Thin-film Thermoelectric Devices Research Articles

Advances in environmentally-friendly organic thermoelectric and organic thin-film transistor materials

<p indent=0mm>Organic semiconductors (OSCs) are significant systems of next-generation optoelectronic materials, which are widely applied in flexible devices, power generation and sensors. However, most of the organic semiconductors are prepared in chlorinated solvents which are harmful to human health and ecological environment. Nowadays, accompanied with the trends of environmentally-friendly optoelectronics, the OSCs processed with green solvents attracted extensive attention and developed rapidly. In this article, we reviewed the advances in environmentally-friendly OSCs in molecular design, solvent selection and processing method. The application of those materials in organic thin-film transistor and organic thermoelectric devices was highlighted. Finally, challenges and perspectives of the environmentally-friendly OSCs are also summarized.

Study of thermoelectric properties in the PEDOT:PSS/Te double-layer thin film devices

The research on the organic hybrid composites have been regarded as important developing direction in the field of thermoelectric fields, which combine the advantages of organic thermoelectric materials and inorganic thermoelectric materials. In this paper, the thermoelectric properties of Tellurium (Te) and Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) hybrid composite thin film and double-layer structured of PEDOT/Te thin film are studied, separately. We found that the PEDOT:PSS/Te double-layer structured thin film thermoelectric devices can greatly improve conductivity and Seebeck coefficient of PEDOT:PSS compared with the hybrid structure of PEDOT:PSS thermoelectric devices. We thought that the semi-metallicity of PEDOT:PSS and Te forms an ohmic contact interface, which is advantageous to charge transfer, so the double-layer structure of thin film devices possess a better carrier transport channels by the interface effect than hybrid structure devices. On the other hand, the C–V results also confirmed that the interfacial charge accumulation in the PEDOT:PSS/Te double-layered thin-film device, when the temperature rises, the Fermi level close to the conduction band as the temperature increases, leading to the number of high-energy carriers increase at the interface, which effectively improves the Seebeck coefficient of PEDOT:PSS. Our research provide experimental and theoretical support for the development of thin-film thermoelectric materials and devices.

Promising Development of Thin Film and Flexible Thermoelectric Devices

The temperature dependences of the thermoelectric properties of Ge2Sb2Te5 (p-type) and Bi2Te2.8Se0.2 (n-type) thin films obtained by magnetron and vacuum thermal evaporation, respectively, are under investigation. The effect of crystallization on the Seebeck coefficient of Ge2Sb2Te5 thin films is established. The temperature dependences of the power factors of the investigated thin films are determined. Film thermoelements based on Ge2Sb2Te5 and Bi2Te2.8Se0.2 materials have sufficiently high thermoelectric properties and can be used for manufacturing flexible film thermoelectric generators (TEGs). Computer simulation of the characteristics of a flexible film TEG using the proposed multi-physics model of a flexible film TEG is carried out. The characteristics of the TEG are determined at various ambient temperatures and various electrical loads, which make it possible to optimize the geometric parameters of thermoelements. The optimal operating temperatures of TEGs for wearable electronics are determined.

CoSb3-Based Thin-Film Thermoelectric Devices with High Performance Via Electrode Optimization

Flexible thin-film thermoelectric devices have been extensively studied for powering wearable electronics, particularly as a power source for self-powered sensors or temperature detection. Skutteru...

On-Demand Mobile CPU Cooling With Thin-Film Thermoelectric Array

On-demand cooling is inevitable to maximize the processor's performance, while fulfilling thermal constraints-this holds more in advanced technologies, where localized hotspots change during runtime. In this work, we propose to adopt an array of thin-film thermoelectric (TE) devices, which is integrated within the chip packaging, for both cooling and energy-harvesting purposes. Each TE device within the array can be during the runtime enabled either for energy harvesting or on-demand cooling. Our approach is implemented and evaluated using a mature finite elements analysis tool in which a commercial multicore mobile chip is modeled after careful calibrations together with state-of-the-art TE devices. Results demonstrate that our approach reduces the peak temperature by up to 24 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> C°C and the average temperature by 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> C°C across various benchmarks with the cost of 67.5 mW. Additionally, the harvested energy from the array of TE devices compensates for 89

α-Cu2Se thermoelectric thin films prepared by copper sputtering into selenium precursor layers

