Unraveling the Electronic Properties of Lead Halide Perovskites with

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Unravelling the electronic properties of lead halide perovskites with surface photovoltage in photoemission studies Fengshuo Zu, Christian M. Wolff, Maryline Ralaiarisoa, Patrick Amsalem, Dieter Neher, and Norbert Koch ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b05293 • Publication Date (Web): 24 May 2019 Downloaded from http://pubs.acs.org on May 24, 2019

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ACS Applied Materials & Interfaces

Unravelling the Electronic Properties of Lead Halide Perovskites with Surface Photovoltage in Photoemission Studies Fengshuo Zu,†,‡ Christian M. Wolff,₸ Maryline Ralaiarisoa,† Patrick Amsalem,† Dieter Neher,₸ Norbert Koch*†,‡ †Institut

für Physik & IRIS Adlershof, Humboldt-Universität zu Berlin, 12489 Berlin,

Germany ‡Helmholtz-Zentrum

Berlin für Materialien und Energie GmbH, 12489 Berlin,

Germany ₸Institut

für Physik und Astronomie, Universität Potsdam, 14776 Potsdam, Germany

KEYWORDS lead halide perovskite films, ultraviolet photoelectron spectroscopy, Kelvin probe, surface band bending, surface photovoltage, surface states

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ABSTRACT

The tremendous success of metal halide perovskites, especially in the field of photovoltaics, has triggered a substantial number of studies towards understanding their opto-electronic properties. However, consensus regarding the electronic properties of these perovskites is lacking due to a huge scatter in reported key parameters, such as work function () and valence band maximum (VBM) values. Here, we demonstrate that the surface photovoltage (SPV) is a key phenomenon occurring at perovskite surfaces that feature a non-negligible density of surface states, which is more the rule than the exception for most materials under study. With ultraviolet photoelectron spectroscopy (UPS) and Kelvin probe, we evidence that even minute UV photon fluxes (500 times lower than used in typical UPS experiments) are sufficient to induce SPV and shift the perovskite  and VBM by several 100 meV compared to the dark. By combining UV and visible light, we establish flat band conditions (i.e., compensate the surface state induced surface band bending) at the surface of four important perovskites, and find that all are p-type in the bulk, despite pronounce n-type surface character in the dark. The present findings highlight that SPV effects must be considered in all surface studies in order to fully understand the perovskites' photo-physical properties.

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1. INTRODUCTION The family of metal halide perovskites, owing to their remarkable optoelectronic properties, has attracted enormous interest, particularly in the field of photovoltaics.1-2 The typical ABX3 perovskite structure has been expanded from simple perovskites, which are primarily (MA/FA)PbX3 (MA = methylammonium, FA = formamidinium)3-4 and CsPbX3 (X = I or Br),5 to a wide range of combinations by mixing of above cations68

and halides,9 opening up a route to tune their optoelectronic properties. As a result,

the mixed perovskites have not only shown improvements in device performance,10 but also enhanced stability and reproducibility.8-9 Over the past few years, numerous studies devoted to the physico-chemical properties of thin film have revealed that the outstanding device performance originates from high optical absorption coefficients,2 long charge-carrier diffusion length,11 and low density of trap states in the bulk.12 For knowledge-based further device optimization, the key electronic properties of perovskite thin films, i.e., work function (), ionization energy (IE), and energy separation of the valence band maximum (VBM) from the Fermi level (EF), as well as the energy levels at interfaces to charge transport layers, as typically employed in solar cells, must be reliably known. There are, however, substantial variations in the reported electronic properties of neat perovskite films, e.g., ranging from p- to n-type, and even conflicting energetics at perovskite-relevant interfaces.13-16 In this context, it was demonstrated that the widely observed n-type behavior of the mixed halide perovskites (MAPbI3-xClx) as measured by ultraviolet photoelectron spectroscopy (UPS) can be attributed to downward surface band bending

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(SBB) induced by donor-type surface states.17 Those conceivably originate from metallic lead (Pb0) and induce Fermi-level pinning close to the conduction band minimum (CBM). Upon illumination with photons of energy larger than the band gap, electron-hole pairs are generated, separated, and redistributed by the electric field. Consequently, the SBB becomes reduced with higher light intensity, and the bands can even be flattened at sufficiently high photon flux. This effect is well known as surface photovoltage (SPV),18-19 and schematically illustrated in Figure 1. Notably, SPV should thus be considered in all studies of semiconductors using above band gap photon illumination, for example, when performing UPS measurements with UV photons of typically 21.22 eV from a He gas discharge lamp.

