Thermoelectric performance of Li doped, p-type Mg₂(Ge,Sn) and comparison with Mg₂(Si,Sn)

Solid solutions with chemical formula Mg₂(Si,Ge,Sn) are promising thermoelectric materials with very good properties for the n-type material but significantly worse for the p-type. For power generation applications good n- and p-type materials are required and it has been shown recently that Li doping can enhance the carrier concentration and improve the thermoelectric properties for p-type Mg₂Si_1-xSn_x. We have investigated the potential of p-type Mg₂(Ge,Sn) by optimizing Mg₂Ge_0.4Sn_0.6 using Li as dopant. We were able to achieve high carrier concentrations (1.4 × 10²⁰ cm⁻³) and relatively high hole mobilities resulting in high power factors of 1.7 × 10⁻³ W m⁻¹ K⁻² at 700 K, the highest value reported so far for this class of material. Exchanging Ge by Si allows for a systematic comparison of Mg₂(Ge,Sn) with Mg₂(Si,Sn) and shows that Si containing samples exhibit a lower power factor but also a lower thermal conductivity resulting in comparable thermoelectric figure-of-merit. The data is furthermore analyzed in the framework of a single parabolic band model to gain insight on the effect of composition on band structure.