Bismuth Telluride Films Research Articles

Performance evaluation of flexible thermoelectric generator with Bi2Te3 thin-film

In the information society, carrying devices such as smartphones and tablets is crucial, and ensuring their power supply is essential. Furthermore, from the perspective of energy security, being able to secure their power supply from low-quality energy sources could reduce dependence on fossil fuels. In this study, we fabricated a flexible thermoelectric generator capable of harvesting energy from a low-temperature difference between a low-temperature heat source and room or outdoor temperature and evaluated its performance. The device was fabricated by depositing an n-type Bismuth telluride (Bi2Te3) film onto a polyimide film substrate using radio frequency (RF) magnetron sputtering. A 3D printer was used to fabricate flexible fins for attaching and bonding the films. Owing to their adaptability, these flexible models can be installed on heat sources of different shapes. The experimental conditions for this study were as follows: the temperature in the test chamber was maintained at 25 ± 1 °C and that of the heat source ranged from 35 to 75 °C; further, thermoelectric elements in numbers varying from 1 to 6 were installed. The maximum open-circuit output voltage |E| and power generation (Pmax) were determined to be 11.4 mV and 69 nW, respectively. The fins with the Bi2Te3 thin film maintained a temperature of approximately 25 to 30 °C at the tip of the fin, even when exposed to the highest temperature (75 °C) of the heat source during the experiment. As a result, the thermoelectric device developed in this study has demonstrated the potential to power sensors operating in the tens of nW range.

Low Contact Resistivity of Bi2Te3‐Based Films and Metals Interfaces Enabled by Orientation Regulation

AbstractThe contact resistivity originating from the metal–semiconductor interface is a crucial factor for conversion efficiency of thermoelectric films devices. Due to anisotropic nature of state‐of‐art Bi2Te3 films, the contact resistivity also needs to match with the different orientations in/out‐plane film configurations. Here, a series of bismuth telluride (Bi2Te3) films with different orientations are deposited, and the contact resistivity between Ti/Cr/Cu and Bi2Te3‐based films is studied in detail. The increased (00l) orientation can effectively reduce the contact resistivity mainly due to the enhanced Tunneling effect. Finally, a low contact resistivity of 0.84 µΩ cm2 is observed in highly (00l)‐orientated Bi2Te3 film in contact with Cr. This work provides an effective method to control contact resistivity by adjusting the orientation of film, and also provides guidance on how to further improve the performance of thin film‐TEDs with different configurations.

Exceptional Performance of Room Temperature Sputtered Flexible Thermoelectric Thin Film Using High Target Utilisation Sputtering Technique

AbstractThe High Target Utilisation Sputtering technique (HiTUS) is of interest for industrial processes, including in roll‐to‐roll manufacturing. This study marks the first application of HiTUS to thermoelectric materials, exemplified by bismuth telluride. The HiTUS technique separates the sputtering power into the plasma power and the target power, with additional kinetic energy in the sputtering particles from the applied electrical field, thus enabling a much wider sputter parameter space to modify the film performance. This study investigates how plasma power, target power, and substrate bias in HiTUS intricately influence crystal orientation/size, elemental composition, surface morphology, and other film properties. These factors subsequently affect carrier density/mobility, and consequently the thermoelectric performance of the bismuth telluride film. These deposited films reach a power factor of 6.5 × 10−4 W m−1 K−2 with a figure of merit ≈0.14 at room temperature, the highest value for room‐temperature sputtered un‐doped bismuth telluride. Subsequent post‐deposition annealing significantly enhances the crystallinity of the film (highly polycrystalline), further improving the power factor to 23.5 × 10−4 W m−1 K‐2, with a figure of merit ≈0.45 at room temperature. The excellent performance of the HiTUS fabricated thermoelectric film opens opportunities for the large‐area manufacture of thin‐film thermoelectric materials and devices.

