This work proposes a novel approach to modulate the nucleation and growth of perovskite crystals in planar perovskite (CH₃NH₃PbI_3−xCl_x) solar cells by intermixing precursor-capped inorganic nanoparticles of PbS. A small amount of dispersed PbS nanoparticles which were covered with perovskite precursor molecules of methylammonium iodide (CH₃NH₃I, MAI) through the ligand-exchange treatment functioned as effective seed-like nucleation sites to promote the formation of perovskite lattice structures. Through this intermixing-seeded growth technique, substantial morphological improvements, such as increased crystal domains, enhanced coverage, and uniformity, were realized in the perovskite thin films, and the corresponding solar cell devices exhibited a promising power conversion efficiency of 17.4%, showing an enhancement of approximately 25% compared to that of the controlled pristine solar cell device. The substantially enhanced crystal orientation, particularly along the direction perpendicular to the substrate, was evident from the synchrotron-based grazing incidence wide-angle X-ray scattering data. This observation was consistent with the enhanced carrier diffusion lengths and excellent reproducibility of high fill factors of the planar heterojunction perovskite devices fabricated through the proposed technique.