Crystallographic, optical, and electronic properties of Li-doped CuIn(S,Se)2, (Cu,Li)In(S,Se)2

(Cu1−xLix)In(S1−ySey)2 (CLISSe) samples with 0.0 ≤ x ≤ 0.4 and y = 0.25, 0.50, and 0.75 were prepared by a mechanochemical process and sequential heating. The X-ray diffraction (XRD) peaks of the (Cu1−xLix)In(S1−ySey)2 shifted to the lower angle side by the substitution of Li atoms for Cu atoms. The solid solution limit of Li for Cu in (Cu1−xLix)In(S1−ySey)2 system widened with increasing Se content, y. The single-phase (Cu1−xLix)In(S1−ySey)2 samples were obtained for the compositions with 0.0 ≤ x ≤ 0.05 for y = 0.25, 0.0 ≤ x ≤ 0.20 for y = 0.50, and 0.0 ≤ x ≤ 0.30 for y = 0.75. The crystallographic parameters such as the lattice constants a and c, c/a, and the atomic coordinate of a S/Se atom for (Cu1−xLix)In(S1−ySey)2 were refined by Rietveld analysis using XRD data. Both the lattice constants a and c of the (Cu1−xLix)In(S1−ySey)2 with a tetragonal chalcopyrite structure increased with increasing Li content, x. The band gaps of (Cu1−xLix)In(S1−ySey)2 solid solutions widened with increasing Li content, x. To understand the band diagram of the solid solutions, the energy level of the valence band maximum (VBM) from the vacuum level was determined from the ionization energy measured by photoemission yield spectroscopy (PYS). The energy level of the conduction band minimum (CBM) was also determined by adding the band-gap energy to the VBM level. The VBM level of the (Cu1−xLix)In(S0.25Se0.75)2 solid solution decreased considerably with increasing Li content, x. The CBM level of the (Cu1−xLix)In(S1−ySey)2 solid solution was approximately constant. The Li doping in CuIn(S,Se)2 is useful for decreasing the VBM of the CuIn(S,Se)2 absorber and increasing the band-gap energy of the CuIn(S,Se)2 absorber without increasing the CBM level.