Realizing high thermoelectric performance in copper sulfide via intermediate doping

Due to its low cost, eco-friendliness, and excellent thermoelectric (TE) performance, copper sulfide (Cu_2−xS) has emerged as a promising TE material. Nevertheless, the trade-off between enhanced electrical conductivity and suppressed thermal conductivity is a critical challenge. This study systematically investigated an intermediate doping approach through the strategic incorporation of copper alloys (bronze, cupronickel, and brass) to optimize the TE performance of Cu_1.8S. The proposed strategy realized partial copper source replacement and solved the problem of excessive Cu vacancy in Cu_1.8S. Comprehensive characterization demonstrated that compared to conventional direct elemental doping, the intermediate doping with Zn, Sn, Pb, and Ni elements achieved superior TE performance. Additionally, this improvement was from dual synergistic mechanisms: the increased solubility limits facilitating optimized carrier concentration, and the in situ formation of the nanoscale second phase effectively scattered phonons. Ultimately, the Cu_1.8S + 5 wt.% bronze + 3 wt.% cupronickel + 2 wt.% brass sample reached a remarkable ZT value of 1.7 at 673 K, which was a 247% enhancement over pristine Cu_1.8S and surpassed all previously reported ZT values for the Cu_1.8S system. This study established a novel paradigm of intermediate doping in optimizing the TE properties, providing a new perspective for other alloy-based TE systems.