First-principles study on crystal structures and bulk modulus of CuInX₂ (X = S, Se, S-Se) solar cell absorber

Chalcopyrite semiconductors are widely used as absorbers in thin film solar cells, especially flexible solar cells, due to their high power conversion efficiency. They also have interesting mechanical properties, making them promising materials for flexible, light, and thin solar cells. In this work, we report the first-principle calculations of the lattice constant and bulk modulus for CuInS₂, CuInSe₂, and CuIn(S,Se)₂ absorber solar materials. All calculations are performed using plane wave as implemented in the Quantum ESPRESSO software package in the framework of density functional theory using PBE-GGA approximations and ultrasoft pseudopotentials. The calculated lattice constant correlates well with the available experimental study. The energy-volume and pressure-volume relations are described using the third order of Birch-Murnaghan's equation of state to calculate the bulk modulus of the absorber solar material, which is associated to the hardness of a material under particular conditions. The values of bulk modulus obtained for CuInS₂ and CuInSe₂ are in good agreement with available theoretical results, except for CuIn(S,Se)₂, which have been calculated and reported for the first time.