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Using a first-principles band structure method we have theoretically studied the effects of Ga additions on the electronic and structural properties of We find that (i) with increasing the valence band maximum of (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 (CIS) and (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 donor level in CGS is much deeper than the 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.