石井 久夫

Charge accumulation at the organic heterointerfaces in multilayer organic light-emitting diodes (OLEDs) is an important process for understanding their device operation, efficiency, and degradation properties. Charge accumulation behavior has typically been analyzed in terms of the energy barrier and difference of the charge carrier mobility across heterointerfaces. In this study, we demonstrate that permanent dipole moments and their orientational order also play a significant role in the charge behavior at organic semiconductor interfaces. The charge accumulation properties of bilayer devices composed of polar or nonpolar molecules deposited on a 4,4'-bis[N-(1-naphthyl)-N-phenylamino] -biphenyl layer between the anode and cathode were examined by displacement current measurement and impedance spectroscopy. In addition, Kelvin probe measurements for the corresponding bilayer structures excluding the cathode were performed to analyze the relationship between the potential profile and charge accumulation properties of the bilayer devices. We found that several polar molecules including tris-(8-hydroxyquinolate) aluminum, 1,3,5-tris(1-phenyl-1H-benzimidazol-2-yl) benzene, 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP), and 1,3-bis[2-(4-tert-butylphenyl)-1, 3,4-oxadiazo-5-yl] benzene (OXD-7) are spontaneously ordered in evaporated films, and orientation polarization remains in bilayer devices. Orientation polarization leads to interface charge which determines the least amount of accumulated charge in the device under operation. The estimated interface charge density for these molecules ranged from -2.3 (OXD-7) to -0.5 (BCP) mC/m(2). Furthermore, impedance spectroscopy revealed that the presence of a permanent dipole moment can suppress the charge dispersion along the interface probably owing to the microscopic potential fluctuation formed in the vicinity of the interface. These results indicate that the permanent dipole moment and orientation polarization contribute to the efficient charge accumulation at organic heterointerfaces and are important parameters for understanding and controlling the charge carrier dynamics in multilayer OLEDs. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4724349]

Electronic structures of a novel interface formed by coverage of organic semiconductor zinc-phthalocyanine (ZnPc) onto a nanometer-thick Ag thin-film with quantized electronic states were investigated by means of angle-resolved photoelectron spectroscopy (ARPES). While the highest occupied molecular orbital position of ZnPc accords with those of a ZnPc overlayer on a bulk Ag substrate having been reported previously, an interface state (IS) revealed different energy position with respect to the bulk case. Absence of the angular-dependence of the IS position strongly suggests that this state is localized at each ZnPc molecule. © 2012 The Surface Science Society of Japan.

We investigate different parameters influencing the occurrence of s-shaped current voltage (j-V) characteristics in planar heterojunction organic solar cells. It is shown how substrate modification, purity of the active organic material as well as variation of the top contact can affect the shape of the j-V curves. The studies are performed on vacuum-evaporated planar heterojunction solar cells with diindenoperylene (DIP) as electron donor and fullerene C-60 as acceptor. The focus is on the fill factor and forward current being the most direct indicators for s-shapes in j-V curves. We find that the main effect of substrate heating during film growth can be assigned to changes in energy barriers rather than to the modification of morphology and crystallinity, which is also influenced by elevated substrate temperatures. The decisive role of the barrier height between the anode work function and the HOMO (i.e., highest occupied molecular orbital) level of the donor is approved by comparing hole-injection layers with different work functions. By using donor materials of different purity we find a correlation between charge carrier mobilities and fill factors. Finally, it is demonstrated that an exciton blocking interlayer is essential to get high fill factors when aluminum is used as top contact, but is dispensable for samarium as cathode material. This finding can be ascribed to the protective effect of the interlayer from aluminum diffusion into the active semiconductor rather than to its role as exciton diffusion barrier. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.3692050]

