The p-n conduction type transition in Ge-incorporated Bi2Te3 thermoelectric materials

Abstract The capability of converting waste heat into electricity characterizes the technology of thermoelectricity as a feasible solution in dealing with the energy issue. The Bi2Te3-based alloys have long been in the spotlight as they show promising zTs and high thermal stability near the room-temperature region. Herein, the experimentally determined 523 K isothermal section of ternary Bi-Ge-Te system concludes that the four ternary compounds, the Ge3Bi2Te6, the Ge1.5Bi1.5Te5, the Ge1Bi2Te4 and the Ge1Bi4Te7, are thermally stabilized and the solubility of Ge in Bi2Te3 could reach ∼5.0 at.%Ge. The thermoelectric performance of various Ge-incorporated Bi2Te3 alloys with compositions of Bi40-yGexTe60-x+y (x = 0–12.5, y = 0–10.0) are evaluated within 300 K-650 K. The conduction type of those alloys transits from p-type, for the un-doped Bi2Te3, to n-type semiconducting as one of the following conditions are satisfied: (1) the concentration of Te drops to less than 60.0 at.%, or (2) the concentration of Te exceeds 60.0 at.% and the ratios of Ge/Bi is larger than 0.2. The zT peak values reach zT∼0.9 at 325 K for p-type alloy P2 (Bi39.0Ge1.0Te60.0), which features a Bi2Te3 single-phase microstructure, and zT∼0.45 at 525 K for n-type alloy N5 (Bi32.5Ge10Te57.5), which falls in a Bi2Te3+Ge1Bi2Te4+Te three-phase region, respectively.