Perovskite solar cells are expected to be the next generation solar cells due to their low cost and easy fabrication process. The purpose of this study is to investigate the effects of co-addition of copper, sodium and ethylammonium (EA) to the CH₃NH₃PbI₃ perovskite compound. The conversion efficiencies were enhanced by the addition of copper and sodium to CH₃NH₃PbI₃. From the first-principles calculations of the band structures, the formation of a shallow band of copper d-orbitals, which functions as an acceptor level, would promote the carrier generation. The additional excitation process from copper d-orbitals to sodium s-orbitals would suppress the carrier recombination, thus improving the conversion efficiency. Furthermore, the stability of the crystals increased by the EA substitution at the CH₃NH₃ site from the results of the total energy calculations, which would lead to suppression of formation of lattice defects. The further addition of EA also improved the conversion efficiencies, which indicates the EA substitution would stabilize the crystal structure even in the presence of copper and sodium.