a-In2Se3 is one of the A2IIIB3IV type semiconductors with a wide band gap. A proper co-doping of Cu and Te leads to the formation of slab Cu2Se, which accounts for the reduction of band gap Eg, with its value decreasing from 1.32 eV for the intrinsic a-In2Se3 to 1.14 eV. This reduction of band gap is responsible for the remarkable improvement in thermoelectric properties, with the maximum power factor p increasing from 0.76′10-4 to 2.8×10-4 W·m-1·K-2 and the highest ZT value from 0.25 to 0.63. A proper co-doping results in a change in morphology from the amorphous-like structure represented by In2Se3 to the visible polycrystalline form, observed using HRTEM, and this change is directly related to the moderate enhancement of lattice thermal conductivity above 500 K