石井 久夫

Illumination stress (IS) and negative bias under illumination stress (NBIS) cause considerable device instability in thin-film transistors based on amorphous In–Ga–Zn–O (a-IGZO). Models using in-gap states are suggested to explain device instability. Therefore, to provide reliably their density of states (DOS), this study investigated the valence band, conduction band, and in-gap states of an a-IGZO thin film. The DOS of in-gap states was directly determined in a dynamic range of six orders of magnitude through constant final state yield spectroscopy (CFS-YS) using low-energy and low-flux photons. Furthermore, light irradiation irreversibly induced extra in-gap states near the Fermi level and shifted the Fermi level to the vacuum level side, which should be related to the device instability due to IS and NBIS. Hard x-ray photoemission spectroscopy and ultraviolet photoemission spectroscopy using synchrotron radiation observed the large DOS of in-gap states near the Fermi level as in previous works. Here, we reveal that they are not intrinsic electronic states of undamaged a-IGZO, but induced by the intense measurement light of synchrotron radiation. This study demonstrates that CFS-YS is useful for determining the reliable DOS of the in-gap states for samples that are sensitive to light irradiation. The absorption spectrum measured through photothermal deflection spectroscopy is interpreted based on DOS directly determined via photoemission spectroscopies. This indicates that the line shape in the energy region below the region assigned to the Urbach tail in previous works actually roughly reflects the DOS of occupied in-gap states.

Abstract Photodiode‐type solar‐blind photodetectors (SBPDs) with the self‐powered feature hold great promise for applications in unattended secure communication, flame detection, and missile warning. However, the responsivity of SBPDs is usually limited due to the severe solar‐blind (SB) light extinction in substrates and charge transport layers. Herein, a spectrally selective hole extraction structure (SHE) is proposed for high‐efficiency perovskite SBPDs. The SHE consisting of a tandem Fabry–Perot cavity and energy‐level‐matched hole transport layer endows the device with narrowband absorption in the SB region and optimized charge extraction capability from the CsPbI₂Br perovskite. The optimized SHE exhibits a peak transmittance of 27% at 255 nm and a half maximum at full width of 28 nm. Under SB light illumination, the champion device achieves a responsivity of 56.20 mA W⁻¹ and a detectivity (D^*) of 2.86 × 10¹³ Jones, which are the record values among the reported results. The approach demonstrated here paves the way for the optical and electrical design of perovskite photodetectors with spectrally selective detection.

In this paper, we present the observation results of the surface potential of micropatterned thick (>1 & mu;m) self-assembled electrets (SAEs) for MEMS vibrational energy harvesters (VEHs). To evaluate the surface potential of micropatterned SAEs, we propose and develop test devices with removable through-hole substrates corresponding to the moving electrodes of SAE-MEMS VEHs. In this study, SAEs are deposited simultaneously on two test devices with different through-hole spacings and on a reference flat substrate using the same vacuum evaporation process. The surface potential of SAEs is proportional to the film thickness, and when the film thickness of the SAE deposited on the flat substrate is 4.48 mu m, the surface potential exceeds 200 V. At this time, in a test device where the average thickness of micropatterned SAEs is 3.08 mu m, the measured surface potential is 68 V. In addition, it is experimentally observed that when micropatterned SAEs are formed using the through-hole structures, the microfabrication process causes the SAE pattern to spread wider than the through-hole dimensions, and the surface profiles are not flat. These findings provide useful insights for the design of SAE-MEMS VEHs using micropatterned SAEs with through-hole structures.

1,3,5,7-Tetrakis(1-phenyl-1H-benzo[d]imidazol-2-yl)adamantane exhibits large and stable surface orientation polarization in vacuum-deposited films due to the subtle orientation selectivity of the molecule and the non-conjugated adamantane core.

Abstract Carrier injection, which is a key factor in controlling and improving organic device properties, has been predominantly studied using the injection barrier height derived from HOMO and LUMO positions. The weak density of states (DOS) within the HOMO–LUMO energy gap is also important to understand the practical injection properties. In this study, the DOS of the α-NPD/electrode model interfaces are investigated using high-sensitivity UV photoemission spectroscopy. The nature of hole injection is discussed based on the observed DOS and a simple simulation. The results indicate that the weak DOS close to the Fermi level is critical for carrier injection.

