PbTe-based alloys are potential mid-temperature thermoelectric materials due to their excellent thermoelectric properties. Formation of intermetallic deteriorates mechanical joint strength and thermoelectric performance as well. In the present work, interfacial reaction, electrical and mechanical behaviors for both p- and n-PbTe joints with an addition of diffusion barrier layer are investigated. The results show that Co–P is a suitable barrier layer for PbTe-based thermoelectric devices with Cu or Ni electrode to inhibit the growth of massive intermetallic compound formation caused by fast interdiffusion in Cu/p-PbTe joints and melting of the joints in Cu/n-PbTe joints. Directly bonded Ni electrode induced formation of needle-like IMCs and disintegration of the electrodes in p- and n-PbTe modules respectively. Severe interfacial problems were overcome by adding a Co–P diffusion barrier, mechanical strength was improved. In addition, the Co–P layers enhanced the thermoelectric properties of the joints. The addition of the barrier layer increased 27% and 109% of the zT for p- and n-type joints, respectively.