Controlling Structural Dimensionality in 2D Perovskites through A-Site Cation Enrichment

Two-dimensional (2D) Ruddlesden–Popper (RP) lead halide perovskites exhibit tunable structures, compositions, and optoelectronic properties, yet achieving precise control over their structural dimensionality in solution-processed thin films remains a significant challenge. Here, we report a simple strategy to regulate the dimensionality of 2D RP perovskites by enriching the A-site cations (FA+/Cs+) in the precursor solution, where FA = formamidinium. Based on a nominal n = 3 composition, systematic increase of the A-site cation ratio yields films with the general formula (4FPEA)2(FA0.3xCs0.7x)2Pb3X8+2x (X = I, Br; x = 1.5–3.0), where 4FPEA = 4-fluorophenylethylammonium. Within an optimal range, excess A-site cations promote the progressive formation of smaller-n phases without compromising film quality or triggering bulk three-dimensional (3D) perovskite formation. This tuning the n-value distribution effect is evidenced by UV–vis absorption, X-ray diffraction (XRD), steady-state photoluminescence (PL), and femtosecond transient absorption spectroscopy (TAS). As the A-site enrichment increases, the dominant phases shift from mixed high-n (quasi-3D) toward lower-n phases and energy funneling to large-n phases is suppressed, resulting in a continuous blue shift of the PL emission─from red to green for iodide-based perovskites and from green to blue for bromide-based analogues. This A-site stoichiometry engineering approach provides a straightforward, scalable route for tailoring the structural dimensionality and emission color of 2D perovskites via solution processing, with potential applications in light-emitting diodes and other optoelectronic devices.