The electrical and optical properties of various types of perovskite halides are dependent on the atomic compositions of the compounds, and their photovoltaic properties and stability are also affected by the interfacial structures of the devices. The aim of the present work was to fabricate and characterize guanidinium [C(NH₂)₃, GA]-, ethylammonium (CH₃CH₂NH₃, EA)-, and rubidium (Rb)-added CH₃NH₃PbI₃ (MAPbI₃) solar cells, which were passivated with decaphenylcyclopentasilane (DPPS) and GA. First-principles calculations on the proposed mixed-cation halides clarified the electronic structures, which were compared with experimental data. The lattice constants of GA/EA-added perovskites increased through the growth of perovskite crystals aged at ~22°C, which caused an increase in photoconversion efficiency. The GA/EA co-addition also improved their photovoltaic properties in an indoor light environment using a white-color LED. The surface passivation of MAPbI₃ using GA and DPPS decreased carrier traps in the perovskite crystal, and the photovoltaic properties were improved. Energy band structures and the partial density of states were investigated in the GA-, EA-, and Rb-modified perovskite compounds using first-principles calculations. The calculations showed that the total energies were reduced by adding GA, EA, or Rb to the perovskites. The bandgap energies were also decreased, which could lead to an increase in current density.