Study on the electrodeposition of Bi2Te3 thin film thermoelectric material in dimethyl sulfoxide

In this paper, the electrochemical behaviors of Bi3+ and Te4+ ions in dimethyl sulfoxide (DMSO) organic solvent were studied by testing the cyclic voltammetry curves and cathodic polarization curves. The diffusion coefficients of Bi3+ and Te4+ ions were investigated and calculated, and reduction difficulty of them was compared. Then, bismuth telluride thermoelectric thin films were prepared using potentiostatic electrodeposition mode in the electrolytes with different compositions at different potentials. The structure, morphology and compositions of the prepared thermoelectric materials were characterized and analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM), and X-ray energy dispersive spectroscopy (EDS). The results show that the redox processes of Bi, Te and bismuth telluride on Au electrode are all completely irreversible processes. The diffusion coefficient of Bi3+ ion has an order of magnitude of approximately 10−7 while that of Te4+ ion is about 10−5. The reduction of Bi3+ ion is more difficult than that of Te4+ ion in dimethyl sulfoxide organic solvent. The prepared thin film thermoelectric materials are bismuth telluride alloy, and the electrolyte composition and deposition potential have great influence on the structure, morphology and composition of the films. Increasing the concentration of Bi3+ ions in the electrolyte can greatly increase the content of element Bi in the deposits. In the electrolyte containing 20 mol·L−1 Bi(NO3)3·5H2O and 10 mol L−1 TeCl4, the bismuth telluride film electrodeposited at − 1.2 V has uniform grain size and compact structure, shows good surface morphology, and the atomic fraction ratio of Bi/Te element in the film is 1:1.80.

Electrochemical and structural investigation of BixTey thin films electrodeposited on ITO substrate

Bismuth telluride films with various stoichiometries BixTey have been successfully elaborated by potentiostatic deposition on indium tin oxide (ITO) substrate from an acidic solution containing a mixture of different concentrations of Bi3+ and HTeO2+ ions. The electrochemical study using cyclic voltammetry and chronoamperometry shows that the electrodeposition of bismuth telluride is a quasi-reversible reaction controlled by the diffusion process and the composition of the reaction mixture controls the BixTey stoichiometry. It was found, from electrochemical and EDS analysis, that a mixture in stoichiometric proportions leads to the formation of bismuth tellurium Bi2Te3, an excess of telluryl ion HTeO2+ gives a product rich in tellurium Bi2-xTe3+x, but a high concentration of Bi3+ (compared to the stoichiometry) gives rise to the electrodeposition of a compound rich in bismuth Bi2+xTe3-x. The chronoamperometric analysis of current density-time transients using Scharifker-Hills model showed that the mechanism of deposition and growth of BixTey on ITO substrate takes place according to a progressive 3D nucleation for low Bi3+ concentration and 3D instantaneous nucleation for higher concentration. The morphology of the BixTey films is strongly depending to the electrolytes composition. The X-ray diffraction characterization shows that the crystalline structure of bismuth tellurium BixTey produced is of the rhombohedral type.

Ultrafast carrier and phonon dynamics in thin films of bismuth telluride on a flexible substrate

Thin films of topological insulators (TIs) possess exotic nonlinear optical properties such as strong light-matter interaction, broadband spectral sensitivity, thickness-dependent tunable bandgap, higher harmonic generation, etc. Due to the presence of metallic surface states with Dirac fermions and an insulating bulk band in TI, they are projected to be a viable material for studying novel physics, resulting in exciting new properties and technologies. The peculiar electron-phonon interactions at the surface have been linked to various unexpected physical features of topological insulators. Although electron behaviour in topological insulators has been extensively investigated on non-flexible substrates, electron-phonon interactions at TI bulk and surfaces states are less well known on a flexible substrate. Because of its potential uses in wearable devices, communications, sensors, and other fields, there is a significant need for the manufacture of high performance flexible optoelectronic responses employing novel exotic materials. In this paper, we preformed ultrafast pump-probe method to explore TI (Bi2Te3) thin films on a flexible PET (polyethylene terephthalate) substrate. We studied the dynamics of Bi2Te3 thin films' hot carrier relaxation progression and coherent phonon behaviour using transient absorbance measurements. Thickness-dependent low-frequency coherent acoustical phonon oscillations are observed in 10 nm thick films, and the changes vanish for 25 nm thick films, and high-frequency optical phonon oscillations are absent in our work. Subpicosecond range of time constant for the photon excitation, diffusion, carrier thermalization, and relaxation are reported. Longer characteristics time was observed for 25 nm film as compared to 10 nm film, and its variation has been discussed here. The thickness-dependent coherent acoustic phonon oscillating in the terahertz frequency range has been experimentally calculated. The film's terahertz frequency response varies with its thickness, allowing it to be employed in future terahertz applications based on flexible topological insulator thin films.

