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 (FAxMA1-xPbI3) 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 1 H-NMR spectra, the influence of the acidity of the solution on the linewidth 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 1 H-NMR and solid-state 13 C-NMR spectroscopic analysis.

Novel multi-layered hybrid perovskites containing carbazole derivatives show superior environmental stability, maintaining solar cell power conversion efficiency.

This study broadens the family of 2D layered perovskites by the self-assembly of organic charge-transfer complexes in their organic layer.