We present an investigation of the interface between organic semiconductor films and metal substrates (organic/metal interface) using photoelectron yield spectroscopy (PYS) as the probing technique. PYS studies were conducted on the pentacene/Au, copper phthalocyanine (CuPc)/Au, and perfluorinated zinc phthalocyanine (F16ZnPc)/Au, and the results were compared with literature results obtained using conventional ultraviolet photoemission spectroscopy (UPS). PYS is advantageous for probing the electronic structure of the organic/metal interface because of the relatively long mean free path of photoexcited electrons with very low kinetic energy in PYS, which enables the detection of the photoelectrons from the metal substrate buried deep in the organic film. We demonstrate herein that the use of PYS reduces the significance of the final state effect of the electronic density surrounding the photohole at the organic molecule generated after the photoemission; this effect is known as the electric polarization effect. Although this effect has a significant influence on the results obtained using conventional UPS, the reduced influence of the final state effect in PYS makes it possible to construct an energy level diagram at the organic/metal interface with greater accuracy than can be achieved with UPS. In addition, a novel mechanism of the photoelectron detection for PYS enables us to apply PYS to very thick organic films, and therefore, PYS provides a reliable value of ionization energy for organic films without the influence of the substrate.Because the interface electronic structure has a significant influence on the carrier injection properties of organic devices, the increased reliability of the information obtained by PYS will render it very useful for the improvement of device performance as well for understanding their operation principles. (C) 2011 Elsevier B. V. All rights reserved.

Tris(8-hydroxyquinolinato) aluminum (Alq(3)) is known to induce negative interface charge in multilayer organic light-emitting diodes (OLEDs) because of orientation polarization when fabricated in dark. Recently, it was reported that light illumination during the deposition of Alq(3) induces bulk charge in addition. In this study, the illumination effect on the performance of Alq(3)-based OLED was examined. In comparison with the device fabricated in dark condition, the slight increase of the current density was observed. Light emission is also increased for constant driving voltage, while the luminous efficiency showed decrease.

For better understanding the operation of organic electronic devices, the information of carrier behavior is indispensable. In this study, we propose a new usage of time-of-flight (TOF) technique to examine the carrier behavior in operating device. From the measurement for lTO vertical bar alpha-NPD vertical bar Alq3 vertical bar Al device, which is a widely investigated organic light emitting diode, we have demonstrated the feasibility of TOF measurement for double-layer device in operating condition. The obtained TOF signal includes two kinds of useful information: (i) carrier transporting nature under the influence of actual current flow and (ii) delayed-transport and blocking of carriers at hetero interface.

Displacement current measurement (DCM) is a simple but powerful tool for exploring charge carrier dynamics in organic semiconductor devices. In the first section, we review the basic concept of DCM and how it detects the charge injection, extraction, accumulation, and trapping behaviors in organic semiconductor devices within a quasistatic regime. Subsequently, we present applications of this technique to investigate the device properties of tris-(8-hydroxyquinolate) aluminum (Alq(3))-based organic light-emitting diodes. We observed that light irradiation during device fabrication induces additional negative space charges and charge traps in the Alq(3) layer. In addition, the device containing the illuminated Alq(3) film exhibits a lower luminous efficiency and shorter lifetime compared to the device fabricated in dark conditions, possibly because of the additional hole accumulation in the illuminated Alq(3) film. DCM detects the formation of charge traps in the aged devices, decay of the negative space charge, and increase in hole injection voltage with device aging. The origins of these behaviors can be attributed to orientation polarization and charge traps in Alq(3) film. (C) 2012 Society of Photo-Optical Instrumentation Engineers (SPIE). [DOI: 10.1117/1.JPE.2.021214]

Displacement current measurements (DCMs) were used to investigate transient carrier behavior during the carrier accumulation process in pentacene (Pn) metal-insulator-semiconductor structures [Au/Pn (100 nm)/SiO2 (100 nm)/p-type Si]. We propose a modified DCM technique that applies a combined voltage waveform of ramp and plateau parts to observe the transient carrier behavior. This allows to simultaneously detect both the effective capacitance of the device and the transient response which reflects the dynamic carrier behavior in the device. In addition, the long decay time during carrier spreading at the Pn/SiO2 interface agrees well with the inverse of the relaxation frequency, which was estimated by capacitance-frequency measurement. This result demonstrates that DCM is a powerful tool for investigating both the quasi-static and dynamic behavior of carriers in organic devices. (C) 2011 Elsevier B.V. All rights reserved.

