Surface Electronic Structure of Hybrid Organo Lead Bromide

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The Surface Electronic Structure of Hybrid Organo Lead Bromide Perovskite Single Crystals Takashi Komesu, Xin Huang, Tula R Paudel, Yaroslav B. Losovyj, Xin Zhang, Eike F. Schwier, Yohei Kojima, Mingtian Zheng, Hideaki Iwasawa, Kenya Shimada, Makhsud I. Saidaminov, Dong Shi, Ahmed L. Abdelhady, Osman M. Bakr, Shuai Dong, Evgeny Y Tsymbal, and Peter A. Dowben J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b08329 • Publication Date (Web): 24 Aug 2016 Downloaded from http://pubs.acs.org on August 26, 2016

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The Journal of Physical Chemistry

The Surface Electronic Structure of Hybrid Organo Lead Bromide Perovskite Single Crystals Takashi Komesu,1,#,* Xin Huang,1,2,# Tula R. Paudel,1* Yaroslav B. Losovyj,3 Xin Zhang,1 Eike F. Schwier,4 Yohei Kojima,5 Mingtian Zheng,5 Hideaki Iwasawa,4 Kenya Shimada,4* Makhsud Saidaminov,6 Dong Shi,6 Ahmed L. Abdelhady,6 Osman M. Bakr,6* Shuai Dong,2 Evgeny Y. Tsymbal,1* and Peter A. Dowben1* 1)

Department of Physics and Astronomy, Jorgensen Hall, 855 North 16th Street, University of

Nebraska-Lincoln, Lincoln, NE, 68588-0299, U. S. A. 2)

Department of Physics, Southeast University, Nanjing 211189, China

3)

Department of Chemistry, A421C 800 E. Kirkwood Ave., Indiana University, Bloomington,

Indiana 47405, U.S.A. 4)

Hiroshima Synchrotron Radiation Center, Hiroshima University, Higashi-Hiroshima 739-

0046, Japan 5)

Graduate School of Science, Hiroshima University, Higashi-Hiroshima 739-8526, Japan

6)

Solar and Photovoltaic Engineering Research Center (SPERC), King Abdullah University of

Science and Technology (KAUST), Thuwal 23955-6900, Saudi Arabia # These authors contributed equally to this work

ACS Paragon Plus Environment

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Corresponding Author *Peter Dowben, Department of Physics and Astronomy, Theodore Jorgensen Hall, 855 North 16th Street, University of Nebraska-Lincoln, Lincoln, NE 68588-0299 U.S.A., tel: 402-4729838; FAX: 402-472-6148; e mail: [email protected]; Osman M. Bakr, Solar and Photovoltaic Engineering Research Center (SPERC), King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Saudi Arabia; e mail: [email protected]


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ABSTRACT

The electronic structure and band dispersion of methylammonium lead bromide, CH3NH3PbBr3, has been investigated through a combination of angle-resolved photoemission spectroscopy (ARPES) and inverse photoemission spectroscopy (IPES), as well as theoretical modelling based on density functional theory. The experimental band structures are consistent with the density functional calculations. The results demonstrate the presence of a dispersive valence band in MAPbBr3 that peaks at the M point of the surface Brillouin zone. The results also indicate that the surface termination of the CH3NH3PbBr3 is the methylammonium bromide (CH3NH3Br) layer. We find our results support models that predict a heavier hole effective mass, in the region of -0.23 to -0.26 me, along the ! (surface Brillouin center) to M point of the surface Brillouin zone. The surface appears to be n-type as a result of an excess of lead in the surface region.

