Electronic structures of model interfaces of organic electroluminescent (EL) devices and porphyrin/metal interfaces were investigated by UV photoemission spectroscopy (UPS). At all the measured interfaces, shift of the vacuum level was observed, showing the formation of an interfacial electric dipole layer. For Alq/sub 3/ (tris(8-hydroxyquinolino) aluminum), TPD (N,N'-diphenyl-N,N'-(3-methylphenyl)-1,1-biphenyl-4,4'-diamine), and DP-NTCI (N,N'-diphenyl-1,4,5,8-naphthyltetracarboxylimide)/metal interfaces, interfacial energy diagrams determined by UPS correspond well with the actually observed carrier-injecting character at the interfaces. For ZnTPP (15,10,15,20-zinc-tetraphenylporphyrin), H/sub 2/TPP (5,10,15,20-tetraphenylporphyrin), and H/sub 2/T(4-Py)P (5,10,15,20-tetra(4-pyridyl) porphyrin)/metal interfaces, the shifts of the vacuum level as well as the energies of the levels in porphyrins could be expressed as a linear function of work function of the metal substrate. The slope of the linear function depended on the compound. These findings are in contrast to the traditional assumption of common vacuum level at the interfaces, For ZnTPP/metal interfaces, sample exposure to oxygen induced energy level shift in close relation with the change of the substrate work function at oxygen exposure. The present results have clearly demonstrated that direct observation of the interfacial electronic structure by microscopic method such as UPS is necessary for understanding the organic electronic devices such as EL devices and organic solar cells.

Energy level alignment at organic/metal interfaces studied by UV photoemission: breakdown of traditional assumption of a common vacuum level at the interface

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...

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

The electronic structures of model interfaces of organic electroluminescent (EL) devices were investigated with UV photoemission spectroscopy (UPS). Interfaces of TTN (tetrathianaphthacene) and TCNQ (tetracyanoquinodimethane) were also studied as extreme cases for hole transport and electron transport material, respectively. For all organic/metal interfaces studied, the work function of metal electrode was changed by deposition of organic layer, i.e., the vacuum level was shifted at the interface, indicating the invalidity of the traditional energy level alignment model where a common vacuum level was assumed at organic/metal interface. At TCNQ/Au, DP-NTCI/Al, which are acceptor/metal interfaces, upward shift of the vacuum level of organic layer relative to that of metal was observed, suggesting the formation of interfacial dipole due to electron-transfer from metal to acceptor. At other organic/metal interfaces, TPD(N, N'-diphenyl-N, N'-(3-methylphenyl)-1, 1'-biphenyl-4, 4'-diamine)/Au or ITO (indium tin oxide), ALq/sub 3/ (tris(8-hydroxyquinolino) aluminum)/Al, DP-NTCl(N, N'-diphenyl-1,4,5,8- naphthyltetracarboxylimide)/Al or Au, downward shift of the vacuum level was observed. Such downward shift has been also observed in our previous study for porphyrin/metal interfaces, and seems to be a trend for organic/metal interfaces at which no electron-transfer from metal to organic layer occurs. This trend suggests that the traditional model tends to underestimate (overestimate) the barrier height for hole (electron) injection. On the other hand, the vacuum level shift at ALq/sub 3//TPD interface was less than 0.1 eV, leading to an apparent applicability of the traditional model. However, it is not always the case for organic/organic interfaces: finite shift of 0.2 eV was observed at TTN/TCNQ interface due to electron-transfer from TTN to TCNQ. Possible origins of vacuum level shift at organic/metal interfaces were also discussed.

Energy-level alignment at model interfaces of organic electroluminescent devices studied by UV photoemission: trend in the deviation from the traditional way of estimating the interfacial electronic structures

The electronic structure of a well-oriented perylene-3,4,9,10-tetracarboxylic acid-dianhydride multilayer prepared on ${\mathrm{MoS}}_{2}$ single crystal surface were studied by angle-resolved ultraviolet photoemission spectroscopy using synchrotron radiation. From the photon energy dependence of normal emission spectra, we observed an intermolecular energy-band dispersion of about 0.2 eV for the highest occupied molecular orbital (HOMO) band of single $\ensuremath{\pi}$ character. The observed energy-band dispersion showed a cosine curve, which originates from the intermolecular $\ensuremath{\pi}\ensuremath{-}\ensuremath{\pi}$ interaction. Analyses using the tight-binding model gave that the transfer integral of about 0.05 eV for the $\ensuremath{\pi}\ensuremath{-}\ensuremath{\pi}$ interaction, the effective mass of HOMO hole ${m}_{h}^{*}{=5.28m}_{0},$ and the hole mobility ${\ensuremath{\mu}}_{h}g3.8{\mathrm{cm}}^{2}/\mathrm{V}\mathrm{}\mathrm{s}.$ This is the first observation of the intermolecular energy-band dispersion of a conventional single-component organic semiconductor only with the weak intermolecular van der Waals interaction.

