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Multi-layered-stack thermoelectric generators using p-type Sb2Te3 and n-type Bi2Te3 thin films by radio-frequency magnetron sputtering
Ken Takayama, Masayuki Takashiri*
Department of Materials Science, Tokai University, 4-1-1 Kitakaname, Hiratsuka, Kanagawa 259-1292, Japan
Article history: Received 25 June 2017 Received in revised form 25 July 2017 Accepted 26 July 2017 Available online 28 July 2017
Keywords: Multi-layered-stack thermoelectric generators Sb2Te3 Bi2Te3 Thermal annealing
abstract
To provide power to electronic sensors operating at low power, we prepared multi-layered-stack thermoelectric generators using radio-frequency (RF) magnetron sputtering. Prior to the preparation of thermoelectric generators, Sb2Te3 and Bi2Te3 thin films were deposited on glass substrates followed by carrying out thermal annealing at temperatures ranging from 200 to 400 C in order to investigate and improve their thermoelectric properties. Both the films exhibited the maximum power factor values measured at room temperature, namely,12.7 mW/(cm$K2) for Sb2Te3 and 10.2 mW/(cm$K2) for Bi2Te3, at an annealing temperature of 300 C. Therefore, the films annealed at 300 _C are suitable for fabricating multi-layered-stack thermoelectric generators. To prepare thermoelectric generators, Sb2Te3 and Bi2Te3 thin films were deposited on the top and bottom sides of 0.3 mm-thick glass substrates, respectively. Eleven sample pieces were connected in series by spraying silver paste to obtain the multi-layered-stack thermoelectric generators. Generators with dimensions of 20 30 mm2 and a thickness of 7 mm were fabricated. The thermoelectric generators exhibited an open circuit voltage of 32 mV and maximum output power of 0.15 mW at a temperature difference (on both ends) of 28 K. copy; 2017 Elsevier Ltd. All rights reserved
1. Introduction
Energy harvesting technology, conversion of ambient energy into electrical energy, has been recently regarded as one of the attractive power sources for obtaining power ranging from nanowatts to milliwatts. This technology can be applied for low-power devices such as wireless sensor nodes [1,2], wearable sensors [3,4], and hearing aids [5]. Various ambient energy sources are suitable for portable energy harvesting systems, including mechanicalenergy[6,7],electromagneticenergy[8,9],thermalenergy [10e12]. Since thermal energy around room temperature (RT) is easily available and accessible dayand night, thermal energy is the most attractive form of energy for use in thermoelectric generators among various energy harvesting systems. In general, there are two types of thermoelectric generators, namely, bulk thermoelectric generators [13,14] and thin-film generators [15e17]. In bulk generators, the temperature difference between the hot and cold sides can be easily achieved, thereby yielding a relatively high electric power. However, it is difficult to
reduce the device size and manufacturing cost of the bulk generatorsowingtothedifficultyinusingthebatchprocess.Ontheother hand,thin-filmgeneratorsexhibitoppositefeaturesascomparedto those of the bulk generators; i.e., it is possible to reduce the device size and manufacturing cost, but it is challenging to produce a high electric power. Therefore, the ideal strategy for fabricating thermoelectric generators is to combine the favorable features of bulk and thinfilm generators. Hence, the fabrication of a multi-layered-stack type generator composed of thermoelectric thin films is promising [18,19]. In this type of generators, a number of thin glass substrates covered with p- and n-type thermoelectric thin films on the sides are fabricated, and p-n couples are formed byconnecting with metal electrodes. Owing to this structure, heat flows parallel to the surface of p- and n-type films and it is possible to easily achieve a temperature difference between the hot and cold sides. One of the key issues in fabricating high-quality thermoelectric generators is to deposit thin films exhibiting good thermoelectric properties. Among thin-film deposition methods, sputtering [20e22], vacuum evaporation [23,24], and electrodeposition [25,26] are regarded as the conventional methods. These deposition methods have relatively high deposition rates as well as a low equipment cost and running cost. In particular, sputtering can be used to deposit uniform thin films on a large area and achieve excellent adhesion between the film and substrate [27,28]. However, the as-deposited thin films deposited using these conventional methods exhibit inferior thermoelectric properties as compared to the corresponding well-established bulk materials [29]. Therefore, additional treatments, including electron beam irradiation [30e32], thermal annealing [33e35], and two-step treatment [36,37], are needed. In this study, we fabricated multi-layered-stack thermoelectric generators composed of p-type Sb2Te3 and n-type Bi2Te3 films using radio-frequency (RF) magnetron sputtering. Sb2Te3 and Bi2Te3 materials were selected as they exhibit good thermoelectric properties around RT [29]. Wefirst examined the thermoelectric properties of p- and n-type thin films as a function of their annealing temperatures. The open circuit voltage (Voc) and maximum output power (Pmax) of the thermoelectric generators were estimated by applying a temperature difference between the ends of the generator.
2. Experimental section
2.1. Thin film deposition and optimization of film properties
To fabricate multi-layered-stack thermoelectric generators exhibiting a high performance, we initially investigated the properties of
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