INTRODUCTION Organic-inorganic perovskites are attractive candidates for next generation photovoltaic cells. The reported efficiency of CH3NH3PbX3 (X=Cl, Br, I) based perovskite thin-film solar cells has seen rapid improvements,1-13 yet few fundamental studies of electronic structure have been performed on such materials. There exist theoretical band structure calculations,13-27 photoemission16,28,29 and combined photoemission and inverse photoemission28 studies. While the formation energy of methylammonium (MA), CH3NH3+, is too high to make MAPbBr3 an n-type material, there is no a priori reason for the surface region to retain the p-type character of the


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bulk,14 so in this regard, the characterization of the surface electronic structure is crucially important. Indeed, knowledge of the surface termination, and the surface band structure are especially important, as is the placement of the surface chemical potential, in the choice of the most effective device contacts.30,31 The band structure is also key to understanding the low effective masses of the carriers in the methylammonium lead halide perovskites.13,24-26,30,32 Without an experimental verification of the effective mass, especially the hole effective mass and the effect of the interface composition,31 the limits to carrier mobility cannot be ascertained in spite of the high mobilities of ∼115 cm2 V– s ref. [33] to ∼217 cm2 V–1s–1 ref. [34] and very long diffusion lengths33,35,36 already measured.

1 –1

Roughly the photocarrier mobility (µ) is related to the effective mass (m*) by µ ~ (q/m*)τ, where τ is the average scattering time. So far, the calculated effective mass seems to depend on the choice of functional. The local density approximation (LDA) over binds metal and ligand, resulting in a dispersive valance band with smaller effective mass, with a reported hole effective mass of -0.12 me (me: mass in a free electron gas solid), along R to Γ direction (for the bulk MAPbBr3).13 Generally though, the generalized gradient approximation (GGA), the HeydScuseria-Ernzerhof (HSE) and GW methodologies correct this overbinding and increases effective mass: a hole effective mass value of approximately -0.3 me was calculated in GGA/HSE.32 In this paper, we present an extensive study of the occupied and unoccupied electronic structure of the hybrid organo-lead trihalide perovskite, MAPbBr3, with an emphasis on the surface of MAPbBr3(001), bringing some clarity to the nature of the surface termination and the controversies surrounding the hole effective mass.


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THE SURFACE BAND STRUCTURE AND THE SURFACE TERMINATION Figure 1b shows the atomic structure of the MAPbBr3 crystal, which has cubic symmetry in the bulk (space group Pm 3 m) and the basis for the theoretical modeling of the electronic structure of MAPbBr3 was performed using density functional theory, the projected augmented wave method,37 and Perdew-Burke-Ernzerhof pseudopotentials,38 as implemented in Vienna ab initio simulation package.39

Figure 1. (a) a MAPbBr3 crystal grown using the antisolvent vapor-triggered crystallization approach.32,34 (b) the atomic structure of the MAPbBr3 crystal.

Figure 2a shows the experimental band structure obtained from angle-resolved photoemission spectroscopy (ARPES) measurements performed at photon energy of 34 eV. It is evident that there is large spectral weight (bright yellow) around the binding energy of –3.0 eV near the Brillouin zone center (the

Γ

point) and below –3.1 eV at the zone edge (kp ∼ 0.6 Å-1). This may

correspond to surface states or a large density of bulk bands. In addition, there is a clear evidence of a band (indicated in Figure 2a with a dashed line as a guide) dispersing towards higher


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energies with increasing kp value. The binding energy of this state changes from about –3.1 eV at kp = 0 (the Γ point) to about –1.8 eV at kp ≈ 0.7 Å-1 (the M point). The absence of spectral density above the top of this band is indicative of the band gap, suggesting that the top of the valence band, or valence band maximum (VBM), lies at binding energy E – EF ≈ –1.8 eV. While the photoelectron mean free path is typical larger than through metal, this is offset by the large Z of lead and bromine, so our expectation is that, at a photon energy of 34 eV, the mean-free-path of photoelectron is in the region of ~5 Å or less.40-42 The comparatively weak band intensity (orange instead of yellow) thus suggests bulk rather than surface character at the top of the valence band. Figures 2b and 2c show the calculated band structure, along the high symmetry line Γ - M of the surface Brillouin zone of the MAPbBr3 (001) slabs, with the MABr (Figure 2b) and PbBr2 (Figure 2c) surface terminations. For both terminations we see dispersive bulk bands (evident from their white color in Figures 2b and 2c) approaching their maxima at the M point. These bands clearly resemble the dispersive band seen in the experimental data (Figure 2a, dashed line). Analysis of the orbital character reveals that these bulk bands are largely composed of the Br-p orbitals with admixture of the Pb-s orbitals.


