Fabrication of Bismuth Telluride-Based Alloy Thin Film Thermoelectric Devices Grown by Metal Organic Chemical Vapor Deposition

Bismuth–antimony–telluride based thin film materials were grown by metal organic vapor phase deposition (MOCVD). A planar-type thermoelectric device was fabricated with p-type Bi_0.4Sb_1.6Te₃ and n-type Bi₂Te₃ thin films. The generator consisted of 20 pairs of p-type and n-type legs. We demonstrated complex structures of different conduction types of thermoelectric elements on the same substrate using two separate deposition runs of p-type and n-type thermoelectric materials. To demonstrate power generation, we heated one side of the sample with a heating block and measured the voltage output. An estimated power of 1.3 μW was obtained for the temperature difference of 45 K. We provide a promising procedure for fabricating thin film thermoelectric generators by using MOCVD grown thermoelectric materials that may have a nanostructure with high thermoelectric properties.     

Printed Se-Doped MA n-Type Bi2Te3 Thick-Film Thermoelectric Generators

In this work, we highlight new materials processing developments and fabrication techniques for dispenser-printed thick-film single-element thermoelectric generators (TEG). Printed deposition techniques allow for low-cost and scalable manufacturing of microscale energy devices. This work focuses on synthesis of unique composite thermoelectric systems optimized for low-temperature applications. We also demonstrate device fabrication techniques for high-density arrays of high-aspect-ratio planar single-element TEGs. Mechanical alloyed (MA) n-type Bi₂Te₃ powders were prepared by taking pure elemental Bi and Te in 36:64 molar ratio and using Se as an additive. X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques were used to characterize the as-milled powders to confirm the Bi₂Te₃ phase formation and particle size below 50???m. Thermoelectric properties of the composites were measured from room temperature to 100°C. We achieved a dimensionless figure of merit (ZT) of 0.17 at 300?K for MA n-type Bi₂Te₃?Cepoxy composites with 2?wt.% Se additive. A 20 single-leg TEG prototype with 5?mm?×?400???m?×?120???m printed element dimensions was fabricated on a polyimide substrate with evaporated gold contacts. The prototype device produced a power output of 1.6???W at 40???A and 40?mV voltage for a temperature difference of 20°C.     

Fabrication and Evaluation of a Thermoelectric Microdevice on a Free-Standing Substrate

Using shadow masks prepared by standard microfabrication processes, we fabricated in-plane thermoelectric microdevices (4 mm × 4 mm) made of bismuth telluride thin films, and evaluated their performance. We used Bi_0.4Te_3.0Sb_1.6 as the p-type semiconductor and Bi_2.0Te_2.7Se_0.3 as the n-type semiconductor. We deposited p- and n-type thermoelectric thin films on a free-standing thin film of Si₃N₄ (4 mm × 4 mm × 4 μm) on a Si wafer, and measured the output voltages of the microdevices while heating at the bottom of the Si substrate. The maximum output voltage of the thermoelectric device was 48 mV at 373 K.     

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.     

Development of Skutterudite Thermoelectric Materials and Modules

Multifilling with La, Ba, Ga, and Ti in p-type skutterudite and Yb, Ca, Al, Ga, and In in n-type skutterudite remarkably reduces their thermal conductivity, resulting in enhancement of their dimensionless figure of merit ZT to ZT?=?0.75 for p-type (La,Ba,Ga,Ti)₁(Fe,Co)₄Sb₁₂ and ZT?=?1.0 for n-type (Yb,Ca,Al,Ga,In)_0.7(Co,Fe)₄Sb₁₂. A thermoelectric module technology suitable for these skutterudites including diffusion barrier and electrode materials has been established. The diffusion barrier materials allow the electrode to coexist stably with the p/n skutterudites in the module??s working temperature range of room temperature to 600°C. Under conditions of hot/cold-side temperatures of 600°C/50°C, a skutterudite module with size of 50?mm?×?50?mm?×?7.6?mm exhibited generation performance of 32?W power output and 8% thermoelectric conversion efficiency.     

Thermoelectric Performance Enhancement of Poly(3,4-ethylenedioxythiophene):Poly(styrenesulfonate) Composite Films by Addition of Dimethyl Sulfoxide and Urea

Significant enhancement of thermoelectric (TE) performance was observed for free-standing poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT: PSS) composite films obtained from a PEDOT:PSS aqueous solution by simultaneous addition of dimethyl sulfoxide (DMSO) and different concentrations of urea. The electrical conductivity was enhanced from 8.16?S?cm^?1 to over 400?S?cm^?1, and the maximum Seebeck coefficient reached a value of 18.81???V?K^?1 at room temperature. The power factor of the PEDOT:PSS composite films reached 8.81???W?m^?1?K^?2. The highest thermoelectric figure of merit (ZT) in this study was 0.024 at room temperature, which is at least one order of magnitude higher than most polymers and bulk Si. These results indicate that the obtained composite films are a promising thermoelectric material for applications in thermoelectric refrigeration and thermoelectric microgeneration.     

