Bismuth telluride-based alloys are the most widely used commercial thermoelectric (TE) material for room temperature refrigeration. Here, we successfully shift up the optimum figure of merit of n-type bismuth-telluride-based TE materials for mid-temperature power generation. SbI3 doping is used to regulate the carrier concentration and Indium alloying to increase the bandgap, suppressing the detrimental bipolar conduction in the mid-temperature range. The lattice thermal conductivity is significantly reduced due to the multiscale microstructures induced via hot deformation. As a result, a peak zT of ∼1.1 was attained at 625 K for Bi1.85In0.15Te2Se + 0.25 wt% SbI3 alloy after hot deformation, showing a great application prospect of this alloy in mid-temperature TE power generation. Display omitted •In alloying successfully broadens the band gap of Bi2Te2Se alloys, suppressing the bipolar effect at higher temperatures.•The lattice thermal conductivity was greatly reduced by multiscale phonon scattering.•Hot deformed n-type Bi1.85In0.15Te2Se + 0.25 wt% SbI3 sample shows excellent TE performance in the range of 500–700 K.