Enhancement of Thermoelectric Performance in Na-Doped Pb_0.6Sn_0.4Te_0.95-xSe_xS_0.05 via Breaking the Inversion Symmetry, Band Convergence, and Nanostructuring by Multiple Elements Doping

Topological insulators have attracted much interest in topological states of matter featuring unusual electrical conduction behaviors. It has been recently reported that a topological crystalline insulator could exhibit a high thermoelectric performance by breaking its crystal symmetry via chemical doping. Here, we investigate the multiple effects of Na, Se, and S alloying on thermoelectric properties of a topological crystalline insulator Pb_0.6Sn_0.4Te. The Na doping is known to be effective for breaking the crystalline mirror symmetry of Pb_0.6Sn_0.4Te. We demonstrate that simultaneous emergence of band convergence by Se alloying and nanostructuring by S doping enhance the power factor and decrease lattice thermal conductivity, respectively. Remarkably, the high power factor of 22.3 μW cm^-1 K^-2 at 800 K is achieved for Na 1%-doped Pb_0.6Sn_0.4Te_0.90Se_0.05S_0.05 mainly due to a relatively high Seebeck coefficient via band convergence by Se alloying as well as the suppression of bipolar conduction at high temperatures by the increase of energy band gap. Furthermore, the lattice thermal conductivity is significantly suppressed by PbS nanoprecipitates without deteriorating the hole carrier mobility, ranging from 0.80 W m^-1 K^-1 for Pb_0.6Sn_0.4Te to 0.17 W m^-1 K^-1 at 300 K for Pb_0.6Sn_0.4Te_0.85Se_0.10S_0.05. As a result, the synergistically combined effects of breaking the crystalline mirror symmetry of topological crystalline insulator, band convergence, and nanostructuring for Pb_0.6Sn_0.4Te_0.95-xSe_xS_0.05 (x = 0, 0.05, 0.1, 0.2, and 0.95) give rise to an impressively high ZT of 1.59 at 800 K for x = 0.05. We suggest that the multiple doping in topological crystalline insulators is effective for improving the thermoelectric performance.