Enhancement of thermoelectric properties by effective K-doping and nano precipitation in quaternary compounds of (Pb_1XK_xTe)_0.70(PbSe)_0.25(PbS)_0.05

We investigated thermoelectric properties in K-doped quaternary compounds of (Pb_1-xK_xTe)_0.70(PbSe)_0.25(PbS)_0.05 (x ≤ 0.03). In terms of two valence bands model, we argue that the L-band approaches to the Σ-band with increasing the K-doping concentration resulting in the increase of carrier concentration and effective mass of carrier due to the increase of band degeneracy. The effective K-doping by x = 0.02 and PbS substitution causes high power factor and low thermal conductivity, resulting in the comparatively high ZT value of 1.72 at 800 K. The low thermal conductivity for (Pb_0.98K_0.02Te)_0.70(PbSe)_0.25(PbS)_0.05 compound is attributed from the lattice distortion and line dislocation in a phase of nano precipitation. By optimizing K-doping and PbS substitution, we achieved the enhancement of practical thermoelectric performance such as average ZT_avg = 1.08, engineering (ZT)_eng = 0.81, maximum efficiency η_max = 11.63%, and output power density P_d = 6.3 W cm^-2, with temperature difference ΔT = 500 K, which has practical applicability in waste heat power generation technologies.