Exploring Structural Nuances in Germanium Halide Perovskites using Solid-state 73Ge and 133Cs NMR Spectroscopy

Metal-halide perovskites remain top candidates for better-performing photovoltaic devices but concerns with leading lead-based materials continue. Ge perovskites remain understudied for use in solar cells compared to their Sn-based counterparts. In this work, we undertake a combined 133Cs and 73Ge solid-state NMR and DFT study of the bulk CsGeX3 (X = Cl, Br, I) series. We show how seemingly small structural variations within germanium halide perovskites have major consequences on their 73Ge and 133Cs NMR signatures and reveal a near cubic-like phase at room temperature for CsGeCl3 with severe local Ge polyhedral distortion. Quantum chemical computations are effective at predicting the structural impact on NMR parameters for 73Ge and 133Cs. This study demonstrates the value of a combined experimental and theoretical approach for investigating attractive materials for energy applications – providing information that is out of reach with conventional characterization methods – and adds the challenging 73Ge nucleus to the toolkit.