(color online) (a) Schematic diagram illustrating the maintenance of highly degenerate electronic bands of the matrix by highly efficient doping (HED), and (b) curve of S versus carrier concentration showing that the nearly non-changed band structure can be obtained via HED (i.e., Al, Ga, In doping)^[60] in Cu₃SbSe₄ with high band degeneracy. Reproduced from Ref. [6] with permission from the American Chemical Society.