Figure 1. Schematic energy level diagram for surface photovoltage (SPV) at the perovskite surface. The black and red solid lines represent the band alignment in dark and under saturation UV and/or visible light illumination, respectively. Red dashed lines depict the situation where illumination intensity is intermediate. The shift in SPV corresponds to the magnitude of changed surface band bending (SBB) due to illumination. EC and EV denote

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conduction and valence band edges, respectively. EF is the Fermi level. E0 refers to the zero of the electrostatic potential, which is equivalent to the surface vacuum level.

In fact, UPS is the most widely applied technique to investigate the electronic structure of a functional materials, from which the occupied density of states (DOS), , and IE can be assessed. However, the possible influence of SPV on UPS measurements has barely been attended to when aiming at assessing perovskite energy levels. Thus, the question arises whether the energy levels as determined by UPS, where high energy photon excitation is inevitable, represent the intrinsic properties of the materials. In this study, focusing on unraveling the impact of SPV, we conducted a detailed study on the electronic properties of perovskites with varied composition, covering a wide range from the prototypical MAPbI3 to more complex and recently used mixed cation and halide perovskites. To warrant high relevance of our results, we used the sample preparation protocols from our recent studies that resulted in over 20% power conversion efficiency in a planar p-i-n architecture solar cells.8,

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With UPS and

complementary Kelvin probe (KP) measurements, we demonstrate that all these perovskite films exhibit pronounced n-type character at the surface due to pronounced SBB in dark. We identify that even low-level UV photon flux on the samples (to create the photoelectrons) induces notable SPV, which increases with photon flux and reaches more than 700 meV for standard laboratory conditions. Consequently, fundamentally different energy level scenarios can be derived from UPS measurements, depending

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solely on the applied UV photon flux. As well, irradiation of samples with white light in the visible regime during UPS measurements induces SPV, and by reaching saturation conditions (flat bands as shown in Figure 1 under saturation illumination) we find that the perovskites investigated here are in fact p-type in the bulk. Our results demonstrate that great care must be taken when interpreting data from measurements that employ above band gap photon irradiation, as an intensity-dependent SBB magnitude must be considered explicitly.

2. EXPERIMENTAL DETAILS 2.1 Sample Preparation 1x1 cm² indium-tin-oxide (ITO) substrates are washed by sonication for 15 min. with acetone, Hellmanex III (3% in deionized water), deionized-water and isopropanol, respectively. After 4 min of microwave plasma (200W), the samples were taken to a N2-filled glovebox and PTAA (Sigma-Aldrich, 1.5 mg/mL in toluene, dissolved over night by stirring) was spin-coated at 6000 rpm for 30 s after spreading around 20 µL on the substrate prior to starting. The substrate was subsequently annealed at 100°C for 10 minutes and left to cool for 5 minutes. The perovskite solution MAPbI3 consisted of 1 mmol PbI2(Alfa Aesar), 1 mmol MAI (dyesol), 0.85 mmol dimethyl sulfoxide (DMSO, Sigma Aldrich) and 0.15 mmol thiourea (Sigma-Aldrich) in 636 µL anhydrous N,NDimethylformamide (DMF, Alfa-Aesar). The perovskite solution MAFA was prepared by dissolving (stirring overnight) MABr (0.2M, dyesol), FAI (1.03M, dyesol), PbI2 (1.13M, Alfa-Aesar) and PbBr2 (0.2M TCI) in anhydrous DMF/DMSO = 4:1 (v/v, Alfa6


