The recent development of quasi-2D perovskite solar cells have drawn significant attention due to the improved stability of these materials and devices against moisture compared to their 3D counterparts. However, the optoelectronic properties of 2D perovskites need to be optimized in order to achieve high efficiency. In this work, the effect of spacer cations, i.e., phenethylammonium (PEA), 4-fluorophenethylammonium (F-PEA), and 4-methoxyphenethylammonium (MeO-PEA) on the optoelectronic properties and device performance of quasi-2D perovskites is systematically studied. It is observed that both larger and more hydrophobic cations can improve perovskite stability against moisture, while larger size can adversely influence the device performance. Interestingly, with F-PEA or MeO-PEA, distribution of n value can be shifted toward high 3D content in quasi-2D perovskite layers, which enables lower bandgaps and possibly lower exciton binding energy. Due to the best charge transport and lowest bandgap, the F-PEAI-based quasi-2D perovskite (n = 5) solar cell shows a highest power conversion efficiency (PCE) of 14.5% with excellent stability in air with a humidity of 40–50%, keeping 90% of the original PCE after 40 d. It is believed that the approach may open a way for the design of new organic spacer cations for stable low-dimensional hybrid perovskites with high performance.