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We applied photoemission tomography (PT) to a unique one-dimensional row structure of a picene multilayer realized on an anisotropic Ag(110) surface. Taking advantage of the simplified structure of the multilayer film, we successfully deconvoluted the photoelectron momentum maps of three frontier orbitals of picene. Thereafter, the clearly deconvoluted experimental momentum maps were compared to the Fourier transform simulation of the molecular orbitals of picene in detail, enabling not only the evaluation of the electronic structure of the picene in the multilayer but also the determination of the molecular orientation in the multilayer within a few degrees. In addition, the PT results indicated the orientation of the molecules in all layers to be flat-lying. The successful demonstration of PT of the multilayer molecular film marks an important step toward the wide-range utilization of the PT technique.

Substituting carbon with silicon in organic molecules and materials has long been an attractive way to modify their electronic structure and properties. Silicon-doped graphene-based materials are known to exhibit exotic properties, yet conjugated organic materials with atomically precise Si substitution have remained difficult to prepare. Here we present the on-surface synthesis of one- and two-dimensional covalent organic frameworks whose backbones contain 1,4-disilabenzene (C4Si2) linkers. Silicon atoms were first deposited on a Au(111) surface, forming a AuSix film on annealing. The subsequent deposition and annealing of a bromo-substituted polyaromatic hydrocarbon precursor (triphenylene or pyrene) on this surface led to the formation of the C4Si2-bridged networks, which were characterized by a combination of high-resolution scanning tunnelling microscopy and photoelectron spectroscopy supported by density functional theory calculations. Each Si in a hexagonal C4Si2 ring was found to be covalently linked to one terminal Br atom. For the linear structure obtained with the pyrene-based precursor, the C4Si2 rings were converted into C4Si pentagonal siloles by further annealing.

[Abstract] Energy levels in an organic semiconductor are mainly determined by the molecular orbital energies of the constituent molecules. Recent studies, however, have revealed that the energy levels can be altered as much as 1 eV by the molecular orientation in the film or the molecular mixing ratio in the binary film, owing to the intermolecular electrostatic interaction. Because of the long-range nature of Coulomb interaction, theory predicts that the electrostatic energy should depend on the sample shape. In this article, we examine the coverage- dependent energy levels of zinc phthalocyanine and per-fluorinated zinc phthalocyanine in the submonolayer region with ultraviolet photoelectron spectroscopy (UPS) and low-energy inverse photoelectron spectroscopy (LEIPS). Using the procedure we reported previously, we separately evaluated the electronic polarization energy and electrostatic energy as a function of coverage. Unlike the electronic polarization, which contributes only as much as 10 meV, the electrostatic energy contributes as much as 120 meV to the coverage-dependent energy shift. We conclude that the shift in energy levels by changing the coverage is attributed to the sample shape-dependent energy level, owing to the long-range nature of the charge–permanent quadrupole interaction.

The conduction band dispersion in methylammonium lead iodide (CH3NH3PbI3) was studied by both angle-resolved two-photon photoelectron spectroscopy (AR-2PPE) with low photon intensity (∼0.0125 nJ/pulse) and angle-resolved low-energy inverse photoelectron spectroscopy (AR-LEIPS). Clear energy dispersion of the conduction band along the Γ-M direction was first observed by these independent methods under different temperatures, and the dispersion was found to be consistent with band calculation under the cubic phase. The effective mass of the electrons at the Γ point was estimated to be (0.20 ± 0.05)m0 at the temperature of 90 K. The observed conduction band energy was different between the AR-LEIPS and AR-2PPE, which was ascribed to the electronic-correlation-dependent difference of initial and final states probing processes. The present results also indicate that the surface structure in CH3NH3PbI3 provides the cubic-dominated electronic property even at lower temperatures.

Dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]thiophene (DNTT) is a p-type organic semiconductor that exhibits high charge-carrier mobility and atmospheric stability. Although it has been proposed that the ...

Electronic devices based on organic semiconductors are a fast-growing field of technology. A detailed understanding of the interplay between optical and electronic properties of organic molecular t...

