Controllable Electrical Contact Resistance between Cu and Oriented-Bi2Te3 Film via Interface Tuning

Enhanced Contact Performance and Thermal Tolerance of Ni/Bi2Te3 Joints for Bi2Te3-Based Thermoelectric Devices

Ni metal has been widely used as a barrier layer in Bi₂Te₃-based thermoelectric devices, which establishes stable joints to link Bi₂Te₃-based legs and electrodes. However, the Ni/Bi₂Te₃ joints become very fragile when the devices were exposed to high temperature, causing severe performance deterioration and even device failure. Herein, stable Ni/Bi₂Te₃ joints have been established by arc spraying of the Ni barrier layer on the Bi₂Te₃-based alloys. The interface microstructure and contact performance including the bonding strength and contact resistivity of the arc-sprayed Ni/Bi₂Te₃ joints are investigated. The results indicate that, as compared with traditional Ni/Bi₂Te₃ joints, the arc-sprayed Ni/Bi₂Te₃ joints have comparably low contact resistivity while possessing a 50% higher bonding strength. Aging the joints as an exposure to high-temperature circumstances, the arc-sprayed Ni/Bi₂Te₃ joints exhibit much better tolerance to the thermal shock with stable bonding strength and contact resistivity. The enhanced interfacial contact performance and thermal tolerance should be attributed to the thick Ni barrier layer and interface reaction layer with good Ohmic contact. This work provides an effective strategy to establish stable joints for the Bi₂Te₃-based thermoelectric devices with improved thermal stability.

The simulated cooling performance of a thin-film thermoelectric cooler with coupled-thermoelements connected in parallel

The thermoelements of the traditional thin-film thermoelectric cooler (TEC) are connected electrically in series, thus the performance of traditional thin-film TEC reduces sharply when there is something wrong with any thermoelement. On account of this deficiency, we proposed a novel thin-film TEC with a couple of thermoelements electrically connected in parallel and then electrically connected in series to the next couple of thermoelements. The performance and reliability of the novel thin-film TEC is compared with the traditional thin-film TEC. The maximum cooling capacity, the maximum cooling temperature, and the coefficient of performance of the novel and the traditional thin-film TEC are systematically studied and compared when 0, 2, and 4 thermoelements are disabled, respectively. The results show that the performance and reliability of the novel thin-film TEC are superior to that of the traditional thin-film TEC, while the optimal electric current of the novel thin-film TEC current is 2.14 times of that for the traditional thin-film TEC. This work is of great significance to improving the performance and reliability of thin-film thermoelectric devices consisting of dozens of small thermoelements.

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A novel thin-film thermoelectric cooler with better reliability was demonstrated.

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The cooling performance of the new thin-film thermoelectric cooler is quantified.

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The new thin-film thermoelectric cooler is less sensitive to the local cracks.

Optimization of Interface Materials between Bi2Te3-Based Films and Cu Electrodes Enables a High Performance Thin-Film Thermoelectric Cooler

Thermoelectric interface materials (TEiMs) are key to optimizing the electrical contact and stability of the interface between thermoelectric material and metal electrode in high-performance thin-film thermoelectric coolers (TECs). Herein, we explored TEiMs applicable to representative Bi-Te films and found that Cr and Ag are effective TEiMs for p-type Bi_0.5Sb_1.5Te₃ and n-type Bi₂Te₃, respectively. By introducing 200 nm Cr and 200 nm Ag as TEiMs for p-type Bi_0.5Sb_1.5Te₃/Cu and n-type Bi₂Te₃/Cu interfaces, Cu diffusion is suppressed, and excellent electrical contact is achieved (1.81 × 10^-12 Ω m² for p-type and 3.32 × 10^-12 Ω m² for n-type) and remains stable after heat treatment (2.37 × 10^-12 Ω m² for p-type and 1.63 × 10^-12 Ω m² for n-type). Furthermore, the cooling flux of TECs with optimized TEiMs increases from 122.74 to 296.56 W/cm², while the performance degradation caused by contact resistance decreases from 50.81 to 4.15%. In addition, our results show that diffusion occurs between not only Cu but also Ag and the thermoelectric material, as TEiMs diffuse slightly. The diffusion of Cu and Ag at the interface can optimize the electrical contact of Bi₂Te₃/Cu but strongly degrade the electrical contacts of Bi_0.5Sb_1.5Te₃/Cu. Our work provides an optimal selection of TEiMs for high-performance Bi-Te thin film coolers and provides guidance for further miniaturization of devices.