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Aesar and Sigma-Aldrich, respectively). In parallel CsI (1.5M, Sigma-Aldrich) and RbI (1.5M Sigma-Aldrich) were dissolved in DMSO and DMF/DMSO (4:1, v/v), respectively. CsMAFA was mixed by combining MAFA:CsI = 960:40 µL/µL to produce a composition with the stoichiometry Cs0.05[MA0.17FA0.83Pb(I0.83Br0.17)3]0.95. An aliquot of this mixture was then further mixed to CsMAFA:RbI = 960:40 µL/µL to produce RbCsMAFA (Rb0.05{Cs0.05[MA0.17FA0.83Pb(I0.83Br0.17)3]0.95}0.95). Typically, 30 µL of either perovskite solution was drop-cast on the 1x1 cm² substrates and spincoated at 3500 prm with 3 s acceleration (1160 rpm/s) for 35 s. 10 s after the start of the spin-coating 100 µL of ethyl acetate (Sigma-Aldrich) were dispensed on the spinning substrate inducing a color change to dark brown (MAPbI3) or reddish-brown (mixed perovskites), depending on the composition. The samples were thereafter annealed for 1 hour at 100°C. All samples, as prepared in the N2-filled glove box, were transferred to the ultrahigh vacuum system for UPS measurements within 24 hours. The samples were never exposed to ambient air or experienced any test measurements before UPS studies, so that UPS studies on fresh samples are guaranteed. 2.2 Photoemission Spectroscopy Measurements. UPS was conducted using a helium discharge lamp (HIS 13 FOCUS GmbH, photon energy 21.22 eV) in a custom ultrahigh vacuum (UHV) system (base pressure: 1 × 10-9 mbar). Filters made from aluminum foil in different thicknesses (attenuation by a factor of ca. 20 and ca. 500, respectively) were placed between UV-lamp and sample to reduce the UV flux. Samples were in part also illuminated with a white light halogen lamp (150 mW/cm2, 1.5 sun) during UPS measurements. All spectra were

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recorded at room temperature and normal emission using a hemispherical analyzer (SPECSPhoibos 100). The energy resolution was set to 130 meV. One secondary electrons cutoff and valence band measurements took ca. 40 seconds and 2 minutes, respectively. Overall, the samples were illuminated under 1.5 sun for less than 6 minutes, to ensure that light-induced surface states17 do not interfere with the measurements. 2.3 Valence Band Maximum Determination. Graphical determination of VBM values using linear and semi-log extrapolation methods was done by determining the intersection of the background line and a tangent line taken at the inflection point of the top valence band. All valence band spectra were normalized to the maximum peak intensity. VBM values in dark (from KP measurements of ) are extrapolated by taking into account the averaged  difference between KP and UPS data, i.e. 𝑉𝐵𝑀𝑑𝑎𝑟𝑘 = (𝑉𝐵𝑀𝑠𝑎𝑡 + ∆𝛷𝑠𝑎𝑡 ― 𝑑𝑎𝑟𝑘) + (𝑉𝐵𝑀𝑈𝑉𝐿 + ∆𝛷𝑈𝑉𝐿 ― 𝑑𝑎𝑟𝑘) 2

, where sat stands for saturation.

2.4 Kelvin Probe Measurements. Measurements were conducted using a KP Technology (KP6500) instrument in a N2 filled glove box. The 2 mm diameter gold tip was calibrated using a highly oriented pyrolytic graphite (HOPG) sample, whose work function was directly measured by photoemission yield spectroscopy (PYS) in air and by UPS in UHV. Therefore, the work function of HOPG in inert atmosphere was estimated to be 4.66 ± 0.09 eV by averaging the values measured by PYS and UPS.

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3. RESULTS AND DISCUSSION 3.1 SPV of MAPbI3 Thin Films We present our results obtained for the prototypical MAPbI3 films in detail first, and further below a summary for the other perovskites, where studies proceeded in full analogy. For the UPS measurements with varied UV photon flux, MAPbI3 films were either exposed to the full flux as provided by our UV-lamp (see Experimental details), or to lower fluxes realized by two attenuation filters of different thicknesses placed between UV-lamp and sample. Hereafter, UVL and UVM denote low (thick filter, attenuation by a factor of ca. 500 fold) and medium (thin filter, attenuation by a factor of ca. 20 fold) UV flux, respectively. UVH refers to high UV flux, i.e., obtained without filter. Figure 2a shows UPS spectra of the MAPbI3 thin film with varied UV photon flux. In the following we discuss the valence band onset determined by extrapolation of the leading edge on a linear photoelectron intensity scale as well as a logarithmic one. The rationale for this is that it was demonstrated that the VBM can be retrieved more accurately for perovskite materials when employing a logarithmic intensity scale due to the proximity of non-dispersive bands to the highly dispersive valence band.2122