Thermoelectric Properties of Al-Doped ZnO Thin Films

We have prepared 2 % Al-doped ZnO (AZO) thin films on SrTiO₃ substrates by a pulsed laser deposition technique at various deposition temperatures (T _dep = 300–600 °C). The thermoelectric properties of AZO thin films were studied in a low temperature range (300–600 K). Thin film deposited at 300 °C is fully c-axis-oriented and presents electrical conductivity 310 S/cm with Seebeck coefficient ?65 μV/K and power factor 0.13 × 10^?3 Wm^?1 K^?2 at 300 K. The performance of thin films increases with temperature. For instance, the power factor is enhanced up to 0.55 × 10^?3 Wm^?1 K^?2 at 600 K, surpassing the best AZO film previously reported in the literature.     

Fabrication of Highly (0?0?l)-Textured Sb2Te3 Film and Corresponding Thermoelectric Device with Enhanced Performance

An approach for fabrication of highly (0?0?l)-textured Sb₂Te₃ thin film with layered structure by the magnetron sputtering method is reported. The composition, microstructure, and thermoelectric properties of the thin films have been characterized and measured by x-ray diffraction, scanning electron microscopy with energy-dispersive x-ray spectroscopy, and a thermoelectric (TE) measurement system, respectively. The results show that well-oriented (0?0?l) Sb₂Te₃ thin film with layered structure is beneficial for improvement of thermoelectric properties, being a promising choice for planar TE devices. The power generation and cooling performance of a layered p-Sb₂Te₃ film device are superior to those of the ordinary thin-film device. For a typical parallel device with 38 layered Sb₂Te₃ film elements, the output voltage, maximum power, and corresponding power density are up to 10.3?mV, 11.1?μW, and 73?mW/cm², respectively, for a temperature difference of 76?K. The device can produce a 6.1?K maximum temperature difference at current of 45?mA. The results prove that enhanced microdevice performance can be realized by integrating (0?0?l)-oriented Sb₂Te₃ thin films with a layered architecture.     

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.     

Power Generation Characteristics of Mg2Si Uni-Leg Thermoelectric Generator

Mg₂Si thermoelectric (TE) elements were fabricated by a plasma-activated sintering method using a commercial polycrystalline n-type Mg₂Si source produced by the Union Material Co., Ltd. This material typically has a ZT value of ??0.6. A monobloc plasma-activated sintering technique was used to form Ni electrodes on the TE elements. The dimensions of a single element were 4.0?mm?×?4.0?mm?×?10?mm, and these were used to construct a TE module comprising nine elements connected in series. To reduce the electrical and thermal contact resistance of the module, each part of the module, i.e., the elements, terminals, and insulating plates, was joined using a Ag-based brazing alloy. In addition, to maintain the temperature difference between the top and bottom of the module, a thermal insulation board was installed in it. The observed values of open-circuit voltage (V _OC) and output power (P) of a uni-leg structure module were 594?mV and 543?mW, respectively, at a maximum ??T?=?500?K.     

Thermoelectric Behavior of Conducting Polymers Hybridized with Inorganic Nanoparticles

We introduce a simple and facile method for fabrication of a poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS)/germanium nanoparticle (Ge NP) composite film with enhanced thermoelectric conversion efficiency. The Ge NP were prepared by mechanical grinding and mixed with solution-phase PEDOT:PSS. The film processability of the composite was excellent and the overall process did not involve complicated synthetic procedures. The thermoelectric power factor of the composite film was optimized to 31.20 μW m^?1 K^?2 by controlling the composition. The composite film had an exceptionally low thermal conductivity of 0.417 W m^?1 K^?1 and the thermoelectric figure of merit (ZT) was maximized at 0.0223 at room temperature. The mechanism for the improvement of the thermoelectric conversion efficiency was investigated by introducing energy models based on interfacial scattering of charge carriers and phonons. We expect that this robust method could lead to a facile route for design of organic–inorganic composite-based thermoelectric materials.     