We present the investigation of the vibrational and electronic states of tris(8-hydroxyquinoline) aluminum (Alq3)/Al (Alq3 on Al) interfaces by using two-color infrared-visible sum frequency generation (SFG) spectroscopy. The visible wavelength dependence of the SFG spectra of the 2 nm thick Alq3/Al consists of the vibrational bands derived from the Alq3 at the Al interfaces. The intensities of the peaks derived from the ring stretching modes of the quinolate ligands were significantly enhanced due to the double resonance effect. In contrast, the SFG electronic spectrum obtained from the output photon energy dependence of the SFG peak amplitudes derived from the C=C bands of the Al on Alq3 interfaces does not show the wavelength dependences, indicating that the electronic-resonance associated with the π-π* transitions in the quinolate rings are almost vanished at the Al deposited on the Alq3. The disappearance of the electronic-resonance of the C=C stretching modes must be caused by the perturbation of the HOMO and LUMO of pristine Alq3 by the interaction with the Al. The spectral features of the two-color SFG spectra of the Al/LiF/Alq3 system show quite different behavior from those of Alq3/Al and Al/Alq3. The shift of the C=C stretching modes toward lower frequencies is indicative of the formation of the Alq3 anionic states upon reaction with Li at the interface. Additional broad bands around 1335 and 1450 cm-1, which show the weak excitation wavelength dependence, might be due to the existence of the Li-reacted graphitic carbon-like Alq3. © 2011 American Chemical Society.

The carrier behavior in pentacene based organic field-effect transistors (OFETs) was investigated by impedance spectroscopy measurements (IS). We clearly observed the carrier injection from the electrode and accumulation at the pentacene/insulator interface. We propose a method for IS in transistor operation by using a battery source for applying the drain voltage. In these measurements, an additional structure is observed where the capacitance gradually increases between injection and accumulation processes. From the comparison of this result and the analyzed curve, which reflects the injection properties from the electrode, we conclude that this additional structure originates from the charge sheet spreading into the channel region. This technique enables us to observe the carrier injection into the channel region and to investigate the charge sheet formation from off-state to on-state and vice versa of OFETs.

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. © 2011 Materials Research Society.

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 the ITO to α-NPD layer and accumulation at the IL/α-NPD. © 2011 Materials Research Society.

2-amino-4,5-imidazoledicarbonitrile (AIDCN) has been considered as a promising material toward organic nonvolatile memory application. Aiming for achieving a deep understanding of the origins of the bistable electric behavior of the AIDCN-based memory devices and, in particular, of the reported drastic improvement of the device performance by replacement of the Al top-electrode material with Ag, we elucidated the electronic structures of the interfaces between AIDCN and electrode metals (Ag and Al), as well as the bulk of AIDCN, by photoemission spectroscopy (PES). Ionization energy of AIDCN was determined to be 6.6 eV that was also certified by photoelectron yield spectroscopy measurements. For the AIDCN/Ag interface, the highest occupied molecular orbital (HOMO)-derived peak was clearly resolved in the PES spectra even when the thickness of the AIDCN overlayers were below the monolayer. The peak position showed significant shifting (similar to 0.5 eV) to the higher binding energy side with formation of the multilayers. Probing angle dependent spectra revealed that the AIDCN monolayer on Ag shows different molecular orientation from that of the multilayers. These results strongly suggest that a sharp interface was formed between the Ag surface and AIDCN overlayers. In contrast, the PES spectra of the AIDCN/Al interface showed not a peak but a diffuse tailing structure in the HOMO region until the bulklike phase was formed. No angular dependent change appeared on the PES spectra in the low coverage region, which suggests the molecular orientation was random. The hole injection barriers, derived from energy offsets between the edges of the HOMO-derived spectral features and the Fermi level, of AIDCN/Ag and AIDCN/Al interfaces are estimated to be 2.0 eV and 1.0 eV, respectively. (C) 2010 American Institute of Physics. [doi:10.1063/1.3481388]

We report space charge formation in tris-(8-hydroxyquinolate) aluminum (Alq(3))-based organic light-emitting diodes induced by light irradiation and ion-gauge (IG) operation during device fabrication. An analysis of the capacitance-voltage curves of the light-treated devices reveals the presence of uniformly distributed negative space charges in the Alq(3) layer. Spatial inhomogeneity of the orientation polarization as well as electrons trapped in the Alq(3) film can be the origin of the negative space charge. We also found that positively charged species can be included in the device due to IG operation.

