Effects of Ga addition to $CuInSe2$ on its electronic, structural, and defect properties

Using a first-principles band structure method we have theoretically studied the effects of Ga additions on the electronic and structural properties of $CuInSe2.$ We find that (i) with increasing $xGa,$ the valence band maximum of $CuIn1−xGaxSe2$ (CIGS) decreases slightly, while the conduction band minimum (and the band gap) of CIGS increases significantly, (ii) the acceptor formation energies are similar in both $CuInSe2$ (CIS) and $CuGaSe2$ (CGS), but the donor formation energy is larger in CGS than in CIS, (iii) the acceptor transition levels are shallower in CGS than in CIS, but the $GaCu$ donor level in CGS is much deeper than the $InCu$ donor level in CIS, and (iv) the stability domain of the chalcopyrite phase increases with respect to ordered defect compounds. Our results are compared with available experimental observations.