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.