Abstract
Thermoelectric devices based on the Seebeck effect can directly convert waste heat to electricity, but the demand for better thermoelectric materials remains unfulfilled. The lead-free halide double perovskites have recently shown promises for their stability and environment-friendly nature, but impacts from intrinsic defects (vacancies and anti-sites) on their thermoelectric performance remains elusive. Combing first-principle calculations, the Boltzmann transport theory, and the defect formation theory, we investigate the thermoelectric properties of lead-free double perovskite Cs_2NaInCl_6, taking the intrinsic defects into consideration. Our results demonstrate that the pristine Cs_2NaInCl_6 presents thermodynamic, mechanical, and dynamic stability. The band edges are mainly comprised of the electrons from In and Cl atoms. Furthermore, the double perovskite has an ultra-low lattice thermal conductivity and a high Seebeck coefficient, while showing a small charge carrier relaxation time and electric conductivity. Promisingly, the maximum ZT values can reach as high as ～ 1.36 and ～ 1.44 at 800 K temperature, respectively, with optimal extrinsic carrier concentrations of ～ 1.9 × 10~(19) and 5.8 × 10~(20) cm~(-3) for N- and P-type carriers (holes and electrons), and the maximum power generation efficiency can reach ～ 14% (for P-type) when the hot-side temperature is 800 K. Among various intrinsic defect types, we determine the preferable defect types in Cs_2NaInCl_6 based on the minimal defect formation energy criteria. The free carriers can be switched from N- to P-type under these two preferable defects, respectively, with Cl and Na vacancies. Even under a very high defect concentration of 2.1 × 10~(19)/cm~3 considered here, the extra carrier concentrations induced by the defects are only ～ 10~(17) cm~(-3) at 800 K, showing strong defect tolerance in carrier concentration. This work suggests that lead-free halide double perovskites are promising thermoelectric materials, and they show strong intrinsic defect tolerance that prevents a negative impact on the extrinsic doping-controlled carrier concentrations.
NSTL
Elsevier