First‐Principles Investigation on the Electronic and Optical Properties of CuGa x In ₁ – x Se ₂ Alloy

The electronic and optical properties of CuGa x In 1−x Se 2 are explored in the full component range through first‐principles calculations. Our findings demonstrate that the lattice parameters decrease as the Ga component increases. CuGa x In 1−x Se 2 is an alloy with a direct bandgap. Its bandgap enlargement is due to the rise of the conduction band minimum (CBM) and the fall of the valence band maximum (VBM). Our findings also show that Cu plays a secondary role in determining the dependence of the VBM on Ga component because the Cu‐3d and Cu‐4p states are nearly independent of the Ga component. The rise of the CBM is caused by the enhanced total coupling, while the fall of the VBM is mainly due to the enhanced total coupling. Analysis of the imaginary part of the dielectric function reveals that the transition energies of the critical points E 0 , E 1 (A), E 1 (B), E 2 (A), E 2 (B), and E 3 move toward the higher‐energy direction approximately linearly in the whole component range. Similar results are also found by analyzing the absorption spectra, refractive index spectra, and reflectivity spectra.