Although silicon solar cells are currently the most common solar cells, they have a complicated fabrication process and are expensive. Recently developed CH₃NH₃PbI₃ (MAPbI₃)-based perovskite compounds have demonstrated numerous advantages, such as tunable band gaps, an easy fabrication process and high conversion efficiencies. 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 this work is investigate the effects of addition of guanidinium [C(NH₂)₃; GA] on MAPbI₃ perovskite solar cells fabricated at a high temperature of 190 °C in atmospheric air. The addition of guanidinium iodide 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 after 1 month. X-ray diffraction showed that the lattice constant of the perovskite crystals was increased by the addition of GA, and the GA addition also improved the surface morphology. First principles calculations on the density of states and band structures showed reduction of the total energy by the GA addition and the effectiveness of the nitrogen atoms in GA.