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In this study, we investigated the effects on the characteristic changes in OLED devices of using self-assembled monolayers with different functional groups as the hole injection layer, resulting in changes in their performance. Thus, we confirmed that it is possible to control the wetting properties, surface roughness, and work function of the indium tin oxide (ITO) surface by introducing self-assembled monolayers (SAMs). The contact angle measurements confirmed that the substrate surface contact angle tended to increase with SAM deposition. In addition, AFM measurements confirmed that the substrate surface roughness tended to decrease when SAM was deposited on the surface. Finally, it was confirmed through the work function measurement results that the work function increased when the ITO surface was modified by SAM. Furthermore, compared to OLEDs using only the ITO anode, the SAM-modified device showed a higher current density (359.68 A/cm2), improved brightness (76.8 cd/cm2), and a smaller turn-on voltage (7 V). This approach provides a simple route for fabricating organic light-emitting diode applications.

In order to enhance the performance of organic light-emitting diodes, fluorinated self-assembled monolayer (F-SAM) with long alkyl chain was introduced to the device between the anode and hole-tran...

A new Gadolinium Fine Aluminum Gallate (Ce:GFAG) scintillation crystal with both high energy resolution and fast timing properties has successfully been grown. Compared to Gd3Al2Ga3O12 (Ce:GAGG), this new inorganic scintillation crystal has a high luminosity similar to and a faster decay time. In this paper, we report on the timing and energy performance results of the new GFAG scintillation crystal read out with digital silicon photomultipliers (dSiPM) for positron emission tomography (PET) application. The best coincidence resolving time (FWHM) of polished 3×3×5 mm3 crystals was 223±6 ps for GFAG crystals compared to 396±28 ps for GAGG crystals and 131±3 ps for LYSO crystals respectively. An energy resolution (511 keV peak of Na-22) of 10.9±0.2% was attained with GFAG coupled to dSiPM after correcting for saturation effect, compared to 9.5±0.3% for Ce:GAGG crystals and 11.9±0.4% for LYSO crystals respectively. It is expected that this new scintillator may be competitive in terms of overall properties such as energy resolution, timing resolution and growing (raw material) cost, compared to existing scintillators for positron emission tomography (PET).

We examined the electro-optical characteristics of organic light emitting diodes according to the N,N'-diphenyl-N,N'-di(m-tolyl)-benzidine (TPD) thicknesses. The thicknesses of TPD were varied from 5 nm to 50 nm. The current density of the device with a TPD thickness of 5 nm was 8.94 times higher than that with a thickness of 50 nm at a driving voltage of 10 V. According to the conduction&ndash, current characteristics of conductors, the current densities improved with a decreasing TPD thickness. Different from the current density&ndash, voltage characteristics, the current efficiency&ndash, current density characteristics showed an improved efficiency with a 50 nm TPD thickness. The current efficiencies of a device with a 5 nm TPD thickness at a driving voltage of 10 V was 0.148 and at a 50 nm TPD thickness 0.993 cd/A, which was 6.7 times higher than the 5 nm TPD thickness. These results indicated that hole transport in Organic Light-Emitting Diode (OLED) devices were more efficient with thin 5 nm TPD than with thick 50 nm TPD, while electron transport was more efficient with thick 50 nm TPD, which caused conflicting results in the current efficiency-current density and current density-voltage characteristics according to TPD thicknesses.

In this study, we investigated the photoelectric effect and optimization of an organic light-emitting diode (OLED) depending on the presence or absence of a fluorinated self-assembled monolayer (FLSAM) and by varying the thickness of N,N'-Di (1-naphthyl)-N,N'-diphenyl-(1,1'-biphenyl)-4,4'-diamine (α-NPD) from 0 nm to 50 nm. The large distinction in electronegativity between the carbon and the fluorine replacing hydrogen in the alkyl chain of FLSAM generates a strong dipole moment to elevate the vacuum level, resulting in a change of the work function. This eliminates the injection barrier between the work function of the ITO modified by FLSAM and the highest occupied molecular orbital (HOMO) level of the hole-transport layer, thus leading to excellent driving voltage characteristics. Devices without FLSAM had a driving voltage more than twice that of devices using with FLSAM. The introduction of α-NPD as the hole-transport layer enhanced the electrical conductivity by facilitating the transport of holes. However, due to the inherent insulating film properties of α-NPD, the increase in its thickness resulted in a decrease in current density.

