Localized van der Waals Gap Engineering Enables High‐Performance n ‐Type Bi 2 Te 3 ‐Based Thermoelectrics

Bismuth telluride (Bi 2 Te 3 )‐based thermoelectrics are indispensable for near‐room‐temperature cooling and low‐grade heat‐to‐electricity conversion. However, in practical Bi 2 Te 3 ‐based devices, n ‐type Bi 2 (Te,Se) 3 often represents the weaker leg relative to its p ‐type counterpart, as suppressing κ L in this system is difficult to decouple from its microstructure‐sensitive electron transport. Here, we report localized van der Waals gap engineering in n ‐type Bi 2 Te 2.3 Se 0.7 through ZnS incorporation. ZnS incorporation forms coherent Zn‐rich nanoclusters and Bi–S–O‐related nanoscale secondary phases. Adjacent to the Zn‐rich nanoclusters, inserted Bi bilayers form within van der Waals gaps, creating localized interlayer phonon‐scattering centers. These interlayer–interfacial structural features enhance phonon scattering while slightly improving carrier mobility at the optimized composition. Further optimization of texture and carrier concentration balances charge carrier and phonon transport. The optimized hot‐deformed Cu 0.01 Bi 2 Te 2.3 Se 0.7 + 0.5 wt% ZnS sample achieves a peak zT of ∼1.30 at 450 K and an average zT of ∼1.17 over 325–475 K; a single‐leg device delivers a conversion efficiency of ∼6.5% under Δ T = 250 K. This work establishes localized van der Waals gap engineering as an effective route to selective phonon scattering in defect‐sensitive n ‐type Bi 2 Te 3 ‐based thermoelectrics.