Figure 5

UPS data of multilayer heterojunctions recorded (a,c) in the low kinetic energy region (the secondary electron cutoff), and (b,d) in the low-binding-energy region (VBM region). (a,b) Cs1–xFAxPbBr3 on b-PEI/ZnO/ITO, (c,d) PVK on Cs1–xFAxPbBr3 /b-PEI/ZnO/ITO. UPS spectra of Cs1–xFAxPbBr3 layers were recorded in binding energy of − 2 to 12 eV for investigating the valence band electronic structures of the heterojunctions.