Crystallographic and optical properties of CuGa3S5, CuGa3Se5 and CuIn3(S,Se)5 and CuGa3(S,Se)5 systems

Abstract To clarify the solid solution regions of CuIn 3 (S x Se 1 − x ) 5 and CuGa 3 (S x Se 1 − x ) 5 systems and their optical properties, we prepared CuIn 3 (S,Se) 5 and CuGa 3 (S,Se) 5 samples by a mechanochemical process and post-heating. Single-phase solid solutions with a tetragonal stannite-type structure could not be obtained for CuIn 3 (S x Se 1 − x ) 5 with 0 ≤ x 3 (S x Se 1 − x ) 5 with 0.0 ≤ x ≤ 1.0. The solid solution region of the CuGa 3 (S x Se 1 − x ) 5 system is much wider than that of the CuIn 3 (S x Se 1 − x ) 5 system. The band gap energy of the CuGa 3 (S x Se 1 − x ) 5 solid solution linearly increased from 1.85 eV of CuGa 3 Se 5 (x = 0.0) to 2.58 eV of CuGa 3 S 5 (x = 1.0). The energy levels of the valence band maxima (VBMs) were estimated from the ionization energies measured by photoemission yield spectroscopy (PYS). The ionization energy of stannite-type CuGa 3 Se 5 (5.69 eV) is approximately equal to that of CuIn 3 Se 5 (5.65 eV). The energy levels of the VBMs of the CuGa 3 (S x Se 1 − x ) 5 solid solution decrease with increasing S content, x = S/(S + Se) ratio. The conduction band minimum (CBM) levels of CuGa 3 (S x Se 1 − x ) 5 are almost constant with x = S/(S + Se) ratio. CuIn 3 Se 5 , CuGa 3 Se 5 , CuGa 3 S 5 and CuGa 3 (S,Se) 5 solid solution are expected to be useful for controlling the valence band offset (Δ E v ) and the conduction band offset (Δ E c ) at the interface between buffer layer and absorber layer in CIGS solar cells.