Stabilizing grain boundaries and surfaces of the perovskite layer is critical to extend device durability. Here we introduced an unexplored sulfonium-based molecule to post-treat formamidinium lead iodide perovskite films, which shows outstanding stability upon light soaking and remarkably remains in black-phase after 2 years ageing under ambient condition without encapsulation. The DMPESI-treated PSCs deliver a breakthrough record in operational stability of highly-efficient PSCs with less than 1% performance loss after more than 4500 h at maximum power point tracking, yielding a theoretical T80 of over 9 years under continuous 1-sun illumination. They also present less than 5% PCE drops under various ageing conditions (including thermal cycling and damp heat test)
