In this work, enhanced photovoltaic performance and light stability of perovskite solar cells (PSCs) was achieved by applying down conversion (DC) CeO2:Eu3+ nanophosphor—TiO2 composite electrodes and the related mechanism are reported. High-yield CeO2:Eu3+ nanocrystals were synthesized by a simple hydrothermal method with combinational use of trisodium phosphate dodecahydrate and sodium hydroxide. Uniform and efficient CeO2:Eu3+ nanophosphors were prepared at an optimized reaction time. The optimal CeO2:Eu3+ nanophosphors were 70 nm in size, with octahedral and mirrorlike facets that provided excellent DC luminescence. The CeO2:Eu3+ nanophosphors grown at the optimal conditions were incorporated into mesoporous TiO2 layers of PSC devices. The PSC device with the CeO2:Eu3+—TiO2 composite electrode exhibited an energy conversion efficiency of 10.8%, which improved efficiency by 35% relative to the referenced one with undoped CeO2 nanocrystals. PSC devices added with undoped and doped CeO2 nanocrystals exhibited significantly better stability toward UV light compared to the bare TiO2 based PSC cell. The fundamental optics behind light propagation and absorption in perovskite solar cell and concept of using DC nanophosphors to improve the device performance has been explored. In this study, an attempt was made to elucidate the multi-roles of DC nanoconverters in mesoporous TiO2 based PSCs.