Effect of Pulse Frequency on Structural and Thermoelectric Properties of Bismuth Telluride Thin Films By Electrodeposition
Authors: Okuhata Mitsuaki , Takashiri Masayuki 
Published: February 2020

Recently, thin film materials for environmental energy harvesting such as solar cells and thermoelectric generators have been growing the interest. In particular, thin film thermoelectric generators are promising devices with applications in sensor nodes for the internet of things (IoT).

To justify a wider range of practical applications for these devices, higher performance thermoelectric materials are needed. Thermoelectric performance is defined by a figure of merit, ZT =
s
S2T/
k, where S is the Seebeck coefficient, s is the electrical conductivity, and k is the thermal conductivity. The figure of merit depends on the temperature, and its maximum value is different for different kinds of thermoelectric materials. Among thermoelectric materials, bismuth telluride (Bi2Te3) and alloys based on it are especially favorable because they exhibit a high figure of merit near room temperature (RT), and can be produced in both n-type and p-type.

Electrodeposition is one of the most important methods of film fabrication, offering low-cost growth of high-quality metal, alloy, and semiconductor films. This is because 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 a pulse-electrodeposition method to improve the thermoelectric performance. We used bismuth telluride (Bi2Te3) as a thermoelectric material because this material exhibits high performance near room temperature (RT). For the electrodeposition, we used a standard three-electrode cell with a potentiostat and a function generator. The working electrode was a stainless steel (SUS304) substrate, chosen because of its excellent corrosion resistance. A platinum-coated titanium mesh on a titanium plate was used as the counter electrode. A Ag/AgCl (saturated KCl) electrode was used as the reference electrode. The pulse frequency was varied from 1 Hz to 10 kHz whereas the mixing ratio (Te/[Te+Bi]) of Bi(NO3)3, and TeO2was constant through all the experiments. To avoid complications in analysis due to the electrical conduction of the stainless steel substrate, the film was fixed on a glass plate using an epoxy resin, followed by thin film removal from the substrate. 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.

The most apparent changes between the thin films with the pulse-electrodeposition and that with the potential constant electrodeposition were the electrical conductivity. The thin films prepared by the potential constant electrodeposition obtained the electrical conductivity and the other properties as follows; s = 135.6 S/cm, S = −57.3 µV/K, and P.F. = 0.44 µW/(cm·K2). The thin films prepared by the pulse-frequency electrodeposition at 10 Hz obtained the electrical conductivity and the other properties as follows; s = 211.8 S/cm, S = −52.9 µV/K, and P.F. = 0.59 µW/(cm·K2). Therefore, we observed that the thermoelectric properties were improved by applying pulse potential to the electrodeposition. To further investigate the effect of pulse frequency of electrodeposition, we plan to analyze the surface morphology and crystallographic properties.