Perovskite solar cells (PSCs) consisting of hybrid organic/lead halide, such as MAPbI3 shows impressive power conversion efficiencies (PCE). Such hybrid perovskites possess certain unique features that are suitable for a wide range of device architectures, but vulnerable to fabrication process, especially sensitive to humidity. Therefore, fabrication of highly efficient PSCs by a fully ambient-process is still a key challenge. Herein, however, we demonstrate a nanoparticle (ZnO)-assisted fast nucleation (NAFN) growth of PbI2 on ZnO which results a pinhole-free morphology of MAPbI3 at fully ambient conditions with no residual PbI2. PSCs boosted by NAFN shows significant improvement in PCEs in straight comparison to devices fabricated under stringent environment control. On extending the NAFN method to SnO2, we report a maximum PCE of 18.3%, demonstrating versatility and potential for commercialization.