Formamidinium lead
triiodide (FAPbI3) 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 (MDACl2) has been used as an additive in FAPbI3. MDA2+has been reported as incorporated into
the perovskite lattice alongside Cl–. However, the
precise function and role of MDA2+remain uncertain. Here,
we grow FAPbI3single crystals from a solution containing
MDACl2(FAPbI3-M). We demonstrate that FAPbI3-M crystals are stable against transformation to the photoinactive
δ-phase for more than one year under ambient conditions. Critically,
we reveal that MDA2+is not the direct cause of the enhanced
material stability. Instead, MDA2+degrades rapidly to
produce ammonium and methaniminium, which subsequently oligomerizes
to yield hexamethylenetetramine (HMTA). FAPbI3crystals
grown from a solution containing HMTA (FAPbI3-H) replicate
the enhanced α-phase stability of FAPbI3-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 FAPbI3-M and FAPbI3-H at ∼0.5
mol % and infer that this addition is responsible for the improved
α-phase stability.