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a)

c)

b) MABr Surface

PbBr2 Surface

-3

Μ

0.4 -1

Γ Μ

0.4

Γ

k|| (Å ) Figure 2. The electronic band structure of MAPbBr3. (a) The experimental band structure obtained from angle resolved photoemission spectroscopy (ARPES), acquired at a photon energy of 34 eV. (b,c) The calculated band structure of a MAPbBr3(001) surface with MABr surface termination (b) and PbBr2 surface termination (c) along the high symmetry M to Γ direction of the surface Brillouin Zone (SBZ). The color in (b) and (c) indicates contributions from the surface (red) or the bulk (light grey) to various bands. The theoretical band binding energies are shifted to match the energy of the experimental valence band at k = 0.6 Å-1.

Comparison of the band structure for two different surface terminations shows that the two calculated band structures differ by the placement of the surface weighted bands (the degree of the surface contribution is indicated in the red contrast in Figures 2b and 2c). At the M point, the surface bands appear at –2.0 eV, –2.6 eV and –1.9 eV for the PbBr2 termination, whereas they lie at –2.4 eV and –3.2 eV for the MABr termination. For both terminations there is a large


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density of surface and bulk bands in a broad energy range around a binding energy of –3 eV, in the region of the Brillouin zone center, i.e. close to the Γ point, consistent with experiment. The experimental band structure (Figure 3a) also shows a large spectral density around –3.0 eV binding energy near the Brillouin zone center and below –3.1 eV binding energy at the Brillouin

M Γ

X

M

Γ

X

Figure 3. The two-dimensional iso-energy band mapping in k-space. Experiment, from ARPES, is in red (left side of each panel), while the calculated plots are in blue (right side of each panel). The experimental images are acquired at binding energies (E – EF) of (a) –1.57 eV and (b) –1.92 eV. The theoretical plots are obtained by integration over the energies E – EV (a) from 0.0 to – 0.5 eV and (b) –0.7 to –0.9 eV, where EV is the energy of the top of the valence band. The center of the two-dimensional Brillouin zone ( Γ ) is in the center of each image at the dividing line between experiment and theory as indicated.

zone edge. In particular, it is evident that the calculated surface band at the energy –2.0 eV near M

point of the surface Brillouin zone for the PbBr2 termination is absent in the experimental

spectra. The top of the valence band near the M point of the surface Brillouin zone has a weak intensity in ARPES, more indicative of a bulk band but not a band with strong surface weight.


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We find that the placement of the experimental bands agrees better with the calculated band structure of the MABr terminated slab. We would like to note that although the spin-orbit coupling is an important relativistic effect, producing changes in the electronic structure of CH3NH3PbBr3, such as band splitting and band gap reduction,23,26,27,43 its role in the context of this work is minimal. This is evident from the supplementary materials (Figure 2S), where we plotted the calculated band structures of the MABr terminated surface with and without spin-orbit coupling. The results indicate only small changes, on the scale of tenths of an eV, are produced by the spin-orbit coupling. These changes in the band dispersion are not resolved by our ARPES measurements and do affect our conclusion regarding the preferential surface MABr-termination. The agreement between the experimental and calculated band structures is also evident from the two-dimensional mapping of electronic states at certain iso-energy cuts at specific binding energies. Figure 3 shows the photoemission response measured by ARPES through the surface Brillouin zone for the top of the valence band at a binding energy (E – EF) of –1.57 eV (Figure 3a, left panel) and slightly below the top of the valence band at a binding energy (E – EF) of – 1.92 eV (Figure 3b, left panel). These experimental iso-energy cuts are in good agreement with the iso-energy cuts obtained from our density functional calculations (Figures 3a and 3b, right panels) for a MABr terminated surface, as seen in Figure 3. Similar iso-energy cuts, also obtained from angle-resolved photoemission of MAPbBr3(001), have been reported elsewhere29 and indicate placement of the valence band maximum at the M point. The additional support for the MABr termination of MAPbBr3 (001) comes from comparative analysis of the surfaces energetics based on the calculated surface grand potentials for the MABr and PbBr2 terminated surfaces of MAPbBr3 (001).44 By examining the two types of surfaces, we


