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

• Published in: Journal of the American Chemical Society Vol. 145; no. 18; pp. 10275 - 10284
• Main Authors: Duijnstee, Elisabeth A., Gallant, Benjamin M., Holzhey, Philippe, Kubicki, Dominik J., Collavini, Silvia, Sturdza, Bernd K., Sansom, Harry C., Smith, Joel, Gutmann, Matthias J., Saha, Santanu, Gedda, Murali, Nugraha, Mohamad I., Kober-Czerny, Manuel, Xia, Chelsea, Wright, Adam D., Lin, Yen-Hung, Ramadan, Alexandra J., Matzen, Andrew, Hung, Esther Y.-H., Seo, Seongrok, Zhou, Suer, Lim, Jongchul, Anthopoulos, Thomas D., Filip, Marina R., Johnston, Michael B., Nicholas, Robin J., Delgado, Juan Luis, Snaith, Henry J.
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 10-05-2023
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/jacs.3c01531

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 FAPbI3 single 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). FAPbI3 crystals 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.