Multifold improvement of thermoelectric power factor by tuning bismuth and antimony in nanostructured n-type bismuth antimony telluride thin films

Abstract

Bismuth telluride (Bi2Te3) based alloys are preferred thermoelectric materials for room temperature thermoelectric applications. Electrical transport and thermopower properties of n-type nanostructured BixSb2-xTe3 films were tuned by post-growth heat treatment. We report that annealing of sputter-deposited nanostructured Bi0.7Sb1.3Te3 films lead to several-fold increases in thermoelectric power factor α2σ, where σ is electronic conductivity and α is Seebeck coefficient. Annealing at Tanneal = 200 °C nearly quadruples the power factor to α2σ = 3.1 μW cm−1 K−2. Spectroscopy and microscopy analyses indicate that the power factor increase is attributable to ~50% decrease in the Bi/Sb ratio, and grain growth, which increase the charge carrier concentration and mobility. The observed Bi depletion is contrary to annealing-induced Te depletion reported in p-type films. For Tanneal > 200 °C, although continued Bi depletion increases σ, a precipitous decrease in α sharply lowers the power factor, yielding tenfold lower α2σ for Tanneal = 400 °C than the as-deposited films. Our findings indicate that post-deposition annealing is a potent way to tune the thermoelectric properties of n-type Bi2Te3-based alloy films to fabricate devices for high-efficiency solid-state refrigeration and power harvesting from waste heat. Keywords: Thermoelectric, Bismuth antimony telluride, N-type, Power factor, Thin film