A polyethyleneimine (PEI) interlayer has been applied on indium tin oxide (ITO) to improve electron injection in organic devices including inverted organic light-emitting diodes (OLEDs). To understand the improvement effect by PEI insertion, the energy level alignment at bis(10-hydroxybenzo[h]quinolinato)beryllium (Bebq2)/PEI/ITO interfaces was investigated by UV photoemission spectroscopy (UPS). The deposition of a PEI layer was found to reduce the absolute work function of ITO by 1.4 eV. The vacuum level shifts at Bebq2/ITO and Bebq2/PEI interfaces were also determined as 0.3 eV and 0.1 eV in the direction to reduce the electron injection barrier, respectively. Thus the work function reduction by PEI and downward vacuum level shift at the Bebq2/PEI interface can contribute to the improvement effect. Kelvin probe measurement revealed the weak orientation polarization in Bebq2 film with the bottom side positively polarized. This polarization polarity is also advantageous for electron injection in inverted devices.

By taking advantage of three-terminal capacitance-voltage (TT-CV) measurement, we investigated a formation of trapped charge in pentacene(Pn)-based organic field-effect transistors (OFETs) during the bias stress measurement. The shift of the turn-on voltage in transfer curve correlated well with the increase of trapped charge estimated from TT-CV curves. Moreover, TT-CV measurement revealed that the trapped charges were distributed inhomogeneously at the vicinity of the pentacene/insulator interface, indicating that the current does not obviously affect their formation. Thus we suggested that the trapped charges are formed by keeping Pn molecules as unstable cation (hole state) by the prolonged bias stress.

Charge injection property of organic thin film devices is a key issue to understand the device operation. Displacement current measurement (DCM) is a powerful technique to probe the charge injection behaviors in terms of a change in the apparent capacitance of test devices. However, it requires to suppress actual current flowing through the device for investigating the details of interface phenomena. We propose here the use of ionic liquids (ILs) as a top contact insulator in organic metal-insulator-semiconductor (MIS) structures. Because of the high stability and dielectric constant of the ILs, the external applied voltage was applied mainly to the organic layer with suppressing the actual current. The DCM curves of Pt wire/IL/α-NPD/ITO structure were measured, and they actually show the signals due to the hole injection from theITO to α-NPD layer and accumulation at the IL/α-NPD.

The channel formation process of a pentacene ambipolar field-effect transistor was studied by displacement current measurement (DCM). We proposed a modified measurement circuit of DCM in order to investigate the channel formation at the organic/insulator interface under transistor operation. We observed an additional terrace structure between the depletion and accumulation states when the drain voltage is applied. The capacitance at the terrace structure corresponds well with that in pinch-off condition. DCM enables us to understand the operation mechanism of the organic FET in more detail.

High-resolution angle resolved ultraviolet photoelectron spectroscopy measurements were conducted on rubrene single crystals successfully through relief of the sample charging assisted by a laser illumination. Significant dispersion of the valence band was clearly resolved. The band width W and the hole effective mass mh* were estimated to be 0.4 eV and 0.7m0, respectively, along the most conductive direction. The present results strongly suggest that the transport nature in rubrene single crystals should be described in the band transport framework of a delocalized charge carrier.

The influences of self-assembled-monolayers (SAMs) modifications on the gold electrodes in organic field effect transistors are studied using benzen-thiols (BzTs) having amino (NH2), nitro (NO2) and methyl (CH3) substituents and pentacene as an organic semiconductor. The field effect transistor (FET) characteristics are found to be very sensitive to the preparation condition of the SAMs modified surfaces. This can be rationalized in terms of the barriers against carrier injections made by SAMs and this situation can be monitored using a surface contact-angle method. The enhancement in the field effect mobilities μ is observed when the SAMs modifications to the electrodes are applied, and this could be caused by the higher efficiency of carrier injection attained by the hole transfer from SAMs to pentacene molecules.