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find that the MABr-terminated surface is more stable than the PbBr2-terminated surface, under the thermodynamic equilibrium conditions of bulk MAPbBr3. Similar first-principles calculations for methylammonium lead iodide perovskite, MAPbI3, have found that the MAIterminated surface is more stable under equilibrium growth conditions.18 The PbI2 surface reduces its surface energy by incorporating various amount of defects.31,45 As seen in Figure 2, neither the experimental band structure nor theory can be reconciled with the smaller effective hole mass values of -0.12 me reported13,32 suggesting that the somewhat larger absolute valued hole effective masses are more realistic. The generalized gradient approximation (GGA) methodology, used here, results in a hole effective mass in the region of 0.23 me for the bulk and -0.24 me for the slab along the ! (surface Brillouin center) to M point of the surface Brillouin zone. These values for the effective mass are closer to the roughly -0.3 me reported elsewhere for the hole effective mass.32

THE INFLUENCE OF SURFACE STOICHIOMETRY While there is qualitative agreement of partial density of states from density functional theory (Figure 4a) with the combined photoemission and inverse photoemission (Figure 4b), the combined photoemission and inverse photoemission place the Fermi level at the bottom of the conduction band. This suggests a MAPbBr3 (001) surface that is n-type. The band gap of MAPbBr3(001) is in the region of 2.16 eV46 to 2.23 eV47 to 2.3 eV,14,48-56 close to the calculated gap of 1.8 eV14,33 to 2.24 eV,57 and this is consistent with our measured gap of 1.8+0.2 eV (undoped) to 2.4+0.3 eV (1% bismuth doped) from combined photoemission and inverse photoemission spectra. The occupied state large binding energies, seen in the angle-resolved photoemission of Figures 2 and 3, combined with the known band gap, suggest a placement of


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the Fermi level close to the conduction band minimum. Yet MAPbBr3 (001) is generally regarded as p-type. This somewhat unexpected behavior of apparent n-type behavior at the surface can be understood as resulting from a bromine deficient surface. The X-ray photoemission (supplementary materials) indicates that the surface stoichiometry is in fact close to MAPbBr2.4 (001) to MAPbBr2.5 (001), which would in fact drive the chemical potential towards the conduction band minimum,14 as is observed. The fact that the band gap is smaller than expected (1.8+0.3 eV versus 2.3 eV) when derived from the combined photoemission and inverse photoemission spectra for undoped MAPbBr3 (001), is consistent with a bromine deficient or lead rich surface. The bromine vacancies or a lead excess will certainly add midgap states,14,58 resulting in a band gap smaller than expected, but also result in a more n-type material.14 These midgap states created by surface defects have been attributed to enhanced surface recombination.58 Over eight samples, we find the bromine to lead ratio to be about 2.36+0.2, in the near surface region, as determined by X-ray photoemission (supplementary materials). This lower than expected ratio, can be attributed to a surface lead species with a 4f core level binding energy of 136.3+0.2 eV, consistent with the accepted binding energy of metallic lead.59,60 If we substract this minority metallic lead component contribution to the Pb 4f X-ray level photoemission spectra, the bromine to lead ratio increases about 2.98+0.1, in the near surface region. This is close the expected value if the surface were stoichiometric. Obviously, even with a freshly cleaved surface, there is a small excess of lead in the crystal that quickly segregates to the surface or near surface region. One obvious consequence is to make the surface appear n-type.


