Predicting copper gallium diselenide and band structure engineering through order-disordered transition

Chalcopyrite CuGaSe2 has been widely studied as a promising light-harvesting material for solar cells, but new stable phases and compounds with other stoichiometries of the Cu-Ga-Se ternary systems could have more desirable properties and their optoelectronic properties could be engineered by controlling the degree of disorder. Employing an ab initio evolutionary variable-composition search and Monte Carlo simulations based on the special quasirandom structures, we identified several stable phases of Cu2Se, Ga2Se3, and their alloys CuGaSe2, CuGa3Se5, CuGa5Se8, and Cu(4)Ga(2)Se(5 )at ambient and high pressures. Computed electronic band structures of these alloys indicate that they are semiconductors with direct band gaps ranging from 0.77 eV of Cu4Ga2Se5 to 2.11 eV of CuGa3Se5. Our results disclose anomalous changes in band gap induced by varying chemical composition and applying high pressure, due to the variation in p-d coupling between Se and Cu atoms. Furthermore, the band gap of CuGaSe2 can vary continuously from 1.64 eV for the ordered chalcopyrite structure to 0.23 eV for the fully disordered structure; thus optical absorption spectra in these alloys could be tuned by controlling the synthesis temperature and annealing time, which determine the degree of ordering.