Percolation Process-Mediated Rich Defects in Hole-Doped PbSe with Enhanced Thermoelectric Performance

PbSe–SnSe solid solutions have been extensively investigated in the fields of topological physics, optoelectronic conversion, and thermoelectric technology. In this study, we show that minute Sn-doped Pb_0.98Na_0.02Se behaves quite differently from conventional solid solutions. A dilute amount of Sn dopants induces rich defects (substitutional Sn atoms, interstitial Sn atoms, and cation vacancies) in hole-doped PbSe. Moreover, with growing concentration of Sn, the dominated defect type varies accordingly due to the percolation process of dopants, leading to anomalies in the dependences of thermoelectric properties on compositions. These rich defects increase the thermoelectric performance of p-type PbSe by (i) enhancement of Seebeck coefficient through increased density of states by interstitial Sn atoms; (ii) reduction of lattice thermal conductivity through strong point defect scattering mostly contributed by interstitial Sn atoms and cation vacancies. Moreover, the dynamic migrations of Sn atoms from the interstitial sites to regular lattice sites at elevated temperatures (>623 K) give rise to diffusion-like atomic vibrations that frustrate heat conduction further. Consequently, 1 mol % Sn doping in Pb_0.98Na_0.02Se yields 20% improvement of thermoelectric performance. This work demonstrates that dilute doping-induced percolation phenomena should be taken into consideration when developing efficient thermoelectric materials.