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Intensity (arb.units) DOS (states/eV)


80

a)

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MABr Surface PbBr2 Surface

40 0

-6

-4

-2

0

2

4

6

b)

-8

-4

EF

4

Binding Energy (eV) Figure 4. The combined density of occupied and unoccupied states. (a) The calculated density of states (DOS) for two possible surface terminations of MAPbBr3: MABr (black line) and PbBr2 (red line). (b) The corresponding experimental results from photoemission, taken at a photon energy of 23.5 eV (filled circles) and inverse photoemission (open circles). The green lines indicate a possible fit of the experimental data. The calculated DOS in (a) is shifted to match the main experimental peak at binding energy –4 eV and the band gap is adjusted to the experimental bulk band gap. Binding energies are given in terms of E – EF.

This lead segregation explains why the surface is readily passivated by oxygen and photoluminescence restored.60 Metallic lead at the surface would enhance the facile oxidization of the surface, as is observed.61


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CONCLUSION In summary, we have investigated methylammonium lead bromide, CH3NH3PbBr3, single crystals using angle resolved photoemission and inverse photoemission spectroscopy. Our results demonstrate the presence of a dispersive valence band which is peaked at the M point of the Brillouin zone. This is consistent with our density functional calculations, which assign this band to the bulk band mostly composed of the Br-p orbitals. Comparison of the experimental and theoretical band structures suggests that the surface termination of the CH3NH3PbBr3 (001) crystal is the methylammonium bromide (CH3NH3Br) layer. This conclusion is in agreement with our theoretical analysis of the surface energetics under thermodynamic equilibrium conditions of bulk CH3NH3PbBr3. The surface is found to be n-type, likely as a result of Br deficiency on the surface.

EXPERIMENTAL The methylammonium lead bromide perovskite, MAPbBr3 or MAPbBr3, samples were created using an antisolvent vapor-triggered crystallization approach33 from the two precursors MABr and PbBr2 using N,N-dimethylformamide (DMF) as the solvent and dichloromethane (DCM) volatile antisolvent to force MAPbBr3 out of solution as larger crystals. The MAPbBr3 singlecrystals (Figure 1a) are in the shape of square wafers (between ~1 mm x 1 mm x 0.2 mm and ~8 mm x 8 mm x 2 mm) and large enough to be suitable for angle-resolved photoemission and inverse photoemission measurements. The experimental electronic structure measurements were performed in several systems. The angle-resolved photoemission spectroscopy (ARPES), exploited p-polarized light dispersed by the linear undulator beamline (BL-1) at the Hiroshima Synchrotron Radiation Center


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(HiSOR) in Hiroshima University, Japan.62 The placement of the Fermi level, EF, was determined from the polycrystalline Au film signal, in electronic contact with MAPbBr3. Inverse photoemission (IPES) spectra were obtained using incident electrons, with varying kinetic energy, while detecting the emitted photons at a fixed energy (9.7 eV) using a channeltron detector. The inverse photoemission spectroscopy resolution was limited by an instrumental linewidth of approximately 400 meV. The material stoichiometry, in the region of the surface, was established by X-ray photoemission using a PHI Versa Probe II instrument equipped with monochromatic Al K(alpha) source, and with the PHI MultiPack v9.0 and/or CasaXPS v.2.3.14 software for data analysis. The X-ray power of 65 W at 15 kV was used for all experiments with 260 μm beam size at the X-ray incidence and take off angles of 45o. All the spectra were collected at room temperature. Throughout, all photoemission and inverse photoemission binding energies are denoted in terms of E-EF, making the occupied state binding energies negative. Each experiment, either angle resolved photoemission or the X-ray photoemission, was taken from a freshly cleaved sample, in ultrahigh vacuum (10-10 mbar).

