Daisuke Yoshimura

Electronic structures of organic molecular materials for organic electroluminescent devices studied by ultraviolet photoemission spectroscopy

Electronic structures of evaporated films of five organic light-emitting and carrier-injecting materials for organic electroluminescent devices were studied by ultraviolet photoemission spectroscopy. The compounds examined were (i) light-emitting materials tris(8-hydroxyquinolino) aluminum (Alq3) and 1,2,3,4,5-pentaphenylcyclopentadiene, (ii) a hole-injecting material N,N′-bis(3-methylphenyl)-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine, and (iii) electron-injecting materials N,N′-diphenyl-1,4,5,8-naphthyletracarboxyldiimide and 1,3,5-tris(5-phenyl-1,3,4-oxadiazol-2-yl)benzene. The spectral features corresponding to the top parts of the valence states, which dominate the electric properties of the materials, were assigned by the comparison with the simulated density of states obtained from PM3 molecular orbital calculations. Using these calculations, the evolution of the electronic structure of each molecule from those of constituent parts was discussed. The characters of the unoccupied states obtained by t...

Ultraviolet photoelectron spectroscopy of poly(pyridine-2,5-diyl), poly(2,2′-bipyridine-5,5′-diyl), and their K-doped states

Ultraviolet photoelectron spectra were measured using synchrotron radiation for two kinds of π‐conjugated polymers, poly(pyridine‐2,5‐diyl) (PPy) and poly(2,2′‐bipyridine‐5,5′‐diyl) (PBPy) which exhibit n‐type electrically conducting properties. The two compounds show similar spectra and they were analyzed with MO calculations and the comparison with the data of related molecules. The ionization threshold energies of PPy and PBPy were found to be 6.3 and 6.35 eV, respectively. These values are higher than those of π‐conjugated conducting polymers capable of p doping. Upon potassium doping of PBPy, two new states appeared in the originally empty energy gap and the intensity of the state at 0.65 eV from EF grows as the doping proceeds. This finding and the change of optical absorption spectra upon doping indicate that bipolaron bands are formed in K‐doped PBPy. While K‐doped PPy also shows similar gap states, it requires higher dopant concentration to create bipolaron bands than in the case of K‐doped PBPy. The difference of the dependence on dopant concentration between K‐doped PPy and K‐doped PBPy is discussed based on the conformational difference between these polymers.Ultraviolet photoelectron spectra were measured using synchrotron radiation for two kinds of π‐conjugated polymers, poly(pyridine‐2,5‐diyl) (PPy) and poly(2,2′‐bipyridine‐5,5′‐diyl) (PBPy) which exhibit n‐type electrically conducting properties. The two compounds show similar spectra and they were analyzed with MO calculations and the comparison with the data of related molecules. The ionization threshold energies of PPy and PBPy were found to be 6.3 and 6.35 eV, respectively. These values are higher than those of π‐conjugated conducting polymers capable of p doping. Upon potassium doping of PBPy, two new states appeared in the originally empty energy gap and the intensity of the state at 0.65 eV from EF grows as the doping proceeds. This finding and the change of optical absorption spectra upon doping indicate that bipolaron bands are formed in K‐doped PBPy. While K‐doped PPy also shows similar gap states, it requires higher dopant concentration to create bipolaron bands than in the case of K‐doped PBPy....

Energy Level Alignment at Alq3/LiF/Al Interfaces Studied by Electron Spectroscopies: Island Growth of LiF and Size-Dependence of the Electronic Structures

