Valence Band Structure Degeneracy Enhanced Thermoelectric Performance in beta-Cu2Se

Cu2Se is known to be a promising p-type thermoelectric material resulting from its extremely low thermal conductivity. On the other hand, compared to other highperformance thermoelectric materials like PbTe, the valence band degeneracy of Cu2Se is 4 times lower, which is believed to be unfavorable to the power factor. In this work, band structure engineering has been applied to increase the band degeneracy of Cu2Se through effective doping. Based on the careful analysis of bonding characters and atomic energy levels in the Cu2Se compound, some defects (the Fe or Mn substitutional defects, FeCu or MnCu, and the copper vacancy, VCu) are predicted to be able to converge two valence band maxima in Cu2Se. The electrical transport properties of Cu2Se with different defects (VCu/FeCu/MnCu) have been studied based on first-principles electronic structure calculations. A Boltzmann transport study demonstrates that the weakened p-d orbital interaction may enhance the electrical transport properties in Cu2Se.