Keisuke Ebata

Polysilane light-emitting diodes

This paper discusses the material factors which have led to a recent breakthrough in polysilane light-emitting diodes (LEDs) made from a diaryl polysilane, poly[bis(p-n-butylphenyl)silane] (PBPS), by comparing them with LEDs which employ a conventional polysilane, poly(methylphenylsilane). In contrast to LEDs based on conventional polysilanes in which a weak ultraviolet electroluminescence (EL) was detected either with a strong broad visible EL or only at low temperatures, room-temperature pure near-ultraviolet EL was observed with a quantum efficiency of 0.1% photons/electron with an electron injecting A1 electrode in PBPS-LEDs. We examined the spectroscopic, electronic and structural properties of PBPS, and ascribed to them the improvements observed in the EL characteristics. We also mention the possible future direction of polysilane LED research and other potential optoelectronics applications of polysilanes to the active medium of lasers. Copyright

Near-ultraviolet electroluminescent performance of polysilane-based light-emitting diodes with a double-layer structure

We fabricated double-layer light-emitting diodes (LEDs) by utilizing poly[bis(p-nbutylphenyl)silane] (PBPS) and oxadiazole derivatives, and investigated their basic LED characteristics. The near-ultraviolet electroluminescence (EL) performance, such as the EL threshold electric field and the current density, depended on the oxadiazole derivatives used as the electron transport materials as well as the components of the EL emission. We observed better EL performance where the EL external quantum efficiency in a double-layer LED with a 2-(4′-tert-butylphenyl)-5-(4″-biphenyl)-1,3,4-oxadiazole based electron transport layer was twice that of a PBPS single-layer LED. By contrast, we observed a worse EL threshold electric field and current density when we used 2,5-bis(1-naphthyl)-1,3,4-oxadiazole as an electron transport material. The reason for the difference in the EL performance was revealed by investigating the charge carrier injection and transport dynamics of the two LEDs.

4,8-Dichloroocta-t-butyltetracyclo[3.3.0.02,7.03,6]octagermane

A polyhedral germane, 4,8-dichloroocta-t-butyltetracyclo[3.3.0.02,7.03,6]octagermane, was prepared by reductive reactions of either 1,2-di-t-butyl-1,1,2,2-tetrachlorodigermane or t-butyltrichlorogermane with lithium naphthalenide. The structure of the new compound was determined by X-ray crystallographic analysis as well as by NMR spectra.

Molecular weight dependent electroluminescence of silicon polymer near-ultraviolet light-emitting diodes

We investigated the electroluminescence (EL) of single-layer near-ultraviolet (NUV) light-emitting diodes (LEDs) made from poly[bis(p-n-butylphenyl)silane] (PBPS) with three different molecular weights (MWs). Although the NUV EL spectra of the three LEDs exhibited no noticeable differences, we observed a marked MW dependence on such aspects of the operating performance as the EL external quantum efficiency, EL threshold current density and electric field, which were improved as the MW of PBPS decreased. The MW dependence of the hole transport behavior suggested that the MW decrease promoted positive space charge formation in the PBPS layer during LED operation. We attributed the origin of the MW dependence of the LED performance to this positive space charge formation, which played an important role in improving the electron–hole supply balance from the external electrodes of the LED.

Preparation and single molecule structure of electroactive polysilane end-grafted on a crystalline silicon surface

Electrically active polysilanes of poly(methylphenylsilane) (PMPS) and poly[bis(p-n-butylphenyl)silane] (PBPS), which are, respectively, known as a good hole transporting material and a near-ultraviolet electroluminescent material, are end-grafted directly on a crystalline silicon surface. The single polysilane molecules are clearly distinguished one from the other on the surface by means of atomic force microscopy observations. End-grafted single molecules of PMPS are observed as dots while end-grafted PBPS appear as worms extending for more than 100 nm on the crystalline silicon surface.

