Regulating phonon-carrier transport by interfacial symmetry breaking in thermoelectric multilayers.

More than half of global primary energy is dissipated as low-grade waste heat, yet thermoelectric conversion remains constrained by the intrinsic coupling between phonon and charge transport. Here, we introduce graded interfacial size distribution as a thermodynamic design variable that breaks translational symmetry in multilayers, enabling anisotropic regulation of phonon-carrier transport. Using bismuth telluride (Bi2Te3)/metal [gold, silver, and platinum (Pt)] multilayers as a model system, we demonstrate that multiscale interface distributions induce broadband phonon suppression through the coexistence of interfacial scattering, coherent interference, and localization. This yields an ultralow cross-plane thermal conductivity of 0.22 watts per meter per kelvin and a high room-temperature ZT of 1.51 in Bi2Te3/Pt films. Concurrently, asymmetric metal-semiconductor interfaces create quasi-two-dimensional accumulation channels that enhance in-plane carrier mobility while preserving energy filtering, delivering a power factor of 176.2 microwatts per centimeter per square kelvin at 300 kelvin. The graded architecture enables high performance in both vertical and flexible planar devices, illustrating a general strategy in which interface distribution, not merely composition, governs anisotropic heat-charge transport. Our findings establish statistical interface engineering as a platform for thermoelectric energy harvesting and solid-state cooling.