In various organic electronic devices, interfaces formed by organic layers can play important roles. We have been studying various organic interfaces for clarifying their structure and electronic structure. In this talk, we will report our recent study of the effect of various types of doping for a variety of dopants – residual impurity, atmospheric gases, and metallic and organic intentional dopants. In particular, detailed and quantitative information about the effect of oxygen from the viewpoint of electronic structure was obtained for titanyl phthalocyanine (TiOPc), and the results corresponded well with the recent report of atmospheric effect on orga nic field effect transistor.

We investigated formation processes of a porous anodic alumina film on a p-type silicon (Si) substrate using infrared absorption spectroscopy in the multiple internal reflection geometry (MIR-IRAS). We observed drastic IR spectral changes when porous anodic alumina film approached interfaces between an aluminum (Al) layer and a Si substrate. The intensity of the IR absorption peaks due to water (H2O) molecules and silicon oxides (SiO2) increased simultaneously with a spike of anodic current density. The IR spectral changes indicated that the penetration of electrolytes brought about inhomogeneous oxidation of a Si substrate surface. We observed that the arrangement of the SiO2 nanodots closely reflected that of pores of a porous anodic alumina film. IR absorption peaks due to porous anodic alumina finally disappeared. The formation of SiO2 nanodots on a Si substrate promoted penetration of electrolytes to peel the porous anodic alumina film off it.

Interfacial band offset and band bending of organic semiconductors are critical to understand and improve organic photovoltaic cells. In this study, the energy level alignment of fullerene(C60) / metal-free phthalocyanine (H2Pc) interface which is one of the model interfaces of organic photovoltaic cells has been investigated using UV and X-ray photoemissions. For both ‘H2Pc on C60’ and “C60 on H2Pc' interfaces, 0.3 eV downward energy level shift was observed in XPS at the interface formation. This energy shift is quite steep in contrast to the band bending observed for C60/metal interfaces in our previous study, where thickness of 500nm was required to achieve 0.21eV band bending to get Fermi level alignment between metal electrode and C60. To clarify the origin of the band bending, the effect of the insertion of C60-H2Pc co-deposited layer between C60 and H2Pc layers was also investigated. The result suggested that possible doping of H2Pc to C60 is not main origin of the observed energy shift. We also found that the vacuum level shift at H2Pc/C60 interface is strongly dependent on the deposition sequence of the interface formation.

Recently we found high and persistent spontaneous buildup of the surface potential upon vacuum vapor deposition of tris(8-hydroxyquinolinato) aluminum(III)(Alq3) on Au substrate under dark conditions (28 V for 560 nm thick Alq3 film). Such giant potential is removed by visible-light irradiation. These natures of the film suggest the possible applications to various organic devices such as memory devices. In this study, we investigated the retention time of the giant potential and the mechanism of the light-induced depolarization in order to discuss the feasibility of the device applications. The observed decay rate of the surface potential in vacuum condition was roughly 10 % loss in 10 years, which is enough for memory devices. As to the decay rate by light-irradiation, the observed rate was successfully reproduced by a theoretical simulation based on the photo-induced randomization of oriented Alq3 molecules.

We report on the recent progress of our continued effort for the preparation and characterization of well oriented films of tetratetracontane (n-C44H90) and perfluorotetracosane (n-C24F50), which are good model molecules of fundamental polymers, polyethylene (CH2)n and poly(tetrafluoroethylen) (CF2)n, vacuum-deposited on metal surfaces. When the surface of the metal substrate was oxidized, the deposited chain molecules are aligned with their molecular axes vertically standing on the surface, while they lie flat on the surface when atomically clean Cu(100) surface was used. In the case of n-C44H90 on Cu (100), low-energy electron diffraction (LEED) studies revealed that the molecules were also azimuthally aligned with the chain axis almost parallel with the <100> and <010> axis, and infrared reflection-absorption spectroscopy (IR-RAS) enabled detailed studies of the film structure at increasing film thickness._By applying the technique of angle-resolved ultraviolet photoelectron spectroscopy (ARUPS) to these flat-lying molecules, the intramolecular energy-band dispersion relation could be determined, The results agreed well with the results deduced from the previous work using vertically standing molecules assuming an inner potential V0. of –5 eV, confirming the validity of the estimation of V0. Excellent agreement was found between the observed and simulated spectra using the independent-atomic-center (IAC) approximation. Also the value of the vacuum level shift by molecular deposition was deduced for TTC/Cu(100) system to be – 0.3 eV. As for n-C24F50, the E(k) relation deduced in our previous study using vertically standing molecules showed discrepancy with band calculations. The detailed reexamination of the experimental data using IAC calculations for the previous report gave more realistic estimation of V0, and the newly deduced E(k) relation using this data showed good correspondence with theoretical band calculations, thus removing the difficulty in the previous work.

