Valence Level Character in a Mixed Perovskite Material and Determination of the Valence Band Maximum from Photoelectron Spectroscopy: Variation with Photon Energy

Abstract

A better understanding of the electronic structure of perovskite materials used in photovoltaic devices is essential for their development and optimization. In this investigation, synchrotron-based photoelectron spectroscopy (PES) was used to experimentally delineate the character and energy position of the valence band structures of a mixed perovskite. The valence band was measured using PES with photon energies ranging from ultraviolet photoelectron spectroscopy (21.2 eV) to hard X-rays (up to 4000 eV), and by taking the variation of the photoionization cross sections into account, we could experimentally determine the inorganic and organic contributions. The experiments were compared to theoretical calculations to further distinguish the role of the different anions in the electronic structure. This work also includes a thorough study of the valence band maximum and its position in relation to the Fermi level, which is crucial for the design and optimization of complete solar cells and their functional properties.

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“…[47] The relative position of the valence band maximum (VBM, indicated by dashed lines) differs by ≈0.3 eV between PbS-PbX 2 and PbS-AI QDs versus the Fermi level at 0 eV in BE (Figure 2h). The valence band maximum is determined with a linear extrapolation of the leading edge of the valence band.…”

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“…Other LHPs, such as MAPbI 3 and CsPbBr 3 indicate a similar band characteristics . Reproduced with permission . Copyright 2017, American Chemical Society.…”

“…The energy level configuration of LHPs is mainly determined by the site‐B and site‐X components . The valence band for LHPs is hybridized by the np orbitals of the halogens and the 6s orbitals of lead, whereas the conductive band is mainly determined by the lead 6p orbitals with a minor contribution from the np orbitals of the halogens (Figure c) . Thus, the bandgap of LHPs can be widely tuned by exchanging halogen species with different np orbital energy levels (Cl‐3p, Br‐4p, and I‐5p) .…”

“…The valence band for LHPs is hybridized by the np orbitals of the halogens and the 6s orbitals of lead, whereas the conductive band is mainly determined by the lead 6p orbitals with a minor contribution from the np orbitals of the halogens (Figure c) . Thus, the bandgap of LHPs can be widely tuned by exchanging halogen species with different np orbital energy levels (Cl‐3p, Br‐4p, and I‐5p) . Changing the halogen species from chlorine to iodine drastically shifts the light emitted from the resulting LHPs from near‐ultraviolet to near‐infrared (Figure d) .…”

“…Unlike classic semiconductors with bandgap defined by the difference between bonding valence band maxima (VBM) and antibonding conduction band minima (CBM), the VBM of LHPs is formed by an antibonding interaction of X‐np orbital and Pb‐6s orbital, and the nonbonding CBM is mainly decided by the Pb‐6p orbitals along with a slight contribution from X‐np orbitals . Thus, the absence of bonding–antibonding interaction between these special CB and VB results in a high defect tolerance, where the highest defect states, primarily A‐cation and X‐anion vacancies, lie within the valence or conduction bands instead of in the bandgap (Figure c) . The potential deep trap states formed by interstitial and antisite defects can be neglected because of their high formation energy.…”