CH₃NH₃PbI₃ perovskite solar cells 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. In order 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 copper (Cu) at the lead site, and potassium (K) or guanidinium (GA) at the CH₃NH₃ site. Additive effects on the photovoltaic properties and crystalline structures are investigated by the experimental results, and theoretical calculation such as electronic structures and thermodynamic stabilities. As a result, the simultaneous addition of Cu and K to the CH₃NH₃PbI₃ perovskite crystal improved the stability and open-circuit voltage, which was due to the suppression of decomposition of the perovskite crystals.