THEORETICAL METHODOLOGY In order to investigate the surface effects, first-principles calculations were carried out using projected augmented wave method (PAW),38 as implemented in the Vienna ab initio simulation package (VASP).39 We constructed symmetric MAPbBr3 slabs with a stacking sequence along the [001] direction. Along this direction the MAPbBr3 crystal consists of alternating chargeneutral MABr and PbBr2 monolayers, allowing for two possible terminations of the slab: MABr termination or PbBr2 termination. The computational slabs contained 7 unit cells (6 unit cells) for MABr termination (PbBr2 termination), as shown in the supplementary materials (Figure S1),


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which were used to obtain the converged electronic structure for each termination. Periodic boundary conditions perpendicular to the plane were maintained by placing 10 Å vacuum regions separating the periodically repeated slabs to minimize the interaction between them. The orientation of the CH3NH3 molecules was assumed to be along the [111] direction with alternating orientation of the dipole moments in the consecutive monolayers in the [001] stacking direction. In the calculations the in-plane lattice parameters were kept fixed, while the atomic positions were relaxed until the forces on each atom was less than |0.02| eV/Å. We used the kinetic energy cutoff of 660 eV and 8×8×1 Monkhorst-Pack k-point mesh for Brillouin zone integration. In the calculations the spin-orbit interaction was ignored. In the calculated band structure, we treat the top and bottom unit cells of the CH3NH3PbBr3 slabs (shown in Figure 1S, of the supplementary materials) as having surface character (colored in red in Figure 2) and the contribution from the rest part of the slab as retaining bulk character (colored in light grey in Figure 2).

SUPPORTING INFORMATION AVAILABLE The supporting information has the two atomic structures of CH3NH3PbBr3 with both the CH3NH3Br (Figure S1(a)) and (b) PbBr2 (Figure S1(b)) terminations, used in the first-principles calculations, in addition to considerable surface structural data (Table S1 and Table S2). Also plotted are the calculated band structures of the MABr terminated surface with and without spinorbit coupling (Figure S2). The Pb 4f (Figure S3) and Br 3d (Figure S4) core level X-ray photoemission spectra (XPS), used here, is also plotted in the supporting information, along with Br/Pb relative concentrations with the metallic Pb component (Table S4), and without the metallic Pb component (Table S3) included.


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AUTHOR INFORMATION The authors declare no competing financial interests.

Acknowledgments This work was supported by the National Science Foundation through the Nebraska MRSEC (Grant No. DMR-1420645) and the Nebraska Center for Energy Science Research, and KAUST. Facility use at HiSOR based on HiSOR proposal number 15-A-18. E.F.S acknowledges financial support from the JSPS postdoctoral fellowship for overseas researchers as well as the Alexander von Humboldt Foundation (Grant No. P13783). X.H. was supported by the China Scholarship Council.

REFERENCES (1) Sum, T. C.; Nripan Mathews, N. Advancements In Perovskite Solar Cells: Photophysics Behind the Photovoltaics. Energy Environ. Sci. 2014, 7, 2518-2534. (2) Gao, P.; Grätzel, M.; Nazeeruddin, M. K. Organohalide Lead Perovskites for Photovoltaic Applications. Energy Environ. Sci. 2014, 7, 2448-2463. (3) Xing, G. C.; Mathews, N.; Sun, S. Y.; Lim, S. S.; Lam, Y. M.; Grätzel, M.; Mhaisalkar, S.; Sum, T. C. Long-Range Balanced Electron- and Hole-Transport Lengths in OrganicInorganic MAPbI3. Science 2013, 342, 344–347. (4) Stranks, S. D.; Eperon, G. E.; Grancini, G.; Menelaou, C.; Alcocer, M. J. P.; Leijtens, T.; Herz, L. M.; Petrozza, A.; Snaith, H. J. Electron-Hole Diffusion Lengths Exceeding 1 Micrometer in an Organometal Trihalide Perovskite Absorber. Science 2013, 342, 341– 344.


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TOC GRAPHIC


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Surface Electronic Structure of Hybrid Organo Lead Bromide

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