Fine tuning of Fermi level by charged impurity-defect cluster formation and thermoelectric properties in n-type PbTe-based compounds

Abstract

There have been significant developments in the enhancement of the thermoelectric (TE) performances of p-type PbTe compounds; however, the TE performances of n-type compounds are not comparable with those of p-type ones. For optimizing the high performance, effective Fermi level tuning is critical. Here, we found that extrinsic doping caused complex defect configurations, resulting in unconventional lattice dynamics and abnormal electronic/thermal transport properties. From the formation energy calculation, the increase in the lattice parameter of PbTe compounds having excess Pb was correlated with the formation of interstitials (Pb_int and Te_int), Pb–Te antisite defects, and cluster-type defects [(Pb_Te–Te_Pb), (Pb–Pb)_Te, (Te–Te)_Pb, and (Pb_int–2Pb_Te)]. When we applied extrinsic n-type doping such as excess Pb and Bi doping, counterpart hole doping-type complex defects were easily generated. The charge compensation effects due to the intrinsic defects such as the V_Te²⁺ and Pb_int²⁺ point defects and cluster-type anionic defects such as Pb_int–2(Pb_Te) resulted in an unconventional electrical transport behavior with respect to the excess Pb concentration in Pb_1+xTe. Bi doping in Pb_1−xBi_xTe also gave rise to complex cluster-type defects such as nBi_Pb–V_Pb (n = 2 and 3) simultaneously. The complex cluster-type defects not only suppressed the doping efficiency but also increased the scattering exponent in the Seebeck coefficient, resulting in the enhancement of the Seebeck coefficient. The charge compensation between the charged impurity and defect cluster doping was beneficial for fine tuning of the Fermi level for optimal carrier concentration.