Conduction Band Convergence and Modular Nanostructures: Driving High Thermoelectric Performance in n-Type PbSe

n-type lead chalcogenides showing high thermoelectric performance are rare due to the larger energy offset between the two lowest energy conduction bands minima, leaving ample opportunity to modulate electronic structure for improving their thermoelectric performance. Here, we present a remarkable thermoelectric figure of merit (zT) of ∼1.8 at 873 K in n-type PbSe doped with MoCl₅ by modulation of the conduction bands, while simultaneously suppressing the phonon transport. Doping MoCl₅ in PbSe induces notable convergence of conduction bands and an increased density of states near the Fermi level, mainly due to the contribution of Mo 4d orbital hybridized with the Se 4p-Pb 6p. This results in an improved Seebeck coefficient, despite maintaining a high n-type charge carrier concentration resulting in an excellent power factor (σS²) of ∼21 µW cm^-1 K^-2 at 873 K for PbSe + 1 mol% MoCl₅. When the solid solution limit of the doping exceeds, it forms unique modular nano-heterostructures (5-30 nm) of PbSe-MoSe₂ misfit layered compounds embedded in PbSe matrix. These nano-heterostructures significantly intensify phonon scattering, leading to an ultralow lattice thermal conductivity (κ_lat) of 0.20 W m^-1 K^-1 at ∼725 K in PbSe + 1 mol% MoCl₅ sample.