藤井 俊治郎

Single-wall carbon nanotubes (SWCNTs) are promising materials for electronic applications, such as transparent electrodes and thin-film transistors. However, the dispersion of isolated SWCNTs into solvents remains an important issue for their practical applications. SWCNTs are commonly dispersed in solvents via ultrasonication. However, ultrasonication damages SWCNTs, forming defects and cutting them into short pieces, which significantly degrade their electrical and mechanical properties. Herein, we demonstrate a novel approach toward the large-scale dispersion of long and isolated SWCNTs by using hydrodynamic cavitation. Considering the results of atomic force microscopy and dynamic light-scattering measurements, the average length of the SWCNTs dispersed via the hydrodynamic cavitation method is larger than that of the SWCNTs dispersed by using an ultrasonic homogenizer.

The chemical vapor transport method was used in this research to synthesize MoS2 bulk. Through mechanical exfoliation, we limited the thickness of MoS2 flakes from 1 to 3 μm. In order to fabricate a p-n homogeneous junction, we used oxygen plasma treatment to transform the MoS2 characteristics from n-type to p-type to fabricate a p-n homogenous junction and demonstrate the charge neutrality point shift from −80 to +102 V successfully using FET measurement. The MoS2 p-n homogeneous junction diode showed an excellent p-n characteristic curve during the measurements and performed great rectifying behavior with 1-10 Vpp in the half-wave rectification experiment. This work demonstrated that MoS2 flake had great potential for p-n diodes that feature significant p-n characteristics and rectifying behavior.

To clarify the nature of defects presented in neutron (n)-irradiated highly oriented pyrolytic graphite (HOPG), in situ X-ray diffraction (XRD) observation at room temperature (RT) and high pressure was conducted with synchrotron radiation (SPring-8). We focused on the graphite (002) [G(002)] peak under compression to 18.1 GPa and also under decompression. For comparison, unirradiated HOPG was also placed in the same high-pressure cell. We found that the G(002) peak can be represented by two components, the S and L peaks, for the n-irradiated HOPG, whereas it can be represented by only one component for the unirradiated HOPG. The d-spacing for the n-irradiated and unirradiated HOPG samples gradually decreased with increasing pressure. At 18.1 GPa, the d-spacing of the S peak of the irradiated sample became almost the same as that of the unirradiated one, but that of the L peak was larger. Under decompression, the behavior of the d-spacing was almost opposite to that under compression, and the d-spacing was restored to its value before compression. Also, taking account of the changes in the peak widths, we referred to and considered irradiation-induced defects of interstitial-type defects existing between the basal planes and in-plane defects of dislocation dipoles as possible defects that affect the changes in the G(002) peak.

© 2019 The Japan Society of Applied Physics. The study used multi-layer graphene, synthesized by thermal chemical vapor deposition, as the material for a homo-junction pn device formation. Due to the oxygen adsorption in air, the synthesized pristine graphene behaved as a p-type semiconductor. In order to modulate the graphene semiconductor type, the nitrogen plasma treatment with radio-frequency power as a parameter was used to transfer the semiconductor characteristics. The graphene-based field-effect transistor transfer characteristics were subsequently measured to confirm whether the modified graphene was p-type or n-type. Different nitrogen doping levels affected the diode properties differently. It demonstrated that the rectification effect of the formed pn device was clearly observed using this simple strategy. We also successfully produced this graphene lateral pn device applying a half-wave rectification circuit. The effective formed pn device can be further applied in bipolar junction transistor development.

Organic solar cells are expected to have superior performance in terms of flexibility and low-cost fabrication. However, conventional organic solar cells usually have issues while using rare earth ITO materials and have poor long-term reliability. To seek possible solutions for these issues, we fabricated an inverted solar cell structure of PEN/PEDOT:PSS/PFN/PTB7: PC71BM/MoO3/Au, and found that the fabricated devices had considerably improved long-term reliability when stored in air without any surface passivation layer. The resultant conversion efficiency of the solar cell was 1.88%, and it decreased to 90% of its initial value after 100 h of storage in air.