Modern opto-electronics technologies are built on the basis of fundamental knowledge about electronic properties of semiconductor materials, which can be attained through accurate analyses made on single crystals of the materials as standard samples. In the cases of organic semiconductor materials, although technical difficulties have obstructed direct measurements of electronic properties in the organic semiconductor single crystals by photoelectron spectroscopy, recent advances in the experimental methodologies have been opening a route for accessing their electronic bands, defect-induced “gap states”, and core levels by using single crystals of the organic semiconductors at higher accuracy and precision. In this review, we summarize recent achievements for the exploration into the electronic structures on organic semiconductor single crystals. The energy-momentum dispersion relations of holes obtained by the angle-resolved ultraviolet photoelectron spectroscopy (ARUPS) measurements are mainly focused, which allow one to deduce one of the most essential properties dominating the charge carrier transport, that is, the effective mass of a quasi-particle, namely dressed charge, and impact of electron–phonon coupling to the band structure. We describe theoretical bases of the ARUPS methodology and peculiar know-hows for adopting this technique to the molecular single crystal samples, which prone to be charged up upon photoelectron emission because they are good electrical insulator.

UVSOR is a Japanese low-energy synchrotron light source that has been operational since 1983. It has 750 MeV electron beam energy and a circumference of 53 m. The machine has undergone two major upgrades and is now called UVSOR-III, with a moderately small emittance of 17 nm·rad and is operated in the top-up mode at 300 mA. It has six undulators. Three in-vacuum undulators provide high-brightness VUV-SX light to the beamlines BL3U, BL4U, and BL6U. BL3U and BL4U are actively used for chemistry and life sciences using in situ and operando measurements, such as soft X-ray absorption and photoelectron spectroscopy. Since 2020, a unique photoelectron analyzer called the Momentum Microscope has been in operation in BL6U. Two variable polarization undulators provide high-brightness VUV light to the beamlines, BL5U and BL7U, which are used for solid-state physics through angle- and spin-resolved photoelectron spectroscopy. BL1U is equipped with two APPLE-II undulators in tandem and is used for various research purposes, including laser Compton scattering gamma rays. In addition to these beamlines, eight beamlines are operational which use bending magnet radiation, ranging from THz to tender X-rays. In 38 years of operation, we have dealt with and properly resolved machine problems, such as water leakage from magnet coils and cooling water flow paths in ultrahigh vacuum. Considering the machine’s declining competitiveness as a light source due to its aging and emerging demands in the low-energy area, we have started a design study on the future plan, as well as discussing it closely with the user community.

Crown-ether (CR) ring molecules are known as host molecules for capturing guest species inside the ring. So far CR molecular films have only been grown by drop-casting a CR solution on an inert sub...

Strong intermolecular electronic coupling and well-ordered molecular arrangements enable efficient transport of both charge carriers and excitons in semiconducting π-conjugated molecular solids. Thus, molecular heteroepitaxy to form crystallized donor–acceptor molecular interfaces potentially leads to a novel strategy for creating efficient organic optoelectronic devices via the concomitance of these two requirements. In the present study, the crystallographic and electronic structures of a heteroepitaxial molecular interface, perfluoropentacene (PFP, C22F14) grown on pentacene single crystals (Pn-SCs, C22H14), were determined by means of grazing-incidence X-ray diffraction (GIXD) and angle-resolved ultraviolet photoelectron spectroscopy (ARUPS), respectively. GIXD revealed that PFP uniquely aligned its primary axis along the [110] axis of crystalline pentacene to form well-crystallized overlayers. Valence band dispersion (at least 0.49 eV wide) was successfully resolved by ARUPS. This indicated a signif...

Interpreting experimental spectra of thin films of organic semiconductors is challenging, and understanding the relationship between experimental data obtained by different spectroscopic techniques requires a careful consideration of the initial and final states for each process. The discussion of spectroscopic data is frequently mired in confusion that originates in overlapping terminology with however distinct meaning in different spectroscopies. Here, we present a coherent framework that is capable of treating on equal footing most spectroscopies commonly used to investigate thin films of organic semiconductors. We develop a simple model for the expected energy level positions, as obtained by common spectroscopic techniques, and relate them to the energies of molecular states. Molecular charging energies in photoionization processes, as well as adsorption energies and the screening of molecular charges due to environmental polarization, are taken into account as the main causes for shifts of the measured spectroscopic features. We explain the relationship between these quantities, as well as with the transport gap, the optical gap and the exciton binding energy. Our considerations serve as a model for weakly interacting systems, e.g., various organic molecular crystals, where wave function hybridizations between adjacent molecules are negligible.