Copper selenide (Cu2Se) is a promising thermoelectric material and α-phase Cu2Se provides relatively safe thermoelectric modules for thin film thermoelectric device in contrast to some toxic materials currently on the market. In this work, nanocrystalline Cu2Se thin film with uniform element distribution was fabricated at room temperature through an effective combination reaction method by implanting sputtered Cu+ ions into Se precursor. Then, self-assembled growth of Cu2Se thin films with desired α-phase and well crystallinity was successfully achieved with optimization of annealing temperature. Interestingly, the growth orientation has obviously affected by the annealing temperature, which significantly affects the thermoelectric properties. Consequently, the Seebeck coefficients increase with the increase in the orientation factor of the (0l0) preferred orientation, which enhances power factor. The high maximum power factor of 9.23 μWcm−1K−2 is achieved for α-Cu2Se thin film with (0l0) preferred orientation, demonstrating that the method used in this work has great potential in developing low-cost and high-performance thermoelectric thin films from relatively earth-abundant elements.

PEDOT:PSS-polyethylene oxide composites for stretchable and 3D-Printed thermoelectric devices

We developed poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) and polyethylene oxide composites and demonstrated their use for soft and stretchable thin film thermoelectric devices. A stable thermoelectric performance can be observed under 20% stretching. Due to the vast processability of the polyethylene oxide polymer, bulk size thermoelectric devices can be produced with scalable manufacturing techniques, such as hot compression molding and 3D printing. These techniques allow for devices with a large area and customized geometry.

Enhanced thermoelectric properties of screen-printed Bi–Sb–Te films on flexible substrate by electrical sintering process

Screen printing is a low-cost solution-based process for preparing thick-film thermoelements, which bridges the technology gap between bulk and thin-film thermoelectric devices. A screen-printed thermoelectric film on a polymer substrate may exhibit moderate flexibility but normally suffer low electrical conductivity due to the presence of porosity or organic residues. In this study, we present a current-assisted sintering process to consolidate screen-printed Bi–Sb–Te (BST) films on a polyimide substrate at the temperature below 350 °C with simultaneous passage of a pulse current of 2 × 103 A/cm2 in peak current density. The electrically sintered BST film shows greatly enhanced electrical conductivity, leading to a 45% higher thermoelectric power factor (20.2 μW/cm⋅K2) than the thermally sintered BST film (13.9 μW/cm⋅K2). The variations of crystal defect concentration, grain size and secondary precipitates in the BST films with increasing pulse current density and sintering temperature are investigated. The influences of grain boundaries and secondary precipitates on carrier scattering behavior and thermoelectric transport properties of the BST films are also discussed.

Highly (1 0 0)-orientated SnSe thin films deposited by pulsed-laser deposition

This work aims at improving the quality of the highly (100)-orientated SnSe thin films for thermoelectric applications. The as-deposited films were obtained by controlling the basic parameters including target-to-substrate distance, deposition time and growth temperature through pulsed-laser deposition. The films quality was further improved by vacuum thermal annealing. The microstructure and crystalline structure of the films were studied by X-ray photoelectron spectroscopy, X-ray diffraction, electron probe micro-analyzer, electron back-scatter diffraction, atomic force microscope and Raman spectroscopy. The SnSe thin films grown on SiO2/Si substrate at 673 K followed by thermal annealing at 673 K for 30 min show the best crystal quality and uniform orientation with mirror-like surface, and the corresponding Seebeck coefficient and power factor are about 383 μV/K and 15.4 μW/m⋅K2, respectively. Angle resolved polarized Raman spectroscopy proved that the surface of the SnSe films is the b-c plane with preferred (100) orientation crystalline over a large area, providing an important way to prepare thermoelectric thin film devices by pulse laser deposition.

In Situ Dynamics during Heating of Copper-Intercalated Bismuth Telluride

Summary The presence of a van der Waals gap within layered materials enables the intercalation of guest species, which can be used for tuning properties. The introduction of intercalant species may also lead to structural transformations across a range of temperatures and use conditions, but we have a limited understanding of the high-temperature evolution of intercalated layered materials. Here, we use in situ transmission electron microscopy to investigate temperature-dependent structural evolution of Bi2Te3, a layered material with well-known application as a thermoelectric, which contains intercalated Cu. As temperature is increased, the initially disordered Cu undergoes local ordering within the van der Waals gap, along with anisotropic sublimation behavior of the crystals. As the temperature is raised further, layered Cu2Te crystals nucleate and grow with a crystallographic relationship to the Bi2Te3 lattice. Notably, the presence of Cu decreases the thermal stability of the Bi2Te3 and substantially alters the transformation pathways during heating.

