Crystallographic, optical, and electronic properties of (Cu,Li)InS2 system

To investigate the effect of a lithium doping in chalcopyrite-type CuInS2 on its crystallographic and optical properties, we prepared (Cu,Li)InS2 samples by a mechanochemical process and sequential heating at 550 °C in H2S gas. The single-phase chalcopyrite-type (Cu1 − xLix)InS2 solid solution could be obtained for the samples with 0.0 ≤ x ≤ 0.10. The lattice constants a and c of tetragonal chalcopyrite-type (Cu1 − xLix)InS2 solid solution increase with increasing Li content. The band gap energy of the (Cu1 − xLix)InS2 solid solution linearly increased from 1.44 eV of CuInS2 (x = 0.0) to 1.54 eV of (Cu0.90Li0.10)InS2 (x = 0.10). We performed first-principles band structure calculations for chalcopyrite-type (Cu1 − xLix)InS2, using the HSE06 nonlocal screened hybrid density functional. Theoretically, the band-gap energies of (Cu1 − xLix)InS2 increase with increasing Li content. The energy levels of the valence band maxima (VBMs) were estimated from the ionization energy measured by photoemission yield spectroscopy. The ionization energy of chalcopyrite-type (Cu1 − xLix)InS2 solid solutions increased from 5.45 eV of CuInS2 (x = 0.0) to 5.79 eV of (Cu0.90Li0.10)InS2 (x = 0.10). The energy level of the VBM of the (Cu1 − xLix)InS2 solid solution considerably decreases with increasing Li content, x. The conduction band minimum (CBM) level of the (Cu1 − xLix)InS2 solid solution also decreases with increasing Li content. The Li-doping in CuInS2 is expected to be useful for decreasing the VBM of CuInS2 absorber and increasing the band-gap energy of CuInS2 absorber without increasing the CBM level.