Perovskite solar cells (PSCs) offer an efficient, inexpensive alternative to current photovoltaic technologies. However, solution-processing PSCs involves volatilising large volumes of toxic solvents, presenting significant challenges for industrial-scale production. Moreover, perovskite materials are compositionally unstable under operational conditions. Here we employ a novel low-toxicity, biorenewable solvent system to process a range of 2D Ruddlesden Popper perovskite (RPP) compositions.

We demonstrate that such 2D RPPs act as highly effective solid-state ‘templates’ for subsequent solution-processed conversion to α-formamidinium lead triiodide (α-FAPbI₃). By tracking the evolution of excitonic photoluminescence from the material progressively throughout conversion we observe the sequence of 2D perovskite intermediates via which this transformation proceeds. It is demonstrated that precise control of the 2D RPP template’s phase-composition is of paramount importance to obtain the favourable optoelectronic and material properties of 2D-templated α-FAPbI₃. From these findings we propose a ‘templated’ growth regime in which the inorganic haloplumbate scaffold of the 2D perovskite is retained during conversion and is effective in templating the subsequent growth of low-defect, large-grain 3D perovskite materials.

Optical-pump terahertz-probe spectroscopy reveals an electron-hole mobility in excess of 60 cm² V^-1 s^-1 for the optimised 3D perovskite, which is among the highest recorded for thin-film α-FAPbI₃. Concurrently, an order of magnitude improvement in charge carrier lifetime compared to conventionally processed α-FAPbI₃ leads to a carrier diffusion length of 5.3 ± 0.2 μm in our material. Moreover, incorporation of 2D-templated α-FAPbI₃ layers into PSCs yields power conversion efficiencies of >21 %.

2D-templated α-FAPbI₃ demonstrates substantially improved stability in comparison to other state-of-the-art perovskite compositions. We find remarkably improved thermal stability of FAPbI₃ made by our route, which we show is due to the combined novel solvent and templated growth mechanisms; retention of low-volatility solvents (e.g. DMF, DMSO) is avoided, while degradation of the FA⁺ cation into sym-triazine is substantially reduced. As a result, PSCs based on 2D-templated FAPbI₃ show no degradation after 2,000 hours at 85 °C and 85% relative humidity (ISOS-D-3). Moreover, the isolated FAPbI₃ thin film material demonstrates heat and light stability of over 3,500 hours and, once incorporated in PSCs, these devices show champion t₈₀ stability of >800 hours in 85 °C, 1 sun equivalent light (ISOS-L-2).