Exploring Structural Nuances in Germanium Halide Perovskites Using Solid-State Ge and Cs NMR Spectroscopy.

Metal halide perovskites remain top candidates for higher-performance photovoltaic devices, but concerns about leading lead-based materials remain. Ge perovskites remain understudied for use in solar cells compared to their Sn-based counterparts. In this work, we undertake a combined Ge and Cs solid-state Nuclear Magnetic Resonance (NMR) spectroscopy and density functional theory (DFT) study of the bulk CsGeX (X = Cl, Br, or I) series. We show how seemingly small structural variations within germanium halide perovskites have major effects on their Ge and Cs NMR signatures and reveal a near-cubic phase at room temperature for CsGeCl with severe local Ge polyhedral distortion. Quantum chemical computations are effective at predicting the structural impact on NMR parameters for Ge and Cs. This study demonstrates the value of a combined solid-state NMR and DFT approach for investigating promising materials for energy applications, providing information that is out of reach with conventional characterization methods, and adds the challenging Ge nucleus to the NMR toolkit.