Targeting the unfavorable phases problem caused by self-assembled multi-quantum well structures in quasi-two-dimensional (quasi-2D) perovskite films, we propose a facile approach by utilizing a series of small molecules with different spatial hindrance to suppress low-n phases and comprehensively explore the underlying regulatory kinetics mechanisms. The size effect enables smaller molecules to engage more intimately with Pb²⁺ sites, fortifying the bond strength between the interacting species. Thus, the deployment of ultrasmall methylamine formate (MAFA) engenders more potent interactions, which in turn are likely to exert a beneficial influence on the assembly of quantum well structures. Upon the optimization of the crystalline growth path, increased concentration of high-n phase is obtained in resultant quasi-2D perovskite films. The significant reduction in charge-transfer barriers and defects leads to a remarkable improvement in carrier transport and suppressed non-radiative recombination. Therefore, an optimum PCE of 21.13% is achieved for MAFA-based devices. Additionally, the unencapsulated MAFA-based devices can maintain over 92.6% of their initial efficiency after aging in air at room temperature at 85 % RH for 1000 h. Meanwhile, the MAFA-based devices retain 90% of initial PCE at 75 °C in air with 40 ± 5% RH for 225 h.