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...

Gedopte Moleküle: Das Standardmodell für die molekulare Dotierung organischer Halbleiter (OSCs) basiert auf ganzzahligem Ladungstransfer zwischen OSC und Dotand. Ein alternatives Modell geht von der Bildung eines Komplexes aus OSC und Dotand aus. Experimente unter systematischer Variation der Akzeptorstärke sprechen klar für letzteres Modell, was zum gezielten chemischen Design effizienterer molekularer Dotanden beitragen könnte.

We exploited the thermal annealing of poly­(3-hexylthiophene) (P3HT) molecularly p-doped with the strong electron acceptor 2,3,5,6-tetrafluoro-7,7,8,8-tetra­cyanoquino­dimethane (F4TCNQ) as a tool for tuning the doping concentration as a quasi singular parameter. Via directed dopant desorption, we could unravel the complex microstructure of this semicrystalline system, leading to a detailed growth model solely based on complementary experimental evidence from scattering and spectroscopic techniques. We find the crystalline portion of p-doped P3HT to comprise regions, where dopant anions pack with the polymer chains in a metastable, cocrystalline structure with additional ionized dopants dispersed in the alkyl chain region of P3HT. Simultaneously, regions exist where the pristine polymer backbones closely pack. The dedoping via dopant desorption through thermal annealing reveals the dopants within the mixed crystalline phase to be thermally least stable. Notably, their initial desorption does not alter the thin film conductivity, which indicates this phase to be not crucial for charge transport. Upon further dopant desorption, the pristine P3HT backbone phase prevails with dopant anions remaining still dispersed in the alkyl chain region of the film. During the entire dedoping, we did not observe indications for the presence of neutral F4TCNQ. Only upon completing the dedoping at 120 °C are both the conductivity and the microstructure of pristine P3HT recovered. We demonstrate that the temperature-induced dedoping provides valuable information on the microstructure of doped organic semiconductors, which remains inaccessible otherwise because of the intrinsic structural and energetic complexity of such systems.

Halogenation of conjugated molecules represents a powerful approach to tune the electronic structure of molecular thin-films through inductive effects and long-range intermolecular electrostatic interactions. The mixing of halogenated molecules with their pristine counterparts has recently proven successful in altering the blend's energy levels to adjust the open-circuit voltage of organic solar cells by the mixing ratio. Here, we show that the prevailing rationale for this effect is not equally valid for different molecular orientations. We provide a comprehensive experimental and theoretical analysis of the prototypical blend formed by pentacene and perfluoropentacene to relate structure with electronic properties. We find a mixed-stack structural motif in standing and lying orientation depending on the substrate nature. In standing orientation, the ionization potential lies in between the values of the pure components, in line with the established picture of averaged molecular quadrupole moments. For the lying orientation, however, we experimentally observe an ionization potential lower than both pristine values, which seems at odds with this simple rationale. Electrostatic simulations based on the knowledge of the atomistic structure of the films capture the complex experimental scenario for both orientations. In particular, the ultra-low ionization potential of films formed by lying molecules is identified as a signature of the monolayer structure, where quadrupolar interactions are responsible for a difference of ca. 0.4 eV in the highest occupied molecular orbital energy as compared to thicker films with the same molecular orientation.

Paper published as part of the special topic on Singlet Fission Note: This paper is part of the JCP Special Collection on Singlet Fission.