Spontaneous orientation polarization (SOP) of amorphous organic semiconducting films has attracted much attention because of its frequent observation in common organic light-emitting diodes (OLEDs) and potential influences on the device properties of OLEDs. On the other hand, the formation mechanism of SOP has been controversial for a long time, ever since its discovery in 2002. Recently, the formation mechanism of SOP was explained in terms of the surface equilibration mechanism of vapor-deposited glasses, and the understanding of SOP has progressed significantly. Based on the improved understanding, some active control methods of SOP have been demonstrated and further influences on the device performance of OLEDs were revealed, suggesting that higher efficiency can be achieved by managing SOP properly. Furthermore, some applications of SOP have also been proposed, such as a self-assembled electret and a tool for evaluating materials properties. In this paper, recent progress in the understanding of SOP and its applications to devices are reviewed.

Electret-based vibrational energy generators (E-VEGs) have attracted considerable attention because they can generate electrical power from ambient vibration. E-VEGs have a capacitor structure in which an electret and air gap are sandwiched by electrodes, and the devices are automatically charged by the electric field of the electret. Although various charging processes for dielectric materials to be polarized have been proposed, they lead to low productivity of the device. Recently, we have developed a self-assembled electret (SAE)-based VEG that does not require any charging process. The SAE was realized by utilizing the spontaneous orientation of polar molecules for organic light-emitting diodes: positive and negative polarization charges exist on the film surface and reverse side. Because an electric field will not be formed outside of the SAE, the driving force inducing charge carriers on the electrodes of the SAE VEG has not been clarified. To clarify the operation mechanism of the SAE VEG, in this study, the relationship between the surface potential of the SAE and the current generated by electrode vibration in the SAE VEG is carefully examined. The surface potential is changed over a wide range from negative to positive by controlling the molecular orientation. In addition to establishing a model for the device operation, we demonstrated that the output power of the SA E-VEG can be easily enhanced simply by increasing the thickness of the SAE.

Electret-based microelectromechanical system (MEMS) vibratory energy harvesting is a key technology for converting the mechanical energy of environmental vibrations into electricity. Unfortunately, conventional electret charging methods generally rely on high-voltage and high-temperature processes that present limitations to MEMS design and production. Here, we show a MEMS post-processed self-assembled electret (SAE) that enables the integration of electrets with MEMS vibratory devices via evaporation as a post-MEMS process. Owing to the spontaneous orientation of polar molecules, the surface potential of the SAE can build up at room temperature in a microscopic region without charging processes, which enhances the design and fabrication flexibility of electret-based MEMS energy harvesters. We develop a MEMS vibratory device followed by post-processing the SAE and confirm induced electrical currents caused by the electrical field of the SAE at the vibrational input. This SAE-based MEMS technology is a promising design guideline for highly integrated single-chip MEMS vibratory energy harvesters. Published under an exclusive license by AIP Publishing.

Recently, a spontaneous orientation polarization (SOP) has attracted much attention as an important factor improving the performance of organic light-emitting devices. However, so far, SOP is reported only for films fabricated by vacuum vapor deposition, and no direct attempt is made to examine the existence of SOP for wet-processed film. To examine this phenomenon, it is necessary to completely exclude light illumination in the sample system, as photocarriers can reduce or eliminate the surface potential caused by the SOP formation. Herein, the SOP of wet-processed tris(8-hydroxyquinolinato)aluminum (Alq(3)) film on an indium-tin-oxide substrate is investigated using a novel rotary Kelvin probe method. Film formation and surface potential measurement are conducted under completely dark condition. A surface potential shift of -0.33 V is observed for a 1 mu m thick film of Alq(3), indicating that the SOP of wet-processed Alq(3) film is negligible, in contrast to vacuum-evaporated film.

