Detrimental Effects of Doping Al and Ba on the Thermoelectric Performance of GeTe

GeTe-based materials are emerging as viable alternatives to toxic PbTe-based thermoelectric materials. In order to evaluate the suitability of Al as dopant in thermoelectric GeTe, a systematic study of thermoelectric properties of Ge_1-xAl_xTe (x = 0⁻0.08) alloys processed by Spark Plasma Sintering are presented here. Being isoelectronic to Ge_1-xIn_xTe and Ge_1-xGa_xTe, which were reported with improved thermoelectric performances in the past, the Ge_1-xAl_xTe system is particularly focused (studied both experimentally and theoretically). Our results indicate that doping of Al to GeTe causes multiple effects: (i) increase in p-type charge carrier concentration; (ii) decrease in carrier mobility; (iii) reduction in thermopower and power factor; and (iv) suppression of thermal conductivity only at room temperature and not much significant change at higher temperature. First principles calculations reveal that Al-doping increases the energy separation between the two valence bands (loss of band convergence) in GeTe. These factors contribute for Ge_1-xAl_xTe to exhibit a reduced thermoelectric figure of merit, unlike its In and Ga congeners. Additionally, divalent Ba-doping [Ge_1-xBa_xTe (x = 0⁻0.06)] is also studied.