Carrier concentration optimization for thermoelectric performance enhancement in n-type Bi2O2Se

The Bi2O2Se-based compounds with an intrinsically low thermal conductivity and relatively high Seebeck coefficient are good candidates for thermoelectric application. However, the low electrical conductivity resulted from carrier concentration of only 1015 cm−3 for pristine material, which is too low for optimized thermoelectrics. As a result, the carrier concentration optimization of Bi2O2Se is important and useful to achieve higher power factor. In this work, the effect of Ge-doping at the Bi site has been investigated systematically, with expectations of carrier concentration optimization. It is found that Ge doping is an efficient method to increase carrier concentration. Due to the largely increased carrier concentration via Ge doping, the room temperature electrical conductivity rises rapidly from 0.03 S/cm in pristine sample to 133 S/cm in x = 0.08 sample. Combined with the intrinsically low thermal conductivity, a maximum ZT value of 0.30 has been achieved at 823 K for Bi1.92Ge0.08O2Se, which is the highest ZT value for Bi2O2Se-based thermoelectric materials.