Understanding the Degradation of Methylenediammonium and Its Role in Phase-Stabilizing Formamidinium Lead Triiodide

[Image: see text] Formamidinium lead triiodide (FAPbI(3)) is the leading candidate for single-junction metal–halide perovskite photovoltaics, despite the metastability of this phase. To enhance its ambient-phase stability and produce world-record photovoltaic efficiencies, methylenediammonium dichloride (MDACl(2)) has been used as an additive in FAPbI(3). MDA(2+) has been reported as incorporated into the perovskite lattice alongside Cl(–). However, the precise function and role of MDA(2+) remain uncertain. Here, we grow FAPbI(3) single crystals from a solution containing MDACl(2) (FAPbI(3)-M). We demonstrate that FAPbI(3)-M crystals are stable against transformation to the photoinactive δ-phase for more than one year under ambient conditions. Critically, we reveal that MDA(2+) is not the direct cause of the enhanced material stability. Instead, MDA(2+) degrades rapidly to produce ammonium and methaniminium, which subsequently oligomerizes to yield hexamethylenetetramine (HMTA). FAPbI(3) crystals grown from a solution containing HMTA (FAPbI(3)-H) replicate the enhanced α-phase stability of FAPbI(3)-M. However, we further determine that HMTA is unstable in the perovskite precursor solution, where reaction with FA(+) is possible, leading instead to the formation of tetrahydrotriazinium (THTZ-H(+)). By a combination of liquid- and solid-state NMR techniques, we show that THTZ-H(+) is selectively incorporated into the bulk of both FAPbI(3)-M and FAPbI(3)-H at ∼0.5 mol % and infer that this addition is responsible for the improved α-phase stability.