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

There have been significant progresses on the enhancement of thermoelectric (TE) performance in p-type PbTe compounds while the TE performances of n-type compounds do not compatible with the p-type ones. For optimizing high performance, an effective Fermi level tuning is critical. Here, we found that the extrinsic doping gives rise to complex defect configurations, resulting in unconventional lattice dynamics and abnormal electronic/thermal transport properties. From the formation energy calculation, the increase of lattice parameter in Pb excess PbTe compounds is correlated with formation of interstitial (Pbint and Teint), Pb-Te antisite defects, and cluster type defects [(PbTe-TePb), (Pb-Pb)Te, (Te-Te)Pb and (Pbint-2PbTe)]. When we apply extrinsic n-type doping such as excess Pb and Bi-doping, counterpart hole doping type complex defects are easily generated. The charge compensation effects due to intrinsic defects such as VTe+2 and Pbint+2 point defects and cluster type anionic defects such as Pbint-2(PbTe) make unconventional electrical transport behavior with respect to Pb excess concentration in Pb1+xTe. Bi-doping in Pb1-xBixTe also gives rise to complex cluster type defects such as nBiPb-VPb (n = 2 and 3) simultaneously. The complex cluster type defects not only suppress the doping efficiency but also increase the scattering exponent in Seebeck coefficient, resulting in the enhancement of Seebeck coefficient. The charge compensation between charged impurity and defect cluster doping is beneficiary for fine tuning of Fermi level for optimal carrier concentration.