Flexible Thermoelectric Devices of Ultrahigh Power Factor by Scalable Printing and Interface Engineering

Printing is a versatile method to transform semiconducting nanoparticle inks into functional and flexible devices. In particular, thermoelectric nanoparticles are attractive building blocks to fabricate flexible devices for energy harvesting and cooling applications. However, the performance of printed devices are plagued by poor interfacial connections between nanoparticles and resulting low carrier mobility. While many rigid bulk materials have shown a thermoelectric figure of merit ZT greater than unity, it is an exacting challenge to develop flexible materials with ZT near unity. Here, a scalable screen‐printing method to fabricate high‐performance and flexible thermoelectric devices is reported. A tellurium‐based nanosolder approach is employed to bridge the interfaces between the BiSbTe particles during the postprinting sintering process. The printed BiSbTe flexible films demonstrate an ultrahigh room‐temperature power factor of 3 mW m^?1 K^?2 and ZT about 1, significantly higher than the best reported values for flexible films. A fully printed thermoelectric generator produces a high power density of 18.8 mW cm^?2 achievable with a small temperature gradient of 80 °C. This screen‐printing method, which directly transforms thermoelectric nanoparticles into high‐performance and flexible devices, presents a significant leap to make thermoelectrics a commercially viable technology for a broad range of energy harvesting and cooling applications.     

Enhanced Thermoelectric Properties of Antimony Telluride Thin Films with Preferred Orientation Prepared by Sputtering a Fan-Shaped Binary Composite Target

p-Type antimony telluride (Sb₂Te₃) thermoelectric thin films were deposited on BK7 glass substrates by ion beam sputter deposition using a fan-shaped binary composite target. The deposition temperature was varied from 100°C to 300°C in increments of 50°C. The influence of the deposition temperature on the microstructure, surface morphology, and thermoelectric properties of the thin films was systematically investigated. x-Ray diffraction results show that various alloy composition phases of the Sb₂Te₃ materials are grown when the deposition temperature is lower than 200°C. Preferred c-axis orientation of the Sb₂Te₃ thin film became obvious when the deposition temperature was above 200°C, and thin film with single-phase Sb₂Te₃ was obtained when the deposition temperature was 250°C. Scanning electron microscopy reveals that the average grain size of the films increases with increasing deposition temperature and that the thin film deposited at 250°C shows rhombohedral shape corresponding to the original Sb₂Te₃ structure. The room-temperature Seebeck coefficient and electrical conductivity range from 101 μV K^?1 to 161 μV K^?1 and 0.81 × 10³ S cm^?1 to 3.91 × 10³ S cm^?1, respectively, as the deposition temperature is increased from 100°C to 300°C. An optimal power factor of 6.12 × 10^?3 W m^?1 K^?2 is obtained for deposition temperature of 250°C. The thermoelectric properties of Sb₂Te₃ thin films have been found to be strongly enhanced when prepared using the fan-shaped binary composite target method with an appropriate substrate temperature.     

Detection of Thermal Radiation,Sensing of Heat Flux,and Recovery of Waste Heat by the Transverse Thermoelectric Effect

The transverse thermoelectric effect is unique in that an output voltage can be extracted in the direction perpendicular to the input temperature gradient. This paper describes how this transverse feature can be exploited to realize simple and promising configurations of thermoelectric devices. For detection of thermal radiation, two-dimensional imaging has been demonstrated by a fabricated sensor array of tilt-oriented Ca _x CoO₂ epitaxial thin film. We have also developed a serpentine heat flux sensor made of multilayered Bi/Cu, and Bi_0.5Sb_1.5Te₃/Ni tubular thermoelectric devices for power generation. The fabrication processes and test results are presented.     

Flexible 3D Porous MoS2/CNTs Architectures with ZT of 0.17 at Room Temperature for Wearable Thermoelectric Applications

Developing materials that possess high electrical conductivities (σ) and Seebeck coefficients (S), low thermal conductivities (κ), and excellent mechanical properties is important to realize practical thermoelectric (TE) devices. Here, 3D hierarchical architectures consisting of hybrid molybdenum disulfide (MoS₂)/carbon nanotubes (CNTs) films are fabricated with the goal of increasing σ and decreasing κ. In these films, perpendicularly orientated CNTs interpenetrate restacked MoS₂ layers to form a 3D architecture, which increases the specific surface area and charge concentration. The MoS₂/20 wt% CNTs film shows high σ (235 ± 5 S?cm^?1), high S (68 ± 2 µV?K^?1), and low κ (19 ± 2 mW?m^?1?K^?1). The corresponding figure of merit (ZT) reaches 0.17 at room temperature, which is 65 times higher than that of pure MoS₂ film. In addition, the MoS₂/20 wt% CNTs film shows a tensile stress of 38.9 MPa, which is an order of magnitude higher than that of a control MoS₂ film. Using the MoS₂/CNTs film as an active material and human body as a heat source, a flexible, wearable TE wristband is fabricated by weaving seven strips of the 3D porous MoS₂/CNTs film. The wristband achieves an output voltage of 2.9 mV and corresponding power output of 0.22 µW at a temperature gradient of about 5 K.     

