  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 highly effective precursor
phases for subsequent transformation to alpha-formamidinium lead triiodide
(FAPbI3), fully processed under ambient conditions. PSCs utilising our 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 alpha-phase.
