Extended Data Fig. 8: XPS spectra.

a-d, XPS spectra of VmB1, VmB1 and SnI₂, 3D perovskite and VmB1-based perovskite, N 1s spectra (a), S 1s spectra (b), Sn 3d spectra (c), I 3d spectra (d). The perovskite with VmB1 has additional N 1s and S 1s signals from VmB1 compared with the control perovskite film, suggesting the existence of VmB1 on the shallow sub-surfaces. Because of the large molecular size, the VmB1 should be expelled to crystal surfaces during crystallization, which can passivate defects of perovskites. When VmB1 is mixed with SnI₂ or introduced into the control perovskite, the N 1s (C-N) and S 1s signals of VmB1 are shifted to higher energies, while the Sn 3d and I 3d signals are shifted to lower energies. This indicates that the –NH₂ group of VmB1 can bind to tin iodide octahedra through hydrogen bond interaction or Lewis acid-base interaction, and the S atom of VmB1 also can bind to tin iodide octahedron through Lewis acid-base interaction. e, Sn 3d spectra of the control, PEAI, VmB1 and PEAI+VmB1 based perovskite films. The black and olive lines are the raw data and background data respectively. The ratio of Sn⁴⁺/(Sn²⁺+Sn⁴⁺) in the control, PEAI, VmB1 and PEAI+VmB1 samples are 21%, 11%, 6% and 6%, respectively.