Here we report, a novel two-step dipping technique via post-immersion polar solvent engineering for controlled secondary grain growth (Ostwald Ripening) to fabricate efficient mixed organic cation based MA(0.6)FA(0.4)PbI(3) perovskite solar cell (PSC) in conjunction with low temperature (140 degrees C) processed sol-gel ZnO ETL for full process compatibility with flexible substrates. The reported MTD-SE method (stands for Modified Two Step Dipping - Solvent Engineering) limits the grain coarsening effect during post-immersion stage of two-step dipping method and provides substantially smooth perovskite surface morphology for enhanced charge transport properties compared to conventional two-step techniques by means of controlled Ostwald Ripening process. The grain coarsening process and concomitant irregular grain size distribution are judiciously controlled by increasing the chemical potential or free energy change (Delta G) of the system at the post-immersion. The photovoltaic performance and photo-current hysteresis phenomena of the reported MTD-SE PSC have been compared with PSCs fabricated with conventional two-step techniques, incorporating 2-Propanol or ethyl alcohol as dipping solvents. The enhanced device performance of MTD-SE PSCs is correlated with the conducive role of the evenly distributed grain boundaries in them, which act as carrier dissociation interfaces and carrier transport pathways to charge selective contacts for superior charge separation and extraction properties. Adding to the merits, MTD-SE PSCs also demonstrate significantly suppressed photo-current hysteretic behaviour which has been elucidated in the context of faster ion migration kinetics with the increased grain boundaries, which exhibit higher ionic diffusivity. The favourable ion migration kinetics with MTD-SE PSC have also been comprehensively analysed from the frequency-dependent capacitive spectra.