LAUSR

Single crystals of SnSe have gained considerable attention in thermoelectrics due to their unprecedented thermoelectric performance. However, polycrystalline SnSe is more favorable for practical applications due to its facile chemical synthesis procedure, processability, and scalability. Though the thermoelectric figure of merit (zT) of p‐type bulk SnSe polycrystals has reached >2.5, zT of n‐type counterpart is still lower and lies around ≈1.5. Herein, record high zT of 2.0 in n‐type polycrystalline SnSe0.92 + x mol% MoCl5 (x = 0–3) samples is reported, when measured parallel to the spark plasma sintering pressing direction due to the simultaneous optimization of n‐type carrier concentration and enhanced phonon scattering by incorporating modular nano‐heterostructures in SnSe matrix. Modular nanostructures of layered intergrowth [(SnSe)1.05]m(MoSe2)n like compounds embedded in SnSe matrix scatters the phonons significantly leading to an ultra‐low lattice thermal conductivity (κlat) of ≈0.26 W m−1 K−1 at 798 K in SnSe0.92 + 3 mol% MoCl5. The 2D layered modular intergrowth compound resembles the nano‐heterostructure and their periodicity of 1.2–2.6 nm in the SnSe matrix matches the phonon mean free path of SnSe, thereby blocking the heat carrying phonons, which result in low κlat and ultra‐high thermoelectric performance in n‐type SnSe.