The electronic structure of rubrene single crystals was studied by angle-resolved ultraviolet photoelectron spectroscopy. A clear energy dispersion of the highest occupied molecular orbital-derived band was observed, and the dispersion width was found to be 0.4 eV along the well-stacked direction. The effective mass of the holes was estimated to be 0.65(+/- 0.1)m(0). The present results suggest that the carrier conduction mechanism in rubrene single crystals can be described within the framework of band transport.

We carried out a displacement current measurement (DCM) study to examine the resistance to hole injection and the electric field distribution in tris(8-hydroxyquinoline) aluminum-based organic light-emitting diodes. The DCM curves of the devices with/without a copper phthalocyanine (CuPc) interlayer of different thicknesses were measured as a function of the sweep rate of applied triangular wave voltage from 1 to 10000 V/s. The presence of a thin CuPc interlayer markedly shifted hole injection voltage to the higher side with increasing sweep rate and resulted in a hump in the backward sweep of the DCM curves. These modulations in DCM curve were reduced or disappeared in the thick CuPc and CuPc-free devices. We found that the modulations originate from the hole injection resistance induced by the CuPc interlayer. The electric field distribution in the devices under operation is estimated taking into account the band bending in the CuPc interlayer.

2-amino-4,5-imidazoledicarbonitrile (AIDCN) is known as a promising material for organic nonvolatile memory application, and it has been reported that the performance of the AIDCN-based devices strongly depends on electrode metals. In the present study, we carried out photoemission spectroscopy (PES) measurements on the organic-on-metals (Ag and Al) and metals-on-organic model interfaces of an AIDCN-based memory device. The AIDCN overlayers on the Ag surface formed a sharp interface without apparent chemical interaction, while clear signs of a chemical reaction between the metal and AIDCN and formation of Al clusters in the AIDCN overlayers were revealed on the Al surface. Tendencies of the vacuum level shift and the highest occupied molecular orbital (HOMO) positions depending on the AIDCN coverage looked similar irrespective of the substrates. In the case of the metals-on-organic interfaces, in contrast, the shifting tendencies were opposite depending on the top-electrodes;the Ag-on-AIDCN interface showed upward band-bending with increasing the metal thickness, whereas downward bend-bending was observed for the Al-on-AIDCN interface. A possible origin of the band-bending is explained as intercalation of Ag or Al that behaves as acceptor- or donor-centers, respectively, into the AIDCN film.

The electronic structure of the interfaces formed after deposition of MoO3 hole-injection layers on top of a polymer light-emitting material, poly(dioctylfluorene-alt-benzothiadiazole) (F8BT), is studied by ultraviolet photoelectron spectroscopy (UPS), X-ray photoelectron spectroscopy and metastable atom electron spectroscopy. Significant band bending is induced in the F8BT film by MoO3 "acceptors" that spontaneously diffuse into the F8BT "host" probably driven by kinetic energy of the desposited hot MoO3. Further deposition leads to the saturation of the band bending accompanied by the formation of MoO3 overlayers. Simultaneously, a new electronic state in the vicinity of the Fermi level appears on the UPS spectra. Since this peak does not appear in the bulk MoO3 film, it can be assigned as an interface state between the MoO3 overlayer and underlying F8BT film. Both band bending and the interface state should result from charge transfer from F8BT to MoO3, and they appear to be the origin of the hold-injection enhancement by the insertion of MoO3 layers between the F8BT light-emitting diodes and top anodes.

