Ultraviolet Photoemission Spectroscopy and Kelvin Probe Measurements on Metal Halide Perovskites: Advantages and Pitfalls

Abstract

In this essay, a case study is presented on the electronic structure of several metal halide perovskites (MHP) using Kelvin probe (KP)-based surface photovoltage (SPV) measurements and ultraviolet photoelectron spectroscopy (UPS), to demonstrate the advantages, but also the pitfalls, of using these techniques to characterize the surfaces of these materials. The first part addresses the loss of halide species from perovskite surfaces upon supra-gap illumination in vacuum. This has the potential to cause both a long-term alteration of the sample work function, as well as a modification This article is protected by copyright. All rights reserved. 2 of the KP tip during SPV measurements. If undetected, this leads to a misinterpretation of the MHP surface potential. The second part illustrates the difficulties in determining the valence band maximum (VBM) of MHP surfaces with UPS, and stresses the importance of taking into account the low density of states (DoS) at the VBM edge. Given this circumstance, specific care must be taken to eliminate measurement artifacts in order to ascertain the presence or absence of low densities of electronic gap states above the VBM. This essay also highlights issues such as film degradation, nonequilibrium situations (e.g. SPV) and satellite emissions, that occur during photoelectron spectroscopy.

Gap States in Methylammonium Lead Halides: The Link to Dimethylsulfoxide?

“…Indeed, all three black spectra seem to point to the presence of a significant density of filled gap states extending to E F , and above, which is unphysical. [26] Equally striking are the differences between the three sets of corrected data. Although the valence bands of the three films are quite similar, the main difference is specifically in the bandgap above the VBM, in the energy window corresponding to the gap states captured by these experiments.…”

“…To shine light on the expectedly low density of states at the top of the MHP valence band and above, UPS data are collected and displayed on a logarithmic scale, an approach previously shown to lead to a significantly more accurate determination of the VBM position on these materials than the standard linear extrapolation of the leading edge of the valence band spectrum typically applied to semiconductors. [25][26][27] The VBM position was determined in two independent ways, which lead to values consistent with each other. Both approaches are described in detail in the Experimental Section in Supporting Information.…”

“…Furthermore, given that the photoemission signature of the gap state is expected to be several orders of magnitude smaller than the average signal from the valence band, it is crucial to remove from the experimental spectra all parasitic components due to satellite emissions from the He-discharge lamp, which would otherwise introduce intensity features overlapping with the energy gap and could be construed as corresponding to gap states. To that end, the contributions from the satellite He I α (23.09 eV), He I β (23.75 eV), and He I γ (24.05 eV) lines [28][29][30] were carefully calibrated using C 60 spectra following a method developed by Zhang et al [26] and removed from the predominantly He I (21.22 eV) spectrum. This procedure is carefully developed and described in great details in a section of Supporting Information, including Figures S3 and S4, Supporting Information.…”

“…To that end, the contributions from the satellite He I α (23.09 eV), He I β (23.75 eV), and He I γ (24.05 eV) lines [ 28–30 ] were carefully calibrated using C 60 spectra following a method developed by Zhang et al. [ 26 ] and removed from the predominantly He I (21.22 eV) spectrum. This procedure is carefully developed and described in great details in a section of Supporting Information, including Figures S3 and S4, Supporting Information.…”

Origin of Sn(ii) oxidation in tin halide perovskites

“…Therefore values obtained with XPS can be misleading and inaccurately represent the real Sn(IV) content in the film. Besides, the influence on Sn(IV) formation from ultra-high vacuum and X-rays, which are claimed to induce degradation in lead-based perovskite films, 7 has not yet been studied in detail. While precisely measuring the proportion of these two oxidation states in perovskite thin films can be tricky, there are other facile and available techniques to investigate the origins of Sn(II) oxidation directly.…”