Low-density electronic states in the energy gap of an amorphous In-Ga-Zn-O film control device performance. Herein, density of states (DOS) distribution from valence band to the in-gap states of 10(14) cm(-3) eV(-1) level was determined using high-sensitivity UV photoemission spectroscopy. Exponential tail states accompanying two energetically-localized states were directly observed as reported previously. The observed slope of the exponential tail state was different from the Urbach energy derived using photothermal deflection spectroscopy, indicating the importance of directly observing the DOS of in-gap states.

In vacuum-deposited film composed of organic polar molecules, polarization charges appear on the film surface owing to spontaneous orientation of the molecule. Because its density (spol) determines an amount of accumulation charge (sacc) in organic light-emitting diodes and output power in polar molecular-based vibrational energy generators (VEGs), control of molecular orientation is highly required. Recently, several groups have reported that dipole-dipole interaction between polar molecules induces anti-parallel orientation which does not contribute to spol. In other words, perturbation inducing the attenuation of the dipole interaction is needed to enhance spol. In this study, to investigate an effect of light irradiation on spol, we prepared 1,3,5-tris(1-phenyl-1Hbenzimidazol-2-yl)benzene (TPBi) film under illumination during its deposition, and evaluated the sacc in TPBi-based bilayer device, which equals to spol. We found that the sacc was increased by light irradiation, indicating that average orientation of TPBi is enhanced. These results suggest that light irradiation during device fabrication is promising process for organic electronic devices including polar molecule-based VEGs.

Direct detection of intrinsic defects in halide perovskites (HaPs) by standard methods utilizing optical excitation is quite challenging, due to the low density of defects in most samples of this family of materials (<= 10(15) cm(-3) in polycrystalline thin films and <= 10(11) cm(-3) in single crystals, except melt-grown ones). While several electrical methods can detect defect densities <10(15) cm-3, such as deep level transient spectroscopy (DLTS) or thermally stimulated current (TSC), they require preparation of ohmic and/or rectifying electrical contacts to the sample, which not only poses a challenge by itself in the case of HaPs but also may create defects at the contact-HaP interface and introduce extrinsic defects into the HaP. Here, we show that low-energy photoelectron spectroscopy measurements can be used to obtain directly the energy position of gap states in Br-based wide-bandgap (E-g > 2 eV) HaPs. By measuring HaP layers on both hole- and electron-contact layers, as well as single crystals without contacts, we conclude that the observed deep defects are intrinsic to the Br-based HaP, and we propose a passivation route via the incorporation of a 2D-forming ligand into the precursor solution.

Electret-based vibrational energy generators (E-VEGs) arc of particular interest since they can generate electrical power from ambient vibration. A challenge is that the charging process is required for making the electret from a dielectric material, and the process is one of the main factors that determine the manufacturing costs of the device. Recently, by utilizing the spontaneous orientation of polar molecules, which have been used for organic light-emitting diodes, we realized a novel E-VEG that does not require any charging process. In this paper, the operation mechanism and stability of the device are discussed. We believe that the E-VEG using spontaneous orientation enables us to reduce the costs. leading to the device's mass commercialization.

Spontaneous orientation polarization (SOP) has been frequently observed in the evaporated films of organic light-emitting diode materials. Because SOP modifies the charge injection and the accumulation properties of the device, understanding and controlling SOP is crucial in optimizing the performance of the device. In this study, we investigated the dominant factors for SOP formation by focusing on intermolecular interactions. We examined the giant surface potential characteristics of coevaporated films incorporating 1,3,5-tris(1-phenyl-1H-benzimidazol-2-yl)benzene (TPBi) that is a typical polar molecule exhibiting SOP. In the coevaporated films of TPBi and nonpolar molecules such as 4,4 '-bis(N-carbazolyl)-1,1 '-biphenyl and 4,4 ',4 ''-tris (carbazol-9-yl)triphenylamine, the orientation degree of the permanent dipole moment (PDM) of TPBi is significantly enhanced with diluted TPBi density, though the enhancement is weak on the film withN,N '-bis(1-naphthyl)-N,N '-diphenyl-1,1 '-biphenyl-4,4 '-diamine. The results indicate that the PDM interaction between polar molecules results as a negative factor for SOP formation. Furthermore, we found that SOP formation is suppressed by the surface treatment of the self-assembled monolayer on the gold substrate, indicating a positive effect of the van der Waals interaction between the molecule and the substrate surface.

