Electronic Structure of CH 3 NH 3 PbX 3 Perovskites: Dependence on the Halide Moiety Rebecka Lindblad,* ,† Naresh K. Jena, ‡ Bertrand Philippe, † Johan Oscarsson, † Dongqin Bi, § Andreas Lindblad, ∥ Suman Mandal, ⊥ Banabir Pal, ⊥ D. D. Sarma, ⊥ Olof Karis, † Hans Siegbahn, † Erik M. J. Johansson, § Michael Odelius, ‡ and Ha ̊ kan Rensmo* ,† † Department of Physics and Astronomy, Molecular and Condensed Matter Physics, Uppsala University, Box 516, 751 20 Uppsala, Sweden ‡ Department of Physics, AlbaNova University Center, Stockholm University, 106 91 Stockholm, Sweden § Department of Chemistry, Ångströ m, Uppsala University, Box 523, 751 20 Uppsala, Sweden ∥ Department of Chemistry, Ångströ m, Inorganic Chemistry, Uppsala University, Box 538, 751 21 Uppsala, Sweden ⊥ Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560012, India ABSTRACT: A combination of measurements using photoelectron spectroscopy and calculations using density functional theory (DFT) was applied to compare the detailed electronic structure of the organolead halide perovskites CH 3 NH 3 PbI 3 and CH 3 NH 3 PbBr 3 . These perovskite materials are used to absorb light in mesoscopic and planar heterojunction solar cells. The Pb 4f core level is investigated to get insight into the chemistry of the two materials. Valence level measurments are also included showing a shift of the valence band edges where there is a higher binding energy of the edge for the CH 3 NH 3 PbBr 3 perovskite. These changes are supported by the theoretical calculations which indicate that the differences in electronic structure are mainly caused by the nature of the halide ion rather than structural differences. The combination of photoelectron spectroscopy measurements and electronic structure calculations is essential to disentangle how the valence band edge in organolead halide perovskites is governed by the intrinsic difference in energy levels of the halide ions from the influence of chemical bonding. ■ INTRODUCTION The recent advancement of perovskites as photovoltaic material has given a new dimension to the conventional solar cell research paradigm. Organolead halide perovskites, in particular, have recently drawn much attention as light absorbers in mesoscopic solar cells. 1−3 In mesoscopic devices, the perovskite is incoorporated into a mesoporous network of a large bandgap semiconductor or insulator. 4−7 Because of the favorable light absorbing properties together with high electron and hole conduction, the perovskite material also works as a thin film absorber in layered structures. 8−11 The general chemical formula of methylammonium halide perovskites is CH 3 NH 3 PbX 3 , where X can be different combinations of I, Br, and Cl. In this study we focus on perovskites including the halide I and Br as in CH 3 NH 3 PbI 3 and CH 3 NH 3 PbBr 3 . Because of its smaller bandgap, CH 3 NH 3 PbI 3 gives a broader light absorption in the visible region compared to CH 3 NH 3 PbBr 3 . CH 3 NH 3 PbBr 3 has, on the other hand, shown the potential to give solar cells with a higher open circuit voltage (V oc ), 12 and nanoparticles of this material show high stability. 13 The function of the solar cells is to a large extent dependent on the energy level alignment between the different materials in the device and also on their electronic properties. Photo- electron spectroscopy is a method to directly probe the occupied electronic levels in a material, and the experimental results are naturally related to calculations of the electronic structure allowing for a decomposition into element-specific contributions. 8,14 In the literature, there are several inves- tigations in which density functional theory (DFT) has been used to calculate the electronic band structure of various perovskites. 15−17 In this study, we exploit the combination of photoelectron spectroscopy measurements using high excita- tion energies (hard X-ray photoelectron spectroscopy, HAXPES) and density functional calculations as a source of information to gain a detailed understanding of the electronic structure of organolead halide perovskites. A recent exper- imental study of the outermost energy levels in perovskite materials on TiO 2 using UV photoelectron spectroscopy (UPS) reveals a valence band edge of CH 3 NH 3 PbBr 3 at a higher binding energy compared to CH 3 NH 3 PbI 3 . 18 In the present study, we use a complementary experimental approach using HAXPES to investigate the electronic structure of these two materials and explain the observed differences using DFT calculations. Received: September 18, 2014 Revised: December 17, 2014 Published: December 18, 2014 Article pubs.acs.org/JPCC © 2014 American Chemical Society 1818 DOI: 10.1021/jp509460h J. Phys. Chem. C 2015, 119, 1818−1825