Synergistic Mixed-Valence Yb Doping Defect Control Enable High-Performance Flexible Films

Flexible thermoelectric generators (TEGs) require materials that combine high power factor (PF) with mechanical durability. p -type Bi-Sb-Te alloys are state-of-the-art near room temperature, but improving their carrier concentration without degrading mobility remains challenging. Here, we introduce a 'defect-balanced' co-doping strategy by adding trace YbTe (0-0.25 mol%) to Bi _0.5 Sb _1.5 Te ₃ thin films. This approach yields a peak PF of ~2.40×10 ³ µW/(m.K ² ) at 300 K for the 0.15 mol% YbTe film an ~85% improvement over an undoped film (~1.30×10 ³ ). First-principles calculations and experiments reveal that the performance boost arises from a unique defect-compensation mechanism: Yb atoms occupy Sb sites in mixed valence (Yb ²⁺ /Yb ³⁺ ), with Yb ²⁺ providing holes, while excess Te from Yb _Te creates Te _Sb antisite donors.