Abstract The vibration-based electret generators (EGs) for energy harvesting have been extensively studied because they can obtain electrical energy from ambient vibrations. EGs exhibit a sandwich structure of electrodes surrounding an air gap and an electret, which is a dielectric material with a quasi-permanent electrical charge or dipole polarisation. Various charging processes have been developed because the surface charge density (σ) of the electret determines the output power of the device. However, such processes are considered to constitute a key productivity-limiting factor from the mass production viewpoint, making their simplification or elimination a highly desired objective. Herein, a model EG that does not require any charging process by utilising the spontaneous orientation polarisation of 1,3,5-tris(1-phenyl-1H-benzimidazole-2-yl)benzene (TPBi) is demonstrated. The surface potential (V_sp) of an evaporated TPBi film has reached 30.2 V at a film thickness of 500 nm without using a charging process. The estimated σ of 1.7 mC m⁻² is comparable with that obtained using a conventional polymer-based electret after charging. Furthermore, V_sp is considerably stable in environmental conditions; thus, TPBi can be considered to be “self-assembled” electret (SAE). Application of SAE leads to developing an EG without requiring the charging process.

Electron-hole pairs at the interface between electron-donating and electron-accepting molecules form charge-transfer excitons (CTEs) via Coulomb attraction. Generally, the attraction energy of the CTE is weaker than that of the Frenkel exciton because of spatial separation of the charge pair; thus, the binding of the CTE is expected to be sensitive to an electric field. Here, the shielding of the binding energy of the CTEs by an internal electric field induced by spontaneous orientation polarization (SOP) of the solid-state donor-acceptor blend film is reported. When the blend film forms a large SOP, the photogenerated CTEs spontaneously dissociate without an external electric field, resulting in carrier diffusion and carrier lifetimes that are in the milliseconds or longer. In the absence of a large SOP, the CTEs preferentially form exciplexes that quickly release their energy as light rather than dissociation. Hence, the control of SOP in the donor-acceptor blend films can provide new insights into exciton binding and facilitate the development of tailored high-performance organic semiconductor devices.

Multi-heme cytochromes (MHCs) are fascinating proteins used by bacterial organisms to shuttle electrons within, between, and out of their cells. When placed in solid-state electronic junctions, MHCs support temperature-independent currents over several nanometers that are 3 orders of magnitude higher compared to other redox proteins of similar size. To gain molecular-level insight into their astonishingly high conductivities, we combine experimental photoemission spectroscopy with DFT+S current-voltage calculations on a representative Gold-MHC-Gold junction. We find that conduction across the dry, 3 nm long protein occurs via off-resonant coherent tunneling, mediated by a large number of protein valence-band orbitals that are strongly delocalized over heme and protein residues. This picture is profoundly different from the electron hopping mechanism induced electrochemically or photochemically under aqueous conditions. Our results imply that the current output in solid-state junctions can be even further increased in resonance, for example, by applying a gate voltage, thus allowing a quantum jump for next-generation bionanoelectronic devices.

In this study, the formation of Ag-S bond was systematically elucidated by thickness-dependent ultraviolet photoelectron spectroscopy (UPS) in order to understand the L-cysteine interaction with silver surface. A clean Ag(111) as the model system for silver surface was used, and L-cysteine films on silver substrate were formed by vacuum evaporation. The orbital configurations at the interface was estimated including work function, secondary electron cutoff (SECO), highest occupied molecular orbital (HOMO) onset, position of an interface state, charge injection barrier, and ionization energy. A clear spectral feature was appeared in between Fermi edge and HOMO of L-cysteine, and the feature can be attributed to the formation of Ag-S bonding. In the case of SECO, the maximum shift was 0.46 eV to the higher binding energy side at the nominal thickness of 1 angstrom. However, from the nominal thickness of 2 angstrom, SECO started to shift to the lower binding energy side, and at 16 angstrom, the SECO shifted to a value of around 0.4 eV to the lower binding energy side to almost cancel the initial vacuum level shift. This behavior can be attributed to weakening of the silver-sulfur bond with increasing of L-cysteine coverage referring to the literature. The photoelectron yield spectroscopy (PYS) was also performed as an additional spectroscopic work, which exhibited that the work function of silver once decreased and then recovered at low coverage. This behavior can also be assigned to a weakening the interaction of L-cysteine with silver by increasing of L-cysteine coverage.

Organic small molecule solar cells are used as a test bed to investigate the influence of film morphology on the density of charge-transfer (CT) states. CT states are considered as precursors for charge generation and their energy as the upper limit for the open-circuit voltage in organic donor-acceptor solar cells. In this study the influence of morphology for two perylene donors [crystalline diindenoperylene (DIP) versus amorphous tetraphenyldibenzoperiflanthene (DBP)] with almost identical ionization energy is investigated. As acceptor material, the fullerene C-60 is used. By combining device measurements with optical and low-energy ultraviolet photoelectron spectroscopy, a comprehensive picture is obtained that describes how morphology and the connected density of states (DOS) affect device performance and the spectroscopic signature of CT states. Especially for the crystalline donor material DIP, strong exponential tail states reaching far into the gap are observed, which can be related to the presence of grain boundaries. A voltage-dependent filling of these states is identified as the origin of a blue shift of electroluminescence spectra with increasing applied voltage. Different approaches are compared to study the influence of static and dynamic disorder in the description of CT emission and absorption spectra of organic solar cells. Despite the fact that both donors yield almost identical CT energy (and, thus, the same open-circuit voltage) the Stokes shift between photocurrent and electroluminescence spectra and, concomitantly, the width of the CT DOS varies by more than a factor of 2. We discuss this observation in terms of the donor-acceptor reorganization energy as well as an additional line broadening by static disorder. Remarkably, the more crystalline donor DIP shows a significant deviation from a Marcus-type description, while this is not the case for the amorphous DBP. This highlights the importance of film morphology in organic solar cells.

If not oriented perfectly isotropically, the strong dipole moment of polar organic semiconductor materials such as tris-(8-hydroxyquinolate)aluminum (Alq(3)) will lead to the buildup of a giant surface potential (GSP) and thus to a macroscopic dielectric polarization of the organic film. Despite this having been a known fact for years, the implications of such high potentials within an organic layer stack have only been studied recently. In this work, the influence of the GSP on hole injection into organic layers is investigated. Therefore, we apply a concept called dipolar doping to devices consisting of the prototypical organic materials N,N'-Di(1-naphthyl)-N,N'-diphenyl-(1,1'-biphenyl)-4,4'-diamine (NPB) as nonpolar host and Alq(3) as dipolar dopant with different mixing ratios to tune the GSP. The mixtures are investigated in single-layer monopolar devices as well as bilayer metal/insulator/semiconductor structures. Characterization is done electrically using current-voltage (I-V) characteristics, impedance spectroscopy, and charge extraction by linearly increasing voltage and time of flight, as well as with ultraviolet photoelectron spectroscopy. We find a maximum in device performance for moderate to low doping concentrations of the polar species in the host. The observed behavior can be described on the basis of the Schottky effect for image-force barrier lowering, if the changes in the interface dipole, the carrier mobility, and the GSP induced by dipolar doping are taken into account.

Abstract Electret generators (EGs) for energy harvesting is a device which ambient vibrations convert into electric energy. They are expected as power supplies for low energy applications such as wireless sensors. In EGs, charges are induced by an electric field of electret, and thus output power is determined by a surface charge density (σ) of the electret. Various charging processes were proposed so far to realize the electret with high σ, however, they were one of problems decreasing productivity. The elimination of the processes is extremely desired. In this study, we developed charged film composed of 1,3,5-tris(1-phenyl-1H-benzimidazol-2-yl) benzene (TPBi) without any charging process. A surface potential (SP) exceeded 46 V for ca. 740 nm thick film due to spontaneous orientation of TPBi, and was sufficiently stable under room light illumination in atmosphere. Finally, we demonstrated EGs utilizing TPBi as electret generated an AC current of the order of nano-ampere in environmental condition, indicating the feasibility of EGs using TPBi film in practical atmosphere. The series of results strongly suggested that application of spontaneous orientation of polar molecules was promising for the realization of EGs with high productivity.

