Fig. 1: Device performance and XPS depth profiles.

a Typical J–V curves for the inverted planar heterojunction PSCs from both the forward and reverse scans. (Inset) A PSC consists of glass/ITO (indium tin oxide)/HTL (8 nm)/perovskite (500 nm)/PCBM (30 nm)/BCP (8 nm)/Cu (100 nm). HTL, hole transporting layer. PCBM (phenyl-C61-butyric acid methyl ester, C₇₂H₁₄O₂) and BCP (bathocuproine, C₂₆H₂₀N₂) are the electron transporting and hole blocking layers, respectively. b Schematic illustration of the PSCs with the electric stress of ±1 V. The whole process was conducted in a dark N₂-filled glove box without exposure to air. c Illustration of two surfaces cleaved from the same device for depth-dependent XPS and UPS analyses. d–f Depth XPS profiles for the BCP side cleaved from the full PSCs with and without electric stress: d 0 V, e −1 V and f + 1 V. The XPS depth profiles for the BCP side with and without electric stress are provided in Supplementary Table 2. Note that the metallic Pb (Pb⁰) was observed from the XPS spectra once sputtering the perovskite absorber (Supplementary Fig. 1).