Power-Generation Performance and Durability of a Skutterudite Thermoelectric Generator

By using a p-type (La, Ba, Ga, Ti)₁(Fe, Co)₄Sb₁₂ skutterudite with a dimensionless figure of merit, ZT, = 0.75 at 500°C and an n-type (Yb, Ca, Al, Ga, In)_0.7(Co, Fe)₄Sb₁₂ skutterudite with ZT = 1.0 at 500°C, we fabricated a thermoelectric power-generation module capable of working at high temperatures (up to 600°C). When its hot and cold sides were at 600°C and 30°C, respectively, the power output of a 50 mm × 50 mm × 7.6 mm skutterudite module was 34 W and its thermoelectric conversion efficiency was 8%. In a durability test with the module’s hot and cold sides continuously maintained at 600°C and 80°C, respectively, for 8000 h, power generation first decreased by approximately 6% in the initial 300 h then remained constant.     

Determination of Thermoelectric Module Efficiency: A Survey

The development of thermoelectrics (TE) for energy conversion is in the transition phase from laboratory research to device development. There is an increasing demand to accurately determine the module efficiency, especially for the power generation mode. For many TE, the figure of merit, ZT, of the material sometimes cannot be fully realized at the device level. Reliable efficiency testing of thermoelectric modules is important to assess the device ZT and provide end-users with realistic values for how much power can be generated under specific conditions. We conducted a general survey of efficiency testing devices and their performance. The results indicated a lack of industry standards and test procedures. This study included a commercial test system and several laboratory systems. Most systems are based on the heat flow meter method, and some are based on the Harman method. They are usually reproducible in evaluating thermoelectric modules. However, different systems often showed large differences that are likely caused by uncertain heat loss and thermal resistance. Efficiency testing is an important capability for the thermoelectric community to improve. A follow-up international standardization effort is planned.     

Engineering Thermal Conductivity for Balancing Between Reliability and Performance of Bulk Thermoelectric Generators

The nanostructuring approach has significantly contributed to the improving of thermoelectric figure‐of‐merit (ZT) by reducing lattice thermal conductivity. Even though it is an effective method to enhance ZT, the drastically lowered thermal conductivity in some cases can cause thermomechanical issues leading to decreased reliability of thermoelectric generators. Here, an engineering thermal conductivity (κ_eng) is defined as a minimum allowable thermal conductivity of a thermoelectric material in a module, and is evaluated to avoid thermomechanical failure and thermoelectric degradation of a device. Additionally, there is dilemma of determining thermoelectric leg length: a shorter leg is desired for higher W kg^?1, W cm^?3, and W The nanostructuring approach has significantly contributed to the improving of thermoelectric figure‐of‐merit (ZT) by reducing lattice thermal conductivity. Even though it is an effective method to enhance ZT, the drastically lowered thermal conductivity in some cases can cause thermomechanical issues leading to decreased reliability of thermoelectric generators. Here, an engineering thermal conductivity (κ_eng) is defined as a minimum allowable thermal conductivity of a thermoelectric material in a module, and is evaluated to avoid thermomechanical failure and thermoelectric degradation of a device. Additionally, there is dilemma of determining thermoelectric leg length: a shorter leg is desired for higher W kg^?1, W cm^?3, and W $^?1, but it raises the thermomechanical vulnerability issue. By considering a balance between the thermoelectric performance and thermomechanical reliability issues, it is discussed how to improve device reliability of thermoelectric generators and the engineering thermal conductivity of thermoelectric materials.     

Thermoelectric Modules Based on Half-Heusler Materials Produced in Large Quantities

Half-Heusler (HH) compounds are some of the most promising candidates among the medium-temperature thermoelectric materials being investigated for automotive and industrial waste heat recovery applications. For n- as well as p-type material, peak ZT values larger than one have been published recently, and first modules have been built. The next step to facilitate the industrialization of thermoelectric module production is upscaling of material synthesis. In this paper, the latest results of the thermoelectric properties of HH compounds produced in kg batches are presented and compared with values published in the literature. The performance of modules built from these materials is analyzed with respect to power output and long-term stability of the material and electrical contacts.     

Fabrication by Coaxial-Type Vacuum Arc Evaporation Method and Characterization of Bismuth Telluride Thin Films

We prepared both n- and p-type bismuth telluride thin films by using a coaxial-type vacuum arc evaporation method. The atomic compositions of the as-grown thin films and several annealed thin films were comparable to that of bulk bismuth telluride. Their thermoelectric properties were measured and found to be comparable to those of bulk materials. The Seebeck coefficient and electrical conductivity of the as-grown thin films were improved by the annealing process. The measured figures of merit (ZT) of the films were 0.86 for the n-type and 0.41 for the p-type at 300 K for annealing temperatures of 573 K and 523 K, respectively.