We report valence and conduction band densities of states measured via ultraviolet and inverse photoemission spectroscopies on three metal halide perovskites, specifically methylammonium lead iodide and bromide and cesium lead bromide (MAPbI3, MAPbBr3, CsPbBr3), grown at two different institutions on different substrates. These are compared with theoretical densities of states (DOS) calculated via density functional theory. The qualitative agreement achieved between experiment and theory leads to the identification of valence and conduction band spectral features, and allows a precise determination of the position of the band edges, ionization energy and electron affinity of the materials. The comparison reveals an unusually low DOS at the valence band maximum (VBM) of these compounds, which confirms and generalizes previous predictions of strong band dispersion and low DOS at the MAPbI3 VBM. This low DOS calls for special attention when using electron spectroscopy to determine the frontier electronic states of lead halide perovskites.

One merit of organic-inorganic hybrid perovskites is their tunable bandgap by adjusting the halide stoichiometry, an aspect critical to their application in tandem solar cells, wavelength-tunable light emitting diodes (LEDs), and lasers. However, the phase separation of mixed-halide perovskites caused by light or applied bias results in undesirable recombination at iodide-rich domains, meaning open-circuit voltage (V) pinning in solar cells and infrared emission in LEDs. Here, we report an approach to suppress halide redistribution by self-assembled long-chain organic ammonium capping layers at nanometer-sized grain surfaces. Using the stable mixed-halide perovskite films, we are able to fabricate efficient and wavelength-tunable perovskite LEDs from infrared to green with high external quantum efficiencies of up to 5%, as well as linearly tuned V from 1.05 to 1.45 V in solar cells.

This study presents a combined experimental and theoretical study of the electronic structure of two two-dimensional (2D) metal halide perovskite films. Ultraviolet and inverse

Using an ultrasensitive, exfoliated 2D perovskite single-crystal sheet/graphene heterostructure device, spontaneous iodide loss is revealed as an important degradation pathway of perovskites, which n-dopes perovskites by generating positively charged iodide vacancies. Furthermore, covering perovskites with graphene can suppress iodide loss, significantly improving perovskite stability. Our work not only provides important insights for future stable perovskite optoelectronic device development, but also demonstrates the potential of graphene as an encapsulant as well as a sensitive diagnostic tool for device and material degradation studies.

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.