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Valence and Conduction Band Densities of States of Metal Halide Perovskites: A Combined Experimental - Theoretical Study James Endres, David A. Egger, Michael Kulbak, Ross A. Kerner, Lianfeng Zhao, Scott H. Silver, Gary Hodes, Barry P. Rand, David Cahen, Leeor Kronik, and Antoine Kahn J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.6b00946 • Publication Date (Web): 01 Jul 2016 Downloaded from http://pubs.acs.org on July 2, 2016
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The Journal of Physical Chemistry Letters
Valence and Conduction Band Densities of States of Metal Halide Perovskites: a Combined Experimental - Theoretical Study James Endres1, David A. Egger2, Michael Kulbak2, Ross A. Kerner1, Lianfeng Zhao1, Scott H. Silver1, Gary Hodes2, Barry P. Rand1, David Cahen2, Leeor Kronik2, and Antoine Kahn1* 1
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Department of Electrical Engineering, Princeton University, Princeton NJ, 08544, USA Department of Materials and Interfaces, Weizmann Institute of Science, Rehovot, 76100, Israel * Corresponding author:
[email protected] 1 ACS Paragon Plus Environment
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Abstract We report valence and conduction band densities of states measured via ultra-violet and inverse photoemission spectroscopies on three metal halide perovskites, specifically methyl ammonium 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.
UPS vs. DFT of MAPbBr3 UPS DFT low DOS near VBM
2.6 eV > EG
EVBM
Logarithmic Intensity Scale
TOC graphics:
Linear Intensity Scale
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4 3 2 1 Binding Energy w.r.t. EF (eV)
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The remarkable progress recently made in the development of halide perovskite (HaP)-based solar cells has spurred considerable interest in the electronic structure of these materials.1,2 Interest is amplified by the seemingly simple, solution-based, low temperature processing that yields HaP films with carrier transport and optical properties comparable to those of standard semiconductors requiring far more constrained and energy intensive processing methods.3 Research efforts have focused among other things on the nature of carrier transport and related electronic structure,4,5 and the presence of defect states in the gap of these semiconductors.6–8 The determination of the electronic structure and chemical state of HaP interfaces with carrier extraction/blocking interlayers and contacts, which play a key role in the optimization of devices, has also been a major point of interest.9–15 In that regard, the electronic structure of the constituent materials, i.e., the energy positions of valence band maximum (VBM) and conduction band minimum (CBM), density of states (DOS) at frontier electronic states, vacuum level (EVAC), ionization energy (IE) and electron affinity (EA), is of paramount importance for understanding and controlling interfaces and optimizing device performance. Ultra-violet photoemission spectroscopy (UPS) and inverse photoemission spectroscopy (IPES) are experimental techniques of choice to investigate the semiconductor valence and conduction band DOS, respectively. However, the determination of the VBM and CBM positions for a new material, or for a material affected by structural disorder or defects, is often hindered by the difficulty to distinguish bulk- from surface- or defect-related states, and pinpoint the energy position of the onset of the valence or conduction band states. Early investigations of standard crystalline semiconductor surfaces like Ge(110) or GaAs(110) showed that simulations of photoemission spectra with theoretical VBM DOS could lead to a precise determination of this onset.16 These studies also showed that a good approximation of the photoemission onset could 3 ACS Paragon Plus Environment
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be obtained from a linear extrapolation to zero of the leading edge of the valence band spectrum. This ad hoc procedure was then widely adopted in the last three decades as a practical approach to photoemission studies of most semiconductors, such as conjugated organic semiconductors and organic/inorganic metal halides.17,18 Problems arise, however, for materials that exhibit soft onsets with a non-linear leading edge of the valence band spectrum, often seen in disordered materials, e.g., polymers, or materials with a significant density of gap states tailing above the VBM. Without theoretical input on the DOS, the determination of the onset on these materials via photoemission becomes difficult and somewhat subjective. Several groups have reported photoemission spectroscopy data on HaP thin films in the past few years, with the goal of determining electronic parameters (VBM, IE, EA) and the relative position of electronic levels across interfaces. Emara et al. showed significant variations in the IE of methylammonium lead iodide (MAPbI3) as a function of preparation method and film stoichiometry.19 Miller et al. reported X-ray photoemission spectroscopy (XPS) measurements of the valence band of MAPbI3 films deposited on various substrates.20 Schulz et al. reported on UPS and IPES measurements of IE and EA of MAPbI3 and its bromide analogue, MAPbBr3, and associated interfaces.9,10 The data presented in the Supplementary Information of this earlier MAPbI3 study
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pointed to the difficulty of evaluating the IE of these materials based on
photoemission data, and the need to carefully look at the DOS in order to pinpoint the VBM position. The present work expands on this point by presenting a combined experimental-theoretical investigation of the DOS of several organic/inorganic metal halides. In addition to providing an identification of the origin of valence and conduction band features, the theoretical simulation of the data helps locate the energy positions of the VBM and CBM, and determine IE and EA, of 4 ACS Paragon Plus Environment
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organic/inorganic lead halides, MAPbI3, MAPbBr3 and cesium lead bromide (CsPbBr3). The combination of valence band experimental and theoretical spectra shows that the DOS at the VBM is remarkably low compared to other semiconductors used in photovoltaics, such as Si or GaAs.16,21 The simple linear extrapolation procedure outlined above, and performed on standard UPS spectra of these materials, misses the VBM and results in an overestimation of IE. The problem is largely corrected by using a logarithmic detection of the photoemission signal, which emphasizes the low DOS part of the valence band photoemission spectrum. By judicious comparison to theoretically computed DOS, based on density functional theory (DFT), it allows for a more accurate definition of the photoemission onset. Experimental valence band (UPS, He I) and conduction band (IPES) spectra taken on MAPbI3, MAPbBr3 and CsPbBr3 samples made at the Weizmann Institute of Science (WIS) are compared with theoretical simulations in Figures 1-3. The experimental plots are obtained from the UPS and IPES data following subtraction of a cubic spline background (not shown here). Both UPS and IPES energy scales are referenced to the Fermi level EF (0 eV), measured separately on a metallic surface in electrical contact with the samples. The energy of the UPS and IPES electrons involved in probing the valence and conduction bands is in the 5-15 eV range, making these techniques sensitive to the top 3 to 4 lattice planes of the material. The valence and conduction band data can therefore be considered as comprising strong contributions from both the surface and bulk DOS of the material. The alignment between experimental and theoretical features is obtained by first shifting the computed VBM and CBM DOS (independently), stretching them (by