Yet, we also include the values obtained on a linear intensity scale (indicated in

brackets with "lin") for better comparison with earlier reports, which did not use the logarithmic scale. Initially, as shown in Figure 2a (middle and right plot) the VBM of MAPbI3 is positioned at 1.37 eV (lin: 1.51 eV) binding energy (BE, with respect to EF)

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when probed using UVL, indicating a strong n-type character of the perovskite surface given the fundamental band gap of ca. 1.7 eV.23-24 When increasing the UV flux, the VBM decreases gradually to 1.07 eV (lin: 1.23 eV) using UVM, and even to 0.88 eV (lin: 1.12 eV) using UVH. Concomitantly, as seen from the secondary electron cutoff (SECO) spectra in Figure 2a (left plot), the sample work function increases from 4.48 eV to 5.09 eV when going from UVL to UVH. We note that illumination induced shifts of  are slightly different from those of the VBM, which is due to the fact that the low kinetic energy SECO provides an area-averaged sample .25-26 Since sample  and VBM move essentially in parallel, this origin for the shift must be of electrostatic origin, and is in fact reminiscent of the typical SPV effect occurring during UPS experiments, which has been reported to induce serious modifications in determining the Schottkybarrier at metal-semiconductor interfaces.27-29 In the present study, the perovskite surface shows pronounced n-type character at low UV flux as a result of downward surface band bending that is induced by surface states (see Introduction). The surface band bending region extends from the very surface to the position in the bulk of the perovskite films where the intrinsic carrier density, and thus intrinsic Fermi level position, is reached. The width of this region depends on the intrinsic carrier density and the density and energy position of the surface states. The surface as investigated by UPS is given by the probing depth of the methods, which is up to 2 nm depending on the electron inelastic mean free path of the investigated material.30 Thus, UPS measurements provide the maximum magnitude of surface band bending, provided that surface photovoltage does not play a role. During the photoemission process, photo-

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generated charges (due to direct excitation by the UV light but also due to excitation by inelastically scattered secondary electrons) are redistributed by the electric field, producing an opposing field, which reduces the initial SBB. This is illustrated in Figure 1 for an n-type surface, where the SPV effect can readily shift the energy levels and eventually lead to a flat band condition at sufficient excitation intensity.

Figure 2. (a) UPS spectra of MAPbI3 thin films measured with varied incident UV flux and additional white light illumination (150 mW/cm2, intensity equivalent to 1.5 sun). Valence bands (VB) in linear and logarithmic intensity scale are presented in middle and right panels, respectively; the left panel shows secondary electron cutoff (SECO) spectra. (b) Summarized work function (Φ) and VBM values using linear and logarithmic extrapolation methods at varied excitation flux. Φ in dark is determined from Kelvin probe (KP) measurements, where the VBM values in dark are extrapolated based on the averaged Φ difference between KP and UPS results (see Experimental details).

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To further investigate the effect of SPV at higher illumination-intensity as provided by our UV-lamp alone (i.e., beyond UVH), UPS spectra were taken while samples were simultaneously illuminated with 150 mW/cm2 (equivalent to 1.5 sun) visible white light, as also indicated in Figure 2a. In this case, additional visible light exposure results in a further increase of Ф up to 5.20 eV, accompanied with a decrease of VBM down to 0.58 eV (lin: 0.75 eV). It is worth stressing that we apparently reach flat band conditions, i.e., the SBB is fully compensated for, when employing UVH + 1.5 sun, as the  and VBM values obtained with UVL + 1.5 sun are almost identical. The fact that the VBM is only 0.58 eV away from EF indicates a p-type character of the materials in the bulk, in agreement with the observations from Hall effect measurements.31-32 Noteworthy, neither UV light nor visible light was found to induce degradation and/or decomposition of the perovskite sample during the measurements, as Ф and VBM measured under UVL conditions after higher light intensity exposure return back to their typical initial values of 4.42 eV and 1.43 eV (lin: 1.52 eV), respectively. Additionally, no changes of valence band shape were observed throughout the whole set of measurements. The Ф and VBM values against varied illumination conditions are summarized in Figure 2b. Notably, the trend of Ф and VBM as a function of illumination conditions straightforwardly demonstrates that the SPV due to very low UV excitation alone (as used in UPS) can strongly affect the determination of the energy levels at perovskite surface. These energy level shifts reflect the change in magnitude of the SBB, which is fully developed only in complete dark conditions (and thus not realistically accessible

