The crystallographic orientation and phase distribution of two-dimensional Ruddlesden−Popper perovskites (2D-RPPs) should be carefully controlled to obtain high-performance 2D-RPP-based optoelectronic devices. However, these characteristics are still unclear. Herein, we systematically examine the formation mechanism of highly oriented multiphase 2D-RPPs. We argue that the 3D-like perovskites containing small organic cations nucleate first with out-of-plane (111) preferential orientation, followed by the further growth of twodimensional perovskites incorporating bulky organic cations owing to the difference in the solubility between small and bulky cations. This spatial segregation of organic cations across the film depth induces the formation of multiple perovskite phases, which produces n-value-graded 2D-RPP films with continually upshifted band energy alignment. Highly oriented multiphase 2D-RPP films with isobutylammonium (isoBA 2 (Cs 0.02 MA 0.64 FA 0.34 ) 4 Pb 5 I 16 ) were successfully employed as a photoabsorbers for perovskite solar cells (PSCs), exhibiting remarkable efficiency of over 16% and significantly enhanced environmental stability compared with their three-dimensional counterparts.O rganic−inorganic hybrid perovskite materials have shown useful optoelectronic properties for application in various devices, including photodetectors, light-emitting diodes, and solar cells. 1−8 Despite their tremendous potential, the intrinsic instability of organic− inorganic hybrid perovskites against moisture, heat, and light limits their commercialization. 9,10 Recently, two-dimensional Ruddlesden−Popper perovskites (2D-RPPs) have been recognized as a new class of materials that enable high performance and long-term stability. Furthermore, 2D-RPPs have a more widely tunable optoelectronic properties, which originate from their structural versatility and quantum confinement effect, and thus, offer a broader application range than their three-dimensional (3D) counterparts. 11−14 The crystal structure of 2D-RPPs is derived from typical 3D perovskite materials with an ABX 3 composition, where A is an univalent organic cation and B is a divalent metal cation, which are octahedrally coordinated with halide ions X. With the introduction of bulky alkylammonium spacer cations, the chemical composition of 2D-RPPs is expressed as A′ 2 A n−1 B n X 3n+1 (n = 1, 2, 3, ..., ∞), where n is the number of inorganic octahedra layers, which are sandwiched between spacer cations A′ to form unit building blocks. 15 The building