Acceptor–acceptor-type conjugated polymer for use in n-type organic thin-film transistors and thermoelectric devices

The development of n -type conjugated polymers (CPs) forming an acceptor–acceptor (A–A)-type structure are quite challenging due to the synthetic limitation of imposing a functional group on the electron deficient aromatic structure. The diketopyrrolopyrroles (DPP) structure exhibits n -type characteristics; however, the extended π-structure enables stannylation at the terminal position. The stannylated DPP is beneficial for making n -type CPs, but its use in the development of n -type CPs and their application in n -type organic thin-film transistors and organic thermoelectric devices have been not extensively studied. In the present study, we synthesized the A–A-type conjugated polymer, poly [((2,5-bis(2-ethylhexyl)-3,6-di(thiophen-2-yl)-2,5-dihydropyrrolo [3,4- c ]pyrrole-1,4-dione)-5,5′-diyl)- alt -((2,7-bis(2-ethylhexyl)benzo [lmn] [3,8]phenanthroline-1,3,6,8(2 H ,7 H )-tetraone)-4,9-diyl))] (PDPP-NDI), by Stille coupling of distannylated DPP and dibrominated naphthalene diimide (NDI). The alternating copolymerization of DPP and NDI moieties led to a low-lying LUMO energy level of −4.4 eV, suitable for electron transport. The n -type organic thin-film transistors were fabricated with PDPP–NDI, and a substantial average electron mobility of 3.78 × 10 −2 cm 2 V −1 s −1 was obtained after thermal annealing. The n -type organic thermoelectric devices were developed via n -doping, and PDPP–NDI exhibited n -type thermoelectric properties, with an average power factor of 1.82 × 10 −3 μW m −1 K −2 . • An acceptor–acceptor-structured n -type alternating copolymer, PDPP–NDI, was developed. • A small bandgap of 1.03 eV and deep LUMO energy level of −4.4 eV were achieved. • The n -type characteristics of PDPP–NDI were successfully utilized in n -type OTFTs and OTE devices. • The device characteristics of both OTFT and OTE devices were fully studied and discussed.

Thermal Treatment Effects on the Thermoelectric Properties of Ni/Si/Si + Sb/Si/Si + Ge/Ni Multilayer Thin Films

The multilayer thermoelectric devices including Ni/200 layers of Si/Si + Sb/200 layers of Si/Si + Ge/Ni thin films were fabricated using electron beam and DC/RF magnetron sputtering deposition systems. The thickness measurements have been performed using Filmetrics UV thickness measurement system. The Au contacts at the bottom and top of the fabricated thermoelectric devices were measured as 100 nm for each side. Ni layer at the bottom is 108 nm and Ni layer at the top of the multilayer structures is 168 nm. The thickness of 200 layers of Si/Si + Ge thin film is 173 nm and the thickness of 200 layers of Si/Si + Sb thin film is 199 nm. The fabricated thermoelectric devices have total of 402 layers of thin films with the total thickness of 648 nm thickness excluding two Au contact layers. The prepared thin film devices were annealed at different temperatures for one hour to improve the thermoelectric properties. The current studied system has reached some remarkable values of Seebeck coefficients when the suitable annealing temperatures and the suitable operating temperatures of Seebeck measurement system were applied. The multilayer thin film system has reached the Seebeck coefficient of -344.8 μV/K when the annealed temperature was 100°C and the operating temperature was 320 K. One of the main problems with the thermoelectric devices is having higher temperature dissipation during the operation of the devices. Ni thin film was used in the fabrication process to remove excess of the heat as a heat sink from the thermoelectric devices. This will bring new approach for the high efficient thermoelectric devices. The goal of the manuscript is to improve the thermoelectric properties of the fabricated thin film thermoelectric devices using Ni thin films and the thermal treatment. The resistivity values decreased when the annealing temperatures increased. The highest power factor values were reached when the thermoelectric devices were annealed at 100°C. Mobility values increased when the suitable temperatures were applied for thermal treatment.

