吉田 弘幸

Carbazole-based self-assembled monolayers have received considerable attention as hole-selective layers (HSLs) in inverted perovskite solar cells. As an HSL, the electron-blocking capability is important and directly related to electron affinity (EA). Low-energy inverse photoelectron spectroscopy (LEIPS) is the most reliable method for EA measurement. However, the intense electron-impact-induced fluorescence from carbazole interferes with their measurement. By improving the photon detector, we were able to measure 2PACz and MeO-2PACz LEIPS spectra and determine their respective EAs of 1.72 and 1.48 eV. These small EA values ensure effective electron-blocking capability of HSLs regardless of the type of perovskite layer.

Record power conversion efficiencies (PCEs) of perovskite solar cells (PSCs) have been obtained with the organic hole transporter 2,2′,7,7′-tetrakis( N , N -di- p -methoxyphenyl-amine)9,9′-spirobifluorene (spiro-OMeTAD). Conventional doping of spiro-OMeTAD with hygroscopic lithium salts and volatile 4- tert -butylpyridine is a time-consuming process and also leads to poor device stability. We developed a new doping strategy for spiro-OMeTAD that avoids post-oxidation by using stable organic radicals as the dopant and ionic salts as the doping modulator (referred to as ion-modulated radical doping). We achieved PCEs of >25% and much-improved device stability under harsh conditions. The radicals provide hole polarons that instantly increase the conductivity and work function (WF), and ionic salts further modulate the WF by affecting the energetics of the hole polarons. This organic semiconductor doping strategy, which decouples conductivity and WF tunability, could inspire further optimization in other optoelectronic devices.

Porphyrin derivatives are promising materials for various thin-film-based devices such as solar cells, field-effect transistors, and gas sensors. Since the molecular aggregation structure in a thin film significantly influences the device performance, controlling the aggregation structure is of crucial importance. In this study, we show that three different crystalline polymorphs of free-base tetraphenylporphyrin (H2TPP) can be made to form in spin-coated thin films depending on the evaporation time of the solvent. The results show that the aggregation structure in the as-spun films is controlled by the solvent evaporation time, and the initial film structure determines the polymorphs obtained after thermal annealing.

<jats:title>Abstract</jats:title> <jats:p>Quasi-2D perovskites passivate the perovskite surface and improve the lifetime of perovskite solar cells. However, their detailed surface structures have never been reported. We studied the surfaces of the solution-processed quasi-2D PEA<jats:sub>2<jats:italic>m</jats:italic> </jats:sub>MA<jats:sub> <jats:italic>n</jats:italic>−2<jats:italic>m</jats:italic> </jats:sub>Pb<jats:italic> <jats:sub>n</jats:sub> </jats:italic>I<jats:sub>3<jats:italic>n</jats:italic> </jats:sub> (PEA: C<jats:sub>6</jats:sub>H<jats:sub>5</jats:sub>CH<jats:sub>3</jats:sub>CH<jats:sub>2</jats:sub>NH<jats:sub>3</jats:sub>, MA: CH<jats:sub>3</jats:sub>NH<jats:sub>3</jats:sub>) perovskites as well as the 2D perovskite formed on top of 3D MAPbI<jats:sub>3</jats:sub> with the thicknesses relevant to practical solar cell (<jats:italic>n</jats:italic> ≈ 400) using ultraviolet photoelectron and metastable-atom electron spectroscopies. We confirmed that PEA segregates to the surface and that the phenyl group (C<jats:sub>6</jats:sub>H<jats:sub>5</jats:sub>) covers the outermost surface of the quasi-2D perovskite. We discuss plausible structures from the concentration dependence of PEA.</jats:p>

Perylene dyes are a representative framework of electron transport (n-type) organic semiconductors. The energy of their electron transport level is the electron affinity (EA), which is an important parameter in selecting the electron transport materials for device application and of the material's electron-accepting ability. Recent studies show that EA may vary by as much as 1 eV depending on the molecular orientation owing to the electrostatic potential generated by the quadrupole moments. Because perylene dyes have a large quadrupole moment, it is essential to discuss EA combined with the molecular orientation in the sample film. In this work, we determine the EAs of perylene diimide derivatives in the solid phase using low-energy inverse photoelectron spectroscopy (LEIPS), which was developed by one of the authors. By changing the substrate and the alkyl-chain length, we systematically investigate the relationship between EA and molecular arrangement in the film to derive the general trend of the EA of perylene dyes. The results show that most of the perylene dyes' EAs are in the range from 3.7 to 4.0 eV.

