Bi₂Te₃ and p-type Sb₂Te₃ thin-film legs on the bottom substrate. The electrodeposited Bi₂Te₃ and Sb₂Te₃ films of 20-μm thickness exhibited Seebeck coefficients of ?59 μV/K and 485 μV/K, respectively. The internal resistance of the thin-film device was measured as 3.7 kΩ, most of which was attributed to the interfacial resistance of the flip-chip joints. The actual temperature difference ΔT _G working across the thin-film legs was estimated to be 10.4 times smaller than the apparent temperature difference ΔT applied across the thin-film device. The thin-film device exhibited an open-circuit voltage of 0.294 V and a maximum output power of 5.9 μW at an apparent temperature difference ΔT of 22.3 K applied across the thin-film device.     

Preparation and Thermoelectric Properties of Doped Bi2Te3-Bi2Se3 Solid Solutions

Since Bi₂Te₃ and Bi₂Se₃ have the same crystal structure, they form a homogeneous solid solution. Therefore, the thermal conductivity of the solid solution can be reduced by phonon scattering. The thermoelectric figure of merit can be improved by controlling the carrier concentration through doping. In this study, Bi₂Te_2.85Se_0.15:D _m (D: dopants such as I, Cu, Ag, Ni, Zn) solid solutions were prepared by encapsulated melting and hot pressing. All specimens exhibited n-type conduction in the measured temperature range (323 K to 523 K), and their electrical conductivities decreased slightly with increasing temperature. The undoped solid solution showed a carrier concentration of 7.37 × 10¹⁹ cm^?3, power factor of 2.1 mW m^?1 K^?1, and figure of merit of 0.56 at 323 K. The figure of merit (ZT) was improved due to the increased power factor by I, Cu, and Ag dopings, and maximum ZT values were obtained as 0.76 at 323 K for Bi₂Te_2.85Se_0.15:Cu_0.01 and 0.90 at 423 K for Bi₂Te_2.85Se_0.15:I_0.005. However, the thermoelectric properties of Ni- and Zn-doped solid solutions were not enhanced.     

Post-annealing Effect on Microstructures and Thermoelectric Properties of Bi0.45Sb1.55Te3 Thin Films Deposited by Co-sputtering

p-Type Bi_0.45Sb_1.55Te₃ thermoelectric (TE) thin films have been prepared at room temperature by a magnetron cosputtering process. The effect of postannealing on the microstructure and TE properties of Bi_0.45Sb_1.55Te₃ films has been investigated in the temperature range from room temperature to 350°C. x-Ray diffraction analysis shows that the annealed films have polycrystalline rhombohedral crystal structure, and the average grain size increases from 36?nm to 64?nm with increasing annealing temperature from room temperature to 350°C. Electron probe microanalysis shows that annealing above 250°C can cause Te reevaporation, which induces porous thin films and dramatically affects electrical transport properties of the thin films. TE properties of the films have been investigated at room temperature. The hole concentration shows a trend from descent to ascent and has a minimum value at the annealing temperature of 200°C, while the Seebeck coefficient shows an opposite trend and a maximum value of 245?μV?K^?1. The electrical resistivity monotonically decreases from 19.8?mΩ?cm to 1.4?mΩ?cm with increasing annealing temperature. Correspondingly, a maximum value of power factor, 27.4?μW?K^?2?cm^?1, was obtained at the annealing temperature of 250°C.