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

To clarify the solid solution regions of CuIn3(SxSe1 − x)5 and CuGa3(SxSe1 − x)5 systems and their optical properties, we prepared CuIn3(S,Se)5 and CuGa3(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 CuIn3(SxSe1 − x)5 with 0 ≤ x < 0.1. On the other hand, we successfully obtained single-phase solid solutions with a tetragonal stannite-type structure for CuGa3(SxSe1 − x)5 with 0.0 ≤ x ≤ 1.0. The solid solution region of the CuGa3(SxSe1 − x)5 system is much wider than that of the CuIn3(SxSe1 − x)5 system. The band gap energy of the CuGa3(SxSe1 − x)5 solid solution linearly increased from 1.85 eV of CuGa3Se5 (x = 0.0) to 2.58 eV of CuGa3S5 (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 CuGa3Se5 (5.69 eV) is approximately equal to that of CuIn3Se5 (5.65 eV). The energy levels of the VBMs of the CuGa3(SxSe1 − x)5 solid solution decrease with increasing S content, x = S/(S + Se) ratio. The conduction band minimum (CBM) levels of CuGa3(SxSe1 − x)5 are almost constant with x = S/(S + Se) ratio. CuIn3Se5, CuGa3Se5, CuGa3S5 and CuGa3(S,Se)5 solid solution are expected to be useful for controlling the valence band offset (ΔEv) and the conduction band offset (ΔEc) at the interface between buffer layer and absorber layer in CIGS solar cells.