Information concerning the unoccupied states of condensed matter is of great relevance to its electronic, optical and chemical properties. These unoccupied states can be directly examined by inverse photoelectron spectroscopy (IPES), which is often regarded as the time-inversion process of photoelectron spectroscopy. The fundamental drawback of IPES is its low signal intensity. Other spectroscopic intensities are enhanced by surface plasmon resonance (SPR), so the intensity of the IPES signal may also be enhanced by SPR. This was, however, impossible because the photon energy involved in conventional IPES exceeds 9 eV and is much higher than the SPR energy of existing materials. In 2012, we developed low-energy IPES (LEIPS), in which the photon energy is <5 eV, which can then be matched with the SPR energy. We demonstrate a five-fold enhancement of the LEIPS signal from a prototypical organic semiconductor, copper phthalocyanine (CuPc), by SPR of Ag nanoparticles.

Eliminating the excess energetic driving force in organic solar cells leads to a smaller energy loss and higher device performance; hence, it is vital to understand the relation between the interfacial energetics and the photoelectric conversion efficiency. In this study, we systematically investigate 16 combinations of four donor polymers and four acceptors in planar heterojunction. The charge generation efficiency and its electric field dependence correlate with the energy difference between the singlet excited state and the interfacial charge transfer state. The threshold energy difference is 0.2 to 0.3 eV, below which the efficiency starts dropping and the charge generation becomes electric field-dependent. In contrast, the charge generation efficiency does not correlate with the energy difference between the charge transfer and the charge-separated states, indicating that the binding of the charge pairs in the charge transfer state is not the determining factor for the charge generation.

Methylammonium lead triiodide (CH3NH3PbI3) is an essential material in prototype perovskite solar cells, and its intrinsic electronic structures have been of great interest. In this study, the clean surface of single crystal samples of CH3NH3PbI3 was elucidated by X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy (UPS), and photoelectron yield spectroscopy (PYS), and was closely compared with as-prepared single crystal surfaces with considerable native impurities. Low-energy UPS and PYS successfully revealed a presence of electronic states exceeding the Fermi level, which are presumably attributed to electrons transferring from the surface impurities into the conduction band of CH3NH3PbI3.

Pentacene attracts a great deal of attention as a basic material used in organic thin-film transistors for many years. Pentacene is known to form a highly ordered structure in a thin film, in which the molecular long axis aligns perpendicularly to the substrate surface, i.e., end-on orientation. On the other hand, the face-on oriented thin film, where the molecular plane is parallel to the substrate, has never been found on an inert substrate represented by SiO2. As a result, the face-on orientation has long been believed to be generated only on specific substrates such as a metal single crystal. In the present study, the face-on orientation grown on a SiO2 surface has first been identified by means of visible and infrared p-polarized multiple-angle incidence resolution spectrometry (pMAIRS) together with two-dimensional grazing incidence X-ray diffraction (2D-GIXD). The combination of the multiple techniques readily reveals that the face-on phase is definitely realized as the dominant component. The face-on film is obtained when the film growth is kinetically restricted to be prevented from transforming into the thermodynamically stable structure, i.e., the end-on orientation. This concept is useful for controlling the molecular orientation in general organic semiconductor thin films.

© 2018 American Chemical Society. A molecular anion of tris(8-hydroxyquinolinato)aluminum (Alq3) was generated by a pulsed discharge to the solid sample under supersonic expansion and its photoelectron spectrum was recorded after mass selection. The vertical detachment energy of Alq3- and the adiabatic electron affinity of Alq3 were determined to be 1.24 ± 0.01 and 0.89 ± 0.04 eV, respectively. By using these energies determined for monomeric Alq3, the reorganization energy for the intermolecular electron transport in bulk Alq3 was estimated to be 0.70 ± 0.08 eV.