The conduction band dispersion in methylammonium lead iodide (CH

The advantages of methylammonium triiodideplumbate (CH

We report on the synthesis of CsPbBr3 perovskite quantum dots (PeQDs) with a high solubility of 75 g/L in toluene and a good film-forming property, as enabled by a dense layer of didodecyldimethylammonium bromide and octanoic acid surface ligands. The crystalline and monodisperse PeQDs feature a cubic-like shape, with an edge length of 10.1 nm, and a high photoluminescence quantum yield of greater than 90% in toluene solution and 36% as a thin film. We find that the PeQDs are n-type doped following the synthesis but also that they can be p-type and additionally n-type doped by in situ electrochemistry. These combined properties render the PeQDs interesting for the emitter in solution-processed light-emitting electrochemical cells (LECs), and we report a PeQD-LEC with air-stabile electrodes that emits with a narrow emission spectrum (lambda(peak) = 514 nm, full width at half-maximum = 24 nm) and a luminance of 250 cd/m(2) at 4 V and a luminance of 1090 cd/m(2) at 6.8 V. To reach this performance, it was critical to include a thin solution-processed layer comprising p-type poly(vinyl carbazole) and a tetrahexylammonium tetrafluoroborate ionic liquid between the PeQD emission layer and the anode in order to compensate for the as-synthesized n-type doping of the PeQDs. Funding Agencies|Stiftelsen J. Gust. Richert; Stiftelsen J.C. Kempes Minnes Stipendiefond; Swedish Energy AgencySwedish Energy Agency [45419-1, 46523-1, 50779-1]; Swedish Research CouncilSwedish Research CouncilEuropean Commission [2017-04380, 2017-04862, 2018-03937, 2019-02345]; Swedish Foundation for Strategic ResearchSwedish Foundation for Strategic Research; Stiftelsen Olle Engkvist ByggmastareSwedish Research Council [1860637, 193-0578]; Bertil & Britt Svenssons stiftelse for belysningsteknik; Swedish Foundation for International Cooperation in Research and Higher Education via an Initiation Grant for Internationalization; JSPS KAKENHIMinistry of Education, Culture, Sports, Science and Technology, Japan (MEXT)Japan Society for the Promotion of ScienceGrants-in-Aid for Scientific Research (KAKENHI) [JP18H03904]

Organic halide perovskites have attracted much attention due to their potential applications in optoelectronic devices. Since the generally higher flexibility compared to their inorganic counterparts, their structures are prone to be more sensitive toward external effects, where the fundamental understanding of their band structure evolutions is still inconclusive. In this study, different electronic structure evolutions of perovskite single crystals are found via angle-resolved photoelectron spectroscopy: (i) Unchanged top valence band (VB) dispersions under different temperatures can be found in the $\mathrm{C}{\mathrm{H}}_{3}\mathrm{N}{\mathrm{H}}_{3}\mathrm{Pb}{\mathrm{I}}_{3}$. (ii) Phase transitions induced the evolution of top VB dispersions, and even a top VB splitting with Rashba effects can be observed in the $\mathrm{C}{\mathrm{H}}_{3}\mathrm{N}{\mathrm{H}}_{3}\mathrm{Pb}{\mathrm{Br}}_{3}$. Combined with low-energy electron diffraction, metastable atom electron spectroscopy, and density functional theory calculation, we confirm that different band structure evolutions observed in these two perovskite single crystals originated from the cleaved top surface layers, where the different surface geometries with $\mathrm{C}{\mathrm{H}}_{3}\mathrm{N}{{\mathrm{H}}_{3}}^{+}\text{\ensuremath{-}}\mathrm{I}$ in $\mathrm{C}{\mathrm{H}}_{3}\mathrm{N}{\mathrm{H}}_{3}\mathrm{Pb}{\mathrm{I}}_{3}$ and Pb-Br in $\mathrm{C}{\mathrm{H}}_{3}\mathrm{N}{\mathrm{H}}_{3}\mathrm{Pb}{\mathrm{Br}}_{3}$ are responsible for finding band dispersion change and appearance of the Rashba-type splitting. Such findings suggest that the top surface layer in organic halide perovskites should be carefully considered to create functional interfaces for developing perovskite devices.

