Two-Dimensional Halide Perovskites Incorporating Straight Chain Symmetric Diammonium Ions, (NH₃C_mH_{2 m}NH₃)(CH₃NH₃)_n-1Pb_nI_{3 n+1} (m = 4-9; N = 1-4)

Low-dimensional halide perovskites have recently attracted intense interest as alternatives to the three-dimensional (3D) perovskites because of their greater tunability and higher environmental stability. Herein, we present the new homologous 2D series (NH₃C_mH_2mNH₃)(CH₃NH₃)_n-1Pb_nI_3n+1 (m = 4-9; n = 1-4), where m represents the carbon-chain number and n equals layer-thickness number. Multilayer (n > 1) 2D perovskites incorporating diammonium cations were successfully synthesized by the solid-state grinding method for m = 4 and 6 and by the solution method for m = 7-9. Structural characterization by single-crystal X-ray diffraction for the m = 8 and m = 9 series (n = 1-4) reveals that these compounds adopt the Cc space group for even n members and Pc for odd n members. The optical bandgaps are 2.15 eV for two-layer (n = 2), 2.01 eV for three-layer (n = 3), and 1.90 eV for four-layer (n = 4). The materials exhibit excellent solution processability, and casting thin-films of the n = 3 members was successfully accomplished. The films show a clear tendency for the higher-m members to have preferred orientation on the glass substrate, with m = 8 exhibiting almost perfect vertical layer orientation and m = 9 displaying both vertical and parallel layer orientation, as confirmed by grazing-incidence wide-angle X-ray scattering (GIWAXS) measurements. The vertical layer orientation for the (NH₃C₈H₁₆NH₃)(CH₃NH₃)₂Pb₃I₁₀ member results in the best thermal, light, and air stability within this series, thus showing excellent potential for solar cell applications.