Thermoelectric Properties of Electrodeposited Bismuth Telluride Thin Films By Electron Beam Irradiation and Thermal Annealing
Authors: Takemori Daichi , Takashiri Masayuki 
Published: February 2020

Recently, thermoelectric materials have attracted considerable interest because of developments in energy harvesting technology. These materials are able to covert thermal energy into electric energy, and vice versa convert electricity into heat. In particular, thermoelectric generators produce electric power from thermal energy for a variety of uses for energy harvesting applications in sensor nodes for the internet of things (IoT). Thermoelectric generators require high energy conversion efficiency (thermoelectric performance), miniaturization, and low manufacturing costs.

The thermoelectric performance is dependent on a dimensionless figure of merit (ZT), which is defined as ZT = σ
S2T/κ, where σ is the electrical conductivity, S is the Seebeck coefficient, T is the absolute temperature, and κ is the thermal conductivity. To improve the thermoelectric performance, the power factor, σS
2, should be maximized and the thermal conductivity should be minimized.

Thin film technology is beneficial from the viewpoint of miniaturization as well as the low manufacturing cost of thermoelectric generators. The advantage of thin film technology is that it allows for deposition of thin films over a large area at once and fabrication of fine structures. In addition, this technology makes it relatively easy to control the crystal growth and atomic composition of the materials. Among many deposition methods, electrodeposition is one of the most favorable because it is very cost effective; this is because of its easy scalability, high deposition rate, and the fact that it involves operation at low temperature with no requirement for vacuum conditions. However, the thin films prepared by electrodeposition generally obtained relatively low thermoelectric performance compared with that by other deposition methods.

Therefore, in this study, we performed an electron beam (EB) irradiation treatment and a thermal annealing to the electrodeposited thin films for improving the thermoelectric performance. We used bismuth telluride (Bi2Te3) as a thermoelectric material because this material exhibits high performance near room temperature (RT). Prior to the electrodeposition, to determine the appropriate potential range for depositing Bi2Te3 films, cyclic voltammetry (CV) was performed with a standard three-electrode cell in an unstirred electrolyte solution consisting of Bi(NO3)3, TeO2, and hydrochloric acid diluted by deionized water. Based on the obtained cyclic voltammogram, it is apparent that the appropriate applied potential for the deposition of Bi2Te3 thin films is in the range of −0.05 to −0.15 V vs Ag/AgCl. We set the applied potential to −0.08 V vs Ag/AgCl at the film deposition.

The thin films were electrodeposited on stainless steel (SUS 304) substrate with different mixing ratio (Te/[Te+Bi]) of Bi(NO3)3, and TeO2. The structural properties were evaluated by means of scanning electron microscopy (SEM) and XRD (X-ray diffraction) analysis. The thermoelectric properties, which were electrical conductivity, Seebeck coefficient and power factor, were measured at RT. As a result, we confirmed that the highest performance was achieved at a mixing ratio of 0.4 and 0.5. The thermal annealing was performed at 300°C for 1 h in mixed gas (argon 95% and hydrogen 5%). The electrical conductivity was improved by the annealing because of high crystallinity, and thus the power factor was also improved.

To further enhance the thermoelectric performance, we investigate the effect of thermal annealing in detail. In addition, we plan to perform the EB irradiation treatment and two-step method which is sequentially treated with EB irradiation and thermal annealing.