Structural correlation of phonon dynamics in Bismuth and Tellurium for the formation of Bismuth Telluride, using ultrafast transient absorption spectroscopy

The charge carrier and phonon dynamics are highly reliant on the material structural features, and understanding their dynamics help in interpreting the underlying phenomenological qualities of any material system. The phonon and carrier dynamics of Bi, Te, and BiTe3 thin films formed on a Si (100) wafer through thermal evaporation are discussed in this article. The Field Emission Scanning Electron Microscopy (FESEM) and X-ray Diffraction (XRD) analysis are used to characterize the structural features of these films. The dynamics of charge carriers and phonons are further determined using femtosecond ultrafast transient absorption spectroscopy (UFTS). A comprehensive analysis of dynamics is carried out by performing multiple excitations probed in broad Near-Infrared (NIR) regimes. The current study would help collect the knowledge about the phonon and charge carrier dynamics of Bismuth, Tellurium, and Bismuth Telluride films for optical, spintronics, and terahertz applications.

Elastic (acrylate/polydimethylsiloxane) substrate-to-coating interlayers for improving the mechanical resilience of thermoelectric films on poly(ethylene terephthalate) during roll-to-roll manufacture and in service operation

Strain-induced failure is a major concern in wearable electronics. Herein, a ~ 90-nm thick bismuth telluride film is fabricated on 125-μm thick polyethylene terephthalate as the basis for a flexible thermoelectric generator. To simulate induced mechanical strains during wearable operation and roll-to-roll manufacture, both tensile and buckling strains are studied. To improve the mechanical resilience, two types of substrate-to-coating interlayers with varying thicknesses are investigated: 0.4–7.7 μm-thick Poly-tripropyleneglycol diacrylate (acrylate) and 2.5–16.8 μm-thick Poly-dimethyl siloxane (PDMS). The thermoelectric performance of the coating is influenced through the stress, applied on the polymer substrate, being transferred through to the coating. Samples exposed to tensile testing recover once the load is released, mitigating against change on the film morphology and thermoelectric performance, in contrast to those measured in-situ under buckling strain. Both interlayers are shown to assist in maintaining the film performance, as fewer cracks are formed in the semiconductor. A thicker interlayer is shown to better mitigate the impact of deformation of the substrate. Furthermore, in fatigue-bending tests, the film grown on a PDMS interlayer shows better cyclical fatigue performance.

Novel Stacking Design of a Flexible Thin‐Film Thermoelectric Generator with a Metal–Insulator–Semiconductor Architecture

AbstractA stacked thermoelectric generator on a flexible polymer sheet is investigated that can utilize a low‐cost high throughput roll‐to‐roll process, employing a metal–insulator–semiconductor structure of <100 nm thick Cu and bismuth telluride films with a ≈1 µm thick acrylate insulating coating. Thermoelectric strips can be stacked and connected in the out‐of‐plane direction, which significantly decreases the size required in the substrate plane and also gives rise to the opportunity for greatly extending power output by stacking thousands of layers. A smooth surface of stacked layers is confirmed due to the nature of the acrylate layer. Room‐temperature sputtering can produce good quality/crystalline films, indicated by X‐ray diffraction and transmission electron microscope. Both experimental and simulation results observe a small temperature gradient across the stack from the bottom heat source to the top free surface. A stacked thermoelectric generator shows comparable performance to an in‐plane device, and most notably, the stacked architecture allows a higher power output without increasing the dimension of the device in the substrate plane, while the thickness is increased within only a µm range. Cyclic buckling fatigue tests suggest that the performance of stacked functional strips can be protected under deformation within the acrylate matrix.

