devices. Due to substantial research efforts, the power conversion efficiency of perovskite-based PV cells has been pushed beyond 20%. [3] The most widely studied materials are the triiodide CH 3 NH 3 PbI 3 and mixed halide CH 3 NH 3 PbI 3−x Cl x perovskites, which can be either solution processed [4] or thermally evaporated. [5] In particular, the mixed halide perovskites have been widely investigated due to their long carrier high diffusion length (over 1 µm [6] ) and low density of trap states, [7] making them especially efficient for charge generation and collection. Though substantial achievements for solar cells using solution processed CH 3 NH 3 PbI 3−x Cl x thin films have been reported, large variations in efficiency still persist among the devices even for nominally identical device configurations, [8,9] indicating significant differences in perovskite film properties. This is certainly to be related to the different preparation methods, e.g., one-step or twostep, [10] solvent treatment, [4] as well as the use of chlorine containing precursors, [11] which can greatly influence the film morphology and charge transport properties.Despite the rapid improvement in device performance, most of the fundamental questions regarding the physico-chemical properties of these materials remain to be unequivocally answered. In this context, the determination of the electronic structure and interface energetics with electron and hole transport layers, as well as contacts are crucial prerequisites to further device optimization.Photoelectron spectroscopy (PES) is the experimental technique of choice to investigate the core level and valence electronic structure of semiconductors. Particularly relevant parameters are the work function (φ), the ionization energy (IE), and the energy difference between the valence band maximum (VBM) and the Fermi level (E VBM − E f , where E f = 0 eV). To date, ultraviolet photoelectron spectroscopy (UPS) studies reporting the valence electronic properties of CH 3 NH 3 PbI 3 and CH 3 NH 3 PbI 3−x Cl x thin films have shown a wide range of VBM binding energies. [12][13][14][15][16] In most UPS studies, E f was reported very close to (or even within) the conduction band edge, thus suggesting n-type character. The transport gap of these two perovskites was determined by Schulz et al. via combining UPS This study investigates the effect of white light illumination on the electronic and chemical properties of mixed halide perovskite (CH 3 NH 3 PbI 3−x Cl x ) thin films and CH 3 NH 3 PbI 3 single crystals using photoelectron and absorption spectroscopy. The pristine materials' surfaces are found to be n-type because of surface band bending due to the presence of donor levels, likely consisting of reduced lead (Pb 0 ) that acts as surface traps. When illuminating the sample with white light (up to 1 sun), the valence features shifted to lower binding energy due to surface photovoltage, i.e., the bulk of the materials is much less n-type. However, the surface photovoltage is only partially revers...

The formation mechanism of perovskite methylammonium lead triiodide (CH3NH3PbI3) was studied with in situ X-ray photoelectron spectroscopy (XPS) on successive depositions of thermally evaporated methylammonium iodide (CH3NH3I) on a lead iodide (PbI2) film. This deposition method mimics the “two-step” synthesis method commonly used in device fabrication. We find that several competing processes occur during the formation of perovskite CH3NH3PbI3. Our most important finding is that during vapour deposition of CH3NH3I onto PbI2, at least two carbon species are present in the resulting material, while only one nitrogen species is present. This suggests that CH3NH3I can dissociate during the transition to a perovskite phase, and some of the resulting molecules can be incorporated into the perovskite. The effect of partial CH3NH3 substitution with CH3 was evaluated, and electronic structure calculations show that CH3 defects would impact the photovoltaic performance in perovskite solar cells. The possibility that not all A sites in the APbI3 perovskite are occupied by CH3NH3 is therefore an important consideration when evaluating the performance of organometallic trihalide solar cells synthesized using typical approaches.

We measured the electronic and the molecular surface structure of pentacene deposited on the (1 1 1)-surfaces of coinage metals by means of ultraviolet photoelectron spectroscopy (UPS) and low-energy electron diffraction (LEED). Pentacene is almost flat-lying in monolayers on all three substrates and highly ordered on Au(1 1 1) and on Cu(1 1 1). On Ag(1 1 1), however, weak chemisorption leads to almost disordered monolayers, both, at room temperature and at 78 K. On Cu(1 1 1) pentacene is strongly chemisorbed and the lowest unoccupied molecular orbital becomes observable in UPS by a charge transfer from the substrate. On Ag(1 1 1) and Cu(1 1 1) multilayers adopt a tilted orientation and a high degree of crystallinity. On Au(1 1 1), most likely, also in multilayers the molecular short and long axes are parallel to the substrate, leading to a distinctively different electronic structure than on Ag(1 1 1) and Cu(1 1 1). Overall, it could be demonstrated that the substrate not only determines the geometric and electronic characteristics of molecular monolayer films but also plays a crucial role for multilayer film growth.

Photovoltaic cells based on halide perovskites and possessing remarkably high power conversion efficiencies have been reported. To push the development of such devices further, a comprehensive and reliable understanding of their electronic properties is essential, but presently not available. To provide a solid foundation for understanding the electronic properties of polycrystalline thin films, we employ single crystal band structure data from angle-resolved photoemission measurements. For two prototypical perovskites (CH3NH3PbBr3 and CH3NH3PbI3) we reveal the band dispersion in two high symmetry directions, and identify the global valence band maxima. With these benchmark data, we construct "standard" photoemission spectra from polycrystalline thin film samples and resolve challenges discussed in the literature of determining the valence band onset with high reliability. Within the framework laid out here, the consistency of relating the energy level alignment in perovskite-based photovoltaic and optoelectronic devices with their functional parameters is substantially enhanced.