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

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.