N‐methyl‐2‐pyrrolidone Iodide as Functional Precursor Additive for Record Efficiency 2D Ruddlesden‐Popper (PEA)2(Cs)n−1PbnI3n+1 Solar Cells

2D perovskite (PEA)2(Cs)n−1PbnI3n+1 (PEA: phenylethylammonium) exhibits more strengthened phase stability than its 3D components under ambient conditions and hence gained great attention in recent years. However, uncontrollable crystallization kinetics in (PEA)2(Cs)n−1PbnI3n+1 leads to difficulty in controlling film morphology and phase‐orientation regulation, resulting in poor power conversion efficiency (PCE). Herein, by incorporating precursor additive N‐methyl‐2‐pyrrolidone iodide (NMPI), the crystallization rate during the deposition of (PEA)2(Cs)n−1PbnI3n+1 film is efficiently regulated. As a result, the 2D or quasi‐2D perovskite solar cell (PSC) delivers record PCEs in all reported 2D or quasi‐2D CsPbX3 families, for instance, the quasi‐2D (n = 20) CsPbI3 PSC exhibits a record PCE of 14.59%, showing significantly enhanced stability. Detailed characterization reveals that the NMPI forms hydrogen bonds with dimethylammonium iodide (DMAI) in the precursor to control crystallization rate for a smooth morphology with small fluctuation, which leads to improved carrier lifetime and reduced trap‐density. More importantly, femtosecond transient absorption (fs‐TA) measurements confirm an improved phase purity and the suppressed nonradiative recombination in quasi‐2D perovskite film. It is believed that this simple additive strategy paves a new route for solving phase transition and crystallization kinetic problems in 2D and quasi‐2D CsPbX3.