Atomic Permutation toward New Ruddlesden–Popper Two-Dimensional Perovskite with the Smallest Interlayer Spacing

We present a homologous series (CH3(NH2)2)2PbX4 (X = Cl, Br, I) of the Ruddlesden–Popper two-dimensional (2D) perovskite series. The new A-site cation (methyl hydrazinium, MHy) stemming from an atomic permutation of formamidine shows unique hydrogen-bond-stabilized 2D structures contrasting ethyl amine (CH3CH2NH3) and formamidine (NHCHNH2) as revealed by single-crystal X-ray diffraction. The subtle manipulation of the tolerance factor leads to (CH3(NH2)2)2PbX4 with the smallest interlayer spacing among the reported R–P 2D perovskites. A combined density functional theory calculation and experimental study assist in interpreting these disparate (electronic) structures/properties brought by similar cations. First-principles calculations also confirm the direct bandgap nature of (CH3(NH2)2)2PbI4 with wide valence and conduction bandwidths. After studying different colloidal behavior in the common polar solvents, we continued with the characterization based on the thin film samples. The homologous series (CH3(NH2)2)(CH3NH3)nPbnI3n+1 (n = 1, 2, 3, 4) multidimensional (MD) perovskite films show a characteristic decrease of the bandgap as n increases. The potential of these MD perovskites toward planar junction solar cell applications was assessed, and an initial power conversion efficiency of 5.83% was obtained from the (CH3(NH2)2)2(CH3NH3)3Pb4I13 perovskite, with reasonable stability after aging for 1200 h. This new cation provides new chances for many more homologous 2D halide perovskites for solid-state photovoltaics and other applications.