Deposition and Fabrication of Sputtered Bismuth Telluride and Antimony Telluride for Microscale Thermoelectric Energy Harvesters

Thermoelectric (TE) n-type bismuth telluride (Bi2Te3) films and p-type antimony telluride (Sb2Te3) films are grown on SiO2/Si substrates via radio frequency magnetron sputtering. The crystal structures and TE properties are characterized for 1 µm and 10 µm films deposited using different deposition conditions and using various heat treatment conditions. Single-target sputtered deposition of n-type Bi2Te3 films resulted in a Te-deficient off stoichiometric films due to the evaporation of tellurium. Two-target co-sputtered deposition using Bi2Te3 and Te targets at room temperature and subsequent anneal at 250°C yielded a 10 µm n-type film with -102 µV/K for the Seebeck coefficient and 0.7 mW/K2.m for the power factor. Similarly, prepared p-type Sb2Te3 film but using a single sputtering target yielded +110 µV/K and 1.3 mW/K2.m for their Seebeck coefficient and power factor respectively. The fabrication of the micro-scale thermoelectric energy generator (µTEG) using separate n-type and p-type wafers is demonstrated.

High power factor due to multi-scale engineering in ultra-thin bismuth telluride films

High thermoelectric (TE) power factors were obtained for bismuth telluride by deploying confinement and multi-scale engineering in synergy. The thickness of the film was kept in the ultra-thin range (41 nm) following which a high magnitude of 1.9 × 104 S m−1 was obtained at room temperature (RT). Films were deposited at an elevated substrate temperature to enhance the grain quality and high mobility bearing (00l) grain growth. Thus, relatively large crystallite sizes (∼26 nm) with less grain boundaries and directional growth with a low defect profile were the prime reasons for highly enhanced electrical conductivity. Apart from the multiple effects that were deployed, ultra-thin dimensions of the films proved to be effective in further enhancing Seebeck coefficient values. The co-alloyed In minimized the hole concentration through reducing antisite defects and also preserved the reduced bipolar effect at elevated temperatures. The inclusion of excess tellurium induced Te segregates in the film that helped in energy dependent scattering of carriers in addition to its donor-like effect. Hot carrier filtering, induced by excess Te along with ultra-thin dimensions resulted in a Seebeck coefficient (S) of −223.6 μV K−1 at RT. A soaring value of −338.1 μV K−1 was obtained at 90 °C. Following the synergetic employment of multiple enhancement strategies, a high power factor of 959.9 μW m−1 K−2 was obtained at room temperature with a towering magnitude of 2537.7 μW m−1 K−2 at 90 °C.

Thermoelectric generators for heat harvesting: From material synthesis to device fabrication

This work reports high performance thermoelectric materials based on an electrochemical deposition method. Ultra-thick (2 mm-thick) and dense electrodeposited thermoelectric films have been successfully synthesized. It is found that at 0.7 at% concentration of nickel doped bismuth telluride, the power factor increases to 2050 µV/m K2 which is approximately 3 times higher than that of an electrodeposited pure bismuth telluride film. Meanwhile, the measurement on a thermal conductivity of the nickel doped bismuth telluride shows an improvement, which is nearly 2 times reduction as compared to the pure bismuth telluride film. As a result, a higher ZT value (0.78) of the nickel doped bismuth telluride is more than 5 times better than that of the electrodeposited pure bismuth telluride under similar evaluation conditions. In addition, a highly scalable synthesis process on a 4-inch wafer size of the thermoelectric material-based electrodeposition technique has been demonstrated. It proves that this electrodeposition could open up a chance to develop mass production for the low cost and high performance thermoelectric materials synthesis. Moreover, four kinds of thermoelectric generators (TEGs) including TEGs #1, #2, #3 and #4 with different parameters have been fabricated, evaluated and compared. The metal doped thermoelectric material devices are able to harvest more power than that of the pure thermoelectric material device. The fabricated device is successfully integrated with a DC-DC booster of which output can reach 3 V when a temperature difference of 4 °C is applied across the TEG. The generated power can be used to light up an LED from this small temperature difference.

Self-powered broadband photo-detection and persistent energy generation with junction-free strained Bi2Te3 thin films.