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by UPS). We argue that any band bending effects on the back contact (substrate) can be ruled out as the SPV readily takes place by varying the UV flux only, which is expected to be attenuated significantly within 20 nm. The exact origin of surface band bending in perovskites is related with the presence of donor-type surface states, likely consisting of metallic Pb on the surface due to iodine deficiency.17, 33-35 To assess the perovskite Ф in complete dark condition, the sample was further investigated by the Kelvin probe (KP) method. The measurement of the contact potential difference (CPD) between the reference electrode and the sample in KP experiments allows to straightforwardly deriving the samples’ Ф with respect to a known reference surface. It is worth noting that for both experiments (UPS and KP) the same set of samples were investigated, ensuring consistent results. We note that, as one would expect, KP in dark yields a  value of (4.22 ± 0.09) eV for MAPbI3, which is yet lower than the value (4.48 eV) determined by UPS at UVL, as shown in Figure S1a of the Supporting Information. The VBM under dark condition is estimated to be at 1.60 eV BE (lin: 1.75 eV), evidencing a highly pronounced SBB at electronic equilibrium. This strongly suggests that probing the energy levels using UPS even at very low photon flux (in our case 500 times lower than in conventional laboratory conditions) can already be misleading when not carefully accounting for SPV.

3.2 SPV of Mixed Perovskite Thin Films To further demonstrate and confirm the universal relevance of SPV in other hybrid perovskite films, experiments analogous to those detailed above were performed on 13



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mixed cations and halides perovskites, which have been extensively explored as light absorber in perovskite solar cells.6, 9, 36-37 Following the above described procedures, UPS and KP measurements were performed on FA0.83MA0.17Pb(I0.83Br0.17)3, Cs0.05[FA0.83MA0.17Pb(I0.83Br0.17)3]0.95

and

Rb0.05{Cs0.05[FA0.83MA0.17Pb(I0.83Br0.17)3]0.95}0.95 thin films. Hereafter, we use the nomenclature of FAMA, CsFAMA and RbCsFAMA to denote the three different compositions. We observed, similarly to the results obtained from MAPbI3, that all samples exhibit excitation-intensity dependent energy level shifts, revealing that strong SPV effects can readily take place even for minute UV flux, and certainly visible light exposure. The corresponding UPS spectra as a function of illumination type and intensity of the mixed perovskites are shown in the Supporting Information (Figures S3 to S5). The Ф and VBM values against varied illumination conditions as obtained from UPS are summarized in Figure 3. Clearly, for all the studied perovskites, the trends, the magnitudes of the energy shifts, as well as the absolute values of their energy levels under illumination are comparable to those observed for MAPbI3, suggesting the same physical processes being at play. As seen from Figure 3a for FAMA, essentially parallel shifts of Ф and VBM take place. Starting at 4.37 eV using UVL, Ф increases with increasing UV flux, and further increases up to 5.25 eV using additional white light illumination at UVM, which is concomitant with the VBM shifting from 1.38 eV (lin: 1.54 eV) down to 0.6 eV (lin: 0.83 eV). The results for CsFAMA and RbCsFAMA thin films exhibit essentially the same trend. For CsFAMA shown in Figure 3b, Ф shifts from 4.29 eV using UVL up to 5.15 eV with additional white light illumination.

