Thin-film Legs Research Articles

Transparent flexible thin-film p\u2013n junction thermoelectric module

Transparent and flexible thermoelectrics has been highly sought after for future wearable devices. However, the main stumbling block to prevent its widespread adoption is the lack of p-type transparent thermoelectrics and the stringent criteria of electrical and thermal properties matching appropriately between p-legs and n-legs. This work demonstrates the fabrication of p-type PEDOT:PSS films whose optical properties, electrical conductivity, thermal conductivity, and Seebeck coefficient were engineered to perfectly match the n-type indium tin oxide (ITO) counterparts. The dense p-type PEDOT:PSS and n-type ITO thin films show a thermoelectric figure of merit of zT = 0.30 and 0.29 at 450 K, and a thermal conductivity of 0.22 and 0.32 W m−1 K−1, respectively. A flexible thermoelectric generator (TEG) module with a high transmittance of >81% in the visible wavelength range of 400–800 nm is fabricated using 10 pairs of p-type PEDOT:PSS and n-type ITO thin film legs. An ultra-high power density of 22.2 W m−2 at a temperature gradient of 80 K was observed, which is the highest power density reported for organic/hybrid-based flexible TEGs so far. Our transparent flexible thin-film p–n junction thermoelectric module with exceptionally high power generation may take a tremendous step forward towards multi-functional wearable devices.

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.

A thin film thermoelectric device fabricated by a self-aligned shadow mask method

A planar thermoelectric device with 20 pairs of Sb2Te3 and Bi2Te3 thin-film legs was fabricated by using a facile self-aligned shadow mask method in combination with a thermal evaporation process. Effects of substrate temperature during the evaporation process on the Seebeck coefficient and electrical conductivity of the thin films have been investigated. The maximum Seebeck coefficient was found to be 158 µV K−1 for the Sb2Te3 film produced at 260 °C and −148 µV K−1 for the Bi2Te3 film produced at 230 °C. The device was fabricated on a glass substrate under the above optimal conditions. At a temperature difference of 90 K, it demonstrated an open-circuit voltage of more than 0.5 V (which was equivalent of a sensitivity of 5.40 mV K−1) and a maximum output power of 1.105 µW.

On-chip thermoelectric module comprised of oxide thin film legs

On-chip thermoelectric thin film modules containing 5 legs of n-type (Al0.02Zn0.98O) and 5 legs of p-type (Ca3Co4O9) were fabricated on Al2O3, SrTiO3 single crystal, and fused silica substrates by pulsed laser deposition technique. Performance of modules was evaluated using ad hoc customized system where the module was set vertically for temperature gradient. The maximum output power (Pmax) was obtained on Al2O3 (0001) single crystal substrate with temperature difference (ΔT)=230°C (Th=300°C): Pmax=29.9pW. The value of maximum output power increases with increase of temperature difference (ΔT). These results are encouraging for the practical applications of thermoelectric oxide thin films.

Micro-Power Generation Characteristics of Thermoelectric Thin Film Devices Processed by Electrodeposition and Flip-Chip Bonding

A thermoelectric thin film device of cross-plane configuration was fabricated by the flip-chip process using an anisotropic conductive adhesive. The Cu/Au bonding bumps electrodeposited on the Ti/Cu/Au electrodes in the top substrate were flip-chip bonded to the 242 pairs of the n-type Bi2Te3 and p-type Sb2Te3 thin film legs electrodeposited on the Ti/Cu/Au electrodes in the bottom substrate. Using the output voltage–current curve, the internal resistance of the thin film device was measured to be 21.4 Ω at temperature differences of 9.8–39.7 K across the device. The thin film device exhibited an open-circuit voltage of 320 mV and a maximum output power of 1.1 mW with a power density of 3.84 mW/cm2 at a temperature difference of 39.7 K applied across the thin film device.

Preparation and Characterization of Bi2Te3/Sb2Te3 Thermoelectric Thin-Film Devices for Power Generation

A device based on a new double-layer-leg thin-film concept has been successfully fabricated by flip-chip bonding of 242 pairs of n-type Bi2Te3 and p-type Sb2Te3 thin-film legs electrodeposited on top substrates to those processed on bottom substrates. Based on the output voltage–current curve, the internal resistance of the double-layer-leg thin-film device was measured to be 3.47 kΩ at an apparent temperature difference of 25.9 K across the device. The actual temperature difference across the thin-film legs was estimated to be 3.51 K, which is ∼13% of the apparent temperature difference ΔT of 25.9 K applied across the thin-film device. The double-layer-leg thin-film device exhibited an open-circuit voltage of 0.43 V and maximum output power of 13.1 μW at an apparent temperature difference ΔT of 25.9 K.

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 ΔTG 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.

