2D-3D-structured formamidine perovskite composites are highlighted due to their enhanced stability in solar cell applications. However, the structural and optical properties of 2D formamidine perovskites remain unclear. In this work, we developed new formamidine-based Ruddlesden-Popper perovskite compounds (BA)(2)(FA)(n-1)PbnI3n+1 (n4) using hot-spin coating. Orthorhombic 2D-layered perovskites were formed with a mixture of 3D formamidine perovskite from (BA)(2)(FA)(n-1)PbnI3n+1 precursors. Their formation highly depended on the solvent used in the precursors, in which dimethyl sulfoxide benefited from the b-axis oriented growth and improved the crystallization of the compounds. The formamidine-based Ruddlesden-Popper perovskites were direct bandgap materials, with the experimental bandgaps of over 1.7eV, and whose average photoluminescence lifetimes were affected by their chemical components and crystal structures. In addition, planar perovskite solar cells were fabricated by employing these films as absorbing layers. The initial devices from the (BA)(2)(FA)(3)Pb4I13 precursors exhibited an efficiency of 2.88% and an open voltage of 0.93V. These results indicated that the formation and stability of 2D-structured perovskites were strongly related to the A-site cations, thus offering new insights into the structures of 2D perovskites and their applications for 2D-3D-structured perovskite solar cells.