The stabilization of grain boundaries and surfaces of the perovskite layer is critical to extend the durability of perovskite solar cells. Here we introduced a sulfonium-based molecule, dimethylphenethylsulfonium iodide (DMPESI), for the post-deposition treatment of formamidinium lead iodide perovskite films. The treated films show improved stability upon light soaking and remains in the black α phase after two years ageing under ambient condition without encapsulation. The DMPESI-treated perovskite solar cells show less than 1% performance loss after more than 4,500 h at maximum power point tracking, yielding a theoretical *T*₈₀ of over nine years under continuous 1-sun illumination. The solar cells also display less than 5% power conversion efficiency drops under various ageing conditions, including 100 thermal cycles between 25 °C and 85 °C and an 1,050-h damp heat test.

Jiajia Suo, Bowen Yang, Edoardo Mosconi, Dmitry Bogachuk, Tiarnan A. S. Doherty, Kyle Frohna, Dominik J. Kubicki, Fan Fu, YeonJu Kim, Oussama Er-Raji, Tiankai Zhang, Lorenzo Baldinelli, Lukas Wagner, Ayodhya N. Tiwari, Feng Gao, Andreas Hinsch, Samuel D. Stranks, Filippo De Angelis, Anders Hagfeldt