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

Thealloyed lead/tin (Pb/Sn) halide perovskites have gained significantattention in the development of tandem solar cells and other optoelectronicdevices due to their widely tunable absorption edge. To gain a betterunderstanding of the intriguing properties of Pb/Sn perovskites, suchas their anomalous bandgap's dependence on stoichiometry, itis important to deepen the understanding of their chemical behaviorand local structure. Herein, we investigate a series of two-dimensionalRuddlesden-Popper (RP) and Dion-Jacobson (DJ) phasealloyed Pb/Sn bromide perovskites using butylammonium (BA) and 3-(aminomethyl)pyridinium(3AMPY) as the spacer cations: (BA)(2)(MA)( n-1)Pb x Sn n-x Br3n+1 (n = 1-3) and (3AMPY)(MA)( n-1)Pb x Sn n-x Br3n+1 (n = 1-3) through a solution-based approach.Our results show that the ratio and site preference of Pb/Sn atomsare influenced by the layer thickness (n) and spacercations (A & PRIME;), as determined by single-crystal X-ray diffraction.Solid-state H-1, Sn-119, and (PbNMR)-Pb-207 spectroscopy analysis shows that the Pb atoms prefer the outerlayers in n = 3 members: (BA)(2)(MA)Pb x Sn n-x Br10 and (3AMPY)(MA)Pb x Sn n-x Br10. Layered 2D DJ alloyed Pb/Sn bromide perovskites (3AMPY)(MA)( n-1)Pb x Sn n-x Br3n+1 (n = 1-3) demonstratemuch narrower optical band gaps, lower energy PL emission peaks, andlonger carrier lifetimes compared to those of RP analogs. Densityfunctional theory calculations suggest that Pb-rich alloys (Pb:Sn & SIM;4:1) for n = 1 compounds are thermodynamicallyfavored over 50:50 (Pb:Sn & SIM;1:1) compositions. From grazing-incidencewide-angle X-ray scattering (GIWAXS), we see that films in the RPphase orient parallel to the substrate, whereas for DJ cases, randomorientations are observed relative to the substrate.