Bismuth telluride (Bi 2 Te 3 )-based thermoelectric materials are well-known for their high figure-of-merit (zT value) in the low-temperature region. Stable joints in the module are essential for creating a reliable device for long-term applications. This study used electroless Co−P to prevent a severe interfacial reaction between the joints of solder and Bi 2 Te 3 . A thick and brittle SnTe intermetallic compound layer was successfully inhibited. The strength of the joints improved, and the fracture mode became more ductile; furthermore, there was no significant degradation of thermoelectric properties after depositing the Co−P layer after long-term aging. The result suggests that electroless Co−P could enhance the interfacial stability of the joints and be an effective diffusion barrier for Bi 2 Te 3 thermoelectric modules.