In this work, bulk heterojunction solar cells based on poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]-thiophenediyl]] and phenyl-C71-butyric-acid-methyl-ester were fabricated using 1,2-dichlolobenzene solutions containing different weight ratios of oleamide. The oleamide layers were self-assembled on the active layer surfaces during the solidification of the active layer after spin coating. A significant increase in open-circuit voltage was observed after the introduction of oleamide at the expense of short-circuit current density. The optimal performance of the solar cell was obtained by spin coating the active layer at 1000 rpm for 60 s using a 1,2-dichlolobenzene solution containing 3% oleamide. The solar cell exhibited a short-circuit current density, an open circuit voltage, a fill factor, and a power conversion efficiency of 13.95 mA/cm(2), 0.79 V, 0.47, and 5.22%, respectively. These solar cell behaviors are discussed on the basis of results of morphological analysis by optical microscopy, atomic force microscopy, and surface energy analysis.

The push coating technique was used to fabricate a bulk heterojunction organic solar cell based on poly[[4,8-bis[(2-ethylhexyl)oxy]benzo [1,2-b:4,5-b0] dithiophene-2,6-diyl] [3-fluoro-2-[(2-ethylhexyl)carbonyl] thieno[3,4-b]-thiophenediyl] (PTB7) and[6,6]-phenyl C71 butyric acid methyl ester (PC71BM). A 1,2-dichlorobenzene solution of PTB7:PC71BM was dropped on a poly(3,4-ethylenedioxythiophene):poly(styrenesulphonate) surface, which was push-coated using a polydimethylsiloxane stamp withdimensions 3 £ 19 mm2. The entire surface under the stamp was covered with only2 mL of the solution. The thickness of the active layer ranged between 25 and 55 nm when a 0200 g weight was placed on the stamp. The push-coated solar cell gaveacceptable performance. The short-circuit current density (8.05 mA/cm2), open circuitvoltage (0.8 V), fill factor (0.41), and power conversion efficiency (2.65%) indicatethat the push coating technique is a suitable solar cell fabrication process that minimises the loss of organic materials.

Bulk-heterojunction solar cells were fabricated using a dichlorobenzene solution of poly[4,8-bis[(2-ethylhexyl)Oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]-thiophenediyl] (PTB7):[6,6]-phenyl-C-71-butyric acid methyl ester (PC71BM) on a flexible indium tin-oxide-coated polyethylene terephthalate substrate. It was found that the performance of the solar cells could be markedly improved by minimizing the spin coating time of a blend of PTB7 and PC71BM to 10 s and maximizing the successive drying and solidification time up to 30 min in a confined Petri dish. As a result, a short-circuit current density of 14.5 mA/cm(2), an open-circuit voltage of 0.62V, and a power conversion efficiency of 3.67% were obtained. These improvements are attributed to the growth of favorable nanostructures during the slow drying process that increased the photocarrier collection efficiency while simultaneously increasing the performance fluctuations of each device. (C) 2014 The Japan Society of Applied Physics

Bulk heterojunction solar cells were fabricated using poly[4,8-bis[(2-ethylhexyl)oxy]benzo [1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]-thiophenediyl] (PTB7) and [6,6]-phenyl C-71 butyric acid methyl ester (PC71BM) after a layer of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) was deposited on a flexible indium tin oxide (110)-coated polyethylene terephthalate substrate. The fabricated structures were Al/LiF/PTB7:PC71BM/PEDOT:PSS/ITO with or. without a lithium fluoride (LiF) buffer layer, and the effect of the LiF buffer layer on the performance of the solar cells was investigated. The LiF layer significantly increased the open-circuit voltages and fill factors of the solar cells, presumably because of the work function shift of the aluminum cathode. As a result, the conversion efficiency increased from 2.31 to 4.02% owing to the presence of the LiF layer. From the results of a stability test, it was concluded-that the inserted LiF layer acted as a shielding and scavenging protector, which prevented the intrusion of some chemical species into the active layer, thereby improving the lifetime of the unpakcaged devices. (C) 2014 The Japan Society of Applied Physics

Nano-indentation manipulator operated in SEM was constructed in order to investigate the electrical contacts on a nanometer scale. Then it is simultaneously possible to observe and control indentation movement and to measure the load force and electrical resistance. Particularly in order to investigate the influence of thick oxide layer on the electrical resistance during indentation, a tin substrate covered with thick tin oxide layer was indented by a tungsten probe. Electrical resistance, measured as a function of indentation depth and load force, drastically decreased in the region where concentric circular and radial cracks and tin penetration in the cracks were found. Resistance decrease was strongly related to the tin appearance on the surface through cracks. The results indicate that the nano-indentation manipulator is powerful tool for the nanometer scale research on electrical contacts.