Partial indium solubility induces chemical stability and colossal thermoelectric figure of merit in Cu2SeElectronic supplementary information (ESI) available. See DOI: 10.1039/c7ee01193h

High thermoelectric figure of merit, ZT ∼ 2.1 at 1000 K, have been reported in Cu 2− x Se-based materials. However, their deployments in operational devices have been hampered by chemical instability and low average ZT ( ZT ave ) values. Here, we demonstrate improved chemical stability and a record high ZT ave ∼ 1.5 over a broad temperature range ( T ≤ 850 K) in Cu 2 Se/CuInSe 2 nanocomposites, with ZT values ranging from 0.6 at 450 K to an unprecedentedly large value of 2.6 at 850 K for the sample with 1 mol% In. This remarkable performance is attributed to the localization of Cu + ions induced by the incorporation of In into the Cu 2 Se lattice, which simultaneously boost the electrical conductivity and reduce the thermal conductivity of the nanocomposites. These findings pave the way for large-scale utilization of Cu 2 Se-based materials in thermoelectric generators. Thermoelectric figure of merit, ZT, exceeding 2.6 at 850 K and copper electromigration inhibition have been demonstrated in indium modified Cu 2 Se.