石井 久夫

Understanding of the electronic structures is indispensable for complete elucidation of the charge carrier behaviors in organic semiconductors. Although recent progress enabling accurate photoemission demonstrations of organic single crystals has greatly promoted such understanding, it had been achieved merely on partially oxidized surfaces by exposure to ambient conditions. In this study, we successfully prepared an oxide-free surface of the pentacene single crystal (PnSC) by cleavage in vacuum. X-ray and ultraviolet photoelectron spectroscopy measurements on the PnSC clean surface revealed improved energetic homogeneity of the C1s level and highest-occupied state in comparison to those of the partially oxidized surface.

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.

Designing molecular p-n heterojunction structures, i.e., electron donor-acceptor contacts, is one of the central challenges for further development of organic electronic devices. In the present study, a well-defined p-n heterojunction of two representative molecular semiconductors, pentacene and C-60, formed on the single-crystal surface of pentacene is precisely investigated in terms of its growth behavior and crystallographic structure. C-60 assembles into a (111)-oriented face-centered-cubic crystal structure with a specific epitaxial orientation on the (001) surface of the pentacene single crystal. The present experimental findings provide molecular scale insights into the formation mechanisms of the organic p-n heterojunction through an accurate structural analysis of the single-crystalline molecular contact.

Ambipolar switching behavior was observed in a silver nanoparticle (AgNP)-based single-electron transistor (SET) with tetra-tert-butyl copper phthalocyanine (ttbCuPc) as a molecular floating gate. Depending on the wavelength of the incident light, the stability diagram shifted to the negative and positive directions along the gate voltage axis. These results were explained by the photoinduced charging of ttbCuPc molecules in the vicinity of AgNPs. Moreover, multiple device states were induced by the light irradiation at a wavelength of 600 nm, suggesting that multiple ttbCuPc molecules individually worked as a floating gate. (C) 2016 The Japan Society of Applied Physics

Upon charge carrier transport behaviors of high-mobility organic field effect transistors of pentacene single crystal, effects of ambient gases and resultant probable 'impurities' at the crystal surface have been controversial. Definite knowledge on the surface stoichiometry and chemical composites is indispensable to solve this question. In the present study, high-resolution x-ray photoelectron spectroscopy (XPS) measurements on the pentacene single crystal samples successfully demonstrated a presence of a few atomic-percent of (photo-) oxidized species at the first molecular layer of the crystal surface through accurate analyses of the excitation energy (i.e. probing depth) dependence of the C1s peak profiles. Particular methodologies to conduct XPS on organic single crystal samples, without any charging nor damage of the sample in spite of its electric insulating character and fragility against x-ray irradiation, is also described in detail.

Tris(8-hydroxyquinoline) aluminum (Alq(3)) has been widely applied as a good electron-injecting layer (EIL) in organic light-emitting diodes. High-sensitivity photoemission measurement revealed a clear photoemission by visible light, although its ionization energy is 5.7 eV. This unusual photoemission is ascribed to Alq(3) anions captured by positive polarization charges. The observed electron detachment energy of the anion was about 1 eV larger than the electron affinity reported by inverse photoemission. This difference suggests that the injected electron in the Alq(3) layer is energetically relaxed, leading to the reduction in injection barrier. This nature is one of the reasons why Alq(3) worked well as the EIL. (C) 2016 The Japan Society of Applied Physics

L-cysteine, one of amino acids, is well known in the bioelectronics field as a linker between proteins and metal electrodes. The interface electronic structures between L-cysteine and metals are therefore very crucial because they dominate the charge carrier injection characteristics into such biological systems. However the interface electronic structures of L-cysteine and metals have been not well understood. In this study, the electronic structures at the interfaces of sequentially deposited L-cysteine layers on the metal surfaces of Au(111), Ag(111), and Cu(111) were investigated by thickness-dependent ultraviolet photoelectron spectroscopy (UPS). At the contact regions, the electronic structures of L-cysteine revealed modification with respect to its bulk phase and significant variation depending on the substrate. The electronic structure at the interfaces including work function, secondary electron cutoff (SECO), highest occupied molecular orbital (HOMO) onset, position of an interface state, charge injection barrier, and ionization energy were estimated for each case.

