This study evaluates the influence of Ocimum lamiifolium leaf extract (OLE) concentration on the green synthesis of zinc oxide nano particles (ZnO NPs) and their performance as an Electron Transport Layer (ETL) in perquisite solar cells (PSCs). Synthesized across five extract-to precursor ratios (1:2 to 3:2), the ZnO NPs followed a parabolic size trend dictated by competitive Ostwald ripening and steric crowding kinetics, which were modeled using atomistic Kinetic Monte Carlo (kMC) simulations. The 2:2 ratio achieved optimal stoichiometric equilibrium, yielding highly stable, pure nano particles with a 3.19 eV optical bandgap. This sample demonstrated an experimental crystalline core size of 10.80 nm (XRD) and an optical confinement size of 12.10 nm (Effective Mass Model), aligning exceptionally well with the kMC prediction of 10.24 nm (18.16% difference). EDX confirmed high purity with a minor 2.04 wt.% carbon signature from biomolecular capping. SCAPS-1D photovoltaic simulations proved that using this optimized ZnO (2:2) sample as an ultra-thin 20 nm ETL minimizes interfacial recombination and resistance, achieving a peak power conversion efficiency (PCE) of 16.03% (Jsc = 23.93 mA/cm2, Voc = 1.11V). Increasing the ETL thickness to 90 nm systematically degraded performance due to bulk carrier trapping. This work highlights kMC validated, biogenic ZnO as a sustainable, efficient architecture for next-generation PSCs.