The electronic structure of the interfaces formed after deposition of MoO3 hole-injection layers on top of a polymer light-emitting material, poly(dioctylfluorene-aft-benzothiadiazole) (F8BT), is studied by ultraviolet photoelectron spectroscopy (UPS), X-ray photoelectron spectroscopy and metastable atom electron spectroscopy. Significant band bending is induced in the F8BT film by MoO3 "acceptors" that spontaneously diffuse into the F8BT "host" probably driven by kinetic energy of the deposited hot MoO3. Further deposition leads to the saturation of the band bending accompanied by the formation Of MoO3 overlayers. Simultaneously, a new electronic state in the vicinity of the Fermi level appears on the UPS spectra. Since this peak does not appear in the bulk MoO 3 film, it can be assigned as an interface state between the MoO 3 overlayer and underlying F8BT film. Both band bending and the interface state should result from charge transfer from F8BT to MoO3, and they appear to be the origin of the hole-injection enhancement by the insertion of MoO3 layers between the F8BT light-emitting diodes and top anodes. © 2009 WlLEY-VCH Verlag GmbH &. Co. KCaA.

The electronic states of single crystalline (SC) rubrene were experimentally observed by photoelectron yield spectroscopy without the sample charging problem. The ionization energy (I-s) in the SC phase was determined to be 4.85 (+0.05) eV, which is reduced by 0.45 eV compared to that of the amorphous film. The changes in the electronic states during photo-oxidation reaction and under ambient air were also observed directly to reveal the further reduction in I-s, which can be attributed to the generation of polar oxide molecules and reversible physisorption of H2O, respectively. (C) 2008 American Institute of Physics. [DOI: 10.1063/1.2998650]

We examined the effects of ambient light on the device properties of an organic light-emitting diode, indium tin oxide/4,4'-bis[N-(1-naphthyl)-N-phenylamino]-biphenyl (alpha-NPD)/tris-(8-hydroxyquinolate) aluminum (Alq(3))/Al, during fabrication using displacement current measurement. Light irradiation induces a shift in the threshold voltage for hole injection and results in the formation of charge traps in the Alq(3) layer. The voltage shift implies a reduction in charge density at the alpha-NPD/Alq(3) interface. The origin of the interfacial charge can be attributed to dipole moment ordering in the Alq(3) layer. (C) 2008 American Institute of Physics.

The electronic structures of various materials have been investigated using photoemission measurements. The sample charge-up problem, however, limits the application of these measurements to insulating materials. In this study, we propose a capacitive photocurrent detection method that permits photoelectron yield spectroscopy measurements of extremely insulating materials in both vacuum and ambient pressure conditions. The mechanism of detection is discussed, and the application to gold and rubrene films on mica substrates is demonstrated. (C) 2008 American Institute of Physics.

We examined the driving mechanism of indium-tin oxide (ITO)/4,4-bis[N-(1- naphthyl)-N-phenyl-amino]biphenyl (α-NPD)/Tris-(8-hydroxiquinolate) aluminum (Alq3)/cathode type organic light-emitting diode (OLED) by using a displacement current measurement (DCM). The DCM enables us to directly calculate the amount of accumulated charge. There exists a maximum value in the amounts of the blocked holes at α-NPD/Alq3 interface. The maximum value was about 120 nC/cm2, this value is consistent with the density of the fixed interfacial charge proposed by Brütting et al. By using hole-only device with Au cathode, we also investigated the hole blocking and the subsequent overflow of hole current beyond the interface. The observed feature can be explained by the hole blocking due to the interfacial charge rather than by that due to the HOMO mismatch at the interface.

For the purpose of close understanding and further improvement of the organic electronics devices, elucidation of the electronic structures of organic-electrode interfaces as well as the organic materials themselves in the ambient pressure condition is highly anticipated. Ultraviolet Photoelectron spectroscopy (UPS) is not a satisfactory technique for this purpose, because UPS needs high vacuum condition in principle and the sample charging problem hinders characterization of the electronic structures of specimens of insufficient conductivity (e.g. organic thick-films and crystals). In the present study, the authors proved that one can observe the occupied electronic structures of insulating materials by photoelectron yield spectroscopy (PYS). A novel mechanism enabling photoelectron measurement on insulators is specified. By adopting this method, the electronic structures of rubrene thin films at a buried interface beneath the rubrene-peroxide layers were directly observed in the ambient air condition. These merits of PYS can be extended to investigate the electronic structure of organic devices.

