Synergistically Optimized Carrier and Phonon Transport Properties in Bi-Cu₂S Coalloyed GeTe

GeTe is an emerging lead-free thermoelectric material, but its excessive carrier concentration and high thermal conductivity severely restrict the enhancement of thermoelectric properties. In this study, the synergistically optimized thermoelectric properties of p-type GeTe through Bi-Cu₂S coalloying are reported. It can be found that the donor behavior of Bi and the substitution-interstitial defect pairs of Cu⁺ ions effectively reduce the hole concentration to an optimal level with carrier mobility less affected. At the same time, Bi-Cu₂S coalloying induces many phonon scattering centers involving stacking faults, nanoprecipitations, grain boundaries and tetrahedral dislocations and suppresses the lattice thermal conductivity to 0.64 W m^-1 K^-1. Consequently, all effects synergistically yield a peak ZT of 1.9 at 770 K with a theoretical conversion efficiency of 14.5% (300-770 K) in the (Ge_0.94Bi_0.06Te)_0.988(Cu₂S)_0.012 sample, which is very promising for mid-low temperature range waste heat harvest.