Bi₂Te₃-based alloys are widely utilized in Peltier coolers owing to their highest thermoelectric performance at nearroom-temperatures. However, their peak dimensionless thermoelectric figure of merit, zT, is limited to a narrow temperature window due to minority carrier excitation emerging upon heating at around 400 K. Here, we show how this issue can be overcome by incorporating a synthetic rickardite mineral, Cu_3−xTe₂, in p-type (Bi,Sb)₂Te₃. The significant enhancement of the electronic and thermal properties could be achieved due to small Cu incorporation into the crystal structure of Bi,Sb)₂Te₃ and homogeneous precipitation of Cu_3−xTe₂ at the grain boundaries. This leads to a high average zT value (zT ave) of 1.22 between 350 and 500 K for two compositions, Bi_0.5Sb_1.5Te₃ (BST-5) and Bi_0.3Sb_1.7Te₃ (BST-3), with peak zT values of 1.32 at 467 K and 1.30 at 400 K, respectively. These high zT values result in a considerably high maximum device ZT of ca. 1.15 and a theoretical efficiency of up to 7% between 325 and 525 K. Additionally, room-temperature micro-hardness is substantially improved, which is desirable for constructing reliable and durable thermoelectric modules.