Designing donor/acceptor (D/A) interfaces that can efficiently generate free carriers is an attractive research target for organic photovoltaics (OPVs). While many reports suggest that the molecular orientation of the donor at the D/A interface influences the free-charge generation and recombination, the effects of the acceptor orientation on these processes remain elusive. In this work, we demonstrate that [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) changes its molecular orientation at the film surface on crystallization, resulting in the preferential surface exposure of the side chains. Photoelectron spectra of amorphous- and crystalline-PC61BM/sexithiophene (6T) interfaces and analysis of the external quantum efficiency and electroluminescence of bilayer OPVs in the charge-transfer absorption range reveal that the orientational change of PC61BM raises the energy of the charge-transfer state at the D/A interface. In addition, the PC61BM side chain at the crystalline-PC61BM/6T interface reduces the electronic coupling between the charge-transfer and ground states, suppressing carrier recombination without sacrificing photocurrent. These two factors lead to the higher open-circuit voltage of crystalline-PC61BM/6T OPV compared with its amorphous counterpart. This work directly links the interface and photovoltaic properties, highlighting the role of the acceptor’s orientation in determining the efficiency of OPVs.

The solvent vapor annealing (SVA) technique is one of the useful post processing techniques of a thin film, which is an alternative technique of the thermal annealing one. SVA has a great advantage that the molecular rearrangement in the film is made moderately by employing an appropriate solvent without the sample heating. The moderate processing is expected to yield a benefit that the molecular coalescence would be suppressed, which would readily keep the continuous surface topography of the film during the annealing, and another benefit that a metastable structure would be obtained. To make the best use of the SVA-specific characteristics, in the present study, a material having a metastable structure is chosen. The sample is zinc tetraphenylporphyrin (ZnTPP) that yields a metastable triclinic crystal structure, which can easily be converted to a monoclinic crystal structure by thermal annealing. A triclinic-structure film of ZnTPP by the combination of a wet process and the thermal annealing has thus never been reported. By choosing a fluorine-containing solvent, which has a low affinity to ZnTPP, a triclinic-structure film has first been obtained by a wet process while the surface continuity is protected.

Energy levels of organic molecular films exert paramount influence on the electronic properties of organic semiconductors. Recently the effect of electrostatic energy was highlighted as an origin of such peculiar phenomena as the molecular-orientation dependence and continuous tuning of energy levels. However, the mechanism has been discussed mostly based on theoretical work and has not been adequately supported by experiments. In this work, we propose a procedure to evaluate the electrostatic and electronic polarization energies in organic films solely from the experimental data obtained by ultraviolet photoelectron and low-energy inverse photoelectron spectroscopies. We apply it to the energy levels of thin films of 6,13-pentacenequinone on SiO2 substrates with three different molecular orientations. The obtained electrostatic energies are fully consistent with the theoretical results at different levels such as the first-principles calculations and the electrostatic energy of charge-multipole interactions. The present work also underlines the importance of the charge-permanent quadrupole interaction which is the leading term of the electrostatic energy in the film of nonpolar molecules.

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. On the basis of 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 development of semiconducting polymers is imperative to improve the performance of polymer-based solar cells (PSCs). In this study, new semiconducting polymers based on naphtho[1,2-c:5,6-c′]bis[1,2,5]thiadiazole (NTz), PNTz4TF2 and PNTz4TF4, having 3,3′-difluoro-2,2′-bithiophene and 3,3′,4,4′-tetrafluoro-2,2′-bithiophene, respectively, are designed and synthesized. These polymers possess a deeper HOMO energy level than their counterpart, PNTz4T, which results in higher open-circuit voltages in solar cells. This concequently reduces the photon energy loss that is one of the most important issues surrounding PSCs. The PNTz4TF4 cell exhibits up to 6.5% power conversion efficiency (PCE), whereas the PNTz4TF2 cell demonstrates outstanding device performance with as high as 10.5% PCE, which is quite high for PSCs. We further discuss the performances of the PSCs based on these polymers by correlating the charge generation and recombination dynamics with the polymer structure and ordering structure. We believe that the results provide new insights into the design of semiconducting polymers and that there is still much room for improvement of PSC efficiency.

