Thermoelectric transport properties of electron- and hole-doped pyrite FeS₂

Pyrite FeS2 is an earth-abundant, nontoxic sulphide mineral with semiconducting behaviour. Hence, it has been investigated for various energy applications, including thermoelectrics using experiments and theoretical studies. Previous studies on electron doped FeS2 have shown discrepancies in the magnitude and sign of room-temperature thermopower (S) depending on the nature of the sample. Co-doped FeS₂ samples have shown similar variations in the magnitude of room-temperature S. Therefore, it is of immense importance to examine the intrinsic thermopower of self- and Co-doped FeS₂ systems. Our study employed three distinct doping schemes of electron doping: explicit Co-substitution, jellium doping, and Fermi shift within the rigid band approximation (RBA) picture, using first principles calculations. The thermopower is overestimated within the RBA picture. However, the observed room-temperature S is under 50 μV/K for all the doping schemes. These low S values stem from a highly dispersive band at the bottom of the conduction band manifold, resulting in low density of states (DOS) and hence reduced thermopower. These findings suggest that electron doping is an ineffective strategy for enhancing the thermoelectric performance of FeS2. Following this observation, we explored hole doping as an alternative approach. This was motivated by the observation of multiple flat bands near the valence band edge in the electronic structure of stoichiometric FeS₂, which contributes to a high DOS in this region. These heavy electronic bands and increased DOS can potentially offer large S and, consequently, a higher thermoelectric figure of merit (ZT), given that ZT ∝ S².