Highly effective solid solution towards high thermoelectric and mechanical performance in Bi–Sb–Te alloys via Trojan doping

Abstract

The optimization of electrical and thermal transport properties of thermoelectric materials greatly relies on achieving higher solubility of doping elements. In this study, we present a novel approach named Trojan doping, which utilizes NaBiS₂ to increase the solubility of Na in p-type Bi_0.42Sb_1.58Te₃ (BST) alloys. NaBiS₂ is just like the Trojan horse, which brings Na ions into the BST matrix. The S elements were proved to play a role in keeping more Na dissolved in the BST matrix. This strategy reduces the total thermal conductivity of the BST alloy while enhancing its electrical transport properties due to the highly effective Na solid solution in the BST system. Among the BST + 4.0 wt% NaBiS₂ samples, the highest peak ZT value of 1.44 was achieved at 373 K with a high ZT platform beyond 1.4, from 323 to 373 K. Furthermore, analysis of the defect formation energy obtained through density functional theory (DFT) calculations revealed that the coordination between Na and S exhibits a lower defect formation energy compared to that exhibited by Te coordination in the vicinity of Na. This reduction in the defect formation energy is attributed to the greater strength and stability of Na–S bonds compared to Na–Te bonds, as confirmed by crystal orbital Hamiltonian calculations. The solid solution of Na and S also resulted in a conversion efficiency of about 6.0% in single-leg devices, indicating their potential application in multi-leg devices. Moreover, the mechanical properties of the sample showed significant improvement. Specifically, the sample with a doping level of 4.0 wt% exhibited a 44.9% increase in hardness and a 41.3% increase in the modulus compared to the pure sample. This work demonstrates the feasibility of increasing the solid solubility of doping elements through Trojan doping, highlighting its potential application in other thermoelectric materials.