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

Abstract

Chalcopyrite ${\mathrm{CuGaSe}}_{2}$ 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 ${\mathrm{Cu}}_{2}\mathrm{Se}$, ${\mathrm{Ga}}_{2}{\mathrm{Se}}_{3}$, and their alloys ${\mathrm{CuGaSe}}_{2}$, ${\mathrm{CuGa}}_{3}{\mathrm{Se}}_{5}$, ${\mathrm{CuGa}}_{5}{\mathrm{Se}}_{8}$, and ${\mathrm{Cu}}_{4}{\mathrm{Ga}}_{2}{\mathrm{Se}}_{5}$ 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 ${\mathrm{Cu}}_{4}{\mathrm{Ga}}_{2}{\mathrm{Se}}_{5}$ to 2.11 eV of ${\mathrm{CuGa}}_{3}{\mathrm{Se}}_{5}$. Our results disclose anomalous changes in band gap induced by varying chemical composition and applying high pressure, due to the variation in $p\text{\ensuremath{-}}d$ coupling between Se and Cu atoms. Furthermore, the band gap of ${\mathrm{CuGaSe}}_{2}$ 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.