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 orientations depending on the substrate nature. In the 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 ultralow 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.

Electronic structures of single crystal pentacene are of great interest for the elucidation of charge carrier transport in organic semiconductor materials. Experimental observation of valence band dispersion was recently achieved on single crystal samples of pentacene; however, its intrinsic properties are still unresolved because past experiments were performed on specimens with surface oxides formed by exposure to the ambient atmosphere. In this work, X-ray photoelectron spectroscopy (XPS) and angle-resolved ultraviolet photoelectron spectroscopy (ARUPS) were conducted on single-crystal pentacene samples prepared without ambient exposure. The XPS results confirmed the reduction of the abundance of oxide impurities on the present samples. The ARUPS measurements clearly resolved the valence band structures of the single-crystal pentacene in four symmetry directions of the surface Brillouin zone, indicating anisotropy of at least a factor of 2.4 for the intermolecular transfer integral and hole effective mass at the valence band maximum.

The structural and photophysical properties of tetradentate Pt(ppzOppz), Pt(ppzOpopy), Pt(ppzOczpy), and Pt(czpyOczpy) have been experimentally and theoretically explored. Single-crystal diffraction measurements provided accurate structural information. Electrochemical and photophysical characterizations revealed internal electronic energy levels in ground and excited states. (Time-dependent) Density functional theory calculation revealed electron distributions in transition processes of S0 → S1 and S1 → T1 → S0. Electronic transition study indicated that Pt(ppzOppz) demonstrated mixed MLCT/LC states and Pt(czpyOczpy) showed MLCT-dominated states in S1 and T1. Both Pt(ppzOpopy) and Pt(ppzOczpy) presented strong delocalized spin transition (DST) during intersystem crossing. Upon frame modification of Pt(ppzOczpy), we found that their S1 and T1 can be independently manipulated. These blue emitters showed a tunable and narrow emission band (the narrowest fwhm was 19 nm) with luminescence efficiency as high a...

Doping plays a crucial role in semiconductor physics, with n-doping being controlled by the ionization energy of the impurity relative to the conduction band edge. In organic semiconductors, efficient doping is dominated by various effects that are currently not well understood. Here, we simulate and experimentally measure, with direct and inverse photoemission spectroscopy, the density of states and the Fermi level position of the prototypical materials C60 and zinc phthalocyanine n-doped with highly efficient benzimidazoline radicals (2-Cyc-DMBI). We study the role of doping-induced gap states, and, in particular, of the difference Δ1 between the electron affinity of the undoped material and the ionization potential of its doped counterpart. We show that this parameter is critical for the generation of free carriers and influences the conductivity of the doped films. Tuning of Δ1 may provide alternative strategies to optimize the electronic properties of organic semiconductors.

[ABSTRACT] In organic semiconductors, the hole and electron transport occurs through the intermolecular overlaps of highest occupied molecular orbitals (HOMO) and lowest unoccupied molecular orbitals (LUMO), respectively. A measure of such intermolecular electronic coupling is the transfer integral, which can experimentally be observed as energy level splittings or the width of the respective energy bands. Quantum chemistry textbooks describe how an energy level splits into two levels in molecular dimers, into three levels in trimers and evolves into an energy band in infinite systems, a process that has never been observed for the LUMO or beyond dimers for the HOMO. In this work, our new technique, low-energy inverse photoelectron spectroscopy, was applied to observe the subtle change of the spectral line shape of a LUMO-derived feature while we used ultraviolet photoelectron spectroscopy to investigate the occupied states. We show at first that tin-phthalocyanine molecules grow layer-by-layer in quasi one-dimensional stacks on graphite, and then discuss a characteristic and systematic broadening of the spectral line shapes of both HOMO and LUMO. The results are interpreted as energy-level splittings due to the intermolecular electronic couplings. Based on the Hückel approximation, we determined the transfer integrals for HOMO-1, HOMO, and LUMO to be ≤15 meV, (100 ± 10) meV, and (128 ± 10) meV, respectively.

The advantages of methylammonium triiodideplumbate (CH3NH3PbI3)-based organic-inorganic hybrid halide perovskite have led to devices with power conversion efficiencies of >20%. The CH3NH3PbI3structure is prone to be more sensitive towards external effects due to its higher flexibility than inorganic counterparts. Nevertheless, a direct photoemission spectroscopy study is still lacking on the density of gap states (DOGS) influenced by air exposure and synchrotron light-induced degradation. In this paper, we investigate the evolution of electronic structure in CH3NH3PbI3single crystals after air exposure and intense synchrotron light irradiation to reveal the effects on its density of states distribution below and above the valence band maximum (VBM) by using ultrahigh-sensitivity photoelectron spectroscopy. We find that the PbI2compounds, decomposed from CH3NH3PbI3after air exposure, could not affect the DOGS distribution but only give the VBM shift in the high binding energy region, which is dramatically different from the impacts of an impurity found for other organic or inorganic counterparts. A further study using intense synchrotron irradiation confirms the decomposed processes for CH3NH3PbI3: (i) the initial degradation would induce the formation of PbI2, which gives a negligible impact on the DOGS above the VBM; (ii) the continuous intense light irradiation could further degrade PbI2to metallic Pb, in which DOGS appears in the energy bandgap.

Electronic charge rearrangement between components of a heterostructure is the fundamental principle to reach the electronic ground state. It is acknowledged that the density of states distribution of the components governs the amount of charge transfer, but a notable dependence on temperature has not yet been considered, particularly for weakly interacting systems. Here, we experimentally observe that the amount of ground state charge transfer in a van der Waals heterostructure formed by monolayer MoS2 sandwiched between graphite and a molecular electron acceptor layer increases by a factor of three when going from 7 K to room temperature. State-of-the-art electronic structure calculations of the full heterostructure that account for nuclear thermal fluctuations reveal intra-component electron-phonon coupling and inter-component electronic coupling as the key factors determining the amount of charge transfer. This conclusion is rationalized by a model applicable to multi-component van der Waals heterostructures.

A many-body photoemission theory based on Keldysh nonequilibrium Green's functions is developed and applied to photoemission from extended states of organic molecules and their crystals, in particular excited by ultraviolet and soft X-ray radiation. In this energy region, electron-phonon interaction and radiation filed screening play some important roles. To incorporate the latter effects, we should use quantum electrodynamics (QED) approach. To discuss the electron-phonon interaction, we apply the Hedin equation including the screened Coulomb interaction mediated by the electron-phonon interaction. This theoretical framework gives us a unified picture of the UPS spectra from organic solids. Furthermore some illustrative numerical calculations are shown to discuss the relative importance of the multiple scatterings, Debye-Waller factors, the peak width of the quasi-particles, and the angular dependence of the phonon satellites. We demonstrate some specific features of ultraviolet photoemission spectroscopy (UPS) spectra from particularly anisotropic organic solids.

Charge-carrier injection in organic electronics is critically influenced by the interface electronic states formed at the organic semiconducting thin films and metal electrode contacts. A practical solution for the issue is to form a charge-transfer (CT) state at the interfaces, which will produce a substantial density of gap states in the vicinity of the Fermi level of metal electrodes and thus reduce significantly the charge-injection barrier at the interfaces. In this study, we use ultraviolet photoelectron spectroscopy and demonstrate the formation of a CT state at diindenoperylene (DIP) and Ag (or Cu) electrode contacts. Remarkably, the CT state occurs irrespective of the electrode crystallinity, crystal grain orientations, or the molecular orientations in the DIP film, hence both the top- or bottom-metal contact conditions result in the formation of the CT state. Theoretical calculation simulations show the sign of strong interaction between those metal atoms and DIP leading to the CT state formation, analogous to the surface-induced aromatic stabilization proposed by Heimel et al. in a literature [Nat. Chem., 5(2013)187.]. Our results aid in understanding the mechanism of the CT state formation and propose the DIP-CT interlayers being novel candidates for the efficient hole-injection layers used in organic electronics.

