P-type PbTe is one of the most representative high-performance thermoelectric materials, while the conversion efficiency of the fabricated module is limited by the relatively low zT of n-type PbTe. Here, we report the optimization of Cu-doped n-type PbTe by tuning the ionic migration energy, aiming for the high-efficiency and robust modules. It is revealed that the strategy of lattice contraction, achieved by Ge/Se co-doping, preserves the excellent carrier mobility from interstitial Cu and suppresses the unstable transport at high temperature. In the optimized sample of Pb_0.94Ge_0.06Cu_0.02Se_0.04Te_0.96, a superior average zT (300–823 K) of 1.04 and a high peak zT of 1.45 at 823 K are obtained. A remarkable conversion efficiency of 10.5% at a temperature difference of 500 K is achieved in the fabricated PbTe-based module.