AN ISOLATED SILICON SINGLE CHAIN END-GRAFTED ONTO A SUBSTRATE SURFACE

We report atomic force microscopy images of one-dimensional silicon chains (polysilanes) individually bonded to a substrate surface [quartz glass (SiO2), sapphire (Al2O3), and Si(111)]. The sample was prepared by a one-to-one chemical reaction between a reactive anchor of alkylbromide on the surface and a polysilanyl lithium. We observed dot images with variable densities which depended on the reactive anchor density. Each dot, with a diameter of about 20 nm and a height of about 5 nm, corresponded to an isolated single polysilane molecule with a collapsed structure typical of usual end-grafted polymers under poor solvent conditions.

Molecular weight dependence of the conformational phase transition and electroluminescence of diarylpolysilane diodes

We investigated the molecular weight dependence of the conformational phase transition behavior of thin films of poly[bis(p-n-butylphenyl)silane] (PBPS), which is a near-ultraviolet (NUV) electroluminescence (EL) material for polysilane-based polymer light-emitting diodes (LEDs). A low molecular weight PBPS with efficient NUV EL exhibited a phase transition from a disordered to an ordered backbone conformation when we increased the temperature. By contrast, the opposite transition behavior was observed for a high molecular weight PBPS. The photoluminescence (PL) quantum yield of the PBPS films changed in association with the thermal phase transition behavior; the PL quantum yield of the low molecular weight PBPS was improved by increasing the temperature, whereas that of the high molecular weight polymer deteriorated. The time dependence of the EL investigation suggests that, although the EL intensity continued to increase for the first few hours, insufficient Joule heat was generated to induce a phase tr...

Conformational phase transition in a high-efficiency near-ultraviolet electroluminescent diarylpolysilane

Abstract We report on the observation of a structural phase transition in a diarylpolysilane, poly[bis( p - n -butylphenyl)silane] (PBPS), which was recently shown to emit high-efficiency room-temperature electroluminescence (EL) in the near-ultraviolet region. Spectroscopic and calorimetric experiments have revealed that this structural phase transition accompanies a conformational change in the Si-backbones of PBPS from the disorder form (lower temperature form) to the trans -planar form (higher temperature form). The spectroscopic changes induced by this structural phase transition are discussed in relation to the EL characteristics of PBPS, and found to have positive effects on them.

High-pressure synthesis, structure and novel photochemical reactions of 7,7,8,8-tetramethyl-7,8-disilabicyclo[2.2.2]octa-2,5-diene

7,7,8,8-Tetramethyl-7,8-disilabicyclo[2.2.2]octa-2,5-diene 1, prepared by the high-pressure reaction (10 000 bar) of 1,1,2,2-tetramethyl-1,2-disilacyclohexa-3,5-diene and phenyl vinyl sulphoxide followed by elimination of benzenesulphenic acid, gave tetramethyldisilene (Me2SiSiMe2) upon photolysis which underwent a photochemical [2 + 4] reaction with benzene at 10 K in an argon matrix to regenerate the precursor.

Soft X-ray emission and absorption spectroscopy for electronic structure analysis of cubic silicon clusters in Si K-shell threshold

Abstract Soft X-ray emission and absorption (SXEA) spectroscopy is presented as a possible method for electronic structure analysis. To demonstrate its feasibility, the electronic structure of silicon backbones in a cubic silicon cluster (octasilacubane) and its related clusters was analyzed by measuring the SXEA spectra in the Si K-shell threshold. Three discrete levels are observed in both Si3p occupied and unoccupied orbitals of octasilacubane, which may be caused by the large degree of degeneracy of the orbitals due to the high symmetry of the cubic silicon backbone structure. The measured narrow energy gap of 2.3 eV between the highest occupied and the lowest unoccupied orbitals of octasilacubane shows that Si3p σ-electrons in octasilacubane are more widely conjugated than those in the related clusters.