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

Dominik J Kubicki,Aydar Rakhmatullin,Clare P Grey,Daniel Prochowicz,Lyndon Emsley,Elodie Salager,Samuel D Stranks

doi:10.1021/jacs.0c00647

Abstract

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 119Sn 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 119Sn 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. 119Sn 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 MASnBr3 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.

Highlights

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, reaching power conversion efficiencies of over 25%.1,4,5OIHPs are represented by the generic ABX3 formula, in which A is typically a small cation such as methylammonium (CH3NH3+, MA), formamidinium (CH3(NH2)2+, FA), and/or cesium ions
The structure aKd)opistemdobnyoMcliAniScnwCilt3hunsldigehrtolyurdiesxtpoertreimd e[nStnaCl cl6o]n4−ditoioctnashe(d29ra[8], which leads to the presence of chemical shift anisotropy (CSA)
We have previously shown that the width of the 14N spinning sidebands (SSB) manifold is related to the cubooctahedral symmetry in lead halide perovskites, with narrower manifolds corresponding to cubooctahedral symmetry closer to cubic; here we show that the same considerations hold for tin(II)

Summary

■ 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, reaching power conversion efficiencies of over 25%.1,4,5. Tin-based materials (Figure 1), while providing lower band gaps than Their lead analogues, essential for tandem solar cells, suffer from easy oxidation and disproportionation which lead to selfdoping, very short charge carrier lifetimes, and in turn poor power conversion efficiencies. Iodide−chloride mixing has been a widely investigated problem in the field of lead halide perovskite photovoltaics, since chloride doping leads to significantly improved thin film crystallinity and carrier diffusion lengths,[24−27] and considerable improvements have been reported for chloride doping in tin(II) halide perovskite based solar cells.[28,29] to the best of our knowledge, there is no direct evidence for I/Cl mixing in the case of tin(II) halide perovskites Another strategy to stabilize tin(II)-based materials is the use of mixed-metal tin(II)-lead(II) halide perovskites, which combine the advantageous optoelectronic properties of leadbased materials while providing band gaps of 1.2−1.3 eV which are close to the optimum required for all-perovskite tandem solar cells.[30−34].

■ RESULTS AND DISCUSSION

■ CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES