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To improve the conversion efficiency of polymer photodetectors (PDs) fabricated by solution process, the properties of fluorene-type polymer photodetectors doped with iridium (Ir) and platinum (Pt) complexes were investigated. The devices based on poly(dioctylfluorene) and poly(dioctylfluorene-co-benzothiadiazole) (F8BT) had violet and blue sensitivity, respectively. Triplet materials can enhance the incident-photon-to-current conversion efficiency of the devices utilizing the fluorene-type polymers when their triplet levels are lower than the lowest excited singlet states of the host and higher than the lowest excited triplet states of the host. The transmission of a moving picture was successfully demonstrated using the bilayer F8BT device with green Ir complex as an opto-electrical conversion device. We demonstrate that the polymer PDs fabricated by solution process can be applied to short-range optical communication fields, such as opto-electrical conversion devices for optical links.
The transient electroluminescence (EL) of phosphorescent organic
light-emitting diodes (OLEDs) was investigated. The behaviors of the
transient characteristics are analyzed using the triplet-triplet
annihilation model. The device exhibited a gradual decrease in quantum
current efficiency owing to the triplet-triplet annihilation at a high
current density. At a higher current density, the reduced rise and decay
times are due to high-density triplet excitons related to the enhanced
triplet-triplet annihilation and the increase of the nonradiative process.
The modulation speed of the devices is mainly limited by the phosphorescent
Organic field effect transistors (OFETs) fabricated on the polymer substrate including conductive organic materials and polymeric gate insulator is one of promising devices for the flexible electronic devices. For OFETs with a conductive organic layer formed on the gate insulating materials, the performance of devices strongly depends on the molecular structure of the gate insulators. In this study, we investigated the effect of hydroxyl group of gate insulating materials on the characteristics of n- and p-type OFETs utilizing [6,6]-phenyl C61-butyric acid methyl ester (PCBM), α, ω-dihexylsexithiophene (DH6T) and pentacene OFETs as a conducting layer, respectively. Poly(p-silsesquioxane) (PSQ) derivatives are used as a polymeric gate insulator, which containes various ratios of phenol-group with a hydroxyl group bonded to a phenyl ring in the side chain of their molecular structures.
We investigated luminescent properties of rare-earth metal complexes and phosphor-rescent molecules doped in a conducting polymer. Electroluminescent (EL) properties in combination of red emissive europium complex of tris(di-benzoylmethane)-mono(4,7-diphenyl-phenanthroline) europium (III) (Eu(dbm)3phen) and blue phosphorescent molecule of bis[(4,6-difluoro-phenyl)-pyridinato-N,C2'] (picolinate) iridium (III) (FIrpic) doped in poly(N-vinyl-carbazole) (PVK) and a new rare-earth complex tris(hexafluoroacethylacetonato) (phenan-throline) samarium(III) [Sm(hfa)3(phen)2] have been investigated as an EL emitter. White EL has been obtained from mixed layer of Eu(dbm)3phen and FIrpic, and EL from Sm complex has been obtained. Energy transfer and the emission mechanisms have been discussed.
The electron injection effects of organic light-emitting diodes (OLED) from the cathode interface between LiF or Li and Al on the tris-(8-hydroxyquinolinato) aluminum (Alq3) emissive layer have been investigated. Efficient electron injection was achieved in the case of thin layer of LiF or Li deposited on Alq3 emissive layer at the thickness of 0.5 nm and 2 nm, respectively. The results indicate that the coexistence of LiF and Al layer or the Li and Al layer on the Alq3 emissive layer result in the efficient electron injection. Electron injection effect in the case of LiF/Al and Li/Al electrode configurations and also the effect of air explosion during the electrode formation have been discussed. In the case of the device with Li, the diffused Li in the Al layer acts as the efficient carrier injection process.
Direct fabrication of organic light emitting diodes (OLED) on a polymeric substrate, i.e., polymeric waveguide substrate to form a flexile optical integrated devices has been realized. The OELD was fabricated by organic molecular beam deposition (OMBD) technique on a polymeric substrate and a glass substrate, for comparison. The device fabricated on a polymeric substrate shows similar device characteristics to that on a glass substrate. Optical signal of faster than 100 MHz has been created by applying pulsed voltage directly to the OLED with emissive layers utilizing rubrene or porphine doped in 8-hydoxyquinolinum aluminum derivatives. Optical signal transmission with OLED fabricated on a polymeric waveguide with optical connectors has been successfully realized. Optical photo detectors (OPD) utilizing phthalocyanine derivatives with superlattice structure provide increased pulse response with input optical signals, and the OPD with 5 MHz of cut-off frequency has been realized with superlattice structure under reverse bias voltage to the OPD.
