In recent years, perovskite solar cells have been expected to become an alternative to silicon solar cells due to their high-power conversion efficiency and low production costs. However, the formation of PbI₂ during long-term storage, the resulting decrease in conversion efficiency, and the decrease in FF due to defects in the perovskite layer are challenges. In this study, rare-earth co-doped perovskite solar cells (Ce, Nd, Gd, Er, Yb) were fabricated to address these challenges. Rare earth elements are characterized by high localization of 4f orbitals and strong Lewis acidity. A decrease in trap density was observed in the Gd-doped device. In addition, an increase in the (100) orientation and an increase in crystal size were observed after ~4 months of storage at room temperature and humidity in a dark place. As a result, the fill factor increased to 0.640. In addition, the Gd-doped device was stored in a dark room at room temperature and humidity for ~4 months, and the conversion efficiency was greatly maintained without degradation. This study shows that rare-earth-doped perovskite solar cells are effective in suppressing PbI₂ formation due to long-term storage, the resulting decrease in conversion efficiency, and the reduction in FF due to grain boundary defects, providing a new strategy for perovskite solar cells.