We investigated the charge transport characteristics of alkanemonothiol (CnH2n+1SH, n = number of carbons) molecular junctions by means cif transition voltage spectroscopy (TVS) based on the observations of the interface electronic structures. The minimum in the Fowler-Nordheim plot was observed at the average positive and negative sample biases of +1.23 and -1.44 V. These voltages (V-min) were insensitive to the molecular length. The low-energy ultraviolet photoelectron spectroscopy (LE-UPS) measurements revealed the presence of an Au-S bond at a binding energy of 1.4 eV with reference to the Fermi level of the Au substrates. The binding energy of the interface electronic state was independent of the molecular length. The TVS results were analyzed based on the LE-UPS results, including the differences in the measurement conditions. The results were consistently explained by the Au-S bond being responsible for V-min at the negative bias. In addition, another interface state was suggested to be responsible for V-min at the positive bias. The effects of the interface electronic structures besides the apparent barrier height should be considered to understand TVS of molecular junctions with wide energy gap molecules.

Although the long-term stability of organic light-emitting diodes (OLEDs) under electrical operation made significant progress in recent years, the fundamental underlying mechanisms of the efficiency decrease during operation are not well understood. Hence, we present a comprehensive degradation study of an OLED structure comprising the well-known green phosphorescent emitter Ir(ppy)(3). We use transient methods to analyze both electrical and optical changes during an accelerated aging protocol. Combining the results of displacement current measurements with time-resolved investigation of the excited states lifetimes of the emitter allows for a correlation of electrical (e.g., increase of the driving voltage due to trap formation) and optical (e.g., decrease of light-output) changes induced by degradation. Therewith, it is possible to identify two mechanisms resulting in the drop of the luminance: a decrease of the radiative quantum efficiency of the emitting system due to triplet-polaron-quenching at trapped charge carriers and a modified charge carrier injection and transport, as well as trap-assisted non-radiative recombination resulting in a deterioration of the charge carrier balance of the device. (C) 2015 AIP Publishing LLC.

We report on the charge carrier dynamics and their degradation phenomena in an organic light-emitting diode (OLED) doped by a thermally activated delayed fluorescence emitter; (4s, 6s)-2,4,5,6-tetra(9H-carbazol-9-yl) isophthalonitrile (4CzIPN). Displacement current measurement (DCM) data revealed the presence of negative interface charge originating from the spontaneous orientation polarization of the electron transport layer (ETL), 1,3,5-tris(1-phenyl-1H-benzimidazol-2-yl) benzene. The negative interface charge acts as a hole reservoir and thus confines the recombination zone near the emission layer (EML); 4,4'-bis(N-carbazolyl)-1,1'-biphenyl (CBP):4CzIPN (5 wt%)/ETL interface. By keeping the recombination zone far from the hole transport layer, 4,40-bis[N-(1-naphthyl)-N-phenylamino]- biphenyl (alpha-NPD), a better electroluminescence efficiency is expected because the Dexter energy transfer from the triplet state of 4CzIPN to that of alpha-NPD is suppressed. Moreover, we found an excellent linear relation between the accumulated hole amount and luminance losses owing to device aging. The results are consistent with hole traps originating at the degradation products of CBP as the main factor of device degradation. We suggest device architectures based on the charge carrier dynamics for better efficiency and lifetime. (C) 2014 Elsevier B.V. All rights reserved.

© Springer Japan 2015. The book covers a variety of studies of organic semiconductors, from fundamental electronic states to device applications, including theoretical studies. Furthermore, innovative experimental techniques, e.g., ultrahigh sensitivity photoelectron spectroscopy, photoelectron yield spectroscopy, spin-resolved scanning tunneling microscopy (STM), and a material processing method with optical-vortex and polarization-vortex lasers, are introduced. As this book is intended to serve as a textbook for a graduate level course or as reference material for researchers in organic electronics and nanoscience from electronic states, fundamental science that is necessary to understand the research is described. It does not duplicate the books already written on organic electronics, but focuses mainly on electronic properties that arise from the nature of organic semiconductors (molecular solids). The new experimental methods introduced in this book are applicable to various materials (e.g., metals, inorganic and organic materials). Thus the book is also useful for experts working in physics, chemistry, and related engineering and industrial fields.

