Menzel Dorothee, Tejada Alvaro, Al-Ashouri Amran, Levine Igal, Guerra Jorge Andres, Rech Bernd, Albrecht Steve, Korte Lars
Young Investigator Group Perovskite Tandem Solar Cells, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Kekuléstraße 5, 12489 Berlin, Germany.
Departamento de Ciencias, Sección Física, Pontificia Universidad Católica del Perú, Av. Universitaria 1801, 15088 Lima, Peru.
ACS Appl Mater Interfaces. 2021 Sep 15;13(36):43540-43553. doi: 10.1021/acsami.1c10171. Epub 2021 Sep 2.
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(MAFA)Pb(IBr) 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 -vectors in the angle-integrated spectra, where the highest one, resulting from the VBM at the -point in the Brillouin zone, is found between -1.50 to -1.37 eV relative to the Fermi energy . We investigate quantitatively the formation of defect states in the band gap up to 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.
我们使用具有非常高动态范围的先进近紫外光电子能谱（PES）在恒定终态模式（CFSYS）下，研究了三阳离子铅卤化物钙钛矿Cs(MAFA)Pb(IBr)，并对价带中的占据态密度（DOS）和带隙有了详细的了解。建立了一个价带模型，该模型在一个单一方程中包括抛物线形的价带边缘和指数衰减的带尾。这使我们能够在角度积分光谱中精确确定不同 - 矢量处的两个价带最大值（VBM），其中最高的一个，由布里渊区 - 点处的VBM产生，相对于费米能量在 -1.50至 -1.37 eV之间被发现。我们定量研究了在样品转移、储存和测量过程中钙钛矿分解时带隙中直至 缺陷态的形成：在基于近紫外的PES过程中，缺陷态密度饱和在一个比价带边缘态密度低约4个数量级的值。然而，即使是短暂的空气暴露或3小时的X射线照射，分别使它们的密度增加了近六倍和约40倍。在真空中长时间储存后，观察到一个明显的缺陷峰的形成。因此，如本文所示的具有建模功能的近紫外CFSYS被证明是表征价带和量化铅卤化物钙钛矿中缺陷态的有力工具。