A novel electron-deficient building unit, dithienylthienothiophenebisimide, and its polymers (PTBIs) are reported. Organic photovoltaic (OPV) cells based on PTBIs as p-type material exhibit 8.0% efficiencies with open-circuit voltages higher than 1 V. Interestingly, PTBIs also function as n-type material in OPVs depending on the molecular structure. These polymers also exhibit p-channel, n-channel, and ambipolar behaviors in field-effect transistors.

Information about the unoccupied states is crucial to both fundamental and applied physics of organic semiconductors. However, there were no available experimental methods that meet the requirement of such research. In this review, we describe a new experimental method to examine the unoccupied states, called low-energy inverse photoemission spectroscopy (LEIPS). An electron having the kinetic energy lower than the damage threshold of organic molecules is introduced to a sample film, and an emitted photon in the near-ultraviolet range is detected with high resolution and sensitivity. Unlike the previous inverse photoemission spectroscopy, the sample damage is negligible and the overall resolution is a factor of two improved to 0.25 eV. Using LEIPS, electron affinity of organic semiconductor can be determined with the same precision as photoemission spectroscopy for ionization energy. The instruments including an electron source and photon detectors as well as application to organic semiconductors are presented. (C) 2015 Published by Elsevier B.V.

When a thin layer of bathocuproine (BCP) is inserted between the metal electrode and the organic layer of the organic semiconductor device, the electron injection/collection efficiency at the interface is significantly improved. However, the mechanism of electron transport through the BCP layer has not been clarified yet. In this study, we directly observed the unoccupied electronic states of the Ag/BCP interface using low-energy inverse photo emission spectroscopy. The result shows that Ag strongly interacts with the BCP molecule and the lowest unoccupied molecular orbital (LUMO) level of the Ag-BCP complex aligns with the Fermi level, indicating that the electron transport occurs through the LUMO level of the complex. With the aid of DFT calculation, we identify the reaction product.

Ionization energy and electron affinity in organic solids are understood in terms of a single molecule perturbed by solid-state effects such as polarization energy, band dispersion, and molecular orientation as primary factors. However, no work has been done to determine the individual contributions experimentally. In this work, the electron affinities of thin films of pentacene and perfluoropentacene with different molecular orientations are determined to a precision of 0.1 eV using low-energy inverse photoemission spectroscopy. Based on the precisely determined electron affinities in the solid state together with the corresponding data of the ionization energies and other energy parameters, we quantitatively evaluate the contribution of these effects. It turns out that the bandwidth as well as the polarization energy contributes to the ionization energy and electron affinity in the solid state while the effect of the surface dipole is at most a few eV and does not vary with the molecular orientation. As a result, we conclude that the molecular orientation dependence of the ionization energy and electron affinity of organic solids originates from the orientation-dependent polarization energy in the film.

The electron affinity (EA) of an organic semiconductor is a measure of the electron transport level. Although reliable values of the EA are required for designing the device architecture of organic light-emitting diodes (OLED), there were no appropriate methods. Recently we have developed low-energy inverse photoemission spectroscopy which enables us to determine the EA of organic materials in solid with the precision required for research of OLED. Using this new technique, we precisely determined EA of typical OLED materials, TCTA, CBP, Ir(ppy)(3), BCP, Alq(3) and Liq as well as a newly developed dopant 4CzIPN. The obtained electron affinities are generally smaller by about 1 eV than the commonly believed values urging the reconsideration of the electron injection/transport mechanisms in OLED. We also compare EAs determined by various experimental and calculation methods for 29 materials. The results show that the reduction potential gives a reasonable estimate rather than the optical gap and ionization energy. (C) 2015 Elsevier B.V. All rights reserved.

The effect of thermal annealing on the energy levels of [6,6]-phenyl-C-61-butyric acid methyl ester (PCBM) films was investigated using ultraviolet photoelectron spectroscopy, X-ray photoelectron spectroscopy, and low-energy inverse photoemission spectroscopy. We observed that thermal annealing at 150 degrees C induces reductions of both the ionization potential (IP) and the electron affinity (EA) with a narrowing of the band gap by 0.1 eV. These changes are associated with crystallization and a 2.54% reduction in the film thickness. Precise measurements of both the IP and EA enabled an evaluation of the effects of the electronic polarization energy in a model based on the charge localized in a single PCBM molecule.