高い電荷移動度を示すペンタセン単結晶は，有機固体内の電荷輸送メカニズムを解明するモデル物質の一つと注目されている。近年，有機半導体結晶の価電子バンド構造の測定が，角度分解型光電子分光法によって可能となった。しかし，これまでの研究では大気曝露を経た試料に対して測定が行われており，試料表面の酸化物の影響が懸念される。本研究では，ペンタセン単結晶を真空内で劈開し，得られた清浄表面のバンド分散を実測した。

In this chapter, we describe technical essences and several example works of angle-resolved photoelectron spectroscopy (ARPES) that is a surface science methodology to map out the electronic band structures of the matters. Successful results of demonstrating the valence band dispersion relations of crystalline organic semiconductor materials are introduced, which were acquired through resolution of inherent “sample charging†problems in photoelectron spectroscopy techniques. The effective mass of the valence hole and intermolecular transfer integral values of van-der-Waals molecular solids were directly derived as fundamental physical properties regulating the charge carrier transport in these solids. In addition, recent ARPES works on novel interface electronic structures of organic semiconducting molecules in contact with “quantum wells†in nanometer-thick metal thin-films are also reviewed.

© Springer Japan 2015. Photoelectron yield spectroscopy (PYS), in which total photoelectron yield is recorded as a function of incident photon energy, has been widely applied to determine the ionization energy of various organic electronic materials. PYS has some advantage complimentary to conventional photoelectron spectroscopy; (i) measurement environment is not limited to vacuum, (ii) sample charge-up problem is practically negligible, (iii) high sensitivity is available in vacuum, and so on. Thus, PYS is a powerful method to explore the electronic structures of organic materials and interfaces in practical situation. In this chapter, first we describe the basic principle and experimental setup of PYS. Then the applications to various organic materials and interfaces are described with the results of combined application of PYS and high sensitivity photoemission spectroscopy.

Electronic structures of donor-acceptor hetero-interfaces are crucial for performance of organic solar cell devices. In the present study, a well-defined organic molecular interface of pentacene single crystal and C&lt inf&gt 60&lt /inf&gt overlayer is elucidated by x-ray and ultraviolet photoelectron spectroscopy (XPS and UPS) and photoelectron yield spectroscopy (PYS). An energy level diagram at this hetero-molecular contact in the "head-on" orientation is derived, which reveals the vacuum level alignment and flat-band conditions.

The attenuation length (AL) of low energy photoelectrons inside a thin film of a pi-conjugated organic semiconductor material, 2,2',2 ''-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole), was investigated using ultraviolet photoelectron spectroscopy (UPS) and photoelectron yield spectroscopy (PYS) to discuss their probing depth in amorphous organic thin films. The present UPS results indicated that the AL is 2-3 nm in the electron energy range of 6.3-8.3 eV with respect to the Fermi level, while the PYS measurements which collected the excited electrons in a range of 4.5-6 eV exhibited a longer AL of 3.6 nm. Despite this still short AL in comparison to a typical thickness range of electronic devices that are a few tens of nm-thick, the photoemission signal penetrating through further thicker (18 nm) organic film was successfully detected by PYS. This fact suggests that the electronic structures of "buried interfaces" inside practical organic devices are accessible using this rather simple measurement technique. (C) 2014 Elsevier B.V. All rights reserved.

© 2014 American Chemical Society. The film morphology and device performance of planar heterojunction solar cells based on the molecular donor material α-sexithiophene (6T) are investigated. Planar heterojunctions of 6T with two different acceptor molecules, the C60 fullerene and diindenoperylene (DIP), have been prepared. The growth temperature of the 6T bottom layer has been varied between room temperature and 100°C for each acceptor. By means of X-ray diffraction and X-ray absorption, we show that the crystallinity and the molecular orientation of 6T is influenced by the preparation conditions and that the 6T film templates the growth of the subsequent acceptor layer. These structural changes are accompanied by changes in the characteristic parameters of the corresponding photovoltaic cells. This is most prominently observed as a shift of the open circuit voltage (Voc): In the case of 6T/C60 heterojunctions, Voc decreases from 0.4 to 0.3 V, approximately, if the growth temperature of 6T is increased from room temperature to 100°C. By contrast, Voc increases from about 1.2 V to almost 1.4 V in the case of 6T/DIP solar cells under the same conditions. We attribute these changes upon substrate heating to increased recombination in the C60 case while an orientation dependent intermolecular coupling seems to change the origin of the photovoltaic gap in the DIP case. (Graph Presented).