An organic light emitting device (OLED) has been successfully fabricated on a thin paper-like polyimide substrate (about 10 μm-thick), which is sandwiched between silicone oxide and silicone nitride films. The emission characteristics of the OLEDs, which consist of diamine derivative (α-NPD) and 8-hydroxyquinoline aluminum (Alq3), are similar to those fabricated on a conventional glass substrate. Since the substrates and the OLEDs are very thin like a paper, the devices can be applicable for paper-like displays.
Organic electroluminescent diode (OELD) has been investigated for use as a light source of polymeric optical integrated devices. The OLED was fabricated by organic molecular beam deposition (OMBD) technique. The OLEDs were fabricated on both glass and polymeric substrates. The device fabricated on a polymeric substrate shows similar device characteristics to those on a glass substrate. Optical signals of faster than 100 MHz has been created by applying pulsed voltages directly to the rubrene doped OLED. Optical photo detectors (OPDs) utilizing superlattice structure phthalocyanines provide increased pulse response with input optical signals and the response was faster than 1 MHz.
Enhanced electroluminescence (EL) from vapor deposited p-sexiphenyl (6p) layer has been observed utilizing heterostructure of p-sexiphenyl emissive layer and N,N'-diphenyl-N,N'-(3-methylphenyl)-1,1'-biphenyl-4,4'-diamine (TPD) hole transporting layer. The EL device consists of an indium-tin oxide (ITO) anode, a hole transporting layer, an emissive layer, and a magnesium containing silver cathode. A heterostructure device with 50 nm-thick p-sexiphenyl and 60 nm-thick TPD shows the enhanced emission from the p-sexiphenyl emissive layer. The electroluminescence from the device shows the emission peak centered at 420 nm. The p-sexiphenyl/TPD heterostructure device emits 2 orders of magnitude higher intensity than the conventional single layer of p-sexiphenyl device. The mechanism of enhanced emission from the heterostructure device has been discussed utilizing energy band diagrams of these materials.
Improvement of electroluminescence (EL) efficiency by utilizing quantum confined structure, so-called type I superlattice, has been discussed. Superlattice structures, which consist of 8-hydroxyquinoline aluminum (Alq3) and 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD), and of 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM) and Alq3, and of (1,10-phenanthroline)-tris -(4,4,4-trifluoro -1-(2-thienyl)-butane-1,3-dionate) europium (III) (Eu(TTA )3phen) and (N,N'-disalicylidene-1,6-hexanediaminate) zinc (II) (1AZM -Hex) are studied. As a result, strong EL emission is observed in the type I superlattice structures. The mechanism of enhancement is discussed using energy band structure.
Organic light emitting diodes (OLEDs) which consists of 8-hydroxyquinoline aluminum (Alq3) and diamine derivative (TPD) were directly fabricated on a polymer waveguide device. Polymer waveguide device consists of deuterated methacrylate polymer core and UV cured epoxy resin cladding. One of the edges of the polymer waveguide was cut in 45 degree, which was served as a mirror, in order to introduce the output light from OLED to the waveguide. Indium-tin oxide (ITO) or semi-transparent aluminum metal was deposited onto the polymer waveguide, which served as anode. The OLED was directly fabricated by evaporation technique at the edge of a waveguide, whose edge served as a mirror.
Emission and transmission characteristics of the light from red-light-emitting OLED are also discussed as a light source for the polymer based waveguide with low transmission loss.
Control of organic interfaces by insertion of a thin inorganic film (SiO) has been investigated for an organic electroluminescent (EL) diode which consists of 8-hydroxyquinoline aluminum (Alq3) and diamine derivative (TPD). In order to evaluate optical quality of the emissive layer at the interface, thin film of emissive marker layer was inserted into the emissive layer at the interface between the emissive layer and the carrier transport layer. The EL emission spectrum and the optical characteristics have been discussed for the EL diode with and without inorganic film.
Multicolor electroluminescent (EL) device which emits red (R), green (G) and blue (B) light has been realized by stacking a two-color emission part and a single-color emission part. The former part consists of two emissive layers of red and blue light, which can be selected by changing the polarity of applied field. The latter part consists of a single emissive layer of green light. The emission color from the R-G-B emission device can be modulated by the combination of applying various voltages to the two-color and to the single-color emission parts, separately.
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