Two-Dimensional\nHalide Perovskites Incorporating Straight\nChain Symmetric Diammonium Ions, (NH₃C_<i>m</i>H_2<i>m</i>NH₃)(CH₃NH₃)_<i>n</i>−1Pb_<i>n</i>I_3<i>n</i>+1 (<i>m</i> = 4–9; <i>n</i> = 1–4)

Abstract

Low-dimensional\nhalide perovskites have recently attracted intense\ninterest as alternatives to the three-dimensional (3D) perovskites\nbecause of their greater tunability and higher environmental stability.\nHerein, we present the new homologous 2D series (NH₃C_<i>m</i>H_2<i>m</i>NH₃)­(CH₃NH₃)_<i>n</i>−1Pb_<i>n</i>I_3<i>n</i>+1 (<i>m</i> = 4–9; <i>n</i> = 1–4), where <i>m</i> represents the carbon-chain number and <i>n</i> equals layer-thickness number. Multilayer (<i>n</i> >\n1) 2D perovskites incorporating diammonium cations were successfully\nsynthesized by the solid-state grinding method for <i>m</i> = 4 and 6 and by the solution method for <i>m</i> = 7–9.\nStructural characterization by single-crystal X-ray diffraction for\nthe <i>m</i> = 8 and <i>m</i> = 9 series (<i>n</i> = 1–4) reveals that these compounds adopt the <i>Cc</i> space group for even <i>n</i> members and <i>Pc</i> for odd <i>n</i> members. The optical bandgaps\nare 2.15 eV for two-layer (<i>n</i> = 2), 2.01 eV for three-layer\n(<i>n</i> = 3), and 1.90 eV for four-layer (<i>n</i> = 4). The materials exhibit excellent solution processability, and\ncasting thin-films of the <i>n</i> = 3 members was successfully\naccomplished. The films show a clear tendency for the higher-<i>m</i> members to have preferred orientation on the glass substrate,\nwith <i>m</i> = 8 exhibiting almost perfect vertical layer\norientation and <i>m</i> = 9 displaying both vertical and\nparallel layer orientation, as confirmed by grazing-incidence wide-angle\nX-ray scattering (GIWAXS) measurements. The vertical layer orientation\nfor the (NH₃C₈H₁₆NH₃)­(CH₃NH₃)₂Pb₃I₁₀ member\nresults in the best thermal, light, and air stability within this\nseries, thus showing excellent potential for solar cell applications.