The carrier extraction property of a prototypical small molecule organic solar cell (OSC) composed of copper phthalocyanine (CuPc), C-60, and bathocuproine (BCP) was studied on the basis of the internal potential distribution and carrier dynamics in the device. The internal potential distribution in the OSC structure at the interfaces and in the bulk region was determined by the Kelvin probe method. Significant potential gradients were found in the CuPc film on indium tin oxide and in the C-60 film on CuPc, consistent with charge transfer through the contacts. Moreover, surface potential of the BCP layer grew linearly with increasing film thickness with a slope of ca. 35 mV/nm (giant surface potential: GSP), which indicated spontaneous orientation polarization in the film. The potential gradient and GSP significantly changed the built-in potential of the device. Current-voltage and modified time-of-flight measurements revealed that the BCP layer worked as an electron injection and extraction layer despite the wide energy gap. These results were discussed based on the contributions of GSP and the gap states in the BCP layer. (C) 2014 AIP Publishing LLC.

The complete electronic structure inside a practical organic photovoltaic (OPV) device consisting of a trilayer structure of copper-phthalocyanine (CuPc), fullerene (C-60), and bathocuproine (BCP) is demonstrated using low-energy ultraviolet photoelectron spectroscopy (LE-UPS) and photoelectron yield spectroscopy (PYS). The molecular orbital energy alignment and electrostatic potential distribution throughout the entire device is illustrated based on the LE-UPS results. A favorable potential gradient to carry the photogenerated holes and electrons is manifested to be built spontaneously in the CuPc and BCP layers, respectively. Furthermore, the ultrahigh sensitivity measurements of LE-UPS clearly unveil the distributions of faint density-of-states in the energy-gap region in the organic films. Substantially barrierless contacts to both electrodes are fulfilled by the existence of these gap states. The electronic structure under simulated sunlight illumination is examined for the purpose of elucidating the electronic structures inside the working devices in the open-circuit condition. These results indicate experimentally the electronic functionalities of each organic material, in particular of the BCP buffer layer, on the cell efficiency.

The electronic structures of pentacene single crystals (SCs) were elucidated by ultraviolet photoelectron spectroscopy (UPS) and photoelectron yield spectroscopy (PYS). An asymmetric HOMO peak profile of the pentacene SCs obtained by UPS exhibits a close similarity to the k-projected density-of-states of the valence band that has been predicted by a theoretical calculation [H. Yoshida and N. Sato, Phys. Rev. B 77, 235205 (2008)]. The ionization energy of the pentacene SCs is successfully determined to be 4.95 (+/- 0.03) eV which is evidently greater than that of the bulk films of pentacene [4.90 (+/- 0.02) eV]. (C) 2014 The Japan Society of Applied Physics

We have proposed a gold nanoparticle (GNP)-based single-electron transistor (SET) doped with a dye molecule, where the molecule works as a photoresponsive floating gate. Here, we examined the source-drain current (I-SD) at a constant drain voltage under light irradiation with various wavelengths ranging from 400 to 700 nm. Current change was enhanced at the wavelengths of 600 and 700 nm, corresponding to the optical absorption band of the doped molecule (copper phthalocyanine: CuPc). Moreover, several peaks appear in the histograms of ISD during light irradiation, indicating that multiple discrete states were induced in the device. The results suggest that the current change was initiated by the light absorption of CuPc and multiple CuPc molecules near the GNP working as a floating gate. Molecular doping can activate advanced device functions in GNP-based SETs. (C) 2014 The Japan Society of Applied Physics

Organic-metal interfaces are key elements in organic-based electronics. The energy-level alignment between the metal Fermi level and the molecular orbital levels determines the injection barriers for the charge carriers at the interfaces, which are crucial for the performance of organic electronic devices. Dipole formation at the interfaces has been regarded as the main factor that affects the energy-level alignment. Several models have been proposed for the mechanism of dipole formation in the context of the interface between organic molecules and a bulk metal crystal surface, at which surface states were mostly used to probe the interfacial properties. Here we report that when the bulk metal crystal is replaced by a uniform metal thin film, the resulting two-dimensional quantum-well states will be able to not only probe but also modify the interfacial electronic structures, such as gap states, that have no counterpart at the organic-bulk crystal interface.

