Chemical Behavior and Local Structure of the Ruddlesden–Popper and Dion–Jacobson Alloyed Pb/Sn Bromide 2D Perovskites

Abstract

The alloyed lead/tin (Pb/Sn) halide perovskites have gained significant attention in the development of tandem solar cells and other optoelectronic devices due to their widely tunable absorption edge. To gain a better understanding of the intriguing properties of Pb/Sn perovskites, such as their anomalous bandgap's dependence on stoichiometry, it is important to deepen the understanding of their chemical behavior and local structure. Herein, we investigate a series of two-dimensional Ruddlesden-Popper (RP) and Dion-Jacobson (DJ) phase alloyed Pb/Sn bromide perovskites using butylammonium (BA) and 3-(aminomethyl)pyridinium (3AMPY) as the spacer cations: (BA)₂(MA)_<i>n</i>-1Pb_<i>x</i>Sn_{<i>n</i>-<i>x</i>}Br_3<i>n</i>+1 (<i>n</i> = 1-3) and (3AMPY)(MA)_<i>n</i>-1Pb_<i>x</i>Sn_{<i>n</i>-<i>x</i>}Br_3<i>n</i>+1 (<i>n</i> = 1-3) through a solution-based approach. Our results show that the ratio and site preference of Pb/Sn atoms are influenced by the layer thickness (<i>n</i>) and spacer cations (A'), as determined by single-crystal X-ray diffraction. Solid-state ¹H, ¹¹⁹Sn, and ²⁰⁷Pb NMR spectroscopy analysis shows that the Pb atoms prefer the outer layers in <i>n</i> = 3 members: (BA)₂(MA)Pb_<i>x</i>Sn_{<i>n</i>-<i>x</i>}Br₁₀ and (3AMPY)(MA)Pb_<i>x</i>Sn_{<i>n</i>-<i>x</i>}Br₁₀. Layered 2D DJ alloyed Pb/Sn bromide perovskites (3AMPY)(MA)_<i>n</i>-1Pb_<i>x</i>Sn_{<i>n</i>-<i>x</i>}Br_3<i>n</i>+1 (<i>n</i> = 1-3) demonstrate much narrower optical band gaps, lower energy PL emission peaks, and longer carrier lifetimes compared to those of RP analogs. Density functional theory calculations suggest that Pb-rich alloys (Pb:Sn ∼4:1) for <i>n</i> = 1 compounds are thermodynamically favored over 50:50 (Pb:Sn ∼1:1) compositions. From grazing-incidence wide-angle X-ray scattering (GIWAXS), we see that films in the RP phase orient parallel to the substrate, whereas for DJ cases, random orientations are observed relative to the substrate.