Thermal degradation of formamidinium based lead halide perovskites into sym-triazine and hydrogen cyanide observed by coupled thermogravimetry - mass spectrometry analysis

J. Mater. Chem. A 2019, 7, 16912-16919. E. J. Juarez-Perez, L. K. Ono, Y. Qi

The thermal stability and decomposition products of formamidinium, a widely used organic cation in
perovskite solar cell formulation, were investigated. The thermal degradation experiments of
formamidinium-based perovskites and their halide precursors were carried out under helium
atmosphere and vacuum at a constant heating rate of 20 C min1. In addition, pulsed heating steps
were employed under illumination/dark conditions to simulate a more realistic working temperature
condition for photovoltaic devices. The identification of gas decomposition products was based on the
quadrupole mass spectrometry technique. The released amounts of sym-triazine, formamidine, and
hydrogen cyanide (HCN) were observed to highly depend on the temperature. For the experimental
conditions used in this study, sym-triazine was obtained as the thermal product of degradation at
temperatures above 95 C. Below this temperature, only formamidine and HCN generation routes were
observed. The energy pathways of formamidinium thermal degradation under photovoltaic working
temperature conditions were further assessed by density functional theory calculations. The results
indicated that formamidinium was more resilient to thermal degradation and the release of irreversible
decomposition products compared to methylammonium because of a larger enthalpy and activation
energy obtained for the decomposition reactions. The HCN instantaneous concentration observed
during the low temperature heating tests and the estimations of the maximum release of HCN
achievable per meter-square of an FA based perovskite based solar cell were compared to acute
exposure guideline levels of airborne HCN concentration.

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