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

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Menzel D.
Tejada A.
Al-Ashouri A.
Levine I.
Guerra Torres, Jorge Andrés
Rech B.
Albrecht S.
Korte L.
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American Chemical Society
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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 Cs0.05(MA0.17FA0.83)0.95Pb(I0.83Br0.17)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 EF. We investigate quantitatively the formation of defect states in the band gap up to EF 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. © 2021 The Authors. Published by American Chemical Society.
The authors acknowledge funding from the Federal Ministry of Education and Research (BMBF) for funding of the Young Investigator Group Perovskite Tandem Solar Cells within the program “Materialforschung für die Energiewende” (grant no. 03SF0540), the Helmholtz Association within the HySPRINT Innovation lab project, and the HyPerCells joint Graduate School. This research was supported by the joint agreement between the DAAD (German Academic Exchange Service) and FONDECYT (National Fund for Scientific, Technological Development and Technological Innovation) under the agreements 57508544 DAAD and 423-2019-FONDECYT. Further support has been provided by the PUCP vice chancellorship for research (VRI, project no. CAP-2019-3-0041/702). The authors thank Thomas Lußky for technical support, Bor Li for part of the sample preparation, and Norbert Koch, David Cahen, Isaac Balberg and Norbert Nickel for fruitful discussions. I.L. thanks the PEROSEED project and AiF project (ZIM-KK5085302DF0) for financial support.
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solar cells, defect states, halide perovskites, near-UV spectroscopy, photoemission spectroscopy