The bottom small n phases in quasi-two-dimensional (Q-2D) perovskite films significantly hinder their photovoltaic performance development due to their severely low conductivity and nonideal band alignment in the corresponding solar cells. In this study, we successfully suppressed the growth of small n phases in Q-2D Ruddlesden-Popper (RP) perovskite (BA2MA4Pb5I16, < n > = 5) films by introducing 2,7-bis(diphenylphosphoryl)-9,9 '-spirobifluorene (SPPO13) as an additive into the perovskite precursor solution. It is interesting to find that the hole transport layer poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA) in our p-i-n device can attract the SPPO13 due to the pi-pi stacking effect. As a result, the SPPO13 concentrates at the bottom, and the coordination between SPPO13 and PbI2 leads to more [PbI6]4- octahedra gathering at the downside of the Q-2D perovskite film. Thereby, more large n phases remain at the bottom, and the unwanted small n phases are suppressed. The optimized device achieves a remarkable power conversion efficiency of 18.41%, which, according to our knowledge, is the highest value for the BA-MA-based perovskite. Moreover, our device also demonstrates outstanding stability, maintaining 99.5% and 95.3% of the initial efficiency after being stored for over 3500 h and under maximum power point tracking operation for over 400 h, respectively. Unlike conventional methods that primarily address bulk or interface properties, this approach uniquely combines pi-pi stacking effects and defect passivation through phosphine oxide groups, leading to enhanced crystallinity, vertical orientation, and suppressed nonradiative recombination. This work provides a new approach to regulate n-phase growth and promote the photovoltaic behavior of Q-2D perovskite solar cells.