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Equivalently, the VBM BE reduces from 1.38 eV (lin: 1.55 eV) to 0.62 eV (lin: 0.83

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eV). For the RbCsFAMA thin film shown in Figure 3c, Ф increases from 4.46 eV and

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saturates at 5.11 eV at saturation illumination intensity (i.e., combined UV and visible

light), and in parallel VBM values change from 1.29 eV (lin: 1.57 eV) to 0.6 eV (0.80 eV). The illumination intensity dependence of  for a CsFAMA film further substantiates that indeed saturation illumination conditions (flat bands) are reached, Figure 3. Summarized  and VBM values (the latter evaluated on linear band logarithmic 17



intensity plots) for different excitation flux for (a) FAMA, (b) CsFAMA and (c) RbCsFAMA thin films. Φ in dark is determined from KP measurements, where the VBM values in dark are extrapolated based on the averaged difference of Φ between KP and UPS results (see Experimental details).

here already at ca. 0.6 sun as shown in Figure S6. Notably, all three mixed perovskite materials exhibit p-type bulk characteristics, as the VBM is closer to EF than the CBM. With KP measurements, as shown in Figure S1, the respective  values in dark conditions were determined to be (4.16 ± 0.09) eV, (4.19 ± 0.09) eV and (4.45 ± 0.09 eV) for FAMA, CsFAMA, and RbCsFAMA, respectively, which are all lower than the values measured by UPS at UVL, in agreement with the observation of MAPbI3 results. These observations clearly demonstrate the significance of SPV on the energy levels as probed in UPS. Already at room temperature and upon UV and or visible light illumination, surface states induced SBB can be largely reduced by the redistribution of photo-generated charge carriers. Consequently, large variations of the key electronic property parameters, i.e., Ф and VBM, of hybrid perovskites can be readily observed by UPS, simply depending on the particular UV photon fluxes and eventually present visible light (e.g., window ports, ion pressure gauges) in different laboratories. Therefore, reduction of the excitation flux in UPS appears as highly recommended in order to approach the perovskite intrinsic electronic structure, and ideally a full characterization of SPV effects can lead to highly reliable data. Furthermore, the present findings may shed light on one key aspect to explain why perovskite based optoelectronic devices perform so well, as surface defect induced SBB can easily be compensated for under illumination, thus not impeding long-carrier lifetimes through 18


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trap-assisted recombination.38-40

4. CONCLUSION For four different hybrid perovskites, we demonstrated that substantial SPV can be induced by visible light, and notably also by UV light as employed in UPS. For perovskite thin films that feature (ubiquitous) surface states and concomitant downward surface band bending, the work function and energy levels, including the VBM, exhibit more than 700 meV variation as function of UV photon flux alone. Consistent with previous reports, the downward surface band bending can be attributed to the presence of donor-type surface states, likely due to elemental lead, which pin EF at (or close to) the CBM. By combining UV and visible photon sample irradiation, we could establish flat band conditions at the surface, and provide evidence that the bulk of these four perovskites has p-type character, despite the pronounce n-type character of the surface in dark. Our results emphasize that SPV must be carefully taken into account when characterizing hybrid perovskite samples where optical excitation is inevitably employed, particularly when performing UPS experiments. When implemented consistently, e.g., as demonstrated here, the great challenge of determining the electronic structure of hybrid perovskites with high confidence can be mastered.

ASSOCIATED CONTENT

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Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. Kelvin probe measurements on the 4 different perovskites in dark, SPV measurements on MAPbI3 sample using Kelvin probe, UPS spectra of the 4 different perovskites as a function of excitation intensity, UV-vis absorption spectra of freshly prepared mixed perovskite films and the corresponding ones after UPS measurements (PDF) AUTHOR INFORMATION Corresponding Author *N. K. email: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT This work was supported by the Helmholtz Energy Alliance "Hybrid Photovoltaics", the Joint Graduate School HyPerCells of the University of Potsdam and the Helmholtz Zentrum Berlin, the Potsdam Graduate School, the DFG (SFB951). P. A. acknowledges the DFG (AM 419/1-1) for financial support.

REFERENCES (1) Snaith, H. J., Perovskites: The Emergence of a New Era for Low-Cost, HighEfficiency Solar Cells. J. Phys. Chem. Lett. 2013, 4, 3623-3630.

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Table of Contents graphic 148x79mm (150 x 150 DPI)


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Figure 1 95x108mm (150 x 150 DPI)



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Figure 3 85x214mm (300 x 300 DPI)


Unraveling the Electronic Properties of Lead Halide Perovskites with

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