The lowest unoccupied molecular orbital (LUMO) levels of six fullerene derivatives frequently used for organic photovoltaic cells are examined in the solid state. We employed a new experimental technique, low-energy inverse photoemission spectroscopy (LEIPS), which allows us to determine the electron affinities of solid samples within the uncertainties of 0.1 eV. Using the precisely determined electron affinities, the energy difference between the ionization energy of donor and the electron affinity of acceptor is compared with the open circuit voltage for various polymer/fullerene combinations taken from the literature. The result suggests that the well-known relation between the energy difference and the open circuit voltage should be reconsidered.

Electron affinity is a fundamental energy parameter of materials. In organic semiconductors, the electron affinity is closely related to electron conduction. It is not only important to understand fundamental electronic processes in organic solids, but it is also indispensable for research and development of organic semiconductor devices such as organic light-emitting diodes and organic photovoltaic cells. However, there has been no experimental technique for examining the electron affinity of organic materials that meets the requirements of such research. Recently, a new method, called low-energy inverse-photoemission spectroscopy, has been developed. A beam of low-energy electrons is focused onto the sample surface, and photons emitted owing to the radiative transition to unoccupied states are then detected. From the onset of the spectral intensity, the electron affinity is determined within an uncertainty of 0.1 eV. Unlike in conventional inverse-photoemission spectroscopy, sample damage is negligible and the resolution is improved by a factor of 2. The principle of the method and several applications are reported.

Electron-donor function of methanofullerenes (MFs) in bulk heterojunction systems is demonstrated by the combination of MFs with the electron-transporting pi-system that has a much higher electron affinity than MFs.

An apparatus for the low-energy inverse photoemission spectroscopy is described. In this technique, low energy electron having kinetic energy below 4 eV is incident to the sample and detect the emitted photons in the near ultraviolet range (below 5 eV, longer than 250 nm) to investigate the unoccupied states of the solid materials. Compared with the prototype apparatus reported previously [H. Yoshida, Chem. Phys. Lett. 539-540, 180-185 (2012)], the collection efficiency of photons is improved by a factor of four and practically any conductive substrates can be used. The overall resolution is 0.27 eV. (C) 2014 AIP Publishing LLC.

An inverse photoemission spectroscopy (IPES) apparatus using a Czerny-Turner grating spectrometer is demonstrated. Previous IPES instruments based on grating spectrometers used a concave grating and operated in the vacuum ultraviolet range. The reflectance of such gratings is lower than 20% and the aberration cannot be finely corrected leading to an energy resolution of up to 0.1 eV. In the present study, employing the low energy IPES regime [H. Yoshida, Chem. Phys. Lett. 539-540, 180 (2012)], incident electrons with a kinetic energy below 5 eV are used, while photon emission in the range of between 250 and 370 nm is analyzed with a 10-cm Czerny-Turner grating spectrometer. The signal intensity is at least 30 times higher than the previous apparatus. The resolution of photon detection is set at 0.07 eV though the ultimate resolution is one order of magnitude higher. The experiment is performed both by sweeping the electron energy (isochromat mode) and by simultaneously analyzing the photon of whole wavelength range (tunable photon energy mode). (C) 2013 AIP Publishing LLC.

The electron affinity of pentacene thin films has been evaluated during the last decades, but it is still under controversial due to varieties of film quality and radiation damages of the films introduced during inverse photoemission spectroscopy (IPES) experiment together with insufficient energy resolution of the instruments. We employed the near-ultraviolet IPES with a better energy resolution 0.27 similar to 0.32 eV and using lower energy electron beams (0 eV &lt;= Ei &lt;= 4.9 eV) to study the unoccupied states of pentacene thin film. Due to a large mean-free-path of the electron in this energy region, the threshold electron affinity of the bulk of pentacene film was precisely determined to be 2.70 +/- 0.03 eV. Using the threshold ionization energy of 4.90 +/- 0.05 eV determined by ultraviolet photoemission spectroscopy, the band-gap energy of the pentacene film is obtained to be 2.2060.06 eV. (C) 2013 AIP Publishing LLC.