We have proposed a simple method of activating advanced functions in single-electron transistors (SETs) based on the specific properties of individual molecules. As a prototype, we fabricated a copper phthalocyanine (CuPc)-doped SET. The device consists of a gold-nanoparticle (GNP)-based SET doped with CuPc as a photoresponsive floating gate. In this paper, we report the details of the photoresponses of the CuPc-doped SET, such as conductance switching, sensitivity to the wavelength of the incident light, and multiple induced states. (C) 2013 The Japan Society of Applied Physics

We have examined the significant enhancement of ambipolar charge injection and transport properties of bottom-contact single crystal field-effect transistors (SC-FETs) based on a new rubrene derivative, bis(trifluoromethyl)-dimethyl-rubrene (fm-rubrene), by employing carbon nanotube ((NT) electrodes. The fundamental challenge associated with fm-rubrene crystals is their deep-lying HOMO and LUMO energy levels, resulting in inefficient hole injection and suboptimal electron injection from conventional Au electrodes due to large Schottky barriers. Applying thin layers of CNT network at the charge injection interface of fm-rubrene crystals substantially reduces the contact resistance for both holes and electrons; consequently, benchmark ambipolar mobilities have been achieved, reaching 4.8 cm(2) V-1 s(-1) for hole transport and 4.2 cm(2) V-1 s(-1) for electron transport. We find that such improved injection efficiency in fm-rubrene is beneficial for ultimately unveiling its intrinsic charge transport properties so as to exceed those of its parent molecule, rubrene, in the current device architecture. Our studies suggest that CNT electrodes may provide a universal approach to ameliorate the charge injection obstacles in organic electronic devices regardless of charge carrier type, likely due to the electric field enhancement along the nanotube located at the crystal/electrode interface.

We propose a modified measurement technique of capacitance for three-terminal devices and apply this method for the evaluation of the channel formation in pentacene field-effect transistors. An additional structure in the capacitance-voltage curves clearly shows the channel formation from the saturation to the linear region in an operating transistor which has not been directly observed in conventional methods. Based on the amount of accumulated charge in the channel region, the validity of the gradual channel approximation model and the usability of a buffer layer are discussed. This method enables the detailed investigation of carrier behaviors in organic transistors through appropriate evaluation of the channel formation process during operation. (C) 2013 Elsevier B.V. All rights reserved.

Organic field effect transistors (OFETs) using crystalline organic semiconductors are of great interest because of their well-defined structural and electronic properties to study the intrinsic charge carrier transport mechanisms in p-conjugated molecular solids, as well as to unravel their potential to be applied as a novel type of electronic device. In the present study, the valence band structure of the channel region of an OFET is proposed based on photoemission results of a well-defined interface between a dielectric molecular monolayer and single crystals of 5,6,11,12-tetraphenyltetracene (rubrene) which is known to exhibit the highest field effect mobility of all organic semiconductors at room temperature. Commensurate growth of clusters of tetratetracontane (TTC; n-C44H90) on the rubrene single crystal surface and their morphological transformation into a uniform overlayer were observed by atomic force microscopy. Photoelectron spectroscopy measurements at various electron take-off angles were then conducted to derive the valance band width of the rubrene single crystal covered by the TTC overlayers. The valence band width at this hetero-interface was found to be equivalent to that of the pristine rubrene, which suggests an unchanged 'band effective mass (h) over bar (2)(d(2)E/dk(parallel to)(2))' of accumulated holes even at the vicinity of hydrocarbon-based gate dielectrics. (C) 2013 Elsevier B.V. All rights reserved.

A tris(7-propyl-8-hydroxyquinolinato) aluminum [Al(7-Prq)3] film shows negative giant surface potential (GSP) because of spontaneous orientation polarization. The polarity of this film is opposite to those of tris-(8-hydroxyquinolate) aluminum films. In Al(7-Prq)3-based organic light-emitting diodes, negative GSP leads to the positive interface charge and governs the electron injection and accumulation properties. In addition, a high resistance to the electron injection at the Al(7-Prq)3/Ca interface is suggested possibly because of the negative polarization charge at the interface. These results show the importance of orientation polarization in controlling the charge injection and accumulation properties and potential profile of the resultant devices. © 2013 AIP Publishing LLC.