Abstract Vibration-driven electret generators (EGs) for energy harvesting have been intensively studied since they are expected to be used as electric source, especially, for low power consumption devices. EGs have generally capacitor structure consisting of air gap and electret which is a dielectric material with a quasi-permanent electrical charge or dipole polarization. Because charges are induced on top and bottom electrodes by the electret, higher surface charge density (σ) is needed to achieve sufficient output power in EGs. Various techniques have been proposed to make electret with high σ; however, they are one of the factors reducing the productivity. Thus the elimination of charging process is crucially required. In this study, we develop electret film without any charging process by utilizing spontaneous orientation polarization of 1,3,5-tris(1-phenyl-1H-benzimidazol-2-yl)benzene (TPBi). A surface potential of TPBi vacuum evaporated film exceeded 46 V at 739 nm, and σ was estimated to be 1.8 mC/m². In addition, the σ was quite stable in dark and vacuum condition. By taking the advantages of TPBi film, we developed a model EG without any charging process. An AC current in nano-ampere order appeared due to the vibration, suggesting the power generation in EG without charging process.

© 2019, © 2019 Taylor &amp; Francis Group, LLC. The electronic structures of materials such as HOMO and LUMO are essential information to understand their functions. Photoemission spectroscopy, which is the most standard technique to examine the electronic structures of various materials, has been not well applied to biomolecules mostly due to sample charging and poor sensitivity. Very recently, we have developed high-sensitivity photoemission spectroscopy (HS-PES) using low photon energy. In this study, HS-PES has been successfully applied to a thermophilic rhodopsin (TR) film. The high sensitivity of our technique enabled to observe the HOMO region of retinal part in TR without any problem due to sample charging. This technique is expected to explore the electronic structures of various proteins.

© 2019 American Physical Society. We propose a simple theory of photoelectron yield spectra (PYS) on the basis of a density-of-states approximation and universal transmission and loss functions. The theory reproduces the known threshold behavior of metals and that of semiconductors in the indirect-transition regime. We show that a Gaussian-shaped density of states leads to a PYS spectrum that increases very nearly as the third power of energy from threshold, indicating that the empirical cube law of PYS of organic matter reflects the Gaussian-broadened highest occupied molecular orbital peak. We show how the peak position and width can be extracted from the PYS spectrum. The results obtained for several organic compounds agree well with ultraviolet photoemission data. We further show that the density of occupied states is approximately proportional to the second derivative of the PYS spectrum in the near-threshold region.

Copyright © 2019 The Institute of Electronics, Information and Communication Engineers. The gap states of tetratetracontane (C 44 H 90 ; TTC), which is a model oligomer of polyethylene, was examined by using high-sensitivity UV photoemission spectroscopy (HS-UPS). The high sensitivity enabled us to directly observe the weak gap states distributed in the HOMO-LUMO gap from the valence band top to 3.0 eV below the vacuum level. On the basis of the density-of-states derived from UPS results, the tribocharging nature of polyethylene was discussed in comparison with our previous result for nylon-6,6 film.

Copyright © 2019 The Institute of Electronics, Information and Communication Engineers. On surfaces of tris-(8-hydroxyquinolate) aluminum (Alq) and tris(7-propyl-8-hydroxyquinolinato) aluminum (Al7p) thin-films, positive and negative polarization charges appear, respectively, owing to spontaneous orientation of these polar molecules. Alq is a typical electron transport material where electrons are injected from cathode. Because the polarization charge exists at the Alq/cathode interface, it is likely that it affects the electron injection process because of Coulomb interaction. In order to evaluate an impact of polarization charge on electron injection from cathode, electron only devices (EODs) composed of Alq or Al7p were prepared and evaluated by displacement current measurement. We found that Alq-EOD has lower resistance than Al7p-EOD, indicating that the positive polarization charge at Alq/cathode interface enhances the electron injection due to Coulomb attraction, while the electron injection is suppressed by the negative polarization charge at the Al7p/Al interface. These results clearly suggest that it is necessary to design organic semiconductor devices by taking polarization charge into account.

