Remarkable Roles of Cu to Synergistically Optimize Phonon and Carrier Transport in n-Type PbTe-Cu₂Te

High thermoelectric performance of n-type PbTe is urgently needed to match its p-type counterpart. Here, we show a peak ZT ∼ 1.5 at 723 K and a record high average ZT > 1.0 at 300-873 K realized in n-type PbTe by synergistically suppressing lattice thermal conductivity and enhancing carrier mobility by introducing Cu₂Te inclusions. Cu performs several outstanding roles: Cu atoms fill the Pb vacancies and improve carrier mobility, contributing to an unexpectedly high power factor of ∼37 μW cm^-1 K^-2 at 423 K; Cu atoms filling Pb vacancies and Cu interstitials both induce local disorder and, together with nano- and microscale Cu-rich precipitates and their related strain fields, lead to a very low lattice thermal conductivity of ∼0.38 Wm^-1 K^-1 in PbTe-5.5%Cu₂Te, approaching the theoretical minimum value of ∼0.36 Wm^-1 K^-1. This work provides an effective strategy to enhance thermoelectric performance by simultaneously improving electrical and thermal transport properties.