Fig. 1. Stability of DMPESI molecule and treated perovskite films under moisture and light-soaking conditions.

a, ¹H NMR spectra of DMPESI in anhydrous DMSO-d6 after six months of storage in ambient air. The H₂O peak in DMSO-d6 is at 3.3 ppm (vertical dashed blue line): the signal is absent in the samples. The chemical structure of DMPESI is shown. b, Photographs of fresh and 24-month aged unencapsulated perovskite film (1.0 cm by 2.0 cm) without and with DMPESI treatment of different concentrations (from left to right: reference, 1 mg ml⁻¹, 3 mg ml⁻¹, 5 mg ml⁻¹, 10 mg ml⁻¹) in ambient air with R.H. 40 ± 20%. c, Photographs of fresh and 24 h aged unencapsulated perovskite film (1.0 cm by 2.0 cm) without and with DMPESI treatment of different concentrations (from left to right: reference, 1 mg ml⁻¹, 3 mg ml⁻¹, 5 mg ml⁻¹, 10 mg ml⁻¹) in high humid condition of R.H. 85∼95%. (d,e,h,i) Hyperspectral photoluminescence microscopy of FAPbI₃ reference and DMPESI-treated films, scale bar = 15 μm. d,e, Photoluminescence (PL) map of reference FAPbI₃ film before (d) and after (e) 10 minutes of light soaking in air at 1-sun illumination intensity with a 405 nm continuous wave laser. f, PL spectra before (solid lines) and after (dashed lines) light from the coloured points marked in panels d and e. The mean PL spectrum of the region is marked in black. I_PL is PL intensity. g, Photoluminescence intensity changes with light soaking of the marked regions in d and the average of the image (black). h,i, Photoluminescence intensity map of DMPESI-treated film before (h) and after (i) 10 minutes of light soaking (1-sun equivalent). j, PL spectra before (solid lines) and after (dashed lines) light soaking of the marked regions in h and i. k, PL intensity changes with light soaking of the marked regions in h and the average of the image (black).