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

Formamidinium lead triiodide (FAPbI) 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) has been used as an additive in FAPbI. MDA has been reported as incorporated into the perovskite lattice alongside Cl. However, the precise function and role of MDA remain uncertain. Here, we grow FAPbI single crystals from a solution containing MDACl (FAPbI-M). We demonstrate that FAPbI-M crystals are stable against transformation to the photoinactive δ-phase for more than one year under ambient conditions. Critically, we reveal that MDA is not the direct cause of the enhanced material stability. Instead, MDA degrades rapidly to produce ammonium and methaniminium, which subsequently oligomerizes to yield hexamethylenetetramine (HMTA). FAPbI crystals grown from a solution containing HMTA (FAPbI-H) replicate the enhanced α-phase stability of FAPbI-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-M and FAPbI-H at ∼0.5 mol % and infer that this addition is responsible for the improved α-phase stability.