Efficiencies >20% are obtained from the perovskite solar cells (PSCs) employing Cs+ and Rb+ based perovskite compositions; therefore, it is important to understand the effect of these inorganic cations specifically Rb+ on the properties of perovskite structures. Here the influence of Cs+ and Rb+ is elucidated on the structural, morphological, and photophysical properties of perovskite structures and the photovoltaic performances of resulting PSCs. Structural, photoluminescence (PL), and external quantum efficiency studies establish the incorporation of Cs+ (x < 10%) but amply rule out the possibility of Rb-incorporation into the MAPbI(3) (MA = CH3NH3+) lattice. Moreover, morphological studies and time-resolved PL show that both Cs+ and Rb+ detrimentally affect the surface coverage of MAPbI(3) layers and charge-carrier dynamics, respectively, by influencing nucleation density and by inducing nonradiative recombination. In addition, differential scanning calorimetry shows that the transition from orthorhombic to tetragonal phase occurring around 160 K requires more thermal energy for the Cs-containing MAPbI(3) systems compared to the pristine MAPbI(3). Investigation including mixed halide (I/Br) and mixed cation A-cation based compositions further confirms the absence of Rb+ from the 3D-perovskite lattice. The fundamental insights gained through this work will be of great significance to further understand highly promising perovskite compositions.