© 2019 The Japan Society of Applied Physics. Spontaneous orientation polarization (SOP) is inherent in the evaporated films of many organic semiconducting molecules with a permanent dipole moment. A significant electric field is formed in the film due to SOP. Consequently, the properties of organic light-emitting diodes (OLEDs) incorporating such films are influenced. The polarization charge appearing at heterointerfaces dominates the charge injection and accumulation properties. Moreover, SOP correlates with device degradation. In this article, we review the SOP of organic semiconductor films and its influences on the device properties of OLEDs.

© 2018 IEEE. ZnO nanorods electron emitter was fabricated with the simple solution and hydrothermal process. ZnO can grow to countless nanorods without vacuum process. The ITO glass was used as an anode plate and the ZnO electron emitter as a cathode. The cathode and anode plate were kept apart by spacer with thickness of 10μm. The current-voltage (I-V) characteristics were measured in the air and in a vacuum in order to determine the performance of ZnO nanorods as the electron emission source. Here we found this device provides a high drive current (&gt;10 micro-amperes) at relatively low drain voltage (10V). This indicates ZnO have high performance for electron emitter.

© 2018 The Japan Society of Applied Physics. This study aims to investigate the electronic structure of negatively charged C60 by applying operando photoelectron yield spectroscopy (PYS) to a C60-based field-effect transistor (FET). After a gate voltage was applied and removed to inject electrons, an onset structure emerged in the PYS spectrum. The onset value of 3.6 eV was comparable to the electron affinity (4.0 eV) obtained using inverse photoemission spectroscopy, suggesting that the observed photoemission originated from the injected anions. These results clearly demonstrate that operando PYS can be a powerful tool for investigating carriers in n-type FETs.

The molecular orientation in organic semiconductor films determines device performances. In particular, the spontaneous orientation of a permanent dipole moment (PDM) along the surface normal direction induces a polarization charge at the hetero-interfaces of stacked multilayer devices, and the interface charge dominates the charge accumulation and injection properties. Spontaneous orientation polarization (SOP) has been observed in the “randomly oriented” films of several organic semiconductor materials, and is potentially inherent in many common materials. Herein, we report that 11 additional molecules of organic light-emitting diode materials, including thermally activated delayed fluorescence emitters, and horizontally oriented emitters and electron transporters, exhibit SOP in their evaporated films. The experimental results clearly indicate that SOP frequently occurs in “horizontally oriented” films as well as “randomly oriented” films. The factors contributing to SOP formation are discussed in terms of the figure of merit per PDM. We found that strong intermolecular interactions tend to reduce the figure of merit. Moreover, we suggest the impact of SOP on device performances.

We applied displacement current measurement (DCM) to an organic solar cell (OSC) with a structure of ITO/CuPc/C60/BCP/Al in order to investigate the origin of a negative capacitance (NC). In DCM curve, an anomalous characteristic that the current in backward scan was larger than that in forward scan was observed. We found that this characteristic originates from a gradual increase of an additional current, leading to NC in DCM. The additional current can be explained by the increase of the device temperature owing to a Joule heat and thermal deactivation of the exciton. Moreover, NC value was observed to be enhanced by light irradiation due to an additional temperature increase.

The dynamic interaction between the traveling charges and the molecular vibrations is critical for the charge transport in organic semiconductors. However, a direct evidence of the expected impact of the charge-phonon coupling on the band dispersion of organic semiconductors is yet to be provided. Here, we report on the electronic properties of rubrene single crystal as investigated by angle resolved ultraviolet photoelectron spectroscopy. A gap opening and kink-like features in the rubrene electronic band dispersion are observed. In particular, the latter results in a large enhancement of the hole effective mass (&gt;1.4), well above the limit of the theoretical estimations. The results are consistent with the expected modifications of the band structures in organic semiconductors as introduced by hole-phonon coupling effects and represent an important experimental step toward the understanding of the charge localization phenomena in organic materials.

