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

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 alpha(2)sigma, where sigma is electronic conductivity and alpha is Seebeck coefficient. Annealing at T-anneal = 200 degrees C nearly quadruples the power factor to alpha(2)sigma = 3.1 mu Wcm(-1) K-2. Spectroscopy and microscopy analyses indicate that the power factor increase is attributable to similar 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 T-anneal >200 degrees C, although continued Bi depletion increases sigma s, a precipitous decrease in alpha sharply lowers the power factor, yielding tenfold lower alpha(2)sigma for T-anneal = 400 degrees 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. (c) 2018 Elsevier Ltd.