Valence and Conduction Band Densities of States of Metal Halide Perovskites: A Combined Experimental-Theoretical Study.

James Endres,Lianfeng Zhao,Leeor Kronik,Ross A Kerner,Barry P Rand,Antoine Kahn,David A Egger,Scott H Silver,Gary Hodes,David Cahen,Michael Kulbak

doi:10.1021/acs.jpclett.6b00946

Abstract

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.

Highlights

Ultraviolet photoemission spectroscopy (UPS) and inverse photoemission spectroscopy (IPES) are experimental techniques of choice to investigate the semiconductor valence and conduction band densities of states (DOS), respectively
The data presented in the Supporting Information of this earlier MAPbI3 study[9] pointed to the difficulty of evaluating the ionization energy (IE) of these materials based on photoemission data, and the need to carefully look at the DOS in order to pinpoint the valence band maximum (VBM) position
The strong tailing at the VBM is a further similarity between the electronic structure of halide perovskite (HaP) and lead-chalcogenides.[3]. We address this issue by analyzing photoemission and inverse photoemission data recorded on a logarithmic scale, with an emphasis on spectral regions of low intensity, as already employed by Schulz et al in their analysis of the electronic structure of MAPbI3 and MAPbBr3 interfaces.[9]

Summary

■ EXPERIMENTAL METHODS

Electron spectroscopy was performed in an ultrahigh vacuum (UHV) analysis chamber (base pressure 10−10 Torr) equipped with an introduction system allowing sample transfer from a nitrogen glovebox without ambient exposure. UPS and IPES measurements were performed in sequences designed to minimize photon and electron irradiation time and impact on the electronic structure of the films. After a 10 min O2 plasma treatment, a Letter compact ∼60 nm TiO2 layer was applied to the clean substrate by spray pyrolysis of a 30 mM titanium diisopropoxide bis(acetylacetonate) (Sigma-Aldrich) solution in isopropanol using air as the carrier gas on a hot plate set to 450 °C, followed by a two-step annealing procedure at 160 and 500 °C, each for 1 h in air. The PU MAPbI3 and CsPbBr3 samples were processed as follows: MAI was synthesized by mixing ethanol:methylamine (Sigma-Aldrich) and aqueous HI (Sigma-Aldrich, stabilized) solutions together (equimolar methylamine to HI). Inorganic CsPbBr3 (PbBr2 and CsBr from Sigma-Aldrich, ≥ 98% and 99.9%, respectively) solutions were prepared in dimethyl sulfoxide Samples were transferred under nitrogen directly to vacuum for UPS/IPES measurements

■ COMPUTATIONAL METHODS

■ ACKNOWLEDGMENTS

Findings

■ REFERENCES