Methylammonium-Free Ink for Low-Temperature Crystallization of α-FAPbI3 Perovskite

Abstract

Formamidinium lead triiodide (FAPbI₃) perovskite without methylammonium and/or Cs cations is considered the most promising candidate for perovskite photovoltaics. However, the crystallization of photoactive α-FAPbI₃ requires high-temperature annealing (≥150 °C) and a controlled humidity environment when methylammonium-containing additives are absent. A methylammonium-free ink is reported that enables low-temperature (≤80 °C) crystallization of photoactive α-FAPbI₃ films, while also demonstrating compatibility with blade-coating large-area films in ambient air. The synergistical effects of methylphenyl sulfoxide and PbCl₂ facilitate the formation of an intermediate phase of nanoscale-disordered δ-FAPbI₃, which dramatically reduces the crystallization temperature of α-FAPbI₃ down to 80 °C and even below. The 80 °C crystalized α-FAPbI₃ exhibits reduced strain and improved uniformity compared to high-temperature annealed counterparts. The synthesized ink and the corresponding intermediate precursor film are also found remarkably stable, allowing open-air processing without the need for humidity control. Highly efficient n–i–p structured α-FAPbI₃ minimodules can be fabricated under an ambient environment RH 50% with the ink at 80 °C, achieving a power conversion efficiency of up to 22.4%. The discovery of the low-temperature FAPbI₃ ink paves a new avenue for printing perovskite solar cells and associated optoelectronic applications, accelerating the commercialization progress of perovskite materials.