Local Structure and Dynamics in Methylammonium, Formamidinium, and Cesium Tin(II) Mixed-Halide Perovskites from 119 Sn Solid-State NMR

Dominik J Kubicki (1) , Daniel Prochowicz (2) , Elodie Salager (3) , Aydar Rakhmatullin (3) , Clare P Grey (4) , Lyndon Emsley (5) , Samuel D Stranks (6)

1 Cavendish Laboratory
2 Institute of Physical Chemistry of the Polish Academy of Sciences
3 CEMHTI - Conditions Extrêmes et Matériaux : Haute Température et Irradiation
4 Department of Chemistry [Cambridge, UK]
5 Laboratory of Magnetic Resonance
6 Department of Chemical Engineering - University of Cambridge

Organic−inorganic tin(II) halide perovskites have emerged as promising alternatives to lead halide perovskites in optoelectronic applications. While they suffer from considerably poorer performance and stability in comparison to their lead analogues, their performance improvements have so far largely been driven by trial and error efforts due to a critical lack of methods to probe their atomic-level microstructure. Here, we identify the challenges and devise a 119 Sn solid-state NMR protocol for the determination of the local structure of mixed-cation and mixed-halide tin(II) halide perovskites as well as their degradation products and related phases. We establish that the longitudinal relaxation of 119 Sn can span 6 orders of magnitude in this class of compounds, which makes judicious choice of experimental NMR parameters essential for the reliable detection of various phases. We show that Cl/Br and I/Br mixed-halide perovskites form solid alloys in any ratio, while only limited mixing is possible for I/Cl compositions. We elucidate the degradation pathways of Cs-, MA-, and FA-based tin(II) halides and show that degradation leads to highly disordered, qualitatively similar products, regardless of the A-site cation and halide. We detect the presence of metallic tin among the degradation products, which we suggest could contribute to the previously reported high conductivities in tin(II) halide perovskites. 119 Sn NMR chemical shifts are a sensitive probe of the halide coordination environment as well as of the A-site cation composition. Finally, we use variable-temperature multifield relaxation measurements to quantify ion dynamics in MASnBr 3 and establish activation energies for motion and show that this motion leads to spontaneous halide homogenization at room temperature whenever two different pure-halide perovskites are put in physical contact. ■ INTRODUCTION Organic−inorganic halide perovskites (OIHPs) have emerged as a new class of materials for solar cells and light emission applications owing to the ease of solution processing, immunity to most defects, and long charge carrier lifetimes, which can be tuned by compositional engineering. 1,2 Following the first report of perovskite-based solar cells (PSC) a decade ago, 3 the field of perovskite-based photovoltaics has been developing at a very fast pace, now reaching power conversion efficiencies of over 25%. 1,4,5 OIHPs are represented by the generic ABX 3 formula, in which A is typically a small cation such as methylammonium (CH 3 NH 3 + , MA), formamidinium (CH 3 (NH 2) 2 + , FA), and/or cesium ions. The inorganic sublattice is composed of [BX 6 ] 4− octahedra, where B is a divalent metal such as Pb 2+ , Sn 2+ , and Ge 2+ or a mixture of monovalent and trivalent metals (e.g., Ag + and In 3+) and X is a halide: I − , Br − , or Cl −. Lead halide

Mots clés: Degradation; Anions; Inorganic compounds; Perovskites; Halogens

Domaines: Chimie; Chimie théorique et/ou physique; Matériaux

Dominik J Kubicki, Daniel Prochowicz, Elodie Salager, Aydar Rakhmatullin, Clare P Grey, et al.. Local Structure and Dynamics in Methylammonium, Formamidinium, and Cesium Tin(II) Mixed-Halide Perovskites from 119 Sn Solid-State NMR. Journal of the American Chemical Society, 2020, 142 (17), pp.7813-7826. ⟨10.1021/jacs.0c00647⟩. ⟨hal-03000541⟩