On surfaces of tris-(8-hydroxyquinolate) aluminum (Alq) and tris(7-propyl-8-hydroxyquinolinato) aluminum (Al7p) thin-films, positive and negative polarization charges appear, respectively, owing to spontaneous orientation of these polar molecules. Alq is a typical electron transport material where electrons are injected from cathode. Because the polarization charge exists at the Alq/cathode interface, it is likely that it affects the electron injection process because of Coulomb interaction. In order to evaluate an impact of polarization charge on electron injection from cathode, electron only devices (EODs) composed of Alq or Al7p were prepared and evaluated by displacement current measurement. We found that Alq-EOD has lower resistance than Al7p-EOD, indicating that the positive polarization charge at Alq/cathode interface enhances the electron injection due to Coulomb attraction, while the electron injection is suppressed by the negative polarization charge at the Al7p/Al interface. These results clearly suggest that it is necessary to design organic semiconductor devices by taking polarization charge into account.

Nowadays, semiconductor quantum dots have attracted intense attention as emissive materials for light-emitting diodes, due to their high photoluminescence quantum yield and the controllability of their photoluminescence spectrum by changing the core diameter. In general, semiconductor quantum dots contain large amounts of organic ligands around the core/shell structure to obtain dispersibility in solution, which leads to solution processability of the semiconductor quantum dot. Furthermore, organic ligands, such as straight alkyl chains, are generally insulating materials, which affects the carrier transport in thin-film light-emitting diodes. However, a detailed investigation has not been performed yet. In this paper, we investigated the luminance characteristics of quantum-dot light-emitting diodes containing ZnCuInS2 quantum dots with different carbon chain lengths of alkyl thiol ligands as emitting layers. By evaluating the CH2/CH3 ratio from Fourier-transform infrared spectra and thermal analysis, it was found that approximately half of the oleylamine ligands were converted to alkyl thiol ligands, and the evaporation temperature increased with increasing carbon chain length of the alkyl thiol ligands based on thermogravimetric analysis. However, the photoluminescence quantum yield and the spectral shape were almost the same, even after the ligand-exchange process from the oleylamine ligand to the alkyl thiol ligand. The peak wavelength of the photoluminescence spectra and the photoluminescence quantum yield were approximately 610 nm and 10%, respectively, for all samples. In addition, the surface morphology of spin coated ZnCuInS2 quantum-dot layers did not change after the ligand-exchange process, and the root-mean-square roughness was around 1 nm. Finally, the luminance efficiency of an inverted device structure increased with decreasing carbon chain length of the alkyl thiol ligands, which were connected around the ZnCuInS2 quantum dots. The maximum luminance and current efficiency were 86 cd/m2 and 0.083 cd/A, respectively.

Transparent organic light-emitting diodes (TOLEDs) were investigated with top electrode of indium-tin-oxide (ITO) by ion-plating method. High deposition rate of 4.4 nm/s was realized without plasma damage of under organic layer. In the TOLEDs with inverted structure, high transmittance of over 75% at 550 nm and bright emission of 1,850 and 1,410 cd/m2, from bottom and top side at 163 mA/cm2, respectively, were obtained.

A highly efficient sky-blue organic light-emitting diode (OLED) based on a thermally-activated delayed fluorescence (TADF) molecule, 1,2-bis(carbazol-9-yl)-4,5-dicyanobenzene (2CzPN), was studied. The sky-blue OLED exhibited a maximum external electroluminescence quantum efficiency (ηEQE) of over 24.0%. In addition, a white OLED using 2CzPN combined with green and orange TADF emitters showed a high ηEQE of 17.3% with a maximum power efficiency of 52.3 lm/W and Commission Internationale de l'Eclairage coordinates of (0.32, 0.43).

This paper presents an experimental investigation on the RF characteristics of a 3W-class cryogenically-cooled receiver amplifier employing a gallium-nitride high electron mobility transistor (GaN HEMT) with a blue light for mobile base stations. In general, a cryogenically-cooled receiver amplifier using a GaN HEMT exhibits unstable DC characteristics similar to those found in the current collapse phenomenon because the GaN HEMT loses thermal energy at cryogenic temperatures. The fabricated cryogenically-cooled receiver amplifier achieves stable DC characteristics by injecting blue light into the GaN HEMT instead of thermal energy. Experimental results show that the amplifier achieves fine stable DC characteristics for deviation in the drain-source current from 42% to 5% and RF characteristics for a maximum power added efficiency from 58% to 68% without and with the blue light at 60,K. The fabricated amplifier is effective in reducing the power consumption at cryogenic temperatures. To the best of our knowledge, this paper is the first report regarding RF characteristics of a cryogenically-cooled receiver amplifier using a blue light for mobile base stations.

The fabricated 1.55 µm high power superluminescent light emitting diodes (SLEDs) with 115 mW maximum output power and 3 dB bandwidth of 50 nm, using active multi-mode interferometer (MMI), showed high coupling efficiency of 66% into single-mode fiber, which resulted in maximum fiber-coupled power of 77 mW.

High luminance and high speed response with the cut-off frequency of more than 50 MHz in multilayer polyfluorene-based light-emitting diodes with an interlayer were achieved. We realized multilayer polyfluorene-based light-emitting diodes for frequency response up to 100 MHz.

White-light emitting diode lamps for general illumination can be realized by a combination of a blue light-emitting diode semiconductor die and phosphors. Newly developed oxynitride and nitride phosphors are promising candidates for this application because they have suitable excitation and emission wavelengths and stable optical properties in a high temperature environment. High brightness warm-white LED lamps have been realized using a yellowish-orange α-SiAlON oxynitride phosphor. High color-rendering index white LED lamps have been also realized using three color oxynitride/nitride phosphors.

In this paper, we describe an accelerative current-programming method for active matrix OLED (AM-OLED) display. This new method uses common source configuration, "Acceleration Control" line and some mechanisms to prevent the programming current from flowing through OLED device. It would solve the basic problem of the current-programming pixel circuit: a long programming period, especially at the dark gray-level. The proposed method accelerates the current programming process at any gray levels, and it would be the solution for the problem.

We have demonstrated improvement in the efficiency of TDAPB-based OLEDs. The external quantum efficiency of 8.2% and a power efficiency of 17.3 lm/W were achieved. The results suggest that using the starburst small-molecule TDAPB allows for easy fabrication and is effective for achieving high efficiencies in simple device structures.

In this letter, we describe a four thin-film-transistor (TFT) pixel circuit based on hydrogenated amorphous silicon (a-Si:H) technology for the active-matrix organic light-emitting diode (AMOLED) display applications. The circuit uses current-writing mechanism and can automatically adjust the threshold-voltage shifts of both the organic light-emitting diodes (OLEDs) and the TFTs induced by the circuit aging or process variations. Experimental results indicate virtually no variation of the output driving current after long-term bias-temperature-stress (BTS).