CH₃NH₃PbI₃ perovskite solar cells are expected to be the next generation solar cells due to their low cost and easy fabrication process. The purpose of the present work is to investigate the effects of co-adding copper (Cu), sodium (Na) and ethylammonium (EA) to the CH₃NH₃PbI₃ perovskite compound. The conversion efficiencies were enhanced by the addition of Cu and Na to CH₃NH₃PbI₃. The first-principles calculations showed formation of a shallow band of Cu d-orbitals, which functions as an acceptor level, would promote the carrier generation. The additional excitation process from Cu d-orbitals to Na s-orbitals would suppress the carrier recombination, thus improving the photovoltaic properties. 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 Cu and Na.