Various models have been proposed, to date, to explain the improvement in organic electroluminescent devices by the insertion of an insulating layer such as LiF between the light emitting tris-(8-hydroxyquinolinato) aluminum (Alq3) layer and the cathode metal deposited on it. Although the average thickness of the LiF for the optimal performance is typically only 0.5 nm, the layer structure of the inserted LiF has been assumed explicitly or implicitly. In order to clarify the growth mode and electronic structure of LiF in the system, the Alq3–LiF–Al system was investigated by ultraviolet photoelectron spectroscopy (UPS), X-ray photoelectron spectroscopy (XPS), and metastable atom electron spectroscopy (MAES). We found island formation of LiF on the Al substrate, and also that the Al surface is only partly covered when a 0.5-nm-thick LiF layer is deposited on it. MAES measurements revealed that island-grown LiF has density of states similar to that of a thick LiF layer except for the existence of defect states, which is probably due to Li vacancies near the interface. MAES also suggested a slight difference in the spatial extention of the wave function of LiF between a 0.5-nm-thick island and a 2.0-nm-thick film, while a distinct difference was found between LiF and Alq3 films. These findings are important in order to discuss their attenuation lengths in relationship to electron injection. We have constructed a realistic energy diagram of the interface, taking account of the island nature of the 0.5-nm-thick LiF layer. This energy diagram suggests it is necessary to reexamine the proposed models.

Electronic structure of 8-hydroxyquinoline aluminum (alq3)/metal interfaces studied by UV photoemission

Abstract The electronic structures of tris(8-hydroxyquinoline) aluminum (Alq 3 ) / metal (Au, Al) interfaces as a model interface of organic electroluminescent (EL) devices were investigated by ultraviolet photoemission spectroscopy (UPS). We found abrupt shifts of the vacuum level of ca. 1 eV at the interfaces in contrast to the traditional assumption with a common vacuum level at the interface. The shift indicates the formation of interfacial dipole with the metal side negatively charged. At Alq 3 / Al interface, the estimated energy position of the lowest unoccupied molecular orbital (LUMO) was very close to the Fermi level of the substrate metal, corresponding to the electron-injecting nature of the interface. This is in contrast to the poor electron-injecting character expected by assuming a common vacuum level where Fermi level of the substrate should be around the center of the gap. The electronic structure of Alq 3 as a solid is also discussed in comparison with the results by semi-empirical molecular orbital calculation.

Electronic structure of Alq3/LiF/Al interfaces studied by UV photoemission

Abstract The electronic structure of tris (8-hydroxyquinolino) aluminum (Alq 3 /LiF/Al system was studied in relation to the enhancement of electron-injection efficiency by the insertion of LiF insulating layer at Alq 3 /Al contact, using UV photoemission spectroscopy (UPS), X-ray photoelectron spectroscopy (XPS), and metastable atom electron spectroscopy (MAES). The observed energy separation between the HOMO of Alq 3 and the Fermi level of Al substrate increased from 2.7eV to 3.0eV by inserting 0.5nm thick LiF layer. This result indicates that the LiF layer induces the decrease of the electron injection barrier. We also found extra states probably caused by the interaction at the Alq 3 /Al interface.The spectral intensity of this extra state decreased with increasing LiF thickness, and vanished at 0.5nm.

Electronic structure of organic carrier transporting material / metal interfaces as a model interface of electroluminescent device studied by UV photoemission

Abstract Electronic structures of N,N′ -diphenyl- N,N′ -(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine (TPD) / metal and N,N′ -diphenyl-1,4,5,8-naphtyltetracarboxylimide (DP-NTCI) / metal interfaces were directly investigated as a model interface of organic electroluminescent (EL) devices using UV photoemission spectroscopy (UPS). At the organic / metal interfaces, abrupt shift of vacuum level was observed, in contrast to the traditional assumption of common vacuum level at the interface. For understanding EL devices, we see the necessity of the direct observation of the interfacial electronic structure by UPS or other techniques for understanding EL devices.

