Abstract
Thermoelectric material, that can convert thermal energy into electrical energy and vice versa, is one potential candidate of green materials. Bismuth telluride based compounds have superior thermoelectric figure of merit at room temperature regime. In this research, Bi2Te3 thin film with inverse opal structure that have three different pore sizes (PS-300 ( = 300 nm), PS-400 ( = 400 nm), and PS-600 ( = 600 nm)) as well as dense Bi2Te3 films were prepared by electrodeposition under a fixed current density of 2.5 mA/cm2. Owing to ordered pore arrangement (closed packing), a systematic study of structure and thermoelectric properties of Bi2Te3 films with the inverse opal structure can be conducted. The aim of this research is to investigate the influence of annealing process on crystallographic microstructure and thermoelectric properties of Bi2Te3 film with different pore sizes, and compare it with the results of dense Bi2Te3 films. The x-ray diffraction results reveal that all the as-deposited specimens have (110) preferred orientation. The intensity of (110) reflection increases with annealing in the early stage due to elimination of vacancies. With extended annealing time, the intensity of (110) reflection decreases while those of other reflections increases due to recrystallization process in the Bi2Te3 films. It is speculated that precipitation of Bi-rich phase and formation of p-type antisite defects compete with each other in the inverse opal structure with different pore size during annealing process. The PS-600 specimen has the smallest number of pores and the best crystallinity, resulting in the largest amount of vacancies eliminated during thermal annealing. Owing to the large number of pores and the thin walls between the pores restrict grain growth and transport of charge carriers, the annealed Bi2Te3 films with inverse opal structure have smaller mobility and in turn higher electrical resistivity than the dense Bi2Te3 films. However, the Bi2Te3 films with inverse opal structure show an enlarged Seebeck coefficient caused by good crystallinity and abundant scattering interfaces.