Perovskite solar cells consisting of CH₃NH₃PbI₃ are expected to be alternative photovoltaic devices of silicon solar cells, because of their high conversion efficiency, easy fabrication process and low cost. On the other hand, they have a serious problem of low durability. To improve the stability and conversion efficiencies of the devices, one of the effective methods is to introduce additives into the perovskite photoactive layer. The purpose of this study is to improve the stability and conversion efficiency of the perovskite solar cells by incorporating cesium (Cs) or formamidinium (FA) at the CH₃NH₃ site. Additive effects on the photovoltaic properties and crystalline structures were investigated by current-voltage measurements, X-ray diffraction, and scanning electron microscopy. The simultaneous co-addition of Cs and FA to the CH₃NH₃PbI₃ perovskite crystal improved the photovoltaic properties, which would be due to the suppression of decomposition of the perovskite crystals.