Low-energy photoelectron spectroscopy combined with photoelectron yield spectroscopy was developed to investigate the buried organic interfaces in the practical device thickness. Ultralow background signal and charging durability were achieved by utilizing monochromatic low-energy photons. C-60/rubrene/Au interfaces were studied as a prototypical system of organic solar cells. The low density of gap states was detected in the rubrene film and a small feature due to the C-60-induced morphological change was observed in the C-60/rubrene interface. (C) 2013 The Japan Society of Applied Physics

Interface electronic structures of four-kinds of electron transporting or hole blocking organic materials (n-type) on a widely-used hole transporting material (p-type) in organic light emitting diodes (OLEDs), N,N′-bis (1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamin (NPB), were investigated by means of photoelectron spectroscopy (PES). 1,3-bis[5-(4-tert-butylphenyl)-1,3,4-oxadiazole-2-yl]benzene (OXD-7) and 2,2′,2″-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi) overlayers show continuous energy shift of each overlayer-derived spectral components and the vacuum level proportional to the thickness. This energy shift is ascribed to a spontaneous building up of the electrostatic potential within the organic layers (giant surface potential; GSP). The energy shift of the overlayers induced by GSP as well as the interface vacuum level shift are adequately taken into account to determine the actual energy barrier heights of the hole conduction levels at the heterojunctions. 4,4′-bis(9-carbazolyl) biphenyl (CBP) and p-bis(triphenylsilyl) benzene (UGH2) induce band bending in the NPB film which presumably results from charge transfer (CT) to the n-type materials from NPB. Despite absence of a practical vacuum level shift and thickness dependent shift of the overlayer-derived electronic states, the CT-derived energy shift of NPB reduces the actual energy barrier height with respect to the nominal barrier height being simply interpreted from PES spectra of a thick overlayer of each material. The energy level diagrams across these 'n-on-p' organic-organic heterojunctions were finely determined based on the above interpretation of the PES spectra. © 2012 Elsevier B.V. All rights reserved.

The valence band structure of rubrene single crystals was experimentally determined by high-resolution angle-resolved and excitation-energy-dependent photoelectron spectroscopy at room temperature. The energy position of the peak derived from the highest occupied molecular orbital did not depend on the excitation energy, reflecting an absence of energy dispersion along the surface normal direction. A two-dimensional valence band dispersion relation over the surface Brillouin zone obtained by angle-resolved photoemission to three critical points was reproduced excellently by a two-dimensional tight binding approximation. Highly anisotropic values of intermolecular transfer integrals to four adjacent molecules were obtained from the present results. (C) 2012 The Japan Society of Applied Physics

Using angle-resolved photoemission spectroscopy, we investigated the interfacial properties between the long-chain normal-alkane molecule n-CH 3(CH 2) 42CH 3 [tetratetracontane (TTC)] and uniform Ag films using the Ag quantum well states. The entire quantum well state energy band dispersions were observed to shift toward the Fermi level with increasing adsorption coverage of TTC up to 1 monolayer (ML). However, the energy shifts upon deposition of 1 ML of TTC are approximately inversely dependent on the Ag film thickness, indicating a quantum-size effect. In the framework of the pushback and image-force models, we applied the Bohr-Sommerfeld quantization rule with the modified Coulomb image potential for the phase shift at the TTC/Ag interface to extract the dielectric constant for 1 ML of TTC. © 2012 American Physical Society.

In organic light emitting diodes, carrier-blocking property at organic hetero interfaces is one of important factors to get better performance, but the factor have not been well investigated experimentally. In this study, we proposed a new usage of time-of-flight (TOF) technique to quantitatively examine the carrier behavior in operating device. The measurement was demonstrated for ITO| α-NPD(1100 nm) |Alq 3(60 nm)|Al(100 nm) device. TOF signal was successfully observed under the influence of actual current flow. For hole transport from ITO to Al electrode, delayed-transport and blocking nature at α-NPD/Alq 3 was clearly observed. In contrast, for electron transport in the same direction, no delayed transport was detected. This result was consistent with the possible energy barrier at the interface, indicating the feasibility of this technique to examine carrier blocking nature in electronic devices. The method to analyze the TOF in detail will be discussed. © c 2012 The Surface Science Society of Japan.

Photoinduced conductance switching was demonstrated in a copper phthalocyanine (CuPc)-doped gold nanoparticle (GNP) transistor formed in a nanogap electrode with a back gate structure. Two specific states were reversibly induced in conductance of the CuPc-doped devices by light irradiation and applied voltages. The probability of occurrence of conductance switching decreased with a reduction in the number of incident photons. In addition, conductance switching was not observed in the devices before CuPc doping. Conductance switching originates from change in the local potential of GNPs, possibly induced by a charge-state bistability of an individual CuPc molecule worked as a floating gate. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4733612]