Enhanced Electrical Transport Properties via Defect Control for Screen-Printed Bi2Te3 Films over a Wide Temperature Range.

The application of screen-printed thin-film thermoelectric (TE) devices is still in its infancy, mainly due to low TE performance of screen-printed films and especially the poor electrical transport properties. Herein, we design and prepare a high-performance screen-printed Bi2Te3 film through introducing excessive Te-based nanosolder (Te-NS) to simultaneously realize the conduction channel construction and defect control. On one hand, the promoted carrier migration makes the electrical conductivity dramatically rise about 7 times, with a maximum power factor of 4.65 μW cm -1 K -2. Meanwhile, the defect formation mechanism in the screen-printed Bi2Te3 film after the introduction of Te-NS is also in-depth studied, and the bipolar conduction is reduced by increased generation of TeBi• and/or more suppression of BiTe', resulting in a postponed temperature of the maximum Seebeck coefficient. Hence, the large engineering power factor is achieved with excellent temperature linearity, indicating a possibility of screen-printed film application in a large temperature region. A TE device with a single leg has been fabricated to further demonstrate the generation validity. An open-circuit voltage of 11.34 mV and a maximum output power of 27.1 μW at a temperature gradient of 105 K have been achieved over a wide temperature range from 303 to 478 K. This study provides a theoretical and practical basis for the performance improvement of screen-printed TE films and devices.

Photo-Thermoelectric Thin-Film Generator and Sensor With Ultrahigh Output Voltage and Large Responsivity

The thin-film thermoelectric device has gained much attention as promising candidate for micro-energy conversion and sensing. However, its relatively low responsivity is one of the main obstacles in light detection field due to the limited response voltage, which is determined by the properties of thermoelectric films as well as the device configuration. In this letter, we develop a compatible micromachining technology including photolithographic and lift-off processes to fabricate thin-film thermoelectric device with high integration of thermocouple on flexible substrates. Meanwhile, the properties of thermoelectric films have been optimized during the fabrication process. In addition, by integrating the photothermal layer and Fresnel lens with increased temperature difference, the assembled device can exhibit an ultrahigh output voltage, which achieves 1.25 V under 1.5 AM 1-sun condition. The light sensing performance of the device is also investigated, and the responsivity reaches up to 24.9 V/W. This work presents a significant progress toward a photo-thermoelectric thin-film sensor in light detecting field.

Enhanced thermoelectric properties in vacuum-annealed Bi0.5Sb1.5Te3 thin films fabricated using pulsed laser deposition

Localized cooling in microelectronics and nanoelectronics as well as energy autonomy in applications such as wireless sensor networks and wearable electronics could be well served by thin-film thermoelectric devices fabricated on rigid and/or flexible substrates. Bi0.5Sb1.5Te3 is considered to be a state-of-the-art p-type thermoelectric material at the desired temperature range, i.e., near room temperature (RT). Fabrication of Bi0.5Sb1.5Te3 thin films with bulklike thermoelectric properties (∼3900 μW/mK2 at 380 K) remains, however, a great challenge. In this study, we have successfully fabricated Bi0.5Sb1.5Te3 thin films on fused silica and Kapton substrates using a two-step process. The films were deposited at RT using pulsed laser deposition and then subjected to a postdeposition ex situ vacuum annealing process. The as-grown films were nearly amorphous. However, the annealing process enhanced both their crystallinity and texture, resulting in thin films with bulklike thermoelectric power factor values. Bi0.5Sb1.5Te3 thin films grown on fused silica and annealed at 350 °C for 16 h exhibit a power factor of 3750 μW/mK2 at 380 K. In addition, Bi0.5Sb1.5Te3 films grown on Kapton and annealed at 250 °C for 5 h and also grown on Kapton substrates at 250 °C exhibit a power factor of 2600 μW/mK2 at 390 K. Both of these power factor values are among the highest reported in the literature to date for Bi0.5Sb1.5Te3 thin films grown on fused silica and Kapton substrates, respectively.

