A Green Solvent System for Precursor Phase-Engineered Sequential Deposition of Stable Formamidinium Lead Triiodide for Perovskite Solar Cells

Perovskite solar cells (PSCs) offer an efficient, inexpensive alternative to current photovoltaic technologies, with the potential for manufacture via high-throughput coating methods. However, challenges for commercial-scale solution-processing of metal-halide perovskites include the use of harmful solvents, the expense of maintaining controlled atmospheric conditions, and the inherent instabilities of PSCs under operation. Here, we address these challenges by introducing a high volatility, low toxicity, biorenewable solvent system to fabricate a range of 2D perovskites, which we use as highly effective precursor phases for subsequent transformation to α-formamidinium lead triiodide (α-FAPbI₃), fully processed under ambient conditions. We probe this transformation, and the resulting α-FAPbI₃ thin film materials, using a combination of in-situ grazing incidence wide-angle X-ray scattering (GIWAXS) with synchrotron radiation, nuclear magnetic and quadrupole resonance (NMR/NQR) spectroscopy, and electron microscopy techniques, and present a complete mechanistic description of the 2D-to-3D transformation process. PSCs utilising our α-FAPbI₃ reproducibly show remarkable stability under illumination and elevated temperature (ISOS-L-2) and “damp heat” (ISOS-D-3) stressing, surpassing other state-of-the-art perovskite compositions. We determine that this enhancement is a consequence of the 2D precursor phase crystallisation route, which simultaneously avoids retention of residual low-volatility solvents (such as DMF and DMSO) and reduces the rate of degradation of FA⁺ in the material. Our findings highlight both the critical role of the initial crystallisation process in determining the operational stability of perovskite materials, and that neat FA⁺-based perovskites can be competitively stable despite the inherent metastability of the α-phase.