Formamidinium Incorporates into Rb-based Non-Perovskite Phases in Solar Cell Formulations

Organic-inorganic hybrid perovskite materials, such as formamidinium lead iodide (FAPbI₃), are among the most promising emerging photovoltaic materials. However, the spontaneous phase transition from the photoactive perovskite phase to an inactive non-perovskite phase complicates the application of FAPbI₃ in solar cells. To remedy this, alkali metal cations, most often Cs⁺, Rb⁺ or K⁺, are included during perovskite synthesis to stabilize the photoactive phase. The atomic-level mechanisms of stabilization are complex. While Cs⁺ dopes directly into the perovskite lattice, Rb⁺ does not, but instead forms an additional non-perovskite phase, and the mechanism by which Rb confers increased stability remains unclear. Here, we use ¹H-⁸⁷Rb double resonance NMR experiments to show that FA⁺ incorporates into the Rb-based non-perovskite phases (FA_yRb_1-yPb₂Br₅ and δ-FA_yRb_1-yPbI₃) for both bromide and iodide perovskite formulations. This is demonstrated by changes in the ¹H and ⁸⁷Rb chemical shifts, ¹H-⁸⁷Rb heteronuclear correlation spectra, and ⁸⁷Rb{¹H} REDOR spectra. Simulation of the REDOR dephasing curves suggests up to ~60 % FA⁺ incorporation into the inorganic Rb-based phase for the bromide system. In light of these results, we hypothesize that the substitution of FA⁺ into the non-perovskite phase may contribute to the greater stability conferred by Rb salts in the synthesis of FA-based perovskites.