Revisiting the determination of the valence band maximum and defect formation in halide perovskites for solar cells: insights from highly sensitive near-UV photoemission spectroscopy

Using advanced near–UV photoemission spectroscopy (PES) in constant final state mode (CFSYS) with a very high dynamic range, we investigate the triple-cation lead halide perovskite Cs 0.05 (MA 0.17 FA 0.83 ) 0.95 Pb(I 0.83 Br 0.17 ) 3 and gain detailed insights into the density of occupied states (DOS) in the valence band and band gap. A valence band model is established which includes the parabolic valence band edge and an exponentially decaying band tail in a single equation. This allows us to precisely determine two valence band maxima (VBM) at different k -vectors in the angle-integrated spectra, where the highest one, resulting from the VBM at the R -point in the Brillouin zone, is found between −1.50 to −1.37 eV relative to the Fermi energy E F . We investigate quantitatively the formation of defect states in the band gap up to E F upon decomposition of the perovskites during sample transfer, storage, and measurements: during near–UV-based PES, the density of defect states saturates at a value that is around 4 orders of magnitude below the density of states at the valence band edge. However, even short air exposure, or 3 h of X-ray illumination, increased their density by almost a factor of six and ∼40, respectively. Upon prolonged storage in vacuum, the formation of a distinct defect peak is observed. Thus, near–UV CFSYS with modeling as shown here is demonstrated as a powerful tool to characterize the valence band and quantify defect states in lead halide perovskites.