Stabilising Dry Vacuum Sublimation for Wide Bandgap Perovskite Solar Cells with Tailored Formamidinium Iodide

Dry vacuum sublimation, a production proven technique in semiconductor manufacturing, is a promising route for perovskite silicon tandem solar cells nearing mass production. However, it is well known that thermal decomposition of organic halides such as formamidinium iodide (FAI) critically disrupts process control and degrades film quality, hindering the adoption of vacuum processes in both research and industry. Here we present a protocol to quantitatively analyse FAI decomposition in situ and introduce an acid modulation strategy that suppresses it by incorporating a stronger acid, methylammonium chloride (MACl), into FAI. We identify a narrow process window that enables co-sublimation of the FAI:MACl mixture during the solid to vapour transition. This tailored precursor enables stable sublimation with suppressed decomposition and yields high quality perovskite films with improved crystallinity and reduced defects. This approach delivers 20.5% power conversion efficiency in wide bandgap inverted perovskite single junctions and, when implemented in monolithic tandems, achieves 29.8% on flat and 28.7 % on fully textured silicon bottom cells. This acid modulation concept establishes a rational route to tailor FAI with molecules of tuned acidity and supports the successful commercialisation of vacuum processed perovskite solar cells.