Tsuyoshi Tsujioka

Selective metal-vapor deposition signifies that metal-vapor atoms are deposited on a hard organic surface, but not on a soft (low glass transition temperature, low T-g) surface. In this paper, we introduce the origin, extension, and applications of selective metal-vapor deposition. An amorphous photochromic diarylethene film shows light-controlled selective metal-vapor deposition, which is caused by a large T-g change based on photoisomerization, but various organic surfaces, including organic crystal and polymers, can be utilized for achieving selective metal-vapor deposition. Various applications of selective metal-vapor deposition, including cathode patterning of organic light-emitting devices, micro-thin-film fuses, multifunctional diffraction gratings, in-plane electrical bistability for memory devices, and metal-vapor integration, have been demonstrated.

Selective metal deposition signifies that metal vapor atoms are deposited on a hard organic surface but not on a soft (low glass transition temperature) surface. This paper introduces the origin, extension, and device applications of selective metal deposition. A photochromic surface shows the light-controlled selective metal deposition phenomenon, but the surface material is not essential to it; various organic surfaces including organic crystal and polymer surfaces can be used. We demonstrate the applications of selective metal deposition, including cathode patterning for organic light-emitting devices, micro thin-film fuses, and multifunctional diffraction grating. Selective metal deposition can be applied to a variety of electric and optic devices.

Photochromic diarylethene crystal films are promising candidates for future organic memory applications but this has not yet been studied. We found that a specific molecule of photochromic diarylethenes (DAEs) formed a crystal film that consisted of extraordinary large crystallites with a 1 cm scale domain. We investigated the carrier injection/transport characteristics in the DAE crystal film and found that photocontrolled current switching using UV (lambda = 365 nm) irradiation showed a large on- off ratio of 500, which was 50- times that of the amorphous film. Positive temperature dependence of the current was observed for the initial colorless crystal film, whereas negative dependence for the colored and decolored films was observed. The current level of the decolored sample was lower than that of the initial colorless state and the difference in the current levels was based on the change from the crystal to the amorphous state. The results are essential for achieving organic memory applications with DAE crystals.

In organic electronics, organic crystals are essential materials, and metal deposition on organic surfaces is an important process. We report light-controlled metal deposition on photosensitive organic crystals. Light irradiation to the photosensitive diarylethene (DAE) crystal surface generated another isomer in the crystal lattice and made the surface soft, which caused the Mg-undeposition effect. The light-controlled selective Mg deposition can be applied to fine metal patterning by laser scanning and maskless evaporation. Fine parallel and grid Mg patterning on the DAE crystal surface are demonstrated successfully. This method is promising for fine electrode patterning of various electronics devices with an organic crystal.

Recent progress in electrical properties of photochromic molecules is reviewed. A typical application of the properties is current switching based on the changes in ionization potential or carrier mobility induced by the photoisomerization of photochromic molecules. Carrier injection-type molecular memories have also attracted wide interest because they are a promising candidate for organic semiconductor devices in the field of organic electronics. Various new applications are proposed using photo-induced electrical, as well as optical, property changes of photochromic molecules. (C) 2010 Elsevier B.V. All rights reserved.

Fine metal patterning with a small work function is indispensable to the high integration of organic electronics devices including organic light-emitting devices and organic memories. Selective metal deposition on photochromic surfaces is a promising method to prepare such metal patterns. We found that the surface physical properties of polymer films for metal deposition changed depending on their drying temperature and the photoisomerization state of the doped-photochromic diarylethene (DAE). The Mg deposition property on the DAE-doped polystyrene (PS) film was dramatically switched by light irradiation. Light-controlled selective Mg deposition for poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) was also achieved. Fine metal patterning with a width of 15 mm on DAE5%-doped polymer film with maskless vacuum evaporation was successfully demonstrated. Light-controlled selective metal deposition will contribute to the large scale integration of polymer electronics.

