Dynamic Disorder in Monolayer and Multilayer 2D Ruddlesden–Popper Lead Iodide Perovskites: Evidence from Solid-State Nuclear Magnetic Resonance and Ultrafast Optical Spectroscopy

Layered 2D Ruddlesden–Popper (RP) lead iodide perovskites are solution process semiconductors with very promising optoelectronic applications, especially in photovoltaics. Their properties, and thus their usefulness in solar cells, crucially depend on the number of layers of lead halide octahedra in the unit cell, with multilayer perovskites usually showing larger photoconversion efficiency than monolayer ones. In the literature, such behavior is attributed to the differences in quantum confinements, while here, evidence is presented that it is also associated with differences in the dynamics of the spacer cations. In this paper, structural and dynamic behavior of the 2D RP BA2MAn–1PbnI3n+1 (BA = butylammonium, MA = methylammonium, with n ranging from 1 to 4) homologous series is investigated by 207Pb, 1H, and 13C solid-state nuclear magnetic resonance. 1H and 13C spectra, as well as variable temperature 13C spin–lattice relaxation times (T1), here exploited for the first time on 2D perovskites, give evidence of a larger dynamic disorder of the spacer cation in the monolayer with respect to multilayers. The results have been cross-examined with ultrafast optical spectroscopy measurements, leading to the interpretation that the larger dynamic disorder in monolayers leads to subpicosecond recombination of optical excitations that is detrimental in solar cells.