Shunjiro Fujii

Simple and Scalable Gel-Based Separation of Metallic and Semiconducting Carbon Nanotubes

We report a rapid and scalable method for the separation of metallic and semiconducting single-wall carbon nanotubes (SWCNTs); the separation is performed by the selective adsorption of semiconducting SWCNTs on agarose gel. The most effective separation was realized by a simple procedure in which a piece of gel containing SWCNTs and sodium dodecyl sulfate was frozen, thawed, and squeezed. This process affords a solution containing 70% pure metallic SWCNTs and leaves a gel containing 95% pure semiconducting SWCNTs. Field-effect transistors constructed from the separated semiconducting SWCNTs have been demonstrated to function without any electrical breakdown.

Efficient field emission from an individual aligned carbon nanotube bundle enhanced by edge effect

The authors report on the field emission from an aligned carbon nanotube (CNT) bundle grown by thermal chemical vapor deposition. The CNT bundle showed a low-threshold electric field of 2.0V∕μm that produced a current density of 10mA∕cm2, sustainable evolution of current density up to 2.8A∕cm2 at 2.9V∕μm, and good emission stability without degradation for 200h of continuous dc emission. By calculating the electric-field distribution, it was found that the electric field was significantly higher at the edge of the CNT bundle than at the center. The excellent field-emission properties of the aligned CNT bundle were attributed to the edge effect and the high-density structure.

Air-stable high-efficiency solar cells with dry-transferred single-walled carbon nanotube films

By using the excellent optical and electrical properties of pristine SWNTs with long bundle lengths, we present single-walled carbon nanotube–silicon (SWNT/Si) solar cells of 11% power conversion efficiency (PCE), prepared without doping. The PCEs of the fabricated solar cells even increased slightly after 10 months of exposure to ambient conditions, without any external protection. The open-circuit voltage of the SWNT/Si solar cells under low light intensities, down to 10 mW cm−2, demonstrated the characteristics of the ideal p–n junction model. The mechanism was discussed, taking into account the effect of varying the interfacial oxide layer thickness between the SWNTs and Si on the solar cell’s performance. The high efficiency and stability demonstrated in this study make SWNT/Si solar cells one of practical choices for next generation energy system.

Performance Enhancement of Thin-Film Transistors by Using High-Purity Semiconducting Single-Wall Carbon Nanotubes

Thin-film transistors (TFTs) using a random network of semiconductor-enriched single-wall carbon nanotubes (SWCNTs) were fabricated on a SiO2/Si substrate. Semiconductor-enriched SWCNTs were extracted from a pristine sample by centrifugation using agarose gel. Prior to depositing the SWCNTs, the substrate surface was modified by self-assembly of a monolayer of aminosilanes to produce an ideal two-dimensional network structure. As a result, all the TFTs fabricated on the substrate had on/off current ratios higher than 104 without electrical breakdown, while TFTs fabricated using pristine SWCNTs had a broad distribution of on/off ratios from 101 to 104. This improvement in transfer characteristics demonstrates a major advantage of using semiconductor-enriched SWCNTs.

Flexible PTB7:PC71BM bulk heterojunction solar cells with a LiF buffer layer

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 C71 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 (ITO)-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.

Performance improvement of flexible bulk heterojunction solar cells using PTB7:PC71BM by optimizing spin coating and drying processes

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-C71-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/cm2, an open-circuit voltage of 0.62 V, 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.

Fabrication of screen-printed field electron emitter using length-controlled and purification-free carbon nanotubes

The authors fabricated a screen-printed field electron emitter using purification-free and length-controlled carbon nanotubes (CNTs). They used vertically aligned CNTs grown on Si substrates by thermal chemical vapor deposition as the source material for fabricating CNT paste. The length of CNTs was controlled by adjusting the growth time. The amounts of amorphous carbon and catalyst in the source material were less than 1 and 0.5wt%, respectively, which obviated the need to purify the CNTs. The emitter fabricated using source CNTs with a length of over 80μm showed good reproducibility of current density (J)–electric field (E) characteristics. With a low threshold field Eth of 1.5V∕μm, J=1mA∕cm2 was produced. The emitter exhibited good emission stability for 100h. It was found that the length distribution of the standing CNTs was determined in a precise manner when long CNTs were used as the source material, which led to a highly reproducible fabrication of field emitters.

Fabrication of Cold Cathode Ionization Gauge Using Screen-Printed Carbon Nanotube Field Electron Emitter

We fabricated a Bayard–Alpert-type ionization gauge using a screen-printed field electron emitter with carbon nanotubes (CNTs) as an electron source. Vertically aligned CNTs grown on Si substrates by thermal chemical vapor deposition was used as the source material for CNT paste to make the screen-printed field emitter. The fabricated gauge responded linearly to the pressure in the range from 10-4 to 10-8 Torr, and exhibited a sensitivity of 13 Torr-1, which is competitive with the commercial gauge using a hot-filament electron source. The ion current of the gauge also showed good stability for more than 15 h, indicating the high quality of the electron emitter. This work demonstrated the feasibility of the vacuum electronic device using the CNT field emitter.

Flexible Organic Solar Cells Based on Spin-Coated Blend Films of a Phenylene-Thiophene Oligomer Derivative and PCBM

Flexible organic solar cells were fabricated on a polyethylene terephthalate (PET) substrate using a phenylene–thiophene oligomer, 3,7-bis[5-(4-n-hexylphenyl)-2-thienyl]dibenzothiophene-5,5-dioxide (37HPTDBTSO), as a photoactive donor. In bulk-heterojunction (BHJ) solar cells, many factors such as blend weight ratio, and solution and substrate temperatures have great effects on the photovoltaic performances. The flexible solar cell fabricated from the blend solution with a weight ratio of 1:2 of 37HPTDBTSO to phenyl-C61-butyric acid methyl ester (PCBM) and a temperature of 50°C, exhibited a high open-circuit voltage of 0.74 V and the highest power conversion efficiency of 0.26%.

Bulk heterojunction organic solar cells fabricated using the push coating technique

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-b′] dithiophene-2,6-diyl] [3-fluoro-2-[(2-ethylhe...