A giant surface potential (GSP) has been observed on a tris-(8-hydroxyquinolate) aluminum (Alq(3)) film deposited on a glass or a metal substrate under dark conditions. However, the effects of a GSP oil the device properties of Alq(3)-based organic light-emitting diodes (OLEDs) has not been considered. In this paper, we report on the effects of ambient light during the fabrication of an Alq3-based OLED on the device properties by displacement current measurement. We found that the light irradiation significantly reduces the density of charge existing at the 4,4'-bis[N-(1-naphthyl)-N-phenylamino]-biphenyl/Alq(3) interface and results in the formation of charge traps in the Alq3 layer. Considering the similarities between the GSP and the interfacial charge, they can be attributed to the same origin; the orientaion polarization of Alq(3) film.

The effect of atmospheric gases on the ionization energy (I) of titanyl phthalocyanine thin film was investigated by an apparatus of photoemission yield spectroscopy developed for the measurements of I for the same specimen both in vacuum and under gaseous atmosphere. It was found that the value of I is affected by the exposure to various ambient gases (i.e., air, nitrogen, and oxygen of 1 atm, and water vapor corresponding to 27% relative humidity at 300 K), and that the effect strongly depends on the gas. The ionization energies in vacuum could be determined as the onset I-0 of the cube-root plot of the photoemission yield as a function of photon energy. When the sample was exposed to gases, the cube-root plot still gives an onset, but often a long tail at the low-energy side with another onset I-t was also observed. The first exposure to air did not affect both I-0 and I-t much, while the following evacuation-exposure cycles caused mostly reversible decrease and increase of I-0 and I-t by about 0.2 and 0.4 eV, respectively. Among the examined constituent gases of air, nitrogen was found to hardly affect both I-0 and I-t except for the small temporal decrease by 0.06 eV at the first exposure. Similar trends of the change of I-0 and I-t with those for air were found for oxygen, with the amplitudes of mostly reversible change being 0.1 eV for I-0 and 0.2 eV for I-t. Overlapped with these changes, a slow decrease of both I-0 and I-t, was also observed with a slope of 0.01 eV per day. For water, the trends were mostly similar with O-2, except that (1) the first exposure to water vapor showed small and large changes in I-0 (0.03 eV increase) and I-t (0.4 eV decrease) and (2) the amplitudes of the mostly reversible change of I-0 (0.3 eV) and I-t (0.05 eV) were much larger and smaller than those for O-2, respectively. A long-term decrease was also observed with a slope of 0.04 eV per day. These results indicate that the atmospheric effect by ambient air on I is mainly caused by water vapor, although oxygen also makes significant contribution. Since the observed trends are rather complex, there seem to be multiple factors affecting I, which is the energy difference between the vacuum level and the highest occupied molecular orbita. Possible microscopic mechanisms of the observed variation of I on these levels are also discussed in terms of the energy change in these levels.

Air-stable n-channel field-effect transistors based on thin films of the compound, N, N'-bis(4-trifluoromethylbenzyl) perylene-3,4,9,10-tetracarboxylic diimide (PTCDI-TFB), were fabricated, and the effects of surface treatment and substrate temperature at the film deposition on the electron mobility of the transistors were studied. The maximum mobility, 4.1 x 10(-2) cm(2) V-1 S-1 in the saturation region (1.7 x 10-2 cm(2) V-1 s(-1) in the linear region), was obtained in air for the film deposited at 95 degrees C on the SiO2 surface modified with hexamethyldisilazane. The high electron affinity of PTCDI-TFB estimated at 4.8 eV by photoelectron yield spectroscopy and UV-Vis absorption spectroscopy, which is ascribable to the trifluoromethylbenzyl groups, is likely to result in the observed stable transistor operation in air. (c) 2007 Elsevier B.V. All rights reserved.

