Benefits of oxygen in CuInSe2 and CuGaSe2 containing Se-rich grain boundaries

Using density functional theory calculation, we show that oxygen (O) exhibits an interesting effect in CuInSe2 and CuGaSe2. The Se atoms with dangling bonds in a Se-rich Sigma 3 (114) grain boundary (GB) create deep gap states due to strong interaction between Se atoms. However, when such a Se atom is substituted by an O atom, the deep gap states can be shifted into valence band, making the site no longer a harmful non-radiative recombination center. We find that O atoms prefer energetically to substitute these Se atoms and induce significant lattice relaxation due to their smaller atomic size and stronger electronegativity, which effectively reduces the anion-anion interaction. Consequently, the deep gap states are shifted to lower energy regions close or even below the top of the valence band. (C) 2014 Elsevier B.V. All rights reserved.