The aim of this paper is the improvement of the quantum efficiency of single crystalline silicon solar cells by applying the down-shift conversion of the solar spectrum. We have used this phenomenon of solar spectra shift in order to transfer incident photons of UV-Blue region to lower energy photons in visible spectra to be adapted to spectral sensitivity of the solar cells. Consequently, the reflection losses are reduced. For this object, we replace the PV encapsulation glass by polymethyl methacrylate (PMMA) doped with optically actives components. The effects of some optical transitions on the incident solar spectrum are modeled and simulated by taking into account the spectra of absorption and emission of each type of dopants used. We have shown an increase of about 9% in short circuit photo-current density (Jsc) and then in the photovoltaic conversion efficiency.