We experimentally demonstrate efficient broadband self-powered photo-detection and power generation in thin films of polycrystalline bismuth telluride (Bi2Te3) semiconductors under inhomogeneous strain. The developed simple, junction-free, lightweight, and flexible photo-detectors are composed of a thin active layer and Ohmic contacts on a flexible plastic substrate, and can operate at room temperature and without application of an external bias voltage. We attribute the observed phenomena to the generation of an electric field due to a spontaneous polarization produced by strain gradient, which can separate both photo-generated and thermally-generated charge carriers in bulk of the semiconductor material, without a semiconductor junction. We show that the developed photo-detectors can generate electric power during both the daytime and the nighttime, by either harnessing solar and thermal radiation or by emitting thermal radiation into the cold sky. To the best of our knowledge, this is the first demonstration of the power generation in a simple junction-free device under negative illumination, which exhibits higher voltage than the previously used expensive commercial HgCdTe photo-diode. Significant improvements in the photo-detector performance are expected if the low-charge-mobility polycrystalline active layer is replaced with high-quality single-crystal material. The technology is not limited to Bi2Te3 as the active material, and offers many potential applications in night vision, wearable sensors, long-range LIDAR, and daytime/nighttime energy generation technologies.

Atomic composition changes in bismuth telluride thin films by thermal annealing and estimation of their thermoelectric properties using experimental analyses and first-principles calculations

Abstract We investigated the structural and thermoelectric properties of bismuth telluride films when the atomic composition is changed by evaporation of Te atoms from the films during thermal annealing. The bismuth telluride films were deposited using a magnetron radio frequency sputtering method without a substrate heating. Thereafter, the films were thermally annealed up to 450 °C in an electric furnace with Ar(95%)–H2(5%) gas mixture. The atomic composition of the as-deposited film was approximately that of Bi2Te3. By increasing the annealing temperature, the Te atoms were evaporated from the films and the composition approached that of Bi1Te1, as discovered from electron probe microanalysis. Furthermore, a change in crystalline phase was observed through X-ray diffraction patterns. When the thermoelectric properties of the films were measured at room temperature, the carrier concentration was significantly affected by the change in atomic composition. We confirmed this finding using energy band structures using first-principles calculations. The highest power factor of 16.9 μW/(cm∙K2) was achieved at an annealing temperature of 300 °C because the film achieved a high crystallinity and crystal orientation with relatively low defects owing to less evaporation of Te atoms.

Effect of annealing temperature on thermoelectric properties of bismuth telluride thick film deposited by DC magnetron sputtering

We report the thermoelectric properties of thick bismuth-telluride (Bi2Te3) films deposited on polyimide substrates by DC magnetron sputtering and annealed at various temperatures (150–350 °C). The influence of annealing temperature on the microstructure and electronic structure of thick Bi2Te3 films is discussed. In this work, the annealed film at 250 °C has the best thermoelectric property due to highest electrical conductivity and Seebeck coefficient. The main effect of annealing temperature was really helpful to improve crystalline structure and enhance carrier mobility, whereas the carrier concentration was reduced due to the volatile of tellurium atom during annealing. Chemical states of bound and unbound atoms (Bi, Bi3+, Te, and Te2‐) on the surface play an important role in electrical properties. The exceed temperature caused the micro-crack formation and affect carrier transport by the scattering. The power factor of Bi2Te3 deposited by DC magnetron sputtering and annealed at 250 °C is comparable to the power factors of thick Bi2Te3 film deposited by various deposition techniques. The output power of single-leg, thick, thermoelectric Bi2Te3 film annealed at 250 °C as a function temperature generated a power of 0.98 μW at a temperature difference of 50 °C.

Thermoelectric behaviour of Bi-Te films on polymer substrates DC-sputtered at room-temperature in moving web deposition

High-throughput roll-to-roll processing could be used to scale up the manufacture of flexible thermoelectric generators. Very thin thermoelectric layers can be manufactured at high throughput speed and low cost and, most importantly, are predicted to possess better thermoelectric properties than thicker layers. Here we present a study on a series of bismuth telluride films of different thickness (few nm to 370 nm), deposited on polymer substrates at room temperature using DC magnetron sputtering. Unlike previous studies of deposition of bismuth telluride films onto heated substrates, an island-growth mode, indicated by AFM, was observed for Bi-Te films grown at room temperature. A period of growth in which the layer only partially coats the substrate, with only imperfect connections between islands, was observed. In this partially coated region, the coating exhibited an extremely high Seebeck coefficient. An energy barrier mechanism, similar to the interface effect in nanomaterials, is proposed to explain this phenomenon, along with a possible quantum confinement effect. We found that a thinner Bi-Te film could generate a greater power factor because of a quasi-decoupling of Seebeck coefficient and electrical resistivity. In addition, ensuring that the sample passed directly under the sputtering target, and using a substrate smoothed with an acrylate layer were found to improve film properties, thus enhancing thermoelectric behaviour.

