Here we report, a controlled primary nucleation aided restricted volume solvent annealing method [NR method, NR stands for nucleation assisted restricted volume solvent annealing (RVSA)] along with mixed organic cation based perovskite film for highly efficient (highest PCE: 16.78%), thermally stable perovskite solar cells (PSCs) with full temperature compatibility for flexible substrates. To the best of our knowledge, this is the highest efficiency ever reported with PSCs incorporating low temperature processed ZnO as electron transport layer (ETL). Unlike the conventional RVSA method [CR method, CR refers to conventional restricted volume solvent annealing (RVSA)], the reported NR method raises the degree of supersaturation of the precursor solution due to high solvent evaporation rate during the crucial primary nucleation phase, that ensures smooth and uniform perovskite grain distribution during the rapid crystal growth phase. NR method also provides better perovskite crystallinity during the annealing induced secondary crystallization phase, as unlike CR, NR perovskite does not contain high amount of residual solvent at this stage of perovskite grain growth. The photovoltaic performance, hysteretic behaviour and device degradation phenomena of NR PSCs have been compared with CR PSCs. Enhanced power conversion efficiency (PCE) attained with NR method has been explained with elaborate surface morphology and topography study, along with charge transport analysis with electrochemical impedance spectroscopy and Mott-Schottky analysis. Substantially mitigated photo-current hysteresis phenomenon with NR method has been elucidated in terms of inherent electrode polarization process. Adding to the merits, NR PSCs also demonstrate over three times higher device stability compared to CR PSCs, which has been investigated with frequency-dependent capacitive spectra and Mott-Schottky characterization of the aged devices.