Thermoelectric Characteristics of n-Type Bi<sub>2</sub>Te<sub>3</sub> and p-Type Sb<sub>2</sub>Te<sub>3</sub> Thin Films Prepared by Co-Evaporation and Annealing for Thermopile Sensor Applications

A thermoelectric thin-film device consisting of n-type Bi2Te3 and p-type Sb2Te3 thin-film legs was prepared on a glass substrate by using co-evaporation and annealing process. The Seebeck coefficient and the power factor of the co-evaporated Bi2Te3 film were −30 µV/K and 0.7 × 10−4 W/K2·m, respectively, and became substantially improved to −160 µV/K and 16 × 10−4 W/K2·m by annealing at 400°C for 20 min. While the Seebeck coefficient of the co-evaporated Sb2Te3 thin film was 72 µV/K, it increased significantly to 165–142 µV/K by annealing at 200–400°C for 20 min. A maximum power factor of 25 × 10−4 W/K2·m was achieved for the co-evaporated Sb2Te3 film by annealing at 400°C for 20 min. A thermopile sensor consisting of 10 pairs of n-type Bi2Te3 and p-type Sb2Te3 thin-film legs exhibited a sensitivity of 2.7 mV/K.

Thermoelectric Thin Film Device of Cross-Plane Configuration Processed by Electrodeposition and Flip-Chip Bonding

Using electrodeposition and flip-chip bonding, a cross-plane thin film device consisting of 242 pairs of the electrodeposited n-type Bi–Te and p-type Sb–Te thin film legs was successfully fabricated. The electrodeposited Bi–Te films with the thickness of 2.5–20.2 µm exhibited the Seebeck coefficients of −52 to −59 µV/K and the power factors of 5.5–5.1 × 10−4 W/m·K2. While the Seebeck coefficient of the Sb–Te film varied from 276 to 485 µV/K, the power factor was changed from 81 × 10−4 to 50 × 10−4 W/m·K2 with increasing the film thickness from 2.2 to 20.5 µm. The internal resistance of the thin film device consisting of 242 pairs of the electrodeposited n–p thin film legs was measured as 3.7 KΩ. The open-circuit voltage and the maximum output power of the thin film device were 0.294 V and 5.9 µW, respectively, with the temperature difference of 22.3 K across the hot and cold ends of the thin film device.

Processing and Thermoelectric Performance Characterization of Thin-Film Devices Consisting of Electrodeposited Bismuth Telluride and Antimony Telluride Thin-Film Legs

Thermopile thin-film devices were fabricated by successive electrodeposition of p-type Sb-Te and n-type Bi-Te films. The thermopile processed with 1-μm-thick SiO2 as an insulating layer on the thin-film legs exhibited sensitivity of 57.5 mV/K, much larger than the 7.3 mV/K measured for a thermopile with an insulating layer of 6-μm-thick photoresist. Sensitivity of 30.4 mV/K was obtained for a thermopile with a 1-μm-thick SiNx insulating layer.

THERMOELECTRIC CHARACTERISTICS OF THE THERMOPILE SENSORS PROCESSED WITH THE ELECTRODEPOSITED Bi–Te AND Sb–Te THIN FILMS

A thermopile sensor composed of 196 pairs of p–n thin film legs was processed on a glass substrate by using successive electrodeposition of the n-type Bi–Te and the p-type Sb–Te thin films. The 5.3-μm thick Bi–Te film, electrodeposited at a constant voltage of -50 mV in the 50-mM electrolyte with the Bi/(Bi + Te) mole ratio of 0.5, exhibited a Seebeck coefficient of -67 μV/K. The 5.2-μm thick Sb–Te film, electrodeposited at a constant voltage of 20 mV in the 70-mM solution with the Sb/(Sb + Te) mole ratio of 0.9, possessed a Seebeck coefficient of 63 μV/K. The thermopile sensor exhibited the sensitivities of 13.1 mV/K with temperature differences smaller than 9 K and of 27.3 mV/K with temperature differences larger than 9 K respectively, across the hot and cold ends.

Thermoelectric Characteristics of the Thermopile Sensors with Variations of the Width and the Thickness of the Electrodeposited Bismuth-Telluride and Antimony-Telluride Thin Films

Thermopile sensors were processed on glass substrates by using successive electrodeposition of p-type Sb-Te and n-type Bi-Te thin films, and their thermoelectric characteristics were measured. The thermopile sensor, consisting of the p-n legs of 2 μm-thickness and 50 μm-width, exhibited the sensitivity of 24.8 mV/K. By changing the width of the p-n thin-film legs from 50 to 100 μm, the sensitivity decreased to 15.4 mV/K because of less pairs of the p-n thin-film legs in the thermopile. With increasing the thickness of the thin-film legs from 2 to 5 μm, the sensitivity was improved to 36.5 mV/K due to higher Seebeck coefficients of the 5 μm-thick Bi-Te and Sb-Te films than those of the 2 μm-thick films.