Calculations of the Stark-effect splitting of the energy levels of Er 3+ ions in SiO2 layers alternating with layers of quasi-ordered Si-nanocrystals have been performed. The splitting is caused by the electric field of image charges at the interface between the layers having different dielectric constants. It has been established, that the splitting increases with increasing difference between the dielectric constants of SiO2 and nanocrystalline layers, and as the ions approach the interface. The results obtained allow us to explain the experimental data on an additional spectral broadening of the Er3+ photoluminescence line at 0.8 eV with increasing thickness of nanocrystalline Si layers. Moreover, a modeling of the spectral line shape has been performed, assuming that it consists of six components, properties of which depend on the additional electric field induced by dielectric function inhomogeneity.