Dynamics of Defects in van der Waals Epitaxy of Bismuth Telluride Topological Insulators

Potential applications in spintronics and quantum information processing have motivated much recent research in epitaxial films of bismuth telluride. This system is also an example of van der Waals (vdW) epitaxy, where the interface coherence between the film and substrate is based on vdW bonds instead of strong ionic or covalent bonds. Because of the weakness of the vdW bonds, the overall quality of the epitaxial films is difficult to control and structural defects are easily introduced with a significant impact on the electronic phase diagram of the epitaxial films. To elucidate the evolution of defects as a function of the growth parameters, we combine nondestructive methods for electrical and structural analysis, as well as to establish intercorrelations between structural features and density of free charge carriers. It clearly shows that point defects and twinned domains favor p-type of charge carriers. Passivation of points defects by formation of metallic bismuth bilayers (BLs) drastically changes...

Flexible Thermoelectric Module Using Bi-Te and Sb-Te Thin Films for Temperature Sensors

Electromotive force is generated by a temperature difference in a thermocouple formed of two different metals, known as the Seebeck effect. Flexible thermoelectric (TE) temperature sensors based on the Seebeck effect can be attached to the body to provide body temperature information to health providers via wireless communications. Materials used in flexible TE temperature sensors have higher Seebeck coefficients than those of traditional thermocouples, and hence generate a high output voltage even with small temperature changes, making them highly suitable for use as body temperature sensors. In this study, a flexible TE temperature sensor was fabricated using a cheap, commercial flexible printed circuit board substrate and TE thin films of n-type bismuth telluride and p-type antimony telluride by co-evaporation. The results show that the flexible TE temperature sensor has a sensitivity of 192.70 μV K−1.

Passively Q-switched erbium fiber laser with a thin crystalline film of bismuth telluride used as a saturable absorber

Performance of a passively Q-switched erbium fiber laser with a narrow bandgap Bi2Te3 semiconductor thin film used as a saturable absorber was studied experimentally. In order to obtain repetitively-pulsed lasing, a Q-switcher was fabricated as a section of the standard SMF-28 optical fiber with a short (∼4 mm in length) chemically etched out tapered section laterally covered with the bismuth telluride film. This Q-switcher was installed in a specially designed circuit of a ring-type erbium fiber laser pumped by a laser diode at 975 nm wavelength. The fiber diameter in the middle of the tapered section was 10-15 μm. Two types of taper coverage were studied: Bi2Te3 flakes suspended in isopropanol and a ZnTe/Bi2Te3 uniform polycrystalline thin-film heterostructure synthesized on a lateral surface of a tapered fiber section via MOCVD. In the first case, we observed laser oscillation at a wavelength of 1565 nm as a train of 7-26 μs pulses with a 86-50 μs repetition period at pump powers varying from 26 to 75 mW. In the second case, CW oscillation in the circuit appeared at a pump power of 30 mW while conversion to the repetitively-pulsed mode with a kilohertz repetition rate occurred at pump powers within the range of 34-40 mW. The results obtained indicate the feasibility of effective interaction of the evanescent field with a bismuth telluride film used as a saturable absorber. This interaction is sufficient to achieve a stable repetitively-pulsed lasing in the circuits of passively Q-switched fiber lasers.

Optical studies on bismuth chalcogenides

Films of bismuth telluride (Bi2Te3) and its selenium (Se) derivatives (Bi2SeTe2, Bi2Se2Te, Bi2Se3) were deposited using electrodeposition on indium tin oxide coated glass substrate (ITO). The structural, morphological and optical properties of the prepared films were studied using X-ray diffraction (XRD), scanning electron microscopy (SEM), raman spectroscopy and UV-Visible spectroscopy. Nanostructured polycrystalline films with rhombohedral crystal structure were obtained. Substitution of Se in place of Te does not induce any structural transition but changes the lattice parameters systematically with Se content. However, a new Raman band is observed around 235 cm-1 for intermediate compositions while the end members exhibit only E2g band. Wavelength selective optical transmission proportional to selenium concentration is observed in UV-Vis spectroscopy studies.