Se-induced enhancement of the high-temperature thermoelectric performance of n-type Cu0.008Bi2(Te,Se)3 alloys due to suppressed bipolar conduction

•Bipolar contribution to thermoelectric properties in Cu0.008Bi2(Te1-xSex)3 was investigated by tuning Te/Se ratio.•Weighted mobility and bipolar thermal conductivity was calculated using two-band model.•Weighted mobility for hole carriers gradually decreased with increasing Se content.•Bipolar thermal conductivity was remarkably reduced from 0.4 to 0.06 W m−1 K−1 in Cu0.008Bi2Te2.1Se0.9.•Peak zT was enhanced to 0.92 in Cu0.008Bi2Te2.1Se0.9 at 440 K.