High Interfacial Thermal Stability of Flexible Flake-Structured Aluminum Thin-Film Electrodes for Bi2Te3-Based Thermoelectric Devices

Environmental thermal energy harvesting based on thermoelectric devices is greatly significant to the advancement of next-generation self-powered wearable electronic devices. However, the rigid electrodes and interface diffusion of electrodes/thermoelectric materials would lead to the wearable discomfort and performance degradation of the thermoelectric device. Here, a flake-structured Al thin-film electrode with high conductivity and excellent reliability is prepared by regulating the microstructure and crystallinity of the films. The as-prepared Al thin film not only maintains its robustness after 1000 bending cycles but also does not delaminate from the substrate when subjected to the 3M tape test, exhibiting excellent flexibility and adhesion to substrate. By comparing with the annealed interface of the double-layer Cu/Bi₂Te₃ film, the interface of the heat-treated Al/Bi₂Te₃ film has almost no element diffusion, demonstrating high interfacial thermal stability. Moreover, a thermoelectric temperature sensor based on the Al thin-film electrode is prepared. The sensitivity of the annealed sensor is still linear, and it can stably monitor the temperature variation, showing high reliability. This discovery could provide a facile and effective strategy to achieving highly reliable thermoelectric devices and flexible electronic devices without any additional diffusion barriers.

Toward Reduced Interface Contact Resistance: Controllable Surface Energy of Sb2Te3 Films via Tuning the Crystallization and Orientation

The electrical contact resistance between a metal and semiconductor is one of the keys to improving the output performance of thin-film thermoelectric devices. Herein, we reduced the interface contact resistance by controlling the surface energy of a Sb₂Te₃ semiconductor via tuning of the crystallization and orientation, preparing an intrinsically compact and flat Sb₂Te₃ film with high surface energy and low roughness, which can give rise to a low average specific contact resistivity (8.2 × 10^-6 Ω cm²) with a Ni/Cu metal. The improvement in interface electrical properties is due to the increase in the surface energy and decrease in the surface roughness of the semiconductor surface, which lead to a transformation from three-dimensional island-shaped nucleation to two-dimensional layered nucleation for surface-attached metal films, forming a longitudinally tight connection contact with a low resistance. This approach allows the resistivity to become close to the fundamental theoretically calculated limit. Our work provides a new idea for reducing the contact resistivity of thin-film thermoelectric devices, which is conducive to supporting the development of thermoelectric semiconductor planarization.

Electrodeposited Thin-Film Micro-Thermoelectric Coolers with Extreme Heat Flux Handling and Microsecond Time Response

Thin-film thermoelectric coolers are emerging as a viable option for the on-chip temperature management of electronic and photonic integrated circuits. In this work, we demonstrate the record heat flux handling capability of electrodeposited Bi₂Te₃ films of 720(±60) W cm^-2 at room temperature, achieved by careful control of the contact interfaces to reduce contact resistance. The characteristic parameters of a single leg thin-film devices were measured in situ, giving a Seebeck coefficient of S = -121(±6) μV K^-1, thermal conductivity of κ = 0.85(±0.08) W m^-1 K^-1, electrical conductivity of σ = 5.2(±0.32) × 10⁴ S m^-1, and electrical contact resistivity of ∼10^-11 Ω m². These thermoelectric parameters lead to a material ZT = 0.26(±0.04), which, for our device structure, allowed a net cooling of ΔT_max = 4.4(±0.12) K. A response time of τ = 20 μs was measured experimentally. This work shows that with the correct treatment of contact interfaces, electrodeposited thin-film thermoelectrics can compete with more complicated and expensive technologies such as metal organic chemical vapor deposition (MOCVD) multilayers.

Enhanced thermoelectric properties of (015) plane-oriented n-type Bi2Se0.5Te2.5 films with wide temperature range stability

Enhanced thermoelectric properties with wide temperature range stability are achieved through a facile post-annealing process.

Effect of graphene nanofillers on the enhanced thermoelectric properties of Bi2Te3 nanosheets: elucidating the role of interface in de-coupling the electrical and thermal characteristics

In this report, we investigate the effect of graphene nanofillers on the thermoelectric properties of Bi₂Te₃ nanosheets and demonstrate the role of interface for enhancing the overall figure of merit (ZT) ∼ 53%. The enhancement in the ZT is obtained due to an increase in the electrical conductivity (∼111%) and decrease in the thermal conductivity (∼12%) resulting from increased conducting channels and phonon scattering, respectively at the interfaces between graphene and Bi₂Te₃ nanosheets. A detailed analysis of the thermal conductivity reveals ∼4 times decrease in the lattice thermal conductivity in contrast to ∼2 times increase in the electronic thermal conductivity after the addition of graphene. Kelvin probe measurements have also been carried which reveals presence of low potential barrier at the interface between graphene and Bi₂Te₃ nanosheets which assist the flow of charge carriers thereby, increasing the mobility of the carriers. Thus, our results reveals a significant decrease in the lattice thermal conductivity (due to the formation of interfaces) and increase in the electron mobility (due to conducting paths at the interfaces) strongly participate in deciding observed enhancement in the thermoelectric figure of merit.