Compared to bipolar-dopable PbTe, the IP of Sb2Si2Te6 is smaller (due to the antibonding Sb-s -Te-p interaction at the VBM), while that of Sc2Si2Te6 is larger, indicating the formation of p-type defects may be more likely for Sb2Si2Te6. Indeed, Sb2Si2Te6 exhibits strong p-type behaviour experimentally, which is attributed to Sb vacancies. The EA of Sc2Si2Te6 is analogous to that of PbTe, n-type Y2Ti2O5S2 and BaSnO3, while that of Sb2Si2Te6 is smaller, suggesting that Sc2Si2Te6 may favour ntype doping.