Richard Murdey

Frontier Electronic Structures in Fluorinated Copper Phthalocyanine Thin Films Studied Using Ultraviolet and Inverse Photoemission Spectroscopies

Ultraviolet photoemission and inverse photoemission spectroscopy was used to compare the electronic structure of copper phthalocyanine, copper octafluorophthalocyanine, and copper hexadecafluorophthalocyanine thin films. A rigid shift to lower energy was observed for both the occupied and unoccupied electronic states as the number of fluorine atoms around the phthalocyanine ring increased. The spectral features of the fluorine-substituted derivatives were otherwise very similar to copper phthalocyanine, and no change in the transport gap energy was observed. Vacuum level shifts were observed at the interface with polycrystalline gold substrates of sufficient magnitude to consistently pin the substrate Fermi level near the middle of the HOMO-LUMO gap. The calculated barrier heights for electron and hole injection across the interface were therefore equal, and no correlation with fluorine substitution was found.

Core excitations of naphthalene: Vibrational structure versus chemical shifts

High-resolution x-ray photoelectron emission (XPS) and near-edge x-ray absorption fine structure (NEXAFS) spectra of naphthalene are analyzed in terms of the initial state chemical shifts and the vibrational fine structure of the excitations. Carbon atoms located at peripheral sites experience only a small chemical shift and exhibit rather similar charge-vibrational coupling, while the atoms in the bridging positions differ substantially. In the XPS spectra, C-H stretching modes provide important contributions to the overall shape of the spectrum. In contrast, the NEXAFS spectrum contains only vibrational progressions from particular C-C stretching modes. The accuracy of ab initio calculations of absolute electronic transition energies is discussed in the context of minute chemical shifts, the vibrational fine structure, and the state multiplicity.

Quantitatively identical orientation-dependent ionization energy and electron affinity of diindenoperylene

Molecular orientation dependences of the ionization energy (IE) and the electron affinity (EA) of diindenoperylene (DIP) films were studied by using ultraviolet photoemission spectroscopy and inverse photoemission spectroscopy. The molecular orientation was controlled by preparing the DIP films on graphite and SiO2 substrates. The threshold IE and EA of DIP thin films were determined to be 5.81 and 3.53 eV for the film of flat-lying DIP orientation, respectively, and 5.38 and 3.13 eV for the film of standing DIP orientation, respectively. The result indicates that the IE and EA for the flat-lying film are larger by 0.4 eV and the frontier orbital states shift away from the vacuum level compared to the standing film. This rigid energy shift is ascribed to a surface-electrostatic potential produced by the intramolecular polar bond (>C−-H+) for standing orientation and π-electron tailing to vacuum for flat-lying orientation.

An Accurate Calculation of Electronic Contribution to Static Permittivity Tensor for Organic Molecular Crystals on the Basis of the Charge Response Kernel Theory

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.

In situ conductance measurements of copper phthalocyanine thin film growth on sapphire [0001]

The current flowing through a thin film of copper phthalocyanine vacuum deposited on a single crystal sapphire [0001] surface was measured during film growth from 0 to 93 nm. The results, expressed as conductance vs. nominal film thickness, indicate three distinct film growth regions. Conductive material forms below about 5 nm and again above 35 nm, but in the intermediate thicknesses the film conductance was observed to decrease with increasing film thickness. With the aid of ac-AFM topology images taken ex situ, the conductance results are explained based on the Stranski-Krastanov (2D + 3D) film growth mechanism, in which the formation of a thin wetting layer is followed by the growth of discrete islands that eventually coalesce into an interpenetrating, conductive network.

Accurate Molecular Orientation Analysis Using Infrared pMAIRS Considering the Refractive Index of the Thin Film Sample

Infrared (IR) p-polarized multiple-angle incidence resolution spectrometry (pMAIRS) is a powerful tool for analyzing the molecular orientation in an organic thin film. In particular, pMAIRS works powerfully for a thin film with a highly rough surface irrespective of degree of the crystallinity. Recently, the optimal experimental condition has comprehensively been revealed, with which the accuracy of the analytical results has largely been improved. Regardless, some unresolved matters still remain. A structurally isotropic sample, for example, yields different peak intensities in the in-plane and out-of-plane spectra. In the present study, this effect is shown to be due to the refractive index of the sample film and a correction factor has been developed using rigorous theoretical methods. As a result, with the use of the correction factor, organic materials having atypical refractive indices such as perfluoroalkyl compounds (n = 1.35) and fullerene (n = 1.83) can be analyzed with high accuracy comparable to a compound having a normal refractive index of approximately 1.55. With this improved technique, we are also ready for discriminating an isotropic structure from an oriented sample having the magic angle of 54.7°.

Spontaneous buildup of surface potential with a thin film of a zwitterionic molecule giving noncentrosymmetric crystal structure

Surface potentials were examined using the Kelvin method for thin films of zwitterionic molecules, pyridinium 1,3-dihydro-1,3-dioxo-2H-inden-2-ylide (PI or IPB) and two nitrogen-substituted derivatives. Spontaneous buildup of the surface potential on the film (5.5 V at a film thickness of 300 nm) was only observed for the compound with unidirectional orientation of the molecular dipole moments in the single crystal. The relationship between the alignment of the molecular dipole moments in the film and the measured surface potentials was investigated using grazing incidence x-ray diffraction, pole-figure measurements, atomic force microscopy, and Kelvin probe force microscopy.

Interfacial charge injection barriers in organic light-emitting diodes: the effect of thin interlayers of organic donor-acceptor molecules TTF and TCNQ

Energy level alignment at interfaces between tetrathiafulvalene (TTF) and tetracyanoquinodimethane (TCNQ) films on representative anode materials Au, ITO and PEDOT-PSS are investigated by ultraviolet photoelectron spectroscopy (UPS). Both materials have broadly uniform behavior independent of the chemical composition of the substrate. The position of the vacuum level of the deposited films is fixed with respect to the substrate Fermi energy, and the hole injection barrier is likewise constant. The magnitude of the vacuum level shift is a simple linear function of the substrate work function. A 4 nm thick interlayer in TTF/TCNQ/Au and TCNQ/TTF/Au compound interfaces does not alter the alignment of the outer material with the substrate. Charge transfer in conjunction with subsequent structural relaxation of the ionic donor or acceptor molecule is proposed to explain the results. Lastly, we discuss how donor-acceptor molecules might be used to mediate charge injection into organic light emitting diodes (OLEDs) and similar organic electronic devices.

An atom-transparent photon block for metal-atom deposition from high-temperature ovens

We describe, in the context of polymer metallization calorimetry, the design and operation of a dual-chopper velocity selector to filter out the radiation background from high-temperature metal-atom oven sources. Particular attention is given to the resultant nonuniform atom flux profile.

Lead‐Free Solar Cells based on Tin Halide Perovskite Films with High Coverage and Improved Aggregation

Two simple methods to improve tin halide perovskite film structure are introduced, aimed at increasing the power conversion efficiency of lead free perovskite solar cells. First, a hot antisolvent treatment (HAT) was found to increase the film coverage and prevent electrical shunting in the photovoltaic device. Second, it was discovered that annealing under a low partial pressure of dimethyl sulfoxide vapor increased the average crystallite size. The topographical and electrical qualities of the perovskite films are substantively improved as a result of the combined treatments, facilitating the fabrication of tin-based perovskite solar cell devices with power conversion efficiencies of over 7 %.