Degradation of the Formamidinium Cation and the Quantification of the Formamidinium–Methylammonium Ratio in Lead Iodide Hybrid Perovskites by Nuclear Magnetic Resonance Spectroscopy

The highest efficiency in perovskite solar cells is currently achieved with mixed-cation hybrid perovskites. The ratio in which the cations are present in the perovskite structure has an important effect on the optical properties and the stability of these materials. At present, the formamidinium cation is an integral part of many of the highest efficiency perovskite systems. In this work, we introduce a nuclear magnetic resonance (NMR) spectroscopy protocol for the identification and differentiation of mixed perovskite phases and of a secondary phase due to formamidinium degradation. The influence of the cooling rate used in a precipitation method on the FA/MA ratio in formamidinium–methylammonium lead iodide perovskites (FA_<i>x</i>MA_1–<i>x</i>PbI₃) was investigated and compared to the FA/MA ratio in thin films. In order to obtain the FA/MA ratio from fast and accessible liquid-state ¹H NMR spectra, the influence of the acidity of the solution on the line width of the resonances was elucidated. The ratio of the organic cations incorporated into the perovskite structure could be reliably quantified in the presence of the secondary phase through a combination of liquid-state ¹H NMR and solid-state ¹³C NMR spectroscopic analysis