Fig. 2: Characterization of perovskite surface energetics.

a UPS spectra of secondary electron cutoff region and valence band region of perovskite films with and without SBI. b Energy level diagram of the SBI-modified perovskite. c EF of the SBI-modified perovskite pinning to negative polaron transport state of PCBM used as ETL. d–f Surface morphology and potential images from KPFM. g Statistical potential distributions of film surfaces.