Abstract
Organic-inorganic hybrid perovskite materials, such as formamidinium lead iodide (FAPbI(3)), 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 FAPbI(3) 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 (1)H-(87)Rb double resonance NMR experiments to show that FA(+) incorporates into the Rb-based non-perovskite phases (FA(y)Rb(1-y)Pb(2)Br(5) and δ-FA(y)Rb(1-y)PbI(3)) for both bromide and iodide perovskite formulations. This is demonstrated by changes in the (1)H and (87)Rb chemical shifts, (1)H-(87)Rb heteronuclear correlation spectra, and (87)Rb{(1)H} 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.