Although silicon is the most common solar cell material, the fabrication process is complicated and expensive. In contrast, the CH₃NH₃PbI₃ (MAPbI₃) perovskite compound have tunable band gaps and easy fabrication process. However, MAPbI₃ compounds are unstable in air due to the migration of CH₃NH₃ (MA). MAPbI₃ crystals are known to be able to control their electronic states by the addition of other cations and anions, and this could be used to improve the stability of the perovskite photovoltaic devices. The purpose of the present work is to investigate the effects of addition of guanidinium C(NH₂)₃ (GA) and cesium (Cs) on MAPbI₃ perovskite solar cells. The addition of GA/Cs and the insertion of decaphenylpentasilane between the perovskite and hole transport layer improved the external quantum efficiency and short-circuit current density, and the conversion efficiencies were stable. First principles calculations on the density of states and band structures showed reduction of the total energy by the GA addition.