Magnetic Resonance for Challenging Quadrupolar Halide Spins in Perovskites

Perovskites are nowadays the star materials for the research of next-generation photovoltaics and optoelectronics (Figure 1). Researchers are lured by their remarkably high power conversion efficiency and wide-range tunability of composition and morphology.^1,2 Despite a promising future, there are challenging questions hampering the industrial applications of perovskites: for example, how to evaluate and control different types of defects and interfacial structures, and how to improve their stability across different environmental, light, and temperature conditions. Addressing these questions requires a continuous pursuit of the structure–property relationship with the help of advanced characterization techniques and theoretical investigations. Solid-state nuclear magnetic resonance (NMR) is playing an indispensable role in the research of perovskites due to its superb spectroscopic resolution of chemical structure for both organic and inorganic components.³

Figure 1.

(a) 3D orthorhombic phase of lead halide perovskite, CsPbX₃. Applications of lead halide perovskite in (b) light-emitting diodes (LEDs) and in (c) solar cells. Reproduced with permission from ref (2). Copyright 2018 Springer Nature.

Perovskites of different types consist of abundant NMR-active nuclei including ¹H, ¹³C, ^14/15N, ^35/37Cl, ^79/81Br, ¹²⁷I, ¹³³Cs, ²⁰⁷Pb, etc. Solid-state NMR can provide detailed information on chemical bond structure, molecular dynamics, spatial distribution, lattice orientation, etc., on either crystalline or disordered phases.⁴⁻⁶ For halide perovskite, ABX₃ (where A is Cs⁺, CH₃NH₃⁺, or CH(NH₂)²⁺; B is Pb, Sn, or Ge; X is Cl, Br, or I), solid-state NMR can be performed on ^35/37Cl, ^79/81Br, and ¹²⁷I spins to examine the local environment of anions. These halide spins have spin quantum numbers of 3/2 or 5/2 and come with large quadrupole moments. Due to the exceedingly broad frequency distributions caused by strong quadrupolar interactions, the acquisition of halide signals with common experimental setups is difficult, as is the interpretation of this data.

The work done by Piveteau et al.³ demonstrated that the challenges of halide signals in lead halide perovskites (CsPbX₃) can be tackled with a versatile choice of both NMR and nuclear quadrupole resonance (NQR) (Figure 2). Here, NQR is a similar technique to NMR, instead using zero magnetic field to collect the signal of quadrupolar nuclei. NQR is suited for spins with large quadrupolar interactions.⁷ The authors also demonstrated, for the first time, the ³⁵Cl and ⁷⁹Br NMR of bulk and nanocrystalline CsPbX₃ collected with the special WURST-CPMG sequence.⁸ In the main text and Supporting Information, the authors provided introductory tutorials of NMR and NQR for halide spins. ³⁵Cl, ⁷⁹Br, and ¹²⁷I NMR or NQR measurements in perovskites are complementary to X-ray techniques for the accurate determination of chemical bond geometry, structural order, and dynamical behavior. NMR and NQR are especially helpful to inspect the disordered phase which is inaccessible by diffraction or to study the local structural distortions in nanocrystals.

Figure 2.

(a) Representative NMR spectra of ¹³³Cs and ²⁰⁷Pb in CsPbX₃. The halide NMR of ³⁵Cl, ⁷⁹Br, and ¹²⁷I is often difficult to obtain due to strong quadrupolar interaction. (b) Nuclear spin energy levels of a spin = 5/2 nucleus (e.g., ¹²⁷I) in a strong magnetic field as for NMR, and in zero magnetic field as for NQR. (c) Simulations of the ¹²⁷I NMR and NQR spectra for bulk γ-CsPbI₃. Reproduced with permission from ref (3). Copyright 2020 American Chemical Society.

One of the key findings in this report is the dramatically reduced spin–spin relaxation time (T₂) of ³⁵Cl and ⁷⁹Br NMR signals in CsPbCl₃ and CsPbBr₃ nanocrystals compared to that of bulk phase. The authors attributed the T₂ reduction in nanocrystals to the anharmonic dynamics driven by low-energy soft phonon-modes based on ab initio molecular dynamics simulations. The current study is a showcase of combinatory NMR and NQR techniques for studying quadrupolar nuclei in perovskite materials. Systematic applications of this characterization strategy on perovskites of diverse chemical compositions, nanoscale morphologies, or in composite matrices are expected to follow soon afterward. Compared with NMR, NQR requires minimal sample preparation and can be applied to materials integrated in working devices. These magnetic resonance studies may also benefit from the emerging dynamic nuclear polarization (DNP) technique that can greatly enhance the signal that enables the detection of defects and surface species.⁹ At the same time, the results of halide NMR and NQR should be coupled with the ¹³³Cs and ²⁰⁷Pb NMR as well as other characterizations¹⁰ to solve critical questions in perovskite research.