Intermolecular energy-band dispersion in PTCDA multilayers

Ultraviolet photoelectron spectroscopy (UPS) has been applied to the investigation of the electronic structure of oligothiophenes with 4–8 thiophene rings. In a series of α‐linked oligomers (αn with n being the number of rings), a systematic evolution of the π band is observed. Several peaks which correspond to the π band are observed in the region of 0.7–3 eV below the Fermi level (EF), and the bandwidth becomes broader with increasing n. The nonbonding π band is observed at 3.5 eV below EF and its energy is almost independent of the number of thiophene units. UPS spectra of α7 and α8 are fairly similar to the spectra of polythiophene, showing that these oligomers are good model compounds of the polymer. The ionization threshold energy of α7 and polythiophene was observed to be 5.3 eV. The effect of irregularity on the π‐electron system was also studied by using oligomers which contain a β linkage or a vinylene group at the middle of the molecule. The UPS spectra showed that the β linkages significantly ...

Ultraviolet photoemission study of oligothiophenes: π-band evolution and geometries

Design and performance of a plane-grating monochromator at UVSOR facility are described. Two monochromators of the present design have been constructed and are used for optical and photoelectron spectroscopy of solids.

A plane-grating monochromator for 2 eV ≤ hv ≤ 150 eV

A grazing incidence constant deviation monochromator with a spherical concave grating was fabricated for use with undulator radiation. It has a simple scanning mechanism with fixed entrance and exit slits, as well as fixed directions of incident and exit beams. It has been installed into an undulator beamline at IMS. Synchrotron radiation from a bending section also can be introduced into the monochromator by using premirrors. The energy resolution of the monochromator is better than 70 meV at the photon energy of 94 eV with 10‐μm slits, when the bending magnet synchrotron radiation is used. The spot size of the zeroth‐order light is 1.5(v)×2.0(h) mm2 at the distance of 1.85 m behind a postmirror. The photon flux of the undulator radiation behind the exit slit is approximately 1×1012 photons/s at the peak, when both the entrance and exit slits are 100 μm (ΔE=0.08 eV) and the stored current is 50 mA.

Constant deviation monochromator for the range 100 Å≤λ≤1000 Å

A constant‐deviation constant‐length spherical grating monochromator was constructed at the bending‐magnet beamline 8B1 of the UVSOR. The monochromator has a simple scanning mechanism with a fixed position of the entrance and exit slits, as well as fixed directions of incident and exit photon beams. The monochromator was designed to cover the photon energy of 31–620 eV with three interchangeable laminar gratings (1080 lines/mm: R=15 m, 540 lines/mm: R=15 m, 360 lines/mm: R=7.5 m). All gratings are original gratings fabricated on synthetic quartz substrates and coated with Au. The resolving power evaluated by ray tracing at the lowest energy of each grating with 10 μm slit width is ∼4400 for two high energy gratings and ∼7000 for a low energy grating. In the preliminary performance check, it was found that the output spectrum of the monochromator with G1 grating extends up to 870 eV with resolving power of ∼4000 at 400 eV with 10 μm slits.

Construction of constant‐deviation constant‐length spherical grating monochromator at UVSOR

A focusing soft x‐ray beam line equipped with a focusing premirror and a double crystal monochromator (DXM) has been constructed at BL1A in the UVSOR. An elliptically bent cylindrical mirror was used as the focusing premirror in order to attain both horizontal and vertical focusing at the sample position. Ray tracing shows that the size and shape of the focused spot at sample position with an elliptically bent cylindrical mirror is almost the same as that of an ellipsoidal mirror. Measured spot size of the monochromatic x‐ray beam at the sample position was about 2 mm wide and 1 mm high, which is in good agreement with the result of ray tracing. Monochromatic x rays were observed up to 4 keV even after reflection by a platinum coated premirror with grazing angle of 1°.

Construction of focusing soft x-ray beamline BL1A at the UVSOR

A new plane‐grating monochromator and calibration chamber were constructed for the radiometric calibration of plasma diagnostic devices. The monochromator is designed to cover a wavelength range of 1.8–238 nm with moderate resolving power of 500±200 in λ/Δλ. To check the performance of the monochromator, spectral distribution and resolution were measured by experiments on photoemission from solids and photoionization of rare gases. The characterization of multilayer reflectors by the use of this beamline is demonstrated.

Kusuo Sakai

Papers

A plane-grating monochromator for 2 eV ≤ hv ≤ 150 eV

Constant deviation monochromator for the range 100 Å≤λ≤1000 Å

Construction of constant‐deviation constant‐length spherical grating monochromator at UVSOR

Construction of focusing soft x-ray beamline BL1A at the UVSOR

A plane-grating monochromator for radiometric calibration