Enhanced Thermoelectric Performance of n-type Bi2O2Se Ceramics Induced by Ge Doping

Ceramic samples with the composition Bi 2− x Ge x O 2 Se 1.01 ( x = 0, 0.05, 0.075, and 0.1) were synthesized by solid-state reaction and compacted using a hot-pressing technique. The prepared materials were characterized by x-ray diffraction analysis, electron microscopy, and measurements of electrical conductivity σ , Seebeck coefficient S , and thermal conductivity in the temperature range 300–780 K. Ge in the Bi 2 O 2 Se host structure led to an increase of the free electron concentration compared to pristine Bi 2 O 2 Se 1.01 . The donor effect is attributed to point substitutional defects in the Bi sublattice— Ge Bi + , and oxygen vacancies V O + 2 producing free electrons. As a result, we observe an increase in the electrical conductivity and decrease in Seebeck coefficient while thermal conductivity κ changes slightly. The highest value of the dimensionless figure of merit ZT = σS 2 T / κ reaches 0.25 for the composition Bi 1.95 Ge 0.05 O 2 Se 1.01 at T = 723 K, which is, to date, the highest ZT value reported for Bi 2 O 2 Se ceramics. Our results suggest that Bi 2 O 2 Se is still worth exploring.