Heavy doping and band engineering by potassium to improve the thermoelectric figure of merit in p-type PbTe, PbSe, and PbTe _{1- y}Se _y

We present detailed studies of potassium doping in PbTe _1-ySe _y (y = 0, 0.15, 0.25, 0.75, 0.85, 0.95, and 1). It was found that Se increases the doping concentration of K in PbTe as a result of the balance of electronegativity and also lowers the lattice thermal conductivity because of the increased number of point defects. Tuning the composition and carrier concentration to increase the density of states around the Fermi level results in higher Seebeck coefficients for the two valence bands of PbTe _1-ySe _y. Peak thermoelectric figure of merit (ZT) values of ∼1.6 and ∼1.7 were obtained for Te-rich K _0.02Pb _0.98Te _0.75Se _0.25 at 773 K and Se-rich K _0.02Pb _0.98Te _0.15Se _0.85 at 873 K, respectively. However, the average ZT was higher in Te-rich compositions than in Se-rich compositions, with the best found in K _0.02Pb _0.98Te _0.75Se _0.25. Such a result is due to the improved electron transport afforded by heavy K doping with the assistance of Se.