Quasi-2-dimensional (quasi-2D) perovskite solar cells (PSCs) have attracted much attention due to their superior stability. However, they suffer from inhomogeneous phase distributions and inefficient exciton dissociations, which hinder charge transport and increase energy loss. Herein, a novel chloride-contained aminidium molecule with Lewis-base properties, butyramidinium chloride (BI), was incorporated into quasi-2D PSCs to improve their performance. Chemical interactions between BI and [PbI₆]⁴⁻ octahedra were identified, which optimized the crystallization of 2D perovskite. The resultant film showed a favorable crystal orientation and a homogenized energy landscape with a reduced fraction of low-n 2D phases. Moreover, temperature-dependent photoluminescence results indicate reduced exciton binding energy and enhanced charge dissociation after adding BI. Consequently, quasi-2D PSCs (n = 4) with BI demonstrated a 25% increase in efficiency compared to control devices, attributed to the enhancement of the solar cell’s open-circuit voltage (V_OC) and fill factor. These results underscore the potential of amidinium-based molecules to improve quasi-2D PSC performance.