High‐Efficiency and Stable Step‐Graded PbTe Thermoelectric Module Achieved by a Synergistic Strategy of Thermal Matching and Diffusion Bonding

ABSTRACT PbTe‐based thermoelectric modules for mid‐temperature waste‐heat recovery are often limited by the performance decay of n ‐type materials across a wide temperature range. Here, we develop a step‐graded n ‐type PbTe leg by diffusion‐bonding two compositionally distinct segments into a seamless monolithic structure, which eliminates resistive interconnects while enabling complementary carrier transport. We show that matching the thermal conductivity at the step interface is critical; this principle not only optimizes the temperature‐dependent performance but also ensures interfacial stability by suppressing detrimental thermal stress. Consequently, a module constructed from such graded legs achieves a high conversion efficiency of 11.5% at a 500 K temperature difference, and notably maintains nearly constant power output for over 100 h of sustained operation, demonstrating exceptional long‐term reliability.