Metal cation doping is an established strategy to increase the efficiency of perovskite solar cells. However, the underlying mechanism is less discussed regarding the doping site. In this paper, the effect of rubidium/potassium as dopant has been evaluated on the CH3NH3PbI3 (MAPbI(3)) perovskite lattice. At the 5% doping level, K cations have significantly improved photovoltaic properties by promoting crystallinity, releasing the strain, reducing the trap states, and boosting all key photovoltaic parameters. Moreover, this study provides a rough description of Rb/K doping mechanisms in the MAPbI(3) lattice, where K cations occupy the interstitial sites while both interstitial and substitutional occupancies may occur for Rb cations. Our findings help to adjust the properties of perovskite layers to design new materials with better photovoltaic performance.