Improvement of thermoelectric properties of flexible Bi2Te3 thin films in bent states during sputtering deposition and post-thermal annealing

High-quality thermoelectric thin films formed on flexible substrates are important for the development of flexible thermoelectric generators as power supplies for wireless sensors and wearable devices. However, the thermoelectric properties of the films formed on flexible substrates are inferior to those of films formed on rigid substrates because flexible substates shrink during the processes of film deposition and post-thermal annealing. To overcome this problem, we propose a modification during these processes. Bismuth telluride thin films were deposited on polyimide substrates using radio-frequency magnetron sputtering. The substrates were bent to a convex or concave shape, followed by thermal annealing under the same bending conditions. The crystallite size and crystal orientation improved when the films were set to a concave shape during the film deposition and post-thermal annealing processes. This phenomenon occurred presumably because the substrate shrinkage was counteracted by the stretching of the substrate, and the strains did not accumulate much in the films, resulting in improved crystal growth. Consequently, the electrical conductivity was increased, and the resulting power factor of 16.1 μW/(cm K2) is 1.4 times higher than that of the Bi2Te3 film formed on a flat substrate.