Fine metal patterning was performed by selective Mg deposition on photopolymers. Mg patterns with a minimum width of 5 mu m were obtained by using maskless vacuum evaporation. The selective deposition originates in the difference of glass transition temperature, microscopically, of surface molecular motion between polymerized and nonpolymerized photopolymer surfaces. The difference between photoreactive small molecules and polymers was also discussed. The selective metal deposition method would be applied to a wide range of organic surfaces; this method showed great potential for the preparation of fine wiring for various organic electronic devices.

Selective metal Mg deposition was achieved for a structure with the 8-nm-thick intermediate layer of Alq3 on a photochromic diarylethene (DAE) layer. Selective deposition for a structure with the Alq3 intermediate layer was attributed to uncolored DAE molecules exposed on the surface due to the migration and the aggregation of Alq3 molecules on the uncolored DAE layer. Laser spot irradiation enables both isomerization and an annealing effect on samples with an Alq3 intermediate layer, thereby achieving selective Mg deposition. Selective deposition for a structure with an intermediate layer could be important for the preparation of patterned cathodes in the field of organic electronics.

We developed an electrode/wiring patterning method that does not employ evaporation shadow masks; this method is based on selective metal deposition of photochromic diarylethene (DAE). In the selective Mg deposition based on the photoisomerization of DAE, Mg vapor atoms are deposited only on colored DAE film obtained upon UV irradiation, but not on uncolored film. We demonstrated fine metal Mg patterning with a minimum width of 3 mu m and the preparation of a patterned cathode. The selective metal deposition method has significant potential for preparing fine electrodes/wiring for various organic electronic devices.

Electrical carrier-injection and transport characteristics of photochromic diarylethene thin films were investigated. Injected and transported carriers (injected current) to the diarylethene film were increased with the existence ratio of the closed-ring form, which was one isomerization state of the diarylethene molecule. A dramatic increase was observed at a 12-percent existence ratio of the closed-ring molecules. A lowering effect of the potential barrier, which is caused by a Poole-Frenkel-like effect with a short distance interaction potential for hole transportation by increasing the existence ratio of the closed-ring molecules is proposed as the mechanism behind this phenomenon. (C) 2003 American Institute of Physics.

A principle of organic memory device using a bistable photochromic molecule is presented that allows extremely high bit densities and very low power consumption. This device is based on an isomerization reaction of photochromic diarylethene molecule via its excited state by an electric carrier injection, not by photon absorption. Experimental data show that the reversible writing and nondestructive reading of information by the carrier injection is feasible. The advantages and properties of such an organic semiconductor memory using a bistable molecule are discussed. (C) 2003 American Institute of Physics.

We proposed and demonstrated a nondestructive readout method using photocurrent detection for photon-mode photochromic memory, The principle of this readout method, which utilized the ionization potential change according to photoisomerzation reaction, was confirmed by using a medium with a photochromic diarylethene layer and phthalocyanine photoabsorbing layer, and by using a near-infrared readout light. We demonstrated perfect nondestructive readout operations over 10(6) times. (C) 2001 American Institute of Physics.

We propose an emitting assist (EA) dopant system for obtaining organic light-emitting diodes (OLEDs) with pure red emission. The EA dopant (rubrene) did not itself emit but assisted the energy transfer from the host (Alq(3)) to the red emitting dopant (DCM2). The cell structure used was {indium tin oxide/hole injection layer [(20 nm), CuPc/hole transport layer (50 nm), NPB/emitting layer (40 nm), Alq(3)+DCM2 (2%)+rubrene (5 wt %)]/MgIn}. (CuPc: Copper (II) phthalocyanine, NPB: N, N-'-Di(naphthalen-1-yl)-N, N-'-diphenyl-benzidine, DCM2: 4-Dicyanomethylene- 2-methyl-6-[2-(2,3,6,7-tetrahydro-1H,5H-benzo[ij]quinolizin-8-yl)vinyl]-4H-pyran). A stable red emission (chromaticity coordinates: x=0.64, y=0.36) was obtained in this cell within the luminance range of 100-4000 cd/m(2). When the cell was not doped with rubrene, the emission color changed from red to orange as the luminance increased. The EA dopant system is a promising method for obtaining red OLEDs. (C) 1999 American Institute of Physics. [S0003- 6951(99)04538-6].