The structure of pn heterojunctions is an important subject in the field of organic semiconductor devices. In this work, the crystallinity of an epitaxial pn heterojunction of C-60 on single crystal pentacene is investigated by non-contact mode atomic force microscopy and high-resolution grazing incidence x-ray diffraction. Analysis shows that the C-60 molecules assemble into grains consisting of single crystallites on the pentacene single crystal surface. The in-plane mean crystallite size exceeds 0.1 mu m, which is at least five time larger than the size of crystallites deposited onto polycrystalline pentacene thin films grown on SiO2. The results indicate that improvement in the crystal quality of the underlying molecular substrate leads to drastic promotion of the crystallinity at the organic semiconductor heterojunction.

We investigated reversible switching behaviors of a molecular floating-gate single-electron transistor (MFG-SET). The device consists of a gold nanoparticle-based SET and a few tetra-tert-butyl copper phthalocyanine (ttbCuPc) molecules; each nanoparticle (NP) functions as a Coulomb island. The ttbCuPc molecules function as photoreactive floating gates, which reversibly change the potential of the Coulomb island depending on the charge states induced in the ttbCuPc molecules by light irradiation or by externally applied voltages. We found that single-electron charging of ttbCuPc leads to a potential shift in the Coulomb island by more than half of its charging energy. The first induced device state was sufficiently stable; the retention time was more than a few hours without application of an external voltage. Moreover, the device exhibited an additional state when irradiated with 700 nm light, corresponding to doubly charged ttbCuPc. The life time of this additional state was several seconds, which is much shorter than that of the first induced state. These results clearly demonstrate an alternative method utilizing the unique functionality of the single molecule in nanoelectronics devices, and the potential application of MFG-SETs for investigating molecular charging phenomena.

Although the contact electrification of insulating polymers has been widely used in various technologies, the mechanism of electrification is still not well understood and several models have been proposed to explain the mechanism. Some of the models assume the existence of bandgap states that can store or release electrons to charge the polymer; however, the density of states in the bandgap region is not well examined. In this study, an approach to directly measure the density of state of insulating polymers using h nu-dependent high-sensitivity ultraviolet photoemission spectroscopy is proposed. Demonstration of the approach to a representative insulating polymer, nylon-6,6, is reported with the estimation of the charge density and charge penetration depth as a function of the work function difference. Published by AIP Publishing.

Organic molecules with a permanent electric dipole moment have been widely used as a template for further growth of molecular layers in device structures. Key properties of the resulting organic films such as energy level alignment (ELA), work function, and injection/collection barrier are linked to the magnitude and direction of the dipole moment at the interface. Using angle-resolved photoemission spectroscopy (ARPES), we have systematically investigated the coverage-dependent work function and spectral line shapes of occupied molecular energy states (MESs) of chloroaluminium-phthalocyanine (ClAlPc) grown on Ag(111). We demonstrate that the dipole orientation of the first ClAlPc layer can be controlled by adjusting the deposition rate and postannealing conditions, and we find that the ELA at the interface differs by similar to 0.4 eV between the Cl up and down configurations of the adsorbed ClAlPc molecules. These observations are rationalized by density functional theory (DFT) calculations based on a realistic model of the ClAlPc/Ag(111) interface, which reveal that the different orientations of the ClAlPc dipole layer lead to different charge-transfer channels between the adsorbed ClAlPc and Ag(111) substrate. Our findings provide a useful framework toward method development for ELA tuning.

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.

We propose a method, called h.-dependent high-sensitivity ultraviolet photoemission spectroscopy, to observe the density of states (DOS) in a very wide range from HOMO to extremely weak gap states (10(22) to 10(15)cm(-3) eV(-1) in density of states). The method was applied to a p-type semiconducting polymer. A series of spectra for hv = 4.4-7.7 eV were recorded, and the DOS was obtained by overlapping the spectral part with a similar line shape between adjacent photon energy spectra to eliminate the photon energy dependence of the photoionization cross section. This method can be applied to both organic and inorganic materials, providing useful information about the DOS of functional materials. (C) 2017 The Japan Society of Applied Physics

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.