In previous inverse photoemission spectroscopy (IPES) experiments, either X-ray (h nu &gt; 1 keV) or vacuum ultraviolet (h nu approximate to 10 eV) photons were detected following the injection of electrons with energies of 10-1000 eV into solid materials. Here, we demonstrate IPES in the near-ultraviolet range (h nu &lt; 5 eV) using electrons with kinetic energies less than 4 eV. The energy resolution of the instrument is attained to be 0.27 eV. From the spectra of copper phthalocyanine films, it is found that damage to the organic sample is significantly reduced, demonstrating that this method is especially suitable for organic semiconducting materials. (C) 2012 Elsevier B.V. All rights reserved.

The C 1s core-level energy difference of surface and bulk regions of organic semiconductor films, Alq 3, bathocuproine, and Cu-phthalocyanine, are reported. For the analysis, we employed a novel method based on angle-resolved photoemission spectroscopy and target factor analysis, which we developed recently [Chem. Phys. Lett.2011, 511, 146-150]. In contrast to the earlier studies, the core energy differences between the surface and bulk layers were much more precisely determined together with the thickness of the surface layer. The results show that the core-level energy of the outermost layer is different by from 0.2 to 0.4 eV for all of the three compounds. The finding suggests that the electrostatic polarization energy plays an important role in screening charge carriers in organic solids. © 2012 American Chemical Society.

The difference in the C1s core level energies between the surface and bulk regions of an organic semiconductor thin film is examined for pentacene and perfluoropentacene (F-pentacene). The experimental method we have recently developed allows us to determine precisely the differences as well as the thickness of the surface region. The C1s binding energies in the outermost layer turn out to be larger than those in the bulk by 0.32 eV (pentacene) and 0.25 eV (F-pentacene). The results are successfully explained by the interaction energy between the point charge and induced dipole moments suggesting the electrostatic polarization energy plays a main role in the difference of the core level energy. (C) 2011 Elsevier B.V. All rights reserved.

We have developed a new experimental method of X-ray photoemission spectroscopy (XPS) that can map out the core-energy levels as a function of depth from the surface of the film. A series of XPS data are recorded with different detection angles and expanded to the Taylor series of angle-averaged spectra using the target factor analysis. This procedure enables conversion of the measured angle variations in XPS to the core energy levels as a function of depth from the surface. This method has been applied to profiling the electronic levels of buried interface between organic semiconductor and metal surfaces. (C) 2011 Elsevier B. V. All rights reserved.

We have developed a new method to calculate the static permittivity tensors of organic molecular crystals by applying the charge response kernel theory (Morita, A.; Kato, S. J. Am. Chem. Soc. 1997, 119, 4021) in which all the parameters were obtained with the density functional theory. The accuracy together with the requirements of the computation was discussed in terms of positions of the charge response sites and choice of a basis set. The calculated permittivities of typical organic compounds turned out to agree with the experimentally obtained values in the deviation of about 7% when a reasonable computational cost was maintained.

The electronic structure of [6,6]-phenyl-C-61-butyric acid methyl ester (PCBM) was studied using ultraviolet photoelectron spectroscopy of vapor and thin film and inverse photoemission spectroscopy of thin film. The threshold ionization energy of PCBM was found to be 7.17 +/- 0.04 eV in gas phase and 5.96 +/- 0.02 eV in solid phase. The threshold electron affinity was 3.9 +/- 0.1 eV in solid phase. These values are 0.4-0.6 eV smaller than C-60. The density functional theory calculations gave consistent results with these trends and suggested that the electron donation from the side chain to C-60 backbone raised the C-60-backbone-derived pi orbitals of PCBM. The polarization energy of PCBM is 1.21 eV, which is almost the same as C-60 but is about 0.5 eV smaller than the value of typical aromatic hydrocarbons. (c) 2008 American Institute of Physics.

Calculations are presented for the energy bands of three polymorphs of pentacene: the thin-film, bulk, and single-crystal phases. In the calculation of the thin-film phase, the structural data from our recent results on the x-ray diffraction reciprocal space mapping were applied. The band structures are essentially two dimensional, i.e., only small dispersions are found along the c(*) direction. The energy dispersion of the thin-film phase is found to be larger and more isotropic than those of the other phases. The energy dispersions of the bands derived from highest occupied molecular orbital (HOMO), HOMO-1, lowest unoccupied molecular orbital (LUMO), and LUMO+1 levels are analyzed by comparing with the corresponding results on the basis of the tight-binding approximation; the dispersions are well described by transfer integrals involving only the nearest neighbor molecules. In accordance with this finding, a simple model is presented to explain the relation between the crystal structure and the energy dispersion. From the calculated bands, the effective masses are derived to discuss the transport properties of the hole and electrons. Angle-integrated photoemission spectra were also measured for the thin-film and bulk phases. Finally, spectral features of the HOMO-derived bands are interpreted by the calculated density of states.