We report computational and experimental studies (i) on the Fermi level (EF) pinning phenomena in weakly interacting electrode-organic semiconductor systems with pentacene, N, N-Di(naphthalene-l-yl)-N, N’-diphenyl-benzidine (α-NPD) and Poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT)], and (ii) to elucidate a common reason of the pinning phenomena which have been widely observed for various organic materials on inert electrode surfaces. For (i), the computed results on the electrode-dependence of EF-HOMO and EF-LUMO distance agreed excellently with UPS and Kelvin-probe results. For (ii) we found theoretically that the pinning phenomena occur at systems even without any electronic states in the HOMO-LUMO gap, and thereby indicate that this is a universal Fermi-level pinning-like phenomena for various band gap materials, even if specific interface states do not exist upon contact. We further obtained for Gaussian-distributed HOMO and LUMO that the minimal hole and electron injection barriers are quantitatively determined by degree of standard deviations of HOMO and LUMO bands, which are in excellent agreement with the experimental results. Furthermore, our results provide a guideline for approaching the “zero” injection barriers.

We report on the growth and characterization of molecular mixed thin films of α-sexithiophene (6T), a well-known organic p-type semiconductor with high hole mobility, together with its perfluorinated counterpart, the so far rarely studied tetradecafluoro-α-sexithiophene (PF6T). Pure and blended thin films of these two molecules with different mixing ratios were grown on silicon oxide in ultrahigh vacuum by coevaporation. The effect of perfluorination and mixing on crystal structure, morphology, electronic, and optical properties was examined. The evolution of the PF6T crystal structure was followed in situ in real time by X-ray scattering. We found a new thin film structure different from the reported bulk phase with molecules either standing-up or lying-down depending on the growth temperature. The different morphologies of pure films and blends were investigated with atomic force microscopy. The impact of mixing on the core-levels and on the highest occupied molecular orbitals of 6T and PF6T is discussed.

The deposition of strong molecular electron acceptors onto ZnO induces a substantial work function (\ensuremath{\phi}) increase due to electron transfer from the inorganic semiconductor to the molecules. The \ensuremath{\phi} increase results from two mechanisms: (i) a change of the surface band bending within ZnO and (ii) an interface dipole between the inorganic surface and the negatively charged acceptors. The molecule adsorption induced upward band bending in ZnO is, however, found to be limited to a few 100 meV, while the \ensuremath{\phi} increase is significantly larger (up to 2.8 eV). We elucidate the origin of limited upward band bending by revealing a notable gap state density-of-states (GDOS) using high-sensitivity photoemission spectroscopy. Upon acceptor-induced upward band bending, the GDOS with a wide energy distribution becomes increasingly unoccupied. This, in turn, makes the interface dipole dominant and limits the ZnO surface band bending changes due to a ``soft'' pinning at the Fermi level.

Generally, the lack of long-range order in materials prevents from experimentally addressing their electronic band dispersion by angle-resolved photoelectron spectroscopy (ARPES), limiting such assessment to single crystalline samples. Here we demonstrate that the ARPES spectra of azimuthally disordered transition metal dichalcogenide (TMDC) monolayers with 2 H phase are dominated by their band dispersion along the two high symmetry directions Γ-K and Γ-M. We exemplify this by analyzing the ARPES spectra of four prototypical TMDCs within a mathematical framework, which allows to consistently explain the reported observations. A robust base for investigating TMDC monolayers significantly beyond single crystal samples is thus established. The complex nature of polycrystalline materials mean that characteristics such as their electronic band structure can be more challenging to interpret than their single crystal counterparts. Here, the authors present a framework based on angle-resolved photoemission spectroscopy to reveal the band dispersions for azimuthally disordered transition metal dichalcogenide polycrystalline monolayers.

