Revealing a New Formamidinium-Based Lead Triiodide Perovskite Material Derived from Chemical Degradation of a Common Additive in High Efficiency Perovskite Solar Cells

Despite its attractive low band gap, a key challenge for formamidinium lead triiodide (FAPbI₃) is to overcome its inherent ambient phase instability. A range of chemical additives have been employed in FAPbI₃-based solar cells to produce record efficiencies and enhance the stability of the photoactive FAPbI₃ α-phase. Here we grow FAPbI₃ single crystals from a solution containing one such common additive (FAPbI₃-A), which has been widely employed in high-performance solution-processed perovskite solar cells. We demonstrate that FAPbI₃-A crystals are stable against transformation to the photoinactive δ-phase for more than one year under ambient conditions, while FAPbI₃ crystals grown from a neat solution degrade within days. Critically, we reveal via nuclear magnetic resonance (NMR) spectroscopy that this common additive is unstable in solution and does not play an active stabilising role itself, instead degrading fully within minutes to a range of stable products via a complex chemical pathway, which we present. We show that FAPbI₃ crystals grown from a solution containing a certain one of these degradation products (FAPbI₃-D) replicate the enhanced α-phase stability of FAPbI₃-A. By performing a combination of liquid- and solid-state NMR measurements, along with X-ray diffraction (XRD) techniques, we find that previously unreported organic species are present in the FAPbI₃-A and FAPbI₃-D crystals. Remarkably, however, these correspond to neither the common additive, any of its stable degradation products nor any of the intermediates in its degradation pathway. Instead we are able to show that interruption of the degradation pathway by reaction of these intermediates with excess FA⁺ generates alternative organic species in-situ. It is these alternative products that are incorporated into the FAPbI₃ bulk material and lead to α-phase stabilisation. We anticipate that presentation of our new understanding of the complex solution chemistry associated with this common high-performance additive will provide crucial insight for other researchers, immediately enabling more effective and controllable use of it, and direct use of its degradation products, in the fabrication of highly phase-stable α-FAPbI₃ perovskite materials.