Recently, it was proposed in the literature that the electron trap on a hydroxyl-containing dielectric interface of an organic field effect transistor (OFET) hinders its n type operation. The authors fabricated pentacene and fullerene OFETs with a hydroxyl-free insulating material, a long-chain alkane, i.e., tetratetracontate (TTC), C44H90 layer coated on the SiO2 dielectric layer. The displacement current measurements clearly demonstrated that the electron trap of the SiO2 surface is suppressed by the TTC layer. For a pentacene FET with an Al electrode and SiO2 dielectric layer, a p type operation was observed, while the operation mode was switched to the n type by the insertion of TTC on the SiO2 interface. By simple patterning of the TTC layer to produce a bipolar injection, the authors fabricated an ambipolar pentacene FET with a single kind of metal electrode. Thus TTC is a good material for the surface modification of a dielectric layer in OFETs. (c) 2007 American Institute of Physics.

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 s 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. © 2007 Materials Research Society.

The atmospheric effect on the ionization energy (I) of titanyl phthalocyanine (TiOPc) thin film was investigated by photoemission yield spectroscopy developed for the measurement of I for the same specimen both in vacuum and under gaseous atmosphere. The variation of I of TiOPc thin film induced by exposure to air was observed.

We have investigated in situ the hybridization and denaturation of DNA in aqueous solution using infrared absorption spectroscopy (IRAS) in the multiple internal reflection (MIR) geometry. We demonstrate that conformational changes of DNA strands due to hybridization (binding of two complementary single-stranded DNAs) and denaturation (separation of double helix at elevated temperatures) are reflected in the infrared absorption spectra in the frequency region where vibrational modes of the bases of DNA appear. Comparison with results of ab initio cluster calculation shows that hybridization produces the specific C=O carbonyl stretching vibration modes in the hydrogen-bonded base pairs. The ratio of absorbance of the C=O stretching peak at 1690 cm(-1) to the absorbance at 1660 cm(-1) provides a definitive metric for determining DNA hybridization. We also reveal that the C=O stretching vibration modes of the bases of a single strand is strongly influenced by the surrounding water molecules that may interact with the C=O groups of the bases. The present results suggest that MIR-IRAS is applicable to label-free, high-sensitive biosensors that provide insight about the gene expression and a variety of biological interactions such as DNA-protein interactions. (C) 2006 American Institute of Physics.

We have examined charge carrier injection from electrodes to tris(8-hydroxyquinoline) aluminum (Alq3) with insulator insertion layers by displacement current measurement (DCM). The inserted materials we used are LiF and a long chain alkane C44H90 (TTC). First, the device structures of Au (or Al)/TTC (or LiF)/Alq3/SiO2/Si were examined by DCM. In the case of the Au/Alq3 interface, we found that the threshold voltage for hole injection from the Au electrode to the Alq3 layer was much reduced by the insertion of a 10-nm-thick TTC layer. In the case of the Al/Alq3 interface, the threshold voltage for electron injection from the Al electrode to the Alq3 layer was reduced by the insertion of a 1-nm-thick TTC layer. The reduction in the threshold voltage was compatible to that in the case of LiF insertion. From these results, we can expect that TTC insertion can reduce the operation voltage of organic light-emitting diodes (OLEDs). However, no improvement in performance was observed in the Al/Alq3/TPD/indium tin oxide (ITO) device, indicating that the reduction in the threshold voltage for carrier injection does not directly correspond to efficient injection in the high current region.

In order to investigate a doping mechanism in organic semiconductors, we have studied the influence of oxygen exposure on pentacene and poly(3-hexylthiophene) (P3HT) field effect transistor (FET) using displacement current measurement (DCM). The DCM results revealed that the oxygen doping effect is small in the dark condition, but is much enhanced by light illumination (photoinduced doping). From the result of infrared reflection absorption in the multiple internal reflection geometry (MIR-IRAS) for P3HT film, we conclude that the charge-transfer reaction between P3HT molecule and O2 molecules is accelerated by the light illumination.