Galvanic exchange reaction involving Te nanowires and Ag ions for n-type Te/Ag2Te thermoelectric nanofilms

One-dimensional silver telluride nanowires (NWs) as potential thermoelectric materials have been studied extensively. At present, most of the efforts are focused on p-type silver telluride, while limited works have been achieved on the preparation of n-type silver telluride. Herein, we prepared n-type Te/Ag2Te NWs by galvanic exchange reaction and fabricated a nanofilm. As a simple and cost-effective method, galvanic exchange reaction was employed to prepare n-type Te/Ag2Te NWs with obtained Seebeck coefficient of − 93.98 μV K−1. The nanostructure and chemical composition as well as structural phase transition of as-prepared n-type Te/Ag2Te NWs were characterized and discussed. Besides, the thermoelectric thin film device consisting of n-type Te/Ag2Te nanofilms and p-type Ag-doped Te nanofilms obtained by one-step hydrothermal method was assembled and exhibited accessible output voltage as 65 mV at the 75 K temperature difference, which may provide the fundament for the development of inorganic thermoelectric nanofilm device.

Flexible thermoelectric modules based on ALD-grown ZnO on different substrates

The authors have designed and tested prototype thin-film thermoelectric devices based on 100–500 nm thick layers of n-type ZnO fabricated with atomic layer deposition on different substrate materials: oxidized silicon, polyethylene naphtalate plastics, and thin flexible glass. In addition, they address the benefits of depositing intermittent organic (benzene) layers within the ZnO matrix through molecular layer deposition for thermal conductivity suppression. Thermoelectric performance of the test devices composed of several ZnO or ZnO:benzene thin-film legs was evaluated by generating the temperature difference using a hotplate and measuring the output voltage at the ends of the circuit in both open circuit and load configurations. The output voltage was found to increase with increasing ZnO film thickness. Most interestingly, the ZnO:benzene superlattice film investigated had better performance compared to plain ZnO of the same thickness, thus opening the way to further developments of thermoelectric thin-film devices.

Thermoelectric Inorganic SiGe Film Synthesized on Flexible Plastic Substrate

For developing thermoelectric thin-film devices using reliable inorganic materials, we investigated the synthesis of SiGe directly onto insulating substrates. An Al-induced layer exchange technique was employed, which enabled us to form SiGe on glass substrates at a low temperature (≤400 °C) in the whole SiGe composition range. The SiGe layers were observed to contain Al atoms in concentrations corresponding to their solid solubility limits during growth, which resulted in the formation of self-organically highly p-doped layers without requiring high-temperature-activation annealing. Results indicated that a high power factor (430 μW/mK2) and a low process temperature were simultaneously achieved for SiGe on glass. A SiGe layer fabricated on a plastic substrate exhibited a power factor of 190 μW/mK2, which is higher than most thermoelectric films directly grown onto a plastic substrate. These achievements will present practical applications for flexible thin-film thermoelectrics with a high reliability.

Instability-induced wrinkling in thermoelectric thin film/substrate structures for thermal protection systems in supersonic space shuttle applications

ABSTRACTThis paper examines the behavior of wrinkling instability of a thermoelectric thin film bonded to substrate. The critical temperature differences for wrinkling occurrence and buckling initiation are obtained. Damage growth following wrinkling is also determined. These critical temperatures can provide guidelines for the design of thermoelectric thin film devices. Numerical results show that the stability of thermoelectric thin film is affected by the electric current. The critical temperature differences become smaller when the electric current density in thermoelectric thin film is higher. Effect of the wavelength of wrinkling on the critical temperature differences of wrinkling occurrence is also identified.

Using high thermal stability flexible thin film thermoelectric generator at moderate temperature

Flexible thin film thermoelectric devices are extensively used in the microscale industry for powering wearable electronics. In this study, comprehensive optimization was conducted in materials and connection design for fabricating a high thermal stability flexible thin film thermoelectric generator. First, the thin films in the generator, including the electrodes, were prepared by magnetron sputtering deposition. The “NiCu–Cu–NiCu” multilayer electrode structure was applied to ensure the thermal stability of the device used at moderate temperature in an air atmosphere. A design with metal layer bonding and series accordant connection was then employed. The maximum efficiency of a single PN thermocouple generator is &amp;gt;11%, and the output power loss of the generator is &amp;lt;10% after integration.