Proceedings of International Conference on Hybrid and Organic Photovoltaics (HOPV23)
Publication date: 30th March 2023
Despite its attractive low band gap, a key challenge for formamidinium lead triiodide (FAPbI3) is to overcome its inherent ambient phase instability. Recently methylenediammonium dichloride (MDACl2) has been used as an additive to produce record perovskite solar cell efficiencies and enhance the stability of the photoactive FAPbI3 α-phase. However, as yet MDACl2 has only been employed in polycrystalline films, where its exact role is 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, while FAPbI3 crystals grown from a neat solution degrade within days. Critically, we reveal by nuclear magnetic resonance (NMR) spectroscopy that the acidic MDACl2 salt is unstable in solution and does not play an active stabilising role itself, instead degrading fully within minutes to a range of stable products. We show that FAPbI3 crystals grown from a solution containing one of these additives (FAPbI3-A) replicate the enhanced α-phase stability of FAPbI3-M. 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-M and FAPbI3-A crystals. However, these correspond to neither MDA2+, its stable degradation products nor any of the intermediates in the degradation pathway of MDA2+. Instead interruption of the degradation pathway by reaction of these intermediates with excess FA+ generates alternative organic species in-situ, which are incorporated into the FAPbI3 bulk material. We anticipate that presentation of our new understanding of the complex solution chemistry associated with the MDACl2 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.
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