The characteristics of a Ag anode with molybdenum oxide (MoOx)-doped N,N′-bis(1-naphthyl)-N,N′-diphenyl-1,1′-diphenyl-1,4′-diamine (α-NPD) are reported. The current efficiency of devices with a Ag anode was achieved over 8.2 cd/A due to the microcavity effect. The current density of the device with a 25% concentration of MoOx was higher than those of a 10% concentration due to the improved charge transfer effect. The device of a Ag anode with a 25% concentration of MoOx indicated an approximate 227% higher current efficiency than that of an ITO anode with a 25% concentration of MoOx. The driving voltage of a Ag anode with a 25% concentration of MoOx was achieved at 4.4V with 10−2 A/cm2.

The electro-optics characteristics of organic light-emitting diodes (OLEDs) with microcavity effect were investigated. The transmittance of the substrate with Ag anode was lower than that of the indium-tin-oxide (ITO) anode due to the opaque characteristics of Ag. The current density of the device with Ag anode was high due to the low sheet resistance of Ag anode. The current efficiency of the device with Ag anode was high due to microcavity structure between Ag anode and Al cathode. Especially, the devices of Ag anode show over 43% higher current efficiency than that of conventional ITO anode. The low transmittance of Ag anode using microcavity effect realized as one kind of Fabry–Perot filters was also verified.

The electrical conductivity behavior of a fluorinated self-assembled monolayer (FSAM) of a molybdenum oxide (MoOx)-doped α-naphthyl diamine derivative (α-NPD) in organic light-emitting diodes (OLEDs) was investigated. The current density of the MoOx-doped α-NPD/FSAM device was proportional to its voltage owing to smooth carrier injection through the FSAM and the high carrier density of its bulk. The temperature-dependent characteristics of this device were investigated. The current density–voltage characteristics at different temperatures were almost the same owing to its very low activation energy. The activation energy of the device was estimated to be 1.056 × 10−2 [eV] and was very low due to the inelastic electron tunneling of FSAM molecules. Copyright © 2014 John Wiley & Sons, Ltd.

We report the electro-optical properties of an organic thin-film by varying the thickness of 1,4,5,8,9,11-hexaazatriphenylene-hexacarbonitrile (HAT(CN)6), included therein as an interlayer. Devices with HAT(CN)6, which are 7 nm thin films used as interlayers, exhibited good current density&ndash, voltage characteristics due to an improved hole injection barrier resulting from carrier ladder effects and carrier transport phenomena. The device without an interlayer showed the worst driving voltage characteristics due to the hole injection barrier. At low driving voltages, a device using 7 nm HAT(CN)6 as an interlayer exhibited a current density about 9.9 times higher than that of a device using 20 nm HAT(CN)6, and showed a current density about 9600 times higher than that of a device without an interlayer. Due to the proper carrier balance, the device using 7 nm HAT(CN)6 as an interlayer achieved a maximum current efficiency of 10.8 cd/A, which was the highest among the devices studied. This shows that the electro-optical properties of devices using HAT(CN)6 as an interlayer are dominated by the holes.

In this report, we discuss the effect of FSAM on both hole-injection and hole-transport functions. The thiophene derivative PEDOT:PSS is widely used as a hole-injection layer of conducting-polymer based LEDs (PLEDs). However, since PEDOT:PSS has some problems of strong acidity and metal cation impurities, a substitute material has been developed as a substitute. We used FSAM as a directly hole-injection layer of PLEDs and a single Alq3 layer. The introduction of FSAM lowered the driving voltage of devices and improved EL efficiency per current. In the former case, the EL properties of PLED with PEDOT:PSS is superior to those of PLED with FSAM. In the latter case, it was found the increase in hole injection contributed to high conduction of OLEDs. We reviewed the basic characteristics of polymer and OLED using FSAM.