Boundary Engineering‐Induced Strong Yet Ductile Bi ₂ Te ₃ Thin Film with High Thermoelectric Performance

Abstract Bismuth telluride (Bi 2 Te 3 )‐based thermoelectrics have emerged as prime candidates for wearable and low‐grade heat harvesting. However, the brittleness and insufficient mechanical strength lead to unsatisfactory machinability and flexibility. Here, this study demonstrates grain size‐dependent strengthening‐to‐softening transition in Bi 2 Te 3 thin films, achieving a maximum strength of 363 MPa, several times greater than single‐crystal bulk counterparts. Remarkably, a novel energy dissipation mechanism mediated by stacking faults‐induced ripplocation structures enables an unprecedented tensile ductility of ≈7.3%. High‐density stacking faults simultaneously suppress the dominant grain boundary scattering on carrier transport, preserving excellent thermoelectric performance (power factor ≈2760 µW m −1 K −2 at 550 K). The fabricated Bi 2 Te 3 ‐based thin‐film devices exhibit superior flexibility (over 10 000 bending cycles), power output, and stability across room‐to‐medium temperatures. This work establishes a novel microstructural design paradigm for next‐generation flexible thermoelectric devices with superior strength‐ductility synergy and thermoelectric performance.