Formation of a crystalline InSe phase from a quaternary single crystal of the Cu–Ag–In–Se system by massive ion motion

The composition and structural properties of a single crystal of the Cu–Ag–In–Se system are analyzed. Laue diffraction shows a single crystal while XRD diffraction and EDAX composition indicate two crystalline phases and two compositions close to Cu0.97Ag0.03In1.75Se2.84 and Cu0.95Ag0.05In2Se3.5 with lattice parameter, a = 5.770 Å and a = 5.790 Å and c/a ≅ 2.0 respectively. Impedance spectroscopy is carried out at temperatures up to 120 °C in a sequential annealing in order to obtain the electrical properties. A motion of two ions is observed and two ionic resistances and activation energies are computed in the 0.15–0.17 eV range and 0.52 eV, respectively. In the successive annealing, the impedance spectra change, probably due to a non-reversible process in the sample. After the impedance analysis, composition measurements and the structural analysis show a massive motion of Ag + Cu and In ions in the slice. These motions produce different phases with very different compositions in different regions. Due to the high disorder in Cu and In sublattices and to the high number of (2VCu + InCu) defect pairs, these ions are easily moved, leading to the formation of an InSe crystalline phase. Ions are rearranged in the chalcopyrite phase region, along with the transformation of In3+ into In2+ chemical species accompanied by the corresponding electron conduction capture. These changes are responsible of the non-reversibility of the process. These results would allow to understand the highest solar energy conversion efficiencies of up to 20.3% observed in CuIn1−xGaxSe2 (CIGS) thin films obtained using a three-stage co-evaporation process. In these films, the CIGS layer reaches a copper rich composition and a quasi-liquid Cu2−ySe phase is formed which enhances crystallization of the absorber layer and also affects the distribution of the group III elements throughout the layer. -- Highlights: ► The paper shows composition, structure in a single crystal of the Cu–Ag–In–Se system. ► Impedance spectra during annealing in the 0–120 °C range shows that the sample is a MIECs. ► Strong changes in the spectra indicate a non-reversible process in the sample. ► After annealing, a massive ion motion along the sample profile is observed. ► The destruction of initial phase and the appearance of other chalcopyrite and an InSe phases.