Co-additive effects of ytterbium (Yb) and neodymium (Nd) for efficient and stable CH₃NH₃PbI₃ (MAPbI₃) perovskite solar cells were investigated. Compared to single addition, co-addition of 0.5 at% Yb and Nd supported improvements in the photovoltaic characteristics, resulting in an increase in open-circuit voltage (Voc), short-circuit current density (Jsc), conversion efficiency (η) and stability. Morphological observation and crystal structure analysis showed that the grain boundaries within the perovskite layer were reduced, and stable, dense films were formed through crystal growth and orientation. Surface observation and composition analysis using energy-dispersive X-ray spectroscopy confirmed the presence of Pb, I, Yb and Nd elements in the perovskite crystal grain. The band structure and density of states predicted the state of carrier mobility near the valence and conduction band states. The electron density distribution of 4f and 5d orbitals of Yb and Nd ions and 5p orbital of the I ion was expected to promote the charge transfer related to the semi-conductive properties. The optical properties from ultraviolet to near-infrared regions were based on the excitation process between the 4f and 5d orbitals of Yb and Nd ions in the crystal. The co-doped system had stabilization while suppressing distortion in the coordination structure. The photovoltaic characteristics were discussed by comparing experimental and calculated results in J-V curves of the solar cell using SCAPS-1D simulation. The photovoltaic characteristics originate from the state of carrier diffusion while suppressing carrier recombination near the grain boundaries and interface between the perovskite film and hole-transporting layer. In the co-addition system, passivation occurred more effectively than in the single-addition system, resulting in photovoltaic characteristics with an increase in Jsc, Voc, η and stability.