Electronic Structures of Porphyrins and their Interfaces with Metals Studied by UV Photoemission

Abstract The electronic structures of porphyrins and their interfaces with metals were investigated by UV photoemission spectroscopy (UPS). The UPS spectral features of porphine, 5,10,15,20-tetraphenylporphynatozinc(II) (ZnTPP), 5,10,15,20-tetra(4-pyridyl)porphyrin (H2T(4-Py)P) and 5,10,15,20-tetraphenylporphyrin (H2TPP) could be assigned by comparison with MOPAC PM3 calculations. The electronic structures of the porphyrins can be regarded as the superposition of those of porphine and the substituents. The UPS results for ZnTPP/metal (Mg, Al, Ag, Au) interfaces indicated that the energies of the levels of ZnTPP relative to the Fermi level of substrate metals could be expressed as linear functions of the work function of metals with a shift of the vacuum level at interface. The slope of the linear functions was about unity. This indicates that the energy levels of ZnTPP are fixed to the vacuum level of substrate metal with constant interfacial dipole. The level positions of ZnTPP at the interface exhibited...

Electronic structure and molecular orientation of well-ordered polyethylene oligomer (n-C44H90) on Cu(100) and Au(111) surfaces studied by UV photoemission and low energy electron diffraction

Abstract The electronic structure and molecular orientation of tetratetracontane (n-C44H90) films on Cu(100) and Au(111) surfaces were investigated by angle-resolved UV photoemission spectroscopy (ARUPS) and low energy electron diffraction (LEED). The observed ARUPS spectra showed the drastic take-off angle dependence due to intramolecular band dispersion. A 2×1-like LEED pattern was observed for both substrates. From these results and theoretical simulation of ARUPS spectra based on independent-atomic center (IAC) approximation, we found that the C–C–C plane of the adsorbed TTC molecule is parallel to the substrate surface and its molecular axis is along a [110] direction for both substrates. We also measured the work function change by adsorption of TTC. The observed values were c.a. −0.3eV and −0.7eV for Cu(100) and Au(111) systems, respectively. Such decrease of the work function indicates the existence of a dipole layer at the interfaces in contrast to the traditional picture of energy level alignment at organic/metal interface assuming a common vacuum level at the interface. The dipole formation in such physisorbed systems can be explained by the polarization of the TTC molecule due to an image force.

The electronic structure of porphyrin/metal interfaces studied by UV photoemission spectroscopy

The electronic structures of the interface between porphyrin and metal (Mg, Al, Ag and Au) were studied by UV photoemission spectroscopy (UPS). The samples were 5,10,15,20-tetraphenylporphynatozinc(ZnTPP), 5,10,15,20-tetra(4-pyridyl) porphyrin(H2T(4-Py)P) and 5,10,15,20-tetra-phenylporphyrin(H2TPP) which are known as organic semiconductors. We found that the energy levels of these porphyrins relative to the Fermi level of the substrate metals could be expressed as linear functions of the work function of metals with a shift of the vacuum level at interface (Δ). This indicates that the energy levels of these porphyrins are fixed to the vacuum level of substrate metal with an interfacial dipole layer. We also found that the magnitude of Δ is constant at ZnTPP/metal interfaces, but depends on the work function of the metal at H2T(4-Py)P and H2TPP/metal interfaces. This work function dependence of Δ might be explained by the existence of interface state.

The electronic structure of porphyrin/metal interfaces studied by ultraviolet photoelectron spectroscopy

Electronic structure of interfaces between 5,10,15,20-tetraphenylporphynatozinc (ZnTPP) and four metals (Mg, Al, Ag, and Au) were studied by ultraviolet photoelectron spectroscopy (UPS). The energy levels of ZnTPP relative to the Fermi level of substrate metals could be expressed as linear functions of the work function of metals with the shift of the vacuum level at interface (Δ). The slope of the linear functions was about unity. This indicates that the energy levels of ZnTPP are fixed to the vacuum level of substrate metal with constant interfacial dipole. 5,10,15,20-Tetra(4-pyridyl)porphyrin (H2T(4-Py)P)/metal and 5,10,15,20-tetraphenylporphyrin (H2TPP)/metal interfaces were also investigated, and similar linearity was observed between the energy levels of porphyrin and the work function of metal with the slope of much smaller than unity. This deviation of the slope from unity might be explained by the existence of interface state.