Decisive effects of atomic vacancies and structural ordering on stable phases and band structures in copper-gallium-chalcogenide compounds

Atomic vacancies usually exist in the Cu-Ga-S ternary system, except for chalcopyrite CuGaS2 as a promising light-harvesting material for solar cells, and are expected to have decisive effects on the structure stability and electronic structure. We demonstrate that ordered arrangements of the straight lines locally formed by atomic vacancies prefer a stable structure through lowering the formation energy. Accidentally, we confirm that a metastable van der Waals P21/c-Cu2S phase shares better optical properties than newly-found ground-state P42-Cu2S, and possesses the photovoltaic-potentially direct band gap of 1.09 eV. We find anomalous changes in band gap induced by varying chemical composition and applying pressure, according to the variation in p-d coupling between S and Cu atoms. Our Monte Carlo simulations together with the special quasirandom structures further suggest that the band gap of CuGaS2 can be tuned continuously from 2.51 eV for the chalcopyrite phase to 0.13 eV for the fully disordered configuration by controlling the degree of ordering, which determined by the synthesis temperature and annealing time experimentally.