Charge transport in molecular thin films is often dominated by incoherent hopping processes, and charge-carrier phonon coupling plays a major role in defining mobilities. Our high resolution angle-resolved ultraviolet photoelectron spectroscopy (ARUPS) study reveals the influence of molecular configuration and packing on the hole-phonon coupling in vacuum-sublimed thin films of rubrene on graphite and allows determining charge reorganization energies. In the contact layer to the substrate rubrene is well-ordered with, for low coverages, the tetracene backbone being almost parallel to the graphite surface forming a loose-packed monolayer. Increasing the coverage leads to an orientational transition and a more tilted orientation of rubrene in a close-packed monolayer. This transition results in dramatic changes of spectral features in ARUPS including the photoelectron angular distribution of the highest occupied molecular orbital derived intensity. The charge reorganization energy, however, only changes sli...

Thickness and substrate dependence of film growth, morphology, unit-cell structure, and electronic structures was thoroughly investigated for picene, the zigzag connected 5-ring molecule, by employing complementary techniques of in situ real-time X-ray reflectivity/diffraction, in situ electron spectroscopies, and atomic force microscopy. A different kind of thickness dependent structural transition was observed on SiO2 and graphite, resulting in a distinct electronic structure. On SiO2 picene films with 3D crystalline domains are formed with nearly upright molecular orientation from the initial growth stage. With increasing the film thickness the in-plane dimensions of the unit cell in the initially grown domains become smaller (in other words, more compressed), and, at the same time, crystalline domains with a more relaxed structure are nucleating on top of the compressed domains. In spite of such structural changes, the electronic structure, namely energy position of the highest occupied molecular orbi...

Photoelectron spectroscopy resolved in three-dimensional momentum space with a microscopic field of view is realized by combining a so-called Momentum Microscope with a soft X-ray synchrotron radiation source. A new experimental station for momentum-resolved photoelectron micro-spectroscopy and spectro-microscopy has been built at BL6U, an undulator-based soft X-ray beamline of the UVSOR synchrotron facility. This experimental station specializes in characterizing the electronic structure of surface atomic sites, thin films, molecular adsorbates, and bulk crystals. The instrument details are described along with possible measurement techniques.

We thank O. Kaveh and D. Schutze for performing conductivity measurements, D. Wohrle for supplying F 8 ZnPc, and M.L. Tietze for insightful discussions. M.S. acknowledges financial support by the German Research Foundation (DFG) through the project MatWorldNet LE-747/44-1, the German Academic Exchange Service within the frame of the IPID4all Program and the Graduate Academy of TU Dresden. A.H. acknowledges financial support from the project UNVEiL of the German Federal Ministry of Education and Research (BMBF). S.K. thanks JSPS for financial support (KAKENHI 26248062). N.U. acknowledges support of the Global-COE Program of MEXT (G03) and 21st Century-COE Program of MEXT(G-4) for developing an ultrahigh-sensitivity UPS system. B.N. received funding from the European Union Seventh Framework Programme under grant agreement no. 607232 (THINFACE). F.O. would like to thank the DFG for financial support (project OR-349/1). Grants for computing time from the Zentrum fur Informationsdienste und Hochleistungsrechnen Dresden (ZIH) are gratefully acknowledged.

Efficient n-doping of organic semiconductors requires electron-donating molecules with small ionization energies, making such n-dopants usually sensitive to degradation under air exposure. A workaround consists in the usage of air-stable precursor molecules containing the actual n-doping species. Here, we systematically analyze the doping mechanism of the small-molecule precursor o-MeO-DMBI-Cl, which releases a highly reducing o-MeO-DMBI radical upon thermal evaporation. n-Doping of N,N-bis(fluoren-2-yl)-naphthalene tetracarboxylic diimide yields air-stable and highly conductive films suitable for application as electron transport layer in organic solar cells. By photoelectron spectroscopy, we determine a reduced doping efficiency at high doping concentrations. We attribute this reduction to a change of the precursor decomposition mechanism with rising crucible temperature, yielding an undesired demethylation at high evaporation rates. Our results do not only show the possibility of efficient and air-stable n-doping, but also support the design of novel air-stable precursor molecules of strong n-dopants.