Diffusion and reaction of aluminum metal species (Al) vacuum deposited on perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) thin films were investigated using angle resolved x-ray photoemission spectroscopy. The acquired data were analyzed assuming that the diffusion of Al is described by a one-dimensional diffusion equation with a time-dependent diffusion coefficient. Depth profiles and diffusion coefficients are obtained for reacted and metallic Al separately. The results show that the metallic Al diffuses rapidly during the deposition while the metal diffusion continues at a lower rate even after the deposition at room temperature. On the other hand, the reacted Al does not diffuse further into the PTCDA layers. (C) 2007 American Institute of Physics.

The structure of the thin film phase of pentacene was investigated using x-ray diffraction reciprocal space mapping (RSM). The crystal structure was found to be triclinic with the following lattice parameters: a=0.593 nm, b=0.756 nm, c=1.565 nm, alpha=98.6 degrees, beta=93.3 degrees, and gamma=89.8 degrees. Atomic positions were determined by comparing the observed RSM diffraction intensities with theoretical calculations. (C) 2007 American Institute of Physics.

The structure of pentacene thin films in the bulk phase having interplane spacing of d(001)=1.44 nm grown on SiO2 was investigated using grazing-incidence x-ray diffraction. The films were prepared with two different methods: one treated with an organic solvent after vacuum deposition and the other thermally treated. Both films are similar in structure to the single crystal reported by Campbell [Acta Crystallogr. 15, 289 (1962)]. (c) 2006 American Institute of Physics.

Cluster anions of acrylonitrile (AN), known to give intracluster anionic polymerization products, were deposited on solid substrates. The obtained films were examined by using infrared absorption spectroscopy, X-ray photoemission spectroscopy, and gel permeation chromatography with the aid of quantum chemical calculations. The acquired spectroscopic data are similar to those of polyacrylonitrile (PAN), while the normal polymerization of AN or reactions related to PAN seemed not to occur noticeably. On the contrary, the product analysis shows that most of the constituent molecules of the films are formed via cyclohexane-1,3,5-tricarbonitrile (CHTCN), a dominant product of the intracluster polymerization of AN, accompanied by fragmentation and dimerization.

The electronic structure of low-lying unoccupied electronic states in a hexatriacontane (n-C36H74) thin film was directly observed using inverse photoemission spectroscopy (IPES). In the IPE measurements it was confirmed that the hexatriacontane thin film is easily degraded by electron irradiation. Therefore by reducing the current density of the incident electrons gradually we finally obtained a reliable IPE spectrum free from radiation damage and/or surface charging. As a result, we conclude the IPE spectrum reported by Dudde and Reihl was influenced by radiation damage. The reliable spectrum shows two features near the vacuum level: they were not resolved in the reported spectrum. Further, it is compared with the reported results from other electron spectroscopies and theoretical calculations concerning the unoccupied electronic states in long chain alkanes. (C) 2002 Elsevier Science B.V. All rights reserved.

Unoccupied electronic states of thin films of 3d-metal phthalocyanines (MPc: M=Mn, Fe, Co, Ni, Cu and Zn) and metal-free phthalocyanine (H2Pc) were investigated by inverse photoemission spectroscopy. The experimental results were analyzed with emphasis on the contribution of the 3d-metal orbitals to the lower unoccupied electronic states of MPc. The difference spectra between the sample MPc (M=Mn, Fe, Ni and Cu) and the reference ZnPc were found to be in fair agreement with the previously reported X-ray absorption spectra in the energy regions corresponding to the core excitation of the central metals, This shows that the difference spectra exhibit the partial density of states attributed to the central metals. A comparison of the 3d-metal orbital contributions was also made between the experimental difference spectra and reported results from extended Huckel and density functional calculations. (C) 2001 Elsevier Science B.V. All rights reserved.