Improving the thermoelectric properties of thick Sb₂Te₃ film via Cu doping and annealing deposited by DC magnetron sputtering using a mosaic target

Highlights

• Thick films of Cu doped Sb₂Te₃ were deposited by mosaic target for the first time.

• Structural measurements indicated that Cu atoms occupy the Sb position in Sb₂Te₃.

• DFT was used to clarify internal mechanism of Cu dopant into the Sb₂Te₃ system.

• PF significant increase after Cu doped at appropriate amount and annealing process.

Abstract

Thick Cu-doped Sb₂Te₃ films were deposited on flexible substrate by DC magnetron sputtering from a mosaic Cu–Sb₂Te₃ target. The Cu-doped Sb₂Te₃ films were vacuum annealed to improve their thermoelectric properties. Density functional theory was used to clarify the internal mechanism of the Cu doped into the Sb₂Te₃ system. The results showed that Cu substitution on a Sb site induced electronic states or impurity peaks of Sb₂Te₃ at a valence band maximum. The carrier concentration of the Cu-doped Sb₂Te₃ films increased as the Cu-doped concentration increased. However, the crystallite size and Seebeck coefficient of the Cu-doped Sb₂Te₃ films decreased as the Cu-doped concentration increased. Post-annealing treatment improved the microstructure and thermoelectric properties of the Cu-doped Sb₂Te₃ films. The maximum electrical conductivity and power factor values of 754.20 S/cm at 50 °C and 1.56 10⁻³ W/mK² at 100 °C were obtained in the annealed film with a Cu-doped concentration of 3 at%.

Introduction

Energy harvesting is the process of changing energy that is a by-product of an energy source used for various applications. The efficiency of energy harvesting with a specific device depends on the type and amount of energy [[1], [2], [3]]. Thermoelectric energy harvesting directly converts thermal energy into electrical energy using thermoelectric materials. The performance of a thermoelectric material is defined by a figure of merit ZT = S²σT/κ and power factor P = S²σ, where σ, S, κ, and T are the electrical conductivity (S/m), Seebeck coefficient (V/K), thermal conductivity (W/mK), and absolute temperature (K), respectively. Applying a thermoelectric material for a power generation device that can be used as a power source is called a thermoelectric module via mixing n-type and p-type thermocouples connected in pairs alternately with ceramic plates on both sides. When the thermoelectric module contacts the heat source and is connected to the external load, electricity is produced. The advantage of thermoelectric modules is that they are small, lightweight, and environmentally friendly materials [4,5]. However, current thermoelectric modules cannot be exposed to heat sources with curved or rough surfaces due to their inflexible physical characteristics and cannot bend [[6], [7], [8]]. As reported previously, this limits their applications. Therefore, flexible thermoelectric modules must be developed that can be effectively used with various types of heat sources [[9], [10], [11]]. To create flexible thermoelectric modules, thin or thick films must be prepared. Nevertheless, in many applications, thermoelectric thin films still face challenges: for horizontal use, the problem involves the difference in the temperature between the hot and cold sides, and for vertical use, the problem is insufficient heat flowing into thermoelectric materials [11,12]. In this study, we proposed thick film to solve these problems. However, the thick film had a low electrical conductivity, and when the thickness increased, the film's porosity increased [[12], [13], [14]]. The aforementioned reasons resulted in lower electrical conductivity and the thermoelectric properties decreased. Producing a flexible thermoelectric thick film with satisfactory electrical conductivity and thermoelectric properties is challenging.

Sb₂Te₃ compounds are favoured by p-type thermoelectric material because Sb₂Te₃ has a high ZT near room temperature [[15], [16], [17]]. The principal structure of Sb₂Te₃ consisted of five atomic Sb and Te hexagonal planes oriented perpendicular along the c-axis with Te(1)-Sb-Te(2)-Sb-Te(1), and there were van der Waals interactions between the layers of Te(1) and Te(1) atomic planes [18,19]. Various strategies have been adopted to improve the thermoelectric performance of Sb₂Te₃. One important strategy is doping, such as doping with Pb, Ag, and Cu [20,21]. Cu is a useful element to increase the thermoelectric properties via increasing the electrical conductivity in Sb₂Te₃ thin films [21].

In this study, we used a mosaic target that consisted of a metallic matrix with other metals inserted. This technique promotes the magnetron sputtering approach for deposition of multicomponent films. The copper content in the Sb₂Te₃ thick films was adjusted by varying the diameter of the Cu plate from 0 to 9 mm. Cu-doped Sb₂Te₃ films were deposited by the DC magnetron sputtering method and the microstructure and thermoelectric properties of the as-deposited and annealed films were investigated. The controlled addition of Cu doping was useful not only to improve the electrical properties but also to tune the thermoelectric properties of the Sb₂Te₃ films. As a result, the power factor significantly increased after doping with an appropriate amount of elemental Cu during the post-annealing process.