Valence Bands Convergence in p-Type CoSb3 through Electronegative Fluorine Filling

Band convergence is considered to be a strategy with clear benefits for thermoelectric performance, generally favoring the co-optimization of conductivity and Seebeck coefficients, and the conventional means include elemental filling to regulate the band. However, the influence of the most electronegative fluorine on the CoSb3 band remains unclear. We carry out density-functional-theory calculations and show that the valence band maximum gradually shifts downward with the increase of fluorine filling, lastly the valence band maximum converges to the highly degenerated secondary valence bands in fluorine-filled skutterudites. The effective degeneracy near the secondary valence band promotes more valleys to participate in electric transport, leading to a carrier mobility of more than the threefold and nearly twofold effective mass for F0.1Co4Sb12 compared to Co4Sb12. This work provides a new and promising route to boost the thermoelectric properties of p-type skutterudites.