The interfaces of organic materials with other solids play important roles in the function of various organic devices such as organic light-emitting diodes (OLEDs), spectral sensitization in photography, organic solar cells, and electrophotography. Also they should be important in future molecular devices, both in the central part of the device and at the connection with outside circuits. However, serious experimental examination of such interfaces has started only recently. In this talk we focus our attention on the organic/metal interfaces, and summarize our understanding about (1) the energy level alignment right at the interface, and (2) possible band bending within an organic layer, mainly using the techniques of UV photoemission spectroscopy (UPS) and Kelvin probe method. As for (1), the formation of electric dipole layer was observed in most organic/metal interfaces, and its origin is discussed. As for (2), recent examination of the existence/absence of band bending in ultrahigh vacuum will be reported. It is also pointed out that there can be much effects of (i) atmosphere at sample preparation and measurements, and (2) chemistry and interdiffusion at the interface.

Despite the fast-paced new developments and improvements of applications incorporating conjugated organic materials, rather little effort is made to obtain well-defined structures and thin films of these in order to exploit the highly anisotropic properties (e.g. conductivity, polarized light emission) of most conjugated organic materials. It has been shown that oligomers, which can easily be evaporated under ultrahigh vacuum conditions, exhibit heteroepitaxial growth on various substrates. Depending on the nature of the interaction of the organic molecules with the substrate different orientations can be obtained, as well as by simply changing the conditions of evaporation. Therefore, by going from amorphous or polycrystalline structures to highly textured or single crystalline ones new insights into the physical properties of the materials can be expected, and also new applications. We studied the formation of well-defined films of the electroluminescent oligomer p -sexiphenyl on various metal and semiconductor surfaces and determined the orientation of the molecules by means of low energy electron diffraction (LEED), infrared reflection absorption spectroscopy (IRRAS) and atomic force microscopy (AFM). Depending on the substrate material we found that the molecules are oriented either parallel or perpendicular to the substrate surface. In addition to that, highly oriented 6P films were doped with potassium, and the change in the molecular vibrational signature was investigated with IR-RAS.

Interfacial electronic structures related to organic electroluminescent (EL) devices were studied by UV photoemission spectroscopy (UPS). The two classes of interfaces studied were: (1) interfaces in a typical multilayer device AI/AIq3TPD/ITO, where Alq3 is tris(8-hydroxyquinolino)- aluminum, TPD is N,N×-diphenyl-NN×-(3-methylphenyl)- 1, 1‘-biphenyl-4,4’-diamine, and ITO is indium tin oxide, and (2) TTN/metals and TCNQ/metals interfaces, where TTN is tetrathianaphthacene and TCNQ is tetracyanoquinodimethane. The UPS studies of the specimen formed by the successive deposition of TPD, Alq3, and Al on ITO revealed interfacial energy diagrams, with the vacuum level shift of - 0.25 eV (downward) and - 0.1 eV (downward) at the TPD / ITO and the Alq3 / TPD interfaces, respectively. The deposition of TTN and TCNQ on metals showed opposite direction of the shift of the vacuum level, with the positive and negative charge at the vacuum side. This can be explained by considering the chargetransfer between the metal and the organic molecule, with these directions being consistent with the electron donating and accepting ability of these molecules.